Autoradiographic localization of H1-histamine receptors in brain using 3H-mepyramine: preliminary studies

3H-Mepyramine (3H-MEP) binds to slide-mounted tissue sections with characteristics indicating the labelling of a H1-histamine receptor. It is saturable (Bmax 12 pg/g tissue), of a high affinity (KD 0.5 mM) and possesses a drug specificity similar to that observed for H1 receptors in a variety of systems. In agreement with other biochemical studies, the association constant and the number of binding sites appear lower in the rat than in the guinea pig. Once the optimal conditions for labelling the receptors were found in the biochemical studies, tissue sections were prepared for autoradiographic studies. The receptor density varied markedly in the different brain regions. For example, there was a high density of H1 receptors in the molecular layer of the guinea pig cerebellum. In the guinea pig hippocampal formation, the highest densities were found in the molecular layer of the dentate gyrus while lower levels were found in the dendritic fields of the pyramidal cells. Significant densities of receptors were found in other areas of the rat and guinea pig brain as well.

The possible role of brain histamine and H1 and H2 receptors in the development of morphine tolerance and physical dependence in mice

The possible role of brain histamine in the mechanisms of morphine tolerance and physical dependence is under investigation in mice. L-histidine and histamine, given during the 'withdrawal' phase, significantly increase tolerance to the analgesic effects of morphine but reduce the degree of physical dependence. Metiamide significantly inhibits tolerance but has no consistent effect on physical dependence. These results suggest that H2 receptors may be involved in the development of morphine tolerance. Mepyramine does not significantly affect tolerance, and with regard to dependence there is an effect only on body weight loss, which is increased. However, combined treatment with metiamide and mepyramine inhibits tolerance significantly more than metiamide alone; and withdrawal jumping is also reduced more significantly by combined treatment than by the separate administration of these drugs. It is suggested that brain histamine is definitely implicated in the mechanisms of the 'withdrawal' phase of morphine tolerance and physical dependence in mice, with H2 receptors probably playing the more important part.

5-Hydroxytryptamine and histamine as mediators of the vascular injury produced by agents which damage mast cells in rats

Each of the four agents, ovomucoid, dextran, 48/80, and testis extract, when injected beneath the skin of the dorsa of the paws of rats produces a local vascular injury characterized by a protein-rich edema. Each agent also produces damage to mast cells. Either 5-hydroxytryptamine or histamine produces a response similar in the gross to that elicited by the agents which damage mast cells; however, neither of these two agents produces mast cell damage. On a weight basis 5-hydroxytryptamine is a much more potent edema-producing agent than histamine. The edema-producing action of 5-hydroxytryptamine can be differentiated from the similar action of histamine by the use of specific antagonists; dibenamine is a 5-hydroxytryptamine antagonist and pyrilamine a histamine antagonist. The edema produced by the mast cell-damaging agents is partially inhibited by dibenamine but is not diminished by pyrilamine. It is completely inhibited by treatment of rats with both drugs. The drugs which inhibit edema do not prevent mast cell damage by ovomucoid, dextran, 48/80, or testis extract. The observations are consistent with the hypothesis that agents which damage mast cells, "release" both 5-hydroxytryptamine and histamine and that in the rat the edema associated with mast cell damage is mediated largely by 5-hydroxytryptamine.