Ventrolateral medullary lesions block the antinociceptive and cardiovascular responses elicited by stimulating the dorsal periaqueductal grey matter in rats

Brainstem Mechanisms of Pain Modulation: A within-Subjects 7T fMRI Study of Placebo Analgesic and Nocebo Hyperalgesic Responses

Pain perception can be powerfully influenced by an individual's expectations and beliefs. Although the cortical circuitry responsible for pain modulation has been thoroughly investigated, the brainstem pathways involved in the modulatory phenomena of placebo analgesia and nocebo hyperalgesia remain to be directly addressed. This study used ultra-high-field 7 tesla functional MRI (fMRI) to accurately resolve differences in brainstem circuitry present during the generation of placebo analgesia and nocebo hyperalgesia in healthy human participants (N = 25, 12 male). Over 2 successive days, through blinded application of altered thermal stimuli, participants were deceptively conditioned to believe that two inert creams labeled lidocaine (placebo) and capsaicin (nocebo) were acting to modulate their pain relative to a third Vaseline (control) cream. In a subsequent test phase, fMRI image sets were collected while participants were given identical noxious stimuli to all three cream sites. Pain intensity ratings were collected and placebo and nocebo responses determined. Brainstem-specific fMRI analysis revealed altered activity in key pain modulatory nuclei, including a disparate recruitment of the periaqueductal gray (PAG)-rostral ventromedial medulla (RVM) pathway when both greater placebo and nocebo effects were observed. Additionally, we found that placebo and nocebo responses differentially activated the parabrachial nucleus but overlapped in engagement of the substantia nigra and locus coeruleus. These data reveal that placebo and nocebo effects are generated through differential engagement of the PAG-RVM pathway, which in concert with other brainstem sites likely influences the experience of pain by modulating activity at the level of the dorsal horn.SIGNIFICANCE STATEMENT Understanding endogenous pain modulatory mechanisms would support development of effective clinical treatment strategies for both acute and chronic pain. Specific brainstem nuclei have long been known to play a central role in nociceptive modulation; however, because of the small size and complex organization of the nuclei, previous neuroimaging efforts have been limited in directly identifying how these subcortical networks interact during the development of antinociceptive and pro-nociceptive effects. We used ultra-high-field fMRI to resolve brainstem structures and measure signal change during placebo analgesia and nocebo hyperalgesia. We define overlapping and disparate brainstem circuitry responsible for altering pain perception. These findings extend our understanding of the detailed organization and function of discrete brainstem nuclei involved in pain processing and modulation.

Using Deep Brain Stimulation to Unravel the Mysteries of Cardiorespiratory Control

This article charts the history of deep brain stimulation (DBS) as applied to alleviate a number of neurological disorders, while in parallel mapping the electrophysiological circuits involved in generating and integrating neural signals driving the cardiorespiratory system during exercise. With the advent of improved neuroimaging techniques, neurosurgeons can place small electrodes into deep brain structures with a high degree accuracy to treat a number of neurological disorders, such as movement impairment associated with Parkinson's disease and neuropathic pain. As well as stimulating discrete nuclei and monitoring autonomic outflow, local field potentials can also assess how the neurocircuitry responds to exercise. This technique has provided an opportunity to validate in humans putative circuits previously identified in animal models. The central autonomic network consists of multiple sites from the spinal cord to the cortex involved in autonomic control. Important areas exist at multiple evolutionary levels, which include the anterior cingulate cortex (telencephalon), hypothalamus (diencephalon), periaqueductal grey (midbrain), parabrachial nucleus and nucleus of the tractus solitaries (brainstem), and the intermediolateral column of the spinal cord. These areas receive afferent input from all over the body and provide a site for integration, resulting in a coordinated efferent autonomic (sympathetic and parasympathetic) response. In particular, emerging evidence from DBS studies have identified the basal ganglia as a major sub-cortical cognitive integrator of both higher center and peripheral afferent feedback. These circuits in the basal ganglia appear to be central in coupling movement to the cardiorespiratory motor program. © 2020 American Physiological Society. Compr Physiol 10:1085-1104, 2020.

The association between serotonin transporter availability and the neural correlates of fear bradycardia

Reduced expression of the serotonin transporter (5-HTT) is associated with susceptibility to stress-related psychopathology, but the underlying mechanisms remain elusive. We investigated whether an aberrant physiological and neural response to threat underlies this increased vulnerability. In a cross-species approach, we investigated the association between genetically encoded differences in 5-HTT expression and the neural correlates of fear bradycardia, a defensive response linked to vigilance. In both humans and rats, reduced 5-HTT expression was associated with exaggerated bradycardia or bradycardia-associated freezing, reduced activity of the medial prefrontal cortex, and increased threat-induced amygdala-periaqueductal grey connectivity and central amygdala somatostatin neuron activity. We have delineated a previously unknown neurogenetic mechanism underlying individual differences in the expression of anticipatory threat responses, contributing to stress sensitivity.

Susceptibility to stress-related psychopathology is associated with reduced expression of the serotonin transporter (5-HTT), particularly in combination with stress exposure. Aberrant physiological and neuronal responses to threat may underlie this increased vulnerability. Here, implementing a cross-species approach, we investigated the association between 5-HTT expression and the neural correlates of fear bradycardia, a defensive response linked to vigilance and action preparation. We tested this during threat anticipation induced by a well-established fear conditioning paradigm applied in both humans and rodents. In humans, we studied the effect of the common 5-HTT-linked polymorphic region (5-HTTLPR) on bradycardia and neural responses to anticipatory threat during functional magnetic resonance imaging scanning in healthy volunteers (n = 104). Compared with homozygous long-allele carriers, the 5-HTTLPR short-allele carriers displayed an exaggerated bradycardic response to threat, overall reduced activation of the medial prefrontal cortex (mPFC), and increased threat-induced connectivity between the amygdala and periaqueductal gray (PAG), which statistically mediated the effect of the 5-HTTLPR genotype on bradycardia. In parallel, 5-HTT knockout (KO) rats also showed exaggerated threat-related bradycardia and behavioral freezing. Immunohistochemistry indicated overall reduced activity of glutamatergic neurons in the mPFC of KO rats and increased activity of central amygdala somatostatin-positive neurons, putatively projecting to the PAG, which—similarly to the human population—mediated the 5-HTT genotype’s effect on freezing. Moreover, the ventrolateral PAG of KO rats displayed elevated overall activity and increased relative activation of CaMKII-expressing projection neurons. Our results provide a mechanistic explanation for previously reported associations between 5-HTT gene variance and a stress-sensitive phenotype.

Restricted lesions of the ventrolateral or dorsal columns of the periaqueductal gray promotes distinct effects on tonic immobility and defensive analgesia in guinea pigs

Tonic immobility (TI) is an innate defensive response exhibited by prey when physical contact with a predator is prolonged and inescapable. This defensive response is able to activate analgesia mechanisms; this activation has adaptive value because, during an attack by a predator, the manifestation of recuperative behaviors can affect the appropriate behavioral defense strategy. Some studies have suggested that similar structures of the central nervous system can regulate the response of both TI and nociception. Thus, this study evaluated the effect of chemical lesion through the administration of ibotenic acid in restricted brain areas of the periaqueductal gray matter (PAG) in guinea pig on the TI response and nociception evaluated in the hot plate test before and after emission of TI. The data showed that an irreversible chemical lesion in the ventrolateral PAG reduced of the TI response as well as defensive antinociception. However, a lesion in the dorsal PAG blocked the defensive antinociception induced by TI but did not alter TI duration. In summary, one could hypothesize that the neural substrates responsible for defensive behavior and antinociception represent similar systems that are distinct in modulation. Thus, the ventrolateral PAG has been associated with the modulation of TI and the defensive antinociception induced by TI. In contrast, the integrity of the dorsal PAG should be necessary for defensive antinociception to occur but not to elicit TI behavior in guinea pigs.

Device-based Therapy for Hypertension

Hypertension continues to be a major contributor to global morbidity and mortality, fuelled by an abundance of patients with uncontrolled blood pressure despite the multitude of pharmacological options available. This may occur as a consequence of true resistant hypertension, through an inability to tolerate current pharmacological therapies, or non-adherence to antihypertensive medication. In recent years, there has been a rapid expansion of device-based therapies proposed as novel non-pharmacological approaches to treating resistant hypertension. In this review, we discuss seven novel devices—renal nerve denervation, baroreflex activation therapy, carotid body ablation, central iliac arteriovenous anastomosis, deep brain stimulation, median nerve stimulation, and vagal nerve stimulation. We highlight how the devices differ, the varying degrees of evidence available to date and upcoming trials. This review also considers the possible factors that may enable appropriate device selection for different hypertension phenotypes.

Lower Functional Connectivity of the Periaqueductal Gray Is Related to Negative Affect and Clinical Manifestations of Fibromyalgia

Fibromyalgia (FM) syndrome is characterized by chronic widespread pain, muscle tenderness and emotional distress. Previous studies found reduced endogenous pain modulation in FM. This deficiency of pain modulation may be related to the attributes of chronic pain and other clinical symptoms experienced in patients with FM. Thus, we tested whether there is a link between the clinical symptoms of FM and functional connectivity (FC) of the periaqueductal gray (PAG), a key node of pain modulation. We acquired resting state 3T functional MRI (rsfMRI) data from 23 female patients with FM and 16 age- and sex- matched healthy controls (HC) and assessed FM symptoms with the Brief Pain Inventory (BPI), Fibromyalgia Impact Questionnaire (FIQ), Hospital Anxiety and Depression Scale (HADS) and Pain Catastrophizing Scale (PCS). We found that patients with FM exhibit statistically significant disruptions in PAG FC, particularly with brain regions implicated in negative affect, self-awareness and saliency. Specifically, we found that, compared to HCs, the FM patients had stronger PAG FC with the lingual gyrus and hippocampus but weaker PAG FC with regions associated with motor/executive functions, the salience (SN) and default mode networks (DMN). The attenuated PAG FC was also negatively correlated with FIQ scores, and positively correlated with the magnification subscale of the PCS. These alterations were correlated with emotional and behavioral symptoms of FM. Our study implicates the PAG as a site of dysfunction contributing to the clinical manifestations and pain in FM.

Respiratory manifestations of panic disorder in animals and humans: a unique opportunity to understand how supramedullary structures regulate breathing

The control of breathing is commonly viewed as being a "brainstem affair". As the topic of this special issue of Respiratory Physiology and Neurobiology indicates, we should consider broadening this notion since the act of breathing is also tightly linked to many functions other than close regulation of arterial blood gases. Accordingly, "non-brainstem" structures can exert a powerful influence on the core elements of the respiratory control network and as it is often the case, the importance of these structures is revealed when their dysfunction leads to disease. There is a clear link between respiration and anxiety and key theories of the psychopathology of anxiety (including panic disorders; PD) focus on respiratory control and related CO2 monitoring system. With that in mind, we briefly present the respiratory manifestations of panic disorder and discuss the role of the dorso-medial/perifornical hypothalamus, the amygdalar complex, and the periaqueductal gray in respiratory control. We then present recent advances in basic research indicating how adult rodent previously subjected to neonatal stress may provide a very good model to investigate the pathophysiology of PD.

Differentiated baroreflex modulation of sympathetic nerve activity during deep brain stimulation in humans

Targeted electric deep brain stimulation in midbrain nuclei in humans alters cardiovascular parameters, presumably by modulating autonomic and baroreflex function. Baroreflex modulation of sympathetic outflow is crucial for cardiovascular regulation and is hypothesized to occur at 2 distinct brain locations. The aim of this study was to evaluate sympathetic outflow in humans with deep brain stimulating electrodes during ON and OFF stimulation of specific midbrain nuclei known to regulate cardiovascular function. Multiunit muscle sympathetic nerve activity was recorded in 17 patients undergoing deep brain stimulation for treatment of chronic neuropathic pain (n=7) and Parkinson disease (n=10). Sympathetic outflow was recorded during ON and OFF stimulation. Arterial blood pressure, heart rate, and respiratory frequency were monitored during the recording session, and spontaneous vasomotor and cardiac baroreflex sensitivity were assessed. Head-up tilt testing was performed separately in the patients with Parkinson disease postoperatively. Stimulation of the dorsal most part of the subthalamic nucleus and ventrolateral periaqueductal gray resulted in improved vasomotor baroreflex sensitivity, decreased burst frequency and blood pressure, unchanged burst amplitude distribution, and a reduced fall in blood pressure after tilt. Stimulation of the dorsolateral periaqueductal gray resulted in a shift in burst amplitude distribution toward larger amplitudes, decreased spontaneous beat-to-beat blood pressure variability, and unchanged burst frequency, baroreflex sensitivity, and blood pressure. Our results indicate that a differentiated regulation of sympathetic outflow occurs in the subthalamic nucleus and periaqueductal gray. These results may have implications in our understanding of abnormal sympathetic discharge in cardiovascular disease and provide an opportunity for therapeutic targeting.

Control of the lungs via the human brain using neurosurgery

Neurosurgery can alter cardiorespiratory performance via central networks and includes deep brain stimulation (DBS), a routinely employed therapy for movement disorders and chronic pain syndromes. We review the established cardiovascular effects of DBS and the presumed mechanism by which they are produced via the central autonomic network. We then review the respiratory effects of DBS, including modulation of respiratory rate and lung function indices, and the mechanisms via which these may occur. We conclude by highlighting the potential future therapeutic applications of DBS for intractable airway diseases.

Control of breathing during exercise

During exercise by healthy mammals, alveolar ventilation and alveolar-capillary diffusion increase in proportion to the increase in metabolic rate to prevent PaCO2 from increasing and PaO2 from decreasing. There is no known mechanism capable of directly sensing the rate of gas exchange in the muscles or the lungs; thus, for over a century there has been intense interest in elucidating how respiratory neurons adjust their output to variables which can not be directly monitored. Several hypotheses have been tested and supportive data were obtained, but for each hypothesis, there are contradictory data or reasons to question the validity of each hypothesis. Herein, we report a critique of the major hypotheses which has led to the following conclusions. First, a single stimulus or combination of stimuli that convincingly and entirely explains the hyperpnea has not been identified. Second, the coupling of the hyperpnea to metabolic rate is not causal but is due to of these variables each resulting from a common factor which link the circulatory and ventilatory responses to exercise. Third, stimuli postulated to act at pulmonary or cardiac receptors or carotid and intracranial chemoreceptors are not primary mediators of the hyperpnea. Fourth, stimuli originating in exercising limbs and conveyed to the brain by spinal afferents contribute to the exercise hyperpnea. Fifth, the hyperventilation during heavy exercise is not primarily due to lactacidosis stimulation of carotid chemoreceptors. Finally, since volitional exercise requires activation of the CNS, neural feed-forward (central command) mediation of the exercise hyperpnea seems intuitive and is supported by data from several studies. However, there is no compelling evidence to accept this concept as an indisputable fact.

Effects of angiotensin (5-8) microinfusions into the ventrolateral periaqueductal gray on defensive behaviors in rats

Peptides of the renin-angiotensin system modulate blood pressure and hydro-electrolyte composition. Angiotensin (Ang) receptors are localized in brain areas related to the regulation of autonomic and endocrine control and involved in sensory perception, memory process and behavioral responses. Among these areas, the ventrolateral periaqueductal gray (vlPAG) is one of the most important structures of the neuronal circuitry controlling the autonomic and behavioral components of emotional states. Although Ang II metabolism in the vlPAG forms several Ang-peptides including Ang (5-8), the role of this tetrapeptide in the organization of defensive responses has not yet been described. To address this issue, the purpose of the present study was to determine the effects of intra-vlPAG injections of Ang (5-8) (0.2, 0.4 and 0.8 nmol/0.25 μL) in rats submitted to the elevated plus-maze (EPM) test. Additionally, it was evaluated the effects of intra-vlPAG Ang (5-8) on the expression of conditioned fear, assessed by the fear-potentiated startle and contextual conditioned freezing tests. The results showed that Ang (5-8) produced an intense, dose-related reduction in the entries into and time spent in the open arms of the EPM, decreased direct exploration and increased risk assessment behaviors. Moreover, intra-vlPAG injections of Ang (5-8) before the test session promoted pro-aversive effects in the FPS and enhanced contextual freezing. Taken together, these results point out to an important anxiogenic-like action for Ang (5-8) in the mediation of defensive behaviors organized in the vlPAG.

Defining the neurocircuitry of exercise hyperpnoea

One hundred years ago in this journal, Krogh and Lindhard published a seminal paper highlighting the importance of the brain in the control of breathing during exercise. This symposium report reviews the historical developments that have taken place since 1913, and attempts to place the detailed neurocircuitry thought to underpin exercise hyperpnoea into context by focusing on key structures that might form the command network. With the advent of enhanced neuroimaging and functional neurosurgical techniques, a unique window of opportunity has recently arisen to target potential circuits in humans. Animal studies have identified a priori sites of interest in mid-brain structures, in particular the subthalamic locomotor region (subthalamic nucleus, STN) and the periaqueductal grey (PAG), which have now been recorded from in humans during exercise. When all data are viewed in an integrative manner, the PAG, in particular the lateral PAG, and aspects of the dorsal lateral PAG, appear to be key communicating circuitry for 'central command'. Moreover, the PAG also fulfils many requirements of a command centre. It has functional connectivity to higher centres (dorsal lateral prefrontal cortex) and the basal ganglia (in particular, the STN), and receives a sensory input from contracting muscle, but, importantly, it sends efferent information to brainstem nuclei involved in cardiorespiratory control.

Conditioned fear is modulated by CRF mechanisms in the periaqueductal gray columns

The periaqueductal gray (PAG) columns have been implicated in controlling stress responses through corticotropin-releasing factor (CRF), which is a neuropeptide with a prominent role in the etiology of fear- and anxiety-related psychopathologies. Several studies have investigated the involvement of dorsal PAG (dPAG) CRF mechanisms in models of unconditioned fear. However, less is known about the role of this neurotransmission in the expression of conditioned fear memories in the dPAG and ventrolateral PAG (vlPAG) columns. We assessed the effects of ovine CRF (oCRF 0.25 and 1.0 μg/0.2 μL) locally administered into the dPAG and vlPAG on behavioral (fear-potentiated startle and freezing) and autonomic (arterial pressure and heart rate) responses in rats subjected to contextual fear conditioning. The lower dose injected into the columns promoted proaversive effects, enhanced contextual freezing, increased the blood pressure and heart rate and decreased tail temperature. The lower dose of oCRF into the vlPAG, but not into the dPAG, produced a pronounced enhancement of the fear-potentiated startle response. The results imply that the PAG is a heterogeneous structure that is involved in the coordination of distinct behaviors and autonomic control, suggest PAG involvement in the expression of contextual fear memory as well as implicate the CRF as an important modulator of the neural substrates of fear in the PAG.

Columnar distribution of catecholaminergic neurons in the ventrolateral periaqueductal gray and their relationship to efferent pathways

The periaqueductal gray (PAG) is a critical brain region involved in opioid analgesia and provides efferents to descending pathways that modulate nociception. In addition, the PAG contains ascending pathways to regions involved in the regulation of reward, including the substantia nigra (SN) and the ventral tegmental area (VTA). SN and VTA contain dopaminergic neurons that are critical for the maintenance of positive reinforcement. Interestingly, the PAG is also reported to contain a population of dopaminergic neurons. In this study, the distribution of catecholaminergic neurons within the ventrolateral (vl) PAG was examined using immunocytochemical methods. In addition, the catecholaminergic PAG neurons were examined to determine whether these neurons are integrated into ascending (VTA, SN) and descending rostral ventral medulla (RVM) efferent pathways from this region. The immunocytochemical analysis determined that catecholaminergic neurons in the PAG are both dopaminergic and noradrenergic and these neurons have a distinct rostrocaudal distribution within the ventrolateral column of PAG. Dopaminergic neurons were concentrated rostrally and were significantly smaller than noradrenergic neurons. Combined immunocytochemistry and tract tracing methods revealed that catecholaminergic neurons are distinct from, but closely associated with, both ascending and descending efferent projection neurons. Finally, by electron microscopy, catecholaminergic neurons showed close dendritic appositions with other neurons in PAG, suggesting a possible nonsynaptic mechanism for regulation of PAG output by these neurons. In conclusion, our data indicate that there are two populations of catecholaminergic neurons in the vlPAG that form dendritic associations with both ascending and descending efferents suggesting a possible nonsynaptic modulation of vlPAG neurons.

The autonomic effects of deep brain stimulation--a therapeutic opportunity

Deep brain stimulation (DBS) is an expanding field in neurosurgery and has already provided important insights into the fundamental mechanisms underlying brain function. One of the most exciting emerging applications of DBS is modulation of blood pressure, respiration and micturition through its effects on the autonomic nervous system. DBS stimulation at various sites in the central autonomic network produces rapid changes in the functioning of specific organs and physiological systems that are distinct from its therapeutic effects on central nervous motor and sensory systems. For example, DBS modulates several parameters of cardiovascular function, including heart rate, blood pressure, heart rate variability, baroreceptor sensitivity and blood pressure variability. The beneficial effects of DBS also extend to improvements in lung function. This article includes an overview of the anatomy of the central autonomic network, which consists of autonomic nervous system components in the cortex, diencephalon and brainstem that project to the spinal cord or cranial nerves. The effects of DBS on physiological functioning (particularly of the cardiovascular and respiratory systems) are discussed, and the potential for these findings to be translated into therapies for patients with autonomic diseases is examined.

Cannabinoid and GABA modulation of sympathetic nerve activity and blood pressure in the dorsal periaqueductal gray of the rat

Sympathoexcitation and increased blood pressure evoked by central networks integrating defensive behavior are fundamental to the acute stress response. A balance between excitatory glutamatergic and inhibitory GABAergic neurotransmission in the dorsal periaqueductal gray (dPAG) results in a tonic level of activity in the alerting system. Neuromodulators such as endocannabinoids have been shown to influence the sympathoexcitatory and pressor components of acute stress in the dPAG, exemplified by the defense response as a model, but the mechanism of integration remains unknown. The present study examines the role of GABA and its interaction with endocannabinoids in modulating sympathetic nerve activity and blood pressure related to the defense response. Microinjection of the broad-spectrum excitatory amino acid dl-homocysteic acid (DLH) identified sites of the defense pathway in the dPAG from which an increase in renal sympathetic nerve activity and blood pressure could be evoked, and subsequent microinjections were made at the same site through a multibarrelled micropipette. Blockade of GABAA receptors or microinjection of the cannabinoid 1 receptor agonist anandamide elicited a renal sympathoexcitation and pressor response. Prior microinjection of the GABAA receptor antagonist gabazine attenuated the sympathoexcitation and pressor response associated with anandamide microinjection. In contrast, the sympathetic response to DLH was enhanced by GABAA receptor blockade. These data demonstrate that sympathoexcitatory neurons in the dPAG are under tonic inhibition by GABA and that endocannabinoids modulate this GABAergic neurotransmission to help regulate components of the defense response.

Mapping the central neurocircuitry that integrates the cardiovascular response to exercise in humans

There are abundant animal data attempting to identify the neural circuitry involved in cardiovascular control. Translating this research into humans has been made possible using functional neurosurgery during which deep brain stimulating electrodes are implanted into various brain nuclei for the treatment of chronic pain and movement disorders. This not only allows stimulation of the human brain, but also presents the opportunity to record neural activity from various brain regions. This symposium review highlights key experiments from the past decade that have endeavoured to identify the neurocircuitry responsible for integrating the cardiovascular response to exercise in humans. Two areas of particular interest are highlighted: the periaqueductal grey and the subthalamic nucleus. Our studies have shown that the periaqueductal grey (particularly the dorsal column) is a key part of the neurocircuitry involved in mediating autonomic changes adapted to ongoing behaviours. Emerging evidence also suggests that the subthalamic nucleus is not only involved in the control of movement, but also in the mediation of cardiovascular responses. Although these sites are unlikely to be the 'command' areas themselves, we have demonstrated that the two nuclei have the properties of being key integrating sites between the feedback signals from exercising muscle and the feedforward signals from higher cortical centres.

Endocannabinoid modulation of sympathetic and cardiovascular responses to acute stress in the periaqueductal gray of the rat

Activation of the sympathetic nervous system is fundamental to the coordinated response to stress or danger. The midbrain periaqueductal gray (PAG) contains the neural substrate required to recruit the sympathetic nervous system and organize the physiological and behavioral responses required to respond to imposed challenges. Endocannabinoids have been shown to influence associated behavioral responses. The defense response was used in this study as a working model to examine endocannabinoid modulation of the sympathetic response to acute stress in the anesthetized rat. Microinjection of the cannabinoid 1 (CB1) receptor agonist anandamide into the defense pathway of the dorsal PAG could elicit an increase in renal sympathetic nerve activity and blood pressure, twitching of the whiskers, and movement of the limbs. The response was attenuated by prior microinjection of the CB1 receptor antagonist AM-281 at the same site. Electrical stimulation of the hypothalamic defense area could evoke similar sympathoexcitatory and pressor responses, which were significantly attenuated by microinjection of AM-281 into the dorsal PAG. These data indicate that endocannabinoids can modulate the sympathetic and cardiovascular components of the acute stress response via CB1 receptors at the level of the PAG.

Identifying cardiovascular neurocircuitry involved in the exercise pressor reflex in humans using functional neurosurgery

Groups III and IV afferents carry sensory information regarding the muscle exercise pressor reflex, although the central integrating circuits of the reflex in humans are still poorly defined. Emerging evidence reports that the periaqueductal gray (PAG) could be a major site for integrating the "central command" component that initiates the cardiovascular response to exercise, since this area is activated during exercise and direct stimulation of the dorsal PAG causes an increase in arterial blood pressure (ABP) in humans. Here we recorded local field potentials (LFPs) from various "deep" brain nuclei during exercise tasks designed to elicit the muscle pressor reflex. The patients studied had undergone neurosurgery for the treatment of movement or pain disorders, thus had electrodes implanted stereotactically either in the PAG, subthalamic nucleus, globus pallidus interna, thalamus, hypothalamus, or anterior cingulate cortex. Fast Fourier transform analysis was applied to the neurograms to identify the power of fundamental spectral frequencies. Our PAG patients showed significant increases in LFP power at frequencies from 4 to 8 Hz (P < 0.01), 8 to 12 Hz (P < 0.001), and 12 to 25 Hz (P < 0.001). These periods were associated with maintained elevated ABP during muscle occlusion following exercise. Further increases in exercise intensity resulted in corresponding increases in PAG activity and ABP. No significant changes were seen in the activity of other nuclei during occlusion. These electrophysiological data provide direct evidence for a role of the PAG in the integrating neurocircuitry of the exercise pressor reflex in humans.

Neural basis for improgan antinociception

Improgan, the prototype compound of a novel class of non-opioid analgesic drugs derived from histamine antagonists, attenuates thermal and mechanical nociception in rodents following intracerebroventricular (i.c.v.) administration. Improgan does not bind to known opioid, histamine or cannabinoid receptors, and its molecular target has not been identified. It is known however, that improgan acts directly in the periaqueductal gray and the rostral ventromedial medulla to produce its antinociceptive effects, and that inactivation of the rostral ventromedial medulla prevents the antinociceptive effect of improgan given i.c.v. Here we used in vivo single-cell recording in lightly anesthetized rats to show that improgan engages pain-modulating neurons in the medulla to produce antinociception. Following improgan administration, OFF-cells, which inhibit nociception, became continuously active and no longer paused during noxious stimulation. The increase in OFF-cell firing does not represent a non-specific neuroexcitant effect of this drug, since ON-cell discharge, associated with net nociceptive facilitation, was depressed. NEUTRAL-cell firing was unaffected by improgan. The net response of rostral ventromedial medulla (RVM) neurons to improgan is thus comparable to that evoked by mu-opioids and cannabinoids, well known RVM-active analgesic drugs. This common basis for improgan, opioid, and cannabinoid antinociception in the RVM supports the idea that improgan functions as a specific analgesic agent.

Opioid microinjection into raphe magnus modulates cardiorespiratory function in mice and rats

The raphe magnus (RM) participates in opioid analgesia and contains pain-modulatory neurons with respiration-related discharge. Here, we asked whether RM contributes to respiratory depression, the most prevalent lethal effect of opioids. To investigate whether opioidergic transmission in RM produces respiratory depression, we microinjected a mu-opioid receptor agonist, DAMGO, or morphine into the RM of awake rodents. In mice, opioid microinjection produced sustained decreases in respiratory rate (170 to 120 breaths/min), as well as heart rate (520 to 400 beats/min). Respiratory sinus arrhythmia, indicative of enhanced parasympathetic activity, was prevalent in mice receiving DAMGO microinjection. We performed similar experiments in rats but observed no changes in breathing rate or heart rate. Both rats and mice experienced significantly more episodes of bradypnea, indicative of impaired respiratory drive, after opioid microinjection. During spontaneous arousals, rats showed less tachycardia after opioid microinjection than before microinjection, suggestive of an attenuated sympathetic tone. Thus, activation of opioidergic signaling within RM produces effects beyond analgesia, including the unwanted destabilization of cardiorespiratory function. These adverse effects on homeostasis consequent to opioid microinjection imply a role for RM in regulating the balance of sympathetic and parasympathetic tone.

The diagonal band of Broca is involved in the pressor pathway activated by noradrenaline microinjected into the periaqueductal gray area of rats

AIMS

The dorsal periaqueductal gray area (dPAG) is involved in cardiovascular modulation. Previously, we reported that noradrenaline (NA) microinjection into the dPAG caused a pressor response that was mediated by vasopressin release into the circulation. However, the neuronal pathway that mediates this response is as yet unknown. There is evidence that chemical stimulation of the diagonal band of Broca (dbB) also causes a pressor response mediated by systemic vasopressin release. In the present study, we evaluated the participation of the dbB in the pressor response caused by NA microinjection into the dPAG as well as the existence of neural connections between these areas.

MAIN METHODS

With the above goal, we verified the effect of the pharmacological ablation of the dbB on the cardiovascular response to NA microinjection into the dPAG of unanesthetized rats. In addition, we microinjected the neuronal tracer biotinylated-dextran-amine (BDA) into the dPAG and looked for efferent projections from the dPAG to the dbB.

KEY FINDINGS

The pharmacologically reversible ablation of the dbB with local microinjection of CoCl(2) significantly reduced the pressor response caused by NA microinjection (15 nmol/50 nL) into the dPAG. In addition, BDA microinjection into the dPAG labeled axons in the dbB, pointing to the existence of direct connections between these areas.

SIGNIFICANCE

The present results indicate that synapses within the dbB are involved in the pressor pathway activated by NA microinjection into the dPAG and direct neural projection from the dPAG to the dbB may constitute the neuroanatomic substrate for this pressor pathway.

Preterm births: can neonatal pain alter the development of endogenous gating systems?

Prematurity is known to affect the development of various neurophysiological systems, including the maturation of pain and cardiac circuits. The purpose of this study was to see if numerous painful interventions, experienced soon after birth, affect counterirritation-induced analgesia (triggered using the cold pressor test) later in life. A total of 26 children, between the ages of 7 and 11 participated in the study. Children were divided into three groups, according to their birth status (i.e., term-born, born preterm and exposed to numerous painful interventions, or born preterm and exposed to few painful interventions). Primary outcome measures were heat pain thresholds, heat sensitivity scores, and cardiac reactivity. Results showed that preterm children and term-born children had comparable pain thresholds. Exposure to conditioning cold stimulation significantly increased heart rate and significantly decreased the thermal pain sensitivity of term-born children. These physiological reactions were also observed among preterm children who were only exposed to a few painful interventions at birth. Changes in heart rate and pain sensitivity in response to conditioning cold stimulation were not observed in preterm children that had been exposed to numerous painful procedures during the neonatal period. These results suggest that early pain does not lead to enhanced pain sensitivity when premature babies become children, but that their endogenous pain modulatory mechanisms are not as well developed as those of children not exposed to noxious insult at birth. Greater frequency of painful procedures also dampened the rise in heart rate normally observed when experimental pain is experienced.

Hypovolemic shock: critical involvement of a projection from the ventrolateral periaqueductal gray to the caudal midline medulla

Previous research has suggested that the ventrolateral column of the periaqueductal gray (vlPAG) plays a crucial role in triggering a decompensatory response (sympathoinhibition, hypotension, bradycardia) to severe blood loss. vlPAG excitation triggers also quiescence, decreased vigilance and decreased reactivity, the behavioral response which usually accompanies hypovolemic shock. The aim of this study was to identify, in unanesthetized rats, the main descending pathway(s) via which vlPAG neurons trigger sympathoinhibition and bradycardia in response to severe blood loss. Firstly, immediate early gene (c-Fos) expression was used to identify vlPAG neurons selectively activated by severe blood loss. Subsequently, the specific medullary projections of these vlPAG neurons were defined by combined c-Fos, retrograde tracing (double-label) experiments. It was found that vlPAG neurons selectively activated by severe hemorrhage project overwhelmingly to the vasodepressor portion of the caudal midline medulla (CMM). Previous studies indicate that this CMM region mediates behaviorally-coupled cardiovascular adjustments and the findings described here fit with the idea that CMM neurons are uniquely recruited by salient challenges, the adaptive responses to which require more than reflexive homeostatic cardiovascular adjustments.

Cardiovascular responses to noradrenaline microinjection in the ventrolateral periaqueductal gray of unanesthetized rats

The periaqueductal gray area (PAG) is a mesencephalic area involved in cardiovascular modulation. Noradrenaline (NA), a neurotransmitter involved in central blood pressure control, is present in the rat PAG. We report here on the cardiovascular effects caused by NA microinjection into the ventrolateral PAG (vlPAG) of unanesthetized rats and the peripheral mechanism involved in their mediation. NA microinjection in the vlPAG of unanesthetized rats evoked dose-related pressor and bradycardiac responses. No significant cardiovascular responses were observed in urethane-anesthetized rats. The pressor response was potentiated by pretreatment with the ganglion blocker pentolinium (5 or 10 mg/kg, intravenously). Pretreatment with the vasopressin antagonist dTyr(CH2)5 (Me)AVP (50 microg/kg, intravenously) blocked the pressor response evoked by the NA microinjection into the vlPAG. Additionally, circulating vasopressin content was found to be significantly increased after NA microinjection in the vlPAG. The results suggest that activation of noradrenergic synapses within the vlPAG modulates vasopressin release in unanesthetized rats.

Respiratory responses elicited by rostral versus caudal dorsal periaqueductal gray stimulation in rats

The periaqueductal gray (PAG) is a central neural region essential for defense behavior and coordination of accompanying autonomic responses. Activation of rostral versus caudal dorsal (dPAG) regions mediates different cardiovascular response patterns. Stimulation of the dPAG also elicits increased respiratory activity, however, it is unknown if there is a regional difference in dPAG modulation of respiratory pattern. The present study was undertaken to identify whether activation of rostral vs caudal dPAG modulates respiration differently. In anesthetized, spontaneously breathing rats, chemical and electrical stimulation in rostral and caudal dPAG evoked an increased respiratory frequency (f(R)) with significant shortening of both inspiratory (Ti) and expiratory time (Te). Stimulation in the dPAG also evoked significant increases in electromyography activity of the diaphragm (dEMG), arterial pressure, and heart rate. Caudal dPAG stimulation evoked a greater increase in f(R) due to a significantly greater decrease in Ti and Te than the rostral dPAG. Caudal dPAG stimulation also evoked a greater increase in baseline dEMG activity and elicited a significantly greater increase in dEMG amplitude above baseline than rostral dPAG. There was a rostro-caudal difference in the post-stimulus respiratory recovery response, with the caudal dPAG eliciting a longer sustained effect. No regional differences were identified in the arterial blood pressure and heart rate during dPAG stimulation. The results demonstrate that the magnitude of the respiratory response during and immediately after activation of the caudal dPAG is greater than during rostral dPAG stimulation.

Midbrain modulation of the cardiac baroreflex involves excitation of lateral parabrachial neurons in the rat

Activation of the dorsal periaqueductal gray (PAG) evokes defense-like behavior including a marked increase in sympathetic drive and resetting of baroreflex function. The goal of this study was to investigate the role of the lateral parabrachial nucleus (LPBN) in mediating dorsal PAG modulation of the arterial baroreflex. Reflex responses were elicited by electrical stimulation of the aortic depressor nerve (ADN) at 5 Hz or 15 Hz in urethane anesthetized rats (n=18). Electrical stimulation of the dorsal PAG at 10 Hz did not alter baseline mean arterial pressure (MAP) but did significantly attenuate baroreflex control of heart rate (HR) evoked by low frequency ADN stimulation. Alternatively, 40 Hz dorsal PAG stimulation increased baseline MAP (43+/-3 mm Hg) and HR (33+/-3 bpm) and attenuated baroreflex control of HR at both ADN stimulation frequencies. Reflex control of MAP was generally unchanged by dorsal PAG stimulation. Bilateral inhibition of neurons in LPBN area (n=6) with muscimol (0.45 nmol per side) reduced dorsal PAG-evoked increases in MAP and HR by 50+/-4% and 95+/-4%, respectively, and significantly reduced, but did not completely eliminate dorsal PAG attenuation of the cardiac baroreflex. Bilateral blockade of glutamate receptors in the LPBN area (n=6) with kynurenic acid (1.8 nmol) had a similar effect on dorsal PAG-evoked increases in MAP, HR and cardiac baroreflex function. Reflex control of MAP was unchanged with either treatment. These findings suggest that the LPBN area is one of several brainstem regions involved in descending modulation of the cardiac baroreflex function during defensive behavior.

Dorsal periaqueductal gray area synapses modulate baroreflex in unanesthetized rats

The dorsal portion of the periaqueductal gray area (dPAG) is involved in behavioral and cardiovascular control. We report the effect of acute and reversible dPAG blockade by local microinjection of either lidocaine or CoCl2 on the baroreflex response of unanesthetized rats. Acute and reversible blockade evoked by lidocaine microinjection into the dPAG did not affect the bradycardic response to mean arterial pressure (MAP) increases evoked by i.v. infusion of phenylephrine. However, lidocaine increased baroreflex gain and tachycardic reflex in response to MAP decreases evoked by i.v. infusion of sodium nitroprusside, thus suggesting an action on the sympathetic component of the baroreflex. The effects of dPAG synapses blockade caused by CoCl2 were similar to those observed after lidocaine microinjection. CoCl2 microinjection also increased baroreflex gain and tachycardiac responses to MAP decreases without affecting the parasympathetic baroreflex component. In conclusion, our data point to a dPAG tonic inhibitory involvement in baroreflex control, specifically modulating the sympathetic baroreflex component. Temporary dPAG ablation by local microinjection of lidocaine increased the sympathetic baroreflex component. Because CoCl2 microinjection had similar effects on the baroreflex, this modulation involves local synaptic neurotransmission within the dPAG.

Identifying cardiorespiratory neurocircuitry involved in central command during exercise in humans

For almost one hundred years, the exact role of human brain structures controlling the cardiorespiratory response to exercise ('central command') has been sought. Animal experiments and functional imaging studies have provided clues, but the underlying electrophysiological activity of proposed relevant neural sites in humans has never been measured. In this study, local field potentials were directly recorded in a number of 'deep' brain nuclei during an exercise task designed to dissociate the exercise from peripheral feedback mechanisms. Several patient groups had electrodes implanted sterotaxically for the treatment of movement disorder or chronic pain. Fast Fourier transform analysis was applied to the neurograms to identify the power of fundamental spectral frequencies. Anticipation of exercise resulted in increases in heart rate, blood pressure and ventilation. The greatest neural changes were found in the periaqueductal grey area (PAG) where anticipation of exercise was accompanied by an increase of 43% in the power of the 12-25 Hz frequency band (P = 0.007). Exercise increased the activity by 87% compared to rest (P = 0.006). Changes were also seen in the 60-90 Hz band when anticipation or exercise increased power by 32% (P = 0.006) and 109% (P < 0.001), respectively. In the subthalamic nucleus there was a reduction in the power of the beta frequency during both anticipation (7.6 +/- 0.68% P = 0.001) and exercise (17.3 +/- 0.96% P < 0.001), whereas an increase was seen with exercise only at higher frequencies (93 +/- 1.8% P = 0.007). No significant changes were seen in the globus pallidus during anticipation of exercise. We provide direct electrophysiological evidence highlighting the PAG as an important subcortical area in the neural circuitry of the cardiorespiratory response to exercise, since stimulation of this structure is known to alter blood pressure in awake humans.

Chemical stimulation of visceral afferents activates medullary neurones projecting to the central amygdala and periaqueductal grey

Cholecystokinin (CCK) stimulates gastrointestinal vagal afferent neurones that signal visceral sensations. We wished to determine whether neurones of the nucleus of the solitary tract (NTS) or ventrolateral medulla (VLM) convey visceral afferent information to the central nucleus of the amygdala (CeA) or periaqueductal grey region (PAG), structures that play a key role in adaptive autonomic responses triggered by stress or fear. Male Sprague-Dawley rats received a unilateral microinjection of the tracer cholera toxin subunit B (CTB, 1%) into the CeA or PAG followed, 7 days later, by an injection of CCK (100 microg/kg, i.p.) or saline. Brains were processed for detection of Fos protein (Fos-IR) and CTB. CCK induced increased expression of Fos-IR in the NTS and the VLM, relative to control. When CTB was injected into the CeA, CTB-immunoreactive (CTB-IR) neurones were more numerous in the rostral NTS ipsilateral to the injection site, whereas they were homogeneously distributed throughout the VLM. Double-labelled neurones (Fos-IR+CTB-IR) were most numerous in the ipsilateral NTS and caudal VLM. The NTS contained the higher percentage of CTB-IR neurones activated by CCK. When CTB was injected into the PAG, CTB-IR neurones were more numerous in the ipsilateral NTS whereas they were distributed relatively evenly bilaterally in the rostral VLM. Double-labelled neurones were not differentially distributed along the rostrocaudal axis of the NTS but were more numerous in this structure when compared with the VLM. NTS and VLM neurones may convey visceral afferent information to the CeA and the PAG.

Stimulating the human midbrain to reveal the link between pain and blood pressure

The periaqueductal grey area (PAG) in the midbrain is an important area for both cardiovascular control and modulation of pain. However, the precise relationship between pain and blood pressure is unknown. We prospectively studied 16 patients undergoing deep brain stimulation of the rostral PAG for chronic pain. Pre-operatively, post-operatively, and at 1 year, pain scores were assessed using both visual analogue scores and the McGill Pain Questionnaire. Patients were tested post-operatively to determine whether electrical stimulation of the PAG would modulate blood pressure. We found that the degree of analgesia induced by deep brain stimulation of the rostral PAG in man is related to the magnitude of reduction in arterial blood pressure. We found that this relationship is linear and is related to reduced activity of the sympathetic nervous system. Thus stimulation of the PAG may partly control pain by reducing sympathetic activity as predicted by William James over a century ago.

Deep brain stimulation can regulate arterial blood pressure in awake humans

The periaqueductal grey matter is known to play a role in cardiovascular control in animals. Cardiovascular responses to electrical stimulation of the periventricular/periaqueductal grey matter were measured in 15 awake human study participants following implantation of deep brain stimulating electrodes for treatment of chronic pain. We found that stimulation of the ventral periventricular/periaqueductal grey matter caused a mean reduction in systolic blood pressure of 14.2+/-3.6 mmHg in seven patients and stimulation of the dorsal periventricular/periaqueductal grey matter caused a mean increase of 16.7+/-5.9 mmHg in six patients. A comparison between ventral and dorsal electrodes demonstrated significant differences (P<0.05). These changes were accompanied by analogous changes in diastolic blood pressure, pulse pressure, maximum dP/dt but not in the time interval between each R wave on the electrocardiogram.

Cardiovascular effects of noradrenaline microinjected into the dorsal periaqueductal gray area of unanaesthetized rats

The periaqueductal grey area (PAG) is a mesencephalic region that is involved in the modulation of cardiovascular changes associated with behavioural responses. Among the neurotransmitters present in the PAG, noradrenaline (NA) is also known to be involved in central nervous system cardiovascular regulation. In the present study we report the cardiovascular effects of the microinjection of NA into the dorsal portion of the PAG (dPAG) of unanaesthetized rats and the peripheral mechanism involved in their mediation. Injection of NA in the dPAG of unanaesthetized rats evoked a dose-dependent pressor response accompanied by bradycardia. The magnitude of the pressor responses was higher at more rostral sites in the dPAG and decreased when NA was injected into the caudal portion of the dPAG. The responses to NA were markedly reduced in urethane-anaesthetized rats. The pressor response was potentiated by i.v. pretreatment with the ganglion blocker pentolinium and blocked by i.v. pretreatment with the vasopressin antagonist dTyr(CH2)5(Me)AVP. The results suggest that activation of noradrenergic receptors within the dPAG can evoke pressor responses, which are mediated by acute vasopressin release.

Involvement of the periaqueductal gray in the hypotensive response evoked by l-glutamate microinjection in the lateral hypothalamus of unanesthetized rats

The lateral hypothalamus (LH) is involved in cardiovascular control. L-glutamate (L-glu) stimulation of the LH of unanesthetized rats evoked hypotensive responses without significant heart rate changes. The neuronal pathway that mediates this response is unknown. There is evidence that the periaqueductal gray (PAG) is involved in the mediation of hypotensive responses evoked by electrical stimulation of the LH. In the present study, we attempted to verify the effect of an acute and reversible pharmacological ablation of the PAG with lidocaine or CoCl(2) on the hypotensive response caused by L-glu injection in the LH of unanesthetized rats. Microinjection of the local anesthetic lidocaine or the unspecific synaptic blocker CoCl(2) in the PAG significantly attenuated the hypotensive effects of L-glu stimulation of the LH, indicating the involvement of local synapses within the PAG in the hypotensive pathway activated by LH glutamatergic receptors. Microinjection of the neuronal tracer biotinylated dextran amine (BDA) in the PAG labeled neuronal cell bodies in the LH, indicating the existence of direct connections between these areas. In conclusion, the present results indicate that the hypotensive response evoked by L-glu stimulation of LH may involve a synaptic relay in the dorsal PAG.

Identification of neural pathways involved in genital reflexes in the female: a combined anterograde and retrograde tracing study

The medial preoptic area (MPOA) is important for reproductive behavior in females. However, the descending pathways mediating these responses to the spinal motor output are unknown. The MPOA does not directly innervate the spinal cord. Therefore, pathways mediating MPOA-induced changes in sexual behavior must relay in the brain. The nucleus paragigantocellularis (nPGi) projects heavily to spinal circuits involved in female sexual reflexes and is involved in the tonic inhibition of genital reflexes. However, the periaqueductal gray (PAG) is also important for female sexual behavior. The present study examined the hypothesis that the MPOA output relays through PAG and the nPGi before descending to the spinal cord. We used anterograde and retrograde tracing techniques to examine the descending pathways and relay sites from the MPOA to the spinal cord and the nPGi in the female rat. Injection of biotinylated dextran amine into the MPOA produced dense labeling in specific regions of the PAG and Barrington's nucleus; anterogradely labeled fibers terminated close to neurons retrogradely labeled from the spinal cord in the PAG, Barrington's nucleus, nPGi, lateral hypothalamus and paraventricular nucleus (PVN). Anterogradely labeled fibers and varicosities were also found close to neurons retrogradely labeled from the nPGi in the PAG, lateral hypothalamus and PVN. These results suggest that the major MPOA output relays in the PAG and nPGi before descending to innervate spinal circuits regulating female genital reflexes and that the MPOA plays a multifaceted role in female reproductive behavior through its modulation of PAG output systems.

Activation of 5-HT1B/1D receptor in the periaqueductal gray inhibits nociception

It is considered that the site of action of the abortive antimigraine compounds acting at serotonin, 5-HT(1B/1D,) receptors (triptans) is the trigeminovascular system. We tested whether there is a non-trigeminal site of action. The 5-HT(1B/1D) agonist, naratriptan, was microinjected into the ventrolateral periaqueductal gray (vlPAG), and activity in the trigeminal nucleus caudalis (TNC) was monitored. Recordings were made from 20 nociceptive neurons in the dorsal horn of the TNC that received convergent input from the dura mater and face. Responses of neurons to dural, facial cutaneous and corneal stimulation were studied before and after injection of naratriptan. Naratriptan decreased the excitability to electrical stimulation of the dura mater as the A-fiber response decreased by 24 +/- 4.1% (p < 0.001) and the C-fiber response decreased by 42 +/- 8.2% (p < 0.001). Spontaneous activity was decreased by 38 +/- 7.5% (p < 0.001). After injection, the mechanical thresholds of the dura mater increased from (n = 14, p < 0.01). Responses to stimulation of the face and cornea were not altered by injection of naratriptan. These results suggest that 5-HT(1B/1D) receptor activation in the vlPAG activates descending pain-modulating pathways that inhibit dural, but not facial and corneal nociceptive input. These findings have implications for the understanding of the action of triptans in migraine and cluster headache, suggesting that brain loci other than the trigeminal nucleus may play a role in the clinical action of triptans.

Electrical stimulation of the dorsal periaqueductal gray decreases volume of the brain infarction independently of accompanying hypertension and cerebrovasodilation

We investigated whether selective stimulation of neurons of the sympathoinhibitory ventral periaqueductal gray (VPAG), or sympathoexcitatory dorsal periaqueductal gray (DPAG), differentially modulates CBF and EEG and exerts neuroprotection. Electrical stimulation of either regions of PAG comparably elevated AP and CBF, whereas chemical stimulation with the D,L-homocysteine produced either sympathoinhibition accompanied by decrease in CBF from ventral region or sympathoexcitation accompanied by increase in CBF from dorsal region in nonspinalized rats. The CBF effects evoked from DPAG and VPAG by chemical stimulation were preserved in spinalized rats supporting that the evoked CBF responses result directly from stimulation and are not secondary to AP changes. Stimulation of either region, whether chemical or electrical, synchronized the EEG. To explore whether PAG stimulation might protect the brain against ischemic injury, in other rats the VPAG or DPAG were stimulated for 1 h (50 Hz, 1 s on/1 s off, 75-100 microA) and the middle cerebral artery occluded 72 h later. Stimulation of the DPAG, but not VPAG, significantly reduced infarction volumes relative to sham-stimulated controls as determined 24 h after occlusion. Elevations of AP and CBF did not differ between groups. We conclude: (a). intrinsic neurons of D- and VPAG differentially regulate CBF; (b). neurons of DPAG are neuroprotective independently of changes in CBF and/or AP. The DPAG effect on infarct volume may be related to the central neuroprotective pathway evoked by stimulation of the cerebellar FN.

Phenotypic identification of rat rostroventrolateral medullary presympathetic vasomotor neurons inhibited by exogenous cholecystokinin

Systemic administration of the gastrointestinal hormone cholecystokinin (CCK) selectively inhibits splanchnic sympathetic vasomotor discharge and differentially affects presympathetic vasomotor neurons of the rostroventrolateral medulla (RVLM). Stimulation of the sympathoexcitatory region of the periaqueductal grey (PAG) produces profound mesenteric vasoconstriction. In this study, our aim was to identify phenotypically different populations of RVLM presympathetic vasomotor neurons using juxtacellular neuronal labelling and immunohistochemical detection of the adrenergic neuronal marker phenylethanolamine-N-methyl transferase (PNMT) and to determine whether the PAG provides functional excitatory input to these neurons. Fifty-eight percent (36/62) of RVLM presympathetic neurons were inhibited by systemic administration of CCK. These cells had conduction velocities (3.6 +/- 0.2 m/sec) in the non-C-fiber range consistent with neurons possessing lightly myelinated spinal axons. Of these, 79% (22/28) were excited by PAG stimulation, and 59% (10/17) were not immunoreactive for PNMT. Conversely, 42% (26/62) of RVLM presympathetic neurons were either unaffected or activated by CCK administration and had slower conduction velocities (1.4 +/- 0.3 m/sec) than cells inhibited by CCK. Fifty percent (11/22) of these cells were driven by PAG stimulation, and most (11/14 or 79%) were PNMT-positive. These results suggest that cardiovascular responses elicited by PAG stimulation occur via activation of non-C1 and C1 RVLM presympathetic neurons. RVLM neurons inhibited by CCK were more likely to be driven by PAG stimulation and may be a subset of neurons responsible for driving gastrointestinal sympathetic vasomotor tone. CCK-induced inhibition of a subpopulation of RVLM presympathetic neurons may be implicated in postprandial hyperemia and postprandial hypotension.