Lateral hypothalamus: early developmental expression and response to hypocretin (orexin)

Hypocretin is a recently discovered peptide that is synthesized by neurons in the lateral hypothalamic area (LH) and is believed to play a role in sleep regulation, arousal, endocrine control, and food intake. These functions are critical for the development of independent survival. We investigated the developmental profile of the hypocretin system in rats. Northern blot analysis showed that the expression of hypocretin mRNA increased from postnatal day 1 to adulthood. Both of the identified hypocretin receptor mRNAs were strongly expressed very early in hypothalamic development, and expression subsequently decreased in the mature brain. Immunocytochemistry revealed hypocretin-2 peptide expression in the cell bodies of LH neurons and in axons in the brain and spinal cord as early as embryonic day 19. Whole-cell patch clamp recordings from postnatal P1-P14 LH slices demonstrated a robust increase in synaptic activity in all LH neurons tested (n = 20) with a 383% increase in the frequency of spontaneous activity upon hypocretin-2 (1.5 microM) application. A similar increase in activity was found with hypocretin-1 application to LH slices. Hypocretin-2 evoked a robust increase in synaptic activity even on the earliest day tested, the day of birth. Furthermore, voltage-clamp recordings and calcium digital imaging experiments using cultured LH cells revealed that both hypocretin-1 and -2 induced enhancement of neuronal activity occurred as early as synaptic activity was detected. Thus, as in the adult central nervous system, hypocretin exerts a profound excitatory influence on neuronal activity early in development, which might contribute to the development of arousal, sleep regulation, feeding, and endocrine control.