Evidence that synaptic transmission between giant interneurons and identified thoracic interneurons in the cockroach is cholinergic

In the cockroach, a population of thoracic interneurons (TIs) receives direct inputs from a population of ventral giant interneuons (vGIs). Synaptic potentials in type-A TIs (TI_As) follow vGI action potentials with constant, short latencies at frequencies up to 200 Hz. These connections are important in the integration of directional wind information involved in determining an oriented escape response. The physiological and biochemical properties of these connections that underlie this decision-making process were examined. Injection of hyperpolarizing or depolarizing current into the postsynaptic TI_As resulted in alterations in the amplitude of the postsynaptic potential (PSP) appropriate for a chemical connection. In addition, bathing cells in zero-calcium, high magnesium saline resulted in a gradual decrement of the PSP, and ultimately blocked synaptic transmission, reversibly. Single-cell choline acetyltransferase (ChAT) assays of vGI somata were performed. These assays indicated that the vGIs can synthesize acetylcholine. Further more, the pharmacological specificity of transmission at the vGI to TI_A connections was similar to that previously reported for nicotinic, cholinergic synapses in insects, suggesting that the transmitter released by vGIs at these sypapses is acetylcholine. © 1992 John Wiley & Sons, Inc.