The termination of histamine transmission in the brain: Membrane localization and isoform studies of histamine N-methyltransferase

The neurotransmitter histamine is inactivated in the CNS by histamine N-methyltransferase (HNMT). However, HNMT has been localized to the cytosol of neurons and an active uptake system has not been shown to exist for histamine. Hence it is unclear how histamine gets from the synapse to the cytosol to reach HNMT. The mechanism of histamine inactivation in the CNS has thus been the focus of the present thesis.; In vivo studies focused on the inactivation of histamine in rodent brain and confirmed that the prototypical H₃ antagonist, thioperamide, was able to increase the synthesis rate of the histamine metabolite tele-methylhistamine (tMH). In addition, GT-2016, a compound previously shown to increase histamine release but not histamine turnover, was shown to significantly antagonize thioperamide-induced increases in the levels of tMH. Therefore, it was hypothesized that GT-2016 was acting by some unknown mechanism (i.e. histamine transport or a novel HNMT isoform) to inhibit the metabolism of histamine in the CNS.; In vitro studies revealed that HNMT activity of synaptosomal membranes was found to be equal to the amount of HNMT activity attributed to intact synaptosomes (14.3 ± 2.2 and 18.2 ± 4.3 pmol/min-tube, respectively). In addition, this HNMT activity could not be liberated by 1 M NaCl, but was liberated by 0.1% Triton X-100 detergent. Therefore, it was concluded that an integrally membrane-bound form of HNMT (MB-HNMT) exists and this enzyme might be responsible for the neuronal inactivation of histamine transmission.; The HNMT gene was then investigated to determine if an isoform could be found to account for MB-HNMT. While these molecular studies did not ultimately lead to the discovery of the molecular identity of MB-HNMT, a novel mRNA species with a new exon (named 2b) was found and named HNMT-Short (HNMT-S). HNMT-S mRNA was detected in human brain by RT-PCR and in human placenta by a Northern blot. Upon expression in COS-7 cells, HNMT-S was found to be devoid of histamine-methylating activity.; Further studies are needed to understand the biological roles for MB-HMT and HNMT-S, both of which were discovered in the present studies.