Chapter 6 L - Histidine Decarboxylase A Marker of Histaminergic Neurons

The role of histamine (HA) as a neurotransmitter in the CNS, although proposed a decade ago (Schwartz, 1975), has been only progressively recognized (reviewed by Schwartz et al., 1980, 1982, 1984; Hough and Green, 1984). One of the key pieces of expenmental evidence has been the demonstration that synthesis of the amine in CNS depends on a specific enzyme, L-histidine decarboxylase (HDC) (Schwartz et al., 1970), localized m a specific set of neurons, therefore constituting a reliable marker. Because HA levels in brain are relatively low (about one tenth those of catecholammes, for instance), presumably reflecting a low number of histammergic neurons, it has long been difficult to measure and localize HDC activity in the CNS. However, several methods are now available for reaching these aims. This chapter will discuss their respective advantages and describe in detail some other selected methods developed and currently utilized in our laboratory.