Pharmacology of a cholecystokinin receptor on 5‐hydroxytryptamine neurones in the dorsal raphe of the rat brain

1 The effect of bath application of sulphated cholecystokinin octapeptide (CCK‐8) was studied on neurones in slices containing rat raphe nucleus. 2 Intracellular recordings were made from neurones in the dorsal raphe nucleus. Some of the neurones with the characteristics of 5‐hydroxytryptamine (5‐HT)‐containing cells which were inhibited by 5‐HT and excited by noradrenaline were excited by cholecystokinin. The response to cholecystokinin was dose‐dependent over the range 10 to 1000 nm. 3 The response to CCK‐8 persisted in the presence of tetrodotoxin. Either reduction of extracellular calcium or addition of 25 mm magnesium did not block the CCK response, suggesting it was mediated by receptors located on the membrane of the raphe neurones. 4 The agonist and antagonist specificity of the CCK response was determined. The CCKB selective agonist, pentagastrin, was inactive when applied at concentrations up to 10 μm. the CCKA receptor antagonist L‐364,718 (1 to 100 nm) blocked the response to cholecystokinin. Much higher (1–10 μm) concentrations of the CCKB receptor antagonist L‐365,260 were required for inhibition of the CCK response. 5 These data support the existence of a CCK receptor, located on raphe neurones in the rat, with a pharmacological profile very similar to that described for the CCKA type.