Spontaneous and stimulated firing in cultured rat suprachiasmatic neurons

Neurons from the suprachiasmatic nucleus (SCN) of the hypothalamus, the site of a circadian pacemaker in mammals, were isolated from embryonic rat. After mechanical dissociation neurons were brought into culture for 1–2 weeks, using a chemically defined medium. Recordings were made from 74 bipolar neurons using two different configurations of the patch-clamp technique. During cell attached patch recordings, 45% of neurons fired spontaneously. The mean firing rate was 0.7 ± 0.6 Hz and the firing pattern was irregular. In whole cell recordings 73% of the investigated neurons showed spontaneous activity with an irregular firing pattern. The mean spontaneous firing rate with an intracellular Cl − concentration of 145 mM was 1.0 ± 0.6 Hz. The resting membrane potential of the bipolar neurons was estimated to be −62 ± 24 mV. An intracellular Cl − concentration of 145 mM depolarised the membrane potential. It also increased the probability of spontaneous firing. A depolarising current stimulus produced an action potential with a threshold voltage of −46 ± 9 mV. Suprathreshold stimuli resulted in repetitive firing with a mean frequency of 12 ± 4 Hz. The minimum interspike interval was 52 ± 14 ms. All action potentials either occurring spontaneously or elicited by current stimuli were abolished by the Na +-channel blocker TTX. These results indicate that our cultured neurons have some electrophysiological properties in common with SCN neurons in brain slices and in vivo.