Burst discharge in mammalian neuroendocrine cells involves an intrinsic regenerative mechanism

Calcium-dependent inward current in Aplysia bursting pace-maker neurones

Depolarizing voltage-clamp pulses elicit a triphasic series of tail currents (phase I, II and III) in Aplysia burst-firing neurones L2-L6. The sequence and time course of the tail currents resemble slow changes in membrane potential which follow bursts in the unclamped cell. The phase II tail current is an inward current with a time course similar to that of the depolarizing after-potential (d.a.p.) which follows bursts in the unclamped cell. The phase II tail current is suppressed by depolarizing pulses which approach ECa, is blocked by Ca2+ current antagonists (Co2+ and Mn2+), and is blocked by intracellular injection of EGTA. The phase II tail current is not blocked by agents which block Na+-dependent action potentials, the Na+-Ca2+ exchange pump, or the Na+-K+ exchange pump. The phase II tail current is not blocked by the elimination of large outward K+ currents which can lead to extracellular K+ accumulation. Thus, the phase II tail current is not generated by any of these processes. The phase II tail current is reduced by about 60% following substitution of tetramethylammonium (TMA+) for external Na+, but is unaffected by reducing external Cl-. The phase II tail current is distinct from a persistent inward Ca2+ current which underlies the negative resistance region of the steady-state current--voltage relation of bursting cells. The persistent inward current is only slightly reduced by TMA+ substitution for Na+, and is enhanced by EGTA injection. Injection of Ca2+ into Aplysia bursting cells elicits a biphasic (inward-outward) current. The inward current can be observed in isolation after blocking the outward component (Ca2+-activated K+ current) with 50 mM-external tetraethylammonium. The Ca2+-elicited inward current has a reversal potential near -22 mV, and is non-selective for Na+, K+ and Ca2+. The reversal potential is unaffected by changes in Cl- and pH. The Ca2+- activated conductance is apparently voltage independent. We propose that the phase II tail current, and hence the d.a.p., is due to the Ca2+-dependent activation of a voltage-independent non-specific cationic conductance. This conductance participates in generating the depolarizing phase of bursting pace-maker activity.

Intrinsic inhibition in magnocellular neuroendocrine cells of rat hypothalamus

Endogenous mechanisms of inhibition in magnocellular neuroendocrine cells were studied with intracellular recordings in the rat hypothalamic slice preparation. Hyperpolarizing after-potentials (duration up to 125 ms) followed single action potentials and after-hyperpolarizations (a.h.p.s) lasting hundreds of milliseconds followed brief evoked spike trains. The amplitude and duration of the a.h.p. increased after spike trains of longer duration or higher frequency. The a.h.p. appears endogenous, rather than synaptically mediated from recurrent inhibition, because it persisted after pharmacological blockade of axonal conduction or of chemical synaptic transmission. The reversal potential of the a.h.p. was at least 20 mV more negative than that of inhibitory post-synaptic potentials. Cl- ionophoresis did not alter the a.h.p. Chelation of intracellular Ca2+ with EGTA injection eliminated the a.h.p. A Ca2+-activated K+ conductance, rather than recurrent synaptic inhibition, apparently causes the a.h.p. and is at least partly responsible for the inhibition after single spikes in magnocellular neurones. During hormone release, this endogenous mechanism may contribute to the post-burst silent period in putative oxytocinergic cells and to the interburst interval in phasic neurones, which are known to fire repetitive bursts associated with vasopressin release.