Lisete C. Michelini

Exercise-induced neuronal plasticity in central autonomic networks: role in cardiovascular control

It is now well established that brain plasticity is an inherent property not only of the developing but also of the adult brain. Numerous beneficial effects of exercise, including improved memory, cognitive function and neuroprotection, have been shown to involve an important neuroplastic component. However, whether major adaptive cardiovascular adjustments during exercise, needed to ensure proper blood perfusion of peripheral tissues, also require brain neuroplasticity, is presently unknown. This review will critically evaluate current knowledge on proposed mechanisms that are likely to underlie the continuous resetting of baroreflex control of heart rate during/after exercise and following exercise training. Accumulating evidence indicates that not only somatosensory afferents (conveyed by skeletal muscle receptors, baroreceptors and/or cardiopulmonary receptors) but also projections arising from central command neurons (in particular, peptidergic hypothalamic pre‐autonomic neurons) converge into the nucleus tractus solitarii (NTS) in the dorsal brainstem, to co‐ordinate complex cardiovascular adaptations during dynamic exercise. This review focuses in particular on a reciprocally interconnected network between the NTS and the hypothalamic paraventricular nucleus (PVN), which is proposed to act as a pivotal anatomical and functional substrate underlying integrative feedforward and feedback cardiovascular adjustments during exercise. Recent findings supporting neuroplastic adaptive changes within the NTS–PVN reciprocal network (e.g. remodelling of afferent inputs, structural and functional neuronal plasticity and changes in neurotransmitter content) will be discussed within the context of their role as important underlying cellular mechanisms supporting the tonic activation and improved efficacy of these central pathways in response to circulatory demand at rest and during exercise, both in sedentary and in trained individuals. We hope this review will stimulate more comprehensive studies aimed at understanding cellular and molecular mechanisms within CNS neuronal networks that contribute to exercise‐induced neuroplasticity and cardiovascular adjustments.

Training-induced, pressure-lowering effect in SHR: Wide effects on circulatory profile of exercised and nonexercised muscles

Abstract—We showed that the training-induced, pressure-lowering effect correlates with decreased arteriole wall/lumen ratio and venule growth in the gracilis muscle. To investigate whether these beneficial changes are tissue-specific or occur in other muscles and tissues, we analyzed the effects of hypertension and training on microcirculatory profile of locomotor/nonlocomotor muscles and another nonmuscular tissue. Spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats were submitted to low-intensity training (50% to 60% maximal exercise capacity, 13 weeks); age-matched control rats were kept sedentary. Trained and sedentary rats were instrumented for pressure and heart rate measurements at rest. Morphometric analyses (arterioles, capillaries, venules) were performed in all tissues. Training attenuated pressure and heart rate only in SHR. Arterioles (inner diameter <30 &mgr;m) were markedly hypertrophied in sedentary SHR, but wall/lumen ratio was equally reduced (≈30%) and normalized by training in locomotor (soleus, gastrocnemius, gracilis) and nonlocomotor skeletal muscles (temporalis) in the myocardium and diaphragm, without changes in the renal cortex. Training also increased venule density (≈2-fold) only in locomotor and nonlocomotor muscles of SHR. Capillary density was similarly increased in all exercised muscles of both groups, with no change in temporalis and kidneys. Data suggest that growth/proliferation of small venules and regression of hypertrophied arteriole wall/lumen ratio are generalized tissue-specific (skeletal muscle) and group-specific (SHR) adjustments to training to reduce local resistance and augment physical capacity of circulation, thus contributing to training-induced pressure-lowering effect. They are accompanied by remodeling of myocardium (cardiac output) and diaphragm arterioles (ventilatory adjustments), stressing the importance of training as a nonpharmacological therapy to control pressure levels in hypertension.

Hypertension and Exercise Training Differentially Affect Oxytocin and Oxytocin Receptor Expression in the Brain

We have previously shown that exercise training activates nucleus tractus solitarii (NTS) oxytocinergic projections, resulting in blunted exercise tachycardia. The objective of this study was to determine the effects of hypertension and training on oxytocin (OT) and OT receptor expression in the hypothalamic paraventricular nucleus (PVN) and projection areas (dorsal brain stem [DBS]). Male, normotensive, Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats were trained (55% maximal exercise capacity, 3 months) or kept sedentary, and pressure was measured weekly. DBS sections were processed for immunohistochemistry (polyclonal guinea pig anti-OT) or in situ hybridization for OT and OT receptor (35S-oligonucleotide probes). Other groups of rats had brains removed and frozen to isolate the DBS and PVN; samples were processed for OT and OT receptor cDNA reverse transcription–polymerase chain reaction amplification with &bgr;-actin as the housekeeping gene. Training was equally effective in improving running distance in both groups, with pressure reduction only in SHR (−10%, P<0.05). In trained WKY, baseline bradycardia (P<0.05) occurred simultaneously with increased NTS OT immunostaining and mRNA expression (+3.5-fold), without any change in OT receptor mRNA expression. PVN OT mRNA and DBS OT receptor mRNA expressions were significantly lower in SHR versus WKY (−39% and −56%, respectively). Training did not alter DBS OT receptor density in the SHR group but increased OT mRNA in both PVN and DBS areas (+78% and +45%, respectively). Our results show a marked hypertension-induced reduction in OT receptor mRNA expression, not altered by training. In contrast, training increased OT mRNA expression in sedentary and hypertensive rats, which may facilitate training-induced cardiac performance.

Exercise training normalizes wall-to-lumen ratio of the gracilis muscle arterioles and reduces pressure in spontaneously hypertensive rats.

Objective To investigate mechanisms underlying the training-induced blood pressure-lowering effect we analyzed the hemodynamic responses and morphometric changes of the skeletal muscle microcirculation of spontaneously hypertensive (SHR) and normotensive Wistar–Kyoto (WKY) rats during an exercise training program. Design Training (50–60% VO2 max) was performed on a treadmill for 13 weeks and control groups were kept sedentary over the same period of time. Trained and sedentary rats were chronically instrumented for hindlimb flow and arterial pressure (AP) recordings under conscious unrestrained conditions. Gracilis and myocardial muscle samples were obtained for morphometric analysis after transcardiac perfusion of fixative. Results SHR, when compared to WKY presented an elevated blood pressure, an increased relative hindlimb vascular resistance, capillary rarefaction in both gracilis and myocardium and an increased wall-to-lumen ratio of gracilis arterioles. Training increased significantly both capillary density and capillary/fiber ratio in the gracilis and myocardium of WKY and SHR groups, causing a complete reversal of capillary rarefaction in trained SHR. In SHR, training also reduced resting blood pressure and caused normalization of both relative hindlimb vascular resistance and gracilis arterioles wall-to-lumen ratio. Regression analysis revealed strong positive correlation between hindlimb vascular resistance and mean AP (MAP) and between arterioles wall-to-lumen ratio and MAP. Conclusions The results suggest that low-intensity training can significantly reduce pressure in SHR while normalizing both the arteriole morphology and the resistance of the skeletal muscle microcirculation.

Training-Induced Pressure Fall in Spontaneously Hypertensive Rats Is Associated With Reduced Angiotensinogen mRNA Expression Within the Nucleus Tractus Solitarii

Knowing that exercise training reduces arterial pressure in hypertensive individuals and that pressure fall is accompanied by blockade of brain renin-angiotensin system, we sought to investigate whether training (T) affects central renin-angiotensin system. Spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto controls (WKY) were submitted to training or kept sedentary (S) for 3 months. After functional recordings, brain was removed and processed for autoradiography (brain stem sequential slices hybridized with 35S-oligodeoxynucleotide probes for angiotensinogen [Aogen] and angiotensin II type 1 [AT1A] receptors). Resting arterial pressure and heart rate were higher in SHRS (177±2 mm Hg, 357±12 bpm versus 121±1 mm Hg, 320±9 bpm in WKYS; P<0.05). Training was equally effective to enhance treadmill performance and to cause resting bradycardia (−10%) in both groups. Training-induced blood pressure fall (−6.3%) was observed only in SHRT. In SHRS (versus WKYS) AT1A and Aogen mRNA expression were significantly increased within the NTS and area postrema (average of +67% and +41% for AT1A and Aogen, respectively; P<0.05) but unchanged in the gracilis nucleus. Training did not change AT1A expression but reduced NTS and area postrema Aogen mRNA densities specifically in SHRT (P<0.05 versus SHRS, with values within the range of WKY groups). In SHRs, NTS Aogen mRNA expression was correlated with resting pressure (y=5.95x +41; r=0.55; P<0.05), with no significant correlation in the WKY group. Concurrent training-induced reductions of both Aogen mRNA expression in brain stem cardiovascular-controlling areas and mean arterial pressure only in SHRs suggest that training is as efficient as the renin-angiotensin blockers to reduce brain renin-angiotensin system overactivity and to decrease arterial pressure.

Modulation of exercise tachycardia by vasopressin in the nucleus tractus solitarii

Our objective was to study the role of vasopressinergic synapses at the nucleus tractus solitarii (NTS) in the modulation of exercise-induced tachycardia. We evaluated the effect of NTS administration of vasopressin (AVP) or vasopressin antagonist (AVPant) on heart rate (HR) and mean arterial pressure (MAP) responses during dynamic exercise in male rats with chronic arterial and NTS cannulas. Sedentary (S) and trained (T) animals were tested at three or four exercise levels (from 0.4 up to 1.4 km/h) after NTS injection of AVP or AVPant 20-30 min before treadmill exercise. Plasma and regional brain levels of AVP were measured in separate groups of S and T rats at rest and immediately after acute exercise. When administered into the NTS, exogenous AVP (20 pmol) caused a small but significant decrease in baseline HR and potentiated the tachycardiac response to mild to moderate exercise intensities (on average, increases of 35-46 beats/min over control tachycardic response). The potentiation of exercise tachycardia by AVP was long lasting and more pronounced in T than in S rats. Even 2 days after NTS AVP injection, there was evidence for an alteration in the HR response to exercise. Mediation by V1 receptors was supported by the blunted tachycardiac response to exercise after administration of a V1 antagonist d(CH2)5Tyr MeAVP into the NTS in both T and S rats (average reductions of 23-34 and 13-19 beats/min below control tachycardia, respectively). No changes were observed in baseline MAP or the exercise-induced pressor responses. There were specific changes in brain stem AVP levels that were related to the exercise treatment. T rats showed a marked increase in dorsal and ventral brain stem AVP content after acute exercise. There were no changes in hypothalamus, median eminence, posterior pituitary, or plasma AVP. These data indicate that vasopressinergic synapses and V1 receptors in the NTS are involved in the potentiation of tachycardic response to exercise. The vasopressinergic mechanism operates in both S and T rats, but training alters the sensitization of V1receptors by AVP.Our objective was to study the role of vasopressinergic synapses at the nucleus tractus solitarii (NTS) in the modulation of exercise-induced tachycardia. We evaluated the effect of NTS administration of vasopressin (AVP) or vasopressin antagonist (AVP(ant)) on heart rate (HR) and mean arterial pressure (MAP) responses during dynamic exercise in male rats with chronic arterial and NTS cannulas. Sedentary (S) and trained (T) animals were tested at three or four exercise levels (from 0.4 up to 1.4 km/h) after NTS injection of AVP or AVP(ant) 20-30 min before treadmill exercise. Plasma and regional brain levels of AVP were measured in separate groups of S and T rats at rest and immediately after acute exercise. When administered into the NTS, exogenous AVP (20 pmol) caused a small but significant decrease in baseline HR and potentiated the tachycardiac response to mild to moderate exercise intensities (on average, increases of 35-46 beats/min over control tachycardic response). The potentiation of exercise tachycardia by AVP was long lasting and more pronounced in T than in S rats. Even 2 days after NTS AVP injection, there was evidence for an alteration in the HR response to exercise. Mediation by V1 receptors was supported by the blunted tachycardiac response to exercise after administration of a V1 antagonist d(CH2)5Tyr MeAVP into the NTS in both T and S rats (average reductions of 23-34 and 13-19 beats/min below control tachycardia, respectively). No changes were observed in baseline MAP or the exercise-induced pressor responses. There were specific changes in brain stem AVP levels that were related to the exercise treatment. T rats showed a marked increase in dorsal and ventral brain stem AVP content after acute exercise. There were no changes in hypothalamus, median eminence, posterior pituitary, or plasma AVP. These data indicate that vasopressinergic synapses and V1 receptors in the NTS are involved in the potentiation of tachycardic response to exercise. The vasopressinergic mechanism operates in both S and T rats, but training alters the sensitization of V1 receptors by AVP.

Endogenous vasopressin modulates the cardiovascular responses to exercise.

Abstract: The role of brain‐stem vasopressinergic projections in the genesis of reflex bradycardia and in the modulation of heart rate control during exercise is discussed on the basis of both changes in endogenous peptide content and heart rate changes observed during exercise. Dynamic running caused an increase in vasopressin content specifically in dorsal and ventral brain‐stem areas. Rats pretreated with vasopressin or the V1 receptor antagonist into the nucleus tractus solitarii (NTS) showed a significant potentiation or a marked blunting of the exercise tachycardia, respectively, without any change in the blood pressure response. It is proposed that long‐descending vasopressinergic pathways from the hypothalamus to the NTS serves as one link between the two main neural controllers of the circulation, that is, the central command and feedback control mechanisms driven by the peripheral receptors signals. Therefore vasopressinergic input contributes to the adjustment of heart rate response (and cardiac output) to the circulatory demand during exercise.

Chronic absence of baroreceptor inputs prevents training-induced cardiovascular adjustments in normotensive and spontaneously hypertensive rats.

We investigate whether arterial baroreceptors mediate the training‐induced blood pressure fall and resting bradycardia in hypertensive (SHR) and normotensive rats (WKY). Male SHR and WKY rats, submitted to sino‐aortic denervation (SAD) or sham surgery (SHAM group), were allocated to training (T; 55% of maximal exercise capacity) or sedentary (S) protocols for 3 months. Rats were instrumented with arterial and venous catheters for haemodynamic measurements at rest (power spectral analysis) and baroreceptor testing. Kidney and skeletal muscles were processed for morphometric analysis of arterioles. Elevated mean arterial pressure (MAP) and heart rate (HR) in SHAM SHRS were accompanied by increased sympathetic variability and arteriolar wall/lumen ratio [+3.4‐fold on low‐frequency (LF) power and +70%, respectively, versus WKYS, P < 0.05]. Training caused significant HR (∼9% in WKY and SHR) and MAP reductions (−8% in the SHR), simultaneously with improvement of baroreceptor reflex control of HR (SHR and WKY), LF reduction (with a positive correlation between LF power and MAP levels in the SHR) and normalization of wall/lumen ratio of the skeletal muscle arterioles (SHR only). In contrast, SAD increased pressure variability in both strains of rats, causing reductions in MAP (−13%) and arteriolar wall/lumen ratio (−35%) only in the SHRS. Training effects were completely blocked by SAD in both strains; in addition, after SAD the resting MAP and HR and the wall/lumen ratio of skeletal muscle arterioles were higher in SHRT versus SHRS and similar to those of SHAM SHRS. The lack of training‐induced effects in the chronic absence of baroreceptor inputs strongly suggests that baroreceptor signalling plays a decisive role in driving beneficial training‐induced cardiovascular adjustments.

Role of endogenous nitric oxide in the nucleus tratus solitarii on baroreflex control of heart rate in spontaneously hypertensive rats.

Objective Toinvestigate the modulatory effect of endogenous nitric oxide (NO) in the nucleus tractus solitarii (NTS) on the baroreceptor reflex control of heart rate in conscious spontaneously hypertensive (SHR) and normotensive (WKY) rats. Design and methods Male age- and weight-matched SHR and WKY chronically instrumented with cannulas in the NTS, artery and vein were used. Basal pressure (AP), heart rate (HR) and reflex HR responses during loading/unloading of baroreceptors (phenylephrine/ sodium nitroprusside, iv) were recorded during vehicle (3 nl/min) NG-monomethyl-L-arginine (L-NMMA) and L-arginine (L-Arg) infusions into the NTS. Constitutive NO synthase (NOS) activity was inferred by 3H-citrulline formation in the dorsal brain stem of other SHR and WKY groups. Results In SHR a small dose of L-NMMA (30 ng/kg/min) restricted to the NTS did not change AP and HR (185 ± 4 mmHg, 373 ± 12 beats/min, respectively), but decreased the HR range (57 ± 7 beats/min, a 34% reduction, P < 0.05) without changing further the impaired gain of baroreceptor reflex control of HR. In the WKY group similar results (significant 32% reduction in HR range, gain unchanged) were only attained with a dose 10 times higher (L-NMMANTS = 300 ng/kg/min), no effect being observed with the small dose (HR range = 163 ± 12 beats/min). In SHR, L-ArgNTS (900 ng/kg/min) did not improve baroreflex control of HR, but restored the depression of HR range when given after L-NMMANTS. Basal NOS activity in the dorsal brain stem was reduced in SHR (P < 0.05) when compared to WKY group. Conclusions NO modulates, at the NTS level, the baroreceptor reflex control of HR in both SHR and WKY not by altering the gain, but by increasing HR range during afferent stimulation. In SHR the depressed NO modulation is in accordance with the smaller NOS activity in the dorsal brain stem.

DIFFERENTIAL EFFECTS OF VASOPRESSINERGIC AND OXYTOCINERGIC PRE‐AUTONOMIC NEURONS ON CIRCULATORY CONTROL: REFLEX MECHANISMS AND CHANGES DURING EXERCISE

1 The role of vasopressinergic and oxytocinergic (VPergic and OTergic, respectively) projections to the brain stem in the modulation of heart rate control is discussed on the basis of both changes in the peptide content of the dorsal brain stem (DBS) and functional effects following reflex‐ and exercise‐induced activation in the presence and/or absence of receptor blockade within the nucleus tractus solitarius (NTS) and/or peripheral autonomic block. 2 Experimental data showed a dual effect of NTS VPergic projections on reflex control: (i) to maintain tonically the reflex sensitivity; and (ii) to reset reflex bradycardia towards higher heart rate values when transiently activated. The VPergic drive causes less sympathetic inhibition during pressure increases, mainly by reducing peripheral information going to NTS second‐order neurons. Treadmill running increases the vasopressin content within the DBS. This activates NTS V1 receptors to cause a significant improvement of exercise tachycardia in both sedentary and trained rats. 3 The OTergic drive to DBS areas (NTS/dorsal motor nucleus of the vagus) is also tonic for baroreceptor reflex control: it improves reflex bradycardia by facilitating vagal outflow to the heart. An acute bout of exercise increases DBS oxytocin (OT) content in trained rats, causing a significant blunting of exercise tachycardia only in this group. In both sedentary and trained groups, basal heart rate varies inversely with DBS OT content, the resting bradycardia of trained rats being associated with higher OT content. 4 Specific coordinated activation of VPergic and OTergic suprabulbar pathways is essential to adjust heart rate and cardiac output to circulatory demand at rest and during exercise in both sedentary and trained individuals.