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Endocrinology Vol. 142, No. 4 1659-1668
Copyright © 2001 by The Endocrine Society


ARTICLES

Immunohistochemical Localization of the Vasopressin V1b Receptor in the Rat Brain and Pituitary Gland: Anatomical Support for Its Involvement in the Central Effects of Vasopressin1

Fernando Hernando, Oscar Schoots, Stephen J. Lolait and J. Peter H. Burbach

Section of Molecular Science (F.H., O.S., J.P.H.B.), Department of Medical Pharmacology, Rudolf Magnus Institute for Neurosciences, University Medical Center, 3584 CG Utrecht, The Netherlands; and Department of Medicine (S.J.L.), University of Bristol, Bristol, BS2 8HW United Kingdom

Address all correspondence and requests for reprints to: J. P. H. Burbach, Rudolf Magnus Institute for Neurosciences, Universiteitsweg 100, 3584 CG Utrecht, The Netherlands. E-mail: j.p.h.burbach{at}med.uu.nl


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Biological effects of vasopressin (VP) are mediated by four different receptors, two of which (the V1a and the oxytocin receptors) have been well characterized in the rodent brain, suggesting that these are the main receptors responsible for the central effects of VP. However, transcripts of the V1b VP receptor (V1bR) have been detected throughout the rat brain by RT-PCR and in situ hybridization, indicating that the V1bR adds to the population of central VP receptors. Because there are no specific ligands for the V1bR, the receptor protein itself has been difficult to visualize. In the present study, the distribution of the V1bR protein was investigated in the rat forebrain, midbrain, hindbrain, and cerebellum by immunohistochemistry using an antiserum raised against a synthetic fragment of the carboxylterminal of the rat V1bR protein. Immunohistochemistry revealed the presence of the V1bR in pituitary corticotrophs as expected. In naive, untreated rats, fiber networks containing V1bR-immunoreactivity were mainly concentrated in the hypothalamus, amygdala, cerebellum, and particularly in those areas with a leaky blood brain barrier or close to the circumventricular organs (medial habenula, subfornical organ, organum vasculosum laminae terminalis, median eminence, and nuclei lining to the third and fourth ventricles). A strikingly dense network was present in the external zone of the median eminence. Colchicine treatment was required to reveal the localization of V1bR-immunoreactive cell bodies. V1bR-containing cell bodies and associated protrusions were mainly located in the hippocampus, caudate putamen, cortex, thalamus, olfactory bulb, and cerebellum. These results demonstrate the widespread distribution of the V1bR protein in the rat brain over multiple, functionally distinct neuronal systems. These data suggest that the V1bR mediates different physiological functions of VP in the brain.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
THE NEUROHYPOPHYSIAL HORMONE vasopressin (VP) regulates multiple functions in the central nervous system (CNS), including learning and memory processes, adaptive and sexual behaviors, blood pressure control, and body temperature. These central functions of VP are mediated by several distinct neuronal systems that innervate hypothalamic and limbic brain areas (for review, see 1). Centrally projecting VP neurons are mainly located in the parvicellular part of the paraventricular hypothalamic nucleus, the suprachiasmatic nucleus (SCN), the bed nucleus of the stria terminalis, the medial amygdala, and the locus coeruleus (2, 3, 4).

Although there are four different G protein-coupled receptors that can be activated by VP [the V1a, V1b, V2, and the receptor for oxytocin (OT)], only two subtypes (the V1a and the OT receptors) have been well characterized in the rat brain, suggesting that they are the main receptors responsible for the central functions of VP (1, 5, 6). The peripheral V2 receptor is expressed almost exclusively in kidney, where it mediates the antidiuretic action of VP (7, 8, 9, 10). To date, the V1b receptor (V1bR) is the most obscure receptor. It has been characterized in the anterior pituitary, where it mediates the VP-stimulated corticotrophin secretion (11, 12). However, previous pharmacological studies implicated the V1bR in central effects of VP, including the control of thermoregulation (13) and the stimulation of CRH secretion from the rat hypothalamus (14). In addition, RT-PCR and in situ hybridization studies revealed that transcripts of the cloned rat V1bR were much more widely distributed in the rat brain than expected (15, 16, 17, 18). However, the lack of specific ligands for the V1bR (5, 19), as well as its presumably low expression level in the rat brain, would make it difficult to detect this receptor by binding studies. Therefore, we decided to use an immunohistochemical approach to determine the localization of the V1bR protein in the rat brain, as well as to validate its reported messenger RNA localization, by studying the sites of V1bR protein synthesis after blocking the axonal transport. The results define a multiple restricted set of V1bR-expressing neurons and terminal fields with the anatomical properties to mediate distinct functions of VP in limbic, neuroendocrine, and autonomic regulation.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Animals
Male Wistar rats, weighing 225–275 g (U:WU, Central Animal Facility, Utrecht University), were housed under a 12-h light, 12-h dark cycle with free access to water and food. All animals were perfused intracardially (see below). To block the axonal transport in some animals, colchicine (50 µg/animal; Sigma, Zwÿndrecht, The Netherlands) was intraventriculary administrated, 48 h before the cardiac perfusion, under Hypnorm (Duphar, Amsterdam, The Netherlands) anesthesia (fentanyl citrate; 0.3 mg/kg, ip). After recovering from the anesthesia, these animals were given buprenorfine (1 ml/kg), with intervals of 8 h.

Antiserum
A polyclonal antiserum was raised against a synthetic peptide (NH2-AKPRPAGSLKDLEQVDGE-COOH) derived from the carboxylterminal of the rat V1bR protein (15). This peptide was coupled to KLH fixed with glutaraldehyde and mixed 1:1 with complete Freund’s adjuvant and im injected into rabbits (for details, see 20). A bleeding before the immunization (preimmune serum) and three different bleedings at 6, 8, and 10 weeks after immunization (immune sera) were obtained. All immune sera gave similar results on tissue sections. We will refer to this antiserum as V1bR antiserum.

Immunoblot analysis
A fragment of the V1bR protein complementary DNA (cDNA) (nucleotides 545-2559) (15) was subcloned into pGEX-3x, transformed in Escherichia coli (strain DH5{alpha}), and expressed as soluble protein, after induction with isopropyl ß-D-thiogalacopyranosido, as described by Smith and Johnson (21). The recombinant plasmid encoded a 27.5-kDa polypeptide fragment of glutathione S-transferase (GST) fused to the V1bR protein (GST-V1bR). The total molecular mass of the GST-V1bR fusion protein was 74.2 kDa. A protein fraction from bacterial cultures expressing GST-V1bR was electrophoretically separated on 11% SDS-polyacrylamide gels, according to the method of Laemmeli (22), and transferred to nitrocellulose membranes. The membranes were blocked with 5% goat serum and 5% dry milk at 4 C overnight and further incubated with the V1bR antiserum, the preimmune serum, or the preabsorbed antiserum (V1bR antiserum preincubated with the peptide fragment used to raise the antiserum) at a dilution of 1:1000 for 1 h. After incubation with a horseradish peroxidase-labeled goat antirabbit IgG (Sigma), at 1:10.000 dilution for 1 h, the immunoreactive bands were visualized with the ECL kit Amersham Pharmacia Biotech (Buckinghamshire, UK). As controls, protein extracted from cultures or bacteria expressing the empty GST vector were used.

Antibody specificity was also determined by using HEK-293 cells transiently transfected with an expression plasmid (pcDNA3.1) encoding the receptor subtypes V1a or V1b, using the Ca2+-phosphate precipitation procedure. These cells were homogenized in buffer (10 nM HEPES, pH 7.4; 0.5 mM EDTA; 50 ìg/ml phenylmethylsulfonylfluoride; 0.5 ìg/ml leupeptine; 0.5 ìg/ìl pepstatine) by sonication, for 20 sec, and membranes were prepared. The protein was extracted with Triton X-100 (final concentration, 1%) and NaCl (final concentration, 0.2 M) and separated in an 11% SDS-polyacrylamide gel, transferred to a nitrocellulose paper, and probed with the antiserum as previously described.

Immunohistochemical characterization of the antiserum
BHK cells grown in DMEM containing 10% FCS were transiently transfected with recombinant plasmid (pcDNA3.1) encoding the rat V1a or V1b receptors, using the Ca2+-phosphate precipitation procedure. Forty-eight hours after transfection, cells were washed and fixed with 4% paraformaldehyde in 0.1 M sodium phosphate buffer, pH 7.4. The fixed cells were further blocked in TNB buffer [0.1 M Tris-HCl, pH 7.5, 0.15 M NaCl, and 0.5% blocking reagent from the TSA amplification kit (NEN Life Science Products, Boston, MA)], followed by incubation with the V1bR antiserum, with or without preabsorption with the peptide fragment used to raise the antiserum, diluted at 1:250 in THZT buffer (50 mM Tris, 0.5 M NaCl, and 0.5% Triton X-100), at room temperature for 1 h, and further at 4 C overnight. Immunohistochemical staining was visualized using the TSA-indirect amplification kit (NEN Life Science Products) as described in the instructions manual. Briefly, after washing the primary antiserum with TNT buffer (0.1 M Tris-HCl, pH 7.5; 0.15 M NaCl; 0.05% Tween 20), the cells were incubated with a biotinylated antirabbit IgG (Vector Laboratories, Inc., Burlingame, CA) in TNB buffer at room temperature for 1 h, followed by a peroxidase-conjugated streptavidine in TNB buffer at room temperature for 30 min. After washing the cells with TNT buffer, the cells were incubated with the biotinyl tyramine at room temperature for 10 min, followed by a fluorescein-conjugated streptavidine. Finally, the cells were embedded in DABCO-Mowiol and analyzed under an inverted fluorescence microscope (Carl Zeiss, Axiovert, Germany).

Immunohistochemistry on primary cultures from anterior pituitary cells
Rats were decapitated, and the anterior pituitary gland was removed under sterile conditions. The tissue was minced with a sterile surgical blade, and the pituitary fragments were incubated with trypsin (0.5 mg/ml) and deoxyribonuclease (10 µg/ml; Sigma) in DMEM (Life Technologies, Inc., Gaithersburg, MD) containing 25 mM HEPES (Sigma) at 37 C for 20 min. The cells were further pelleted by centrifugation and resuspended in trypsin inhibitor (10 µg/ml; Sigma) and deoxyribonuclease (10 ìg/ml) in DMEM containing 25 mM HEPES and were passed through a sterile pipette tip for 10 min. Finally, the cells were pelleted by centrifugation and resuspended in DMEM containing 10% FCS and plated at a density of 200,000 cells/well in 2-cm2, 24-well plastic cluster dishes (Costar, Cambridge, MA). Cell cultures were incubated, at 37 C, in an humidified atmosphere of 95% air-5% CO2. After 3–4 days of culture, cells were fixed using 4% paraformaldehyde in 0.1 M phosphate buffer, pH 7.4, for 20 min and further washed several times with Tris-buffered saline (TBS) (50 mM Tris and 0.15 mM NaCl). The cells were incubated with V1bR antiserum together with a monoclonal antiserum against ß-endorphin (provided by Dr. Wiegant, Rudolf Magnus Institute for Neurosciences, Utrecht University) in THZT buffer, for 1 h at RT and overnight at 4 C. After washing several times with TBS, cells were incubated with 1:200 dilution of conjugated fluorescein isothiocyanate-goat antirabbit IgG (DAKO Corp., Glostrup, Germany) together with a dilution of 1:500 of conjugated Texas Red-goat antimouse IgG (DAKO Corp.) for 2 h at room temperature. The cells were further washed several times with TBS, embedded in DABCO-Mowiol, and examined under a inverted fluorescence microscope (Carl Zeiss).

Immunohistochemistry on tissue sections
Rats were anesthetized with Nembutal (sodium pentobarbital, 1 ml/kg, Sanofi Pharmaceuticals, Inc., Libourne, France) and intracardially perfused with NaCl 0.9% (200 ml) followed by a fixative solution (400 ml). Different fixatives were tested: 1) periodate-lysine-paraformaldehyde mixture made as described by Mc Lean and Nakane (23), with the exception that the final solution was adjusted to pH 7.4 with 10 N NaOH; 2) 4% paraformaldehyde; 3) 4% paraformaldehyde and 2% glutaraldehyde; and 4) Bouin’s solution (1.17% picric acid with 9% formalin and 6% acetic acid). After perfusion, the brain and pituitary gland were dissected, immersed in the same fixative solution, on a rocking table, at 4 C overnight, rinsed in 50 mM Tris- 0.9% NaCl buffer (pH 7.6) (TBS), and subsequently transversely sectioned with a vibratome at 50-µm thickness. The sections were incubated with the V1bR antiserum (dilution at 1:500) at room temperature for 1 h and at 4 C overnight, on a rocking table, followed by incubation with goat antirabbit IgG (DAKO Corp., dilution 1:100) and rabbit-peroxidase-antiperoxidase complex (dilution 1:1000; DAKO Corp.) at room temperature for 1 h each. Between the antiserum incubations, the sections were rinsed four times in TBS for 15 min. All antisera were diluted in THZT buffer. After the last washing step, 0.05% 3–3'-diaminobenzidine tetrahydrochloride (DAB; Sigma), dissolved in TBS, was added to the sections with 0.01% H2O2 and 0.2% nickel ammonium sulfate for 5–15 min (time determined empirically) to visualize the peroxidase. After mounting, the sections were dehydrated and embedded in Entellan (Merck, Amsterdam, The Netherlands) and examined by light microscopy.


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Antiserum specificity
The V1bR antiserum specificity was extensively evaluated on protein blots and cultured cells. In Western blot analyses of proteins obtained from HEK-293 cells expressing the V1bR, the V1bR antiserum reacted with a smeared band of about 50–55 kDa when the samples were not heated before loading into the gel (Fig. 1Go). However, when the samples were boiled, the band appeared at the top of the separating gel. These bands did not appear in protein extracts from HEK-293 cells transfected with the recombinant V1a receptor (V1aR) protein. The immunoreactivity was completely abolished when V1bR antiserum was preabsorbed with the immunizing peptide (Fig. 1Go). In addition, on Western blots of proteins obtained from bacteria expressing the GST-V1bR fusion protein, the V1bR antiserum specifically reacted with a band of about 70–75 kDa (Fig. 1Go). No bands were immunodetected in proteins extracted from bacteria expressing the empty GST vector. Preabsorbed serum from the same rabbit was negative.



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Figure 1. Western blot analysis of V1b receptor protein immunoreactivity. Lanes 1–4, Protein extracted from HEK-293 cells transiently transfected with an expression plasmid for the V1aR or V1b receptor. The samples were boiled (lines 1 and 2), 25 µg protein or boiling was omitted (lines 3 and 4), 75 µg protein just before loading into the gel. Lanes 5–7, Protein (5 µg/line) from Escherichia coli (DH5{alpha}), transformed with either a plasmid expressing GST or a GST V1bR fusion protein (GST-V1b), and further induced (+) with IPTG. Total protein was extracted and separated on 11% SDS-polyacrylamide gel. After transfer to a nitrocellulose filter, the proteins were immunodetected with the V1bR antiserum (lanes 1–6) or the preabsorbed antiserum (lane 7) at 1:1000 dilution. Molecular-mass markers (kDa) are shown on the left. Arrowhead, A specific band in lane 2 stacked at the top of the separating gel; vertical bars, V1bR-immunoreactive bands.

 
In immunohistochemistry on cultured cells, approximately 30–40% of BHK cells transiently transfected with cDNA encoding the V1bR were immunolabeled with the V1bR antiserum (Fig. 2BGo). No specific signal was found when cells were transfected with either vector alone (data not shown) or with cDNA encoding the V1aR (Fig. 2AGo). In addition, the immunostaining was abolished using the preabsorbed antiserum (Fig. 2CGo).



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Figure 2. Immunohistochemical characterization of the V1bR antiserum in cultured BHK cells transfected with an expression vector for the V1a (A) receptor or the V1b (B and C) receptor. After 3 days post transfection, the cells were fixed with 4% paraformaldehyde in 0.1 M PBS and further immunostaining using the V1bR antiserum (A and B) or the preabsorbed antiserum (C), at 1:300 dilution. Note a clear immunopositive staining in extensions of cells transfected with the V1b receptor recombinant protein (arrowheads in B). Bar, 30 µm.

 
Immunohistochemistry in the rat pituitary gland
Because the anterior pituitary gland is the only well-established tissue of V1bR expression, vibratome sections of the rat pituitary gland were analyzed by immunohistochemistry. As shown in Fig. 3AGo, a subpopulation of cells (2–5%) in this gland was stained with the V1bR antiserum after fixation with Bouin’s solution. The other fixations did not result in immunostaining. V1bR immunoreactivity (V1bR-ir) was also present in the posterior pituitary gland in structures resembling pituicytes (Fig. 3BGo). The intermediate lobe was negative (data not shown). In primary cultures of anterior pituitary cells, about 10% of cells were stained for V1bR-ir. Double-labeling experiments showed that 90–95% of V1bR-immunoreactive cells were also stained with a monoclonal ß-endorphin antibody (Fig 4Go).



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Figure 3. V1b receptor protein immunoreactivity in the pituitary gland. A, Immunostaining in a subpopulation of cells in the anterior lobe (AP) after fixation with Bouin. Arrowheads mark the immunoreactivity concentrated in the plasma membrane; B, immunoreactivity in the neuronal lobe (NL) resemble pituicytes (arrowheads); bar, 30 µm.

 


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Figure 4. Colocalization of V1b receptor immunoreactivity in POMC cells of anterior pituitary cells in culture. Dispersed anterior pituitary cells were cultured in DMEM containing 10% FCS for 4 days. After fixation in 4% paraformaldehyde, cells were stained with the V1bR antiserum (1:500), together with a monoclonal antibody against ß-endorphin (1:2000). The V1bR antiserum was detected with a conjugated fluorescein isothiocyanate-goat antirabbit IgG; and the ß-endorphin antibody, with a conjugated Texas Red-goat antimouse IgG. Top panels show the same overview of pituitary cells in culture, and panels at the bottom of the figure represent another field at a higher magnification, showing cells in more detail. A and A', V1bR- immunolabeling; B and B', ß-endorphin-labeling; C and C', both labeling together, showing colocalization in some cells. Note the characteristic punctuate staining shown by the V1bR antiserum. Bars, 20 µm.

 
V1b receptor protein mapping in the rat brain
The immunolocalization of the V1bR protein in the rat brain was performed using Bouin’s fixation. This fixation resulted in the best preservation of the V1bR-ir. V1bR-ir was mapped throughout the brain of the Wistar rat with and without colchicine pretreatment. In naive rats, theV1bR immunostaining was mainly localized in neuronal fibers, whereas cell bodies were not visualized. However, after colchicine treatment, we could detect numerous groups of distinct neuronal cell bodies specifically immunostained for V1b-R protein. In addition, colchicine treatment decreased the number and intensity of V1bR-ir in fibers. The staining observed with the V1bR antiserum was abolished when the antiserum was preabsorbed with the peptide used for immunization. Preimmune serum was completely negative. Table 1Go summarizes the mapping of the V1bR-ir in the rat brain.


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Table 1. Distribution of V1b receptor-like immunoreactivity in the rat brain

 
Telencephalon
As shown in Fig. 5BGo, moderate-to-intense cellular V1bR-ir was seen in the olfactory tubercle and piriform cortex after colchicine treatment. At the most rostral level, the staining in the olfactory tubercle was extended to the Calleja island. However, the caudal part of Calleja Island was negative. The immunoreactivity displayed characteristic staining in the plasma membrane of cells and neuronal protrusions (see detail on Fig. 5BGo). Fibers were also stained more caudally in the lateral nucleus of the olfactory tract. Colchicine treatment also revealed a similar staining pattern of cells in layers II-III and V of the cerebral cortex (Fig. 5AGo). Although V1bR-ir could be observed in most of the cortex, the most intense staining occurred in the cingulate and frontal cortex, whereas the parietal cortex contained moderate immunoreactivity.



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Figure 5. Light microscopical distribution of the V1b receptor protein immunoreactivity in the cerebral cortex (A) and olfactory system (B). A, Moderate-to-intense immunostaining is shown in layer II-III and V of the cingulate cortex. Arrowheads mark stained cells; B, intense immunostaining shown in the olfactory tubercle (Tu) and piriform cortex (Pir). As indicated in the insert, immunostaining was mainly concentrated in the plasma membrane of cells and dendrites. Bars: A and B, 75 µm; inset, 40 µm.

 
In the basal forebrain, the posterior part of bed nucleus of the stria terminalis and the diagonal band of Broca showed occasional specifically stained fibers in naive rats. Intensely immunostained fibers were present in the organum vasculosum laminae (OVLT) lining to the third ventricle (see Fig. 7AGo). After colchicine treatment, V1bR immunoreactive cells were detected in the fundus of striatum.



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Figure 7. V1b receptor immunoreactivity in circumventricular organs. A, Intense immunostaining in fibers located in the OVLT; B, scatter fibers immunostained (arrows) in the SFO; C, stained cell layer at the dorsal part of the paraventricular thalamic nucleus (PVA) from a colchicine-treated animal; 3V, third ventricle; ox, optic chiasm; D3V, dorsal third ventricle. Bars: A and B, 75 µm; C, 50 µm.

 
The nucleus accumbens and caudate putamen (CPu) showed moderate-to-intense V1bR-ir at all rostrocaudal levels in cells and dendrites, after colchicine treatment (Fig. 6AGo). The characteristics of this staining were similar to those found in the cortex and olfactory system, being concentrated in the plasma membranes of cells. The taenia tecta also presented numerous V1bR-immunoreactive cells in colchicine-treated animals. In these areas, no staining was found in fibers.



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Figure 6. V1b receptor protein immunoreactivity in CPu and hippocampus. A, Cell bodies immunostained in the caudal part of the CPu. Immnuoreactivity was also extended to the central amygdala (Ce). Cells are shown in more detail, at a higher magnification, in the insert. The immunostaining was mainly concentrated in the plasma membrane and dendrites. B and C, V1bR immunoreactivity in the pyramidal cell layer (Py) from the CA1 hippocampal field of Ammon’s horn. B shows a section from an untreated animal where a diffuse immunostaining can be observed. After colchicine treatment (section shown in C), immunoreactive cell bodies and dendrite appeared clearly stained. LGP, Lateral globus pallidus; BLA, basolateral amygdala; ec, external capsule; Rad, hippocampal stratus radiatum; Or, hippocampal stratus oriens. Bars: A, 150 µm; B and C, 75 µm; inset, 25 µm.

 
In colchicine-treated rats, intense immunostaining was present in the pyramidal cell layer of the hippocampus (Fig. 6CGo). The CA1 and CA2 fields of Ammon’s horn were stained at all rostrocaudal levels. However, the CA3 field was only stained at its caudal level. The granule cell layer from the dentate gyrus was also immunopositive for the V1bR antibody, from rostral to caudal. Some untreated animals also displayed V1bR-ir in these areas, but staining was only weak (Fig. 6BGo).

In colchicine-treated rats, V1bR-ir was found in different amygdaloid nuclei. Cell bodies and dendrites were present in the central amygdala (Fig. 6AGo). This group of cells seems to belong to the same group of V1b-immunopositive cells located along the CPu. Fibers were found to be restricted to two amygdaloid nuclei, the anterior cortical amygdaloid nucleus and the posteromedial cortical amygdaloid nucleus, as found in untreated rats.

Diencephalon
The subfornical organ (SFO) displayed V1bR-ir fibers in untreated rats (Fig. 7BGo). The medial habenula displayed a similar fiber staining, being slightly more intense (see Fig. 9BGo). However, after colchicine treatment, the immunoreactivity of these fibers disappeared almost completely, but many cell bodies and dendrites were immunostained instead. In colchicine-treated animals, V1bR-ir cell bodies also appeared in the habenular commissure.



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Figure 9. Distribution of the V1b receptor immunoreactivity in the medial habenula and cerebellum. A, Fibers stained (arrowheads) in the medial habenula (MHb) from a nontreated animal. B, A section showing the medial habenula from a colchicine-treated animal. Note that the fiber staining has disappeared, and cell bodies are now stained. C and D, Immunostained cells located in the granule cell layers form the cerebellum. Note that lobule 10 (C), close to the fourth ventricle, displayed a more intense staining than lobule 7 (D). Pictures C and D both are from the same section. Arrowheads (inset, top-right), V1b receptor immunoreactivity concentrated in the plasma membranes. Bars: A and B, 100 µm; C and D, 50 µm; inset, 30 µm.

 
Moderate V1bR-ir was found in cell bodies located in the caudal two-thirds of different dorsolateral thalamic nuclei (for specification see Table 1Go). This staining was concentrated in the plasma membranes and could be observed only after colchicine treatment. The most dorsal part of the paraventricular thalamic nucleus showed a layer of V1bR-ir cells just ventral to the dorsal part of the third ventricle (Fig. 7CGo). In addition, some fibers were observed in the periventricular nucleus lining the third ventricle in naive rats.

In the rostral region of the hypothalamus, corresponding to the preoptic area and the anterior hypothalamic area, a low-to-moderate number of V1bR-ir fibers were observed. More prominent V1bR-ir was present at the medial level of the hypothalamus close to the third ventricle. Occasional fibers were found in the SCN. Immunoreactive fibers were present also along the most ventral part of the caudal region of the medial preoptic area running from the supraoptic nuclei to the periventricular nucleus.

Intense V1bR-ir was seen in median eminence (Fig. 8AGo). The staining found in this region was the most intense of the entire brain, showing a dense innervation by fibers, especially in the external layer, that project to the anterior pituitary lobes. Less intense fiber staining was also found in the internal zone of the median eminence. The most posterior part of the hypothalamus, corresponding to the mammillary bodies, presented a moderate fiber staining. At this level, the most pronounced immunoreactivity was concentrated at the medial part, just dorsal to the mammillary recess of the third ventricle (Fig. 8BGo).



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Figure 8. V1b receptor immunoreactivity in hypothalamic circumventricular organs. A, Intense fiber staining in the median eminence, mainly concentrated in its external layer. Note that the periventricular nucleus (Pe) also contains some stained fibers (arrows). B, V1bR-positive fibers stained (arrowheads) in the medial mammillary nucleus (MM) dorsal to the mammillary recess from the third ventricle (Mre). Bars: A, 50 µm; B, 75 µm.

 
Pons and medulla
Naive rats did not display V1bR-ir in these structures. After colchicine treatment, only two nuclei in the brain stem, the ventral cochlear nucleus and the nucleus of the trapezoid body, displayed some V1bR-ir just above background. The medulla presented no specific V1bR-ir.

Cerebellum
V1bR-ir was found in all cerebellar lobes. Colchicine-treated rats displayed immunoreactive cell bodies in the granule cell layers (Fig. 9Go, C and D). Interestingly, lobes 1 and 10, which are located close to the fourth ventricle, presented the most intense staining. These cells project immunopositive dendrites to the white matter. Additional V1bR-ir was found in fibers located in the white matter.


    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The present study provides the first characterization of the V1bR protein in a mammalian brain. Furthermore, it also describes its cellular distribution, which is critical for understanding the mechanisms of action of VP in the brain. The specificity of the antiserum used to detect the V1bR protein was a particular concern in this study, in view of our earlier experiences with antisera against G protein-coupled receptors, particularly the OT and V1aRs (24, 25). We could substantiate the specificity by several approaches:

1) Western blotting, using a homogenate of proteins extracted from HEK-293 cells that were transiently transfected with an expression plasmid for the V1bR, revealed a specific protein band. This band was about 5–10 kDa higher than expected (15, 16, 17). The increased molecular mass may be attributable to glycosylation, because the V1bR has one consensus N-linked glycosylation site at Asn-21 (15). This band does not appear in protein extracts from HEK-293 cells transfected with a V1aR expression plasmid. Interestingly, when the protein sample was boiled just before loading, the specific band at 50–55 kDa disappeared, and a band just at the beginning of the separating gel appeared, indicating that the protein may be aggregated. In addition, using a protein extract from bacteria expressing the GST-V1bR fusion protein, the antibody also revealed a band that corresponded to the expected molecular mass for the GST-V1bR fusion protein. The fact that the V1bR antiserum did not cross-react with any bacterial protein or GST further supported its specificity for the V1bR protein. The specific bands were not detected when the antiserum was preabsorbed with the peptide used to raise the antibody.

2) BHK cells transiently transfected with aV1bR expression plasmid showed a specific staining, when compared with BHK cells transfected with the V1aR protein. The number of stained cells corresponded to the transfection efficiency obtained for this cell line with a LacZ expression vector (not shown).

3) The specific staining was abolished using the preabsorbed antiserum.

4) The V1bR-ir was found in a subpopulation of cells in the anterior pituitary gland as expected from physiological studies (11, 12, 14). Double-labeling experiments with a ß-endorphin antiserum in primary cultures of anterior pituitary cells demonstrated that about 90–95% of V1bR-positive cells were corticotropes. The presence of the V1bR protein in corticotrope pituitary cells has been previously well characterized (11, 12, 15).

In naive rat brain, mostly fiber systems were stained, whereas cell bodies were clearly revealed after colchicine treatment. It should be noted, however, that weakly stained V1bR-ir cells were found in some untreated animals, suggesting that V1bR-expression levels vary between animals. The observation that the naive Brattleboro rat, which is deficient in VP synthesis, shows a similar intense V1bR staining in cells as the colchicine-treated animals (manuscript in preparation) suggests that the V1bR found at the cell body may have functional significance and not simply represent an apparent accumulation of protein to be transported. Variation in brain V1aR-transcript levels between different rats has also been noted previously (26). These variations may represent differences in the physiological state and VP tonus of the animals. Notably, the increased V1bR-ir observed in cell bodies after colchicine treatment of normal rats was mainly located in the plasma membranes and not in the cytosol, as expected when axonal transport is blocked (4).

At the regional level, V1bR-ir was present in different restricted areas of the rat fore- and midbrain and cerebellum. Comparison of these patterns with the VP receptor distribution in the rat brain, as mapped by autoradiographic studies using radiolabeled VP or nonselective VP antagonists, shows areas of clear overlap, such as the piriform cortex, nucleus accumbens, fundus striati, and hippocampus (27, 28, 29). However, there is a significant number of brain areas that were intensely labeled with VP ligands that did not display any V1bR-ir. These areas mainly include the anterior olfactory system, lateral septum, lateral hypothalamus, stigmoid nucleus, arcuate nucleus, different nuclei of the mid- and hindbrain, and choroid plexus (for review, see 19). The binding of VP to these latter structures is likely attributable to the presence of V1aR, because some recently designed V1aR-specific antagonists show the same distribution pattern of VP binding previously established with the nonselective ligands (6, 30, 31). The lack of highly specific ligands for the V1bR (5) makes it difficult to establish whether the nonselective radiolabeled VP ligands were able to detect the V1bR in the rat brain. On the other hand, V1bR-ir was detected in different areas, where binding of radiolabeled VP has not been detected, including the olfactory tubercle, cortex, OVLT, CPu, taenia tecta, different amygdaloid nuclei, medial habenula, SFO, median eminence, mammillary nuclei, and cerebellum. Different explanations could clarify these mismatches (for review, see 32). The sensitivity of the methods to demonstrate binding of the nonselective radiolabeled VP ligands may not be sufficiently high to reveal the low expression levels of the V1bR, by light microscopic autoradiography, in these latter brain structures.

Previous studies, using RT-PCR, have demonstrated a widespread expression of V1bR transcripts in the adult rat CNS, including the olfactory bulb, CPu, septum, cortex, hippocampus, hypothalamus, and cerebellum (15, 16). This distribution correlates very well with the localization of the V1bR-immunoreactive cells in the colchicine-treated animals. V1bR expression has also been studied by in situ hybridization (18). A comparison between the distribution of the V1bR transcripts (18) with the localization of the V1bR-immunoreactive cell bodies from the present work shows overlapping areas, e.g. piriform cortex, taenia tecta, and hippocampus. However, we also found V1bR-immunoreactive cells in brain regions not described by in situ hybridization studies. These areas include the olfactory tubercle, cerebral cortex, basal ganglia, thalamus, and cerebellum. The fact that we could only observe clearly stained cell bodies after colchicine-treatment in the mentioned brain regions may indicate that the expression levels are very low and possibly below detection by in situ hybridization. On the other hand, cells were not labeled with the V1bR antiserum in brain nuclei reported to have V1bR transcripts like supraoptic nuclei, SCN, and the dorsal raphe nucleus. This would indicate that the V1bR receptor is not synthesized efficiently from the transcripts in these areas, or rapidly transported to axon terminals in the same or other brain regions, where the receptor might be functional. The staining in the internal zone of the median eminence may point to V1bR protein production by magnocellular neurons. However, the staining in the posterior lobe of the pituitary gland seemed associated with pituicytes and not with axonal fibers. This finding could explain the labeling of this gland using [3H]VP, a labeling previously suggested to be attributable to binding of VP to neurophysin (29).

This regional distribution for the V1bR protein in the rat brain may have important functional implications. It suggests that some central functions of VP, previously thought to be exclusively mediated by the V1a and OT receptors, may be (at least partially) caused by the activation of the V1bR in the brain. From a functional point of view, the neuronal and fiber systems containing V1bR-ir can be divided in two different groups. First, they encompass structures that carry the V1bR protein only in fiber systems. This has been observed in brain areas either with a deficient blood-brain-barrier or close to the cerebroventricular system (SFO, OVLT, median eminence, periventricular thalamic and hypothalamic nuclei, medial habenula, mammillary bodies), which are involved in homeostatic control circuits, and would serve as a window for VP in the blood circulation. For instance, the OVLT plays an important role in osmoperception and in the regulation of pyrogen-induced febrile responses (33, 34, 35). In addition, the SFO participates in the central thirst mechanism and the control of VP release (36, 37). In these brain regions, the staining found was mainly located in fibers. This cellular localization of the V1bR may have a major implication on the modes of action of VP in the CNS that regulate the secretion of neurotransmitters or neuropeptides produced by these neurons. The intensely V1bR-ir-stained fibers located in the external zone of the median eminence, being, thus far, the most intense in the entire brain, are remarkable in this view. Here it may allow VP to modulate the release of hypothalamic peptides in the control of pituitary function.

Second, multiple V1bR-immunoreactive brain areas present exclusively a staining pattern limited to cell bodies, e.g. periform cortex, olfactory tubercle, hippocampus. The most intense staining of perikarya was found in the hippocampus, were VP is known to modulate learning and memory processes (1). The septum, cortical layers, and several thalamic areas represent other areas associated with central actions of VP. The CPu is a region where binding has not been noted previously for VP or VP analogues; nor has V1bR messenger RNA expression detected by in situ hybridization (19, 18). Nevertheless, VP neurons have been reported to innervate these areas (3, 19); and thus, the peptide may exert a yet-unknown action.

Taken together, the V1bR protein has been found in a surprising number of individual areas of the rat brain. These results suggest that some known central functions of VP could be attributed, at least partially, to the activation of the V1bR in the brain. These include processes of learning and memory mediated by the hippocampus or amygdala, neuroendocrine regulation mediated by hypothalamic peptide systems, and the control of homeostatic circuits in areas related to the cerebroventricular system. The future challenge is to dissect the physiological brain functions of VP mediated by the V1bR. Because the required ligands are not available, this task needs the creation of mice carrying a null mutation of this receptor, which has recently been accomplished (38). These V1bR null mutants display behavioral alterations, e.g. reduced aggression and social memory, that may be attributable to absence of the V1bR in brain structures described in this study.


    Acknowledgments
 
We wish to thank Drs. K. J. de Vries and M. Verhage for help with Western blot analysis, Dr. F. W. van Leeuwen for help with initial immunohistochemistry of receptors, Dr. M. P. Smidt for improving digital images, and Dr. V. M. Wiegant for kindly providing a ß-endorphin antibody.


    Footnotes
 
1 This project was supported by TMR Grant ERBFMBI CT-96-1315 (to F.H. from the European Commission) and by Grant 970-10-015 from The Netherlands Organization for Scientific Research (NWO). Back

Received September 29, 2000.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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