An electrophysiological study of 5‐hydroxytryptamine receptors of neurones in the molluscan nervous system

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Abstract
1. 5‐Hydroxytryptamine (5‐HT) has been iontophoretically applied to the membrane of central neurones of Cryptomphallus aspersa; CILDA neurones (cells with inhibition of long duration) (Gerschenfeld & Tauc, 1964) are the only cells sensitive to 5‐HT. The responses to 5‐HT is always a depolarization. The CILDA cells studied were also depolarized by ACh.2. From experiments in which pulses of 5‐HT and ACh were applied from a double‐barrelled micropipette to the CILDA cell soma, it has been calculated that 5‐HT and ACh receptors were located at different distances from the injecting micropipette tip. It has also been calculated from the diffusion equation that in the same CILDA cell a 5‐HT concentration of 8·2 × 10−9 M and a ACh concentration of 1·3 × 10−8 M caused a similar peak depolarization.3. CILDA neurones show ‘anomalous’ rectification. 5‐HT increases the membrane conductance of CILDA.4. 5‐HT receptors of CILDA neurone are desensitized by repeated application of 5‐HT. The desensitization lasts for ca. 40 sec.5. 5‐HT receptors are blocked by lysergic acid diethylamide and its derivatives. Morphine chlorhydrate blocks them non‐competitively.6. Some inhibitors of monoamine oxidase (trancylpromine, isocarboxazide, iproniazide and nialamide) have been tested. They do not prolong the action of 5‐HT, but block the 5‐HT receptors.7. No crossed desensitization between 5‐HT and ACh has been observed. Atropine blocks both ACh‐receptors and 5‐HT receptors, 5‐HT receptors appear to be blocked to a greater extent.8. The data presented support the assumption of a excitatory transmitter role of 5‐HT to CILDA neurones, but further evidence is necessary to confirm this hypothesis.