Overflow of endogenous norepinephrine from PVH nucleus of DOCA-salt hypertensive rats.

Previous studies from this laboratory demonstrated that there was enhanced basal and evoked (K+ depolarization) overflow of endogenous norepinephrine (NE) into the perfusate of a push-pull cannula placed in the paraventricular nucleus of the hypothalamus (PVH) of conscious freely moving spontaneously hypertensive rat (SHR) compared with Wistar-Kyoto (WKY) or Sprague-Dawley (SD) rats. The present study was carried out to determine whether results obtained with SHR were specific to this genetic model of hypertension by examining NE release in deoxycorticosterone acetate (DOCA)-salt hypertension. DOCA-salt hypertension was produced in 8-wk-old uninephrectomized SD rats by administering a 50-mg DOCA Silastic pellet subcutaneously 7 days postnephrectomy and providing 0.9% NaCl + 0.2% KCl drinking solution at libitum for 3 wk. Sham-implanted animals received normal tap water. Blood pressure was similar to that of 8- to 10-wk-old SHR. Basal release of NE as well as release after K+ added to the push-pull cannula or sodium nitroprusside or phenylphrine administered intravenously was determined. It was observed that there was no difference in basal overflow or after K+ administration in DOCA-salt hypertensive rats compared with sham animals. Similarly, the increase in NE overflow due to sodium nitroprusside or the decrease due to phenylphrine was similar between DOCA-salt rats or sham controls. This was in sharp contrast to what was observed in SHR: basal or K(+)-evoked release was significantly greater in SHR than WKY, SD, DOCA-salt, or DOCA-sham controls. It is concluded that central noradrenergic activity involving the PVH is not altered in DOCA-salt hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-dependent overflow of endogenous norepinephrine from paraventricular hypothalamic nucleus of hypertensive rats.

The relationship between age and central noradrenergic neuronal activity of the paraventricular hypothalamic nucleus (PVH) was examined in 7- to 10-, 12- to 14-, and 30- to 36-wk-old Sprague-Dawley (SD), Wistar-Kyoto (WKY), and spontaneously hypertensive rats (SHR). As an index of noradrenergic activity, endogenous norepinephrine (NE) overflow was assessed utilizing a miniaturized push-pull cannula assembly in unanesthetized freely moving rats. NE overlow under basal, 56 mM K+ stimulation, and in response to pressor/depressor drugs, were examined in all three strains at all ages. Significant increases in basal and K(+)-stimulated overflow of endogenous NE from the PVH were observed in all ages of SHR compared with normotensive controls with the greatest percent increase occurring during the development of hypertension in SHR. In addition, a reciprocal relationship exists with respect to blood pressure and overflow of NE from the PVH such that increases/decreases in blood pressure elicit decreases/increases in NE overflow in all strains at all ages examined. However, developing hypertensive SHR exhibited attenuated decreases in overflow of NE from the PVH compared with age-matched controls and established hypertensive SHR. These results suggest that noradrenergic pathways of the PVH contribute to the development and maintenance of arterial pressure hemostasis and that enhanced central noradrenergic neuronal activity is greatest during the development of hypertension in SHR.

Release of norepinephrine from the paraventricular hypothalamic nucleus of hypertensive rats.

The push-pull cannula was used to examine the release of endogenous norepinephrine (NE) from the paraventricular hypothalamic nucleus (PVH) of unanesthetized freely moving 7- to 10- and 12- to 14-wk-old Sprague-Dawley (SD), Wistar-Kyoto (WKY), and spontaneously hypertensive (SHR) rats. Basal NE release, K+ (56 mM) stimulation-induced NE release, and NE release in response to pressor/depressor drugs were examined in all three strains at both ages. Significant increases in basal and K+-stimulated release of endogenous NE from the PVH were observed in 7- to 10- and 12- to 14-wk-old SHR compared with the normotensive control rats suggesting that enhanced central noradrenergic nerve activity may be involved in the development and maintenance of hypertension in the SHR. In addition, a reciprocal relationship exists with respect to blood pressure and NE release from the PVH, i.e., decreases in blood pressure elicit increases in NE release, and increases in blood pressure elicit decreases in NE release in all three strains at both age groups, suggesting that the noradrenergic pathways of the PVH contribute to the maintenance of arterial blood pressure homeostasis.

Involvement of Gi in the inhibition of adenylate cyclase by cannabimimetic drugs.

The cellular mechanism of action of the cannabimimetic drugs is examined using cultured cells. In membranes from N18TG2 neuroblastoma cells and the neuroblastoma X glioma hybrid cells, NG108-15, the psychoactive cannabinoid drugs and their nantradol analogs could inhibit adenylate cyclase activity. This response was not observed in either the soluble adenylate cyclase from rat sperm or membrane-bound adenylate cyclases from C6 glioma or S49 lymphoma cells. This cellular selectivity provides further evidence for the existence of specific receptors for the cannabimimetic compounds. Receptor-mediated inhibition of adenylate cyclase requires the presence of a guanine nucleotide-binding protein complex, Gi. Gi can be functionally inactivated as a result of an ADP-ribosylation modification catalyzed by pertussis toxin. The present study demonstrates that pertussis toxin treatment of cells abolished the cannabimimetic response in intact cells and in membranes derived therefrom. The action of pertussis toxin required NAD+ as substrate for in vitro modification of neuroblastoma membranes. Furthermore, pertussis toxin was able to catalyze the labeling of a neuroblastoma membrane protein in vitro using [32P] NAD+ under conditions similar to those by which attenuation of the cannabimimetic inhibition of adenylate cyclase could be demonstrated. This evidence demonstrates the requirement for a functional Gi in the action of cannabimimetic drugs.