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 nucl eus is believed to play a role in hippocampo-septo-hypothalamic relationshi ps. mdn neurones display prominent low-threshold Ca2+ spikes, which differ in their propensity to trigger either a burst of Na+ spikes or a single spi ke. In the present study, whole-cell voltage-clamp experiments were carried out on thick slices at 34 degrees C to characterize the pharmacological an d physical properties of the transient Ca2+ current (I-T) underlying the lo w-threshold spikes. Recorded cells were dye-labelled and identified as belo nging to the mdn. In bursting and nonbursting neurones, I,was reduced by am iloride (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 a ctivated between -80 mV and -50 mV and the mean peak current was 1050 pA. S teady-state activation and inactivation curves were fitted by a Boltzmann e quation. 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 activat ion occurred within 500 ms. When compared with results on other I-T, the pr esent data show that the current possesses distinct pharmacological and phy sical properties. Nevertheless, all investigated cells displayed a homogeno us profile of I-T, suggesting that the differences in spike pattern between mdn neurones are not due to different populations of Ca2+ channels.