1. 1.

    Ionophoresis of acetylcholine (ACh) onto the cell body membrane of an identified giant interneurone (GI2) in the central nervous system of the cockroach Periplaneta americana induced a depolarizing response at resting potential which was attributed to a population of extrasynaptic ACh receptors. 2. The sensitivity of the cell body membrane of GI 2 to ionophoresis of ACh was determined. 3. Perfusion of 1.0 × 10−6M neostigmine, an inhibitor of acetylcholinesterase, potentiated the ACh sensitivity of the cell body membrane of GI2. This indicated that a high acetylcholinesterase activity was present in the periphery of the sixth abdominal ganglion (A6). 4. The nicotinic antagonist, α-bungarotoxin (at a concentration of 1.0 × 10−7M) was found to block the ACh response of the cell body membrane of GI2. By contrast, the muscarinic antagonist, quinuclidinyl benzilate, (at concentrations up to 1.0 × 10−5 M) had no detectable effect on the ACh response. 5. It is suggested that an extrasynaptic nicotinic type of ACh receptor is present on the cell body membrane of GI2.

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