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The entopeduncular nucleus (EP) is the rodent equivalent of the internal globus pallidus (GPi) in primates. Despite its important role in basal ganglia output generation, the intrinsic properties of EP neurons are poorly understood. We obtained whole-cell recordings from EP neurons in brain slices from male Sprague-Dawley rats (15-19 d. age). Recordings typically exhibited regular tonic spiking even with the excitatory and inhibitory synaptic inputs blocked. However, several characteristics of spiking properties were heterogeneous between recorded neurons. These variable characteristics included spike width, shape of the fast and slow spike after-hyperpolarization, amount of sag with negative current pulses, and post-hyperpolarization rebounds. Spike rate transients in response to current level changes were generally small, indicating that EP neurons generally show little spike rate adaptation. We applied 5 mM Cs to some cells to determine the contribution of IH to spiking characteristics. Cs blocked the sag with negative current pulses when present, and could also lead to sometimes pronounced changes in spontaneous spike rate. Rebounds following hyperpolarization, when present, were not affected by 5 mM Cs. In another group of cells, we applied the specific SKCa blocker apamin. Such SKCa block could lead to a transition from a regular pacemaking to a irregular bursty firing pattern, and a distinct component of slow spike after-hyperpolarization was abolished. A larger sample of neurons recorded with specific blockers will be obtained to determine the relation of the heterogeneous properties of baseline spiking to determine the effects of specific channel block.

Authors:	*S. LI, D. JAEGER; Biol., Emory Univ., Atlanta, GA
The entopeduncular nucleus (EP) is the rodent equivalent of the internal globus pallidus (GPi) in primates. Despite its important role in basal ganglia output generation, the intrinsic properties of EP neurons are poorly understood. We obtained whole-cell recordings from EP neurons in brain slices from male Sprague-Dawley rats (15-19 d. age). Recordings typically exhibited regular tonic spiking even with the excitatory and inhibitory synaptic inputs blocked. However, several characteristics of spiking properties were heterogeneous between recorded neurons. These variable characteristics included spike width, shape of the fast and slow spike after-hyperpolarization, amount of sag with negative current pulses, and post-hyperpolarization rebounds. Spike rate transients in response to current level changes were generally small, indicating that EP neurons generally show little spike rate adaptation. We applied 5 mM Cs to some cells to determine the contribution of IH to spiking characteristics. Cs blocked the sag with negative current pulses when present, and could also lead to sometimes pronounced changes in spontaneous spike rate. Rebounds following hyperpolarization, when present, were not affected by 5 mM Cs. In another group of cells, we applied the specific SKCa blocker apamin. Such SKCa block could lead to a transition from a regular pacemaking to a irregular bursty firing pattern, and a distinct component of slow spike after-hyperpolarization was abolished. A larger sample of neurons recorded with specific blockers will be obtained to determine the relation of the heterogeneous properties of baseline spiking to determine the effects of specific channel block.

Support:	NINDS RO1 NS039852