In Vivo Modulation of Sigma Receptor Sites by Calcitonin Gene‐related Peptide in the Mouse and Rat Hippocampal Formation: Radioligand Binding and Electrophysiological Studies

Abstract

Possible interactions between sigma (sigma) receptor sites and calcitonin gene-related peptides (CGRP) were investigated using receptor subtype-related analogues and fragment in in vivo [3H](+)SKF 10 047/sigma binding in the hippocampus, and electrophysiological recording of the N-methyl-D-aspartate (NMDA)-induced activation of CA3 pyramidal neurons, two well-established sigma assays. In both paradigms, CGRP and the agonist [Cys(ACM)2,7]hCGRPalpha modulated sigma systems. In vivo binding experiments demonstrated that CGRP and [Cys(ACM)2,7]hCGRPalpha inhibited 25-40% of specific [3H](+)SKF 10 047 labelling in the mouse hippocampal formation while the purported antagonist hCGRP8-37 was inactive. The specificity of this modulation was demonstrated further by the lack of effect of other vasoactive peptides, including the atrial natriuretic peptide, substance P, and its N-terminal fragment, substance P1-7. In the CA3 subfield of the rat dorsal hippocampus, hCGRP alpha decreased (up to 61%) the NMDA-induced activation of the pyramidal neurons. Conversely, the linear analogue [Cys(ACM)2,7]hCGRP alpha enhanced (by 85%) the NMDA-induced activation of CA3 pyramidal neurons, while the antagonistic fragment hCGRP8-37 had no effect. Haloperidol, a high-affinity sigma receptor ligand, inhibited by 90% the in vivo [3H](+)SKF 10 047 labelling, and prevented the modulation of the NMDA-induced activation by hCGRP alpha and [Cys(ACM)2,7]hCGRP alpha. It thus appears that CGRP can modulate sigma-related systems in the hippocampal formation.