Depolarizing action of cholecystokinin on rat supraoptic neurones in vitro

1. Cholecystokinin is co-localized within the oxytocin- and, to a lesser extent, vasopressin-synthesizing magnocellular neurones in the hypothalamic supraoptic and paraventricular nuclei. These nuclei are also prominent binding sites for cholecystokinin. In the present study we used intracellular current- and voltage-clamp recordings from fifty-seven supraoptic nucleus cells, maintained in superfused explants of rat hypothalamus, to assess their membrane responses to exogenous cholecystokinin and define the nature of their cholecystokinin receptors. 2. In a majority of the fifty-seven cells tested, bolus infusions into the superfusion media of cholecystokinin fragments (maximum concentrations estimated at 0.3-15 microM) were followed within 1-5 s by a transient and reversible membrane depolarization. Active peptides included sulphated cholecystokinin octapeptide (26-33) (28 of 33 cells responded), non-sulphated cholecystokinin octapeptide (26-33) (21 of 25 cells responded), cholecystokinin tetrapeptide (30-33) (20 of 24 cells responded and caerulein (4 of 4 cells responded). None of five cells responded to cholecystokinin (26-28). Depolarizing responses to cholecystokinin analogues persisted in the presence of tetrodotoxin (0.2-0.4 microM), and in Ca(2+)-free solutions containing MnCl2 (2.5 mM). 3. Under voltage clamp, cholecystokinin fragments evoked an inward current accompanied by an increase in membrane conductance. The amplitude of the inward current varied linearly as a function of membrane voltage, with an extrapolated reversal potential of approximately -15 mV. Reversal potentials were not altered by chloride injection. These features suggest that cholecystokinin activates a non-selective cationic conductance. 4. Active cholecystokinin analogues were approximately equipotent in their depolarizing actions, a feature that supports the activation of cholecystokinin-B type receptors. Moreover bath application of 200 nM L-365,260, an antagonist with a high affinity for cholecystokinin-B receptors, reversibly attenuated the cholecystokinin-induced responses in four of six cells tested. 5. These observations indicate that cholecystokinin can directly influence the excitability of rat supraoptic nucleus neurones and provide evidence for an additional site where this peptide may act within the hypothalamo-neurohypophysial axis.