Inhibition of histamine synthesis influences the development of hypertension in spontaneously hypertensive rats

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Histamine-mediated neuronal death in a rat model of Wernicke's encephalopathy

Three experiments were conducted to examine the role of histamine in neuronal degeneration in a rat model of Wernicke's encephalopathy induced by an acute bout of pyrithiamine-induced thiamine deficiency (PTD). In the first experiment, histamine levels in medial thalamus of freely moving PTD rats measured by microdialysis were increased (180% of controls) at a prelesion stage of thiamine deficiency (treatment day 12) and further elevated 48 hr later (380%) in the same animals when necrosis was evident. Histamine levels in dialysates of the hippocampus collected simultaneously from the same animals were unchanged at either stage of thiamine deficiency. Glutamate levels in microdialysates from the same animals were unchanged at the prelesion stage but were significantly elevated on the second collection day. In a second experiment, separate groups of PTD and pairfed control (CT) rats were infused continuously with either alpha-fluoromethylhistidine (FMH; 80 mg/day, i.p.), an irreversible inhibitor of histamine synthesis, or saline. FMH pretreatment produced a significant protection against PTD-induced neuronal loss within the midline-intralaminar and anteromedial thalamic nuclei, but had no effect on damage to ventrolateral nuclei, anteroventral nucleus, or the mammillary bodies. In a third study, histamine (80 micrograms, free base) or vehicle was directly infused into the same region of medial thalamus dialyzed in experiment 1. Histamine infusion into prelesion PTD but not CT animals resulted in severe neuronal loss and gliosis. Infusion of vehicle into the same regions of PTD and CT rats produced a mild gliosis restricted to the needle tract with no evidence of neuronal loss. These observations together with recent evidence of a histamine enhancement of glutamate receptor activation suggest that early histamine release may contribute significantly to glutamate-N-methyl-D-aspartate (NMDA)-mediated excitotoxic neuronal death in thiamine deficiency-induced Wernicke's encephalopathy.