(Hypertension. 1995;26:1125-1128.)
© 1995 American Heart Association, Inc.
Articles |
Role of Periaqueductal Gray Matter in Hypertension in Spontaneously Hypertensive Rats
From the Department of Physiological Sciences, Biomedical Center, UFES, Vitória, Brazil.
Correspondence to Dr Luiz Carlos Schenberg, Department of Physiological Sciences, Biomedical Center, UFES, Av. Marechal Campos 1468 (Maruípe), 29040-090 Vitória, ES, Brazil.
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Abstract |
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Abstract We performed experiments to study the effects of electrolytic lesions of periaqueductal gray matter on mean blood pressure, heart rate, and cardiac baroreflex in adult male spontaneously hypertensive rats. Cardiac baroreflex was assessed by the administration of randomly assigned doses of phenylephrine (0.3 to 5.0 µg/kg IV) or sodium nitroprusside (1.5 to 50 µg/kg IV) to unanesthetized rats. Bilateral lesions of the periaqueductal gray matter (0.5 mA/5 s) were then performed with rats under sodium pentobarbital anesthesia (35 mg/kg IP). Twenty hours after lesion, cardiac baroreflex was retested. Baroreflex data were analyzed by sigmoidal curve fitting. Lesion rats (n=12) showed a significant decrease in both the gain (
=-0.89±0.38
beats per minute [bpm]/mm Hg, P<.05) and curve midpoint
(=-15±6 mm Hg, P<.05) of the cardiac baroreflex.
Moreover, despite a moderate increase in heart rate (=34±10 bpm,
P<.01), resting mean blood pressure was significantly
decreased 24 hours after the lesions (=-19±5 mm Hg,
P<.01). No significant changes in cardiac baroreflex were
observed in sham-lesion rats (n=12). Histological
examination showed circumscribed bilateral damage of dorsolateral
periaqueductal gray matter. Dorsolateral periaqueductal gray matter is
an area of the brain putatively related to fear and anxiety. It also
projects onto premotor sympathetic neurons in the medulla. Although
electrolytic lesions damage neurons as well as fibers of passage, these
data suggest that dorsolateral periaqueductal gray matter has a far
greater influence on resting cardiovascular control in
spontaneously hypertensive rats than was previously suspected.
Key Words: rats, inbred, SHR • periaqueductal gray • baroreflex
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Introduction |
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The dorsolateral division of the PAG is an area of the brain putatively related to anxiety, fear, and aggressiveness.1 2 3 In rats, PAG stimulation produces a patterned cardiovascular response characterized by sympathetic overactivity.4 This response has been termed the cardiovascular defense reaction. The sympathetic overactivity that is part of the defense reaction has long been considered to contribute to cardiovascular disorders and hypertension.5 6 However, no increase or negligible increases in resting BP were found after long-term stimulation of the hypothalamic defense areas of Wistar-Kyoto rats.7 8 Likewise, experiments carried out in SHR did not provide consistent results9 10 : SHR sympathetic nerve activity showed an increased responsiveness to stimulation of the hypothalamus,9 but lesions of the dorsomedial PAG of juvenile SHR did not prevent the establishment of hypertension.10 Nevertheless, it has been shown that lesions of pontomedullary ventrolateral PAG are effective in reducing resting BP in awake normotensive cats.11
Stimulation of dorsolateral and ventrolateral PAG has been shown to exert sympathoexcitatory and sympathoinhibitory actions, respectively.12 13 14 15 There is indeed a rich interplay of connections between the PAG and medullary areas involved in cardiovascular control. Thus, whereas the dorsolateral PAG sends excitatory projections to premotor sympathetic neurons of the rostroventrolateral medulla,13 14 15 the ventrolateral PAG, which seems to inhibit the dorsolateral PAG,13 receives afferents from the nucleus tractus solitarius.16 Finally, it is a well-known fact that both the cardiac and vasomotor components of the baroreceptor reflex are modulated by the stimulation of defense areas, particularly those in the hypothalamus and PAG.4 However, the involvement of the PAG in the tonic and reflex control of resting BP in SHR has not been tested thus far. As a preliminary study we sought to investigate the effects of electrolytic or sham lesions of dorsolateral PAG on mean BP and HR resting levels as well as on cardiac baroreflex in SHR.
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Methods |
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The femoral arteries and veins of adult male SHR (250 to 300 g) were cannulated with rats under ether anesthesia. The catheters were tunneled under the skin and exposed through a hole at the nape. After a recovery period of 8 to 12 hours the rats were instrumented for BP and HR recordings and left undisturbed for 30 minutes to acclimate to the environment. For assessment of cardiac baroreflex, randomly assigned doses of either phenylephrine (0.3 to 5.0 µg/kg IV, Sigma Chemical Co) or sodium nitroprusside (1.5 to 50 µg/kg IV, Sigma) were administered. Overall, 15 doses or so were used for the determination of each barocurve. Immediately thereafter, rats were anesthetized with sodium pentobarbital (35 mg/kg IP, Cristalia Laboratories) and fixed in a stereotaxic frame (David Kopf Instruments). Stainless steel electrodes (0.3-mm OD), insulated in all length except at the cross section of the tip, were placed into the PAG at a 10° angle to avoid rupturing the sagittal sinus. Bilateral electrolytic lesions (0.5 mA/5 s anodal current) of the dorsolateral PAG were then performed (n=12). BP and HR recordings as well as cardiac baroreflex evaluations were repeated 24 hours after the lesions. Analogous procedures were carried out in sham-lesion SHR (n=12) except that no current was passed.
At the end of the experiments the brains were perfused sequentially with 0.9% NaCl and 0.9% NaCl plus 10% formaldehyde fixative. The brains were removed and sectioned (60 µm thick) in a freezing microtome (Ernst Leitz). Fresh sections were then mounted uncovered on glass slides, placed on the stage of a photographic enlarger, and reproduced, as if they were negatives, on photographic paper. The contour of the lesions was plotted on diagrams drawn from the rat brain atlas.17
Maximal increases or decreases in HR and corresponding changes in BP after phenylephrine and nitroprusside injections were computed. HR barocurves in response to changes in BP were fitted according to Marquardt's algorithm for least-squares estimation of nonlinear parameters.18 In our case, symmetrical sigmoid barocurves were fitted according to the logistic model19 :
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where Y is the maximal value of F(x); ß is the curvature parameter; and X50 is the mean value. Accordingly, sigmoid barocurves can be fitted as
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where B is the maximal bradycardic response; R is the HR
response range (if T is the maximal tachycardic response, then
R=T-B); ß is the parameter that governs
the slope of the barocurve, ie, the gain of the baroreflex; BP is
the maximal change in BP after drug injection; and
BP50, or reflex midpoint, is the BP level at which
the baroreflex shows its maximal gain (the BP level that corresponds to
the point of inflection of the sigmoid). While the maximal gain of the
reflex, ie, the slope at the point of inflection of the sigmoid, is
G=-ß/4; the average gain, which is the slope of the curve
between 0.2 and 0.8 R, is Gm=-ß/4.56. The equation
can thus be rearranged as shown below.
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This method provides least-squares estimates of Gm and BP50. The reflex set point is defined as the point on the curve that reflects the resting levels of HR and BP. Prelesion and postlesion data and parameters were compared with the use of Student's paired t test. Results are reported as mean ±SEM. Differences were considered significant at a value of P<.05.
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Results |
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Before surgery, BP values in sham- and PAG-lesion SHR groups were 173±5 and 168±4 mm Hg, respectively. Resting HR levels were 356±9 and 340±4 bpm, respectively. Despite a moderate but significant increase in resting HR (
=34±10 bpm, P<.01), resting BP
was significantly decreased 24 hours after the lesions
(=-19±5 mm Hg, P<.01) (Table, Figs 1 and 2). Furthermore, the
PAG-lesion group showed a significant decrease in both the gain
(=-0.89±0.38 bpm/mm Hg, P<.05) and the midpoint
(=-15±6 mm Hg, P<.05) of the cardiac baroreflex,
displacing the set point of the latter to the left (Table, Fig 3). The bradycardic plateau of the PAG-lesion group also
increased significantly (=36±8 bpm, P<.005). No
significant changes were observed in either the tachycardic plateau or
reflex range of the PAG-lesion group. Finally, no changes were observed
in any parameter of the sham-lesion group (Table).
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Histological examination of the brains revealed that 10 of 12 PAG-lesion rats presented circumscribed bilateral damage of intermediate and caudal levels of dorsolateral PAG and deep collicular layers exclusively. In 2 rats lateral PAG was also damaged to some extent (Fig 4).
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Discussion |
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An increase in resting HR of approximately 10% was observed 24 hours after lesioning of dorsolateral PAG in SHR. Despite this, a reduction of 13% was observed in resting BP. Consequently, a decrease in peripheral resistance rather than in cardiac output was likely to be the cause of the decrease in resting BP. It should be noted, however, that a decrease in cardiac output owing to a reduction in systolic volume could not be discarded on the basis of the present data.
The gain of cardiac baroreflex was markedly attenuated after the lesions in the dorsolateral PAG. In fact, whereas an average gain of -3.08 bpm/mm Hg was observed before lesions, a gain of only -2.19 bpm/mm Hg was found 24 hours after lesions. Accordingly, a decrease in BP of 19 mm Hg should produce an increase in HR of 42 bpm the day after the lesions. This figure is quite similar to the observed increase in HR of 34±10 bpm. The attenuation of the baroreflex gain thus seems to be the most probable cause of the increase in resting HR as well as of the elevation of the bradycardic plateau of the postlesion barocurve. In line with the present data, it has been shown that suitable electrical stimuli applied to PAG have the opposite effect; ie, they produce either a leftward shift of the cardiac barocurve4 or an increase in the gain of the vasomotor barocurve.15 Even though we did not evaluate the vasomotor counterpart of the reflex, the change in the cardiac baroreflex we observed would more likely favor an increase rather than a decrease in resting BP. Accordingly, the decrease in the gain of the cardiac baroreflex further supports the assumption that the hypotensive effect of the lesion in the dorsolateral PAG does not rely on the changes in baroreflex activity.
Finally, considering the putative involvement of dorsolateral PAG in fearlike behaviors, it could be argued that the hypotensive effect of the lesion was due to the attenuation of postsurgery stress. It should be noted, however, that no difference was found between presurgery and postsurgery BP levels in sham-lesion SHR. Therefore, postsurgery stress does not seem to contribute significantly to resting BP levels the day after surgery.
The development in the last decades of genetically prone strains of hypertensive rats has produced a remarkable explosion of experimental data on the pathophysiology of hypertension. Regarding the SHR strain, both cardiovascular and behavioral responses to environmental stress seem to be augmented.20 21 22 23 24 25 26 Nonetheless, the contribution of the defense areas to tonic and reflex control of BP in SHR remains unclear. Thus, although there are several studies on the role of these areas in SHR reactivity to stressful stimuli, few studies have investigated the effects of lesions of these areas on tonic and reflex control of resting BP in nonstressed awake SHR.
The present study identified the dorsolateral PAG as a relevant area contributing to the hypertension of nonstressed SHR. In fact, previous studies have shown that lesions of defense areas such as the central nucleus of the amygdala27 28 and the dorsolateral PAG10 could attenuate the development of hypertension in SHR. Yet, the stress involved in tail-cuff plethysmography prevented conclusive interpretation of these data.10 Our experiments were carried out on freely moving SHR bearing indwelling cannulas, thus avoiding the stressful maneuvers of tail-cuff plethysmography. Therefore, a decrease in environmental stress does not seem to be the cause of the hypotensive effect of the lesions in dorsolateral PAG. Furthermore, whereas the present data assign a role of PAG in the reflex control of resting BP, the hypotensive effect of its lesion was unlikely to be due to the change in baroreflex function. Notably, a quite similar reduction in resting BP was reported after lesions of pontomedullary ventrolateral PAG of normotensive freely moving cats.11 Together, the present study and the previous one11 suggest a functional relationship between dorsolateral and pontomedullary ventrolateral PAG. Although electrolytic lesions damage both neurons and fibers of passage, the data thus far reported suggest that dorsolateral PAG has a far greater influence on resting BP levels in nonstressed SHR than was previously suspected.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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Research support was provided by the Brazilian Council for the Development of Science and Technology (CNPq) and Brazilian Agency for the Funding of Studies and Projects (FINEP), Brazil.
Received June 18, 1995; first decision August 18, 1995; accepted September 21, 1995.
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References |
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