Serotonin depolarizes type A and C primary afferents: an intracellular study in bullfrog dorsal root ganglion.
Intracellular recordings were obtained from the somata of type A and C primary afferents in the isolated bullfrog dorsal root ganglion (DRG) preparation. Bath application of serotonin (5-HT) in concentrations of 0.25-1.0 mM led to slow and fast depolarizing responses. Slow, maintained 5-HT depolarizations were observed in 47% of type A and 70% of type C neurons. These slow depolarizations were associated with an underlying increase in input resistance (Rin). In some type A neurons, the Rin increase was masked by a decrease in Rin due to depolarization-induced rectification. The slow 5-HT depolarization of type A, but not type C neurons showed pronounced tachyphylaxis to repeated 5-HT applications. In type C afferents, serotonin's slow action was often accompanied by spontaneous firing. Manganese decreased slow 5-HT depolarizations of both cell types. A slow depolarization and excitation of type C afferents by methysergide and cinanserin was also observed. Fast transient 5-HT depolarizations accompanied by a rapid decrease in Rin were observed in 7% of type A and 24% of type C neurons. In some DRG cells the fast and slow depolarizations combined to form a biphasic response. The actions of 5-HT reported here resemble in some ways 5-HT responses recorded extracellularly from the spinal terminations of primary afferents.
D000818:Animals; D002928:Cinanserin / Q000494:pharmacology; D004594:Electrophysiology; D005260:Female; D005727:Ganglia, Spinal / Q000187:drug effects*; D007425:Intracellular Membranes / Q000187:drug effects; D008297:Male; D008345:Manganese / Q000494:pharmacology; D008784:Methysergide / Q000494:pharmacology; D009431:Neural Conduction; D009475:Neurons, Afferent / Q000145:classification / Q000187:drug effects*; D011892:Rana catesbeiana; D012701:Serotonin / Q000494:pharmacology*
D012701:Serotonin; D008345:Manganese; D002928:Cinanserin; D008784:Methysergide