Different Effects of 5-HT1 and 5-HT2 Receptor Agonists on Excitability Modulation of Motoneurons in Frog Spinal Cord

Abstract

Effects of 5-HT₁ and 5-HT₂ receptor agonists and antagonists on membrane properties of motoneurons in the isolated lumbar segment of the frog spinal cord were investigated using intracellular recordings. Application of a 5-HT_2A,B,C receptor agonist α-Me-5-HT evoked depolarization of the motoneuronal membrane. The co-application of α-Me-5-HT and a specific 5-HT_2B receptor antagonist SB 206553 did not result in depolarization. α-Me-5-HT reduced the amplitude of medium afterhyperpolarization and increased the number of antidromic action potentials (APs). The application of an antagonist SB 206553 abolished these effects. A 5-HT_1A/7 receptor agonist 8-OH-DPAT had a time-dependent effect on the number of antidromic APs, evoking an initial short-term excitation followed by an inhibition. The data obtained in our experiments indicate the presence of 5-HT_1A/7 and 5-HT_2B,C receptors on the postsynaptic membrane of motoneurons. We suggest a possible co-modulation of the accommodative properties of motoneurons by the two types of serotonin receptors, 5-HT_2B,C and 5-HT_1A.