5-Hydroxytryptamine effects on the somata of bullfrog primary afferent neurons.

Intracellular recordings were made from neurons in the isolated dorsal root ganglia of bullfrogs. 5-Hydroxytryptamine was applied by superfusion and by ionophoresis. The most common response to 5-hydroxytryptamine in C neurons was a membrane hyperpolarization and this was observed in 80% of cells. This was due to an increase in membrane potassium conductance because it reversed its polarity at about -90 mV. It was blocked by removal of calcium or addition of calcium blockers. (+)-Tubocurarine, methysergide, ketanserin, quipazine, picrotoxin, caffeine and ouabain blocked this response. The next most common response in C neurons was a fast depolarization, particularly readily observed when 5-hydroxytryptamine was applied by ionophoresis. Since this response reversed its polarity at about -10 mV and was blocked by removal of sodium, this was due to an increase in membrane conductance to both sodium and potassium ions. This response was reduced by superfusion of acetylcholine and gamma-aminobutyric acid. (+)-Tubocurarine, quipazine, picrotoxin and caffeine blocked the response. A small proportion of C neurons (16%) responded to superfusion of 5-hydroxytryptamine with a slow depolarization accompanied by an increase in input resistance. This response reversed its polarity at about -90 mV and, therefore, is presumed to result from potassium inactivation. It was blocked by methysergide and ketanserin but not by (+)-tubocurarine or quipazine. A few type A neurons (8%) caused a fast and transient depolarization like the fast depolarization of C neurons. About half of the A neurons showed a slow depolarization associated with a fall in input resistance. This slow response was assumed to be due to an increase in membrane conductance to both potassium and calcium ions because the response reversed its polarity at about -65 mV and was sensitive to change in external concentrations of those ions. This slow response was blocked by (+)-tubocurarine, methysergide, ketanserin, picrotoxin, caffeine and ouabain but not by quipazine. The effects of 5-hydroxytryptamine are discussed in relation to the similar actions described on a variety of other vertebrate and invertebrate nerve cells. The findings imply that dorsal root ganglion cells of bullfrogs are sensitive to 5-hydroxytryptamine and causes multiple types of 5-hydroxytryptamine responses.