Serotonin and GABA-induced depolarizations of frog primary afferent fibers.

The interaction of gamma-aminobutyric acid (GABA) and serotonin (5-HT) on primary afferent terminals of the isolated frog spinal cord was investigated by sucrose gap recordings from dorsal roots. Application of 5-HT (1.0-100 microM) to the Ringer's solution significantly reduced afferent terminal depolarizations elicited by concentrations of GABA ranging from 0.1 to 1.0 mM. The reductions of GABA-depolarizations which were produced by 1.0 microM 5-HT were mimicked by the 5-HT1A agonists 8-OH-DPAT (8-hydroxy-2-(n-dipropylamino)tetralin) and ipsapirone. The effects of ipsapirone were reversed by the 5-HT1A antagonist spiperone. The decreases of GABA-depolarizations produced by high doses of 5-HT were duplicated by application of alpha-methyl-5-HT, a 5-HT1C/2 agonist and reversed by superfusion of the cord with manserin, a 5-HT1C/2 antagonist. The presumptive 5-HT1A receptor-mediated effects of 1.0 microM 5-HT and 8-OH-DPAT appeared to result from a direct action on afferent terminals because the reduction of GABA responses was unchanged by addition of TTX to the Ringer's solution. In contrast, the putative 5-HT1C/2 receptor actions of 100 microM 5-HT and alpha-methyl-5-HT were substantially reduced by TTX and are presumably caused by activation of receptors located on interneurons. GABAB receptors did not appear to be affected by addition of 5-HT at low or high concentrations because baclofen-induced afferent terminal hyperpolarizations remained unchanged during exposure to 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)