The pharmacological sensitivity of hyperpolarizing components of spike train after‐potentials was examined in sixty‐one magnocellular neurones of the rat supraoptic nucleus using intracellular recording techniques in a brain slice preparation. In 26 % of all neurones a slow after‐hyperpolarization (AHP) was observed in addition to a fast AHP. In 31 % of all neurones a depolarizing after‐potential (DAP) was observed. The fast AHP was blocked by apamin whereas the slow AHP was blocked by charybdotoxin (ChTX). The DAP was enhanced by ChTX or a DAP was unmasked if not present during the control period. Low concentrations of TEA (0.15–1.5 mm) induced effects on the slow AHP and the DAP essentially resembling those of ChTX. The same was true for the effects of CoCl2 (1 mm). Spike train after‐potentials were not affected by either iberiotoxin (IbTX), a selective high‐conductance potassium (BK) channel antagonist, or margatoxin (MgTX), a Kv1.3 α‐subunit antagonist. Kv1.3 α‐subunit immunohistochemistry revealed that these units are not expressed in the somato‐dendritic region of supraoptic neurones. The effects of ChTX, IbTX, MgTX, TEA, CoCl2 and CdCl2 on spike train after‐potentials are interpreted in terms of an induction of the slow AHP by the activation of calcium‐dependent potassium channels of intermediate single channel conductance (IK channels). The results suggest that at least the majority of supraoptic magnocellular neurones share the capability of generating both a slow AHP and a DAP. The slow AHP may act to control the expression of the DAP, thus regulating the excitability of magnocellular neurones. The interaction of the slow AHP and the DAP may be important for the control of phasic discharge.

The contribution of Ca(2+)-activated K(+) channels to hyperpolarizing after-potentials (HAP) of action potentials, to spike-frequency adaptation and thus to the shaping of discharge pattern, was examined in rat supraoptic magnocellular neurosecretory cells. In addition, the expression of BK channels and SK3 subunits of SK channels was studied using double immunofluorescence detection. The presence of BK channels and SK3 subunits was detected in many supraoptic neurones containing either vasopressin or oxytocin. Current-clamp recordings of current-induced spike trains revealed that HAPs comprise a fast and a slow HAP (fHAP and sHAP). Correlation analyses revealed that the increase of the fHAP in amplitude and spike broadening were correlated to a moderate gradual increase of the interspike interval and thus to weak spike-frequency adaptation. By contrast, marked prolongation of the interspike interval and strong spike-frequency adaptation depended on the appearance and on the amplitude of the sHAP. The sHAP and spike-frequency adaptation were blocked by cadmium, as well as by the SK channel antagonist apamin. The fHAP was attenuated by the BK channel antagonist iberiotoxin (IbTX), by the BK/IK channel antagonist charybdotoxin (ChTX) and by apamin. ChTX attenuated fHAPs throughout the entire spike train. By contrast, the IbTX-induced attenuation of the fHAP was restricted to the initial part of the spike train, while the apamin-induced attenuation slowly increased with the progression of the spike train. These results suggest that strong spike-frequency adaptation in supraoptic neurones essentially depends on the generation of the sHAP by activation of SK channels. Comparison of effects of IbTX, ChTX and apamin suggests a complementary contribution of SK-, BK- and IK-channels to fHAPs.

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Electrophysiological properties and ionic basis of subthreshold oscillation of the membrane potential were examined in 104 magnocellular neurones of the rat supraoptic nucleus using intracellular recording techniques in a brain slice preparation. Subthreshold oscillation of the membrane potential occurring in all neurones examined was voltage dependent. Oscillation was initiated 7‐12 mV negative to the threshold of fast action potentials. Oscillation was the result of neither excitatory nor inhibitory synaptic activity nor of electric coupling. Frequency analyses revealed a broad band frequency distribution of subthreshold oscillation waves (range 10‐70 Hz). The frequency band of 15‐33 Hz was observed in neurones depolarized close to the threshold of discharge. Subthreshold oscillation was blocked by TTX (1.25‐2.5 μM) as well as by TEA (15 mM). Subthreshold oscillation was not blocked by low Ca2+‐high Mg2+ superfusate, CdCl2, TEA (1‐4.5 mM), 4‐aminopyridine, apamin, charybdotoxin, iberiotoxin, BaCl2, carbachol and CsCl. During application of TTX, stronger depolarization induced high‐threshold oscillation of the membrane potential at a threshold of about ‐32 mV. These oscillation waves occurred at a mean frequency of about 35 Hz and were blocked by CdCl2. Effects of ion channel antagonists suggest that subthreshold oscillation is generated by the interaction of a subthreshold sodium current and a subthreshold potassium current. The generation of high‐threshold oscillation during TTX involves a high‐threshold calcium current. Subthreshold oscillation of the membrane potential may be important for the inter‐neuronal synchronization of discharge and for the amplification of synaptic events.