The histaminergic tuberomamillary nucleus (TMN) controls arousal and attention and the firing of TMN neurons is state-dependent, active during waking, silent during sleep. Thyrotropin-releasing hormone (TRH) promotes arousal and combats sleepiness associated with narcolepsy. Single-cell RT-PCR (scRT-PCR) demonstrated variable expression of the two known TRH receptors in the majority of TMN neurons. TRH increased the firing rate of most (ca 70%) TMN neurons. This excitation was abolished by the TRH receptor antagonist chlordiazepoxide (50mM). In the presence of tetrodotoxin TRH depolarized TMN neurons without obvious change of their input resistance. This effect reversed at the potential typical for nonselective cation channels. The potassium channel blockers barium and cesium did not influence the TRH-induced depolarization. TRH effects were antagonized by inhibitors of the Na⁺/Ca²⁺ exchanger, KB-R7943 and benzamil. The frequency of GABAergic spontaneous inhibitory postsynaptic currents was either increased (TTX-insensitive-) or decreased (TTX-sensitive- sIPSCs) by TRH, indicating a heterogeneous modulation of GABAergic inputs by TRH. Facilitation but not depression of sIPSC frequency by TRH was missing in the presence of the k-opioid receptor antagonist nor-binaltorphimine. Montirelin (TRH analogue, 1mg/kg ip) induced waking in wild type mice but not in histidine decarboxylase knockout mice lacking histamine. Inhibition of histamine synthesis by (S)-a-fluoromethylhistidine blocked the arousal effect of montirelin in WT mice.

We conclude that direct receptor-mediated excitation of rodent TMN neurons by TRH demands activation of nonselective cation channels as well as electrogenic Na⁺/Ca²⁺ exchange.