A projection from the ventral tegmental area to the periaqueductal gray involved in cardiovascular regulation

Noninvasive Ultrasound Stimulation of Ventral Tegmental Area Induces Reanimation from General Anaesthesia in Mice

Evidence in animals suggests that deep brain stimulation or optogenetics can be used for recovery from disorders of consciousness (DOC). However, these treatments require invasive procedures. This report presents a noninvasive strategy to stimulate central nervous system neurons selectively for recovery from DOC in mice. Through the delivery of ultrasound energy to the ventral tegmental area, mice were aroused from an unconscious, anaesthetized state in this study, and this process was controlled by adjusting the ultrasound parameters. The mice in the sham group under isoflurane-induced, continuous, steady-state general anaesthesia did not regain their righting reflex. On insonation, the emergence time from inhaled isoflurane anaesthesia decreased (sham: 13.63 ± 0.53 min, ultrasound: 1.5 ± 0.19 min, p < 0.001). Further, the induction time (sham: 12.0 ± 0.6 min, ultrasound: 17.88 ± 0.64 min, p < 0.001) and the concentration for 50% of the maximal effect (EC50) of isoflurane (sham: 0.6%, ultrasound: 0.7%) increased. In addition, ultrasound stimulation reduced the recovery time in mice with traumatic brain injury (sham: 30.38 ± 1.9 min, ultrasound: 7.38 ± 1.02 min, p < 0.01). This noninvasive strategy could be used on demand to promote emergence from DOC and may be a potential treatment for such disorders.

Midbrain control of breathing and blood pressure: The role of periaqueductal gray matter and mesencephalic collicular neuronal microcircuit oscillators

Neural circuitry residing within the medullary ventral respiratory column nuclei and dorsal respiratory group interact with the Kölliker-Fuse and medial parabrachial nuclei to generate the core breathing rhythm and pattern during resting conditions. Triphasic eupnea consists of inspiratory [I], post-inspiratory [post-I], and late-expiratory [E2] phases. Mesencephalic zones exert modulatory influences upon respiratory rhythm-generating circuitry, sympathetic oscillators, and parasympathetic nuclei. The earliest evidence supporting the existence of midbrain control of breathing derives from studies conducted by Martin and Booker in 1878. These authors demonstrated electrical stimulation of the deep layers of the mesencephalic colliculi in the rabbit augmented ventilation and sequentially elicited chest wall tremors and tetany. Investigations performed during the past several decades would demonstrate stimlation of distributed zones within the midbrain reticular formation elicits starkly disparate effects upon respiratory phase switching. Schmid, Böhmer, and Fallert demonstrated electrical stimulation of the nucleus rubre and emanating axon bundles alternately elicits or inhibits the activity of medullary expiratory- or inspiratory-related units and phrenic nerve discharge with differential latency. A series of studies would later indicate the red nucleus mediates hypoxic ventilatory depression. Periaqueductal gray matter neurons exhibit extensive afferent and efferent interconnectivity with suprabulbar, brainstem, and spinal cord zones aptly positioning these units to modulate breathing, autonomic outflow, nociception locomotion, micturtion, and sexual behavior. Experimental stimulatory activation of the tectal colliculi and periaqueductal gray matter via electrical current or glutamate, D,L-homocysteinic acid, or bicuculline microinjections coordinately modulates neuromotor inspiratory bursting frequency and amplitude and discharge of pre-Bötzinger complex, ventrolateral medullary late-I and post-I, and ventrolateral nucleus tractus solitarius decrementing early-I and augmenting and decrementing late-I neurons, elicits expiratory outflow and vocalization, and blunt the Hering-Breuer reflex in unanesthetzed decerebrate and anesthetized preprations of the cat and rat. Stimulation of the mesencephalic colliuli or dorsal divisions of the PAG potently amplifes renal sympathetic neural efferent activity, dynamic arterial pressure magnitude, and myocardial contraction frequency and elicits various behavioral defense responses. Elicited physiological effects exhibit extensive locoregional heterogeneity and variably enlist requisite contributions from the dorsomedial hypothalamus and/or lateral parabrachial nuclei. Stimulation of the dorsal mesencephalon occasionally elicits dynamic increases of arterial pressure magnitude exhibiting prominent oscillatory variability coherent with phrenic nerve discharge, perhaps by generating intra-neuraxial hysteresis, serving to intermittently deliver blood to organ vascular beds under high pressure in order to prevent organ edema, microcirculatory dysfunction, and downregulation of vascular smooth muscle alpha adrenergic receptors. Chemosensitive mesencephalic caudal raphé units and projections of hypoxia-sensitive units in the caudal hypothalamus to the periaqueductal gray matter may imply the existence of a diencephalo-smesencephalic chemosensitive network modulating breathing and sympathetic discharge.

Relative contributions and mapping of ventral tegmental area dopamine and GABA neurons by projection target in the rat

The ventral tegmental area (VTA) is a heterogeneous midbrain structure that contains dopamine (DA), GABA, and glutamate neurons that project to many different brain regions. Here, we combined retrograde tracing with immunocytochemistry against tyrosine hydroxylase (TH) or glutamate decarboxylase (GAD) to systematically compare the proportion of dopaminergic and GABAergic VTA projections to 10 target nuclei: anterior cingulate, prelimbic, and infralimbic cortex; nucleus accumbens core, medial shell, and lateral shell; anterior and posterior basolateral amygdala; ventral pallidum; and periaqueductal gray. Overall, the non-dopaminergic component predominated VTA efferents, accounting for more than 50% of all projecting neurons to each region except the nucleus accumbens core. In addition, GABA neurons contributed no more than 20% to each projection, with the exception of the projection to the ventrolateral periaqueductal gray, where the GABAergic contribution approached 50%. Therefore, there is likely a significant glutamatergic component to many of the VTA's projections. We also found that VTA cell bodies retrogradely labeled from the various target brain regions had distinct distribution patterns within the VTA, including in the locations of DA and GABA neurons. Despite this patterned organization, VTA neurons comprising these different projections were intermingled and never limited to any one subregion. These anatomical results are consistent with the idea that VTA neurons participate in multiple distinct, parallel circuits that differentially contribute to motivation and reward. While attention has largely focused on VTA DA neurons, a better understanding of VTA subpopulations, especially the contribution of non-DA neurons to projections, will be critical for future work.

GABA modulates baroreflex in the ventral tegmental area in rat

There are some reports demonstrating the cardiovascular functions of the ventral tegmental area (VTA). About 20-30% of the VTA neurons are GABAergic, which might play a role in baroreflex modulation. This study was performed to find the effects of GABA(A), GABA(B) receptors and reversible synaptic blockade of the VTA on baroreflex. Drugs were microinjected into the VTA of urethane anesthetized rats, and the maximum change of blood pressure and the gain of the reflex bradycardia in response to intravenous phenylephrine (Phe) injection were compared with the preinjection and the control values. Microinjection of bicuculline methiodide (BMI, 100 pmol/100 nl), a GABA(A) antagonist, into the VTA strongly decreased the Phe-induced hypertension, indicating that GABA itself attenuated the baroreflex. Muscimol, a GABA(A) agonist (30 mM, 100 nl), produced no significant changes. Baclofen, a GABA(B) receptor agonist (1000 pmole/100 nl), moderately attenuated the baroreflex, however phaclofen, a GABA(B) receptor antagonist (1000 pmole/100 nl), had no significant effect. In conclusion, for the first time, we demonstrated that GABA(A) receptors of the VTA strongly attenuate and GABA(B) receptors of the VTA moderately attenuate baroreflex in rat.

Cardiovascular responses of the anterior claustrum; its mechanism; contribution of medial prefrontal cortex

The anterior claustrum (CLa) has bilateral connections with the areas involved in cardiovascular regulation, though its role in cardiovascular control is not yet understood. This study was performed to find the cardiovascular responsive region of the CLa by stimulating all parts of the CLa with l-glutamate, and to find the possible mechanisms mediating its responses in urethane-anesthetized rats. We also investigated the possible involvement of the medial prefrontal cortex in the cardiovascular responses of the CLa. The effect of microinjection of l-glutamate (50-100 nl, 0.25 M) was tested throughout the Cla and only in one area at 2.7 mm rostral to bregma, 1.8-2.0 midline and 4.5-5.6mm vertical, significant decreases in arterial pressure were elicited (-21.71±2.1 mmHg, P<0.001, t-test) with no significant change in heart rate. Administration (i.v.) of the muscarinic receptor blocker, atropine, had no effect on the change in mean arterial pressure in response to glutamate stimulation, suggesting that the parasympathetic system was not involved in this response. However, administration (i.v.) of the nicotinic receptor blocker, hexamethonium dichloride abolished the depressor response to glutamate, suggesting that CLa stimulation decreases sympathetic outflow to the cardiovascular system. In addition, microinjection of the reversible synaptic blocker, cobalt chloride, into the medial prefrontal cortex greatly attenuated the depressor response elicited by microinjection of glut into the CLa. Thus for the first time, we found the cardiovascular responsive region of the anterior claustrum. Also we showed that its response is mediated through the medial prefrontal cortex.

Central command: control of cardiac sympathetic and vagal efferent nerve activity and the arterial baroreflex during spontaneous motor behaviour in animals

Feedforward control by higher brain centres (termed central command) plays a role in the autonomic regulation of the cardiovascular system during exercise. Over the past 20 years, workers in our laboratory have used the precollicular-premammillary decerebrate animal model to identify the neural circuitry involved in the CNS control of cardiac autonomic outflow and arterial baroreflex function. Contrary to the traditional idea that vagal withdrawal at the onset of exercise causes the increase in heart rate, central command did not decrease cardiac vagal efferent nerve activity but did allow cardiac sympathetic efferent nerve activity to produce cardiac acceleration. In addition, central command-evoked inhibition of the aortic baroreceptor-heart rate reflex blunted the baroreflex-mediated bradycardia elicited by aortic nerve stimulation, further increasing the heart rate at the onset of exercise. Spontaneous motor activity and associated cardiovascular responses disappeared in animals decerebrated at the midcollicular level. These findings indicate that the brain region including the caudal diencephalon and extending to the rostral mesencephalon may play a role in generating central command. Bicuculline microinjected into the midbrain ventral tegmental area of decerebrate rats produced a long-lasting repetitive activation of renal sympathetic nerve activity that was synchronized with the motor nerve discharge. When lidocaine was microinjected into the ventral tegmental area, the spontaneous motor activity and associated cardiovascular responses ceased. From these findings, we conclude that cerebral cortical outputs trigger activation of neural circuits within the caudal brain, including the ventral tegmental area, which causes central command to augment cardiac sympathetic outflow at the onset of exercise in decerebrate animal models.

High frequency stimulation of the subthalamic nucleus increases c-fos immunoreactivity in the dorsal raphe nucleus and afferent brain regions

High frequency stimulation (HFS) of the subthalamic nucleus (STN) is the neurosurgical therapy of choice for the management of motor deficits in patients with advanced Parkinson's disease, but this treatment can elicit disabling mood changes. Our recent experiments show that in rats, HFS of the STN both inhibits the firing of 5-HT (5-hydroxytryptamine; serotonin) neurons in the dorsal raphe nucleus (DRN) and elicits 5-HT-dependent behavioral effects. The neural circuitry underpinning these effects is unknown. Here we investigated in the dopamine-denervated rat the effect of bilateral HFS of the STN on markers of neuronal activity in the DRN as well as DRN input regions. Controls were sham-stimulated rats. HFS of the STN elicited changes in two 5-HT-sensitive behavioral tests. Specifically, HFS increased immobility in the forced swim test and increased interaction in a social interaction task. HFS of the STN at the same stimulation parameters, increased c-fos immunoreactivity in the DRN, and decreased cytochrome C oxidase activity in this region. The increase in c-fos immunoreactivity occurred in DRN neurons immunopositive for the GABA marker parvalbumin. HFS of the STN also increased the number of c-fos immunoreactive cells in the lateral habenula nucleus, medial prefrontal cortex but not significantly in the substantia nigra. Collectively, these findings support a role for circuitry involving DRN GABA neurons, as well as DRN afferents from the lateral habenula nucleus and medial prefrontal cortex, in the mood effects of HFS of the STN.

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.

Corticotropin-releasing factor receptor-1: a therapeutic target for cardiac autonomic disturbances

Corticotropin-releasing factor (CRF), a neuropeptide involved in triggering a myriad of responses to fear and stress, is favourably positioned in the CNS to modulate the sympathetic and parasympathetic branches of the cardiac autonomic nervous system. In vivo studies suggest that central CRF inhibits vagal output and stimulates sympathetic activity. Therefore, CRF may function to inhibit exaggerated vagal activation that results in severe bradycardia or even vasovagal syncope. On the other hand, CRF receptor-1 (CRF(1)) antagonists increase cardiac vagal and decrease sympathetic activity, thereby also implicating CRF(1) as a therapeutic target for autonomic disturbances resulting in elevated sympathetic activity, such as hypertension and coronary heart disease. The central distribution of CRF(1) and the cardiovascular effects of CRF(1) agonists and antagonists, suggest it mediates CRF-induced autonomic changes. However, there is insufficient information regarding the autonomic effects of CRF(2)-selective compounds to rule out CRF(2) contribution. This review provides an update on the autonomic effects of CRF and the neuronal projections thought to mediate these cardiovascular responses.

Evidence that hemorrhagic hypotension is mediated by the ventrolateral periaqueductal gray region

Severe hemorrhage lowers arterial pressure by suppressing sympathetic activity. This study tested the hypothesis that the decompensatory phase of hemorrhage is mediated by the ventrolateral periaqueductal gray (vlPAG), a region importantly involved in the autonomic and behavioral responses to stress and trauma. Neuronal activity in the vlPAG was inhibited with either lidocaine or cobalt chloride 5 min before hemorrhage (2.5 ml/100 g body wt) was initiated in conscious, unrestrained rats. Bilateral injection of lidocaine (0.5 microl of a 2% or 1 microl of a 5% solution) into the caudal vlPAG delayed the onset and reduced the magnitude of the hypotension produced by hemorrhage significantly. In contrast, inactivation of the dorsolateral PAG with lidocaine was ineffective. Cobalt chloride (5 mM; 0.5 microl), which inhibits synaptic transmission but not axonal conductance, also attenuated hemorrhagic hypotension significantly. Microinjection of lidocaine or cobalt chloride into the vlPAG of normotensive, nonhemorrhaged rats did not influence cardiovascular function. These data indicate that the vlPAG plays an important role in the response to hemorrhage.

Activation of the ventrolateral periaqueductal gray reduces locomotion but not mean arterial pressure in awake, freely moving rats

Activation of the ventrolateral periaqueductal gray produces immobility and antinociception. It has been argued that these behaviors are part of either a defensive fear response to threat or a recuperative quiescence response to deep tissue injury. Data collected in anesthetized animals showing that activation of the ventrolateral periaqueductal gray has a hypotensive effect supports the quiescence hypothesis. Our objective was to determine whether activation of the ventrolateral periaqueductal gray in awake, freely moving rats results in a decrease in blood pressure as it does in anesthetized animals. Changes in blood pressure produced by microinjection of the neuroexcitant D,L-homocysteic acid were measured using radio telemetry while rats were awake and while anesthetized with pentobarbital. Consistent with earlier reports, microinjection of D,L-homocysteic acid into the ventrolateral periaqueductal gray caused a decrease in blood pressure in anesthetized rats. In contrast, microinjection at the same ventrolateral periaqueductal gray sites while rats were awake had no effect on blood pressure, even though the animals became immobile and heart rate decreased. Thus, the immobility evoked from ventrolateral periaqueductal gray is not associated with a fall in mean arterial pressure. Two conclusions can be drawn from these data. (1) Caution must be used in generalizing from data collected in anesthetized animals. (2) The ventrolateral periaqueductal gray is as likely to contribute to defensive fear as to recuperative quiescence.