Heterogeneity of Histaminergic Neurons

The central histaminergic system has a complex neuroanatomical and functional organisation. It originates in a small area of the posterior hypothalamus, the tuberomammillary nucleus (TMN). Despite the restricted location of cell bodies, anatomical studies showed that histamine neurons project to almost the entire brain. Indeed, neuronal histamine (HA) has been proven to modulate a plethora of body functions. The TMN was initially considered a single functional entity with neurons working in a coordinated and synchronous way. Recently though, several works are indicating that histaminergic neurons are organised in heterogeneous populations with distinct roles. Accumulating evidence based on multiple techniques suggests different properties among histamine TMN neurons. Although further studies are needed to fully characterise the organisation of the central histaminergic system and its activation following specific stimuli, interesting observations are emerging on the selective activity of clusters of histaminergic neurons according to the homeostatic or behavioural status. The heterogeneity of histamine neurons might represent the key for a fine-tuned modulation of specific functions regulated by neuronal HA. With the present chapter, we analyse the main findings and discuss future directions.