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 (IT) underlying the low-threshold spikes. Recorded cells were dye-labelled and identified as belonging to the mdn. In bursting and non-bursting neurones, IT was reduced by amiloride (1 microM) and octanol (1 mM), and during replacement of Ca2+ by Ba2+. The Ca2+ channel blocker mibefradil (10 microM) had only a slight blocking action. Nifedipine (100 microM) and flunarizine (1 microM) had no effect. IT 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 IT, the present data show that the current possesses distinct pharmacological and physical properties. Nevertheless, all investigated cells displayed a homogenous profile of IT, suggesting that the differences in spike pattern between mdn neurones are not due to different populations of Ca2+ channels.