Possible inhibitory role of histamine H2 receptors in the control of basal TSH secretion in male rats: studies with dimaprit, a selective H2 receptor agonist

Dimaprit--induced neurotoxicity

The neurotoxicity of the histamine H2 agonist dimaprit was characterized. Dimaprit (100 micrograms administered into the lateral cerebral ventricle) induced a large area of brain necrosis 1-3 days later which was uniformly lethal. Lower doses caused dose - related effects on survival, gross brain pathology and body weight. Experiments with other H2 agonists and H2 antagonists, together with studies by others demonstrating a similar toxicity of the congener homodimaprit suggest that the neurotoxicity of dimaprit is independent of brain H2 receptors. Although dimaprit is a useful tool for the characterization of H2 receptor responses, the present results show that this agent must be used with caution, if at all, in classifying brain H2-receptor mediated events.

Histamine excites arcuate neurons in vitro through H1 receptors

The electrophysiological effects of histamine on neurons of the hypothalamic arcuate nucleus of the female rat were tested with extracellular single unit recordings in an in vitro slice preparation. Histamine increased the spontaneous neuronal firing rate in 63% of the arcuate cells tested. An inhibitory response to histamine was seen in only one of the 117 neurons tested. The excitatory response to histamine showed dose dependency and was stable during synaptic blockade (by high magnesium and low calcium concentrations) and across a temperature range of 29-37 degrees C. Administration of histaminergic type 1 (pyrilamine and chlorpheniramine) and type 2 (cimetidine) receptor blockers revealed that the excitatory responses to histamine were mediated by type 1 receptors. The same neurons were also tested for responses to norepinephrine, serotonin, acetylcholine and substance P. A significant correlation was found between responses to histamine and substance P: all units excited by substance P were also excited by histamine. This subclass of histamine-responsive arcuate neurons may play a role in the regulation of the anterior pituitary, since histamine and substance P have similar effects on LH and prolactin secretion.