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Activation of presynatic histamine [H.sub.3] receptors ([H.sub.3]R) down-regulates norepinephrine exocytosis from cardiac sympathetic nerve terminals, in both normal and ischemic conditions. Analogous to the effects of [[alpha].sub.2]-adrenoceptors, which also act prejunctionally to inhibit norepinephrine release, [H.sub.3]R-mediated antiexocytotic effects could result from a decreased [Ca.sup.2+] influx into nerve endings. We tested this hypothesis in sympathetic nerve terminals isolated from guinea pig heart (cardiac synaptosomes) and in a model human neuronal cell line (SH-SY5Y), which we stably transfected with human [H.sub.3]R cDNA (SH-SY5Y-[H.sub.3]). We found that reducing [Ca.sup.2+] influx in response to membrane depolarization by inhibiting N-type [Ca.sup.2+] channels with [omega]-conotoxin ([omega]-CTX) greatly attenuated the exocytosis of [[sup.3]H]norepinephrine from both SH-SY5Y and SH-SY5Y-[H.sub.3] cells, as well as the exocytosis of endogenous norepinephrine from cardiac synaptosomes. Similar to [omega]-CTX, activation of [H.sub.3]R with the selective [H.sub.3]R-agonist imetit also reduced both the rise in intracellular [Ca.sup.2+] concentration ([Ca.sub.i]) and norepinephrine exocytosis in response to membrane depolarization. The selective [H.sub.3]R antagonist thioperamide prevented this effect of imetit. In the parent SH-SY5Y cells lacking [H.sub.3]R, imetit affected neither the rise in [Ca.sub.i] nor [[sup.3]H]norepinephrine exocytosis, demonstrating that the presence of [H.sub.3]R is a prerequisite for a decrease in [Ca.sub.i] in response to imetit and that [H.sub.3]R activation modulates norepinephrine exocytosis by limiting the magnitude of the increase in [Ca.sub.i]. Inasmuch as excessive norepinephrine exocytosis is a leading cause of cardiac dysfunction and arrhythmias during acute myocardial ischemia, attenuation of norepinephrine release by [H.sub.3]R agonists may offer a novel therapeutic approach to this condition.

The histaminergic system (histamine and its [H.sub.1]-receptor) of the central nervous system has been implicated in control of food intake. The reported studies were designed to examine the effects of food restriction and very low (1%) protein diets on central nervous system [H.sub.1]-receptors in male and female rats. In a series of experiments, groups of rats were freely fed a 25% protein diet, a 1% protein diet, or fed the 25% protein diet at 4 g/100 g body weight for 14-20 d. When freely fed 25% protein diets, females had higher whole-brain [H.sub.1]-receptor binding than males on d I (female 122.36 [+ or -] 4.53 and male 65.78 [+ or -] 3.82 pmol/g protein; P < 0.001). Changing diets affected central [H.sub.1]-receptor binding in both males and females (P < 0.003). When rats were fed both restricted levels of food and 1% protein diets, the receptor binding of males increased by d 5 whereas that of females decreased by d 5 (P < 0.001). When fed 1% protein diets, females had decreased [H.sub.1]-receptor binding (98.4 [+ or -] 2.38 pmol/g protein) and that in males increased to 119.81 [+ or -] 5.09 pmol/g protein. After 15 d, females had eaten significantly more food than males: females 166 [+ or -] 4.9 g, males 124 [+ or -] 1.9 g (P < 0.0007). Males had a significantly greater weight loss than females: males -28.8 [+ or -] 2.6 g, females -17.08 [+ or -] 0.97 g (P < 0.0007). When fed restricted diets, females had decreased [H.sub.1]-receptor binding (93.81 [+ or -] 5.58 pmol/g) whereas binding in males increased to 111.27 [+ or -] 8.55 pmol/g. Preliminary saturation binding studies indicated that restricted food intake lowered receptor density (females consuming 25% protein: 715 [+ or -] 30 pmol/g protein; female restricted: 467 [+ or -] 28 pmol/g protein, P < 0.05), while 1% protein increased receptor sensitivity, i.e., lowered [K.sub.D] (males consuming 25% protein: 15.3 [+ or -] 1.8 nmol; males fed low protein: 2.8 [+ or -] 0.27 nmol). This study suggests that dietary manipulation affects central [H.sub.1]-receptor binding in a gender-specific manner, thereby modulating central histaminergic activity during food or protein deficit. INDEXING KEY WORDS: * receptor * growth * histamine * rats * gender * diet

From pharmacological studies using histamine antagonists and agonists, it has been demonstrated that histamine modulates many physiological functions of the hypothalamus, such as arousal state, locomotor activity, feeding, and drinking. Three kinds of receptors ([H.sub.1], [H.sub.2], and [H.sub.3]) mediate these actions. To define the contribution of the histamine [H.sub.1] receptors (H1R) to behavior, mutant mice lacking the H1R were generated by homologous recombination. In brains of homozygous mutant mice, no specific binding of [3H]pyrilamine was seen. [3H]Doxepin has two saturable binding sites with higher and lower affinities in brains of wild-type mice, but H1R-deficient mice showed only the weak labeling of [3H]doxepin that corresponds to lower-affinity binding sites. Mutant mice develop normally, but absence of H1R significantly increased the ratio of ambulation during the light period to the total ambulation for 24 hr in an accustomed environment. In addition, mutant mice significantly reduced exploratory behavior of ambulation and rearings in a new environment. These results indicate that through H1R, histamine is involved in circadian rhythm of locomotor activity and exploratory behavior as a neurotransmitter.

Histamine [H.sub.2] receptors transfected in Chinese hamster ovary (CHO) cells are time- and dose-dependently upregulated upon exposure to the [H.sub.2] antagonists cimetidine and ranitidine. This effect appears to be [H.sub.2] receptor-mediated as no change in receptor density was observed after [H.sub.1] or [H.sub.3] antagonist treatment or after incubation with the structural analogue of cimetidine, VUF 8299, which has no [H.sub.2] antagonistic effects. By using transfected CHO cells expressing different densities of wild-type [H.sub.2] receptors or an uncoupled [H.sub.2][Leu.sup.124]Ala receptor, the histamine [H.sub.2] receptor was found to display considerable agonist-independent [H.sub.2] receptor activity. Cimetidine and ranitidine, which both induce [H.sub.2] receptor upregulation, actually functioned as inverse agonists in those cell lines displaying spontaneous agonist-independent [H.sub.2] receptor activity. Burimamide, on the other hand, was shown to act as a neutral antagonist and did as expected not induce [H.sub.2] receptor upregulation after long-term exposure. The displayed inverse agonism of [H.sub.2] antagonists appears to be a mechanistic basis for the observed [H.sub.2] antagonist-induced [H.sub.2] receptor upregulation in transfected CHO cells. These observations shed new light on the pharmacological classification of the [H.sub.2] antagonists and may offer a plausible explanation for the observed development of tolerance after prolonged clinical use.

In myocardial ischemia, adrenergic nerves release excessive amounts of norepinephrine (NE), causing dysfunction and arrhythmias. With anoxia and the concomitant ATP depletion, vesicular storage of NE is impaired, resulting in accumulation of free NE in the axoplasm of sympathetic nerves. Intraneuronal acidosis activates the [Na.sup.+]/[H.sup.+] exchanger (NHE), leading to increased [Na.sup.+] entry in the nerve terminals. These conditions favor availability of the NE transporter to the axoplasmic side of the membrane, causing massive carrier-mediated efflux of free NE. Neuronal NHE activation is pivotal in this process; NHE inhibitors attenuate carrier-mediated NE release. We previously reported that activation of histamine [H.sub.3] receptors ([H.sub.3]R) on cardiac sympathetic nerves also reduces carrier-mediated NE release and alleviates arrhythmias. Thus, [H.sub.3]R activation may be negatively coupled to NHE. We tested this hypothesis in individual human SKNMC neuroblastoma cells stably transfected with [H.sub.3]R cDNA, loaded with the intracellular pH (p[H.sub.i]) indicator BCECF. These cells possess amiloride-sensitive NHE. NHE activity was measured as the rate of [Na.sup.+]-dependent p[H.sub.i] recovery in response to an acute acid pulse ([NH.sub.4]CI). We found that the selective [H.sub.3]R-agonist imetit markedly diminished NHE activity, and so did the amiloride derivative EIPA. The selective [H.sub.3]R antagonist thioperamide abolished the imetit-induced NHE attenuation. Thus, our results provide a link between [H.sub.3]R and NHE, which may limit the excessive release of NE during protracted myocardial ischemia. Our previous and present findings uncover a novel mechanism of cardioprotection: NHE inhibition in cardiac adrenergic neurons as a means to prevent ischemic arrhythmias associated with carrier-mediated NE release.