Figure 4.

Schematic representation of the brain histaminergic projections involved in cognition. Histamine‐producing cell bodies are restricted to the TMN and histaminergic fibres project to various brain regions including the prefrontal cortex, nucleus basalis magnocellularis (NBM), the striatum (Str), the amygdala (Amy), the hippocampus (Hip), the cerebellum (Cer) and spinal cord (SC). Histamine effects in the brain are mediated by four receptors (H₁ to H₄ R) with different distribution across the brain. In recognition memory, the role of specific receptor activation in discrete brain regions was just marginally studied as most studies were performed using systemically injected compounds (see text). Only two studies analysed the effects of local drug infusions: H₁ and H₂ antagonists, as well as an H₃ agonist directly infused into the hippocampal CA1 region blocked long‐term consolidation of object recognition memory (da Silveira et al., 2013). An H₂ antagonist delivered into hippocampal CA1 or basolateral amygdala impaired social recognition memory (Garrido Zinn et al., 2016).