Antagonistic effects of growth hormone-releasing factor and somatostatin on brain histamine.
Studies on the morphological distribution of histamine (HA)-secreting neurons and their hypothalamic projections suggest that HA may play a key role in regulating neuroendocrine functions, some of which have been recently elucidated. To investigate possible interactions between the somatotropinergic and histaminergic systems in the brain, the effects of GRF-44 [0.1-10 micrograms intracerebroventricular (icv)] and somatostatin (SS-14; 0.1-10 micrograms, icv) on HA in five different parts of the hypothalamo-hypophyseal system, and in the hippocampus and frontal cortex, were studied using a highly sensitive HPLC system for determination of HA. GRF-44 (1 microgram, icv) elicited significant (P less than 0.005) increases in the concentration of HA in the anterior hypothalamus, posterior hypothalamus, median eminence, adenohypophysis, neurohypophysis, frontal cortex, and, to a lesser extent, in the hippocampus, after a clear time-dependent pattern with maxima 15 min after injection. In contrast, SS-14 (1 microgram, icv) significantly (P less than 0.005) decreased the levels of HA in all areas studied, except in the neurohypophysis. The SS-induced HA levels reached minima 30 min after injection. The antagonistic effects of GRF-44 and SS-14 on the release of brain HA were dose dependent, showing an inverse linear correlation within the range 0.1-10 micrograms in the anterior hypothalamus (r = -0.59) and posterior hypothalamus (r = -0.75). Responses of HA to GRF-44 and SS-14 (range: 0.1-10 micrograms) also exhibited an inverse linear correlation in the median eminence (r = -0.90) and adenohypophysis (r = -0.58), while in the hippocampus and frontal cortex the antagonistic effects of GRF and SS displayed an inverse curvilinear correlation. SS-14 ED50 values ranged from 0.6 to 1.75 nmol with Emax of 0.65-6.10 nmol. GRF-44 ED50 values ranged from 0.02-0.3 nmol and the Emax values oscillated between 0.2 and 1.90 nmol in the regions studied. The greatest responses of HA to GRF-44 and SS-14 were obtained in the hypothalamo-hypophyseal system. Although brain HA is present in both the neuronal and the mast cell compartments, changes induced in the concentration of HA by centrally administered GRF-44 and SS-14 appear to occur mostly in the neuronal compartment. Therefore, it is likely that the somatotropinergic and histaminergic systems reciprocally interact at the central level to regulate still unknown neuroendocrine functions.
D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D013007:Growth Hormone-Releasing Hormone / Q000008:administration & dosage / Q000494:pharmacology*; D006632:Histamine / Q000378:metabolism*; D007276:Injections, Intraventricular; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D013004:Somatostatin / Q000008:administration & dosage / Q000494:pharmacology*; D013997:Time Factors
D013004:Somatostatin; D006632:Histamine; D013007:Growth Hormone-Releasing Hormone