Fast and slow ipsilateral and contralateral spinal reflexes in the neonate rat are modulated by 5-HT

Abstract

1. 1. Three ipsilateral (MSR, PSR, IPSI SLOW) and two contralateral segmental reflexes (CON FAST, CON SLOW) were recorded from L4 or L5 ventral roots of the neonate rat spinal cord in vitro. MSR, PSR and CON FAST were evoked from lower threshold afferents; more intense stimulation evoked IPSI SLOW and CON SLOW. 2. 2. Kainate/AMPA receptors were involved in mediation of MSR, PSR, CON FAST, IPSI SLOW and CON SLOW and NMDA receptors in mediation of CON FAST, IPSI SLOW and CON SLOW. 3. 3. All five reflexes were depressed by 5-HT (IC 50 1.2–7.9 μM; order of sensitivity, CON SLOW > CON FAST = IPSI SLOW > MSR = PSR); and by 5-CT (IC 50 1.9–8.8 nM; order of sensitivity, MSR > IPSI SLOW = CON FAST = CON SLOW > PSR). α-Me-5-HT also depressed all five reflexes. 4. 4. Dipropyl-5-CT selectively depressed MSR and CON SLOW (IC 50 90–170 nM) but was less potent than 5-CT. 8-OH-DPAT selectively depressed MSR (IC 50 1.1 μM), IPSI SLOW and CON SLOW (IC 50 5.8–7.6 μM), while methysergide depressed only MSR (IC 50 26 nM). 5. 5. Phenyl biguanide and m-chlorophenyl biguanide (5-HT 3 receptor agonists) had no significant effects on any reflex. 6. 6. It is concluded that a 5-HT 1-like receptor mediates depression of the MSR. A different receptor or a mixed population of receptors, but not 5-HT 3 receptors, mediate inhibition of PSR, CON FAST, IPSI SLOW and CON SLOW.