One of the earliest discoveries to stem from the development of the Falck-Hillarp technique to visualize bioaminergic neurons159 was the existence of extensive plexuses of monoaminergic and serotonergic nerve fibres in the LHA.160161 Direct projections from the noradrenergic nexus in the locus coeruleus to the LHA have since been confirmed with several techniques; e.g.74,73 162 163. Recently, it was shown that both somatic and dendritic compartments of Hcrt cells are targeted by noradrenergic afferents (identified as immunoreactive for the biosynthetic enzyme, dopamine-P-hydroxylase).164 Both a and P-adrenoceptors are expressed in the LHA165 and noradrenalin hyperpolarizes both Hcrt and MCH cells via postsynaptic mechanisms.88 166 These data suggest that Hcrt neurons are regulated via inhibitory feedback from the locus coeruleus. Lastly, there is a parallel, but less extensive, input to the LHA from adrenergic neurons in the lower brain stem.167
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