Basal Ganglia Output Neurons Research Articles

Striatal dopamine-glutamate interactions reflected in substantia nigra reticulata firing.

To gain insight into the role of striatal dopamine in basal ganglia functioning, dopaminergic drugs alone and in combination with the glutamate receptor agonist kainic acid were infused in the lateral striatum via a microdialysis probe, while single-unit recordings of substantia nigra reticulata neurons were made in chloral hydrate-anaesthetized rats. Striatal infusion of dopaminergic drugs did not significantly affect the firing rate of substantia nigra reticulata neurons, which was related to the low activity of striatal cells under basal conditions, illustrated by the lack of effect of striatal infusion of TTX on substantia nigra reticulata activity. Under glutamate-stimulated conditions, striatal infusion of d-amphetamine potentiated the inhibition of substantia nigra reticulata neurons induced by striatal kainic acid. Thus, under stimulated but not basal conditions, the modulatory role of dopamine in the striatum could be demonstrated. Dopamine potentiated the inhibitory effect of striatal kainic acid on the firing rate of the basal ganglia output neurons.

The substantia nigra as a site of synaptic integration of functionally diverse information arising from the ventral pallidum and the globus pallidus in the rat

Voluntary behaviour in mammals requires the integration of information from different parts of the cerebral cortex, notably the limbic, associative and sensorimotor areas, in a neural network that eventually controls the muscles. One region of the brain that has been proposed to subserve such a function are the basal ganglia which receive inputs from all cortical areas. Although information from different cortical areas passes through the basal ganglia as a series of separate parallel pathways there are several sites where integration of the diverse information could occur. In this study we the identify a neural network at the synaptic level that may underlie a powerful mechanism for the integration, within the basal ganglia, of the diverse types of information arising from the cortex. By double anterograde tracing and immunocytochemistry at both the light and electron microscopic levels, we show that individual neurons in the substantia nigra pars reticulata and dopaminergic neurons in the pars compacta each receive multiple GABAergic synaptic inputs both from neurons in the ventral pallidum (which receive input from limbic areas via the nucleus accumbens) and from neurons in the globus pallidus (which receive input from associative and sensorimotor cortices via the neostriatum). Thus, information subserving functions such as emotion, motivation, cognition and movement converges onto basal ganglia output neurons, leading eventually to the muscles, and also on to the dopaminergic neurons which themselves subserve an integrative role by modulating the flow of information from the cortex through the basal ganglia at the level of the neostriatum and nucleus accumbens.

Activity of neurons of the subthalamic nucleus in relation to motor performance in the cat.

The activity of subthalamic nucleus neurons related to motor performance was studied in three unrestrained cats operantly conditioned to perform a lever-release movement. The movement was initiated either rapidly after the trigger stimulus (a brief sound) in a simple reaction-time paradigm or after a delay in trials identified by a tone cue. These paradigms were randomly presented. The activity of 171 neurons was recorded in the contralateral and in the ipsilateral subthalamic nucleus, with respect to the performing limb. The mean spontaneous activity of cells in the ipsilateral side (18.5 +/- 13.8 imp/s, mean +/- SD) was higher than that in the contralateral side (8.5 +/- 8.1 imp/s). A total of 145 cells (85%) presented significant changes in activity in relation to the lever-release movement (task-related cells). The remaining 26 cells were either related to other events of the task (n = 15; lever-press or reinforcement occurrence) or not related at all to the task performance (n = 11). The majority of changes of activity of task-related cells were initial increases in discharge, which started on average, 127 ms before movement onset and lasted several hundreds of milliseconds. These increases in discharge were more frequent in the contralateral side (75 of 80 task-related cells, 94%) than in the ipsilateral side (43 of 65 task-related cells, 66%). The changes in activity, either increases or decreases, occurred early after the trigger stimulus, since 62% of them had a latency of less than 100 ms. Although the mean latency of initial increases was rather similar in both sides (97 ms contralateral versus 104 ms ipsilateral), the contralateral side was characterized by a high proportion of very early responses (less than 20 ms). For most neurons, the early changes in activity described above were absent after the trigger stimulus in the delayed condition. For certain neurons, the changes in activity prior to movement were different in reaction-time condition and in delayed condition, showing that the pattern of activity preceding movement might depend on the temporal requirements for motor initiation. The results suggest that a significant proportion of subthalamic cells are involved in the preparation and the initiation phases of the lever-release movement studied, although other hypotheses (e.g. stimulus-related responses) cannot be definitely ruled out. The timings and patterns of the changes in activity observed in the subthalamic nucleus in the present study, and in the pallidal complex previously, cannot be explained easily by the classical scheme where the external pallidum inhibits the subthalamic nucleus. The results suggest rather that the subthalamic nucleus, driven by a yet-to-be-determined excitatory input, exerts an excitatory influence on the pallidum and plays a crucial role in the control of the basal ganglia output neurons.

Dopamine modulation of the effects of gamma-aminobutyric acid on substantia nigra pars reticulata neurons.

Studies were conducted to assess whether basal ganglia output neurons originating in the substantia nigra pars reticulata might be affected by dopamine released from dendrites of neighboring substantia nigra pars compacta neurons. Dopamine applied by iontophoresis increased the baseline firing rates of approximately half of the substantia nigra pars reticulata cells tested. The more significant finding, unrelated to the increase in firing, was the ability of dopamine to attenuate the inhibitory responses of these cells to iontophoretically applied gamma-aminobutyric acid. These findings suggest a role for dopamine as a neuromodulator and further suggest that it can act at sites beyond the striatum to modify transmission from the basal ganglia to motor nuclei.