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Reduced activity of locus coeruleus neurons in hypertensive rats
Neuroscience Letters, 61 (1985) 25-29
25
Elsevier Scientific Publishers Ireland Ltd. NSL 03562 REDUCED ACTIVITY OF LOCUS COERULEUS NEURONS IN HYPERTENSIVE RATS
H.R. OLPE t, K. BERECEK 2, R.S.G. JONES j, M.W. STEINMANN l, Ch. SONNENBURG t and K.G. HOFBAUER t ZResearch Department, Pharmaceuticals Division, Ciba-Geigy Ltd., CH-4002 Basel (Switzerland) and 2Cardiovascular Research and Training Center, University of Alabama in Birmingham, Birmingham, AL 35294 (U.S.A.)
(Received September 26th, 1984; Revised version received July 1lth, 1985; Accepted July 12th, 1985) Key words:
locus coeruleus - blood pressure - spontaneously hypertensive rat - deoxycorticosterone acetate (DOCA-salt) rat
The influence of blood pressure on locus coeruleus (LC) neuronal activity was investigated in chloral hydrate anesthetized rats. Mean spontaneous firing rate of LC neurons was reduced by 25~o in deoxycorticosterone acetate (DOCA-salt) treated Sprague-Dawley rats and by 19~o in spontaneously hypertensive rats (SHR) as compared to their corresponding control animals. In acute experiments performed on normotensive rats, peripherally induced blood pressure changes elicited reciprocal changes in cell firing of the majority of LC neurons. The findings suggest that LC may have a role in long-term regulation of blood pressure. Evidence suggests a c o n t r i b u t i o n o f the p e r i p h e r a l a n d central n o r a d r e n e r g i c system in h y p e r t e n s i o n in s p o n t a n e o u s l y hypertensive ( S H R ) a n d d e o x y c o r t i c o s t e r o n e acetate ( D O C A - s a l t ) hypertensive rats [3]. A p a t h o g e n e t i c role o f n o r a d r e n a l i n e ( N A ) is suggested by the finding that d e p l e t i o n o f whole b r a i n N A prevents hypertension in D O C A - s a l t treated a n d S H R [I]. This n o t i o n is s u p p o r t e d by experiments in which electrical s t i m u l a t i o n o f the locus coeruleus (LC), the m a i n source o f N A o f the brain, has been shown to increase b l o o d pressure (BP) [4, 7, 17, 21]. These findings conflict with a study in which lesioning o f L C by 6 - h y d r o x y d o p a m i n e (6O H D A ) induced an increase in BP [14]. These latter o b s e r v a t i o n s a n d the results o b t a i n e d with central injections o f N A suggest a d e p r e s s a n t role o f central catecholamines on BP [1, 2, 5]. Here we s t u d y the r e a c t i o n o f n e u r o n s o f the L C to acute a n d chronic changes in BP. LC n e u r o n s have been r e p o r t e d to be sensitive to acute p e r i p h e r a l l y induced BP changes [6, 15, 19, 20]. M a l e s p o n t a n e o u s l y hypertensive rats (X = 257 g, 250-305 g S H R ) a n d the corres p o n d i n g W i s t a r - K y o t o , c o n t r o l rats ( W K Y , m e a n b o d y wt., 283 g, 250-310 g) were o b t a i n e d from T N O , Zeist, the N e t h e r l a n d s . M a l e S p r a g u e - D a w l e y rats ( R A I f ( S P F ) Sisseln, Switzerland) were used for e x p e r i m e n t s on D O C A - s a l t hypertension. A n i m a l s were u n i l a t e r a l l y n e p h r e c t o m i z e d u n d e r ether anesthesia 6 weeks p r i o r to the e x p e r i m e n t p r o p e r a n d d i v i d e d into two groups. The e x p e r i m e n t a l g r o u p (n = 7) was given D O C A (200 m g / k g ) in a single, s u b c u t a n e o u s silastic i m p l a n t . The c o n t r o l 0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
26 group received silastic implants without DOCA. All animals were fed on standard chow ad libitum and received a solution containing 0.9~0 NaC1 and 0.2'~o KC1 as drinking fluid, One week prior to the experiment, rats received an indwelling femoral arterial catheter. Twenty-four hours after catheterization, BP was monitored directly in conscious rats. On the day of the experiment, animals were anesthetized with chloral hydrate (400 mg/kg i.p.) and placed in a stereotaxic apparatus. The LC was approached stereotaxically from above (1.7 mm posterior, 1.1 mm lateral, 6.5-7 mm below cortical surface). The incisor bar was set 5 mm above the interaural line. Neuronal activity of single noradrenergic neurons was measured extracellularly by means of conventional electrophysiological techniques [ 16]. Noradrenergic neurons were identified on the basis of several physiological criteria as described previously [16]. Ten neurons were recorded in each rat for periods of 3 min each. Thus 70 neurons were recorded in each group of animals. Hypertensive and the corresponding control rats were studied alternatively on consecutive days. The effects of short-lasting (1-2 min) acute BP changes on LC firing induced by NA (0.03-3 #g) or acetylcholine (ACh) (0.0003-0.3 ktg) administered into the jugular vein was investigated on 39 neurons in 12 rats (Sprague-Dawley) anesthetized with chloral hydrate. In separate experiments the effect of prolonged infusions (10-20 min) of NA (2.5-10 ~g/min) on the firing rate of 17 LC neurons was investigated in 8 Sprague-Dawley rats. BP was monitored by means of a cannula inserted into the carotid artery and recorded on a Grass polygraph. N A and ACh induced changes in BP in both directions of 20-50 mmHg. Measurements of BP in conscious hypertensive and control rats were taken prior to the experiment with the animals placed in a restraining cage. In SHR and W K Y systolic blood pressure was measured by tail plethysmography. The influence of chloral hydrate anesthesia on BP in SHR and W K Y control animals (n = 6) was determined in a separate experiment. Statistical significance of the difference between neuronal firing rates recorded in hypertensive and normotensive animals was assessed by means of the Mann-Whitney U-test [10]. Since both, short and prolonged increases in BP elicited qualitatively identical reTABLE I BLOOD PRESSURE RESPONSIVENESS The responsiveness of LC neurons to acute BP changes induced by i.v. administration of either NA (increase) or ACh (decrease) is depicted. The results were obtained from 56 LC neurons recorded in 20 chloral hydrate-anesthetized rats. Blood pressure
Cellular response inhibited activated no effect
Increased
Decreased
27 5 8
0 10 6
27
suits, the data were pooled in Table I. In normotensive animals BP increases were accompanied by decreases (Fig. 1) in neuronal activity in 27 of the 40 neurons investigated (Table I). Eight neurons were not affected and 5 neurons were reversibly activated. By contrast, BP decreases reversibly activated the majority of neurons tested. In corresponding groups of chloral hydrate-anesthetized rats, mean BP values were considerably lower but the relative difference in BP was not different from that recorded in conscious animals (data not shown). Fig. 2A, B depicts the frequency distribution patterns of LC neurons in hypertensive and control animals. It can be seen that there are less fast firing neurons and correspondingly more slowly firing neurons in hypertensive than in control rats. The mean firing rate of LC neurons amounted to 2.70 + 0.1 in W K Y and 2.2 + 0.1 in SHR representing a reduction of 19~ ( P < 0.01). Similar results were obtained in DOCAsalt hypertensive rats. The mean arterial BP was 192_ 3 mmHg in DOCA-salt and 1174-3.5 mmHg in control rats (P
NA 5 jug/min
200 150
•
•
__
.............
: ........................................
I
I
I
0 J
III
I
0 •
I minute
NA 5 iJg/min 01 -I-
1150
(n 4) O. (n
l minute
Fig. 1. The effect of acute prolonged increases in BP on two LC neurons. The upper panel shows the activity of a neuron which responded with a reduction in firing rate. This response was observed in the majority of neurons investigated. The lower panel shows a neuron reacting with an increase in firing rate. This effect was observed in a few neurons only.
2~
A
B
number of neurons
HI
~
control (WKY)
number of neurons
Sprague Dawlev
coro,
Fig. 2. A: frequency distribution patterns of noradrenergic neurons of LC recorded in 7 control and 7 SHR are depicted. Seventy neurons were recorded for periods of 3 min in each group of rats. B: frequency distribution patterns of LC neurons recorded in 7 control and 7 DOCA-salt hypertensive Sprague-Dawley rats are shown. Seventy neurons were recorded in each group of rats.
in control and 1.4_.0.1 Hz in DOCA-salt rats. This reduction in mean firing rate o / by 5J,, for the D O C A group was significant at the 0.01 level. The present electrophysiological findings demonstrate that the activity of the main group of noradrenergic neurons is affected in two forms of hypertension, one a genetic form and the other an acquired form. The findings are in keeping with an earlier study [20]. These results suggest an inhibitory function of LC on BP. If LC neurons function as a tonic inhibitory system on BP, the reduced activity in hypertensive rats may be one reason why BP increases. These speculations are based on the assumption that the LC is indeed an essential structure for BP regulation. However, it is more likely that the LC belongs to a complicated, widely distributed central system controlling BP. The fact that two pathogenetically different forms of hypertension are accompanied by reduced activity of LC neurons suggests that this effect is rather a consequence than a cause of BP increase indicating that these neurons may represent part of a negative feedback system. It is difficult to relate the present electrophysiological studies to neurochemical data, since these results are controversial [! !--13, 18]. It was interesting to find that the mean spontaneous firing of normotensive W K Y (2.7 +_0.1 Hz) and Sprague-Dawley rats (1.9 + 0. I Hz) differ, suggesting strain differences in the activity of this brain area. It would be premature to draw firm conclusions, but the phenomenon needs to be systematically investigated. t Baum, T. and Shropshire, A.T., Reduction of sympathetic outflow by central administration of LDOPA, dopamine and norepinephrine, Neuropharmacology, 12 (1973) 49 56. 2 Bhargava, K.P., Mishra, N. and Tangri, K.K., An analysis of central adrenoceptors for control of cardiovascular function, Br. J. Pharmacol., 45 (1972) 596-602. 3 Brody, M.J., Haywood, J.R. and Touw, K.B., Neural mechanisms in hypertension, Ann. Rev. Physiol., 42 (1980) 441--453. 4 Chida, K., Kawamura, H. and Hatano, M., Participation of the locus coeruleus in DOCA-salt hypertensive rats, Brain Res.. 273 (1983) 53-58.
29 5 De Jong, W., Noradrenaline: central inhibitory control of blood pressure and heart rate, Eur. J. Pharmacol., 29 (1974) 179-181. 6 Elam, M., Yao, T., Svensson, T.H. and Thoren, P., Regulation of locus coeruleus neurons and splanchnic, sympathetic nerves by cardiovascular afferents, Brain Res., 290 (1984) 281-287. 7 Fallert, M. and Polc, P., Blutdruckeffekte aus dem Locus coeruleus, dem ponto-bulbiiren Raphe-System und der medullfiren Formatio reticularis beim Kaninchen, Arch. Kreislaufforsch., 62 (I 970) 153166. 8 Gianutsos, G. and Moore, K.E., Epinephrine contents of sympathetic ganglia in brain regions of spontaneously hypertensive rats at different ages, Proc. Soc. Exp. Biol. Med., 158 (1978) 137-147. 9 Kawamura, H., Gunn, C.G. and Frrhlich, E.D., Cardiovascular alteration by nucleus locus coeruleus in spontaneously hypertensive rat, Brain Res., 140 (1978) 137-147. 10 Mann, H.B., and Whitney, D.R., On a test of whether one of two random variables is stochastically larger than the other, Ann. Math. Statist., 18 (1947) 50-60. 11 Nagaoka, A. and Lovenberg, W., Regional changes in the activities of the adrenergic biosynthetic enzymes in the brains of hypertensive rats, Eur. J. Pharmacol., 43 (1977) 297-306. 12 Nagatsu, T., Ikuta, K., Numata, Y., Kato, T., Sano, M., Nagatsu, I., Umezawa, H., Matsuzaki, M. and Takeuchi, T., Vascular and brain dopamine-/~ hydroxylase activity in young spontaneously hypertensive rats, Science, 191 (1976) 290-291. 13 Nakamura, K. and Nakamura, K., Role of brainstem spinal noradrenergic and adrenergic neurons in the development and maintenance of hypertension in spontaneously hypertensive rats, NaunynSchmiedeberg's Arch. Pharmacol., 305 (1978) 127-133. 14 Ogawa, M., Fujita, Y. and Ozaki, M., Experimental central hypertension produced by chemical degradation of the locus coeruleus, Jap. Circ. J., 43 (1979) 89-98. 15 Olpe, H.R., Jones, R.S.G. and Hauser, K., Influences of blood pressure on locus coeruleus activity, Neurosci. Lett., Suppl. 14 (1983) $267. 16 Olpe, H.R., Jones, R.S.G. and Steinmann, M.W., The locus coeruleus: actions of psychoactive drugs, Experientia, 39 (1983) 242-249. 17 Przuntek, H. and Phillipu, A., Reduced pressor responses to stimulation of the locus coeruleus after lesion of the posterior hypothalamus, Naunyn-Schmiedeberg's Arch. Pharmacol., 276 (1973) 119-122. 18 Saavedra, J.M., Grobecker, H. and Axelrod, J., Changes in central catecholinergic neurons in the spontaneously (genetic) hypertensive rat, Circ. Res., 42 (1978) 529-534. 19 Svensson, T.H. and Thor~n, P., Brain noradrenergic neurons in the locus coeruleus; inhibition by blood volume load through vagal afferents, Brain Res., 172 (1979) 174-178. 20 Svensson, T.H., Engberg, G. and Thorrn, P., Activity of brain noradrenergic neurons in spontaneously hypertensive rats, Acta Physiol. Scand., Suppl. 473 (1979) 12. 21 Ward, D.G. and Gunn, C.G., Locus coeruleus complex: elicitation of a pressor response and a brain stem region necessary for its occurrence, Brain Res., 107 (1976) 401-406.
25
Elsevier Scientific Publishers Ireland Ltd. NSL 03562 REDUCED ACTIVITY OF LOCUS COERULEUS NEURONS IN HYPERTENSIVE RATS
H.R. OLPE t, K. BERECEK 2, R.S.G. JONES j, M.W. STEINMANN l, Ch. SONNENBURG t and K.G. HOFBAUER t ZResearch Department, Pharmaceuticals Division, Ciba-Geigy Ltd., CH-4002 Basel (Switzerland) and 2Cardiovascular Research and Training Center, University of Alabama in Birmingham, Birmingham, AL 35294 (U.S.A.)
(Received September 26th, 1984; Revised version received July 1lth, 1985; Accepted July 12th, 1985) Key words:
locus coeruleus - blood pressure - spontaneously hypertensive rat - deoxycorticosterone acetate (DOCA-salt) rat
The influence of blood pressure on locus coeruleus (LC) neuronal activity was investigated in chloral hydrate anesthetized rats. Mean spontaneous firing rate of LC neurons was reduced by 25~o in deoxycorticosterone acetate (DOCA-salt) treated Sprague-Dawley rats and by 19~o in spontaneously hypertensive rats (SHR) as compared to their corresponding control animals. In acute experiments performed on normotensive rats, peripherally induced blood pressure changes elicited reciprocal changes in cell firing of the majority of LC neurons. The findings suggest that LC may have a role in long-term regulation of blood pressure. Evidence suggests a c o n t r i b u t i o n o f the p e r i p h e r a l a n d central n o r a d r e n e r g i c system in h y p e r t e n s i o n in s p o n t a n e o u s l y hypertensive ( S H R ) a n d d e o x y c o r t i c o s t e r o n e acetate ( D O C A - s a l t ) hypertensive rats [3]. A p a t h o g e n e t i c role o f n o r a d r e n a l i n e ( N A ) is suggested by the finding that d e p l e t i o n o f whole b r a i n N A prevents hypertension in D O C A - s a l t treated a n d S H R [I]. This n o t i o n is s u p p o r t e d by experiments in which electrical s t i m u l a t i o n o f the locus coeruleus (LC), the m a i n source o f N A o f the brain, has been shown to increase b l o o d pressure (BP) [4, 7, 17, 21]. These findings conflict with a study in which lesioning o f L C by 6 - h y d r o x y d o p a m i n e (6O H D A ) induced an increase in BP [14]. These latter o b s e r v a t i o n s a n d the results o b t a i n e d with central injections o f N A suggest a d e p r e s s a n t role o f central catecholamines on BP [1, 2, 5]. Here we s t u d y the r e a c t i o n o f n e u r o n s o f the L C to acute a n d chronic changes in BP. LC n e u r o n s have been r e p o r t e d to be sensitive to acute p e r i p h e r a l l y induced BP changes [6, 15, 19, 20]. M a l e s p o n t a n e o u s l y hypertensive rats (X = 257 g, 250-305 g S H R ) a n d the corres p o n d i n g W i s t a r - K y o t o , c o n t r o l rats ( W K Y , m e a n b o d y wt., 283 g, 250-310 g) were o b t a i n e d from T N O , Zeist, the N e t h e r l a n d s . M a l e S p r a g u e - D a w l e y rats ( R A I f ( S P F ) Sisseln, Switzerland) were used for e x p e r i m e n t s on D O C A - s a l t hypertension. A n i m a l s were u n i l a t e r a l l y n e p h r e c t o m i z e d u n d e r ether anesthesia 6 weeks p r i o r to the e x p e r i m e n t p r o p e r a n d d i v i d e d into two groups. The e x p e r i m e n t a l g r o u p (n = 7) was given D O C A (200 m g / k g ) in a single, s u b c u t a n e o u s silastic i m p l a n t . The c o n t r o l 0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
26 group received silastic implants without DOCA. All animals were fed on standard chow ad libitum and received a solution containing 0.9~0 NaC1 and 0.2'~o KC1 as drinking fluid, One week prior to the experiment, rats received an indwelling femoral arterial catheter. Twenty-four hours after catheterization, BP was monitored directly in conscious rats. On the day of the experiment, animals were anesthetized with chloral hydrate (400 mg/kg i.p.) and placed in a stereotaxic apparatus. The LC was approached stereotaxically from above (1.7 mm posterior, 1.1 mm lateral, 6.5-7 mm below cortical surface). The incisor bar was set 5 mm above the interaural line. Neuronal activity of single noradrenergic neurons was measured extracellularly by means of conventional electrophysiological techniques [ 16]. Noradrenergic neurons were identified on the basis of several physiological criteria as described previously [16]. Ten neurons were recorded in each rat for periods of 3 min each. Thus 70 neurons were recorded in each group of animals. Hypertensive and the corresponding control rats were studied alternatively on consecutive days. The effects of short-lasting (1-2 min) acute BP changes on LC firing induced by NA (0.03-3 #g) or acetylcholine (ACh) (0.0003-0.3 ktg) administered into the jugular vein was investigated on 39 neurons in 12 rats (Sprague-Dawley) anesthetized with chloral hydrate. In separate experiments the effect of prolonged infusions (10-20 min) of NA (2.5-10 ~g/min) on the firing rate of 17 LC neurons was investigated in 8 Sprague-Dawley rats. BP was monitored by means of a cannula inserted into the carotid artery and recorded on a Grass polygraph. N A and ACh induced changes in BP in both directions of 20-50 mmHg. Measurements of BP in conscious hypertensive and control rats were taken prior to the experiment with the animals placed in a restraining cage. In SHR and W K Y systolic blood pressure was measured by tail plethysmography. The influence of chloral hydrate anesthesia on BP in SHR and W K Y control animals (n = 6) was determined in a separate experiment. Statistical significance of the difference between neuronal firing rates recorded in hypertensive and normotensive animals was assessed by means of the Mann-Whitney U-test [10]. Since both, short and prolonged increases in BP elicited qualitatively identical reTABLE I BLOOD PRESSURE RESPONSIVENESS The responsiveness of LC neurons to acute BP changes induced by i.v. administration of either NA (increase) or ACh (decrease) is depicted. The results were obtained from 56 LC neurons recorded in 20 chloral hydrate-anesthetized rats. Blood pressure
Cellular response inhibited activated no effect
Increased
Decreased
27 5 8
0 10 6
27
suits, the data were pooled in Table I. In normotensive animals BP increases were accompanied by decreases (Fig. 1) in neuronal activity in 27 of the 40 neurons investigated (Table I). Eight neurons were not affected and 5 neurons were reversibly activated. By contrast, BP decreases reversibly activated the majority of neurons tested. In corresponding groups of chloral hydrate-anesthetized rats, mean BP values were considerably lower but the relative difference in BP was not different from that recorded in conscious animals (data not shown). Fig. 2A, B depicts the frequency distribution patterns of LC neurons in hypertensive and control animals. It can be seen that there are less fast firing neurons and correspondingly more slowly firing neurons in hypertensive than in control rats. The mean firing rate of LC neurons amounted to 2.70 + 0.1 in W K Y and 2.2 + 0.1 in SHR representing a reduction of 19~ ( P < 0.01). Similar results were obtained in DOCAsalt hypertensive rats. The mean arterial BP was 192_ 3 mmHg in DOCA-salt and 1174-3.5 mmHg in control rats (P
NA 5 jug/min
200 150
•
•
__
.............
: ........................................
I
I
I
0 J
III
I
0 •
I minute
NA 5 iJg/min 01 -I-
1150
(n 4) O. (n
l minute
Fig. 1. The effect of acute prolonged increases in BP on two LC neurons. The upper panel shows the activity of a neuron which responded with a reduction in firing rate. This response was observed in the majority of neurons investigated. The lower panel shows a neuron reacting with an increase in firing rate. This effect was observed in a few neurons only.
2~
A
B
number of neurons
HI
~
control (WKY)
number of neurons
Sprague Dawlev
coro,
Fig. 2. A: frequency distribution patterns of noradrenergic neurons of LC recorded in 7 control and 7 SHR are depicted. Seventy neurons were recorded for periods of 3 min in each group of rats. B: frequency distribution patterns of LC neurons recorded in 7 control and 7 DOCA-salt hypertensive Sprague-Dawley rats are shown. Seventy neurons were recorded in each group of rats.
in control and 1.4_.0.1 Hz in DOCA-salt rats. This reduction in mean firing rate o / by 5J,, for the D O C A group was significant at the 0.01 level. The present electrophysiological findings demonstrate that the activity of the main group of noradrenergic neurons is affected in two forms of hypertension, one a genetic form and the other an acquired form. The findings are in keeping with an earlier study [20]. These results suggest an inhibitory function of LC on BP. If LC neurons function as a tonic inhibitory system on BP, the reduced activity in hypertensive rats may be one reason why BP increases. These speculations are based on the assumption that the LC is indeed an essential structure for BP regulation. However, it is more likely that the LC belongs to a complicated, widely distributed central system controlling BP. The fact that two pathogenetically different forms of hypertension are accompanied by reduced activity of LC neurons suggests that this effect is rather a consequence than a cause of BP increase indicating that these neurons may represent part of a negative feedback system. It is difficult to relate the present electrophysiological studies to neurochemical data, since these results are controversial [! !--13, 18]. It was interesting to find that the mean spontaneous firing of normotensive W K Y (2.7 +_0.1 Hz) and Sprague-Dawley rats (1.9 + 0. I Hz) differ, suggesting strain differences in the activity of this brain area. It would be premature to draw firm conclusions, but the phenomenon needs to be systematically investigated. t Baum, T. and Shropshire, A.T., Reduction of sympathetic outflow by central administration of LDOPA, dopamine and norepinephrine, Neuropharmacology, 12 (1973) 49 56. 2 Bhargava, K.P., Mishra, N. and Tangri, K.K., An analysis of central adrenoceptors for control of cardiovascular function, Br. J. Pharmacol., 45 (1972) 596-602. 3 Brody, M.J., Haywood, J.R. and Touw, K.B., Neural mechanisms in hypertension, Ann. Rev. Physiol., 42 (1980) 441--453. 4 Chida, K., Kawamura, H. and Hatano, M., Participation of the locus coeruleus in DOCA-salt hypertensive rats, Brain Res.. 273 (1983) 53-58.
29 5 De Jong, W., Noradrenaline: central inhibitory control of blood pressure and heart rate, Eur. J. Pharmacol., 29 (1974) 179-181. 6 Elam, M., Yao, T., Svensson, T.H. and Thoren, P., Regulation of locus coeruleus neurons and splanchnic, sympathetic nerves by cardiovascular afferents, Brain Res., 290 (1984) 281-287. 7 Fallert, M. and Polc, P., Blutdruckeffekte aus dem Locus coeruleus, dem ponto-bulbiiren Raphe-System und der medullfiren Formatio reticularis beim Kaninchen, Arch. Kreislaufforsch., 62 (I 970) 153166. 8 Gianutsos, G. and Moore, K.E., Epinephrine contents of sympathetic ganglia in brain regions of spontaneously hypertensive rats at different ages, Proc. Soc. Exp. Biol. Med., 158 (1978) 137-147. 9 Kawamura, H., Gunn, C.G. and Frrhlich, E.D., Cardiovascular alteration by nucleus locus coeruleus in spontaneously hypertensive rat, Brain Res., 140 (1978) 137-147. 10 Mann, H.B., and Whitney, D.R., On a test of whether one of two random variables is stochastically larger than the other, Ann. Math. Statist., 18 (1947) 50-60. 11 Nagaoka, A. and Lovenberg, W., Regional changes in the activities of the adrenergic biosynthetic enzymes in the brains of hypertensive rats, Eur. J. Pharmacol., 43 (1977) 297-306. 12 Nagatsu, T., Ikuta, K., Numata, Y., Kato, T., Sano, M., Nagatsu, I., Umezawa, H., Matsuzaki, M. and Takeuchi, T., Vascular and brain dopamine-/~ hydroxylase activity in young spontaneously hypertensive rats, Science, 191 (1976) 290-291. 13 Nakamura, K. and Nakamura, K., Role of brainstem spinal noradrenergic and adrenergic neurons in the development and maintenance of hypertension in spontaneously hypertensive rats, NaunynSchmiedeberg's Arch. Pharmacol., 305 (1978) 127-133. 14 Ogawa, M., Fujita, Y. and Ozaki, M., Experimental central hypertension produced by chemical degradation of the locus coeruleus, Jap. Circ. J., 43 (1979) 89-98. 15 Olpe, H.R., Jones, R.S.G. and Hauser, K., Influences of blood pressure on locus coeruleus activity, Neurosci. Lett., Suppl. 14 (1983) $267. 16 Olpe, H.R., Jones, R.S.G. and Steinmann, M.W., The locus coeruleus: actions of psychoactive drugs, Experientia, 39 (1983) 242-249. 17 Przuntek, H. and Phillipu, A., Reduced pressor responses to stimulation of the locus coeruleus after lesion of the posterior hypothalamus, Naunyn-Schmiedeberg's Arch. Pharmacol., 276 (1973) 119-122. 18 Saavedra, J.M., Grobecker, H. and Axelrod, J., Changes in central catecholinergic neurons in the spontaneously (genetic) hypertensive rat, Circ. Res., 42 (1978) 529-534. 19 Svensson, T.H. and Thor~n, P., Brain noradrenergic neurons in the locus coeruleus; inhibition by blood volume load through vagal afferents, Brain Res., 172 (1979) 174-178. 20 Svensson, T.H., Engberg, G. and Thorrn, P., Activity of brain noradrenergic neurons in spontaneously hypertensive rats, Acta Physiol. Scand., Suppl. 473 (1979) 12. 21 Ward, D.G. and Gunn, C.G., Locus coeruleus complex: elicitation of a pressor response and a brain stem region necessary for its occurrence, Brain Res., 107 (1976) 401-406.