Orexin modulates lateral habenula neuronal activity

The LHb is involved in the regulation of ascending monoamine and acetylcholine neurotransmission linking limbic forebrain and midbrain structures. LHb has been implicated in sleep-wake cycles, receiving inputs from regions involved in circadian functions and from the retina. Furthermore, in vitro circadian rhythms in both firing rate and c-fos expression have been shown. Orexin is expressed by neurons of the LHA and the presence of its receptors has been shown in the LHb. Because we know extremely little regarding the physiology of this input we recorded the acitivity of LHb cells both in vitro and in vivo following both orexin challenge and electrical stimulation.LHb neurons firing characteristics could be classified from extracellular recordings in vitro as either: (1) high-frequency tonic (mean rate =??); (2) low-frequency tonic (single spikes, sometimes preceded by one or more spike bursts); and (3) bursting. Orexin A (50, 100, 300 nM; n = 67) elicited either a significant inhibition of the firing activity (49% of cells;) or had no effect. No significant effects were observed with regards to the bursting cells. This result is supported by whole-cell recordings where bath application of orexin A (50, 100 or 300 nM) elicited either hyperpolarization (n=??) or had no effect on the membrane potential of recorded cells. Consistent with in vitro findings, single rat LHb neurons recorded extracellularly in vivo showed in some cases a decrease in the mean firing rate following i.c.v. administration of orexin A (30 ???M or 100 ???M). Finally, electrical stimulation of the LHA in vivo using single stimuli (0.3 Hz), paired pulses (50 msec, 0.3 Hz) or six consecutive trains of pulses (10, 20, 30 Hz; 10 stimuli per train) elicited an inhibition of spontaneously active neurons recorded extracellularly. These data support an orexin-dependent inhibitory modulation of LHb neurons. Intracellular in vitro data support this conclusion by showing membrane potential oscillations following orexin perfusion; moreover, neurons with more depolarized cell membrane seemed to respond less to orexin administration, possibly accounting for the potentials different responses observed in the extracellular preparations both in vitro and in vivo.