Glutamate modulates the firing rate in oculomotor nucleus motoneurons as a function of the recruitment threshold current

Key points This study deals with the cellular mechanisms involved in firing rate modulation in vivo in the oculomotor system where there are requirements for high firing rates by motoneurons. The study demonstrates that glutamate effects depend on the recruitment threshold and, presumably, motoneuron size. Mid- and high-threshold motoneurons in response to glutamate decrease their voltage threshold and strengthened the tonic and phasic components of the firing rate. In a functional context, motoneurons could be recruited at lower recruitment threshold and could generate a strong muscle contraction under glutamate modulation to perform saccadic eye movements with different velocities and/or to maintain the eye in different eccentric positions in the orbit. Our results suggest that the recruitment and firing behaviour of ocular motoneurons can be modified in vivo by glutamatergic synaptic inputs. It provides a link between cellular function and behavioural motoneuron output. Abstract Studies in alert preparations have demonstrated that ocular motoneurons exhibit a phasictonic firing rate related to eye velocity and position, respectively. The slopes of these relationships are higher in motoneurons with higher recruitment threshold and have been proposed to depend upon synaptic input. To investigate this hypothesis, motoneurons of the rat oculomotor nucleus were recorded in a brain slice preparation in control conditions and during glutamate (5 mu m) application to the bath. Glutamate did not affect membrane potential or input resistance, but produced a decrease in rheobase and depolarization voltage as a function of the current needed for generating a maintained repetitive discharge (recruitment threshold current). In addition, glutamate compressed the range of recruitment threshold current (0.10.4 nA) as compared to the control (0.150.7 nA). Glutamate exposed motoneurons showed an increase in the tonic frequency gain and the peak frequency. Such increments depended on the recruitment threshold current and the last recruited motoneurons almost doubled the tonic frequency gain (35.2 vs. 57.9 spikes s-1 nA-1) and the peak frequency (52.4 vs. 102.6 spikes s-1). Finally, glutamate increased the spike frequency adaptation due to a significant increase in the phasic firing component as compared to the tonic one. In conclusion, glutamate modulates tonic and phasic discharge properties as a function of the recruitment threshold current and, presumably, motoneuron size. These findings contribute to understand the link between cellular functions and motoneuron discharge during oculomotor behaviour.