On the Role of Galanin, Substance P and Other Neuropeptides in Primary Sensory Neurons of the Rat: Studies on Spinal Reflex Excitability and Peripheral Axotomy.

The interaction of intrathecally (i.t.) applied galanin (GAL) with substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), somatostatin (SOM) and C-fibre conditioning stimulation (CS) with regard to their effects on the spinal nociceptive flexor reflex was studied in decerebrate, spinalized, unanaesthetized rats with intact or sectioned sciatic nerves. SP, CGRP, VIP and SOM applied onto the surface of lumbar spinal cord or a brief CS train (1 Hz, 20 s) to the sural nerve facilitated the flexor reflex for several minutes in animals with intact or sectioned nerves. Pretreatment with GAL, which by itself had a biphasic effect on the flexor reflex in a dose-dependent manner, antagonized the reflex facilitation induced by sural CS before and after sciatic nerve section. SP-induced facilitation of the flexor reflex was antagonized by GAL in rats with intact sciatic nerves, but not after nerve section. In contrast, VIP-induced reflex facilitation was antagonized by GAL only after sectioning of the sciatic nerve. GAL was effective in antagonizing the facilitatory effect of CGRP under both situations, but had no effect on SOM-induced facilitation. A parallel immunohistochemical study revealed that after sciatic nerve section GAL-like immunoreactivity (LI) and VIP-LI are increased in the dorsal root ganglia and that these two peptides coexist in many cells. The present results indicate that GAL antagonizes the excitatory effect of some neuropeptides which exist in the spinal cord. This antagonism could explain the inhibitory effect of GAL on C-fibre CS-induced facilitation of the flexor reflex, which is presumably due to the release of some of these neuropeptides from the terminals of primary afferents. Furthermore, the interaction between GAL and other neuropeptides is altered by sciatic nerve section, paralleling changes in the levels of these neuropeptides in primary afferents and their pattern of coexistence after nerve section. It is proposed that SP and CGRP are important mediators of the spinal flexor reflex in intact rats. However, after axotomy VIP may replace SP in this capacity, paralleling the decrease in SP and marked increase in VIP levels. In general the study provides further support for involvement of peptides in sensory function.