Anxiolytic-like effects produced by bilateral lesion of the periaqueductal gray in mice: Influence of concurrent nociceptive stimulation

Whole-Brain Connectome of GABAergic Neurons in the Mouse Zona Incerta

The zona incerta (ZI) is involved in various functions and may serve as an integrative node of the circuits for global behavioral modulation. However, the long-range connectivity of different sectors in the mouse ZI has not been comprehensively mapped. Here, we obtained whole-brain images of the input and output connections via fluorescence micro-optical sectioning tomography and viral tracing. The principal regions in the input-output circuits of ZI GABAergic neurons were topologically organized. The 3D distribution of cortical inputs showed rostro-caudal correspondence with different ZI sectors, while the projection fibers from ZI sectors were longitudinally organized in the superior colliculus. Clustering results show that the medial and lateral ZI are two different major functional compartments, and they can be further divided into more subdomains based on projection and input connectivity. This study provides a comprehensive anatomical foundation for understanding how the ZI is involved in integrating different information, conveying motivational states, and modulating global behaviors.

A 7-Tesla MRI Study of the Periaqueductal Grey: Resting State and Task Activation Under Threat

The periaqueductal grey (PAG) is a region of the midbrain implicated in a variety of behaviors including defensive responses to threat. Despite the wealth of knowledge pertaining to the differential functional roles of the PAG columns in nonhuman and human research, the basic functional connectivity of the PAG at rest has not been well characterized. Therefore, the current study utilized 7-Tesla MRI to characterize PAG functional connectivity at rest and task activation under uncertain threat. A sample of 53 neurologically healthy undergraduate participants (Mage=22.2, SDage=3.62) underwent structural and resting state functional MRI scans. Supporting previous work, voxel-wise analyses showed the PAG is functionally connected to emotion regulation and fear networks. Comparison of functional connectivity of PAG columns did not reveal any significant differences. Thirty-five participants from the same sample also completed an uncertain threat task with blocks of 3 conditions-No shock, Predictable shock, and Unpredictable shock. There were no robust activity differences within the PAG columns or the whole PAG across conditions, though there was differential activity at the voxel level in the PAG and in other regions theoretically relevant to uncertain threat. Results of this study elucidate PAG connectivity at rest and activation in response to uncertain threat.

Repeated exposure of naïve and peripheral nerve-injured mice to a snake as an experimental model of post-traumatic stress disorder and its co-morbidity with neuropathic pain

Confrontation of rodents by natural predators provides a number of advantages as a model for traumatic or stressful experience. Using this approach, one of the aims of this study was to investigate a model for the study of post-traumatic stress disorder (PTSD)-related behaviour in mice. Moreover, because PTSD can facilitate the establishment of chronic pain (CP), and in the same way, patients with CP have an increased tendency to develop PTSD when exposed to a traumatic event, our second aim was to analyse whether this comorbidity can be verified in the new paradigm. C57BL/6 male mice underwent chronic constriction injury of the sciatic nerve (CCI), a model of neuropathic CP, or not (sham groups) and were submitted to different threatening situations. Threatened mice exhibited enhanced defensive behaviours, as well as significantly enhanced risk assessment and escape behaviours during context reexposure. Previous snake exposure reduced open-arm time in the elevated plus-maze test, suggesting an increase in anxiety levels. Sham mice showed fear-induced antinociception immediately after a second exposure to the snake, but 1 week later, they exhibited allodynia, suggesting that multiple exposures to the snake led to increased nociceptive responses. Moreover, after reexposure to the aversive environment, allodynia was maintained. CCI alone produced intense allodynia, which was unaltered by exposure to either the snake stimuli or reexposure to the experimental context. Together, these results specifically parallel the behavioural symptoms of PTSD, suggesting that the snake/exuvia/reexposure procedure may constitute a useful animal model to study PTSD.

Activation of the Intrinsic Pain Inhibitory Circuit from the Midcingulate Cg2 to Zona Incerta Alleviates Neuropathic Pain

Neuropathic pain is one of the most common and notorious neurological diseases. The changes in cerebral structures after nerve injury and the corresponding contributions to neuropathic pain are not well understood. Here we found that the majority of glutamatergic neurons in the area 2 of midcingulate cortex (MCC Cg2^Glu) were inhibited by painful stimulation in male mice. Optogenetic manipulation revealed that these neurons were tonically involved in the inhibitory modulation of multimodal nociception. We further identified the projections to GABAergic neurons in the zona incerta (ZI^GABA) mediated the pain inhibitory role. However, MCC Cg2^Glu became hypoactive after nerve injury. Although a brief activation of the MCC Cg2^Glu to ZI^GABA circuit was able to relieve the aversiveness associated with spontaneous ongoing pain, consecutive activation of the circuit was required to alleviate neuropathic allodynia. In contrast, glutamatergic neurons in the area 1 of MCC played opposite roles in pain modulation. They became hyperactive after nerve injury and only consecutive inhibition of their activity relieved allodynia. These results demonstrate that MCC Cg2^Glu constitute a component of intrinsic pain inhibitory circuitry and their hypoactivity underlies neuropathic pain. We propose that selective and persistent activation of the MCC Cg2^Glu to ZI^GABA circuit may serve as a potential therapeutic strategy for this disease.SIGNIFICANCE STATEMENT Glutamatergic neurons in the area 2 of midcingulate cortex (MCC Cg2^Glu) are tonically involved in the intrinsic pain inhibition via projecting to GABAergic neurons in the zona incerta. They are hypoactive after nerve injury. Selective activation of the circuit compensates the reduction of its analgesic strength and relieves neuropathic pain. Therefore, MCC Cg2^Glu and the related analgesic circuit may serve as therapeutic targets for neuropathic pain. In contrast, MCC Cg1^Glu have an opposite role in pain modulation and become hyperactive after nerve injury. The present study provides novel evidence for the concept that neuropathic pain is associated with the dysfunction of endogenous pain modulatory system and new perspective on the treatment of neuropathic pain.

Interplay between 5-HT2C and 5-HT1A receptors in the dorsal periaqueductal gray in the modulation of fear-induced antinociception in mice

The confinement of rodents to the open arm of the elevated-plus maze provokes antinociception (OAA). As a type of defensive reaction, the OAA has been investigated through systemic and intramesencephalic (e.g., dorsal portion of the periaqueductal gray - dPAG) injections of anxiolytic-like drugs [e.g., serotonergic (5-HT) receptor agonists or antagonists]. Here we investigated the effects of (i) intra-dPAG injections of a 5HT_2C receptor agonist (MK-212; 0.21 or 0.63 nmol) and antagonist (SB 242084; 0.01, 0.1 or 1.0 nmol); (ii) combined injections of SB 242084 and MK-212 into the dPAG; (iii) combined injections of SB 242084 with 8-OHDPAT (10 nmol) into the dPAG on the OAA in male Swiss mice. Nociception was assessed with the writhing test induced by acetic acid injection. Results showed that (i) intra-dPAG injection of MK-212 (0.63 nmol) increased the OAA; (ii) intra-dPAG SB 242084 (1.0 nmol) prevented the OAA; (iii) SB 242084 (0.1 nmol, a dose devoid of intrinsic effect on nociception) blocked the OAA enhancement provoked by MK-212 and enabled 8-OH-DPAT to prevent the OAA. These results suggest that OAA is mediated by 5-HT_2C receptors within the dPAG. Intra-dPAG SB242084 administration provoked similar results on the effects produced by MK-212 and 8-OH-DPAT on OAA. In addition, the dPAG 5-HT_1A and 5-HT_2C receptors interact each other in the modulation of OAA.

Amygdaloid involvement in the defensive behavior of mice exposed to the open elevated plus-maze

Previous studies have shown that the exposure to an open elevated plus maze (oEPM, an EPM with all four open arms) elicits fear/anxiety-related responses in laboratory rodents. However, very little is known about the underlying neural substrates of these defensive behaviors. Accordingly, the present study investigated the effects of chemical inactivation of the amygdala [through local injection of cobalt chloride (CoCl₂: a nonspecific synaptic blocker)] on the behavior of oEPM-exposed mice. In a second experiment, the pattern of activation of the basolateral (BLA) and central (CeA) nuclei of the amygdala was assessed through quantification of Fos protein expression in mice subjected to one of several behavioral manipulations. To avoid the confound of acute handling stress, 4 independent groups of mice were habituated daily for 10days to an enclosed EPM (eEPM) and, on day 11 prior to immunohistochemistry, were either taken directly from their home cage (control) or individually exposed for 10min to a new clean holding cage (novelty), an eEPM, or the oEPM. An additional group of mice (maze-naïve) was not subjected to either the habituation or exposure phase but were simply chosen at random from their home cages to undergo an identical immunohistochemistry procedure. Results showed that amygdala inactivation produced an anxiolytic-like profile comprising reductions in time spent in the proximal portions of the open arms and total stretched attend postures (SAP) as well as increases in time spent in the distal portions of the open arms and total head-dipping. Moreover, Fos-positive labeled cells were bilaterally increased in the amygdaloid complex, particularly in the BLA, of oEPM-exposed animals compared to all other groups. These results suggest that the amygdala (in particular, its BLA nucleus) plays a key role in the modulation of defensive behaviors in oEPM-exposed mice.

Critical neuropsychobiological analysis of panic attack- and anticipatory anxiety-like behaviors in rodents confronted with snakes in polygonal arenas and complex labyrinths: a comparison to the elevated plus- and T-maze behavioral tests

Objective:

To compare prey and snake paradigms performed in complex environments to the elevated plus-maze (EPM) and T-maze (ETM) tests for the study of panic attack- and anticipatory anxiety-like behaviors in rodents.

Methods:

PubMed was reviewed in search of articles focusing on the plus maze test, EPM, and ETM, as well as on defensive behaviors displayed by threatened rodents. In addition, the authors' research with polygonal arenas and complex labyrinth (designed by the first author for confrontation between snakes and small rodents) was examined.

Results:

The EPM and ETM tests evoke anxiety/fear-related defensive responses that are pharmacologically validated, whereas the confrontation between rodents and snakes in polygonal arenas with or without shelters or in the complex labyrinth offers ethological conditions for studying more complex defensive behaviors and the effects of anxiolytic and panicolytic drugs. Prey vs. predator paradigms also allow discrimination between non-oriented and oriented escape behavior.

Conclusions:

Both EPM and ETM simple labyrinths are excellent apparatuses for the study of anxiety- and instinctive fear-related responses, respectively. The confrontation between rodents and snakes in polygonal arenas, however, offers a more ethological environment for addressing both unconditioned and conditioned fear-induced behaviors and the effects of anxiolytic and panicolytic drugs.

Rat exposure in mice with neuropathic pain induces fear and antinociception that is not reversed by 5-HT2C receptor activation in the dorsal periaqueductal gray

Previous studies have demonstrated that serotonin 5-HT2C receptors in the dorsal periaqueductal gray (dPAG) mediate both anxiety and antinociception in mice submitted to the elevated plus maze. The present study examined the effects of intra-dPAG infusion of the serotonin 5-HT2C receptor agonist (MK-212) in the defensive reactions and antinociception in mice with neurophatic pain confronted by a predator. Neuropathic pain was induced by chronic constriction injury (CCI) of the sciatic nerve, and predator confrontation was performed using the rat exposure test (RET). Our results demonstrated that both sham-operated and CCI mice exhibited intense defensive reactions when confronted by rats. However, rat-exposed CCI mice showed reduced pain reactivity in comparison to CCI mice exposed to a toy rat. Intra-dPAG infusion of MK-212 prior to predator exposure did not significantly alter defensive or antinociceptive responses. To our knowledge, our results represent the first evidence of RET-induced antinociception in mice. Moreover, the results of the present study suggest that 5-HT2C receptor activation in the dPAG is not critically involved in the control of predator-evoked fearful or antinociceptive responses.

Role of TRPV1 channels of the dorsal periaqueductal gray in the modulation of nociception and open elevated plus maze-induced antinociception in mice

Recent findings have identified the presence of transient receptor potential vanilloid-1 (TRPV1) channels within the dorsal portion of the periaqueductal gray (dPAG), suggesting their involvement in the control of pain and environmentally-induced antinociception. Environmentally, antinociception may be achieved through the use of an open elevated plus maze (oEPM, an EPM with 4 open arms), a highly aversive environmental situation. Here, we investigated the role of these TRPV1 channels within the dPAG in the modulation of a tonic pain and in the oEPM-induced antinociception. Male Swiss mice, under the nociceptive effect of 2.5% formalin injected into the right hind paw, received intra-dPAG injections of the TRPV1 agonist (capsaicin: 0, 0.01, 0.1 or 1.0 nmol/0.2 μL; Experiment 1) or antagonist (capsazepine: 0, 10 or 30 nmol/0.2 μL; Experiment 2) or combined injections of capsazepine (30 nmol) and capsaicin (1.0 nmol) (Experiment 3) and the time spent licking the formalin-injected paw was recorded. In Experiment 4, mice received intra-dPAG capsazepine (0 or 30 nmol) and were exposed to the oEPM or to a control situation, an enclosed EPM (eEPM; an EPM with 4 enclosed arms). Results showed that while capsaicin (1 nmol) decreased the time spent licking the formalin-injected paw, capsazepine did not change nociceptive response. Capsazepine (30 nmol) blocked pain inhibition induced by capsaicin and mildly attenuated the oEPM-induced antinociception. Our results revealed an important role of TRPV1 channels within the dPAG in the modulation of pain and in the phenomenon known as fear-induced antinociception in mice.

Alcohol effects on synaptic transmission in periaqueductal gray dopamine neurons

The role of dopamine (DA) signaling in regulating the rewarding properties of drugs, including alcohol, has been widely studied. The majority of these studies, however, have focused on the DA neurons located in the ventral tegmental area (VTA), and their projections to the nucleus accumbens. DA neurons within the ventral periaqueductal gray (vPAG) have been shown to regulate reward but little is known about the functional properties of these neurons, or how they are modified by drugs of abuse. This lack of knowledge is likely due to the highly heterogeneous cell composition of the vPAG, with both γ-aminobutyric acid (GABA) and glutamate neurons present in addition to DA neurons. In this study, we performed whole-cell recordings in a TH-eGFP transgenic mouse line to evaluate the properties of vPAG-DA neurons. Following this initial characterization, we examined how both acute and chronic alcohol exposure modify synaptic transmission onto vPAG-DA neurons. We found minimal effects of acute alcohol exposure on GABA transmission, but a robust enhancement of glutamatergic synaptic transmission in vPAG-DA. Consistent with this effect on excitatory transmission, we also found that alcohol caused an increase in firing rate. These data were in contrast to the effects of chronic intermittent alcohol exposure, which had no significant impact on either inhibitory or excitatory synaptic transmission on the vPAG-DA neurons. These data add to a growing body of literature that points to alcohol having both region-dependent and cell-type dependent effects on function.

Activation of 5-HT(2C) receptors in the dorsal periaqueductal gray increases antinociception in mice exposed to the elevated plus-maze

Several findings have pointed to the role of the dorsal periaqueductal gray (dPAG) serotonin 5-HT(1A) and 5-HT(2A-C) receptor subtypes in the modulation of defensive behavior in animals exposed to the elevated plus-maze (EPM). Besides displaying anxiety-like behavior, rodents also exhibit antinociception in the EPM. This study investigated the effects of intra-dPAG injections of 5-HT(1A) and 5-HT(2B/2C) receptor ligands on EPM-induced antinociception in mice. Male Swiss mice received 0.1 μl intra-dPAG injections of vehicle, 5.6 and 10 nmol of 8-OHDPAT, a 5-HT(1A) receptor agonist (Experiment 1), or 0.01, 0.03 and 0.1 nmol of mCPP, a 5-HT(2B/2C) receptor agonist (Experiment 2). Five minutes later, each mouse received an intraperitoneal injection of 0.6% acetic acid (0.1 ml/10 g body weight; nociceptive stimulus) and was individually confined in the open (OA) or enclosed (EA) arms of the EPM for 5 min, during which the number of abdominal writhes induced by the acetic acid was recorded. While intra-dPAG injection of 8-OHDPAT did not change open-arm antinociception (OAA), mCPP (0.01 nmol) enhanced it. Combined injections of ketanserin (10 nmol/0.1 μl), a 5-HT(2A/2C) receptor antagonist, and 0.01 nmol of mCPP (Experiment 3), selectively and completely blocked the OAA enhancement induced by mCPP. Although intra-dPAG injection of mCPP (0.01 nmol) also produced antinociception in EA-confined mice (Experiment 2), this effect was not confirmed in Experiment 3. Moreover, no other compound changed the nociceptive response in EA-confined animals. These results suggest that the 5-HT(2C) receptors located within the PAG play a role in this type of environmentally induced pain inhibition in mice.

Contribution of the rostral ventromedial medulla to post-anxiety induced hyperalgesia

Rats exposed to an elevated plus maze (EPM) with four open arms display antinociception while on the maze and hyperalgesia immediately upon removal. Little is known about the neural mechanisms underlying EPM-induced antinociception and the subsequent hyperalgesia except that the antinociception is not mediated by endogenous opioids. The objective of the present study was to test the hypothesis that endogenous cannabinoids and/or the rostral ventromedial medulla (RVM) contributes to EPM-induced antinociception. Administration of the CB1 receptor antagonist AM251 (1mg/kg, i.p.) had no effect on baseline nociception to formalin administration into the hindpaw or on the antinociception produced by placing a rat on the open EPM. Likewise, inactivation of the RVM by microinjecting the GABA(A) receptor agonist muscimol (10 ng/0.5 μL) had no effect on the antinociceptive effect of placing a rat in the EPM. However, RVM inactivation blocked the hyperalgesia produced upon removal from the EPM. Although distinct classes of RVM neurons inhibit and facilitate nociception, the present data demonstrate that the antinociception induced by the EPM and the subsequent hyperalgesia is mediated by distinct neural pathways.

Corticosterone does not change open elevated plus maze-induced antinociception in mice

It has been demonstrated that the exposure of rodents to the standard elevated plus-maze (sEPM: 2 open and 2 enclosed arms) elicits defensive behavioral reactions and antinociception and also activates the hypothalamo-pituitary-adrenal (HPA) axis. We have recently reported that EPM-induced antinociception is particularly observed when rats and mice are exposed to a totally open EPM (oEPM: 4 open arms). Given that the oEPM seems to be a more aversive situation than the sEPM, we hypothesized that oEPM exposure would induce higher plasma levels of corticosterone than sEPM exposure in mice. In this study, we investigated the influence of exposure to eEPM (enclosed EPM: 4 enclosed arms), sEPM or oEPM on plasma corticosterone levels in mice, with or without prior nociceptive stimulation (2.5% formalin injection into the right hind paw). We also tested whether the nociceptive response in the formalin test and oEPM-induced antinociception are altered by adrenalectomy. Results showed that oEPM-exposed mice spent less time licking the injected paw than sEPM- and eEPM-exposed animals. All three types of EPM exposure increased plasma corticosterone when compared to the basal group, but sEPM- and oEPM-exposed mice showed higher corticosterone levels than eEPM-exposed mice. Prior nociceptive stimulation (formalin injection) did not enhance the plasma corticosterone response induced by the three types of EPM exposure. Indeed, formalin injection appeared to provoke a ceiling effect on plasma corticosterone concentration. Furthermore, neither the nociceptive response in the formalin test nor oEPM-induced antinociception was changed by adrenalectomy. Present results suggest that oEPM antinociception does not depend on corticosterone release in mice.

Environmentally induced antinociception and hyperalgesia in rats and mice

Stress can enhance and inhibit nociception depending on the situation. Thus, simply shifting the context from the elevated plus maze (EPM) which has been shown to produce stress-induced antinociception to a different environment could produce drastic and rapid changes in nociception. The present experiment tested this hypothesis by assessing nociception in rats and mice during and immediately after removal from the maze. Experiment 1 found hyperalgesia in female and male rats tested on the hot plate immediately after exposure to the elevated plus maze. This hyperalgesia occurred with or without the added stress of a hind paw formalin injection and regardless of whether rats were exposed to an EPM with open (oEPM) or enclosed (eEPM) arms despite a clear antinociceptive effect while on the oEPM. Experiment 2 showed a similar shift from antinociception to nociception on the formalin test in mice immediately after removing them from the EPM. These data demonstrate that a mild stressor such as the EPM can produce both antinociception and hyperalgesia depending on the context. This shift from antinociception to hyperalgesia occurs rapidly and is evident in mice, male and female rats, and with the hot plate and formalin tests.

Ventrolateral periaqueductal gray lesion attenuates nociception but does not change anxiety-like indices or fear-induced antinociception in mice

The exposure of rodents to an open elevated plus-maze (oEPM: four open arms raised from the floor) elicits naloxone-insensitive antinociception. Midazolam infusion into the dorsal portion of the periaqueductal gray (dPAG), a structure of the descending inhibitory system of pain, failed to alter oEPM-induced antinociception. Chemical lesion of dorsomedial and dorsolateral PAG attenuated defensive behavior in the standard EPM (sEPM), an animal model of anxiety, but failed to change oEPM-induced antinociception. The present study investigated the effects of bilateral lesion, with the injection of NMDA (N-methyl-D-aspartic acid), of the ventrolateral column of PAG (vlPAG) (i) on nociceptive response induced by 2.5% formalin injected into the right hind paw (nociception test) in mice exposed to the enclosed EPM (eEPM: four enclosed arms - a non-aversive situation) or to the oEPM and (ii) on anxiety indices in mice exposed to the sEPM without prior formalin injection. Results showed that oEPM-induced antinociception was not altered by lesion of vlPAG. Nevertheless, the lesion reduced the nociceptive response in mice exposed to the eEPM and increased general locomotor activity during the eEPM and oEPM exposure. Furthermore, vlPAG lesion did not alter anxiety-like indices in mice exposed to the sEPM. The results suggest that vlPAG does not play a role in oEPM-induced antinociception or in defensive reactions assessed in the sEPM. Moreover, vlPAG inactivation induces pain inhibition in mice not exposed to an aversive situation and seems to increase general activity.