Inhibition of an identified snail postsynaptic neuron evoked by stimulation of a peptidergic interneuron initiating bursting pacemaker activity in another neuron

Beating neuron V8, whose activity is abolished upon interneuronal stimulation, has been found and identified in the CNS of the snail, Helix pomatia. It was found earlier that stimulation of the same interneuron initiates bursting pacemaker activity in another neuron. Under voltage clamp conditions interneuronal stimulation evoked a slow outward current in the V8 neuron with a latency of about 2 s. Intracellular injection of tetraethylammonium or Cs+ ions into the interneuron greatly increased the slow outward current amplitude. External application of CdCl2 (1 mM), LaCl3 (5 mM) or replacement of Ca2+ by Mg2+ ions reversibly abolished transmission between the interneuron and the V8 neuron. The neuronal membrane conductance increased by about three times on the maximum of the slow outward current development. Computer fitting showed that a decay of the slow outward current can be approximated by the sum of two exponentially decaying components with time constants of about 1 s and 16 s. Extrapolated reversal potential for a fast (1 s) component of the current was about -70 mV. A slow (16 s) component asymptotically decreased upon hyperpolarization and it was observed at even more negative potential than Ek. Theophylline (1 mM) and 3-isobutyl-1-methylxanthine (0.1 mM) reversibly increased the slow outward current amplitude, whereas imidazole (5 mM) and tolbutamide (5 mM) caused its reversible decrease. It is concluded that inhibitory monosynaptic transmission exists between the interneuron and the beating V8 neuron. Interneuronal simulation evokes an increase in the membrane potassium conductance and probably a decrease in stationary potential-dependent sodium conductance of the postsynaptic V8 neuron. The cellular cyclase system seems to be involved in the postsynaptic response.