Presynaptic inhibition produced by an identified presynaptic inhibitory neuron. II. Presynaptic conductance changes...

We have examined the morphology and pharmacology of the L32 neurons, identified cells that mediate presynaptic inhibition in the Aplysia abdominal ganglion, to gain insight into the putative transmitter released by the L32 cells. We analyzed the fine structure of the synaptic release sites of L32 cells stained with horseradish peroxidase. Each varicosity of L32 was found to contain two general classes of vesicles. One class of vesicles is large (mean long diameter of 98 nm) and contains an electron-dense core that typically filled or nearly filled each vesicle profile. The second class of vesicles is smaller (mean long diameter of 67 nm) and relatively electron lucent. The size, distribution, and morphology of the vesicle population in L32's terminals was similar to that described at the synapses of the identified histaminergic neuron C2 in Aplysia (2). These morphological observations suggested that L32 cells might be histaminergic. Among the various putative transmitters tested, histamine was most effective in mimicking the postsynaptic effects of L32 cells onto L10, and onto other follower cells of L32 in the abdominal ganglion. Histamine also caused inhibition of transmitter output from L10. Both the IPSP produced by L32 in L10 and the response of L10 to histamine could be reversibly blocked by cimetidine, a histamine antagonist in Aplysia (14). These results support, but do not establish the identification of histamine as the putative transmitter of L32 cells. Histamine mimics the action of L32 in mediating presynaptic inhibition allowing us to examine in more detail the conductance changes in L10 underlying presynaptic inhibition. Voltage-clamp analysis revealed that histamine blocked the voltage-dependent Ca2+ current and increased a voltage-dependent K+ current in L10, much as did L32. Both of these changes are likely to act synergistically to inhibit transmitter release. Reduction of Ca2+ current in L10 would directly inhibit transmitter release from L10 directly by decreasing the amount of Ca2+ entering during spike depolarization. The increase in K+ current would act indirectly to reduce transmitter release from L10, by hyperpolarizing L10 and decreasing the amplitude and duration of spikes in L10, as well as reducing the steady-state Ca2+ influx. These results support the idea that in Aplysia presynaptic inhibition is caused primarily by a direct transmitter-mediated reduction in presynaptic Ca2+ current and secondarily by a hyperpolarization of the presynaptic neuron due to a transmitter-mediated increase in a K+ current.(ABSTRACT TRUNCATED AT 400 WORDS)