Synaptic Inhibition of the External Globus Pallidus

The external globus pallidus (GPe) is central to the basal ganglia, a large group of highly interconnected subcortical nuclei. Most synapses formed on and by GPe neurons are inhibitory. Most inhibitory synapses in the GPe arise from spiny projection neurons (SPNs) of the striatum, the input nucleus of the basal ganglia. The remaining inhibitory synapses arise locally from GPe neurons. To determine how GPe neurons integrate local and striatal synaptic inhibition, we isolated pallido-pallidal and striato-pallidal connections in slice preparations of the mouse GPe, performed whole cell voltage clamp recordings of the synaptic currents produced by the firing of SPNs and other GPe neurons, and performed perforated patch current clamp recordings of their effects on firing activity. The firing of GPe neurons produces synaptic inhibition of neurons in the substantia nigra pars reticulata (SNr), an output nucleus of the basal ganglia. To determine the fidelity of communication by the GPe, we performed perforated patch clamp recordings of GPe neurons in slice preparations, measured their entrainment by a wide range of input current frequencies, estimated the synaptic current this entrained firing would impose on SNr neurons, applied this current to SNr neurons, and measured the entrainment of SNr neurons by the original frequencies. Our findings indicate that the external globus pallidus is a network of inhibitory neurons that faithfully communicates a wide range of frequencies from the striatum to the substantia nigra.