Multimodal convergence in the pedunculopontine tegmental nucleus: motor, sensory and theta-frequency inputs influence activity of single neurons

The pedunculopontine tegmental nucleus of the brainstem has wide ranging interconnections and neuronal-behavioural correlates. It is implicated in the regulation of movement, in sensorimotor integration, and may be disordered as part of the neuropathology of the movement disorder Parkinson’s disease. We used single neuron recordings in free moving rats to determine if neuronal activity showed temporal correlation with components of skilled forelimb movement. We also tested with tone stimuli to find if individual neurons respond to both motor and sensory events. We found that PPTg neurons showed activity modulations time locked to specific task phases, similar to activity seen elsewhere in voluntary movement control circuits, such as the motor cortex. Many also responded to auditory inputs, allowing for sensorimotor integration at the cellular level. We further tested for effects of acute drug-induced parkinsonism on the firing rate of the neurons. Acute parkinsonian akinesia was associated with reduction in resting firing rate and increased firing regularity, consistent with reduced excitability in this state. In addition, under normal conditions we detected oscillatory activity in the theta range in some neurons, particularly during performance of the reach-to-grasp movement, which often showed temporal specificity. Together, these data firstly extend the concept of the PPTg as an integrative structure in generation of complex movements, by showing that sensory and motor-related information converge on single neurons; and, secondly for the first time demonstrates that this function extends to the highly coordinated control of the forelimb during skilled reach to grasp movement.