CALCIUM-ACTIVATED HYPERPOLARIZATIONS IN RAT LOCUS-CERULEUS NEURONS IN-VITRO

1. Intracellular recordings were made from rat locus coeruleus (LC) ne urons in completely submerged brain slices. Trains of action potential s in LC neurons were followed by a prolonged post-stimulus hyperpolari zation (PSH). If trains were elicited with depolarizing current pulses of sufficient intensity, PSH was composed of a fast, early component (PSH(E)) and a slow, late component (PSH(L)). PSH which followed train s elicited with lower intensity depolarizing current pulses consisted only of PSH(L). 2. Both PSH(E) and PSH(L) were augmented by increasing the number of action potentials in the train and both were associated with an increase in membrane conductance. The reversal potential for PSH(E) was -108 mV and for PSH(L) it was -114 mV. 3. When a hybrid vol tage clamp protocol was used, the current underlying PSH (I(PSH)) was observed to consist of an early, rapidly decaying component, I(E), fol lowed by a late, slower decaying component, I(L). The time course of d ecay of I(PSH) was biexponential with the time constant of decay of I( L) more than one order of magnitude larger than the time constant of d ecay of I(E). An increase in the concentration of external K+ shifted the reversal potentials for I(E) and I(L) in the depolarizing directio n; the mean value of shift per tenfold increase in external K+ concent ration was 57.1 mV for I(E) and 57.6 mV for I(L). 4. Both PSH(E) and P SH(L) were inhibited by lowering the external Ca2+ concentration or by application of the Ca2+ channel blockers Cd2+ (200-500 muM) or nifedi pine (100 muM). Intracellular injection of EGTA abolished both compone nts of PSH. Increasing the external Ca2+ concentration augmented both PSH components. 5. Superfusion of dantrolene (25 muM) or ryanodine (20 muM) decreased the amplitude and duration of PSH(L) with much less ef fect on PSH(E). 6. d-Tubocurarine (20-200 muM) selectively blocked PSH (E) with no effect on PSH(L); this effect is the same as that of apami n which we have previously described. Superfusion with charybdotoxin ( 40 nM) or TEA (400 muM-1 mM) did not reduce PSH(E) or PSH(L). 7. Inhib ition of I(A) by 4-aminopyridine or 2,4-diaminopyridine also did not r educe either component of PSH. In fact, these agents slightly augmente d both components of PSH; this effect was probably secondary to the pr olongation of action potential duration. Superfusion of TEA in concent rations of 2-10 mM increased the size and duration of PSH(L) and incre ased the duration but decreased the size of PSH(E). 8. Superfusion wit h noradrenaline (10 muM), dibutyryl cAMP or 8-Br-cAMP (1 mM) did not a ffect either component of PSH. Carbachol (40-100 muM) increased the si ze and duration of PSH(L) without changing PSH(E). 9. The results indi cate that PSH in LC neurones is mediated by two types of Ca2+-dependen t K+ conductances with different time courses and pharmacological prop erties. The early component is selectively blocked by apamin and d-tub ocurarine, while the late component is reduced by blockers of Ca2+-ind uced Ca2+ release and enhanced by muscarinic agonists.