Dorsal Periaqueductal Gray Research Articles

Stimulation of the midbrain periaqueductal gray modulates preinspiratory neurons in the ventrolateral medulla in the rat in vivo

The midbrain periaqueductal gray (PAG) is involved in many basic survival behaviors that affect respiration. We hypothesized that the PAG promotes these behaviors by changing the firing of preinspiratory (pre-I) neurons in the pre-Bötzinger complex, a cell group thought to be important in generating respiratory rhythm. We tested this hypothesis by recording single unit activity of pre-Bötzinger pre-I neurons during stimulation in different parts of the PAG. Stimulation in the dorsal PAG increased the firing of pre-I neurons, resulting in tachypnea. Stimulation in the medial part of the lateral PAG converted the pre-I neurons into inspiratory phase-spanning cells, resulting in inspiratory apneusis. Stimulation in the lateral part of the lateral PAG generated an early onset of the pre-I neuronal discharge, which continued throughout the inspiratory phase, while at the same time attenuating diaphragm contraction. Stimulation in the ventral part of the lateral PAG induced tachypnea but inhibited pre-I cell firing, whereas stimulation in the ventrolateral PAG inhibited not only pre-I cells but also the diaphragm, leading to apnea. These findings show that PAG stimulation changes the activity of the pre-Bötzinger pre-I neurons. These changes are in line with the different behaviors generated by the PAG, such as the dorsal PAG generating avoidance behavior, the lateral PAG generating fight and flight, and the ventrolateral PAG generating freezing and immobility. J. Comp. Neurol. 521: 3083–3098, 2013. © 2013 Wiley Periodicals, Inc.

Comparing neural response to painful electrical stimulation with functional MRI at 3 and 7 T

Progressing from 3T to 7T functional MRI enables marked improvements of human brain imaging in vivo. Although direct comparisons demonstrated advantages concerning blood oxygen level dependent (BOLD) signal response and spatial specificity, these mostly focused on single brain regions with rather simple tasks. Considering that physiological noise also increases with higher field strength, it is not entirely clear whether the advantages of 7T translate equally to the entire brain during tasks which elicit more complex neuronal processing. Therefore, we investigated the difference between 3T and 7T in response to transcutaneous electrical painful and non-painful stimulation in 22 healthy subjects. For painful stimuli vs. baseline, stronger activations were observed at 7T in several brain regions including the insula and supplementary motor area, but not the secondary somatosensory cortex (p<0.05 FWE-corrected). Contrasting painful vs. non-painful stimulation limited the differences between the field strengths to the periaqueductal gray (PAG, p<0.001 uncorrected) due to a similar signal increase at 7T for both the target and specific control condition in most brain regions. This regional specificity obtained for the PAG at higher field strengths was confirmed by an additional spatial normalization strategy optimized for the brainstem. Here, robust BOLD responses were obtained in the dorsal PAG at 7T (p<0.05 FWE-corrected), whereas at 3T activation was completely missing for the contrast against non-painful stimuli. To summarize, our findings support previously reported benefits obtained at ultra-high field strengths also for complex activation patterns elicited by painful electrical stimulation. However, this advantage depends on the region and even more on the contrast of interest. The greatest gain at 7T was observed within the small brainstem region of the PAG, where the increased field strength offered marked improvement for the localization of activation foci with high spatial specificity.

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 1.0μg/0.2μL) 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.

Descending control of the respiratory neuronal network by the midbrain periaqueductal grey in the rat in vivo

Emotional reactions such as vocalization take place during expiration, and thus expression of emotional behaviour requires a switch from inspiration to expiration. I investigated how the midbrain periaqueductal grey (PAG), a known behavioural modulator of breathing, influences the inspiratory-to-expiratory phase transition. Contemporary models propose that late inspiratory (late-I) and post-inspiratory (post-I) neurones found in the medulla, which are active during the inspiratory-to-expiratory phase transition are involved in converting inspiration to expiration. I examined the effect of excitatory amino acid (d,l-homocysteic acid; DLH) stimulation of the PAG on the discharge function of late-I and post-I neurones. The data show a topographical organization of DLH-induced late-I and post-I neuronal modulation within the PAG. Dorsal PAG stimulation induced tachypnoea and caused excitation of both the late-I and post-I neurones. Lateral PAG induced inspiratory prolongation and caused an excitation of late-I neurones but inhibition of post-I neurones. Ventrolateral PAG induced expiratory prolongation and caused a persistent activation of post-I neurones. As well, PAG stimulation modulated both the late-I and post-I cells for least two-three breaths even prior to the change in respiratory motor pattern. This indicates that the PAG influences the late-I and post-I cells independent of pulmonary or other sensory afferent feedback. I conclude that the PAG modulates the activity of the medullary late-I and post-I neurones, and this modulation contributes to the conversion of eupnoea into a behavioural breathing pattern.

Analgesia induced by localized injection of opiate peptides into the brain of infant rats.

Stimulation of a variety of brain sites electrically or by opiates activates descending inhibitory pathways to attenuate noxious input to the spinal cord dorsal horn and produce analgesia. Analgesia induced by electrical stimulation of the periaqueductal grey (PAG) of the midbrain or medial rostral ventral medulla (RVM) matures late, towards the end or past the pre-weaning period. Descending facilitation takes precedence over inhibition. Yet opiates injected intracerebroventricularly or directly into the PAG induce analgesia relatively early in development. Our goal was to re-examine the role of opiates specific to individual receptor types in analgesia at several supraspinal sites. Antinociception was tested following microinjection of DAMGO (μ-opiate agonist), DPDPE (∂-opiate agonist) or U50,488 (κ-opiate agonist) into the PAG, RVM or dorsal lateral pons (DLP) in 3-, 10- and 14-day-old rats. DAMGO produced analgesia at 3 days of age at each brain area; the RVM was the most effective and the dorsal PAG was the least effective site. DPDPE produced modest analgesia at 10 and 14 days of age at the ventral PAG, RVM or DLP, but not the dorsal PAG. U50,488H was ineffective at all sites and all ages. Antinociception could be elicited at all three sites by DAMGO as early as 3 days of age and DPDPE at 10 and 14 days of age. The degree of analgesia increased gradually during the first 2 weeks of life, and likely reflects the maturation of connections within the brain and of descending inhibitory paths from these sites.

The Dorsolateral Periaqueductal Gray and Its Role in Mediating Fear Learning to Life Threatening Events

The dorsolateral column of the periaqueductal gray (dlPAG) integrates aversive emotional experiences and represents an important site responding to life threatening situations, such as hypoxia, cardiac pain and predator threats. Previous studies have shown that the dorsal PAG also supports fear learning; and we have currently explored how the dlPAG influences associative learning. We have first shown that N-methyl-D-aspartate (NMDA) 100 pmol injection in the dlPAG works as a valuable unconditioned stimulus (US) for the acquisition of olfactory fear conditioning (OFC) using amyl acetate odor as conditioned stimulus (CS). Next, we revisited the ascending projections of the dlPAG to the thalamus and hypothalamus to reveal potential paths that could mediate associative learning during OFC. Accordingly, the most important ascending target of the dlPAG is the hypothalamic defensive circuit, and we were able to show that pharmacological inactivation using β-adrenoceptor blockade of the dorsal premammillary nucleus, the main exit way for the hypothalamic defensive circuit to thalamo-cortical circuits involved in fear learning, impaired the acquisition of the OFC promoted by NMDA stimulation of the dlPAG. Moreover, our tracing study revealed multiple parallel paths from the dlPAG to several thalamic targets linked to cortical-hippocampal-amygdalar circuits involved in fear learning. Overall, the results point to a major role of the dlPAG in the mediation of aversive associative learning via ascending projections to the medial hypothalamic defensive circuit, and perhaps, to other thalamic targets, as well. These results provide interesting perspectives to understand how life threatening events impact on fear learning, and should be useful to understand pathological fear memory encoding in anxiety disorders.

Stimulation of the dorsal periaqueductal gray enhances spontaneous recovery of a conditioned taste aversion

Due to its relevance to clinical practice, extinction of learned fears has been a major focus of recent research. However, less is known about the means by which conditioned fears re-emerge (i.e., spontaneously recover) as time passes or contexts change following extinction. The periaqueductal gray represents the final common pathway mediating defensive reactions to fear and we have reported previously that the dorsolateral PAG (dlPAG) exhibits a small but reliable increase in neural activity (as measured by c-fos protein immunoreactivity) when spontaneous recovery (SR) of a conditioned taste aversion (CTA) is reduced. Here we extend these correlational studies to determine if inducing dlPAG c-fos expression through electrical brain stimulation could cause a reduction in SR of a CTA. Male Sprague-Dawley rats acquired a strong aversion to saccharin (conditioned stimulus; CS) and then underwent CTA extinction through multiple non-reinforced exposures to the CS. Following a 30-day latency period after asymptotic extinction was achieved; rats either received stimulation of the dorsal PAG (dPAG) or stimulation of closely adjacent structures. Sixty minutes following the stimulation, rats were again presented with the saccharin solution as we tested for SR of the CTA. The brain stimulation evoked c-fos expression around the tip of the electrodes. However, stimulation of the dPAG failed to reduce SR of the previously extinguished CTA. In fact, dPAG stimulation caused rats to significantly suppress their saccharin drinking (relative to controls) – indicating an enhanced SR. These data refute a cause-and-effect relationship between enhanced dPAG c-fos expression and a reduction in SR. However, they highlight a role for the dPAG in modulating SR of extinguished CTAs.

Personality Trait Predictors of Placebo Analgesia and Neurobiological Correlates

Personality traits have been shown to interact with environmental cues to modulate biological responses including treatment responses, and potentially having a role in the formation of placebo effects. Here, we assessed psychological traits in 50 healthy controls as to their capacity to predict placebo analgesic effects, placebo-induced activation of μ-opioid neurotransmission and changes in cortisol plasma levels during a sustained experimental pain challenge (hypertonic saline infused in the masseter muscle) with and without placebo administration. Statistical analyses showed that an aggregate of scores from Ego-Resiliency, NEO Altruism, NEO Straightforwardness (positive predictors) and NEO Angry Hostility (negative predictor) scales accounted for 25% of the variance in placebo analgesic responses. Molecular imaging showed that subjects scoring above the median in a composite of those trait measures also presented greater placebo-induced activation of μ-opioid neurotransmission in the subgenual and dorsal anterior cingulate cortex (ACC), orbitofrontal cortex, insula, nucleus accumbens, amygdala and periaqueductal gray (PAG). Endogenous opioid release in the dorsal ACC and PAG was positively correlated with placebo-induced reductions in pain ratings. Significant reductions in cortisol levels were observed during placebo administration and were positively correlated with decreases in pain ratings, μ-opioid system activation in the dorsal ACC and PAG, and as a trend, negatively with NEO Angry Hostility scores. Our results show that personality traits explain a substantial proportion of the variance in placebo analgesic responses and are further associated with activations in endogenous opioid neurotransmission, and as a trend cortisol plasma levels. This initial data, if replicated in larger sample, suggest that simple trait measures easily deployable in the field could be utilized to reduce variability in clinical trials, but may also point to measures of individual resiliency in the face of aversive stimuli such as persistent pain and potentially other stressors.

The negative effects of alcohol hangover on high-anxiety phenotype rats are influenced by the glutamate receptors of the dorsal midbrain

Alcoholism is a chronic disorder characterized by the appearance of a withdrawal syndrome following the abrupt cessation of alcohol intake that includes symptoms of physical and emotional disturbances, anxiety being the most prevalent symptom. In humans, it was shown that anxiety may increase the probability of relapse. In laboratory animals, however, the use of anxiety to predict alcohol preference has remained difficult. Excitatory amino acids as glutamate have been implicated in alcohol hangover and may be responsible for the seizures and anxiety observed during withdrawal. The dorsal periaqueductal gray (DPAG) is a midbrain region critical for the modulation/expression of anxiety- and fear-related behaviors and the propagation of seizures induced by alcohol withdrawal, the glutamate neurotransmission being one of the most affected. The present study was designed to evaluate whether low- (LA) and high-anxiety rats (HA), tested during the alcohol hangover phase, in which anxiety is the most prevalent symptom, are more sensitive to the reinforcing effects of alcohol when tested in a voluntary alcohol drinking procedure. Additionally, we were interested in investigating the main effects of reducing the excitatory tonus of the dorsal midbrain, after the blockade of the ionotropic glutamate receptors into the DPAG, on the voluntary alcohol intake of HA and LA motivated rats that were made previously experienced with the free operant response of alcohol drinking. For this purpose, we used local infusions of the N-metil D-Aspartato (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-kainate receptors antagonist DL-2-Amino-7-phosphonoheptanoic acid – DL-AP7 (10nmol/0.2μl) and l-glutamic acid diethyl ester – GDEE (160nmol/0.2μl), respectively. Alcohol intoxication was produced by 10 daily bolus intraperitonial (IP) injections of alcohol (2.0g/kg). Peak-blood alcohol levels were determined by gas-chromatography analysis in order to assess blood-alcohol content. Unconditioned and conditioned anxiety-like behavior was assessed by the use of the fear-potentiated startle procedure (FPS). Data collected showed that anxiety and alcohol drinking in HA animals are positively correlated in animals that were made previously familiarized with the anxiolytic effects of alcohol. In addition, anxiety-like behavior induced during alcohol hangover seems to be an effect of changes in glutamatergic neurotransmission into DPAG possibly involving AMPA/kainate and NMDA receptors, among others.

The panicolytic-like effect of fluoxetine in the elevated T-maze is mediated by serotonin-induced activation of endogenous opioids in the dorsal periaqueductal grey

Serotonin (5-HT), opioids and the dorsal periaqueductal grey (DPAG) have been implicated in the pathophysiology of panic disorder. In order to study 5-HT-opioid interaction, the opioid antagonist naloxone was injected either systemically (1 mg/kg, i.p.) or intra-DPAG (0.2 μg/0.5 μL) to assess its interference with the effect of chronic fluoxetine (10 mg/kg, i.p., daily for 21 days) or of intra-DPAG 5-HT (8 μg/0.5 μL). Drug effects were measured in the one-escape task of the rat elevated T-maze, an animal model of panic. Pretreatment with systemic naloxone antagonized the lengthening of escape latency caused by chronic fluoxetine, considered a panicolytic-like effect that parallels the drug's therapeutic response in the clinics. Pretreatment with naloxone injected intra-DPAG antagonized both the panicolytic effect of chronic fluoxetine as well as that of 5-HT injected intra-DPAG. Neither the performance of the inhibitory avoidance task in the elevated T-maze, a model of generalized anxiety nor locomotion measured in a circular arena was affected by the above drug treatments. These results indicate that the panicolytic effect of fluoxetine is mediated by endogenous opioids that are activated by 5-HT in the DPAG. They also allow reconciliation between the serotonergic and opioidergic hypotheses of panic disorder pathophysiology.

Effects of neurokinin-1 and 3-receptor antagonists on the defensive behavior induced by electrical stimulation of the dorsal periaqueductal gray

The dorsal periaqueductal gray (dPAG) is the main output structure for the defensive response to proximal aversive stimulation. Panic-like responses, such as freezing and escape behaviors, often result when this structure is electrically stimulated. Freezing also ensues after termination of the dPAG stimulation (post-stimulation freezing (PSF)). GABA and 5-HT have been proposed as the main neuromediators of these defense reactions. Neurokinins (NKs) also play a role in the defense reaction; however, it is unclear how the distinct types of NK receptors are involved in the expression of these fear responses. This study investigated the role of NK-1 and NK-3 receptors in the unconditioned defensive behaviors induced by electrical stimulation of the dPAG of rats, with and without previous experience with contextual fear conditioning (CFC). Spantide (100 ρmol/0.2 μl) and SB 222200 (50 and 100 ρmol/0.2 μl), selective antagonists of NK-1 and NK-3 receptors, respectively, were injected into the dPAG. Injection of spantide had antiaversive effects as determined by stimulation of the dPAG in naive animals and in animals subjected previously to CFC. SB 222200 also increased these aversive thresholds but only at doses that caused a motor deficit. Moreover, neither spantide nor SB 222200 influenced the PSF. The results suggest that NK-1 receptors are mainly involved in the mediation of the defensive behaviors organized in the dPAG. Because dPAG-evoked PSF was not affected by intra-dPAG injections of either spantide or SB 222200, it is suggested that neurokinin-mediated mechanisms are not involved in the processing of ascending aversive information from the dPAG.

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.

Mammal-Like Organization of the Avian Midbrain Central Gray and a Reappraisal of the Intercollicular Nucleus

In mammals, rostrocaudal columns of the midbrain periaqueductal gray (PAG) regulate diverse behavioral and physiological functions, including sexual and fight-or-flight behavior, but homologous columns have not been identified in non-mammalian species. In contrast to mammals, in which the PAG lies ventral to the superior colliculus and surrounds the cerebral aqueduct, birds exhibit a hypertrophied tectum that is displaced laterally, and thus the midbrain central gray (CG) extends mediolaterally rather than dorsoventrally as in mammals. We therefore hypothesized that the avian CG is organized much like a folded open PAG. To address this hypothesis, we conducted immunohistochemical comparisons of the midbrains of mice and finches, as well as Fos studies of aggressive dominance, subordinance, non-social defense and sexual behavior in territorial and gregarious finch species. We obtained excellent support for our predictions based on the folded open model of the PAG and further showed that birds possess functional and anatomical zones that form longitudinal columns similar to those in mammals. However, distinguishing characteristics of the dorsal/dorsolateral PAG, such as a dense peptidergic innervation, a longitudinal column of neuronal nitric oxide synthase neurons, and aggression-induced Fos responses, do not lie within the classical avian CG, but in the laterally adjacent intercollicular nucleus (ICo), suggesting that much of the ICo is homologous to the dorsal PAG.

GABAergic control of micturition within the periaqueductal grey matter of the male rat.

In urethane-anaesthetised rats continuous infusion of saline into the bladder (6 ml h⁻¹) evoked periodic sharp rises in intravesicular pressure accompanied by rhythmic bursting of external urethral sphincter (EUS) EMG and expulsion of urine from the urethral meatus. Microinjection of the GABA agonist muscimol (250 pmol) into the caudal ventrolateral periaqueductal grey (PAG), but not at other sites in the PAG, either depressed reflex voiding frequency (-60%, n = 7) and tonic EUS EMG activity (-38%, n = 6) or completely inhibited voiding (four sites). Microinjection of the GABA antagonist bicuculline (BIC; 1 nmol) into the same region, to reduce ongoing GABA tone, increased reflex voiding frequency (+467%, n = 16) and tonic activity in the EUS (+56%, n = 7) whilst bursting activity in the EUS became desynchronised. Although muscimol failed to change reflex micturition when microinjected into the dorsal caudal PAG, microinjection of BIC at these sites evoked pronounced autonomic arousal and increased reflex voiding frequency (+237%, n = 34). The results demonstrate that the functional integrity of synapses in the caudal ventrolateral PAG is essential to permit micturition. Transmission through the region is normally regulated by a tonic GABAergic inhibitory influence. In contrast, the functional integrity of the dorsal caudal PAG is not essential for reflex micturition. However, micturition may be initiated from this region via projections to the caudal ventrolateral PAG, as part of the behavioural response to psychological threat or other stressful stimuli.

Long lasting effects of predator stress on pCREB expression in brain regions involved in fearful and anxious behavior

Predator stress is one animal model of posttraumatic stress disorder (PTSD). Neural plasticity in amygdala afferent and efferent pathways underlies anxiogenic effects of predator stress. Predator stress increases pCREB expression in these pathways 20 min after stress, implicating pCREB in stress-induced neural plasticity. Here we examined impact of predator stress on pCREB expression 6–24 h and 7 days after stress in amygdala pathways and in the supramammillary nucleus (SuM). Patterns of change in pCREB expression were complex, time dependent, column dependent in the periaqueductal gray (PAG), and AP plane dependent in the amygdala. In contrast to past work at 20 min after stress, there were no stress-induced increases in pCREB in the amygdala in the anterior AP plane or in the lateral PAG at 6 h onward after stress. However, dorsal PAG pCREB was increased bilaterally at 24 h and 7 days after stress. In the mid AP plane of all amygdala nuclei there were bilateral stress-induced increases in pCREB at 6 h followed by decreases at 24 h post stress. A similar pattern was observed in the posterior AP plane. In addition, we found a persistent increase (6 h to 7 days after stress) in pCREB expression in the SuM. Further study of this nucleus as a contributor to fear sensitization following predator stress is warranted. Overall, these data highlight persistent neuroplastic changes in key brain areas following traumatic stress. Identification of these changes may aid in understanding the neural mechanisms underlying acquired anxiety disorders such as PTSD.

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.

Cardiovascular effects of acetylcholine microinjection into the ventrolateral and dorsal periaqueductal gray of rats

In the present study, we describe the cardiovascular effects of local acetylcholine (Ach) microinjection into both the ventrolateral (vlPAG) and dorsal (dPAG) periaqueductal gray areas of anesthetized rats and the possible local receptors involved with these responses. Microinjection of Ach (9, 27, 45 or 81nmol/50nL) into the vlPAG caused dose-related depressor responses. These hypotensive responses were blocked by local pretreatment with increasing doses of the nonselective muscarinic antagonist atropine (1, 3 or 9nmol/50nL). The microinjection of Ach into the dPAG caused no significant cardiovascular responses in anesthetized rats. In conclusion, the present findings suggest that a cholinergic system present in the vlPAG, but not in the dPAG, is involved with cardiovascular system control. Moreover, these cardiovascular responses evoked by Ach are mediated by muscarinic receptors.

5-HT2C receptor regulation of defensive responses in the rat dorsal periaqueductal gray

Activation of 5-HT2C receptors in limbic structures such as the amygdala and hippocampus increases anxiety. Indirect evidence obtained with non-selective 5-HT2C-interacting drugs suggests that the same may occur in the dPAG, a brainstem region consistently implicated in the genesis/regulation of panic attacks. In this study we used more selective agonists and antagonists to unveil the role played by dPAG 5-HT2C receptors in the regulation of anxiety- and panic-related defensive behaviors. Our results showed that intra-dPAG microinjection of the endogenous agonist 5-HT (20 nmol) or the 5-HT2C receptor agonists MK-212 (1 and 10 nmol) and RO-600175 (40 nmol) significantly increased inhibitory avoidance acquisition in rats tested in the elevated T-maze, suggesting an anxiogenic effect. 5-HT, but not the two 5-HT2C receptor agonists, inhibited escape performance. In the elevated T-maze, inhibitory avoidance and escape responses have been related to generalized anxiety and panic attacks, respectively. The behavioral effects caused by 5-HT and MK-212 were fully blocked by previous local microinjection of the 5-HT2C receptor antagonist SB-242084. Intra-dPAG injection of MK-212 also failed to affect escape expression in another test relating this behavior to panic, the electrical stimulation of the dPAG. Overall, the results indicate that 5-HT2C receptors in the dPAG are preferentially involved in the regulation of defensive behaviors related to anxiety, but not panic. This finding extends to the dPAG the prominent role that has been attributed to 5-HT2C receptors in anxiety generation.

The Dual Role of Serotonin in Defense and the Mode of Action of Antidepressants on Generalized Anxiety and Panic Disorders

Antidepressants are widely used to treat several anxiety disorders, among which generalized anxiety disorder (GAD) and panic disorder (PD). Serotonin (5-HT) is believed to play a key role in the mode of action of these agents, a major question being which pathways and receptor subtypes are involved in each type of anxiety disorder. The dual role of 5-HT in defense hypothesis assumes that 5-HT facilitates defensive responses to potential threat, like inhibitory avoidance, related to anxiety, whereas it inhibits defensive responses to proximal danger, like one-way escape, related to panic. The former action would be exerted at the forebrain, chiefly the amygdala and medial prefrontal cortex (PFC), while the latter would be exerted at the dorsal periaqueductal gray (DPAG) matter of the midbrain. The present review is focused on studies designed to test this hypothesis, performed in animal models of anxiety and panic, as well as in human experimental anxiety tests. The reviewed results suggest that chronic, but not acute, administration of antidepressants suppress panic attacks by increasing the release of 5-HT and enhancing the responsivity of post-synaptic 5-HT1A and 5-HT2A receptors in the DPAG. The attenuation of generalized anxiety, also caused by the same drug treatment, would be due to the desensitization of 5-HT2C receptors and, less certainly, to increased stimulation of 5-HT1A receptors in forebrain structures. This action would result in less activation of the amygdala, medial PFC and insula by warning signals, as shown by the reviewed results obtained with functional neuroimaging in healthy volunteers and patients with anxiety disorders.