Components of after-hyperpolarization in magnocellular neurones of the rat supraoptic nucleus in vitro

The release of the peptide hormones vasopressin and oxytocin strongly depends on the pattern of neuronal activity of magnocellular neurosecretory neurones located in the supraoptic and paraventricular nuclei (see Poulain & Wakerley, 1982, for review). During physiological stimulation of these magnocellular neurones an increase of the discharge rate and, in a subpopulation of neurones, the transition from continuous to phasic activity was observed. The excitability and the discharge pattern of magnocellular neurones of the supraoptic nucleus (SON) strongly depend on depolarizing and hyperpolarizing after-potentials (Andrew & Dudek, 1984a,b). When a train of action potentials is evoked by the injection of a depolarizing direct current, an after-hyperpolarization (AHP) or a sequence of an AHP and a depolarizing after-potential (DAP) follows the termination of the train of spikes. The DAP was reported to be more frequently observed in vasopressin neurones than in oxytocin neurones (Armstrong et al. 1994). Sufficient summation of DAPs induces a plateau potential that gives rise to repetitive neuronal discharge (Andrew & Dudek, 1984a; for review, see Legendre & Poulain, 1992). Both the AHP (Andrew & Dudek, 1984b) and the DAP (Bourque, 1986; Andrew, 1987; Li et al. 1995) are generated by the activation of calcium-dependent mechanisms. During neuronal discharge, the intracellular calcium concentration is increased by the activation of voltage-gated calcium channels. T-, N-, L-, and P-type calcium channels, as well as two novel types of voltage-activated calcium channels, are of functional importance in magnocellular SON neurones (Fisher & Bourque, 1995). The ionic mechanisms involved in the generation of the DAP are still obscure. A reduction of an outward potassium current is suggested to be involved in the induction of the DAP (Li & Hatton, 1997b). The AHP was demonstrated to be evoked by an activation of calcium-dependent potassium channels (Andrew & Dudek, 1984b; Bourque et al. 1985; Bourque & Brown, 1987; Kirkpatrick & Bourque, 1996). Due to differing pharmacological properties, two major types of calcium-dependent potassium channel can be discriminated (for review, see Rudy, 1988; Storm, 1990; Brown et al. 1990). Calcium-dependent potassium channels with a small single channel conductance (SK channels) are selectively blocked by the bee venom toxin apamin, are insensitive to tetraethylammonium (TEA) and have little or no voltage dependence. Calcium-dependent potassium channels with a large single channel conductance (BK or maxi-K channels) are blocked by the scorpion venom peptide charybdotoxin (ChTX; Leiurus quinquestriatus; for review, see Garcia et al. 1995), are very sensitive to TEA and are strongly voltage dependent. BK channels have been demonstrated to be very selectively blocked by the scorpion venom peptide iberiotoxin (IbTX; Butus tamulus; see Garcia et al. 1991). When compared with the action of IbTX, the action of ChTX is less specifically directed against BK channels, since it also blocks calcium-dependent potassium channels with an intermediate single channel conductance (IK channels) which are also sensitive to TEA but not to apamin or IbTX (for review, see McManus, 1991). In addition to its action on calcium-activated potassium channels, ChTX binds to the α-subunit Kv1.3 of the Shaker-related subfamily of rat Kv channels and thus blocks voltage-gated potassium channels containing the Kv1.3 α-subunit (for review, see Garcia et al. 1995). Kv1.3 α-subunit-containing potassium channels are selectively blocked by the scorpion venom peptide margatoxin (MgTX; Centruroides margaritatus;Garcia-Calvo et al. 1993). In supraoptic neurones, a major portion of the AHP was demonstrated to be blocked by apamin (Bourque & Brown, 1987; Armstrong et al. 1994), suggesting at least an important contribution by the SK type of calcium-dependent potassium channels to the genesis of AHPs. In the present study the contribution of ChTX-, IbTX- and MgTX-sensitive potassium channels to the generation of AHPs in magnocellular neurones of the rat SON was examined in brain slices. The effects of these toxins on spike train after-potentials were compared with the effects of apamin. This pharmacological approach allows the selective dissection of AHP components induced by the activation of a broad spectrum of calcium-activated potassium channels. Preliminary accounts of this study have appeared in abstract form (Greffrath & Boehmer, 1997).