Modulation of frog spinal cord interneuronal activity by activation of 5-HT 3 receptors

Abstract

Motoneuron membrane potentials were recorded from the ventral roots of isolated, hemisected frog spinal cords using sucrose gap techniques. The effects of the selective 5-HT 3 agonist 2-methyl-serotonin (2-Me-5HT) on the changes in motoneuron membrane potential produced by dorsal root stimulation and by superfusion of excitatory amino acid agonists were evaluated. Application of 2-Me-5HT (100 μM) did not alter motoneuron membrane potential, but did substantially reduce ( ∼ 20%) the polysynaptic ventral root potentials evoked by dorsal root stimulation. 2-Me-5HT did not change motoneuron depolarizations generated by addition to the Ringer's solution of the excitatory amino acid agonists AMPA (10–30 μM), kainate (30 μM), or t-ACPD (100 μM), but NMDA-induced motoneuron depolarizations (100 μM) were significantly and reversibly reduced ( ≈ 20%) by exposure to 2-Me-5HT (100 μM). 2-Me-5HT-evoked decreases of NMDA depolarizations were blocked by the 5-HT 3 antagonists ICS 205 930 (50–100 μM) and d-tubocurarine (3–10 μM), but not by MDL 72222 (20–100 μM), the 5-HT 2 receptor antagonist ketanserin (10 μM), or the 5-HT 1A/5-HT 2A antagonist spiperone (10 μM). Two lines of evidence support the hypothesis that the effects of 2-Me-5HT are generated by an indirect mechanism involving interneurons: (1) TTX (0.781 μM) eliminated the effect of 2-Me-5HT on NMDA-induced motoneuron depolarizations, and (2) 2-Me-5HT reduced spontaneous ventral root potentials that result from interneuronal discharges. We attempted to establish the identity of a putative transmitter released by interneurons responsible for the effects on NMDA-depolarizations produced by 2-Me-5HT, but the AMPA receptor antagonist, CNQX (10 μM), the GABA A receptor antagonist, bicuculline (50 μM), the GABA B receptor antagonist, saclofen (100 μM), the opioid antagonist, naloxone (100 μM), and the adenosine antagonists, CPT (20–100 μM) and CSC (10–100 μM) did not alter 2-Me-5HT-induced reductions of NMDA-depolarizations. In sum, the site of interaction between 2-Me-5HT and NMDA appears to be at interneuronal locus, but the mechanism remains unclear.