Functional activation of the periaqueductal gray matter during conditioned and unconditioned fear in guinea pigs confronted with the Boa constrictor constrictor snake

The periaqueductal gray matter (PAG) is an essential structure involved in the elaboration of defensive responses, such as when facing predators and conspecific aggressors. Using a prey vs predator paradigm, we aimed to evaluate the PAG activation pattern evoked by unconditioned and conditioned fear situations. Adult male guinea pigs were confronted either by a Boa constrictor constrictor wild snake or by the aversive experimental context. After the behavioral test, the rodents were euthanized and the brain prepared for immunohistochemistry for Fos protein identification in different PAG columns. Although Fos-protein-labeled neurons were found in different PAG columns after both unconditioned and conditioned fear situations at the caudal level of the PAG, we found greater activation of the lateral column compared to the ventrolateral and dorsomedial columns after predator exposure. Moreover, the lateral column of the PAG showed higher Fos-labeled cells at the caudal level compared to the same area at the rostral level. The present results suggested that there are different activation patterns of PAG columns during unconditioned and conditioned fear in guinea pigs. It is possible to hypothesize that the recruitment of specific PAG columns depended on the nature of the threatening stimulus.

Magnetic Resonance Image Texture Analysis of the Periaqueductal Gray Matter in Episodic Migraine Patients without T2-Visible Lesions

OBJECTIVE: The periaqueductal gray matter (PAG), a small midbrain structure, presents dysfunction in migraine. However, the precise neurological mechanism is still not well understood. Herein, the aim of this study was to investigate the texture characteristics of altered PAG in episodic migraine (EM) patients based on high resolution brain structural magnetic resonance (MR) images. MATERIALS AND METHODS: The brain structural MR images were obtained from 18 normal controls (NC), 18 EM patients and 16 chronic migraine (CM) patients using a 3T MR system. A PAG template was created using the International Consortium Brain Mapping 152 gray matter model, and the individual PAG segment was developed by applying the deformation field from the structural image segment to the PAG template. A grey level co-occurrence matrix was used to calculate the texture parameters including the angular second moment (ASM), contrast, correlation, inverse difference moment (IDM) and entropy. RESULTS: There was a significant difference for ASM, IDM and entropy in the EM group (998.629 ± 0.162 × 10−3, 999.311 ± 0.073 × 10−3, 916.354 ± 0.947 × 10−5) compared to that found in the NC group (998.760 ± 0.110 × 10−3, 999.358 ± 0.037 × 10−3 and 841.198 ± 0.575 × 10−5) (p < 0.05). The entropy was significantly lower among the patients with CM (864.116 ± 0.571 × 10−5) than that found among patients with EM (p < 0.05). The area under the receiver operating characteristic curve was 0.776 and 0.750 for ASM and entropy in the distinction of the EM from NC groups, respectively. ASM was negatively related to disease duration (DD) and the Migraine Disability Assessment Scale (MIDAS) scores in the EM group, and entropy was positively related to DD and MIDAS in the EM group (p < 0.05). CONCLUSION: The present study identified altered MR image texture characteristics of the PAG in EM. The identified texture characteristics could be considered as imaging biomarkers for EM.

P2X7R in periaqueductal gray matter participates in nociception modulation of chronic neuropathic pain / 临床麻醉学杂志

Objective To observe the distribution and possible mechanism of P2X7R in periaq-ueductal gray matter (PAG)in a rat model with chronic neuropathic pain in vivo.Methods The in-trathecal catheterization and sciatic nerve injury (SNI)were performed.All animals were randomly assigned into 3 groups with 26 rats in each,which was group Sham,control group (group C)and brilliant blue G (BBG)group (group BBG),respectively.Normal saline or BBG 10 μl were intrathe-cally injected after SNI and repeated for seven days.Paw-withdrawal mechanical thresholds (PWT) were measured on day 0,day 7,day 14,and day 21 after SNI.The rats were sacrificed and PAG tis-sues were collected on day 14 and day 21,separately.The distributions of P2X7R were observed by immunofluorescence.The protein contents of P2X7R and GFAP were assessed by Western blot assays.Results The P2X7R was expressed in PAG in rats.The PWTs of the control group showed a significant decrease during the 21-day period compared with the sham group.The P2X7R signals were predominantly expressed in astrocytes in PAG after SNI.Both P2X7R and GFAP expression remark-ably increased.Administration of BBG increased the PWTs,and inhibited the P2X7R and GFAP ex-pressions compared with those atthe same point of time of the control group.Conclusion These results indicated that P2X7R in PAG might participate in nociception modulation in the midbrain in chronic neuropathic pain.

The hypothalamic-pituitary-adrenal axis in anxiety and panic

This review article focuses on the differential activation of the hypothalamic-pituitary-adrenal (HPA) axis in generalized anxiety and panic. The results of experimental studies that assayed adrenocorticotropic hormone, cortisol and prolactin show that real-life panic attacks as well as those induced by selective panicogenic agents, such as lactate and carbon dioxide, do not activate the HPA axis. Accordingly, experiments carried out in two animal models of panic, namely electrical stimulation of the dorsal periaqueductal gray matter of the rat and the escape from the open arm of the elevated T maze, have shown that in neither case stress hormones are increased in the plasma. Also in humans, reported results have shown that neither cortisol nor prolactin levels were increased following simulated public speaking, an experimental task that has been related to panic, in either healthy volunteers or patients with panic disorder diagnosis. Therefore, although the panic attack causes a major sympathetic stimulation, it has little effect on the HPA axis. In contrast, anticipatory or generalized anxiety activates both the HPA and the sympatho-adrenal axes.

Towards a translational model of panic attack

About 20 years ago, Deakin and Graeff proposed that whereas generalized anxiety disorder is produced by the overactivity of 5-HT excitatory projections from dorsal raphe nucleus to the areas of prefrontal cortex and amygdala which process distal threat, panic attacks are a dysfunction of 5-HT inhibitory projections from dorsal raphe nucleus to the dorsal periaqueductal gray matter, thereby releasing the responses to proximal threat, innate fear or anoxia. Besides, they suggested that the decrease in 5-HT1A neurotransmission in the hippocampus results in learned helplessness and depression. Accordingly, the Deakin Graeff hypothesis provided a unified frame to the widespread use of 5-HT selective reuptake inhibitors in generalized anxiety, panic disorder and depression. Competitor hypotheses implicate panic attacks with the abnormal functioning of locus coeruleus, basolateral amygdala, dorsomedial hypothalamus or an as-yet-unknown suffocation alarm system. Conversely, cognitive psychologists suggest that panic attacks result from the catastrophic (cortical) interpretation of bodily symptoms. In any event, translational models of panic attack are expected to reproduce the main features of clinical panic, namely, the patient's higher sensitivity to both lactate and CO2, the drug specific sensitivity, the lack of stress hormone responses during panic attacks, the higher vulnerability of women and the high comorbidity with agoraphobia, major depression and childhood separation anxiety. Therefore, here we review the main steps in the experimental approach to anxiety disorders which are paving the route towards a translational model of panic attack.

Role of nitric oxide in the periaqueductal gray in defensive behavior in mice: influence of prior local N-methyl-D-aspartate receptor activation and aversive condition

Glutamate N-methyl-D-aspartate (NMDA) receptor activation within the dorsal column of the periaqueductal gray (dPAG) leads to antinociceptive, autonomic, and behavioral responses characterized as the fear reaction. Activation of NMDA receptors in the brain increases nitric oxide (NO) synthesis, and NO has been proposed to be a mediator of the aversive action of glutamate. This paper reviews a series of studies investigating the effects of neuronal NO synthase (nNOS) inhibition in the dPAG of mice in different aversive conditions. nNOS inhibition by infusion of Nω-propyl-L-arginine (NPLA) prevents fear-like reactions (e.g., jumping, running, freezing) induced by NMDA receptor stimulation within the dPAG and produces anti-aversive effects when injected into the same midbrain site in mice confronted with a predator. Interestingly, nNOS inhibition within the dPAG does not change anxiety-like behavior in mice exposed to the elevated plus maze (EPM), but it reverses the effect of an anxiogenic dose of NMDA injected into the same site in animals subjected to the EPM. Altogether, the results support a role for glutamate NMDA receptors and NO in the dPAG in the regulation of defensive behaviors in mice. However, dPAG nitrergic modulation of anxiety-like behavior appears to depend on the magnitude of the aversive stimulus.

Inhibitory Mechanism of Periaqueductal Gray Matter on Neuropathic Pain in Rat

OBJECTIVE: Using Lee et al (1996) model, we assessed the effect of opioid within the PAG on the manifestations of the neuropathic pain, and we studied the effects of naloxone on the analgesic effects of opioid. METHOD: Under pentobarbital anesthesia, male Sprague-Dawley rats were implanted with cannula in the ventral (n=10) and dorsal (n=6) PAG after the unilateral tibial and sural nerves were ligated and cut, leaving the common peroneal nerve intact. Pain sensitivity was assessed using the von Frey filament (8 mN) and acetone applied to the sensitive area for 1 week postoperatively. Rats with neuropathic pain were intracerebrally microinjected with DAMGO (0.1microgram/5microliter) and enkephaline (20microgram/5microliter) into the ventral and dorsal PAG and the pain sensitivity was assessed. Naloxone was injected to assess the observed change of pain sensitivity. RESULTS: Intracerebral microinjection of DAMGO and enkephaline into the ventral PAG, but not the dorsal PAG, increased the pain threshold which was reversed by naloxone. CONCLUSION: The results suggest that stimulation of the ventral PAG in neuropathic rats may reduce neuropathic pain via opioid-mediating pathway of the descending pain inhibition system.