Modulatory effects of the GABAergic basal ganglia neurons on the PPN and the muscle tone inhibitory system in cats

Pedunculopontine tegmental nucleus (PPN) contributes to the control of muscle tone by modulating the activities of pontomedullary reticulospinal systems during wakefulness and rapid eye movement (REM) sleep. The PPN receives GABAergic projection from the substantia nigra pars reticulata (SNr), an output nucleus of the basal ganglia. Here we examined how GABAergic SNr-PPN projection controls the activity of the pontomedullary reticulospinal tract that constitutes muscle tone inhibitory system. Intracellular recording was made from 121 motoneurons in the lumbosacral segments in decerebrate cats (n = 14). Short train pulses of stimuli (3 pulses with 5 ms intervals, 10-40 mu A) applied to the PPN, where cholinergic neurons were densely distributed, evoked eye movements toward the opposite side and bilaterally suppressed extensor muscle activity. The identical PPN stimulation induced IPSPs, which had a peak latency of 40-50 ms with a duration of 40-50 ms, in extensor and flexor motoneurons. The late-latency IPSPs were mediated by chloride ions. Microinjection of atropine sulfate (20 mM, 0.25 mu l) into the pontine reticular formation (PRF) reduced the amplitude of the IPSPs. Although conditioning stimuli applied to the SNr (40-60 mu A and 100 Hz) alone did not induce any postsynaptic effects in motoneurons, they reduced the amplitude of the PPN-induced IPSPs. Subsequent injection of bicuculline (5 mM, 0.25 mu l) into the PPN blocked the SNr effects. Microinjections of NMDA (5 mM, 0.25 mu l) and muscimol (5 mM, 0.25 mu l) into the SNr reduced and increased the amplitude of the PPN-induced IPSPs, respectively. These results suggest that GABAergic basal ganglia output controls postural muscle tone by modulating the activity of cholinergic PPN neurons which activate the muscle tone inhibitory system. The SNr-PPN projection may contribute to not only control of muscle tone during movements in wakefulness but also modulation of muscular atonia of REM sleep. Dysfunction of the SNr-PPN projection may therefore be involved in sleep disturbances in basal ganglia disorders.