Saccadic premotor burst neurons and histochemical correlates of their firing patterns in rhesus monkey

Bursting behavior of brainstem premotor burst neurons (BNs) is essential for initiation of saccades and calibrating their metrics. Several ion channel families such as voltage-gated potassium (Kv) channels, low-voltage-activated calcium (Cav3) channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are major regulators of the bursting in neurons. Therefore, it was speculated that ion channels with rapid kinematics are essential for characteristic firing patterns of the BNs and rapid saccade velocities. However, the expression patterns of ion channels are yet to be confirmed. Confirmation would not only support the neuromimetic model predictions for saccade generation in brainstem, but also contemporary views that channelopathies can cause saccade disorders in humans. As proof of concept, we examined excitatory BNs in the rostral interstitial nucleus of medial longitudinal fasciculus (RIMLF, vertical saccades) and inhibitory BNs in nucleus paragigantocellularis dorsalis (PGD, horizontal saccades) histochemically in macaque monkeys. We found strong expression of Kv channels, which enable rapid-firing, as well as HCN1&2 and Cav3.2&3.3, which enable post-inhibitory rebound bursting, in both BN populations. Moreover, PGD was found to host multiple neuron groups in terms of calretinin immunoreactivity. Our results provide histochemical evidence that supports models proposing post-inhibitory rebound facilitates bursting in BNs. Furthermore, our findings support the notion that deductions can be made about electrophysiological firing properties by histochemical examination of functional groups within the brainstem saccadic circuitry. This development is an important building block supporting the concept of channelopathies in saccadic disorders. Future histological studies in humans will confirm this approach for saccadic disorders.