Pallidal inputs to subthalamus: Intracellular analysis

Neuronal responses of the subthalamic nucleus (STH) to stimulation of the globus pallidus (GP) and the substantia nigra (SN) were studied by intracellular recording in the decorticated rat. (1) GP and SN stimulation evoked antidromic spikes in STH neurons with a mean latency of 1.2 ms and 1.1 ms, respectively. Based on the above latencies, the mean conduction velocity of the STH neurons projecting toward GP was estimated to be 2.5 m/s, and that toward SN was 1.4 m/s. Many STH neurons could be activated following stimulation of both GP and SN, indicating that single STH neurons project to two diversely distant areas. In spite of differences in conduction distance of GP and SN from STH, differences in the conduction velocities of bifurcating axons make it possible for a simultaneous arrival of impulses in the target areas to which these STH neurons project. (2) GP stimulation evoked short duration (5-24 ms) hyperpolarizing potentials which were usually followed by depolarizing potentials with durations of 10-20 ms. These potentials were tested by intracellular current applications and intracellular injections of chloride ions. The results indicated that the hyper- and depolarizing potentials were IPSPs and EPSPs respectively. These IPSPs were considered to be monosynaptic in nature since changes in the stimulus intensities of GP did not alter the latency of IPSPs. The mean latency of the IPSPs was 1.3 ms. Based on the above mean latency the mean conduction velocity of GP axons projecting to STH was estimated to be 3.8 m/s. (3) Analysis of electrical properties of STH neurons indicated that: (i) input resistance estimated by a current-voltage relationship ranged from 9 to 28 M omega; (ii) the membrane showed rectification in the hyperpolarizing direction; (iii) direct stimulation of neurons by depolarizing current pulses produced repetitive firings with frequencies up to 500 Hz. (4) Morphology of the recorded STH neurons was identified by intracellular labeling of neurons with horseradish peroxidase. Light microscopic analysis indicated that the recorded neurons were Golgi type I neurons with bifurcating axons projecting toward GP and SN.