Rhythmic activities of the sympatho-excitatory neurons in the medulla of rabbits: neurons controlling cutaneous vasomotion

Evidence for differential control of muscle sympathetic single units during mild sympathoexcitation in young, healthy humans

Two subpopulations of muscle sympathetic single units with opposite discharge characteristics have been identified during low-level cardiopulmonary baroreflex loading and unloading in middle-aged adults and patients with heart failure. The present study sought to determine whether similar subpopulations are present in young healthy adults during cardiopulmonary baroreflex unloading ( study 1) and rhythmic handgrip exercise ( study 2). Continuous hemodynamic and multiunit and single unit muscle sympathetic nerve activity (MSNA) data were collected at baseline and during nonhypotensive lower body negative pressure (LBNP; n = 12) and 40% maximal voluntary contraction rhythmic handgrip exercise (RHG; n = 24). Single unit MSNA responses were classified as anticipated or paradoxical based on whether changes were concordant or discordant with the multiunit MSNA response, respectively. LBNP and RHG both increased multiunit MSNA burst frequency (∆5 ± 3 bursts/min, P < 0.001; ∆5 ± 8 bursts/min, P = 0.005), burst amplitude (∆5 ± 7%, P = 0.04; ∆13 ± 14%, P < 0.001), and total MSNA (∆302 ± 191 AU/min, P = 0.001; ∆585 ± 556 AU/min, P < 0.001). During LBNP and RHG, 43 and 64 muscle single units were identified, respectively, which increased spike frequency (∆9 ± 11 spikes/min, P < 0.001; ∆10 ± 19 spikes/min, P < 0.001) and the probability of multiple spike firing (∆10 ± 12%, P < 0.001; ∆11 ± 26%, P = 0.001). During LBNP and RHG, 36 (84%) and 39 (61%) single units possessed anticipated firing responses (∆12 ± 10 spikes/min, P < 0.001; ∆19 ± 19 spikes/min, P < 0.001), whereas 7 (16%) and 25 (39%) single units exhibited paradoxical reductions (∆−3 ± 1 spikes/min, P = 0.003; ∆−4 ± 5 spikes/min, P < 0.001). The observation of divergent subpopulations of muscle sympathetic single units in healthy young humans during two mild sympathoexcitatory stressors supports differential control at the fiber level as a fundamental characteristic of human sympathetic regulation.

NEW & NOTEWORTHY The activity of muscle sympathetic single units was recorded during cardiopulmonary baroreceptor unloading and rhythmic handgrip exercise in young healthy humans. During both stressors, the majority of single units (84% and 61%) exhibited anticipated behavior concordant with the integrated muscle sympathetic response, whereas a smaller proportion (16% and 39%) exhibited paradoxical sympathoinhibition. These results support differential control of postganglionic muscle sympathetic fibers as a characteristic of human sympathetic regulation during mild sympathoexcitatory stress.

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Spatiotemporal Structure and Dynamics of Spontaneous Oscillatory Synchrony in the Vagal Complex

Fundamental structure and dynamics of spontaneous neuronal activities without apparent peripheral inputs were analyzed in the vagal complex (VC), whose activities had been generally thought to be produced almost passively to peripheral cues. The analysis included the caudal nucleus of the tractus solitarius—a main gateway for viscerosensory peripheral afferents and involved dynamically and critically in cardiorespiratory brainstem networks. In the present study, a possibility of self-organized brain activity was addressed in the VC. While VC neurons exhibited sparse firing in anesthetized rats and in in vitro preparations, we identified peculiar features of the emergent electrical population activity: (1) Spontaneous neuronal activity, in most cases, comprised both respiration and cardiac cycle components. (2) Population potentials of polyphasic high amplitudes reaching several millivolts emerged in synchrony with the inspiratory phase of respiratory cycles and exhibited several other characteristic temporal dynamics. (3) The spatiotemporal dynamics of local field potentials (LFPs), recorded simultaneously over multiple sites, were characterized by a stochastic emergence of high-amplitude synchrony. By adjusting amplitude and frequency (phase) over both space and time, the traveling synchrony exhibited varied degrees of coherence and power with a fluctuating balance between mutual oscillators of respiratory and cardiac frequency ranges. Full-fledged large-scale oscillatory synchrony over a wide region of the VC emerged after achieving a maximal stable balance between the two oscillators. Distinct somatic (respiratory; ~1 Hz) and visceral (autonomic; ~5 Hz) oscillators seemed to exist and communicate co-operatively in the brainstem network. Fluctuating oscillatory coupling may reflect varied degrees of synchrony influenced by the varied amplitude and frequency of neuronal activity in the VC. Intranuclear micro-, intrabulbar meso-, and wide-ranging macro-circuits involving the VC are likely to form nested networks and strategically interact to maintain a malleable whole-body homeostasis. These two brainstem oscillators could orchestrate neuronal activities of the VC, and other neuronal groups, through a phase-phase coupling mechanism to perform specific physiological functions.

Intrinsic chemosensitivity of rostral ventrolateral medullary sympathetic premotor neurons in the in situ arterially perfused preparation of rats

Brainstem hypoperfusion is a major excitant of sympathetic activity triggering hypertension, but the exact mechanisms involved remain incompletely understood. A major source of excitatory drive to preganglionic sympathetic neurons originates from the ongoing activity of premotor neurons in the rostral ventrolateral medulla (RVLM sympathetic premotor neurons). The chemosensitivity profile of physiologically characterized RVLM sympathetic premotor neurons during hypoxia and hypercapnia remains unclear. We examined whether physiologically characterized RVLM sympathetic premotor neurons can sense brainstem ischaemia intrinsically. We addressed this issue in a unique in situ arterially perfused preparation before and after a complete blockade of fast excitatory and inhibitory synaptic transmission. During hypercapnic hypoxia, respiratory modulation of RVLM sympathetic premotor neurons was lost, but tonic firing of most RVLM sympathetic premotor neurons was elevated. After blockade of fast excitatory and inhibitory synaptic transmission, RVLM sympathetic premotor neurons continued to fire and exhibited an excitatory firing response to hypoxia but not hypercapnia. This study suggests that RVLM sympathetic premotor neurons can sustain high levels of neuronal discharge when oxygen is scarce. The intrinsic ability of RVLM sympathetic premotor neurons to maintain responsivity to brainstem hypoxia is an important mechanism ensuring adequate arterial pressure, essential for maintaining cerebral perfusion in the face of depressed ventilation and/or high cerebral vascular resistance.

The rostral parvicellular reticular formation neurons mediate lingual nerve input to the rostral ventrolateral medulla

In rats that had been anesthetized by urethane-chloralose, we investigated whether neurons in the rostral part of the parvicellular reticular formation (rRFp) mediate lingual nerve input to the rostral ventrolateral medulla (RVLM), which is involved in somato-visceral sensory integration and in controlling the cardiovascular system. We determined the effect of the lingual nerve stimulation on activity of the rRFp neurons that were activated antidromically by stimulation of the RVLM. Stimulation of the lingual trigeminal afferent gave rise to excitatory effects (10/26, 39%), inhibitory effects (6/26, 22%) and no effect (10/26, 39%) on the RVLM-projecting rRFp neurons. About two-thirds of RVLM-projecting rRFp neurons exhibited spontaneous activity; the remaining one-third did not. A half (13/26) of RVLM-projecting rRFp neurons exhibited a pulse-related activity, suggesting that they receive a variety of peripheral and CNS inputs involved in cardiovascular function. We conclude that the lingual trigeminal input exerts excitatory and/or inhibitory effects on a majority (61%) of the RVLM-projecting rRFp neurons, and their neuronal activity may be involved in the cardiovascular responses accompanied by the defense reaction.

Differential responses of sympathetic premotor neurons in the rostral ventrolateral medulla to stimulation of the dorsomedial hypothalamus in rabbits

Electrical stimulation of the posterior dorsomedial hypothalamus (DMH) elicits a defense response, including vasodilation in the skeletal muscles and vasoconstriction in the viscera. To examine whether sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM) participate in these differential vascular responses, RVLM neuron activity, renal sympathetic nerve activity (RSNA), renal vessel conductance (RVC), skeletal muscular vessel conductance (MVC), arterial pressure (AP), and heart rate (HR) were simultaneously measured in urethane-anesthetized, vagotomized, and immobilized rabbits. Electrical stimulation of the DMH increased RSNA, MVC, AP, and HR but decreased RVC. The RVLM neurons were classified into three groups according to their responses to tetanic (10s) stimulation of the DMH. Twenty neurons (Type I) were excited, 17 (Type II) were inhibited, and 2 (Type III) did not respond. To the short-train (100 ms) stimulation, all of the Type I neurons showed excitation; in contrast, 12 Type II neurons showed biphasic response that was early excitation followed by inhibition. The remainder showed only inhibition. Type III neurons also did not respond to the short-train stimulation. These results indicated that regional differences in responses of sympathetic nerves in the defense response are supported by functional differentiation of sympathetic premotor neurons in the RVLM.

The dorsomedial hypothalamus: a new player in thermoregulation

Neurons in the dorsomedial hypothalamus (DMH) play key roles in physiological responses to exteroceptive (“emotional”) stress in rats, including tachycardia. Tachycardia evoked from the DMH or seen in experimental stress in rats is blocked by microinjection of the GABAA receptor agonist muscimol into the rostral raphe pallidus (rRP), an important thermoregulatory site in the brain stem, where disinhibition elicits sympathetically mediated activation of brown adipose tissue (BAT) and cutaneous vasoconstriction in the tail. Disinhibition of neurons in the DMH also elevates core temperature in conscious rats and sympathetic activity to least significant difference interscapular BAT (IBAT) and IBAT temperature in anesthetized preparations. The latter effects are blocked by microinjection of muscimol into the rRP, while microinjection of muscimol into either the rRP or DMH suppresses increases in sympathetic nerve activity to IBAT, IBAT temperature, and core body temperature elicited either by microinjection of PGE2 into the preoptic area (an experimental model for fever), or central administration of fentanyl. Neurons concentrated in the dorsal region of the DMH project directly to the rRP, a location corresponding to that of neurons transsynaptically labeled from IBAT. Thus these neurons control nonshivering thermogenesis in rats, and their activation signals its recruitment in diverse experimental paradigms. Evidence also points to a role for neurons in the DMH in thermoregulatory cutaneous vasoconstriction, shivering, and endocrine adjustments. These directions provide intriguing avenues for future exploration that may expand our understanding of the DMH as an important hypothalamic site for the integration of autonomic, endocrine, and behavioral responses to diverse challenges.

Differential responsiveness of RVLM sympathetic premotor neurons to hypoxia in rabbits

To determine whether differential sympathetic nerve responses to hypoxia are explained by opposing effects of hypoxia upon sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM), the cardiac sympathetic nerve and the renal sympathetic nerve were recorded in anesthetized and vagotomized rabbits. Renal sympathetic nerve was activated by the injection of sodium cyanide solution close to the bifurcation of the common carotid artery and/or by inhalation of hypoxic gas (3% oxygen-97% nitrogen). On the other hand, cardiac sympathetic nerve was inhibited by these stimuli. Barosensitive (inhibited by the stimulation of baroreceptor afferents) reticulospinal (antidromically activated by the stimulation of the spinal cord) neurons in the RVLM were divided into three groups according to their responses to hypoxic stimulation: neurons (Type I, n = 25), the activity of which was inhibited by the injection of sodium cyanide solution close to the bifurcation of the common carotid artery and/or by inhalation of hypoxic gas, neurons (Type II, n = 99), the activity of which was facilitated by the same stimulation, and neurons (Type III, n = 11), the activity of which was not changed. These data indicated that the differential responses of cardiac and renal sympathetic nerves might be due to opposing effects of hypoxia on individual RVLM neurons.

Activation of 5-HT1A receptors in rostral medullary raphé inhibits cutaneous vasoconstriction elicited by cold exposure in rabbits

In both conscious and anesthetized rabbits, we determined whether microinjection of a 5-hydroxytryptamine (5-HT) 1A receptor agonist 8-hydroxy-2-(di-n-propylaminio) tetralin (8-OH-DPAT) into the medullary raphé/parapyramidal region inhibits thermoregulatory vasoconstriction and whether microinjection of a 5-HT1A receptor antagonist (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride) (WAY-100635) into the raphé reverses the cutaneous vasomotor changes induced by intravenous administration of 8-OH-DPAT. In conscious rabbits with measuring ear pinna blood flow, after microinjection of 8-OH-DPAT (3-5 nmol in 300-500 nl) into the raphé, transferring the animal from a warm cage (25-28 degrees C) to a cold cage (5-10 degrees C) did not reduce the ear pinna flow (from 57 +/- 7 cm/s to 59 +/- 3 cm/s, P > 0.05, n = 5), unlike Ringer-treated animals. Microinjection of WAY-100635 (5 nmol in 500 nl) into the raphé reversed ear pinna flow changes induced by intravenous administration of 8-OH-DPAT (0.1 mg/kg, i.v.). In anesthetized rabbits with measuring postganglionic ear pinna sympathetic nerve activity, microinjection of 8-OH-DPAT (1-2 nmol in 100-200 nl) into the raphé reduced resting ear pinna sympathetic nerve activity to 14 +/- 4% of pre-injection level (P < 0.01, n = 12) and attenuated increases in ear pinna sympathetic nerve activity normally elicited by cooling the animal's trunk. WAY-100635 (2 nmol into 200 nl) into the raphé reversed inhibition of ear pinna sympathetic nerve activity elicited by 8-OH-DPAT (0.1 mg/kg, i.v.). The activation of 5-HT1A receptors expressed on the medullary raphé neurons results in reversal of cold-elicited cutaneous vasoconstriction possibly through inhibition of sympathetic premotor neurons that innervate sympathetic preganglionic neurons controlling cutaneous vasomotion.

Reticulospinal neurons inactivated by warming of the preoptic area and anterior hypothalamus of rabbits

To identify the premotor neurons for vasoconstrictors of the skin, activities of reticulospinal neurons in the rostroventral medulla, the ear sympathetic nerve (ESNA) and the renal sympathetic nerve (RSNA) were recorded in anesthetized and immobilized Japanese White or New Zealand White rabbits. Two groups of neurons were identified according to their responses to thermal stimulation of the preoptic area and the anterior hypothalamus (POAH) and to electrical stimulation of baroreceptor afferents, the aortic nerve (AN). Neurons (Type I neurons, n = 21) whose activity was inhibited by warm stimulation of the POAH but not inhibited by the AN stimulation were located in sites medial to the rostral ventrolateral medulla (RVLM). The other neurons (Type II neurons, n = 20) whose activity was not inhibited by warm stimulation of the POAH but inhibited by the AN stimulation were located in the RVLM. Because the time course of the inhibitory response of Type I neurons to warm stimulation of the POAH was very similar to that of the inhibitory response of the ESNA and activities of these neurons and the ESNA were not inhibited by the stimulation of the AN, it was suggested the Type I neurons might participate in regulation of activity of the vasoconstrictors of the ear skin. The Type II neurons are considered to be the barosensitive RVLM neurons that regulate systemic arterial pressure by controlling the activity of visceral or muscular sympathetic vasoconstrictors or cardiac sympathetic fibers.

Activation of slowly conducting medullary raphé-spinal neurons, including serotonergic neurons, increases cutaneous sympathetic vasomotor discharge in rabbit

Neurons in the rostral medullary raphé/parapyramidal region regulate cutaneous sympathetic nerve discharge. Using focal electrical stimulation at different dorsoventral raphé/parapyramidal sites in anesthetized rabbits, we have now demonstrated that increases in ear pinna cutaneous sympathetic nerve discharge can be elicited only from sites within 1 mm of the ventral surface of the medulla. By comparing the latency to sympathetic discharge following stimulation at the ventral raphé site with the corresponding latency following stimulation of the spinal cord [third thoracic (T3) dorsolateral funiculus] we determined that the axonal conduction velocity of raphé-spinal neurons exciting ear pinna sympathetic vasomotor nerves is 0.8 ± 0.1 m/s ( n = 6, range 0.6–1.1 m/s). Applications of the 5-hydroxytryptamine (HT)2A antagonist trans-4-((3 Z)3-[(2-dimethylaminoethyl)oxyimino]-3-(2-fluorophenyl)propen-1-yl)-phenol, hemifumarate (SR-46349B, 80 μg/kg in 0.8 ml) to the cerebrospinal fluid above thoracic spinal cord (T1-T7), but not the lumbar spinal cord (L2-L4), reduced raphé-evoked increases in ear pinna sympathetic vasomotor discharge from 43 ± 9 to 16 ± 6% ( P < 0.01, n = 8). Subsequent application of the excitatory amino acid (EAA) antagonist kynurenic acid (25 μmol in 0.5 ml) substantially reduced the remaining evoked discharge (22 ± 8 to 6 ± 6%, P < 0.05, n = 5). Our conduction velocity data demonstrate that only slowly conducting raphé-spinal axons, in the unmyelinated range, contribute to sympathetic cutaneous vasomotor discharge evoked by electrical stimulation of the medullary raphé/parapyramidal region. Our pharmacological data provide evidence that raphé-spinal neurons using 5-HT as a neurotransmitter contribute to excitation of sympathetic preganglionic neurons regulating cutaneous vasomotor discharge. Raphé-spinal neurons using an EAA, perhaps glutamate, make a substantial contribution to the ear sympathetic nerve discharge evoked by raphé stimulation.

Spinal 5-HT2A receptors regulate cutaneous sympathetic vasomotor outflow in rabbits and rats; relevance for cutaneous vasoconstriction elicited by MDMA (3,4-methylenedioxymethamphetamine, “Ecstasy”) and its reversal by clozapine

We determined whether spinal 5-hydroxytryptamine 2A (5-HT2A) receptors contribute to resting cutaneous sympathetic vasomotor activity, and to increases in activity elicited by electrical stimulation of the medullary raphe/parapyramidal region, and whether these receptors are involved in the cutaneous vasoconstricting action of systemically administered MDMA (3,4-methylenedioxymethamphetamine, "Ecstasy") and its reversal by clozapine. Experiments were conducted in urethane-anesthetized rabbits and rats. Administration of the 5-HT2A antagonist, trans-4-((3Z)3-[(2-Dimethylaminoethyl)oxyimino]-3-(2-fluorophenyl)propen-1-yl)-phenol, hemifumarate (SR 46349B, 0.1 mg/kg, i.v.) inhibited resting ear pinna sympathetic vasomotor nerve discharge and reduced the extent to which raphe/parapyramidal electrical stimulation caused ear pinna (rabbit) and tail (rat) artery blood flow to fall. Clozapine (0.125-0.5 mg/kg, i.v.) also reduced the fall in ear pinna blood flow elicited by raphe/parapyramidal stimulation. In rabbits, after inactivation of raphe/parapyramidal function by local microinjection of muscimol (1 nmol in 100 nl), the 5-HT2A agonist R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI, 50 microg/kg, i.v.) increased ear pinna sympathetic nerve activity from 3+/-2% to 129+/-5% of pre-muscimol levels (P<0.01, n=6), and this increase was abolished by section of the ipsilateral cervical sympathetic nerve trunk. MDMA (2 mg/kg, i.v.) after muscimol decreased ear pinna blood flow from 33+/-10 to 2+/-1 cm/s (P<0.01, n=5) and increased ear pinna sympathetic nerve activity from 8+/-4% to 120+/-41% of pre-muscimol levels (P<0.01, n=6). The MDMA-elicited increase in nerve activity was abolished by SR 46349B. Data suggest that spinal 5-HT2A receptors contribute to sympathetically induced cutaneous vasoconstriction regulated by raphe/parapyramidal neurons in the brainstem, and that these receptors contribute to the cutaneous vasoconstricting action of MDMA and its reversal by clozapine.

Different types of barosensory synaptic inputs to rostral ventrolateral medulla neurons of the rat

Neurons situated in the rostral ventrolateral medulla (RVLM) with descending axons to the spinal cord and that are modulated by different baroreceptor inputs are considered the main central generators of vasomotor activity. In the urethane-anesthetized, curarized rat, we recorded intracellular potentials from 14 neurons located in the RVLM and investigated their barosensory properties by analysis of the relation between neuronal membrane potential (MP), including spike potentials, and high-pressure barosensory activity, which was indicated by arterial blood pressure (BLPR). Time-domain (cross-correlations or triggered averaging) and frequency-domain (autospectra and coherences) analysis showed that 7 of 14 neurons had cardiac-cycle-correlated rhythms. EXCITATORY CARDIAC-CYCLE-RELATED MODULATION: One type of barosensitive neuron, with strong cardiac-related activity, was antidromically activated from the spinal cord and received inhibitory inputs from aortic nerve stimulation. These neurons had strong pulse-modulated activity consisting of EPSPs and spike potentials locked to the cardiac cycle and occurring at the end of diastole. INHIBITORY CARDIAC-CYCLE-RELATED MODULATION: Another type of barosensitive neuron showed hyperpolarizations locked to the cardiac cycle that started during late diastole and ended during the systolic period, but which had little relation to spike firing. The hyperpolarizations might be due to either IPSPs or disfacilitation. RESPIRATORY AND CARDIAC MODULATION: Some neurons also showed modulation of synaptic potentials and/or spike firing locked to the oscillation produced by ventilator pressure. It is suggested that the different types of cardiac- and respiratory-related rhythm reflect different functional roles of neurons in baroreceptor regulation of vasomotor activity.

Clozapine reverses hyperthermia and sympathetically mediated cutaneous vasoconstriction induced by 3,4-methylenedioxymethamphetamine (ecstasy) in rabbits and rats

Life-threatening hyperthermia occurs in some individuals taking 3,4-methylenedioxymethamphetamine (MDMA, ecstasy). In rabbits, sympathetically mediated vasoconstriction in heat-exchanging cutaneous beds (ear pinnae) contributes to MDMA-elicited hyperthermia. We investigated whether MDMA-elicited cutaneous vasoconstriction and hyperthermia are reversed by clozapine and olanzapine, atypical antipsychotic agents. Ear pinna blood flow and body temperature were measured in conscious rabbits; MDMA (6 mg/kg, i.v.) was administered; and clozapine (0.1-5 mg/kg, i.v.) or olanzapine (0.5 mg/kg, i.v.) was administered 15 min later. One hour after MDMA, temperature was 38.7 +/- 0.5 degrees C in 5 mg/kg clozapine-treated rabbits and 39.0 +/- 0.2 degrees C in olanzapine-treated rabbits, less than untreated animals (41.5 +/- 0.3 degrees C) and unchanged from pre-MDMA values. Ear pinna blood flow increased from the MDMA-induced near zero level within 5 min of clozapine or olanzapine administration. Clozapine-induced temperature and flow responses were dose-dependent. In urethane-anesthetized rabbits, MDMA (6 mg/kg, i.v.) increased ear pinna postganglionic sympathetic nerve discharge to 217 +/- 33% of the pre-MDMA baseline. Five minutes after clozapine (1 mg/kg, i.v.) discharge was reduced to 10 +/- 4% of the MDMA-elicited level. In conscious rats made hyperthermic by MDMA (10 mg/kg, s.c.), body temperature 1 hr after clozapine (3 mg/kg, s.c.) was 36.9 +/- 0.5 degrees C, <38.6 +/- 0.3 degrees C (Ringer's solution-treated) and not different from the pre-MDMA level. One hour after clozapine, rat tail blood flow was 24 +/- 3 cm/sec, greater than both flow in Ringer's solution-treated rats (8 +/- 1 cm/sec) and the pre-MDMA level (17 +/- 1 cm/sec). Clozapine and olanzapine, by interactions with 5-HT receptors or by other mechanisms, could reverse potentially fatal hyperthermia and cutaneous vasoconstriction occurring in some humans after ingestion of MDMA.