Histamine synthesis in rat hypothalamus is not acutely regulated via histidine decarboxylase

There is considerable support for histamine acting as a neurotransmitter in the central nervous system. Since the synthesis of many biogenic amine neurotransmitters is highly regulated on a short-term basis, we investigated whether a similar regulation exists for histamine synthesis. Previous studies have indicated that histidine transport into nerve terminals is not a regulatory step in histamine synthesis. In this study we examined histidine decarboxylase activity as a possible regulatory point. Using a variety of depolarization methods and hypothalamic and striatal tissue slices and/or synaptosomes, we found no alterations in histidine decarboxylase activity. Based on these and previous studies we conclude that histamine synthesis in the hypothalamus, and perhaps throughout the CNS, does not have mechanisms for rapid short-term regulation. These data support the idea that histamine acts as a neuromodulator and does not play a critical role in acute regulation of CNS function at least in some brain regions.