Mechanism of irregular firing of suprachiasmatic nucleus neurons in rat hypothalamic slices.

The mechanisms of irregular firing of spontaneous action potentials in neurons from the rat suprachiasmatic nucleus (SCN) were studied in hypothalamic slices using cell-attached and whole cell recording. The firing pattern of spontaneous action potentials could be divided into regular and irregular, based on the interspike interval (ISI) histogram and the membrane potential trajectory between action potentials. Similar to previous studies, regular neurons had a firing rate about >3.5 Hz and irregular neurons typically fired about <3.5 Hz. The ISI of irregular-firing neurons was a linear function of the sum of inhibitory postsynaptic potentials (IPSPs) between action potentials. Bicuculline (10-30 microM) suppressed IPSPs and converted an irregular pattern to a more regular firing. Bicuculline also depolarized SCN neurons and induced bursting-like activity in some SCN neurons. Gabazine (20 microM), however, suppressed IPSPs without depolarization, and also converted irregular activity to regular firing. Thus GABAA receptor-mediated IPSPs appear responsible for irregular firing of SCN neurons in hypothalamic slices.