Michael J. Brody

Factors influencing the altered pain perception in the spontaneously hypertensive rat

Summary Recent studies have demonstrated a hypoalgesia in hypertensive subjects. This study reports and evaluates factors responsible for the expression of the hypoalgesic behavior demonstrated by genetically hypertensive rats of the Okamoto-Aoki strain (SHR) as compared to normotensive age-matched Wistar-Kyoto rats (WKY). Analgesiometric assays were conducted by the hot plate method. SHRs hypoalgesic behavior was reversed by subcutaneously administered naloxone. The intravenous administration of naloxone did not alter arterial pressure of heart rate in either SHR or WKY. Subcutaneous administration of the peripherally acting ganglionic blocker hexamethonium bromide at a dose which lowered mean arterial blood pressure and thus decreased tonic baroreceptor stimulation, concomitantly reversed the SHR hypoalgesic behavior and induced a hyperalgesia in WKY. Denervation of the sinoaortic baroreceptors failed to alter the hypoalgesic behavior demonstrated by SHR. Denervation of the right vagal nerve trunk with associated cardiopulmonary baroreceptor afferents resulted in a reduction of the SHR hypoalgesic behavior and produced a hyperalgesic behavior in WKY as compared to age-matched sham operated controls over a 4 week period. These data suggest a possible physiological role for vagal afferent systems in the concomitant regulation of resting arterial blood pressure and responsiveness to aversive environmental stimuli. A discussion of the interaction between blood pressure and pain regulatory systems as potential substrates associated with the onset and maintenance of hypertension is provided.

Vasoconstrictor hyperresponsiveness: an early pathogenic mechanism in the spontaneously hypertensive rat.

Factors which play a primary role in the initiation and development of hypertension in spontaneously hypertensive rats (SHR) are incompletely defined. To test the possibility that early changes in vascular function play a primary etiologic role, hindquarters of 3-week-old SHR and Wistar-Kyoto normotensive rats (WKY) were perfused at constant flow with plasma substitute. The vasculature of SHR exhibited higher resistance to flow than that of WKY. The threshold constrictor response to norepinephrine (NE) was elicited at a significantly lower concentration (6X) than required in WKY, while threshold to BaCl2 was not different. At concentrations of BaCl2 above threshold, SHR exhibited marked hyperresponsiveness compared to WKY. This resulted in a greater maximum response and thus a steeper slope. The ED50 for BaCl2 was not different. A similar dose--response relationship (greater maximum, steeper slope) was observed with NE except that the ED50 as well as threshold was significantly lower in SHR than in WKY. These data show that vasoconstrictor hyperresponsiveness and increased vascular resistance are present at the time when the hypertension is first detectable. The hyperresponsiveness includes two distinct components: (1) A specific hypersensitivity to NE and (2) non-specific hyperresponsiveness which could derive from altered excitation--contraction coupling and/or from a structural mechanism already present when pressure differences begin to appear.

Neurogenic and Humoral Factors Controlling Vascular Resistance in the Spontaneously Hypertensive Rat

The mechanism of sustained hypertension in spontaneously hypertensive rats has not been elucidated. In the present investigation, vasoconstrictor responses to a variety of neurogenic and humoral interventions were studied in the perfused hindquarters of Okamoto spontaneously hypertensive rats and normotensive Wistar rats. In addition, central sympathetic electrical discharge was measured. Vasoconstrictor responses in the hindquarters to lumbar sympathetic nerve stimulation were unchanged or reduced in the spontaneously hypertensive rats, but the responses to intra-arterially administered norepinephrine and epinephrine were enhanced. Vascular responses to intra-arterially administered tyramine, angiotensin, and barium chloride were also greater in the spontaneously hypertensive rats. Vascular resistance was significantly higher in the spontaneously hypertensive rats, and this difference remained following bilateral lumbar sympathectomy. Despite elevated systemic blood pressure, integrated nerve activity at rest was not different in the spontaneously hypertensive rats. The inverse relationship between arterial blood pressure and sympathetic nerve discharge was not different between spontaneously hypertensive and control rats when pressure was either raised or lowered. Changes in efferent sympathetic discharge produced by activation of chemoreceptors (asphyxia) were somewhat less in the spontaneously hypertensive rats. The contribution of low-pressure baroreceptors (45° tilt) to activation of sympathetic vasomotor tone was not different in the spontaneously hypertensive rats despite a greater decline in systemic blood pressure during the procedure. These data demonstrate that established hypertension in the spontaneously hypertensive rat does not derive from either enhanced central adrenergic discharge or altered central integration of afferent information from peripheral sensory receptors but may result from humoral (e.g., increased reactivity to vasoconstrictors) or structural factors.

Central amygdaloid nucleus lesion attenuates exaggerated hemodynamic responses to noise stress in the spontaneously hypertensive rat

The regional hemodynamic basis of the cardiovascular response to acute noise stress in spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats and the role of the central amygdaloid nucleus in mediating this response was investigated. Using the pulsed Doppler flow probe technique it was determined that in response to noise, SHR exhibit a significantly greater percent increase in renal and mesenteric vascular resistance than WKY. Vascular responses in the hindquarter were similar in both groups. Bilateral lesion of the central amygdaloid nucleus or its output pathways to the brainstem decreased the cardiovascular response to noise in both SHR and WKY, with SHR and WKY lesion rats responding similarly. The central amygdaloid nucleus appears to participate in the cardiovascular response to acute noise stress in SHR and WKY. Although other structures in the limbic system network may contribute to integration of responses that involve the amygdala the present data suggest that this structure may play a central role in mediating the exaggerated cardiovascular responsiveness of SHR to environmental stress.

Role of sympathetic nerve activity in the generation of vascular nitric oxide in urethane-anesthetized rats.

The aim of the present study was to examine the involvement of the sympathetic nervous system in the generation or release of vascular nitric oxide. In urethane-anesthetized rats, the administration of the novel nitric oxide synthesis inhibitor L-jV-nitro arginine (LNA) (0.02 mmol/kg i.v.) increased mean arterial pressure and renal, mesenteric, and hindquarter vascular resistances. The intravenous administration of L-arginine (60 nig/kg plus 12 mg/kg/ min i.v.) produced small reductions in arterial pressure and vascular resistances and abolished the hemodynamic effects of LNA. Pretreatment with the ganglion blocking agent chlorisondamine lowered mean arterial pressure and vascular resistances, abolished the LNA-induced pressor and renal vasoconstrictor response, and attenuated the increases in mesenteric and hindquarter resistances. In contrast, the vasodilator hydralazine lowered mean arterial pressure and vascular resistances to levels equivalent to that of ganglionic blockade; however, the subsequent administration of LNA still produced significant increases in arterial pressure and regional vascular resistances. In ganglion-blocked rats in which pressure and vascular resistances were returned to normal levels by infusion of arginine vasopressin or phenylephrine, the pressor and vasoconstrictor effects of LNA were restored. However, phenylephrine was significantly more efficacious and markedly exaggerated the action of LNA. These results suggest that the sympathetic nervous system plays an important role in modulating the synthesis or release of vascular nitric oxide through the effects of 1) normal sympathetic discharge, 2) humoral activation of α-adrenergic receptors, and 3) vascular tone per se.

The central and peripheral effects of Captopril (SQ 14225) on the arterial pressure of the spontaneously hypertensive rat

An orally active inhibitor of the enzyme that converts angiotensin I to angiotensin II (All) and degrades bradykinin has recently been shown to lower blood pressure in spontaneously hypertensive (SH) rats when administered orally, but to have no depressor effect on normotensive Wistar-Kyoto ( W K Y ) r a t s 7,1°. Since Captopril is a small molecule (2-D-methyl-3-mercapto-propranoyl-L-proline) it might be able to cross the blood-brain barrier and exert its hypotensive action in the brain. On the other hand, it could act peripherally. I f it crosses the blood-brain barrier the action might be due to the inhibition of angiotensin II formation since it has been shown that saralasin, an angiotensin II antagonist, lowers blood pressure in SH rats when injected into the brainS, 11. Therefore, to determine if Captopril has a central action we have compared administration of the same dose intracerebroventricularly (i.v.t.) and intravenously (i.v.) to SH rats. We reasoned that if the same dose of Captopril produces a greater hypotensive response i.v.t, than i.v. it must be acting centrally and not merely leaking into the blood. This was found to be the case. Six male SH rats, 4--5 months of age, weighing 300-350 g, were anesthetized with 20 ~ chloral hydrate (2 ml/kg). Stainless-steel cannulae were implanted into the left lateral ventricles and heparinized saline-filled catheters were introduced into the left femoral arteries and veins. The venous catheter was polyethelene tubing (PE 50). The arterial catheters were Silastic inside the artery. The Silastic part of the catheter was attached to polyethylene tubing (PE 50) outside the artery. Both catheters were exteriorized by placing them subcutaneously to a cut in the skirt over the back of the rat approximately at the level of the scapulae. The catheters were held in place on the back by wound clips and cranioplastic dental cement. The rats were allowed to recover for at least one day after the operation. All experiments were performed on conscious, unrestrained rats. Mean arterial

VASCULAR REACTIVITY IN EXPERIMENTAL DIABETES MELLITUS.

Vascular reactivity was examined in vessels of the perfused hindquarters of alloxan diabetic and control rats. A significant increase was noted in responsiveness of the diabetic animals to intra-arterial injection of epinephrine, norepinephrine, and synthetic angiotensin. No difference was observed in the vasoconstrictor effects of lumbar sympathetic nerve stimulation or intra-arterial administration of tyramine. Results obtained in diabetic animals were compared with those in animals treated with reserpine. It was found that the reserpinized animals exhibited increased vascular responsiveness only to the catecholamines, and not to angiotensin, vasopressin, tyramine, or barium chloride. Whereas the vascular smooth muscle of diabetic rats was sensitized to epinephrine and norepinephrine, isolated atria from these animals responded normally to the catecholamines. The 24-hour urinary excretion of the catecholamines was significantly elevated in diabetic animals. The data suggest that enhanced vascular reactivity seen in diabetic rats does not result from neuropathy of the sympathetic nervous system. The possible contributions of factors including alloxan per se and body weight to development of increased vascular reactivity in diabetes were considered, but the mechanism for the increase in vascular responsiveness remains unidentified.

Role of central catecholamines in the control of blood pressure and drinking behavior.

The role of central nervous system (CNS) catecholamines in the development of hypertension and the control of drinking behavior was assessed in rats by depleting these amines with 6-hydroxydopamine (6-OHDA). Intraventricular administration of 6-OHDA completely prevented the development of one-kidney renal hypertension and abolished the associated increase in water consumption. 6-OHDA-treated rats showed deficits in drinking behavior when challenged with subcutaneous injections of angiotensin II (AII) and hypertonic sodium chloride. The acute pressor responses produced by intraventricular injections of AII and carbachol were virtually abolished by central catecholamine depletion. However, drinking produced by central cholinergic stimulation remained intact while AII drinking was significantly reduced. These data demonstrate that the integrity of CNS catecholamines is required for the development of one-kidney renal hypertension and the increased drinking which accompanies it. In addition, destruction of central catecholamine-containing neurons allows for a specific dissociation of the pressor and drinking responses produced by central cholinergic but not AII stimulation.

Contribution of the sympathetic nervous system to vascular resistance in conscious young and adult spontaneously hypertensive rats.

SUMMARY Although evidence exists for exaggerated sympathetic nervous system activity in spontaneously hypertensive rats (SHR), there are no studies in conscious animals that directly demonstrate that this increased activity is functionally involved in the elevated vascular resistance of these animals. In our present study, 8-week-old and 13-week-old SHR and Wistar Kyoto controls (WKY) were chronically Instrumented with arterial and venous catheters and miniaturized pulsed Doppler flow probes on the renal and mesenteric arteries and lower abdominal aorta. While the rats were conscious and unrestrained, bexamethonium was administered intravenously to block sympathetic nervous system transmission. Prior to bexamethonlum, the mean arterial pressure of young SHR and WKY averaged 123 ± 5 and 109 ± 4 nun Hg respectively (p < 0.05), while adult SHR and WKY averaged 159 ± 7 and 128 ± 3 mm Hg respectively (p < 0.05). Hexamethonium produced an equivalent fall in arterial pressure of young SHR (-32%) and WKY (-30%) and adult SHR (-39%) and WKY (-41%). Vascular resistance was reduced by hexamethonium in the kidney, gut, and hindquarters, but the percent changes were not significant between SHR and WKY. These data suggest that, in both young and adult SHR, vascular resistance and arterial pressure are sustained at elevated levels by some other mechanism than aeurally-derived vasoconstrictor tone.

A comparison of the hemodynamic effects produced by electrical stimulation of subnuclei of the paraventricular nucleus

The paraventricular nucleus of the hypothalamus (PVN) is composed of magnocellular and parvocellular subdivisions. Magnocellular neurosecretory neurons project to the neurohypophysis while parvocellular neurons send monosynaptic axonal projections to autonomic regulatory areas in the brainstem and spinal cord. In the present study, we investigated the hemodynamic effects produced by selective magnocellular or parvocellular stimulation. In urethane anesthetized rats with intact baroreflexes, magnocellular and parvocellular stimulation produced only slight differences in hemodynamic responses, however, following acute sinoaortic denervation a clear difference was observed. Parvocellular stimulation produced an increase in arterial pressure and vasoconstriction in gut, kidney and skeletal muscle. Magnocellular stimulation produced little effect on arterial pressure and marked vasodilation in the hindquarters. Blockade of peripheral vasopressin vascular receptors did not affect the vasoconstrictor response produced by stimulation of PVN. These data are consistent with the hypothesis that the long descending neural projections of the parvocellular PVN subserve a selective vasoconstrictor function.