Role of glycinergic inhibition in shaping activity of saccadic burst neurons.
ABSTRACT The immediate premotor signals for saccades are created at the level of medium-lead burst neurons (MLBNs). During fixations, MLBNs receive tonic inhibition from omnipause neurons (OPNs), which use glycine as a neurotransmitter. To elucidate the role of this inhibition, we studied discharge patterns of horizontal MLBNs following iontophoretic application of strychnine, a glycine-receptor antagonist, in alert cats. Three-barrel micropipettes were used for extracellular recording and iontophoresis. After application of strychnine, MLBNs exhibited spontaneous discharge and visual responses during intersaccadic intervals. Spikes were evoked by single-pulse stimulation of the contralateral superior colliculus (SC). These results show that MLBNs receive substantial excitatory input during intersaccadic intervals and that inhibitory action of OPNs is indeed necessary to prevent MLBNs from firing. Strychnine also affected saccade-related activity of MLBNs. The burst of activity, as in normal conditions, declined rapidly before the end of saccades but was followed by low rate spike activity, which continued beyond the end of saccades. This suggests that in normal conditions, the termination of saccades is determined by resumed inhibitory action of OPNs and not by termination of excitatory input to MLBNs. In addition, the firing rate and the number of spikes during saccades increased after strychnine application, suggesting that MLBNs receive glycinergic inhibition of non-OPN origin as well. We conclude that glycinergic inhibition plays essential roles in the maintenance of stable fixation, the termination of saccades, and the regulation of saccade size and velocity.
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ABSTRACT: Despite a wealth of in vitro and modelling studies it remains unclear how neuronal populations in the cerebellum interact in vivo. We address the issue of how the cerebellar input layer processes sensory information, with particular focus on the granule cells (input relays) and their counterpart inhibitory interneurones, Golgi cells. Based on the textbook view, granule cells excite Golgi cells via glutamate forming a negative feedback loop. However, Golgi cells express inhibitory mGluR2 receptors suggesting an inhibitory role for glutamate. We set out to test this glutamatergic paradox in Golgi cells. Here we show that granule cells and Golgi cells interact through extra-synaptic signalling mechanisms during sensory information processing, as well as synaptic mechanisms. We demonstrate that such interactions depend on granule cell-derived glutamate acting via inhibitory mGluR2 receptors leading causally to the suppression of Golgi cell activity for several hundreds of milliseconds. We further show that granule cell-derived inhibition of Golgi cell activity is regulated by GABA-dependent extra-synaptic Golgi cell inhibition of granule cells, identifying a regulatory loop in which glutamate and GABA may be critical regulators of Golgi cell–granule cell functional activity. Thus, granule cells may promote their own prolonged activity via paradoxical feed-forward inhibition of Golgi cells, thereby enabling information processing over long timescales.The Journal of Physiology 06/2011; 589(Pt 15):3837-54. · 4.38 Impact Factor
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ABSTRACT: The frontal eye field (FEF) has a strong influence on saccadic eye movements with the head restrained. With the head unrestrained, eye saccades combine with head movements to produce large gaze shifts, and microstimulation of the FEF evokes both eye and head movements. To test whether the dorsomedial FEF provides commands for the entire gaze shift or its separate eye and head components, we recorded extracellular single-unit activity in monkeys trained to make large head-unrestrained gaze shifts. We recorded 80 units active during gaze shifts, and closely examined 26 of these that discharged a burst of action potentials that preceded horizontal gaze movements. These units were movement or visuomovement related and most exhibited open movement fields with respect to amplitude. To reveal the relations of burst parameters to gaze, eye, and/or head movement metrics, we used behavioral dissociations of gaze, eye, and head movements and linear regression analyses. The burst number of spikes (NOS) was strongly correlated with movement amplitude and burst temporal parameters were strongly correlated with movement temporal metrics for eight gaze-related burst neurons and five saccade-related burst neurons. For the remaining 13 neurons, the NOS was strongly correlated with the head movement amplitude, but burst temporal parameters were most strongly correlated with eye movement temporal metrics (head-eye-related burst neurons, HEBNs). These results suggest that FEF units do not encode a command for the unified gaze shift only; instead, different units may carry signals related to the overall gaze shift or its eye and/or head components. Moreover, the HEBNs exhibit bursts whose magnitude and timing may encode a head displacement signal and a signal that influences the timing of the eye saccade, thereby serving as a mechanism for coordinating the eye and head movements of a gaze shift.Neuroscience 08/2012; 225:213-36. · 3.12 Impact Factor