The discharge of subthalamic neurons is modulated by inhibiting the nitric oxide synthase in the rat

The effects induced on the discharge of subthalamic spontaneously active neurons by inhibiting the enzyme nitric oxide synthase was studied in two groups of urethane-anesthetized rats. In the first group of animals (n = 10), the activity of subthalamic single units was recorded before and after the systemic administration of 7-nitro-indazole (7-NI, 50 mg/kg i.p.), a selective inhibitor of neuronal nitric oxide synthase. In the second group of rats (n = 15), Nω-nitro-l-arginine methyl ester (L-NAME), another inhibitor of nitric oxide synthase, was iontophoretically administered while performing single unit extracellular recordings. The activity of most tested spontaneously discharging neurons (8/10) was influenced by 7-NI administration, which always caused a statistically significant decrease in the firing rate of the responsive cells. In contrast, the iontophoretic administration of L-NAME, although influencing many cells (24/32), did not have univocal effects: in fact, 18 cells were inhibited while 6 neurons were excited in a statistically significant manner. We hypothesize that nitric oxide neurotransmission could exert a tonic modulatory influence upon spontaneously discharging subthalamic neurons, with a prevalent excitatory effect.