The neuropeptide thyrotropin-releasing hormone (TRH) was originally described as the hormone that regulates the hypothalamus-pituitary-thyroid axis, but its additional role as a neurotransmitter is now widely recognised. Central effects of TRH include the regulation of energy balance and cognitive arousal (1), though the mechanisms involved are not yet clear. A few areas in the brain that express TRH send their projections to the lateral hypothalamus, e.g. the dorsomedial hypothalamus (2). The latter is critical for the temporal organisation of food entrainment of circadian rhythms. On the other hand, the lateral hypothalamus is the only brain region to contain hypocretin (orexin) cells. These cells are essential for cognitive arousal and feeding behaviour; their loss leads to obesity and narcolepsy, whereas their activation promotes wakefulness (3). Considering that both TRH and hypocretin have a role in the regulation of sleep and feeding, and that some of the afferents to the lateral hypothalamus (where hypocretin cells are located) arise from TRH-expressing areas, we tested the effects of TRH on the activity of hypocretin cells. We performed whole-cell recordings from hypocretin cells using brain slices taken from 13-22 day-old mice. These mice expressed enhanced green fluorescent protein under the hypocretin promoter, thus allowing us to unambiguously identify hypocretin cells as described previously (4). TRH was applied extracellularly at concentrations of 100, 250 or 500 nM while cells were recorded in voltage- or current-clamp mode. TRH depolarized and increased the firing rate in all hypocretin cells tested (n=22) in a dose-dependent manner. Membrane depolarization persisted in the presence of tetrodotoxin (n=6 cells) or in a low Ca²⁺/high Mg²⁺ extracellular solution (n=4 cells), suggesting a post-synaptic mechanism. When we analysed the effects of TRH on action potentials, we found that their width increased (control 1.15±0.05 ms, TRH 1.40±0.10 ms, p=0.006, n=11), whereas their firing threshold remained unchanged (control -36.1±0.9 mV, TRH -35.4±1.2 mV, p>0.1, n=11). We also measured the effects on cell excitability by quantifying input-output gain using frequency-current plots, and found that TRH significantly increased gain in 9 of 10 hypocretin cells. Finally, our voltage-clamp recordings revealed that the TRH-induced depolarization involved the activation of an inward current that depended on extracellular Na⁺. In addition, we found that the peptide also increased post-synaptic voltage-gated K⁺ and Ca²⁺ currents. These results show that TRH directly and potently stimulates hypocretin neurones. The actions of TRH on hypocretin cells may contribute to the physiological regulation of rhythms in feeding behaviour and cognitive arousal.