Tracer and electrical coupling of rat suprachiasmatic nucleus neurons.

Whole-cell recording from single neurons of the suprachiasmatic nucleus with an electrode containing the tracer neurobiotin resulted in the staining of multiple neurons in 30% of the cases. Typically, one neuron was darkly stained with dendritic processes and an axon clearly visible while other neurons were lightly stained. The darkly-stained cells were identified as the recorded neuron and tracer-coupled to one to five lightly stained neurons. The resting membrane potential, input membrane conductance, membrane capacitance, the decay time constant and the maximum H-current amplitude of the recorded neurons with tracer-coupled cells were not significantly different from those of neurons not showing tracer coupling. Stimulation of the preoptic area activated an antidromic action potential or an all-or-none small slow inward current in some neurons when the synaptic transmission was blocked by a calcium-free/Mn2+ solution. The small slow inward current did not "collide" with an orthodromically activated action spike suggesting that the current represents the signal from an electrotonically-coupled neuron. In addition, the frequency of biphasic field currents from a neighbouring cell firing were increased by depolarization and decreased by hyperpolarization of the recorded cell. These data demonstrate a chemical and electrical low-resistance coupling of suprachiasmatic nucleus neurons, which could be important in synthesizing the suprachiasmatic nucleus circadian rhythm.