Ghrelin is mainly described for its effects on feeding behavior and metabolism. However, CNS distribution of its receptor (type 1a GHSR), and modulation of neurotransmission in hypothalamus suggest broader effects than originally predicted. Systemically administrated ghrelin inhibits inflammatory pain after behavioral observations. Therefore, we investigated the expression and function of type 1a GHSR in mouse spinal cord by molecular biology, biochemistry, histology, and electrophysiology. The mRNA and protein were detected in tissue extracts by RT-PCR and Western blotting. In situ, receptor mRNA and immunoreactivity were localized to cell bodies within the the medial aspect of deep dorsal horn. Patch clamp recordings on laminae IV-VI demonstrated that bath-applied ghrelin (100nM) induced a strong increase of spontaneous GABA/glycine-mediated current frequency (463±93% of the control) and amplitude (150±16% of the control) in about 60% of recorded neurons. Specificity of type 1a GHSR activation was confirmed by the lack of effect of the de-acylated form of ghrelin (des-acyl-ghrelin), and after pre-incubation with the specific receptor antagonist [D-Lys³]-GHRP-6. In the presence of TTX, effect of the peptide was strongly reduced, mainly indicating an action potential-dependent mechanism. The functional link between ghrelin and pain was confirmed by inhibition in vitro of the c-fos response to capsaicin activation of nociceptive fibers, after quantification of Fos-immunoreactive nuclei in laminae IV-VI.

Our results are the first demonstration of the presence of functional type 1a GHSRs in spinal cord, and indicate that ghrelin may exert antinociceptive effects by directly increasing inhibitory neurotransmission in a subset of deep dorsal horn neurons.