Intracellular study of rat substantia nigra pars reticulata neurons in an in vitro slice preparation: electrical membrane properties and response characteristics to subthalamic stimulation.

The electrical membrane properties of substantia nigra pars reticulata (SNR) neurons and their postsynaptic responses to stimulation of the subthalamic nucleus (STH) were studied in an in vitro slice preparation. SNR neurons were divided into two types based on their electrical membrane properties. Type-I neurons possessed (1) spontaneous repetitive firings, (2) short-duration action potentials, (3) less prominent spike accommodations, and (4) a strong delayed rectification during membrane depolarization. Type-II neurons had (1) no spontaneous firings, (2) long-duration action potentials, (3) a prominent spike accommodation, (4) a relatively large post-active hyperpolarization, and (5) a less prominent delayed rectification. These membrane properties were very similar to those observed in substantia nigra pars compacta (SNC) neurons in slice preparations. Features common to both types of neurons include that (1) the input resistance was similar, (2) they showed an anomalous rectification during strong hyperpolarizations, and (3) they were capable of generating Ca potentials. Intracellular responses of both types of SNR neurons to STH stimulation consisted of initial short-duration monosynaptic excitatory postsynaptic potentials (EPSPs) and a short-duration inhibitory postsynaptic potential (IPSP) followed by a long-duration depolarization. The IPSP was markedly suppressed by application of bicuculline methiodide and the polarity was reversed by intracellular injection of Cl-. In the preparations obtained from internal capsule-transected rats, STH-induced EPSPs had much longer durations than those observed in the normal preparations, while the amplitude of IPSPs and succeeding small-amplitude long-duration depolarizations was small. The results indicated that SNR contains two electrophysiologically different types of neurons, and that both types of neurons receive monosynaptic EPSPs from STH and IPSPs from areas rostral to STH.