Effects of galanin receptor agonists on locus coeruleus neurons

Galanin exerts an inhibitory effect on locus coeruleus (LC) neurons via a postsynaptic, as yet unidentified galanin receptor. Using an in vitro intracellular recording technique the effect of two galanin receptor agonists on LC neurons was investigated. Bath application of [Sar(1), D-Ala(12)]gal(1-16)-NH(2) (AR-M961), an agonist both at galanin R1 and R2 (GALR1, GALR2) receptors, evoked a reversible membrane hyperpolarization and inhibition of spike discharge in all LC neurons tested (n=42). The action of AR-M961 was blocked by tetraethylammonium chloride. Hyperpolarizing responses induced by AR-M961 were retained in the presence of tetrodotoxin and high Mg(2+)/low Ca(2+) media. The selective GALR2 agonist Gal(2-11)-NH(2) (AR-M1896) only caused inhibition of spike discharge and a slight hyperpolarization in 26 of 34 LC neurons tested, and was on a molar basis much weaker than AR-M961. These results suggest that it mainly is the GALR1 receptor that mediates hyperpolarization of LC neurons.

Receptor subtype-specific pronociceptive and analgesic actions of galanin in the spinal cord: selective actions via GalR1 and GalR2 receptors

Galanin is a 29-aa neuropeptide with a complex role in pain processing. Several galanin receptor subtypes are present in dorsal root ganglia and spinal cord with a differential distribution. Here, we describe a generation of a specific galanin R2 (GalR2) agonist, AR-M1896, and its application in studies of a rat neuropathic pain model (Bennett). The results show that in normal rats mechanical and cold allodynia of the hindpaw are induced after intrathecal infusion of low-dose galanin (25 ng per 0.5 microl/h). The same effect is seen with equimolar doses of AR-M1896 or AR-M961, an agonist both at GalR1 and GalR2 receptors. In allodynic Bennett model rats, the mechanical threshold increased dose-dependently after intrathecal injection of a high dose of AR-M961, whereas no effect was observed in the control or AR-M1896 group. No effect of either of the two compounds was observed in nonallodynic Bennett model rats. These data indicate that a low dose of galanin has a nociceptive role at the spinal cord level mediated by GalR2 receptors, whereas the antiallodynic effect of high-dose galanin on neuropathic pain is mediated by the GalR1 receptors. Thus, a selective GalR1 agonist may be used to treat neuropathic pain.

The glycine residue in cyclic lactam analogues of galanin(1-16)-NH2 is important for stabilizing an N-terminal helix

The neuropeptide galanin is a 29- or 30-residue peptide whose physiological functions are mediated by G-protein-coupled receptors. Galanin's agonist activity has been shown to be associated with the N-terminal sequence, galanin(1-16). Conformational investigations previously carried out on full-length galanin have, furthermore, indicated the presence of a helical conformation in the neuropeptide's N-terminal domain. Several cyclic lactam analogues of galanin(1-16)-NH2 were prepared in an attempt to stabilize an N-terminal helix in the peptide. Here we describe and compare the solution conformational properties of these analogues in the presence of SDS micelles as determined by NMR, CD, and fluorescence spectroscopy. Differences in CD spectral profiles were observed among the compounds that were studied. Both c[D4, K8]Gal(1-16)-NH2 and c[D4,K8]Gal(1-12)-NH2 adopted stable helical conformations in the micelle solution. On the basis of the analyses of their respective alpha H chemical shifts and NOE patterns, this helix was localized to the first 10 residues. The distance between the aromatic rings of Trp2 and Tyr9 in c[D4, K8]Gal(1-16)-NH2 was determined to be 10.8 +/- 3 A from fluorescence resonance energy transfer measurements. This interchromophore spacing was found to be more consistent with a helical structure than an extended one. Removal of the Gly1 residue in compounds c[D4,K8]Gal(1-16)-NH2 and c[D4, K8]Gal(1-12)-NH2 resulted in a loss of helical conformation and a concomitant reduction in binding potency at the GalR1 receptor but not at the GalR2 receptor. The nuclear Overhauser enhancements obtained for the Gly1 deficient analogues did, however, reveal the presence of nascent helical structures within the N-terminal sequence. Decreasing the ring structure size in c[D4, K8]Gal(1-16)-NH2 by replacing Lys8 with an ornithine residue or by changing the position of the single lysine residue from eight to seven was accompanied by a complete loss of helical structure and dramatically reduced receptor affinity. It is concluded from the data obtained for the series of cyclic galanin(1-16)-NH2 analogues that both the ring structure size and the presence of an N-terminal glycine residue are important for stabilizing an N-terminal helix in these compounds. However, although an N-terminal helix constitutes a predominant portion of the conformational ensemble for compounds c[D4,K8]Gal(1-16)-NH2 and c[D4, K8]Gal(1-12)-NH2, these peptides nevertheless are able to adopt other conformations in solution. Consequently, the correlation between the ability of the cyclic galanin analogues to adopt an N-terminal helix and bind to the GalR1 receptor may be considered as a working hypothesis.