Role of glutamate in coupling between bilaterally paired circadian clocks in Bulla gouldiana.

A group of electrically coupled basal retinal neurons (BRN) in the eye of the marine snail Bulla gouldiana generate a circadian rhythm in the frequency of compound action potentials (CAPs). CAPs are conducted to the contralateral retina via the optic nerves and the cerebral commissures to synchronize the rhythms of both eyes. CAPs can induce an excitatory postsynaptic potential (EPSP) in the contralateral BRNs that can lead to action potential generation. The pathway and mechanism of this bilateral coupling signal have not been elucidated, but the evidence suggests monosynaptic connections between the populations of pacemaker cells in both retinae. The study was designed to further characterize the coupling signal and investigate the role of glutamate as a neurotransmitter in this pathway. We found evidence supporting our hypothesis that glutamate, previously identified in BRNs by an immunocytological study, is involved in bilateral coupling. First, a combination of extracellular and intracellular electrophysiological recordings revealed that both electrically and optically evoked CAPs generate excitatory synaptic potentials and action potentials in contralateral BRNs. Application of glutamate also led to increased neuronal activity of individual BRNs both in the intact retina as well when isolated in cell culture. Lastly, glutamate-induced inward currents were characterized in cultured BRNs using perforated-patch recordings. The reversal potential was close to 0 mV, and the currents were sensitive to N-methyl-d-aspartic acid (NMDA) and non-NMDA antagonists. NMDA and AMPA, as well as aspartate, also induced distinct inward currents in BRNs. We conclude that glutamate can be used by BRNs as a transmitter to influence electrical activity in the contralateral pacemaker population. We propose that glutamate is required for synchronizing of the bilaterally paired retinal clocks producing a unified circadian timing signal.