Properties of T-type calcium current in enkephalinergic neurones in guinea-pig hypothalamic slices

The guinea-pig hypothalamic magnocellular dorsal nucleus (mdn) exclusively contains enkephalinergic neurones providing inputs to the septum. This nucleus is believed to play a role in hippocampo-septo-hypothalamic relationships. mdn neurones display prominent low-threshold Ca2+ spikes, which differ in their propensity to trigger either a burst of Na+ spikes or a single spike. In the present study, whole-cell voltage-clamp experiments were carried out on thick slices at 34 degrees C to characterize the pharmacological and physical properties of the transient Ca2+ current (I-T) underlying the low-threshold spikes. Recorded cells were dye-labelled and identified as belonging to the mdn. In bursting and nonbursting neurones, I,was reduced by amiloride (1 mu M) and octanol (1 mM), and during replacement of Ca2+ by Ba2+. The Ca2+ channel blocker mibefradil (10 mu M) had only a slight blocking action. Nifedipine (100 mu M) and flunarizine (1 mu M) had no effect. I-T activated between -80 mV and -50 mV and the mean peak current was 1050 pA. Steady-state activation and inactivation curves were fitted by a Boltzmann equation. The half-activation voltage was -70 mV, slope factor=3.6, and half-inactivation voltage was about -80 mV, slope factor=4.5. Time-to-peak and time constant of inactivation were voltage dependent. Recovery from activation occurred within 500 ms. When compared with results on other I-T, the present data show that the current possesses distinct pharmacological and physical properties. Nevertheless, all investigated cells displayed a homogenous profile of I-T, suggesting that the differences in spike pattern between mdn neurones are not due to different populations of Ca2+ channels.