Lesions of the anteroventral third ventricle and development of stress-induce hypertension in the borderline hypertensive rat

Blood pressure regulation in stress: focus on nitric oxide-dependent mechanisms

Stress is considered a risk factor associated with the development of various civilization diseases including cardiovascular diseases, malignant tumors and mental disorders. Research investigating mechanisms involved in stress-induced hypertension have attracted much attention of physicians and researchers, however, there are still ambiguous results concerning a causal relationship between stress and long-term elevation of blood pressure (BP). Several studies have observed that mechanisms involved in the development of stress-induced hypertension include increased activity of sympathetic nervous system (SNS), glucocorticoid (GC) overload and altered endothelial function including decreased nitric oxide (NO) bioavailability. Nitric oxide is well known neurotransmitter, neuromodulator and vasodilator involved in regulation of neuroendocrine mechanisms and cardiovascular responses to stressors. Thus NO plays a crucial role in the regulation of the stress systems and thereby in the BP regulation in stress. Elevated NO synthesis, especially in the initial phase of stress, may be considered a stress-limiting mechanism, facilitating the recovery from stress to the resting levels via attenuation of both GC release and SNS activity as well as by increased NO-dependent vasorelaxation. On the other hand, reduced levels of NO were observed in the later phases of stress and in subjects with genetic predisposition to hypertension, irrespectively, in which reduced NO bioavailability may account for disruption of NO-mediated BP regulatory mechanisms and accentuated SNS and GC effects. This review summarizes current knowledge on the role of stress in development of hypertension with a special focus on the interactions among NO and other biological systems affecting blood pressure and vascular function.

Effect of chronic social stress on nitric oxide synthesis and vascular function in rats with family history of hypertension

Genetic predisposition and psychosocial stress are known risk factors in the aetiology of hypertension. The aim of this study was to investigate the as yet unknown role of nitric oxide (NO) in mechanisms of social stress-induced hypertension in rats with a family history of hypertension. Male adult rats used in the study were offspring of normotensive (Wistar) dams and spontaneously hypertensive sires. The rats were exposed to 6-week crowding stress (5 rats/cage, 200 cm2/rat). Control rats were kept four per cage (480 cm2/rat). Blood pressure was determined non-invasively on the tail. Basal blood pressure of all rats was 131 +/- 2 mm Hg. Crowding stress increased significantly blood pressure (p < 0.02 vs. basal value). Crowding had no influence on NO synthase activity in the left ventricle, adrenal glands and kidney. However, crowding stress reduced significantly NO synthase activity in the aorta by 37% (p < 0.01 vs. control). Acetylcholine-induced relaxation and noradrenaline-induced vasoconstriction of the femoral artery were reduced in stressed rats by 58% (p < 0.001) and 41% (p < 0.003), respectively. On balance then, the results indicate that chronic social stress produced by crowding was associated with reduced vascular NO synthesis and altered vascular function in adult borderline hypertensive rats of normotensive mothers.

Lesions of the anteroventral third ventricle region (AV3V) disrupt cardiovascular responses to an elevation in core temperature

Blood flow is redistributed from the viscera to the periphery during periods of heat stress to maximize heat loss. The heat-induced redistribution of blood flow is strongly influenced by nonthermal inputs such as hydration status. At present, little is known about where thermal and nonthermal information is integrated to generate an appropriate effector response. Recently, the periventricular tissue that surrounds the anteroventral third ventricle (AV3V) has been implicated in the integration of thermal and osmotic information. The purpose of the present study was to determine the effects of electrolytic lesions of the AV3V on the cardiovascular response to a passive heat stress in unanesthetized, free-moving male Sprague-Dawley rats. Core temperature was elevated at a constant rate of approximately 0.03 degrees C/min in sham- and AV3V-lesion rats using an infrared heat lamp. Changes in mesenteric and hindquarter vascular resistance were determined using Doppler flow probes, and heat-induced salivation was estimated using the spit-print technique. The rise in mean arterial pressure (MAP), heart rate (HR), and mesenteric resistance in response to elevations in core temperature were all attenuated in AV3V-lesion rats; however, hindquarter resistance was unaffected. Heat-induced salivation was also diminished. In addition, AV3V-lesion rats were more affected by the novelty of the experimental environment, resulting in a higher basal core temperature, HR, and MAP. These results indicate that AV3V lesions disrupt the cardiovascular and salivatory response to a passive heat stress in rats and produce an exaggerated stress-induced fever triggered by a novel environment.

Stress causes decrease in vascular relaxation linked with altered phosphorylation of heat shock proteins

Cyclic nucleotide-dependent vascular relaxation is associated with increases in the phosphorylation of a small heat shock protein (HSP), HSP20. An increase in phosphorylation of another small HSP, HSP27, is associated with impaired cyclic nucleotide-dependent vascular relaxation. Expression of HSPs is altered by exposure to several types of cellular stress in vitro. To determine if behavioral stress in vivo alters vascular expression and phosphorylation of the small HSPs and cyclic nucleotide-dependent vascular relaxation, borderline hypertensive rats were stressed by restraint and exposure to air-jet stress 2 h/day for 10 days or remained in their home cage. Stress impaired relaxation of aorta to forskolin, which activates adenylyl cyclase, and sodium nitroprusside, which activates guanylyl cyclase. This was associated with an increase in the aortic expression and phosphorylation of HSP27, which was localized to the vascular smooth muscle, but a decrease in the amount of phosphorylated (P)-HSP20. To determine if P-HSP27 inhibits phosphorylation of HSP20, P-HSP27 was added to a reaction mixture containing recombinant HSP20 and the catalytic subunit of cAMP-dependent protein kinase. P-HSP27 inhibited phosphorylation of HSP20 in a concentration-dependent manner. These data demonstrate that P-HSP27 can inhibit phosphorylation of HSP20. The increase in P-HSP27 and decrease in P-HSP20 were associated with reduced cyclic nucleotide-dependent vascular smooth muscle relaxation in response to behavioral stress in vivo, an effect similar to that observed previously in response to cellular stress in vitro.

Anteroventral third ventricle periventricular tissue contributes to cardiac baroreflex responses

1. The studies reviewed in the present paper demonstrate that the anteroventral third ventricle (AV3V) region contains tissue that can modify cardiac baroreflex sensitivity in response to circulating angiotensin (Ang)II and hyperosmolality. 2. The response to hyperosmolality appears to be mediated by noradrenergic receptors. Although the role of noradrenergic receptors in the AV3V region in modification of baroreflex-induced responses to AngII has not been directly tested, this neurotransmitter is a good candidate for control of heart rate because noradrenaline in the AV3V region is critical for mediating other responses to AngII. 3. Results from studies indicate that the AV3V region is part of a central nervous system circuit involved in modulation of cardiac baroreflex sensitivity by circulating substances, possibly acting at the organum vasculosum lamina terminalis. 4. The findings extend the role of the AV3V periventricular tissue as a central site integrating autonomic nervous system function by demonstrating that this brain area contributes to cardiac function, in addition to its well-characterized role in sympathetic nervous system regulation of blood pressure and mechanisms of fluid and electrolyte regulation.

Periventricular anteroventral third ventricle lesions diminish the pressor response produced by systemic injection of the N-methyl-D-aspartate receptor antagonist MK-801

This study examined whether electrolytic ablation of the periventricular anteroventral third ventricle (AV3V) would affect the increases in mean arterial blood pressure (MAP) and heart rate (HR) in conscious rats produced by systemic injection of the centrally acting N-methyl-D-aspartate (NMDA) receptor ion-channel blocker, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (MK-801; 250 microgram/kg, i.v.). MK-801 produced a smaller increase in MAP in rats with AV3V lesions than in sham-lesion rats (+36+/-2% vs. +52+/-5%, respectively, P<0.05). In contrast, MK-801 produced similar increases in HR in the AV3V- and sham-lesion rats (+28+/-3% vs. +22+/-4%, respectively, P>0.05). These findings demonstrate that the MK-801-induced pressor response is dependent upon the integrity of the AV3V region, whereas the MK-801-induced tachycardia is not.

Effects of chronic lesions of the anteroventral third ventricle region on baroreceptor reflex function in conscious rats

This study determined baroreceptor reflex (BR) function in conscious rats which had received sham or electrolytic lesions of the anteroventral third ventricle (AV3V) 54-56 days previously. Resting mean arterial pressure (MAP) and heart rate (HR) values of the AV3V-lesion rats were similar to those of sham-lesion rats (P>0.05 for both comparisons). The sensitivity of the BR-mediated tachycardia in AV3V-lesion was greater than in sham-lesion rats (-9. 92+/-1.00 vs. -4.54+/-0.45 bpm/mmHg, P<0.05). The sensitivity of the BR-mediated bradycardia in AV3V-lesion rats was also greater than in rats with sham lesions (-3.56+/-0.38 vs. -2.06+/-0.42 bpm/mmHg, P<0. 05). The AV3V lesions did not affect other BR parameters. These findings demonstrate that chronic lesions of the AV3V region increase the sensitivity of the baroreceptor HR reflex in conscious rats.

Role of the anterior region of the third ventricle in the cardiovascular responses produced by systemic injection of a nitric oxide synthase inhibitor

This study examined whether a prior electrolytic lesion of the tissue surrounding the anteroventral third ventricle (AV3V) would affect the increase in mean arterial blood pressure (MAP) and the fall in heart rate (HR) produced by systemic injection of the nitric oxide synthesis (NOS) inhibitor, NG-nitro-L-arginine methyl ester (L-NAME; 25 micromol/kg, i.v.) in conscious rats. L-NAME produced a smaller increase in MAP in AV3V-lesion than in sham-lesion rats (+19+/-3 vs. +40+/-3 mmHg, respectively; P<0.05). In contrast, L-NAME produced similar falls in HR in the AV3V-lesion and sham-lesion rats (-103+/-15 vs. -97+/-8 bpm, respectively; P<0.05). These findings demonstrate that the L-NAME-induced pressor response is dependent upon the integrity of the AV3V region, whereas the L-NAME-induced bradycardia is not.

AV3V lesions attenuate the cardiovascular responses produced by blood-borne excitatory amino acid analogs

Systemic injections of the excitatory amino acid (EAA) analogs, kainic acid (KA) and N-methyl-D-aspartate (NMDA), produce a pressor response in conscious rats that is caused by a centrally mediated activation of sympathetic drive and the release of arginine vasopressin (AVP). This study tested the hypothesis that the tissue surrounding the anteroventral part of the third ventricle (AV3V) plays a role in the expression of the pressor responses produced by systemically injected EAA analogs. Specifically, we examined whether prior electrolytic ablation of the AV3V region would affect the pressor responses to KA and NMDA (1 mg/kg iv) in conscious rats. The KA-induced pressor response was smaller in AV3V-lesioned than in sham-lesioned rats (11 +/- 2 vs. 29 +/- 2 mmHg; P < 0.05). After ganglion blockade, KA produced a pressor response in sham-lesioned but not AV3V-lesioned rats (+27 +/- 3 vs. +1 +/- 2 mmHg; P < 0.05). The KA-induced pressor response in ganglion-blocked sham-lesioned rats was abolished by a vasopressin V1-receptor antagonist. Similar results were obtained with NMDA. The pressor response to AVP (10 ng/kg iv) was slightly smaller in AV3V-lesioned than in sham-lesioned ganglion-blocked rats (45 +/- 3 vs. 57 +/- 4 mmHg; P < 0.05). This study demonstrates that the pressor responses to systemically injected EAA analogs are smaller in AV3V-lesioned rats. The EAA analogs may produce pressor responses by stimulation of EAA receptors in the AV3V region, or the AV3V region may play an important role in the expression of these responses.

Vascular and hemodynamic effects of behavioral stress in borderline hypertensive and Wistar-Kyoto rats

In borderline hypertensive rats (BHR), behavioral stress produces hypertension, which has been attributed to increases in sympathetic nervous system activity and peripheral changes in vascular structure. However, the mechanisms mediating development of stress-induced hypertension have not been well defined. Experiments were designed to determine hemodynamic effects and changes in small mesenteric artery (approximately 300 microns) vascular reactivity in response to 10 days of air-jet stress (2 h/day) in BHR and in Wistar-Kyoto (WKY) rats. The acute stress-induced increase in mean arterial pressure (AP) was impaired in WKY rats compared with BHR on day 1, and habituation developed to the increase in AP in BHR, but not WKY rats. Conversely, WKY rats adapted to the stress-induced tachycardia to a larger extent than BHR. The mechanisms mediating endothelium-dependent relaxation to acetylcholine (ACh) were altered in small mesenteric arteries isolated from WKY rats and BHR after 10 days of air-jet stress. Inhibition of nitric oxide synthase activity had a significantly larger inhibitory effect on ACh-induced relaxation in vessels from stressed compared with control BHR. Also, cyclooxygenase products contributed to ACh-induced relaxation of small mesenteric arteries from stressed WKY rats, but not control WKY rats. Endothelium-independent relaxation to nitroprusside was impaired in vessels from stressed WKY rats, but not stressed BHR. Finally, contraction to phenylephrine was impaired in vessels from stressed BHR, but not WKY rats. In conclusion, changes in vascular reactivity induced by air-jet stress appear to correlate with, and may contribute to, the differential hemodynamic adaptations to stress observed in WKY rats and BHR.

Effects of subfornical organ lesions on sympathetic nerve responses to insulin

Although insulin exerts potent excitatory effects on the sympathetic nervous system, the mechanisms of insulin-induced activation remain unclear. To demonstrate a central nervous system site of sympathoexcitation, we recently found that destruction of tissues surrounding the anteroventral third ventricle region abolishes elevations in sympathetic nerve activity to intravenous insulin administration. Anteroventral third ventricle lesions may eliminate sympathoexcitation by destroying cell bodies in the lesioned area or by interrupting fibers of passage from the subfornical organ. To determine whether the lesions abolish sympathetic increases by disrupting efferent fibers from the subfornical organ, we measured lumbar sympathetic activity in anesthetized anteroventral third ventricle-lesioned (n = 4) and subfornical organ-lesioned (n = 12) rats before and during intravenous insulin at 0.13 U/h while maintaining euglycemia. Additional sham-lesioned rats received infusion of insulin (n = 10) and the vehicle for insulin (n = 10). Insulin administration in sham-lesioned rats elevated lumbar activity from 100% to 171 +/- 14% (+/-SE), whereas vehicle infusion did not alter sympathetic activity (100% to 113 +/- 11%). In anteroventral third ventricle-lesioned rats, insulin failed to increase sympathetic nerve activity (100% to 119 +/- 14%). Importantly, rats with subfornical organ lesions had increases in nerve activity that were indistinguishable from increases observed in insulin-infused sham-lesioned rats (100% to 163 +/- 21%). These findings indicate that whereas the anteroventral third ventricle region itself is crucial for sympathoexcitation to insulin, the subfornical organ and fibers originating from the subfornical organ traversing the anteroventral third ventricle area are not essential in mediating elevations in lumbar sympathetic nerve activity to hyperinsulinemia.

Anteroventral third ventricle lesions attenuate pressor responses to serotonin in anesthetized rats

When administered intravenously, serotonin (5-hydroxytryptamine; 5-HT) evokes a triphasic blood pressure response, consisting of the Bezold-Jarisch-associated depressor response, a pressor action, and long-lasting depressor response. Because the pressor response may, in part, be caused by central nervous system (CNS) activation by 5-HT, we predicted that destruction of the anteroventral third ventricle (AV3V) region, an area rich in 5-HT receptors, would attenuate increases in blood pressure to intravenous 5-HT. In anesthetized sham-lesioned and AV3V-lesioned Sprague-Dawley rats, we measured mean arterial pressure (MAP), heart rate (HR), and lumbar sympathetic nerve activity (SNA) to increasing bolus doses of intravenous 5-HT (1, 2.5, 5, 10, 25 mu g/kg), before and after blockade of bradycardia using methylatropine (200 mu g/kg). In all rats, bolus injections of 5-HT elicited bradycardia accompanied by a fall in lumbar SNA and an initial hypotension followed by a pressor response and a longer lasting hypotensive response. The bradycardia, reduction in lumbar SNA, and both depressor responses were equivalent in sham-lesioned and AV3V-lesioned groups. Importantly, AV3V lesions attenuated pressor responses to increasing doses of 5-HT (3 +/- 1, 6 +/- 4, 6 +/- 4, 17 -/+ 4 35 +/- 3 mmHg) compared to sham-lesioned controls (6 +/- 3, 16 +/- 7, 33 +/- 5, 54 +/- 4, 51 +/- 6 mmHg; P < 0.0001). This attenuation was conserved following blockade of bradycardia with methylatropine (P < 0.01). In summary, pressor responses to intravenous 5-HT are diminished by AV3V lesions. These data indicate that the pressor component of the blood pressure response to intravenous 5-HT is partly dependent upon interaction with the CNS.

Anteroventral third ventricle lesions abolish lumbar sympathetic responses to insulin

Insulin has been shown to increase sympathetic nerve activity. Because evidence shows that insulin acts within the central nervous system, we hypothesized that lesions of the anteroventral third ventricle region, an area rich in insulin receptors, would abolish sympathetic responses to hyperinsulinemia. We measured mean arterial pressure and lumbar sympathetic nerve activity in fasted, anesthetized sham-lesioned (n = 8) and lesioned (n = 8) rats before and after intravenous insulin infusion at 0.13 U/h during euglycemic clamp. Additional sham-lesioned (n = 10) and lesioned (n = 5) rats received vehicle infusion. Insulin-infused sham-lesioned rats had substantially greater increases in lumbar sympathetic nerve activity (+83 +/- 18%) than vehicle-infused sham-lesioned rats (+27 +/- 4%). Most importantly, insulin-infused lesioned rats had increases in sympathetic activity (+32 +/- 11%) that were no greater than lesioned rats receiving vehicle (+23 +/- 16%). Blood pressure was not altered by insulin or vehicle. To test the possibility that lesions of the anteroventral third ventricle region nonspecifically suppress sympathetic excitatory responses, we evaluated reflex increases in lumbar sympathetic activity to nitroglycerin in sham-lesioned (n = 5) and lesioned (n = 8) rats. Rats with lesions and sham lesions showed comparable increases in lumbar nerve activity during nitroglycerin-induced hypotension. In summary, increases in sympathetic nerve activity to intravenous insulin infusion are abolished by anteroventral third ventricle lesions. These data indicate that the integrity of this brain region is necessary for activation of lumbar sympathetic nerve activity by systemic administration of insulin.

Effects of long-term air jet noise and dietary sodium chloride in borderline hypertensive rats

The hypothesis that simultaneous exposure to a high (8%) sodium chloride diet and behavioral stress (air jet noise) would act synergistically to increase blood pressure was investigated in male borderline hypertensive rats. Rats were fed either a 1% or an 8% sodium chloride diet beginning at 6 weeks of age. Rats in the Air Noise condition were restrained and exposed to random blasts of air jet noise for 2 h/d, 5 d/wk, from 7 to 17 weeks of age. Controls either were placed in identical restrainers and test chambers but not exposed to air jet noise (Restrained Control) or were left undisturbed (Maturation Control). Biweekly indirect blood pressure measurements showed that by 17 weeks of age, the high-sodium chloride diet and air jet noise exposure produced additive increases in blood pressure. Direct blood pressure measurements at 18 weeks of age confirmed the higher systolic pressures in borderline hypertensive rats exposed to both an 8% sodium chloride diet and air jet noise. After ganglionic blockade, the blood pressure of rats in the Air Noise group remained higher than that of Restrained and Maturation Controls, suggesting that the increased blood pressure of air jet noise-exposed rats was not maintained by increased autonomic activity. Blood pressure after maximal vasodilation by hydralazine was increased in rats exposed to both an 8% sodium chloride diet and air jet noise compared with other groups. Baroreceptor reflex sensitivity (tested by graded doses of angiotensin II) did not differ among groups.(ABSTRACT TRUNCATED AT 250 WORDS)

The borderline hypertensive rat (BHR) as a model for environmentally-induced hypertension: a review and update

In recent years, the burgeoning disciplines of health psychology and behavioral medicine have renewed interest in the important role that environmental factors can play in the disease process. Nowhere is this concern more well-founded than in the area of cardiovascular disorders, particularly hypertension. Epidemiologists and clinicians have long suspected that stressful life events can be a sufficient trigger for the expression of hypertension in some individuals. To understand better the ways in which these variables interact in the disease process, researchers have tried, with limited success, to produce experimental hypertension in animals by exposing them to stressful environmental paradigms. Additionally, recent investigations using the borderline hypertensive rat (BHR) have demonstrated the important role genetic factors can play in mediating both the behavioral and cardiovascular responses to environmental stressors. The current paper will review these attempts and discuss recent data from experiments using a relatively new animal model that appears to be especially appropriate for the study of environmental-genetic factors in the elaboration of essential hypertension. We will also discuss potential mechanisms by which environmental stress influences arterial pressure and suggest avenues for further inquiry into the stress-disease relationship.

Glycyrrhetinic acid, an inhibitor of 11 beta-hydroxysteroid dehydrogenase, alters local cerebral glucose utilization in vivo

11 beta-Hydroxysteroid dehydrogenase (11 beta-OHSD) metabolizes corticosterone (B) to inactive 11-dehydrocorticosterone and thus protects the non-specific renal mineralocorticoid receptor from exposure to B in vivo. There is regional 11 beta-OHSD mRNA expression and bioactivity in brain in vitro, but any in vivo function is unknown. We used the [14C]2-deoxyglucose technique in conscious rats to investigate whether 11 beta-OHSD inhibition with glycyrrhetinic acid alters local cerebral metabolic activity. We found increased glucose use in subregions of the hypothalamus, hippocampus, neocortex and subthalamus. Thus, 11 beta-OHSD may play a role in regulating the effects of B in the brain, in vivo.

Role of anteroventral third ventricle and vasopressin in renal response to stress in borderline hypertensive rats

The borderline hypertensive rat is the first filial offspring of the spontaneously hypertensive rat and the Wistar-Kyoto rat. In response to acute environmental stress (air jet), the borderline hypertensive rat exhibits a diuretic response, whereas the parental strains exhibit an antidiuretic response (spontaneously hypertensive rat) or no change in urine flow rate (Wistar-Kyoto rat). This study sought to investigate the role of the periventricular tissue surrounding the anteroventral third ventricle and vasopressin release in the diuretic response of the borderline hypertensive rat to acute environmental stress. Sixteen-week-old borderline hypertensive rats who had consumed a 1% NaCl diet for 10-12 weeks were given either electrolytic lesions of the anteroventral portion of the third ventricle or sham lesions. When exposed to acute environmental stress 4 weeks later, the increase in volume of dilute urine seen in the sham-lesion rats was not observed in the lesion rats. Plasma vasopressin concentration was decreased by acute environmental stress in the sham-lesion rats (15.2 +/- 4.0 to 10.9 +/- 1.7 pg/ml, p less than 0.05) but was unchanged in the lesion rats (12.3 +/- 2.0 to 13.4 +/- 4.0 pg/ml). In a separate group of intact borderline hypertensive rats, a constant intravenous infusion of vasopressin prevented the diuretic response to acute environmental stress. The results suggest that acute environmental stress produces a diuresis in the borderline hypertensive rats via a decrease in plasma vasopressin concentration that is dependent on the integrity of the periventricular tissue of the anteroventral portion of the third ventricle.

Renal manifestations of NaCl sensitivity in borderline hypertensive rats

Compared with the normotensive Wistar-Kyoto rat, the spontaneously hypertensive rat exhibits exaggerated alterations in renal sympathetic nerve activity and excretory function during volume expansion (exaggerated natriuresis) and environmental stress (antinatriuresis). The borderline hypertensive rat is the first filial offspring of the spontaneously hypertensive rat and the Wistar-Kyoto rat and develops hypertension with increased dietary NaCl intake. The present investigation sought to determine whether the dietary NaCl intake-induced transition from the normotensive state of the Wistar-Kyoto parent to the hypertensive state of the spontaneously hypertensive parent in the borderline hypertensive rat was accompanied by a similar transition of the renal sympathetic nerve activity and excretory responses to volume expansion and environmental stress. Borderline hypertensive rats fed a 1% NaCl diet remained normotensive and exhibited renal sympathetic nerve activity and excretory responses to volume expansion and environmental stress that were similar to those of their Wistar-Kyoto parent. Borderline hypertensive rats fed an 8% NaCl diet developed hypertension and exhibited responses that were similar to those of their spontaneously hypertensive parent. Thus, the dietary NaCl intake-induced transition from the normotensive state of the Wistar-Kyoto parent to the hypertensive state of the spontaneously hypertensive parent in the borderline hypertensive rat was accompanied by a similar transition of the renal sympathetic nerve activity and excretory responses to volume expansion and environmental stress. The results suggest that increased dietary NaCl intake is able to induce or unmask the capabilities for these responses, which are genetically conveyed to the borderline hypertensive rat by the spontaneously hypertensive rat parent in latent forms.

Dam strain affects cardiovascular reactivity to acute stress in BHR

The effect of maternal strain on reactivity to acute stress was studied in F1 reciprocals produced by crossing the spontaneously hypertensive rat (SHR) with its normotensive progenitor, the Wistar-Kyoto (WKY). This F1 generation, known as the borderline hypertensive rat (BHR), is genetically predisposed to develop hypertension in response to chronic stress or high dietary sodium. Reciprocals, considered to be genetically equivalent aside from sex-linked traits, differ in strain of dam during intrauterine and preweanling development. At 17 weeks of age, reciprocal F1 males did not differ in open-field behavior (squares crossed, rearings, and defecation measured over 3 days in 15-min sessions) or in home-cage measurements of mean arterial pressure (MAP) and heart rate (HR). However, different patterns of cardiovascular reactivity were displayed to transfer and footshock. While WKY-mothered rats reacted with graded pressor responses, SHR-mothered rats responded maximally to transfer, showed no additional increase to footshock, and maintained peak responding after footshock was terminated. Such reactivity differences may mediate the impact of environmental variables on the genetic disposition to hypertension.

Lesions of the anteroventral third ventricle prevent salt-induced hypertension in the borderline hypertensive rat

Many forms of experimental hypertension depend on the integrity of the periventricular tissue surrounding the anteroventral third ventricle. The current investigation examined the extent to which this forebrain area is necessary for the elaboration of salt-induced hypertension in the borderline hypertensive rat. Eight-week-old male rats were given either electrolytic lesions of the anteroventral portion of the third ventricle region or sham lesions. All rats were then placed on a high salt diet (8% NaCl) for 10 weeks. At the conclusion of this dietary period, direct measurement of resting mean arterial pressure revealed that borderline hypertensive rats with lesions of the anteroventral portion of the third ventricle had significantly lower blood pressure (128.4 +/- 5.1 mm Hg) compared with sham-operated rats (148.1 +/- 4.1 mm Hg).