Infraslow histaminergic dynamics govern priming states to gate moment-to-moment memory accessibility

Memory expression fluctuates even in response to identical cues, which suggests that ongoing brain states bias memory accessibility. However, the cellular and circuit principles governing these state-dependent fluctuations remain unclear. Here, we show that spontaneous pre-cue activity of histaminergic neurons in the hypothalamic tuberomammillary nucleus (TMN) modulates the expression of reward-associative memory in mice. TMN histaminergic activity exhibited infraslow dynamics (0.05-0.1 Hz) that closely tracked an integrated brain-body state. Closed-loop cue delivery during high histaminergic states enhanced memory expression. Brief optogenetic activation or inhibition of these neurons before the cue bidirectionally modulated memory expression, and direct activation of histaminergic terminals in the basolateral amygdala (BLA) was sufficient to enhance memory expression. Furthermore, histaminergic inhibition before the cue impaired the cue-evoked BLA population response. Thus, ongoing histaminergic activity exerts an infraslow, state-setting influence that primes BLA circuits for robust cue responses and, in turn, modulates moment-to-moment memory accessibility.