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Laboratory of Neuroendocrinology, The Babraham Institute, Cambridge, United Kingdom CB2 4AT; and the Department of Neuroendocrinology, Division of Neuroscience, Imperial College Medical School (H.E.M., G.E.G.), London, United Kingdom W6 8RF
Address all correspondence and requests for reprints to: Dr. Allan E. Herbison, Laboratory of Neuroendocrinology, The Babraham Institute, Cambridge, United Kingdom CB2 4AT. E-mail: allan.herbison{at}bbsrc.ac.uk
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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, but expressed androgen receptors in a sexually dimorphic
manner. These results demonstrate that a sex difference in SOM
biosynthesis, which persists into adulthood, develops between P1 and P5
in PeN neurons. Despite the absence of estrogen receptor- in these
neurons, the organizational influence of testosterone only occurs after
its aromatization to estrogen. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Like GH secretion, the described sex differences in SOM and GHRH neurons are determined by perinatal organizational and adult activational influences of gonadal steroids. Neonatal gonadectomy of the male rat results in feminization of the GH secretory profile (8, 9) as well as SOM mRNA expression in the PeN and GHRH cell number and mRNA content (10). Equally, perinatal and adult administration of testosterone to female rats masculinizes the GHRH and SOM neuronal cell populations and provides a partial masculinization of GH secretion (10, 11). Such observations suggest that the perinatal testosterone surge in the rat (12) organizes the SOM and GHRH neurons so that testosterone can act in the postpubertal male to establish the male pattern of GH secretion.
The SOM neurons of the PeN play a major role in determining the profile of GH secretion (3, 4), and the elevated SOM mRNA expression in these cells in the male is thought to be related to the low baseline GH concentrations found in this sex (3, 7). The stimulatory effect of gonadal steroids on the SOM mRNA content of neurons in the PeN of the adult male has been shown to depend upon an androgen receptor (AR)-mediated mechanism (13, 14) and may represent a direct action of testosterone on SOM neurons as they express ARs in a sexually dimorphic fashion (15). However, the organizational influences of testosterone on this cell population are less clear. For example, SOM neurons differentiate relatively early during embryogenesis (16), and it is not known at what time sex differences first appear within the PeN population. Argente and co-workers (6) demonstrated that SOM mRNA expression was elevated in males on postnatal day 10 (P10), but earlier time points were not examined. Furthermore, although it is established that testosterone acts through the AR to regulate SOM mRNA expression in the adult (13, 14), it is not clear whether this is also the case during development, when testosterone often engenders sex differences in the brain after its aromatization to estrogen (17).
Hence, in the present series of studies we set out to describe and characterize the development of sex differences in the SOM neurons of the PeN, first by examining the time point at which differences in SOM mRNA and peptide content first appear between males and females, and second by establishing the gonadal steroid dependence of this ontogenetic dimorphism and investigating the manner in which it may be exerted on developing SOM neurons.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Exp 1: profile of neonatal SOM mRNA expression
Male and female rats (n = 6 for each sex and group) were
killed on the afternoon of P1, P5, or P10. Animals were decapitated,
and brains were quickly removed. Blocks containing the hypothalamus
were rapidly frozen on dry ice and stored at -70 C. Fresh-frozen
sections (15 µm thick) were cut in the coronal plane (plates 22–26
of Swanson) on a cryostat (Bright, Huntingdon, UK) and thaw-mounted
onto Vectabond-coated slides (Vector Laboratories, Peterborough, UK).
Sections were kept at -70 C until used.
For in situ hybridization experiments, an antisense
oligonucleotide complimentary to bases 310–339 of the rat prepro-SOM
complementary DNA (which codes for part of SOM-14) was synthesized.
This nucleotide sequence has been used previously to detect SOM
mRNA-expressing cells in the rat brain (18). The probe was 3'-end
labeled with [-35S]deoxy-ATP (1000–1500 Ci/mmol; New
England Nuclear, Boston, MA) using terminal deoxynucleotidyl
transferase (50 U; Pharmacia, St. Albans, UK), resulting in a specific
activity of approximately 109 cpm/µg. The radiolabeled
probe was purified by filtration on a Sephadex G-50 column. Frozen
sections were warmed to room temperature with a hair dryer and then
fixed with 4% paraformaldehyde in 0.1 M phosphate buffer
for 20 min. Fixed sections were rinsed in 0.1 M PBS treated
with 0.1% diethylpyrocarbonate. Sections were then dehydrated though
70%, 80%, 90%, 95%, and 100% alcohol and allowed to air-dry. Two
hundred and fifty microliters of hybridization buffer [4 x
saline sodium citrate (SSC), 50% deionized formamide, 10% dextran
sulfate, 1 x Denhardt’s solution, 250 µl/ml sheared salmon
testis DNA, and 0.3% ß-mercaptoethanol] containing the
35S-labeled SOM probe (28 fmol/µl equivalent to 2 x
103 cpm/µl) were then applied to each slide containing
six brain sections. Hybridization was carried out in humidified
chambers at 37 C overnight. Hybridized sections were then washed in
1 x SSC at room temperature, three times in 1 x SSC at 55 C
(30 min each), and again in 1 x SSC for 1 h at room
temperature. All of the 1 x SSC solutions contained 0.001%
ß-mercaptoethanol. After a quick wash in water, hybridized sections
were placed in a 300 mM ammonium acetate-70% ethanol
solution and then absolute alcohol for 30 sec each and allowed to
air-dry before dipping in Ilford K-5 nuclear track emulsion and exposed
for 2–4 days in light-tight boxes. At the appropriate exposure time,
as determined by test slides, all slides were photodeveloped with
Ilford phenisol (diluted 1:5 in distilled water, 5 min at 20 C). After
developing, slides were counterstained lightly with methylene blue and
coverslipped. Hybridization specificity was assessed by use of
competition experiments in which radiolabeled probes were hybridized to
sections in the presence of an excess (25-fold) of unlabeled probe.
For the analysis of SOM mRNA expression, slides were numbered randomly so that the operator was unaware of the experimental group. Using a Seescan (Cambridge, UK) analyzer coupled to a Leica Orthoplan microscope (Milton Keynes, UK), a quantitative analysis of somatostatin mRNA expression was undertaken by determining, under brightfield illumination, the relative silver grain density overlying individual cells in the PeN and central nucleus of the amygdala (CNA) at the coronal levels equivalent to plates 23 and 26 of the Swanson atlas (19), respectively. For each animal, 30 cells were analyzed in each region from 1–3 sections. For the PeN, the field of image analysis was positioned over the dorsal aspect of the periventricular area, and all positively hybridized cells (typically 6–8 cells) within the field of view were analyzed. The field of image analysis was then moved caudally, and an additional set of cells was analyzed until either no more hybridized cells could be found and analysis of another section was begun, or a total of 30 cells had been reached. Previous studies in our laboratory have shown that SOM neurons in the dorsal and ventral aspects of the PeN exhibit an identical pattern of silver grain density and that this responds to gonadal steroids in an identical fashion (Simonian, S. X., and A. E. Herbison, unpublished observations). Initially, the number of silver grains overlying cells in the excess unlabeled probe control were determined, and in experimental sections, only those cells expressing numbers of silver grains greater than 5 times the control value were used for analysis. For each rat and region, an average silver grain count per µm2/cell was determined, and these values were combined to give experimental group means. In addition, frequency histograms were constructed by allocating the relative proportions of PeN cells expressing different numbers of silver grains per µm2 in 0.1-U bins. All values are expressed as the mean ± SEM, and statistical comparisons were made using the nonparametric Mann-Whitney U test. P < 0.05 was considered statistically significant.
Exp 2: profile of neonatal SOM peptide content in PeN
P1, P5, and P10 male and female pups (n = 6 for each sex
and group) were decapitated, their brains were quickly removed, and
tissue was cut so that a block containing the hypothalamus was mounted
on a cutting platform ready for sectioning with a Vibratome. The
hypothalamic block was immersed in a bath of ice-cold Krebs buffer (pH
7.4), and coronal brain slices 300 µm thick were cut from the level
of the optic chiasm through to the mediobasal hypothalamus. The coronal
brain slice containing the PeN at the coronal level equivalent to that
of plate 23 of Swanson (19) was selected from each animal, and a
unilateral punch including all of the PeN was taken. The thickness of
the coronal slice in P1 rats was reduced to 200 µm to ensure that SOM
cell populations outside the PeN were excluded from the punch, which
extended in all cases from the level of the fornix to just above the
optic chiasm in the dorso-ventral orientation and included all of the
periventricular region out to the level of the fornix medio-laterally.
Our previous immunocytochemical and in situ hybridization
experiments had shown that the SOM neurons sampled by this method are
almost exclusively from the PeN population. Tissue punches were
collected in 10 mM HCl on ice, and after homogenization and
centrifugation, the supernatant was collected and stored at -80 C. The
SOM content was later measured in duplicate by RIA as described
previously (20). The sensitivity of the assay was 5–10 pg/ml, with
intra- and interassay coefficients of variation of 3.5% and 7.2%,
respectively. All values are expressed as the mean ±
SEM, and statistical comparisons were made using Student’s
t test. P < 0.05 was considered
statistically significant.
Exp 3: effects of gonadal steroid manipulations on neonatal SOM
mRNA
Gonadectomy (GDX) and SILASTIC brand (Dow Corning, Midland, MI)
capsule implantation were performed within 12 h of birth under
cold anesthesia. Neonates were placed on ice, the gonads were removed,
and the abdominal incision was closed with surgical silk before the
pups were warmed to 37 C and returned to their mothers. Sham operations
involved anesthetizing the pups and making incisions as described for
GDX, but closing the wound without removal of the gonads.
Four experimental manipulations were undertaken. 1) Male rats were GDX (n = 6) and killed alongside sham-treated males (n = 4) and unoperated females (n = 8) on the afternoon of P5. 2) Male rats were GDX and given daily sc (50 µl) injections of ethyl oleate vehicle alone (n = 5) or vehicle containing dihydrotestosterone (DHT; 50 µg/50 µl oil; n = 5) or estradiol benzoate (EB; 1 µg/50 µl oil; n = 5) and killed with sham-treated males (n = 6) on the afternoon of P5. 3) Male rats received sc injections of either ethyl oleate (50 µl; n = 6) or cyproterone acetate (CP; 500 µg/50 µl oil; n = 5) within 12 h of birth and were killed on the afternoon of P5. 4) Male rats received a sc SILASTIC capsule implant (Sani-tech, Havant, UK; id, 1.02 mm; od, 2.18 mm; length, 5 mm) containing either crystalline 1,4,6-androstatriene-3,17-dione (ATD; Steraloids, La Jolla, CA; n = 6) or SILASTIC glue (n = 6). Rats were then killed on the afternoon of P5. Each of these treatment regimens has been described previously for the manipulation of gonadal steroid levels in the neonatal rat (21, 22, 23). Animals were decapitated, and brains were quickly removed and processed for in situ hybridization as described above.
Exp 4: estrogen receptor (ER) and AR expression by neonatal SOM
neurons
Male (n = 5) and female (n = 4) neonates were
anesthetized with Avertin (2% tribromethanol, 1 ml/100 g BW, ip) on P5
and perfused transcardially with heparinized 0.9% saline followed by
approximately 4 ml of a 4% paraformaldehyde 0.1 M
phosphate buffer (pH 7.6) solution. Brains were removed, postfixed for
1 h at room temperature in the same fixative, and cryoprotected
overnight in a 30% sucrose/0.05 M Tris-buffered saline
(TBS) solution. Coronal sections (25 µm thick) containing the PeN
were cut the next day on a freezing microtome and collected into TBS as
three sets of sections.
After washing in a 40% methanol-TBS solution containing 1%
H2O2, one set of sections was incubated in a
monoclonal mouse antiserum directed against the N-terminal domain of
human ER (ID5, 1:10, gift from G. Delsol, Toulouse, France), and
another set of sections was incubated in a polyclonal rabbit antiserum
directed against the N-terminal region of the androgen receptor (AR;
PG-21-18, 1.0 µg/ml, gift from Dr. G. Prins, Chicago, IL) for 40
h at 4 C. This was followed by either biotinylated horse antimouse Igs
(1:200; Vector) or biotinylated goat antirabbit Igs (1:200; Vector) and
the Vectastain Elite kit (1:50; Vector) for 90 min each at room
temperature. Immunoreactivity was visualized using a nickel-enhanced
3,3'-diaminobenzidene tetrachloride (DAB) procedure. The production and
specificity of the ER and AR antibodies were reported previously (15, 24, 25, 26).
Double staining was carried out as described previously (15, 27) by washing ER- and AR-stained sections in a 40% methanol-TBS solution containing 1% H2O2, washing in TBS, and then placing sections in a polyclonal rabbit antiserum specific for somatostatin (1:3000; gift from F. Vandesande, Belgium) for 40 h, followed by incubation in biotinylated goat antirabbit Igs (1:200; Vector) and streptavidin-biotinylated horseradish peroxidase complex (1:200, Amersham) each for 90 min at room temperature. Somatostatin immunoreactivity was visualized using the glucose oxidase technique with DAB only. The production and specificity of the SOM antiserum were reported previously (15, 27, 28). All primary and secondary antibodies were diluted in TBS-0.3% Triton-0.25% BSA, whereas tertiary Igs were diluted in TBS alone.
An analysis of double labeled brain sections was undertaken in the PeN. Brain sections at the level of the anterior hypothalamus containing the greatest numbers of SOM-immunoreactive cell profiles in the PeN were selected, and cell counts were made from a minimum of three sections per animal. The number of DAB-stained cell profiles with and without black nuclear immunoreactivity was counted in the PeN of each section, and these values were averaged for each animal and combined to give the mean ± SEM for each group.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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).
Elsewhere in these sections, the SOM hybridization signal was strong
within neurons of the amygdala, hippocampus, and cortex (18). Silver
grain clusters over cells were absent when sections underwent
hybridization in the presence of an excess of unlabeled probe.
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). Histograms of the relative
proportions of cells expressing different levels of silver grains per
µm2/cell revealed normally distributed populations of
cells at each developmental age and sex (not shown). The relative
hybridization signal in the CNA increased from P1 to P10 in both male
and female rats (male, 1.10 ± 0.05 (P1), 1.45 ± 0.09 (P5),
1.79 ± 0.05 (P10); female, 1.16 ± 0.06 (P1), 1.35 ±
0.08 (P5), 1.95 ± 0.07 (P10) silver
grains/µm2·cell), but was not sexually dimorphic at any
age (Fig. 2B).
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). Although SOM
peptide content was not increased further between P5 and P10 in females
(1420 ± 168 pg SOM/PeN punch on P10), it was elevated further in
males and was approximately 44% higher in males (2056 ± 85 pg
SOM/PeN punch) than in females on P10 (P < 0.05; Fig. 3).
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). Hybridization signal in the CNA of
the same brain sections was not different among sham-treated males
(1.09 ± 0.07 silver grains/µm2·cell), GDX males
(1.20 ± 0.07 silver grains/µm2·cell), and intact
females (1.14 ± 0.05 silver grains/µm2·cell)
(data not shown).
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).
GDX males treated with EB displayed silver grain levels per cell
(0.52 ± 0.07 silver grains/µm2·cell) comparable
to those in sham-treated males (0.54 ± 0.01 silver
grains/µm2·cell), although they failed to reach a
statistically significant increase compared with those in oil-treated
GDX males (Fig. 4B). This resulted from a single estrogen-treated
animal failing to show an increase in hybridization signal. Gonadectomy
or neonatal steroid replacements had no effect on hybridization signal
in the CNA of the male rat (sham GDX, 0.92 ± 0.03; GDX + oil,
0.94 ± 0.03; GDX + DHT, 0.93 ± 0.05; GDX + EB, 0.97 ±
0.08 silver grains/µm2·cell).
Treatment of intact male rats on the day of birth with the androgen
receptor antagonist CP had no effect on relative SOM mRNA expression in
either the PeN (male, 0.75 ± 0.03; male + CP, 0.78 ± 0.07
silver grains/µm2/cell; Fig. 5) or CNA (male, 0.99 ± 0.06; male
+ CP, 1.18 ± 0.07 silver grains/µm2·cell) of P5
rats.
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). No difference was seen in the CNA
between these groups (male 1.74 ± 0.02; male + ATD, 1.70 ±
0.04 silver grains/µm2·cell).
Exp 4: ER and AR expression by neonatal SOM neurons
Cells immunoreactive for SOM were identified throughout the
coronal brain sections as described previously in the
neonatal rat brain (29). Within the hypothalamus of P5 rats,
prominent cell body staining was observed in the PeN, and
immunoreactive cells were also detected in ventrolateral aspects of
the ventromedial nucleus (VMNvl). Cell counts revealed no significant
difference in the number of SOM-immunoreactive cell profiles detected
within the PeN of P5 rats (males, 122 ± 3; females, 136 ± 6
profiles/section).
Immunoreactivity for the ER and AR appeared as a black nuclear reaction
product restricted to the nucleus of the cell. In some cases, ER
staining was also detected in the cytoplasm of cells exhibiting strong
nuclear staining. Cells exhibiting ER immunoreactivity were found in
high density within the medial preoptic nucleus, bed nucleus of the
stria terminalis, arcuate nucleus, and VMNvl, whereas smaller
populations of cells were identified within the PeN extending from the
preoptic area to the mediobasal hypothalamus. Cells exhibiting AR
immunoreactivity within the hypothalamus were found in the medial
preoptic nucleus and arcuate nucleus, and in lower numbers within the
PeN. Double labeled cells were identified as cells displaying both
brown cytoplasmic (somatostatin immunoreactivity) and black nuclear
staining (ER or AR immunoreactivity; Fig. 6). Within the PeN, only a small minority
of SOM neurons was found to express ER immunoreactivity, and this
amounted to less than 0.3% of all SOM neurons (Fig. 6A; 0.27 ±
0.12% in P5 females; 0.12 ± 0.08% in males). This was not due
to a failure of the double labeling procedure, as many cells
coexpressing SOM and ER immunoreactivities were found in the VMNvl
(Fig. 6B), and numerous single labeled ER-immunoreactive cells were
found adjacent to the SOM neurons in the PeN in both sexes (Fig. 6A).
In contrast, a substantial number of SOM neurons in the PeN expressed
AR immunoreactivity (Fig. 6, C and D) in the P5 male rat (42.5 ±
0.7 double labeled profiles/section representing 34 ± 7% of all
SOM neurons), and this was significantly greater (P <
0.05, by Mann-Whitney U test) than the number of SOM plus AR cells
encountered in the female PeN (2.0 ± 1.1 double labeled
profiles/section, representing 3 ± 1% of all SOM neurons; Fig. 7).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Gonadal steroid induction of SOM sex difference in PeN
We demonstrate that cellular SOM mRNA expression in P5 males is
reduced to female levels when male rats are GDX within 12 h of
birth. This result indicated the gonadal steroid dependence of the SOM
mRNA sexual dimorphism, and a subsequent investigation was designed to
determine which gonadal steroid receptor was involved. Although the
neonatal administration of estradiol to GDX male rats elevated SOM mRNA
expression to levels similar to those observed in intact male rats, it
did not, due to a single animal, increase transcript content
significantly compared with that in oil-treated GDX males. In a further
study we used ATD to prevent the aromatization of testosterone to
estradiol and found that neonatal ATD treatment reduced the SOM mRNA
expression of PeN neurons in all males to female levels. Together these
observations provide clear evidence that SOM mRNA expression is
sexually differentiated in PeN neurons through an ER-mediated
mechanism. In two separate experiments involving administration of the
nonaromatizable androgen DHT to GDX males and use of an AR antagonist
in intact males, we could find no evidence for a role of ARs in this
process. This is unlikely to result from a false negative, as the exact
same neonatal ATD and DHT protocols have been used by others to
demonstrate the AR dependence of other mRNAs in the developing rat
brain (22, 23). Hence, we conclude that the SOM neurons of the PeN are
sexually differentiated by estrogen after the aromatization of
testosterone over P1 to P5 when males have relatively high levels of
circulating testosterone concentrations (12). This critical period of
sensitivity and involvement of ERs is similar to that reported for a
number of morphological and biochemical sex differences observed within
the preoptic area of the rat (21, 31, 32, 33).
Mechanism of estrogen action
Although it is clear that ERs are involved in generating the sex
differences in SOM expression by PeN neurons in neonatal rats, the
mechanisms and pathways used by estrogen are unclear. Our double
labeling immunocytochemical studies show an almost complete absence of
ER immunoreactivity in SOM neurons of the PeN in both male and
female P5 rats. We have similarly failed to detect ER
immunoreactivity in SOM neurons of the PeN of adult rats (27). Hence,
it would seem that estrogen does not influence SOM neurons in any
direct manner involving ER during postnatal development. The recent
discovery of ERß (34, 35) raises the interesting possibility that the
sexually differentiating actions of estrogen on SOM neurons in the PeN
may occur directly through this receptor. Whether this is the case
awaits further investigation. Although recent data suggest that ERß
mRNA is indeed expressed in the PeN of neonatal rats (DonCarlos, L.,
personal communication), using one of the available antibodies we have
failed to detect ERß immunoreactivity in SOM neurons of the adult rat
PeN despite showing its presence in numerous oxytocin neurons of the
caudal paraventricular nucleus (36).
Estrogen may influence developing SOM neurons in the PeN in a nongenomic or indirect manner. A number of different neuronal cell populations innervate hypophysiotropic SOM neurons (37) and could conceivably be involved in the transsynaptic mediation of estrogen’s sexually differentiating actions. As aromatase is found concentrated in the preoptic area, ventromedial nucleus, and amygdala of the neonatal rat (38), neurons projecting from these regions to the SOM cells may be particularly important. In this respect it is noteworthy that GHRH neurons are located near aromatase-expressing cells (38), that they express ERs (39), and that GHRH stimulates SOM mRNA expression (40). As GH itself is involved in the feedback regulation of SOM biosynthesis in the PeN (41, 42), an additional possibility is that estrogen may cause the sexually dimorphic pattern of neonatal SOM mRNA expression by engendering sex differences in the GH feedback mechanism (43, 44).
It is interesting to note that we have detected a sexually dimorphic pattern of AR expression within SOM neurons of the PeN on P5. This result is very similar both qualitatively and quantitatively to that reported by us for adult rats (15) and suggests that this sexually dimorphic pattern of AR expression is generated before P5 and remains unchanged into adulthood. It is not clear what function this sexually dimorphic AR expression serves. Certainly, we seem to have excluded a role for AR-mediated mechanisms in the neonatal organization of sex differences in SOM mRNA expression. As the direct AR-dependent activation of SOM mRNA expression in the PeN of the adult male rat is involved in the generation of the male-type pattern of GH secretion (3, 7, 13, 15, 45), it may be that the sex differences in AR synthesis by SOM neurons are determined during the neonatal period in preparation for their role in the adult. Although such reasoning must remain speculative, it is intriguing to note that a striking developmental switch occurs in the steroid receptor mechanism regulating SOM mRNA expression within the PeN; in the neonate it is the ER, whereas in the adult it is the AR (13, 14).
Functional significance of neonatal sex differences in SOM
neurons
The elevated GH concentrations found at the time of birth fall
rapidly over the first 2 weeks of life to reach low levels that remain
depressed until the onset of puberty (1, 3, 30, 46). A variety of
experimental approaches suggest that the developmental increase in SOM
secretion from the PeN plays a role in reducing GH secretion over this
early neonatal period; a clear inverse correlation exists between
hypothalamic SOM content and GH concentrations (30), GH release from
the pituitary begins to be inhibited by SOM in the first neonatal week
(46, 47), and the administration of SOM antibodies partially prevents
the neonatal fall in GH levels (48). We now support further this
concept by showing that an increase in SOM biosynthesis occurs over P1
to P10 in the PeN neurons themselves.
The functional relevance of the sex difference in SOM biosynthesis reported here is less clear in terms of neonatal GH secretion. Despite our observation of greater SOM expression in PeN neurons of the neonatal male, investigators have not noted any sex differences in circulating GH concentrations until the time of puberty (1, 30). Thus, the sexually dimorphic SOM expression engendered by gonadal steroids during the neonatal period may only become functionally relevant after puberty when enhanced SOM secretion by PeN neurons is implicated in defining the male pattern of GH secretion (3, 49). As such, the SOM neurons of the PeN may use the neonatal testosterone surge to trigger molecular events within the cell that, aside from elevating SOM biosynthesis immediately, result in their increased biosynthetic capacity as adults. This hypothesis is supported by the observation that testosterone exposure in the adult can only elevate SOM mRNA content to normal male levels when the animal has been exposed to testosterone as a neonate (10).
In summary, we report here that SOM biosynthesis increases in PeN
neurons during the first 10 days of life and that a marked sex
difference develops over this time. The elevated levels of SOM mRNA
content found in the male result from actions of estrogen after the
aromatization of testosterone around P5, but do not appear to involve a
direct activation of ER within these cells. A sexually dimorphic
pattern of AR expression is found to exist in P5 rats, but, in contrast
to that in the adult rat, is unlikely to be involved in regulating SOM
biosynthesis during the neonatal period. Together, these observations
indicate that the early neonatal period is an important time during
which the SOM neurons of the PeN become sexually differentiated and
that this is very likely to have enduring consequences for SOM neuron
function in the adult.
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Acknowledgments |
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Footnotes |
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2 Lister Institute-Jenner Fellow.
Received July 24, 1997.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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and ß immunoreactivity by
oxytocin neurons of rat paraventricular nucleus. J Neuroendocrinol 9:803–806[CrossRef][Medline]
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