Tomoko Nakamoto

Coactivation of renal sympathetic neurons and somatic motor neurons by chemical stimulation of the midbrain ventral tegmental area

We examined whether neurons in the midbrain ventral tegmental area (VTA) play a role in generating central command responsible for autonomic control of the cardiovascular system in anesthetized rats and unanesthetized, decerebrated rats with muscle paralysis. Small volumes (60 nl) of an N-methyl-D-aspartate receptor agonist (L-homocysteic acid) and a GABAergic receptor antagonist (bicuculline) were injected into the VTA and substantia nigra (SN). In anesthetized rats, L-homocysteic acid into the VTA induced short-lasting increases in renal sympathetic nerve activity (RSNA; 66 ± 21%), mean arterial pressure (MAP; 5 ± 2 mmHg), and heart rate (HR; 7 ± 2 beats/min), whereas bicuculline into the VTA produced long-lasting increases in RSNA (130 ± 45%), MAP (26 ± 2 mmHg), and HR (66 ± 6 beats/min). Bicuculline into the VTA increased blood flow and vascular conductance of the hindlimb triceps surae muscle, suggesting skeletal muscle vasodilatation. However, neither drug injected into the SN affected all variables. Renal sympathetic nerve and cardiovascular responses to chemical stimulation of the VTA were not essentially affected by decerebration at the premammillary-precollicular level, indicating that the ascending projection to the forebrain from the VTA was not responsible for evoking the sympathetic and cardiovascular responses. Furthermore, bicuculline into the VTA in decerebrate rats produced long-lasting rhythmic bursts of RSNA and tibial motor nerve discharge, which occurred in good synchrony. It is likely that the activation of neurons in the VTA is capable of eliciting synchronized stimulation of the renal sympathetic and tibial motor nerves without any muscular feedback signal.

Central command does not decrease cardiac parasympathetic efferent nerve activity during spontaneous fictive motor activity in decerebrate cats

To examine whether withdrawal of cardiac vagal efferent nerve activity (CVNA) predominantly controls the tachycardia at the start of exercise, the responses of CVNA and cardiac sympathetic efferent nerve activity (CSNA) were directly assessed during fictive motor activity that occurred spontaneously in unanesthetized, decerebrate cats. CSNA abruptly increased by 71 ± 12% at the onset of the motor activity, preceding the tachycardia response. The increase in CSNA lasted for 4-5 s and returned to the baseline, even though the motor activity was not ended. The increase of 6 ± 1 beats/min in heart rate appeared with the same time course of the increase in CSNA. In contrast, CVNA never decreased but increased throughout the motor activity, in parallel with a rise in mean arterial blood pressure (MAP). The peak increase in CVNA was 37 ± 9% at 5 s after the motor onset. The rise in MAP gradually developed to 21 ± 2 mmHg and was sustained throughout the spontaneous motor activity. Partial sinoaortic denervation (SAD) blunted the baroreflex sensitivity of the MAP-CSNA and MAP-CVNA relationship to 22-33% of the control. Although partial SAD blunted the initial increase in CSNA to 53% of the control, the increase in CSNA was sustained throughout the motor activity. In contrast, partial SAD almost abolished the increase in CVNA during the motor activity, despite the augmented elevation of 31 ± 1 mmHg in MAP. Because afferent inputs from both muscle receptors and arterial baroreceptors were absent or greatly attenuated in the partial SAD condition, only central command was operating during spontaneous fictive motor activity in decerebrate cats. Therefore, it is likely that central command causes activation of cardiac sympathetic outflow but does not produce withdrawal of cardiac parasympathetic outflow during spontaneous motor activity.

Centrally evoked increase in adrenal sympathetic outflow elicits immediate secretion of adrenaline in anaesthetized rats.

To examine whether feedforward control by central command activates preganglionic adrenal sympathetic nerve activity (AdSNA) and releases catecholamines from the adrenal medulla, we investigated the effects of electrical stimulation of the hypothalamic locomotor region on preganglionic AdSNA and secretion rate of adrenal catecholamines in anaesthetized rats. Pre‐ or postganglionic AdSNA was verified by temporary sympathetic ganglionic blockade with trimethaphan. Adrenal venous blood was collected every 30 s to determine adrenal catecholamine output and blood flow. Hypothalamic stimulation for 30 s (50 Hz, 100–200 μA) induced rapid activation of preganglionic AdSNA by 83–181% depending on current intensity, which was followed by an immediate increase of 123–233% in adrenal adrenaline output. Hypothalamic stimulation also increased postganglionic AdSNA by 42–113% and renal sympathetic nerve activity by 94–171%. Hypothalamic stimulation induced preferential secretion of adrenal adrenaline compared with noradrenaline, because the ratio of adrenaline to noradrenaline increased greatly during hypothalamic stimulation. As soon as the hypothalamic stimulation was terminated, preganglionic AdSNA returned to the prestimulation level in a few seconds, and the elevated catecholamine output decayed within 30–60 s. Adrenal blood flow and vascular resistance were not affected or slightly decreased by hypothalamic stimulation. Thus, it is likely that feedforward control of catecholamine secretion from the adrenal medulla plays a role in conducting rapid hormonal control of the cardiovascular system at the beginning of exercise.

Sympathetic cholinergic nerve contributes to increased muscle blood flow at the onset of voluntary static exercise in conscious cats

We examined whether a sympathetic cholinergic mechanism contributed to increased blood flow of the exercising muscle at the onset of voluntary static exercise in conscious cats. After six cats were operantly conditioned to perform static bar press exercise with a forelimb while maintaining a sitting posture, a Transonic or pulsed Doppler flow probe was implanted on the brachial artery of the exercising forelimb, and catheters were inserted into the left carotid artery and jugular vein. After the baseline brachial blood flow and vascular conductance decreased and became stable in progress of postoperative recovery, the static exercise experiments were started. Brachial blood flow and vascular conductance began to increase simultaneously with the onset of exercise. Their initial increases reached 52 +/- 8% and 40 +/- 6% at 3 s from the exercise onset, respectively. Both a sympathetic ganglionic blocker (hexamethonium bromide) and atropine sulfate or methyl nitrate blunted the increase in brachial vascular conductance at the onset of static exercise, whereas an inhibitor of nitric oxide synthesis (N(omega)-nitro-l-arginine methyl ester) did not alter the increase in brachial vascular resistance. Brachial blood flow and vascular conductance increased during natural grooming behavior with the forelimb in which the flow probe was implanted, whereas they decreased during grooming with the opposite forelimb and during eating behavior. Thus it is likely that the sympathetic cholinergic mechanism is capable of evoking muscle vasodilatation at the onset of voluntary static exercise in conscious cats.

Differential contribution of central command to the cardiovascular responses during static exercise of ankle dorsal and plantar flexion in humans

To examine whether central command contributes differently to the cardiovascular responses during voluntary static exercise engaged by different muscle groups, we encouraged healthy subjects to perform voluntary and electrically evoked involuntary static exercise of ankle dorsal and plantar flexion. Each exercise was conducted with 25% of the maximum voluntary force of the right ankle dorsal and plantar flexion, respectively, for 2 min. Heart rate (HR) and mean arterial blood pressure (MAP) were recorded, and stroke volume, cardiac output (CO), and total peripheral resistance were calculated. With voluntary exercise, HR, MAP, and CO significantly increased during dorsal flexion (the maximum increase, HR: 12 ± 2.3 beats/min; MAP: 14 ± 2.0 mmHg; CO: 1 ± 0.2 l/min), whereas only MAP increased during plantar flexion (the maximum increase, 6 ± 2.0 mmHg). Stroke volume and total peripheral resistance were unchanged throughout the two kinds of voluntary static exercise. With involuntary exercise, there were no significant changes in all cardiovascular variables, irrespective of dorsal or plantar flexion. Furthermore, before the force onset of voluntary static exercise, HR and MAP started to increase without muscle contraction, whereas they had no significant changes with involuntary exercise at the moment. The present findings indicate that differential contribution of central command is responsible for the different cardiovascular responses to static exercise, depending on the strength of central control of the contracting muscle.

Increased oxygenation of the cerebral prefrontal cortex prior to the onset of voluntary exercise in humans

To determine whether output from the forebrain (termed central command) may descend early enough to increase cardiac and renal sympathetic outflows at the onset of voluntary exercise, we examined the changes in regional tissue blood flows of bilateral prefrontal cortices with near-infrared spectroscopy, precisely identifying the onset of voluntary ergometer 30-s exercise at 41 ± 2% of the maximal exercise intensity in humans. Prefrontal oxygenated-hemoglobin (Oxy-Hb) concentration was measured as index of regional blood flow unless deoxygenated-hemoglobin concentration remained unchanged. Prefrontal Oxy-Hb concentration increased significantly (P < 0.05) 5 s prior to the onset of exercise with arbitrary start, whereas such increase in prefrontal Oxy-Hb was absent before exercise abruptly started by a verbal cue. Furthermore, the increase in prefrontal Oxy-Hb observed at the initial 15-s period of exercise was greater with arbitrary start than cued start. The prefrontal Oxy-Hb, thereafter, decreased during the later period of exercise, irrespective of either arbitrary or cued start. The reduction in prefrontal Oxy-Hb had the same time course and response magnitude as that during motor-driven passive exercise. Cardiac output increased at the initial period of exercise, whereas arterial blood pressure and total peripheral resistance decreased. The depressor response was more pronounced (P < 0.05) with arbitrary start than cued start. Taken together, it is suggested that the increase in prefrontal Oxy-Hb observed prior to the onset of voluntary exercise may be in association with central command, while the later decrease in the Oxy-Hb during exercise may be in association with feedback stimulated by mechanical limb motion.

The effects of adrenalectomy and autonomic blockades on the exercise tachycardia in conscious rats

Heart rate (HR) during exercise is controlled by cardiac sympathetic (CSNA) and vagal (CVNA) efferent nerve activity and plasma catecholamines. To determine their relative contribution to the exercise tachycardia, we examined the effects of adrenalectomy (ADX) and autonomic blockades on the HR response during treadmill exercise for 32min in 13 conscious rats. The baseline HR was not influenced by ADX, suggesting no significant role of adrenal catecholamines on the baseline HR. Since the baseline HR was increased 61beats/min by atropine methyl nitrate (1.5mg/kg) and decreased 26beats/min by atenolol (3mg/kg), CVNA determined the baseline HR more than CSNA. ADX did not affect the immediate increase in HR at 0-12s from the exercise onset but reduced the subsequent increase in HR at 13-30s. These increases in HR at the early period of exercise were more blunted by atenolol than atropine. On the other hand, the peak tachycardia response of 99+/-8beats/min at the end of exercise, which was the same between the intact and ADX conditions, was blunted to 73% by atenolol, to 77% by atropine, and to 35% by combined atenolol and atropine, respectively. In conclusion, it is likely that the tachycardia at the beginning of dynamic exercise is predominantly determined by the cardiac autonomic nerve activity, especially by a prompt increase in CSNA, and that the hormonal mechanism due to adrenal epinephrine contributes to a further increase in HR approximately in 13s from the onset of exercise.

Muscle receptors close to the myotendinous junction play a role in eliciting exercise pressor reflex during contraction.

Although a muscle mechanosensitive reflex contributes to regulation of the cardiovascular responses during exercise, the precise location of muscle mechanoreceptors responding to contraction has not been identified yet. We have recently reported that mechanosensitive receptors located at or close to the myotendinous junction play a role in eliciting the cardiovascular responses to passive stretch of skeletal muscle. The mechanoreceptors located at or near the myotendinous junction are hypothesized to respond to static contraction as well. To test this hypothesis, we had two interventions for the reflex cardiovascular responses to static contraction of the triceps surae muscle with the same tension development in decerebrate or pentobarbital-anesthetized rats; cutting the Achilles tendon and local injection of lidocaine into the myotendinous junction. The cardiovascular responses were evoked by static contraction regardless of the achillotomy, suggesting that mechanoreceptors terminating in the more distal part of the cut Achilles tendon did not contribute to the reflex cardiovascular responses. Lidocaine (volume, 0.04-0.1 ml) injected into the myotendinous junction blunted the reflex cardiovascular responses, indicating that muscle afferent fibers terminating at or passing through the myotendinous junction contribute to the exercise pressor reflex. The achillotomy did not affect the cardiovascular responses to passive stretch with the same tension as static contraction, but the localized injection of lidocaine similarly blunted the responses to passive stretch as contraction. We conclude that the mechanosensitive receptors eliciting the reflex cardiovascular responses may at least partly locate close to the myotendinous junction, to monitor tension development during muscular activity.

The enhancing effect of propofol anesthesia on skeletal muscle mechanoreflex in conscious cats

To test the hypothesis that a muscle mechanosensitive reflex is suppressed in the conscious condition, we examined the effect of propofol anesthesia on the cardiovascular responses to passive mechanical stretch of the hindlimb triceps surae muscle in five conscious cats. The triceps surae muscle was manually stretched for 30 s by extending the hip and knee joints and subsequently by dorsiflexing the ankle joint. Mean arterial blood pressure (MAP) and heart rate (HR) slightly increased or decreased during passive mechanical stretch of the muscle in the conscious condition. At 5-17 min after intravenously administering propofol (8.5+/-1 mg/kg), the identical passive stretch of the triceps surae muscle was able to induce the substantial cardiovascular responses; HR and MAP increased by 13+/-3 beats/min and 25+/-4 mm Hg, respectively, and the cardiovascular responses were sustained throughout the passive stretch. In contrast, stretching skin on the triceps surae muscle evoked a smaller pressor response in the anesthetized condition. When propofol anesthesia became light in the recovery period and the animals started to show spontaneous body movement, the cardiovascular responses to passive muscle stretch were blunted again. It is therefore concluded that passive mechanical stretch of skeletal muscle is capable of evoking the reflex cardiovascular responses, which is suppressed in the conscious condition but enhanced by propofol anesthesia.

Cardiac parasympathetic outflow during dynamic exercise in humans estimated from power spectral analysis of P-P interval variability.

What is the central question of this study? Should we use the high‐frequency (HF) component of P–P interval as an index of cardiac parasympathetic nerve activity during moderate exercise? What is the main finding and its importance? The HF component of P–P interval variability remained even at a heart rate of 120–140 beats min−1 and was further reduced by atropine, indicating incomplete cardiac vagal withdrawal during moderate exercise. The HF component of R–R interval is invalid as an estimate of cardiac parasympathetic outflow during moderate exercise; instead, the HF component of P–P interval variability should be used.