Distinct receptors, second messengers and conductances underlying the dual responses to serotonin in an identified leech neurone.

1. Pressure-sensitive mechanosensory (P) neurones of the leech Hirudo medicinalis produce two responses to serotonin (5-HT): activation of a Cl- conductance and of a non-selective monovalent cation conductance. The effects of channel blockers, the receptor pharmacology and the second-messenger dependence of these responses were studied in voltage-clamped P cells in culture. Antagonists were applied by superfusion and agonists by pressure ejection. 2. Zn2+ (100 mumol l-1) and H+ (pH 6.5 and lower) reversibly reduced the Cl- conductance activated by 5-HT. The cation conductance was impermeant to calcium ions and was reduced by micromolar concentrations of the Na+ channel inhibitors amiloride and 3,4-dichlorobenzamil. 3. High concentrations of antagonists or agonists of 5-HT1 receptors and an antagonist of 5-HT3 receptors had no effect on either response of P cells to 5-HT. Micromolar concentrations of ketanserin or cyproheptadine, which selectively antagonize 5-HT2 receptors, reduced the cation but not the Cl- conductance. From these results, the receptor underlying the cation conductance appears to be of the 5-HT2 subtype, whereas the receptor activating the Cl- conductance does not fit within the mammalian classification scheme. 4. Brief (less than 500 ms) application of membrane-permeant agonists of the second messenger cyclic AMP elicited a Cl- conductance, whereas antagonists of cyclic-AMP-dependent protein kinase A reversibly suppressed the Cl- conductance elicited by 5-HT and by cyclic AMP agonists. Compounds affecting other second messenger pathways were without effect on the Cl- conductance. It therefore appears that the Cl- conductance is activated by cyclic-AMP-dependent protein kinase A. 5. Cyclic nucleotide agonists and antagonists were without effect on the cation conductance. However, brief application of phorbol esters, which activate protein kinase C, elicited an amiloride-sensitive cation current. An inhibitor of protein kinase C reduced the cation conductance activated by 5-HT and by phorbol esters. Therefore, the cation conductance appears to depend on activation of protein kinase C. 6. We conclude that 5-HT activates two types of receptor coupled to separate ionic channels via different second messenger pathways in P cells. A receptor that is distinct from the mammalian subtypes activates Cl- channels via cyclic-AMP-dependent protein kinase A. 5-HT2 receptors appear to activate cation channels by means of protein kinase C.