Contextual Modulation of Substantia Nigra Pars Reticulata Neurons

Abstract

Neurons in the substantia nigra pars reticulata (SNr) are known to encode saccadic eye movements within some, but not all, behavioral contexts. However, the precise contextual factors that effect the modulations of nigral activity are still uncertain. To further examine the effect of behavioral context on the SNr, we recorded the activity of 72 neurons while monkeys made saccades during a delayed saccade task and during periods of free viewing. We quantified and compared the movement fields of each neuron for saccades made under three different conditions: 1) spontaneous saccades, which shifted gaze during periods of free viewing when no stimuli were presented and no reinforcements were delivered; 2) fixational saccades, which brought gaze into alignment with a fixation target at the start of a delayed saccade trial, were necessary for trial completion, but were not directly followed by reinforcement; and 3) terminal saccades, which brought gaze into alignment with a visual target at the end of a delayed saccade trial and were directly followed by reinforcement. For three of the four SNr neuron classes, saccade-related modulations were only present before terminal saccades. For the fourth class, discrete pausers, saccade-related modulations were substantially larger for terminal saccades than for fixational saccades, and modulations were absent for spontaneous saccades. These results and other recent work on the basal ganglia suggest that some saccade-related signals in the SNr may be influenced by the reinforcement associated with a particular saccadic eye movement.

GABA, Not Glutamate, Controls the Activity of Substantia Nigra Reticulata Neurons in Awake, Unrestrained Rats

“…Under these "quiet, resting conditions," environmental factors as well as physiological cycles can affect SNr cells and, consequently, change their responsiveness to GLU. Several studies revealed that the activity of SNr neurons is modulated by environmental context (Handel and Glimcher, 2000;Gulley et al, 2002) and by continuous oscillations of the wake-sleep cycle naturally occurring during quiet, resting conditions during day- time recording (Miller et al, 1983). Because the activity of serotonin-containing neurons and serotonin release varies depending on the sleep-wakefulness cycle (for review, see Portas et al, 2000) and because this neurotransmitter has direct postsynaptic effects on SNr neurons via 5-HT 2c receptors and is involved in presynaptic regulation of GABA release via 5-HT 1b receptors (Rick et al, 1995;Stanford and Lacey, 1996), fluctuations in serotonin activity may be a factor determining the variability of basal activity and GLU responsiveness of SNr neurons.…”

Subcortical processes of motor response inhibition during a stop signal task

“…Notably, although the connections within the cortico-striato-thalamic "loops" are well defined chemically (excitatory or inhibitory), none of them have individually been assigned a specific role in motor response inhibition. For instance, despite the focus of earlier unit recording studies on saccade-related tonic-pause neurons, more recent work have documented saccade-related "burst" neuronal responses, in the pars reticulata of the substantia nigra, an output nucleus of the basal ganglia (Bayer et al, 2004;Handel and Glimcher, 1999;Handel and Glimcher, 2000;Hikosaka and Wurtz, 1983a;Hikosaka and Wurtz, 1983b;Sato and Hikosata, 2002).…”

Behavioral Electrophysiology of Psychostimulants

“…In rats working for sucrose reinforcement, for example, a dose of amphetamine that activated behavior without impairing performance had no effect on the pattern of the SNr response, but significantly altered the relative magnitude of the neuronal change in relation to specific components of the operant task (Gulley et al, 2002a). Context also appears critical for SNr involvement in normal, non-drug-related movement (Handel and Glimcher, 2000). Further research is needed to assess how striatal information is integrated into ongoing SNr activity.…”