Neuroscience Letters, 110 (1990) 91-96
91
Elsevier Scientific Publishers Ireland Ltd.
NSL 06679
Vasomotor neurons in the rostral ventrolateral medulla are organized topographically with respect to type of vascular bed but not body region R . M . McAllen 1 a n d R . A . L . D a m p n e y 2 1Howard FIorey Institute of Physiology and Experimental Medicine, University of Melbourne, Melbourne, Vic. (Australia) and 2Department of Physiology, University of Sydney, Sydney, N.S.W. (Australia) (Received 16 June 1989; Revised version received 24 October 1989; Accepted 25 October 1989)
Key words." Medulla oblongata; Blood pressure regulation; Sympathetic activity; Muscle blood flow; Renal nerve; Microinjection Microinjections of sodium glutamate (200-800 pmol) were made into sites within the rostral ventrolateral medulla of anaesthetized cats in which the adrenal glands had been removed and the baroreceptors denervated, while arterial pressure, renal sympathetic nerve activity and hindlimb and forelimb blood flows were measured. Hindlimb and forelimb vascular resistances were affected in a parallel fashion by injections at different sites, but there was a clear dissociation between evoked changes in renal nerve activity and either forelimb or hindlimb vascular resistance. The results indicate that ventrolateral medullary vasomotor neurons are organized topographically with respect to the type, rather than body position, of the vascular beds they control.
Neurons of the rostral ventrolateral medulla are known to play a major role in the tonic and reflex control of blood pressure [3, 6, 11]. Inhibition or destruction of these neurons causes a large fall in arterial pressure, while excitation produces a rise in blood pressure due to sympathetic vasoconstriction [3, 6]. Neurons in the pressor part of the rostral ventrolateral medulla, which has been termed the subretrofacial (SRF) nucleus, drive the sympathetic output to the heart, blood vessels and adrenal medulla, but apparently not that to non-cardiovascular tissues [9]. Recent work has indicated that different sympathetic vasomotor nerves may be activated selectively from different parts of the SRF nucleus. For example, Dampney and McAllen [4] found that small injections of L-glutamate into the medial part of the SRF nucleus selectively activated cutaneous vasoconstrictor fibres, while more lateral injections activated muscle vasoconstrictor fibres. Similarly, Lovick [8] found that stimulation of SRF neurons at different rostrocaudal levels produced different
Correspondence." R.A.L. Dampney, Department of Physiology, University of Sydney, Sydney, N,S.W. 2006, Australia. 0304-3940/90/$ 03.50 © 1990 Elsevier Scientific Publishers Ireland Ltd.
92 patterns of changes in renal, mesenteric and hindlimb vascular beds. These different patterns of regional vascular changes evoked by stimulation of SRF neurons cannot simply be attributed to secondary influences (e.g. baroreceptor reflexes or respiratory changes) since they are still observed following baroreceptor denervation and are independent of respiratory events [4]. It is still uncertain, however, what is the principle underlying the topographic organization of the SRF nucleus. Is it the type of vascular bed on which an SRF cell acts (e.g. skeletal muscle, kidney etc.) or is it the location of the bed within the body (i.e. the spinal segmental origin of its innervation)? We report here an experiment designed to distinguish between these two possibilities. In animals with the vagi, aortic and carotid sinus nerves sectioned and the adrenal glands removed (to eliminate secondary effects arising from baroreceptor reflexes and circulating catecholamines), we compared the vasomotor effects in forelimb and hindlimb skeletal muscle (same type of vascular bed, but innervated by widely separated spinal segments) with those in the kidney (different type, but innervated from intermediate spinal segments), evoked by small injections of glutamate at different sites in the SRF nucleus. If the organizing principle of the SRF nucleus is the type of vascular bed, forelimb and hindlimb responses should be well correlated with each other, regardless of the rostrocaudal position of the injection site; if it is spinal segmental level, the limbs should be separately represented, with renal vascular responses evoked preferentially from sites in between. Four cats were anaesthetized with chloralose (70 mg/kg, i.v.) following induction with halothane. The trachea was cannulated, and the femoral vein and artery on the right side were cannulated. The animals were ventilated with oxygen-enriched air, and the end-tidal CO2 monitored and maintained close to 4%. Both adrenal glands were removed via a retroperitoneal approach, with care being taken to preserve the splanchnic nerves and their connections to the kidney. With the head held supine in a stereotaxic frame, the medulla was exposed by a ventral approach as previously described [9]. In all 4 experiments an electromagnetic flow probe (Narco Bio-systems) was placed on the left iliac artery for the measurement of hindlimb blood flow, and in 3 experiments a flow probe was also placed on the left subclavian artery for the measurement of forelimb blood flow. Snares were placed downstream to the probes to allow zero flow readings to be checked at intervals. In all experiments but one, the circulation to the paws of these limbs was occluded with ligatures. The renal nerve on the same side was cut close to the hilum; its activity was recorded on a pair of silver wire electrodes, filtered (100-1000 Hz bandpass), rectified and integrated (reset every 5 s). Renal nerve fibres behave in a functionally homogeneous manner, and renal nerve activity is believed to be an accurate measure of sympathetic vasoconstrictor output to the kidney [5]. Both vagosympathetic trunks (including the aortic nerve) and carotid sinus nerves were cut. Blood pressure, left hindlimb and forelimb blood flows, integrated left renal nerve activity and heart rate were recorded on a chart recorder. Injections ( 1 4 nl) of a solution of 0.2 M sodium glutamate (pH 7.4) containing a small amount of wheat germ agglutinin-horseradish peroxidase (final concentration
93 0.01%) were made into different sites within the ventral medulla via a micropipette held in a micromanipulator. The tip of the micropipette was positioned 0.7 mm below the surface in each case. The injections were made by air pressure and the volume monitored microscopically as previously described [4]. At the end of the experiments the animals were perfused transcardially with heparinized saline followed by a mixture of 1% paraformaldehyde and 1.25% glutaraldehyde. The centres of injection sites were reconstructed from 50/tm frozen sections processed according to the tetramethylbenzidine procedure [10] and counterstained with neutral red. To quantitate the effects of each glutamate microinjection on cardiovascular variables, the maximum changes (calculated as a percentage of the pre-injection levels) in hindlimb vascular resistance, forelimb vascular resistance and integrated renal nerve activity within 30 s following the injection were calculated. The baseline stability of these variables was determined by measuring, in each case, their relative standard deviation during the 30 s period immediately before each injection. A total of 35 injections were made into the region of the left (ipsilateral) SRF nucleus. In 26 cases a positive response was elicited (defined as a 25% or greater increase in hindlimb or forelimb vascular resistance, or integrated renal nerve activity, above pre-injection levels). The rostrocaudal positions of these 26 sites were mapped with respect to the caudal pole of the facial nucleus (CP7). The centres of the injection sites at which significant increases in skeletal muscle vascular resistance were produced extended from the level 0.6 mm rostral to CP7 to a distance of 1.4 mm caudal to CP7, for both the forelimb and hindlimb beds. In comparison, the centres of injection sites at which significant increases in renal nerve activity were produced extended from the level 0.9 mm rostral to CP7 to the level 1.0 mm caudal to CP7. Thus, although there was considerable overlap, the renal vasoconstrictor sites tended to be located more rostrally than those sites affecting the limbs. In 20 of the 26 cases where positive responses were obtained, forelimb and hindlimb blood flows were measured simultaneously. At 18 sites, glutamate microinjection elicited increases of 25% or more in both forelimb and hindlimb vascular resistance (e.g. Fig. 1), at one site only hindlimb resistance increased significantly (by 37%) and at the remaining site neither increased significantly. The relative magnitudes of forelimb and hindlimb responses differed between experiments, but maintained a fairly constant ratio within each individual animal (Figs. 1, 2A). Renal nerve activity was measured in all 26 cases where glutamate elicited a positive response. In 18 cases, renal nerve activity increased by 25% or more (e.g. Fig. 1B), but at the remainder renal nerve activity was little affected (e.g. Fig. 1A). The lack of response at such sites was not due to the fact that the basal renal nerve activity was already at a maximal level, since in all such cases increases above this basal level were readily evoked by stimulation of other sites in the same experiment. The relationship between evoked changes in hindlimb vascular resistance, forelimb vascular resistance and integrated renal sympathetic nerve activity (measured as a percentage of the pre-injection level) was examined in detail, by determining the rank correlation coefficient (rs) for different combinations of paired measurements [7]. The degree of correlation between the increase in renal nerve activity and the increase in
94
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B
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~
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CALCULATED 200 1 VASCULAR RESISTANCE (% control) 100 4 MEAN ARTERIAL PRESSURE (mmHg)
2°°1 5O
HINDLIMB FLOW (ml/min)
• HINDLIMB ~~ u FORELIMB
i
1 min
Fig. I. Examples, from the same experiment, of the cardiovascular effects of microinjection of sodium glutamate (0.2 M solution) into sites within the SRF nucleus. The volume of the injection was I nl in (A) and 4 nl in (B). Note the parallelism between the changes in hindlimb and forelimb resistance, but the dissociation between changes in renal sympathetic nerve activity and either forelimb or hindlimb resistance. The arrows indicate the times of the glutamate injections.
either forelimb or hindlimb vascular resistance was much less than in the case where the increases in forelimb and hindlimb vascular resistance were compared (Fig. 2). For the grouped data from all experiments, the values of rs for the different combinations of paired measurements were 0.71 (forelimb vs hindlimb), 0.18 (renal vs forelimb) and 0.30 (renal vs hindlimb). The higher value for the first combination (forelimb vs hindlimb) was statistically significantly different from both of the values for the other two combinations (P < 0.05). The better correlation between forelimb and hindlimb responses than that of either with the renal nerve response cannot be explained by differences in the intrinsic baseline variability or noise: the relative standard deviations of the hindlimb vascular resistance, forelimb vascular resistance and renal nerve activity during the pre-injection control period were similar (5.95, 7.56 and 6.66%, respectively). Nor can it be attributed to differences in sensitivity to secondary baroreflexes or to the influence
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Fig. 2. Results from one experiment showing the relationship between the changes in hindlimb vascular resistance and (A) forelimb vascular resistance, (B) renal nerve activity, evoked by glutamate injections at different sites in the SRF nucleus. Note that in (A) the responses were highly correlated (r~=0.96, P < 0 . 0 0 5 ) , whereas in (B) the degree of correlation was much less (r~=0.48, P > 0 . 1 ) . The correlation between evoked changes in forelimb vascular resistance (not shown) and renal nerve activity in this experiment was also relatively p o o r (r~ = 0.40, P > 0.1 ).
of circulating catecholamines: in all experiments the baroreceptors were denervated and the adrenal glands were removed. The results confirm previous findings that there are sub-groups of neurons within the SRF nucleus that preferentially or exclusively control the symapthetic outflows to different vascular beds [4, 8]. It is clear, however, that such subgroups are not simply arranged rostrocaudally according to the segmental location of the sympathetic preganglionic neurons that they control. If such were the case, then sites producing significant increases in hindlimb and forelimb vascular resistance would be separate within the SRF nucleus, while renal vasoconstrictor sites would be in between. As described above, the forelimb and hindlimb vasoconstrictor responses were obtained from sites with a similar rostrocaudal distribution, while renal vasoconstrictor sites tended to be located at more rostral levels of the SRF nucleus. On the other hand, the results are entirely consistent with the hypothesis that SRF neurons are arranged topographically according to the type of vascular bed that they control. Thus, glutamate injections into different sites within the SRF nucleus produced highly correlated changes in hindlimb and forelimb skeletal muscle vascular resistance, regardless of the site of injection. It is possible that forelimb and hindlimb vasoconstrictor neurons are innervated by axon collaterals of the same SRF cells. This would be consistent with electrophysiological evidence that single neurons in the rostral ventrolateral medulla may innervate widely separated spinal segments [2]. Alternatively, or additionally, it is possible that the forelimb and hindlimb beds are controlled by separate but overlapping populations of SRF cells. Whatever the exact details of the organizational principles underlying the topographic arrangement of SRF neurons, our results fit well with observations by others
96 on the firing p a t t e r n s o f s y m p a t h e t i c n e r v e s i n n e r v a t i n g d i f f e r e n t v a s c u l a r beds. O b s e r v a t i o n s in c o n s c i o u s h u m a n
subjects h a v e d e m o n s t r a t e d
t h a t there is a p r o -
n o u n c e d p a r a l l e l i s m in the d i s c h a r g e p a t t e r n o f s y m p a t h e t i c v a s o c o n s t r i c t o r fibres i n n e r v a t i n g skeletal m u s c l e in the a r m a n d leg, l e a d i n g to the s u g g e s t i o n t h a t v a s o c o n s t r i c t o r fibres i n n e r v a t i n g b l o o d vessels in e n d - o r g a n s o f the s a m e t y p e m a y be d r i v e n by a c o m m o n s o u r c e w i t h i n the c e n t r a l n e r v o u s system [12]. A t the s a m e time, the s y m p a t h e t i c o u t p u t to the different l i m b s c a n n o t be e n t i r e l y c o n t r o l l e d by a c o m m o n g r o u p o f c e n t r a l n e u r o n s , since a d i s s o c i a t i o n o f the a c t i v i t y o f s y m p a t h e t i c n e r v e s s u p p l y i n g leg a n d a r m m u s c l e has b e e n o b s e r v e d in c o n s c i o u s h u m a n s d u r i n g m e n t a l stress [1]. T h i s s t u d y was s u p p o r t e d by the N a t i o n a l H e a l t h a n d M e d i c a l R e s e a r c h C o u n c i l , the N a t i o n a l H e a r t F o u n d a t i o n , a n d the A u s t r a l i a n B r a i n F o u n d a t i o n . W e t h a n k Mr. J a i m i e P o i s o n f o r e x c e l l e n t t e c h n i c a l assistance. I Anderson, E.A., Wallin, B.G. and Mark, A.L., Dissociation of sympathetic nerve activity in arm and leg muscle during mental stress, Hypertension, 9 (1987) Suppl. III: IIl-114-III-119. 2 Barman, S.M. and Gebber, G.L., Axonal projection patterns of ventrolateral medullospinal sympathoexcitatory neurons, J. Neurophysiol., 53 (1985) 1551 1566. 3 Dampney, R.A.L., Goodchild, A.K. and Tan, E., Vasopressor neurons in the rostral ventrolateral medulla of the rabbit, J. Auton. Nerv., Syst., 14 (1985) 239 254. 4 Dampney, R.A.L. and McAllen, R.M.~ Differential control of sympathetic fibres supplying hindlimb skin and muscle by subretrofacial neurones in the cat, J. Physiol. (Lond.), 395 (1988) 41 56. 5 Dorward, P.K., Burke, S.L., Jfinig, W. and Cassell, J., Reflex responses to baroreceptor, chemoreceptor and nociceptor inputs in single renal sympathetic neurones in the rabbit and the effects of anaesthesia on them, J. Auton. Nerv. Syst., 18 (1987) 39 54. 6 Feldberg, W. and Guertzenstein, P.G., Vasopressor effects obtained by drugs acting on the ventral surface of the brain stem, J. Physiol. (Lond.), 258 (1976) 337 355. 7 Gardner, M.J. and Altman, D.G., Statistics with Confidence, Br. Med. J. London, 1989, 326 pp. 8 Lovick~ T.A., Differential control of cardiac and vasomotor activity by neurones in nucleus paragigantocellularis lateralis in the cat, J. Physiol. (Lond.), 389 (1987) 23 36. 9 McAllen, R.M., Action and specificity of ventral medullary vasopressor neurones in the cat, Neuroscience, 18 (1986) 51 59. 10 Mesulam, M.-M., Principles of horseradish peroxidase neurohistochemistry and their applications for tracing neural pathways axonal transport, enzyme histochemistry and light microscopic analysis. In M.-M. Mesulam (Ed.), Tracing Neural Connections with Horseradish Peroxidase, Wiley, Chichester, 1982, pp. 1 151. 11 Reis, D.J., Morrison, S. and Ruggiero, D.A., The Cl area of the brainstem in tonic and reflex control of blood pressure, Hypertension, l I (1988) Suppl. I: I 8-I 13. 12 Sundl6f, G. and Wallin, B.G., The variability of muscle nerve sympathetic activity in resting recumbent man, J. Physiol. (Lond.), 272 (1977) 383-397.
91
Elsevier Scientific Publishers Ireland Ltd.
NSL 06679
Vasomotor neurons in the rostral ventrolateral medulla are organized topographically with respect to type of vascular bed but not body region R . M . McAllen 1 a n d R . A . L . D a m p n e y 2 1Howard FIorey Institute of Physiology and Experimental Medicine, University of Melbourne, Melbourne, Vic. (Australia) and 2Department of Physiology, University of Sydney, Sydney, N.S.W. (Australia) (Received 16 June 1989; Revised version received 24 October 1989; Accepted 25 October 1989)
Key words." Medulla oblongata; Blood pressure regulation; Sympathetic activity; Muscle blood flow; Renal nerve; Microinjection Microinjections of sodium glutamate (200-800 pmol) were made into sites within the rostral ventrolateral medulla of anaesthetized cats in which the adrenal glands had been removed and the baroreceptors denervated, while arterial pressure, renal sympathetic nerve activity and hindlimb and forelimb blood flows were measured. Hindlimb and forelimb vascular resistances were affected in a parallel fashion by injections at different sites, but there was a clear dissociation between evoked changes in renal nerve activity and either forelimb or hindlimb vascular resistance. The results indicate that ventrolateral medullary vasomotor neurons are organized topographically with respect to the type, rather than body position, of the vascular beds they control.
Neurons of the rostral ventrolateral medulla are known to play a major role in the tonic and reflex control of blood pressure [3, 6, 11]. Inhibition or destruction of these neurons causes a large fall in arterial pressure, while excitation produces a rise in blood pressure due to sympathetic vasoconstriction [3, 6]. Neurons in the pressor part of the rostral ventrolateral medulla, which has been termed the subretrofacial (SRF) nucleus, drive the sympathetic output to the heart, blood vessels and adrenal medulla, but apparently not that to non-cardiovascular tissues [9]. Recent work has indicated that different sympathetic vasomotor nerves may be activated selectively from different parts of the SRF nucleus. For example, Dampney and McAllen [4] found that small injections of L-glutamate into the medial part of the SRF nucleus selectively activated cutaneous vasoconstrictor fibres, while more lateral injections activated muscle vasoconstrictor fibres. Similarly, Lovick [8] found that stimulation of SRF neurons at different rostrocaudal levels produced different
Correspondence." R.A.L. Dampney, Department of Physiology, University of Sydney, Sydney, N,S.W. 2006, Australia. 0304-3940/90/$ 03.50 © 1990 Elsevier Scientific Publishers Ireland Ltd.
92 patterns of changes in renal, mesenteric and hindlimb vascular beds. These different patterns of regional vascular changes evoked by stimulation of SRF neurons cannot simply be attributed to secondary influences (e.g. baroreceptor reflexes or respiratory changes) since they are still observed following baroreceptor denervation and are independent of respiratory events [4]. It is still uncertain, however, what is the principle underlying the topographic organization of the SRF nucleus. Is it the type of vascular bed on which an SRF cell acts (e.g. skeletal muscle, kidney etc.) or is it the location of the bed within the body (i.e. the spinal segmental origin of its innervation)? We report here an experiment designed to distinguish between these two possibilities. In animals with the vagi, aortic and carotid sinus nerves sectioned and the adrenal glands removed (to eliminate secondary effects arising from baroreceptor reflexes and circulating catecholamines), we compared the vasomotor effects in forelimb and hindlimb skeletal muscle (same type of vascular bed, but innervated by widely separated spinal segments) with those in the kidney (different type, but innervated from intermediate spinal segments), evoked by small injections of glutamate at different sites in the SRF nucleus. If the organizing principle of the SRF nucleus is the type of vascular bed, forelimb and hindlimb responses should be well correlated with each other, regardless of the rostrocaudal position of the injection site; if it is spinal segmental level, the limbs should be separately represented, with renal vascular responses evoked preferentially from sites in between. Four cats were anaesthetized with chloralose (70 mg/kg, i.v.) following induction with halothane. The trachea was cannulated, and the femoral vein and artery on the right side were cannulated. The animals were ventilated with oxygen-enriched air, and the end-tidal CO2 monitored and maintained close to 4%. Both adrenal glands were removed via a retroperitoneal approach, with care being taken to preserve the splanchnic nerves and their connections to the kidney. With the head held supine in a stereotaxic frame, the medulla was exposed by a ventral approach as previously described [9]. In all 4 experiments an electromagnetic flow probe (Narco Bio-systems) was placed on the left iliac artery for the measurement of hindlimb blood flow, and in 3 experiments a flow probe was also placed on the left subclavian artery for the measurement of forelimb blood flow. Snares were placed downstream to the probes to allow zero flow readings to be checked at intervals. In all experiments but one, the circulation to the paws of these limbs was occluded with ligatures. The renal nerve on the same side was cut close to the hilum; its activity was recorded on a pair of silver wire electrodes, filtered (100-1000 Hz bandpass), rectified and integrated (reset every 5 s). Renal nerve fibres behave in a functionally homogeneous manner, and renal nerve activity is believed to be an accurate measure of sympathetic vasoconstrictor output to the kidney [5]. Both vagosympathetic trunks (including the aortic nerve) and carotid sinus nerves were cut. Blood pressure, left hindlimb and forelimb blood flows, integrated left renal nerve activity and heart rate were recorded on a chart recorder. Injections ( 1 4 nl) of a solution of 0.2 M sodium glutamate (pH 7.4) containing a small amount of wheat germ agglutinin-horseradish peroxidase (final concentration
93 0.01%) were made into different sites within the ventral medulla via a micropipette held in a micromanipulator. The tip of the micropipette was positioned 0.7 mm below the surface in each case. The injections were made by air pressure and the volume monitored microscopically as previously described [4]. At the end of the experiments the animals were perfused transcardially with heparinized saline followed by a mixture of 1% paraformaldehyde and 1.25% glutaraldehyde. The centres of injection sites were reconstructed from 50/tm frozen sections processed according to the tetramethylbenzidine procedure [10] and counterstained with neutral red. To quantitate the effects of each glutamate microinjection on cardiovascular variables, the maximum changes (calculated as a percentage of the pre-injection levels) in hindlimb vascular resistance, forelimb vascular resistance and integrated renal nerve activity within 30 s following the injection were calculated. The baseline stability of these variables was determined by measuring, in each case, their relative standard deviation during the 30 s period immediately before each injection. A total of 35 injections were made into the region of the left (ipsilateral) SRF nucleus. In 26 cases a positive response was elicited (defined as a 25% or greater increase in hindlimb or forelimb vascular resistance, or integrated renal nerve activity, above pre-injection levels). The rostrocaudal positions of these 26 sites were mapped with respect to the caudal pole of the facial nucleus (CP7). The centres of the injection sites at which significant increases in skeletal muscle vascular resistance were produced extended from the level 0.6 mm rostral to CP7 to a distance of 1.4 mm caudal to CP7, for both the forelimb and hindlimb beds. In comparison, the centres of injection sites at which significant increases in renal nerve activity were produced extended from the level 0.9 mm rostral to CP7 to the level 1.0 mm caudal to CP7. Thus, although there was considerable overlap, the renal vasoconstrictor sites tended to be located more rostrally than those sites affecting the limbs. In 20 of the 26 cases where positive responses were obtained, forelimb and hindlimb blood flows were measured simultaneously. At 18 sites, glutamate microinjection elicited increases of 25% or more in both forelimb and hindlimb vascular resistance (e.g. Fig. 1), at one site only hindlimb resistance increased significantly (by 37%) and at the remaining site neither increased significantly. The relative magnitudes of forelimb and hindlimb responses differed between experiments, but maintained a fairly constant ratio within each individual animal (Figs. 1, 2A). Renal nerve activity was measured in all 26 cases where glutamate elicited a positive response. In 18 cases, renal nerve activity increased by 25% or more (e.g. Fig. 1B), but at the remainder renal nerve activity was little affected (e.g. Fig. 1A). The lack of response at such sites was not due to the fact that the basal renal nerve activity was already at a maximal level, since in all such cases increases above this basal level were readily evoked by stimulation of other sites in the same experiment. The relationship between evoked changes in hindlimb vascular resistance, forelimb vascular resistance and integrated renal sympathetic nerve activity (measured as a percentage of the pre-injection level) was examined in detail, by determining the rank correlation coefficient (rs) for different combinations of paired measurements [7]. The degree of correlation between the increase in renal nerve activity and the increase in
94
1--
A
B
INTEGRATED RENALNERVE ACTIVITY (arbitrary units) 0-FORELIMB FLOW (ml/min)
o_!
~
1
0 '~ -'1-.... h 0
CALCULATED 200 1 VASCULAR RESISTANCE (% control) 100 4 MEAN ARTERIAL PRESSURE (mmHg)
2°°1 5O
HINDLIMB FLOW (ml/min)
• HINDLIMB ~~ u FORELIMB
i
1 min
Fig. I. Examples, from the same experiment, of the cardiovascular effects of microinjection of sodium glutamate (0.2 M solution) into sites within the SRF nucleus. The volume of the injection was I nl in (A) and 4 nl in (B). Note the parallelism between the changes in hindlimb and forelimb resistance, but the dissociation between changes in renal sympathetic nerve activity and either forelimb or hindlimb resistance. The arrows indicate the times of the glutamate injections.
either forelimb or hindlimb vascular resistance was much less than in the case where the increases in forelimb and hindlimb vascular resistance were compared (Fig. 2). For the grouped data from all experiments, the values of rs for the different combinations of paired measurements were 0.71 (forelimb vs hindlimb), 0.18 (renal vs forelimb) and 0.30 (renal vs hindlimb). The higher value for the first combination (forelimb vs hindlimb) was statistically significantly different from both of the values for the other two combinations (P < 0.05). The better correlation between forelimb and hindlimb responses than that of either with the renal nerve response cannot be explained by differences in the intrinsic baseline variability or noise: the relative standard deviations of the hindlimb vascular resistance, forelimb vascular resistance and renal nerve activity during the pre-injection control period were similar (5.95, 7.56 and 6.66%, respectively). Nor can it be attributed to differences in sensitivity to secondary baroreflexes or to the influence
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150
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Fig. 2. Results from one experiment showing the relationship between the changes in hindlimb vascular resistance and (A) forelimb vascular resistance, (B) renal nerve activity, evoked by glutamate injections at different sites in the SRF nucleus. Note that in (A) the responses were highly correlated (r~=0.96, P < 0 . 0 0 5 ) , whereas in (B) the degree of correlation was much less (r~=0.48, P > 0 . 1 ) . The correlation between evoked changes in forelimb vascular resistance (not shown) and renal nerve activity in this experiment was also relatively p o o r (r~ = 0.40, P > 0.1 ).
of circulating catecholamines: in all experiments the baroreceptors were denervated and the adrenal glands were removed. The results confirm previous findings that there are sub-groups of neurons within the SRF nucleus that preferentially or exclusively control the symapthetic outflows to different vascular beds [4, 8]. It is clear, however, that such subgroups are not simply arranged rostrocaudally according to the segmental location of the sympathetic preganglionic neurons that they control. If such were the case, then sites producing significant increases in hindlimb and forelimb vascular resistance would be separate within the SRF nucleus, while renal vasoconstrictor sites would be in between. As described above, the forelimb and hindlimb vasoconstrictor responses were obtained from sites with a similar rostrocaudal distribution, while renal vasoconstrictor sites tended to be located at more rostral levels of the SRF nucleus. On the other hand, the results are entirely consistent with the hypothesis that SRF neurons are arranged topographically according to the type of vascular bed that they control. Thus, glutamate injections into different sites within the SRF nucleus produced highly correlated changes in hindlimb and forelimb skeletal muscle vascular resistance, regardless of the site of injection. It is possible that forelimb and hindlimb vasoconstrictor neurons are innervated by axon collaterals of the same SRF cells. This would be consistent with electrophysiological evidence that single neurons in the rostral ventrolateral medulla may innervate widely separated spinal segments [2]. Alternatively, or additionally, it is possible that the forelimb and hindlimb beds are controlled by separate but overlapping populations of SRF cells. Whatever the exact details of the organizational principles underlying the topographic arrangement of SRF neurons, our results fit well with observations by others
96 on the firing p a t t e r n s o f s y m p a t h e t i c n e r v e s i n n e r v a t i n g d i f f e r e n t v a s c u l a r beds. O b s e r v a t i o n s in c o n s c i o u s h u m a n
subjects h a v e d e m o n s t r a t e d
t h a t there is a p r o -
n o u n c e d p a r a l l e l i s m in the d i s c h a r g e p a t t e r n o f s y m p a t h e t i c v a s o c o n s t r i c t o r fibres i n n e r v a t i n g skeletal m u s c l e in the a r m a n d leg, l e a d i n g to the s u g g e s t i o n t h a t v a s o c o n s t r i c t o r fibres i n n e r v a t i n g b l o o d vessels in e n d - o r g a n s o f the s a m e t y p e m a y be d r i v e n by a c o m m o n s o u r c e w i t h i n the c e n t r a l n e r v o u s system [12]. A t the s a m e time, the s y m p a t h e t i c o u t p u t to the different l i m b s c a n n o t be e n t i r e l y c o n t r o l l e d by a c o m m o n g r o u p o f c e n t r a l n e u r o n s , since a d i s s o c i a t i o n o f the a c t i v i t y o f s y m p a t h e t i c n e r v e s s u p p l y i n g leg a n d a r m m u s c l e has b e e n o b s e r v e d in c o n s c i o u s h u m a n s d u r i n g m e n t a l stress [1]. T h i s s t u d y was s u p p o r t e d by the N a t i o n a l H e a l t h a n d M e d i c a l R e s e a r c h C o u n c i l , the N a t i o n a l H e a r t F o u n d a t i o n , a n d the A u s t r a l i a n B r a i n F o u n d a t i o n . W e t h a n k Mr. J a i m i e P o i s o n f o r e x c e l l e n t t e c h n i c a l assistance. I Anderson, E.A., Wallin, B.G. and Mark, A.L., Dissociation of sympathetic nerve activity in arm and leg muscle during mental stress, Hypertension, 9 (1987) Suppl. III: IIl-114-III-119. 2 Barman, S.M. and Gebber, G.L., Axonal projection patterns of ventrolateral medullospinal sympathoexcitatory neurons, J. Neurophysiol., 53 (1985) 1551 1566. 3 Dampney, R.A.L., Goodchild, A.K. and Tan, E., Vasopressor neurons in the rostral ventrolateral medulla of the rabbit, J. Auton. Nerv., Syst., 14 (1985) 239 254. 4 Dampney, R.A.L. and McAllen, R.M.~ Differential control of sympathetic fibres supplying hindlimb skin and muscle by subretrofacial neurones in the cat, J. Physiol. (Lond.), 395 (1988) 41 56. 5 Dorward, P.K., Burke, S.L., Jfinig, W. and Cassell, J., Reflex responses to baroreceptor, chemoreceptor and nociceptor inputs in single renal sympathetic neurones in the rabbit and the effects of anaesthesia on them, J. Auton. Nerv. Syst., 18 (1987) 39 54. 6 Feldberg, W. and Guertzenstein, P.G., Vasopressor effects obtained by drugs acting on the ventral surface of the brain stem, J. Physiol. (Lond.), 258 (1976) 337 355. 7 Gardner, M.J. and Altman, D.G., Statistics with Confidence, Br. Med. J. London, 1989, 326 pp. 8 Lovick~ T.A., Differential control of cardiac and vasomotor activity by neurones in nucleus paragigantocellularis lateralis in the cat, J. Physiol. (Lond.), 389 (1987) 23 36. 9 McAllen, R.M., Action and specificity of ventral medullary vasopressor neurones in the cat, Neuroscience, 18 (1986) 51 59. 10 Mesulam, M.-M., Principles of horseradish peroxidase neurohistochemistry and their applications for tracing neural pathways axonal transport, enzyme histochemistry and light microscopic analysis. In M.-M. Mesulam (Ed.), Tracing Neural Connections with Horseradish Peroxidase, Wiley, Chichester, 1982, pp. 1 151. 11 Reis, D.J., Morrison, S. and Ruggiero, D.A., The Cl area of the brainstem in tonic and reflex control of blood pressure, Hypertension, l I (1988) Suppl. I: I 8-I 13. 12 Sundl6f, G. and Wallin, B.G., The variability of muscle nerve sympathetic activity in resting recumbent man, J. Physiol. (Lond.), 272 (1977) 383-397.
