Involvement of prelimbic medial prefrontal cortex in panic-like elaborated defensive behaviour and innate fear-induced antinociception elicited by GABAA receptor blockade in the dorsomedial and ventromedial hypothalamic nuclei: role of the endocannabinoid CB1 receptor

Cannabidiol in the dorsal hippocampus attenuates emotional and cognitive impairments related to neuropathic pain: The role of prelimbic neocortex-hippocampal connections

BACKGROUND AND PURPOSE

Chronic neuropathic pain (NP) is commonly associated with cognitive and emotional impairments. Cannabidiol (CBD) presents a broad spectrum of action with a potential analgesic effect. This work investigates the CBD effect on comorbidity between chronic NP, depression, and memory impairment.

EXPERIMENTAL APPROACH

The connection between the neocortex and the hippocampus was investigated with biotinylated dextran amine (BDA) deposits in the prelimbic cortex (PrL). Wistar rats were submitted to chronic constriction injury (CCI) of the sciatic nerve and CA1 treatment with CBD (15, 30, 60 nmol).

KEY RESULTS

BDA-labeled perikarya and terminal buttons were found in CA1 and dentate gyrus. CCI-induced mechanical and cold allodynia increased c-Fos protein expression in the PrL and CA1. The number of astrocytes in PrL and CA1 increased, and the number of neuroblasts decreased in CA1. Animals submitted to CCI procedure showed increasing depressive-like behaviors, such as memory impairment. CBD (60 nmol) treatment decreased mechanical and cold allodynia, attenuated depressive-associated behaviors, and improved memory performance. Cobalt chloride (CoCl2: 1 nM), WAY-100635 (0.37 nmol), and AM251 (100 nmol) intra-PrL reversed the effect of CA1 treatment with CBD (60 nmol) on nociceptive, cognitive, and depressive behaviors.

CONCLUSION

CBD represents a promising therapeutic perspective in the pharmacological treatment of chronic NP and associated comorbidities such as depression and memory impairments. The CBD effects possibly recruit the CA1-PrL pathway, inducing neuroplasticity. CBD acute treatment into the CA1 produces functional and molecular morphological improvements.

Alpha1- and Beta-norepinephrinergic receptors of dorsomedial and ventromedial hypothalamic nuclei modulate panic attack-like defensive behaviour elicited by diencephalic GABAergic neurotransmission disinhibition

Gamma-aminobutyric acid (GABA) disinhibition in medial hypothalamus (MH) nuclei of rats elicits some defensive reactions that are considered panic attack-like behaviours. Recent evidence showed that the norepinephrine-mediated system modulates fear-related defensive behaviours organised by MH neurons at least in part via noradrenergic receptors recruitment on midbrain tegmentum. However, it is unknown whether noradrenergic receptors of the MH also modulate the panic attack-like reactions. The aim of this work was to investigate the distribution of noradrenergic receptors in MH, and the effects of either α1-, α2- or β-noradrenergic receptors blockade in the MH on defensive behaviours elaborated by hypothalamic nuclei. Defensive behaviours were evaluated after the microinjection of the selective GABAA receptor antagonist bicuculline into the MH that was preceded by microinjection of either WB4101, RX821002, propranolol (α1-, α2- and β-noradrenergic receptor selective antagonists, respectively), or physiological saline into the MH of male Wistar rats. The α1-, α2- and β-noradrenergic receptors were found in neuronal perikarya of all MH nuclei, and the α2-noradrenergic receptor were also found on glial cells mainly situated in the ventrolateral division of the ventromedial hypothalamic nucleus. The α1- and β-noradrenergic receptors blockade in the MH decreased defensive attention and escape reactions elicited by the intra-MH microinjections of bicuculline. These findings suggest that, despite the profuse distributions of α1-, α2- and β-noradrenergic receptors in the MH, both α1- and β-noradrenergic receptor- rather than α2-noradrenergic receptor-signalling in MH are critical for the neuromodulation of panic-like behaviour.

Effect of electrical and chemical (activation versus inactivation) stimulation of the infralimbic division of the medial prefrontal cortex in rats with chronic neuropathic pain

Neuropathic pain (NP) represents a complex disorder with sensory, cognitive, and emotional symptoms. The medial prefrontal cortex (mPFC) takes critical regulatory roles and may change functionally and morphologically during chronic NP. There needs to be a complete understanding of the neurophysiological and psychopharmacological bases of the NP phenomenon. This study aimed to investigate the participation of the infralimbic division (IFL) of the mPFC in chronic NP, as well as the role of the N-methyl-D-aspartic acid receptor (NMDAr) in the elaboration of chronic NP. Male Wistar rats were submitted to the von Frey and acetone tests to assess mechanical and cold allodynia after 21 days of chronic constriction injury (CCI) of the sciatic nerve or Sham-procedure ("false operated"). Electrical neurostimulation of the IFL/mPFC was performed by low-frequency stimuli (20 μA, 100 Hz) applied for 15 s by deep brain stimulation (DBS) device 21 days after CCI. Either cobalt chloride (CoCl2 at 1.0 mM/200 nL), NMDAr agonist (at 0.25, 1.0, and 2.0 nmol/200 nL) or physiological saline (200 nL) was administered into the IFL/mPFC. CoCl2 administration in the IFL cortex did not alter either mechanical or cold allodynia. DBS stimulation of the IFL cortex decreased mechanical allodynia in CCI rats. Chemical stimulation of the IFL cortex by an NMDA agonist (at 2.0 nmol) decreased mechanical allodynia. NMDA at any dose (0.25, 1.0, and 2.0 nmol) reduced the flicking/licking duration in the cold test. These findings suggest that the IFL/mPFC and the NMDAr of the neocortex are involved in attenuating chronic NP in rats.

Unravelling the dorsal periaqueductal grey matter NMDA receptors relevance in the nitric oxide-mediated panic‑like behaviour and defensive antinociception organised by the anterior hypothalamus of male mice

RATIONALE

Previous studies suggested that the dorsal column of the periaqueductal grey matter (dPAG) can be a target of neural pathways from hypothalamic nuclei involved in triggering fear-related defensive responses. In turn, evidence is provided suggesting that microinjection of the nitric oxide (NO) donor SIN-1 into the anterior hypothalamus (AH) of mice evokes panic-like behaviours and fear-induced antinociception. However, it is unknown whether the dPAG of mice mediates these latter defensive responses organised by AH neurons.

OBJECTIVES

This study was designed to examine the role of dPAG in mediating SIN-1-evoked fear-induced defensive behavioural and antinociceptive responses organised in the AH of mice.

METHODS

First, neural tract tracing was performed to characterise the AH-dPAG pathways. Then, using neuropharmacological approaches, we evaluated the effects of dPAG pretreatment with either the non-selective synaptic blocker cobalt chloride (CoCl₂; 1 mM/0.1 μL) or the competitive N-methyl-D-aspartate (NMDA) receptor antagonist LY235959 (0.1 nmol/0.1 μL) on defensive behaviours and antinociception induced by microinjections of SIN-1 in the AH of male C57BL/6 mice.

RESULTS

AlexaFluor488-conjugated dextran-labelled axonal fibres from AH neurons were identified in both dorsomedial and dorsolateral PAG columns. Furthermore, we showed that pre-treatment of the dPAG with either CoCl₂ or LY235959 inhibited freezing and impaired oriented escape and antinociception induced by infusions of SIN-1 into the AH.

CONCLUSIONS

These findings suggest that the panic-like freezing and oriented escape defensive behaviours, and fear-induced antinociception elicited by intra-AH microinjections of SIN-1 depend on the activation of dPAG NMDA receptors.

Neostriatum neuronal TRPV1-signalling mediates striatal anandamide at high concentration facilitatory influence on neostriato-nigral dishinhibitory GABAergic connections

RATIONALE

Several lines of evidence have demonstrated that the cannabinoid type 1 receptor (CB₁) is found in the caudate nucleus and putamen (CPu) in addition to the substantia nigra pars reticulata (SNpr). Here, we investigated the role of endocannabinoid neuromodulation of striato-nigral disinhibitory projections on the activity of nigro-collicular GABAergic pathways that control the expression of unconditioned fear-related behavioural responses elicited by microinjections of the GABA_A receptor selective antagonist bicuculline (BIC) in the deep layers of the superior colliculus (dlSC).

METHODS

Fluorescent neural tract tracers were deposited in either CPu or in SNpr. Wistar rats received injection of vehicle, anandamide (AEA), either at low (50 pmol) or high (100 pmol) concentrations in CPu followed by bicuculline microinjections in dlSC.

RESULTS

Connections between CPu, the SNpr and dlSC were demonstrated. The GABA_A receptor blockade in dlSC elicited panic-like behaviour. AEA at the lowest concentration caused a panicolytic-like effect that was antagonised by the CPu pretreatment with AM251 at 100 pmol. AEA at the highest concentration caused a panicogenic-like effect that was antagonised by the CPu pretreatment with 6-iodonordihydrocapsaicin (6-I-CPS) at different concentrations (0.6, 6, 60 nmol).

CONCLUSION

These findings suggest that while pre-synaptic CB1-signalling subserves an indirect facilitatory effect of AEA on striato-nigral pathways causing panicolytic-like responses through midbrain tectum enhanced activity, post-synaptic TRPV₁-signalling in CPu mediates AEA direct activation of striato-nigral disinhibitory pathways resulting in increasing dlSC neurons activity and a panicogenic-like response. All these actions seem to depend on the interface with the nigro-collicular inhibitory GABAergic pathways.

Physical, Emotional, and Social Pain During COVID-19 Pandemic-Related Social Isolation

The socio-emotional condition during the COVID-19 pandemic subsidises the (re)modulation of interactive neural circuits underlying risk assessment behaviour at the physical, emotional, and social levels. Experiences of social isolation, exclusion, or affective loss are generally considered some of the most “painful” things that people endure. The threats of social disconnection are processed by some of the same neural structures that process basic threats to survival. The lack of social connection can be “painful” due to an overlap in the neural circuitry responsible for both physical and emotional pain related to feelings of social rejection. Indeed, many of us go to great lengths to avoid situations that may engender these experiences. Accordingly, this work focuses on pandemic times; the somatisation mentioned above seeks the interconnection and/or interdependence between neural systems related to emotional and cognitive processes such that a person involved in an aversive social environment becomes aware of himself, others, and the threatening situation experienced and takes steps to avoid daily psychological and neuropsychiatric effects. Social distancing during isolation evokes the formation of social distress, increasing the intensity of learned fear that people acquire, consequently enhancing emotional and social pain.

Major Phytocannabinoids and Their Related Compounds: Should We Only Search for Drugs That Act on Cannabinoid Receptors?

The most important discoveries in pharmacology, such as certain classes of analgesics or chemotherapeutics, started from natural extracts which have been found to have effects in traditional medicine. Cannabis, traditionally used in Asia for the treatment of pain, nausea, spasms, sleep, depression, and low appetite, is still a good candidate for the development of new compounds. If initially all attention was directed to the endocannabinoid system, recent studies suggest that many of the clinically proven effects are based on an intrinsic chain of mechanisms that do not necessarily involve only cannabinoid receptors. Recent research has shown that major phytocannabinoids and their derivatives also interact with non-cannabinoid receptors such as vanilloid receptor 1, transient receptor ankyrin 1 potential, peroxisome proliferator-activated receptor-gamma or glitazone receptor, G55 protein-coupled receptor, and nuclear receptor, producing pharmacological effects in diseases such as Alzheimer's, epilepsy, depression, neuropathic pain, cancer, and diabetes. Nonetheless, further studies are needed to elucidate the precise mechanisms of these compounds. Structure modulation of phytocannabinoids, in order to improve pharmacological effects, should not be limited to the exploration of cannabinoid receptors, and it should target other courses of action discovered through recent research.

Cannabidiol in the prelimbic cortex modulates the comorbid condition between the chronic neuropathic pain and depression-like behaviour in rats: The role of medial prefrontal cortex 5-HT1A and CB1 receptors

The prelimbic division (PrL) of the medial prefrontal cortex (mPFC) is a cerebral division that is putatively implicated in the chronic pain and depression. We investigated the activity of PrL cortex neurons in Wistar rats that underwent chronic constriction injury (CCI) of sciatic nerve and were further subjected to the forced swimming (FS) test and mechanical allodynia (by von Frey test). The effect of blockade of synapses with cobalt chloride (CoCl₂), and the treatment of the PrL cortex with cannabidiol (CBD), the CB₁ receptor antagonist AM251 and the 5-HT_1A receptor antagonist WAY-100635 were also investigated. Our results showed that CoCl₂ decreased the time spent immobile during the FS test but did not alter mechanical allodynia. CBD (at 15, 30 and 60 nmol) in the PrL cortex also decreased the frequency and duration of immobility; however, only the dose of 30 nmol of CBD attenuated mechanical allodynia in rats with chronic NP. AM251 and WAY-100635 in the PrL cortex attenuated the antidepressive and analgesic effect caused by CBD but did not alter the immobility and the mechanical allodynia when administered alone. These data show that the PrL cortex is part of the neural substrate underlying the comorbidity between NP and depression. Also, the previous blockade of CB₁ cannabinoid receptors and 5-HT_1A serotonergic receptors in the PrL cortex attenuated the antidepressive and analgesics effect of the CBD. They also suggest that CBD could be a potential medicine for the treatment of depressive and pain symptoms in patients with chronic NP/depression comorbidity.

Effects of GABAA receptors in nucleus cuneiformis on the cannabinoid antinociception using the formalin test

RATIONALE

Nucleus cuneiformis (NC), a reticular nucleus of the midbrain, is a part of the descending pain modulatory system and therefore has an important role in pain perception.

OBJECTIVES

Considering the abundance of GABA_A and cannabinoid receptors in the NC and also the bidirectional roles for GABA in controlling nociception, the present study examined the effects of bilateral intra-NC microinjection of different doses of the GABA_A receptor agonist, muscimol, and the GABA_A receptor antagonist, bicuculline, on pain modulation using formalin test. We also assessed interaction between canabinergic and GABAergic systems in the NC during this test.

METHODS

Rats were exposed to intra-NC microinjection of bicuculline (50,100, and 200 ng/side) or muscimol (60, 120, and 240 ng/side) and then subjected to the formalin test. In another set of experiments, the effects of muscimol (60 ng/side) or bicuculline (50 ng/side) administration 5 min before a cannabinoid receptor agonist WIN 55,212-2 (5, 10, and 20 μg/side) microinjection into NC on the formalin test were evaluated.

RESULTS

Microinjection of bicuculline and muscimol into the NC decreased and increased pain responses, respectively, in a dose-dependent manner during both phases of the test. Microinjection of WIN 55,212-2 into the NC significantly reduced pain responses in a dose-dependent manner. Microinjection of bicuculline or muscimol in combination with WIN 55,212-2 into the NC respectively potentiated and attenuated WIN 55,212-2-induced antinociception in the formalin test.

CONCLUSIONS

This study shows that GABA in the NC is involved in pain modulation and suggests the existence of a GABA_A-mediated inhibitory system in the NC on pain control. Furthermore, it seems that the antinociceptive effect of WIN 55,212-2 in the formalin test is mediated partly by the activity of local GABA_A receptors in the NC.

An Adapted Chronic Constriction Injury of the Sciatic Nerve Produces Sensory, Affective, and Cognitive Impairments: A Peripheral Mononeuropathy Model for the Study of Comorbid Neuropsychiatric Disorders Associated with Neuropathic Pain in Rats

BACKGROUND

Chronic constriction injury (CCI) is a model of neuropathic pain induced by four loose ligatures around the sciatic nerve. This work aimed to investigate the sensory, affective, cognitive, and motor changes induced by an adaptation of the CCI model by applying a single ligature around the sciatic nerve.

METHODS

Mechanical allodynia was measured from day 1 to day 28 postsurgery by the von Frey test. The beam walking test (BWT) was conducted weekly until 28 days after surgery. Anxiety- and depression-like behaviors, and cognitive performance were assessed through the open field (OF), forced swimming (FS), and novel object recognition (NOR) tests, respectively, 21 days after surgery.

RESULTS

The two CCI models, both Bennett and Xie's model (four ligatures of the sciatic nerve) and a modification of it (one ligature), induced mechanical allodynia, increased immobility in the FS, and reduced recognition index in the NOR. The exploratory behavior and time spent in the central part of the arena decreased, while the defensive behavior increased in the OF. The animals subjected to the two CCI models showed motor alterations in the BWT; however, autotomy was observed only in the group with four ligatures and not in the group with a single ligature.

CONCLUSIONS

Overall these results demonstrate that our adapted CCI model, using a single ligature around the sciatic nerve, induces sensory, affective, cognitive, and motor alterations comparable to the CCI model with four ligatures without generating autotomy. This adaptation to the CCI model may therefore represent an appropriate and more easily performed model for inducing neuropathic pain and study underlying mechanisms and effective treatments.

THC and CBD produce divergent effects on perception and panic behaviours via distinct cortical molecular pathways

Clinical and pre-clinical evidence demonstrates divergent psychotropic effects of THC vs. CBD. While THC can induce perceptual distortions and anxiogenic effects, CBD displays antipsychotic and anxiolytic properties. A key brain region responsible for regulation of cognition and affect, the medial prefrontal cortex (PFC), is strongly modulated by cannabinoids, suggesting that these dissociable THC/CBD-dependent effects may involve functional and molecular interplay within the PFC. The primary aim of this study was to investigate potential interactions and molecular substrates involved in PFC-mediated effects of THC and CBD on differential cognitive and affective behavioural processing. Male Sprague Dawley rats received intra-PFC microinfusions of THC, CBD or their combination, and tested in the latent inhibition paradigm, spontaneous oddity discrimination test, elevated T-maze and open field. To identify local, drug-induced molecular modulation in the PFC, PFC samples were collected and processed with Western Blotting. Intra-PFC THC induced strong panic-like responses that were counteracted with CBD. In contrast, CBD did not affect panic-like behaviours but blocked formation of associative fear memories and impaired latent inhibition and oddity discrimination performance. Interestingly, these CBD effects were dependent upon 5-HT_1A receptor transmission but not influenced by THC co-administration. Moreover, THC induced robust phosphorylation of ERK1/2 that was prevented by CBD, while CBD decreased phosphorylation of p70S6K, independently of THC. These results suggest that intra-PFC infusion of THC promotes panic-like behaviour associated with increased ERK1/2 phosphorylation. In contrast, CBD impairs perceptive functions and latent inhibition via activation of 5-HT_1A receptors and reduced phosphorylation of p70S6K.

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.

Attenuation of fear-conditioned analgesia in rats by monoacylglycerol lipase inhibition in the anterior cingulate cortex: Potential role for CB2 receptors

BACKGROUND AND PURPOSE

Improved understanding of brain mechanisms regulating endogenous analgesia is important from a fundamental physiological perspective and for identification of novel therapeutic strategies for pain. The endocannabinoid system plays a key role in stress-induced analgesia, including fear-conditioned analgesia (FCA), a potent form of endogenous analgesia. Here, we studied the role of the endocannabinoid 2-arachidonoyl glycerol (2-AG) within the anterior cingulate cortex (ACC; a brain region implicated in the affective component of pain) in FCA in rats.

EXPERIMENTAL APPROACH

FCA was modelled in male Lister-hooded rats by assessing formalin-evoked nociceptive behaviour in an arena previously paired with footshock. The effects of intra-ACC administration of MJN110 (inhibitor of monoacylglycerol lipase [MGL], the primary enzyme catabolizing 2-AG), AM630 (CB2 receptor antagonist), AM251 (CB1 receptor antagonist) or MJN110 + AM630 on FCA were assessed.

KEY RESULTS

MJN110 attenuated FCA when microinjected into the ACC, an effect associated with increased levels of 2-AG in the ACC. This effect of MJN110 on FCA was unaltered by co-administration of AM251 but was blocked by AM630, which alone reduced nociceptive behaviour in non-fear-conditioned rats. RT-qPCR confirmed that mRNA encoding CB1 and CB2 receptors was detectable in the ACC of formalin-injected rats and unchanged in those expressing FCA.

CONCLUSION AND IMPLICATIONS

These results suggest that an MGL substrate in the ACC, likely 2-AG, modulates FCA and that within the ACC, 2-AG-CB2 receptor signalling may suppress this form of endogenous analgesia. These results may facilitate increased understanding and improved treatment of pain- and fear-related disorders and their co-morbidity.

Medial prefrontal cortex diclofenac-induced antinociception is mediated through GPR55, cannabinoid CB1, and mu-opioid receptors of this area and periaqueductal gray

Supraspinal mechanisms of non-steroidal anti-inflammatory drug (NSAID)-induced antinociception are not well understood. In the present study, the possible antinociceptive mechanisms induced by intra-medial prefrontal cortex (intra-mPFC) microinjection of diclofenac were investigated after blockade of GPR55, cannabinoid CB1, and mu-opioid receptors in this area and ventrolateral periaqueductal gray (vlPAG). For drug delivery, unilateral (left side) of mPFC and bilateral (right and left sides) of vlPAG were surgically cannulated. Formalin test was induced by subcutaneous injection of a diluted formalin solution into the right vibrissa pad. A typical biphasic (neurogenic and inflammatory phases) pain behavior was produced following formalin injection. Microinjection of diclofenac (2.5, 5, and 10 μg/0.25 μL) into the mPFC suppressed both phases of pain. Intra-mPFC microinjection of naloxonazine (a mu-opioid receptor antagonist, 1 μg/0.25 μL) and AM251 (a cannabinoid CB1 receptor antagonist, 1 μg/0.25 μL) increased both phases of pain intensity. In addition, intra-mPFC-microinjected diclofenac-induced antinociception was inhibited by prior intra-mPFC and intra-vlPAG administration of naloxonazine and AM251. On the other hand, intra-mPFC and intra-vlPAG microinjection of AM251 (0.25 μg/0.25 μL) decreased pain severity which was inhibited by prior administration of ML193. The above-mentioned drugs did not alter locomotor activity. In conclusion, diclofenac suppressed both the neurogenic and inflammatory phases of formalin-induced orofacial pain at the level of mPFC. GPR55, cannabinoid CB1, and mu-opioid receptors of the mPFC and vlPAG might be involved in the mPFC analgesic effects of diclofenac.

Characterization of the sensory, affective, cognitive, biochemical, and neuronal alterations in a modified chronic constriction injury model of neuropathic pain in mice

The chronic constriction injury (CCI) of the sciatic nerve is a nerve injury-based model of neuropathic pain (NP). Comorbidities of NP such as depression, anxiety, and cognitive deficits are associated with a functional reorganization of the medial prefrontal cortex (mPFC). Here, we have employed an adapted model of CCI by placing one single loose ligature around the sciatic nerve in mice for investigating the alterations in sensory, motor, affective, and cognitive behavior and in electrophysiological and biochemical properties in the prelimbic division (PrL) of the mPFC. Our adapted model of CCI induced mechanical allodynia, motor, and cognitive impairments and anxiety- and depression-like behavior. In the PrL division of mPFC was observed an increase in GABA and a decrease in d-aspartate levels. Moreover an increase in the activity of neurons responding to mechanical stimulation with an excitation, mPFC (+), and a decrease in those responding with an inhibition, mPFC (-), was found. Altogether these findings demonstrate that a single ligature around the sciatic nerve was able to induce sensory, affective, cognitive, biochemical, and functional alterations already observed in other neuropathic pain models and it may be an appropriate and easily reproducible model for studying neuropathic pain mechanisms and treatments.

NO in the dPAG modulates panic-like responses and ASIC1a expression in the prefrontal cortex and hippocampus in mice

Panic disorder (PD) is a multifactorial neuropsychiatric disorder. Our previous study has demonstrated that the nitric oxide (NO) pathway and the acid-sensing ion channel 1a (ASIC1a) level in the dorsal midbrain periaqueductal gray (dPAG) are involved in the modulation of panic-like responses. In addition, the prefrontal cortex (PFC) and the hippocampus also play a role in panic-like responses. However, no studies have investigated the protein level of ASIC1a in the PFC and hippocampus in a mouse model of panic-like disorders after alteration of the NO pathway in the dPAG. We investigated the production of a panic attack with intra-dPAG injections of SNAP, an NO donor, and 7-NI, an nNOS inhibitor. Moreover, we measured ASIC1a protein levels in the PFC and hippocampus. The rat exposure test (RET) is frequently used as an animal model of panic. In our study, C57BL/6 mice received an intra-dPAG injection of SNAP or 7-NI before RET; neurobehavioral tests were then conducted, followed by mechanistic evaluation through western blot analysis in the PFC and hippocampus. An intra-dPAG infusion of SNAP significantly increased the panic-like effect, whereas treatment with 7-NI decreased fear behavior. Mice treated with SNAP/7-NI showed significantly increased/decreased ASIC1a expression in the PFC, and a decreasing/increasing trend in the hippocampus. The present study suggests that the NO pathway in the dPAG plays a key role in panic-like responses in mice confronted by a rat, further, NO intra-dPAG injection also modulates the ASIC1a expression levels in the PFC and hippocampus.

The prefrontal cortical endocannabinoid system modulates fear-pain interactions in a subregion-specific manner

BACKGROUND AND PURPOSE

The emotional processing and coordination of top-down responses to noxious and conditioned aversive stimuli involves the medial prefrontal cortex (mPFC). Evidence suggests that subregions of the mPFC [infralimbic (IfL), prelimbic (PrL) and anterior cingulate (ACC) cortices] differentially alter the expression of contextually induced fear and nociceptive behaviour. We investigated the role of the endocannabinoid system in the IfL, PrL and ACC in formalin-evoked nociceptive behaviour, fear-conditioned analgesia (FCA) and conditioned fear in the presence of nociceptive tone.

EXPERIMENTAL APPROACH

FCA was modelled in male Lister-hooded rats by assessing formalin-evoked nociceptive behaviour in an arena previously paired with footshock. The effects of intra-mPFC administration of AM251 [cannabinoid type 1 (CB₁ ) receptor antagonist/inverse agonist], URB597 [fatty acid amide hydrolase (FAAH) inhibitor] or URB597 + AM251 on FCA and freezing behaviour were assessed.

KEY RESULTS

AM251 attenuated FCA when injected into the IfL or PrL and reduced contextually induced freezing behaviour when injected intra-IfL but not intra-PrL or intra-ACC. Intra-ACC administration of AM251 alone or in combination with URB597 had no effect on FCA or freezing. URB597 attenuated FCA and freezing behaviour when injected intra-IfL, prolonged the expression of FCA when injected intra-PrL and had no effect on these behaviours when injected intra-ACC.

CONCLUSIONS AND IMPLICATIONS

These results suggest important and differing roles for FAAH substrates or CB₁ receptors in the PrL, IfL and ACC in the expression of FCA and conditioned fear in the presence of nociceptive tone.

LINKED ARTICLES

This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Transgenic autoinhibition of p21-activated kinase exacerbates synaptic impairments and fronto-dependent behavioral deficits in an animal model of Alzheimer's disease

Defects in p21-activated kinase (PAK) lead to dendritic spine abnormalities and are sufficient to cause cognition impairment. The decrease in PAK in the brain of Alzheimer's disease (AD) patients is suspected to underlie synaptic and dendritic disturbances associated with its clinical expression, particularly with symptoms related to frontal cortex dysfunction. To investigate the role of PAK combined with Aβ and tau pathologies (3xTg-AD mice) in the frontal cortex, we generated a transgenic model of AD with a deficit in PAK activity (3xTg-AD-dnPAK mice). PAK inactivation had no effect on Aβ40 and Aβ42 levels, but increased the phosphorylation ratio of tau in detergent-insoluble protein fractions in the frontal cortex of 18-month-old heterozygous 3xTg-AD mice. Morphometric analyses of layer II/III pyramidal neurons in the frontal cortex showed that 3xTg-AD-dnPAK neurons exhibited significant dendritic attrition, lower spine density and longer spines compared to NonTg and 3xTg-AD mice. Finally, behavioral assessments revealed that 3xTg-AD-dnPAK mice exhibited pronounced anxious traits and disturbances in social behaviors, reminiscent of fronto-dependent symptoms observed in AD. Our results substantiate a critical role for PAK in the genesis of neuronal abnormalities in the frontal cortex underlying the emergence of psychiatric-like symptoms in AD.

The Effects of Extended Pain on Behavior: Recent Progress

Chronic pain is frequently associated with anxiety, depression, and cognitive dysfunction. This review discusses recent work in rodents that contributes to the understanding of their neurobiological links. Brain regions that contain circuits that mediate persistent changes in behavior that are caused by nerve injury or joint inflammation include the rostral anterior cingulate and other parts of the medial prefrontal cortex, the basolateral and central nucleus of the amygdala, and the nucleus accumbens. Functional changes, including increases in the activity within specific neuronal pathways and in the levels of specific synaptic components, that are associated with the behavior changes, or are in some cases necessary for them, have recently been identified. Broadly projecting modulatory systems and widely expressed factors such as cytokines and growth factors also contribute to pain-associated behavior. Integrating these observations and determining their causal relationships is now critical for the identification of therapeutic targets and the design of appropriate interventions.

Decrease in NMDA receptor-signalling activity in the anterior cingulate cortex diminishes defensive behaviour and unconditioned fear-induced antinociception elicited by GABAergic tonic inhibition impairment in the posterior hypothalamus

Acute γ-aminobutyric acid (GABA) disinhibition in the posterior hypothalamus (PH) elicits defensive reactions that are considered anxiety- and panic attack-like behaviour, and these defensive reactions are followed by antinociception. Evidence indicates that the PH connects with the medial prefrontal cortex, particularly the anterior cingulate cortex (ACC), which seems to regulate these unconditioned fear-induced defensive responses. However, few studies have shown the participation of cortical regions in the control of behavioural and antinociceptive responses organised by diencephalic structures. It has been suggested that the glutamatergic system can mediate this cortical influence, as excitatory imbalance is believed to play a role in both defensive mechanisms. Thus, the aim of the present study was to investigate the involvement of ACC glutamatergic connections via blockade of local N-methyl-D-aspartate (NMDA) receptors to elaborate panic-like defensive behaviours and unconditioned fear-induced antinociception organised by PH neurons. Wistar rats were treated with microinjections of 0.9% NaCl or LY235959 (a selective NMDA receptor antagonist) in the ACC at different concentrations (2, 4 and 8 nmol/0.2μL), followed by GABA_A receptor blockade in the PH. Defensive reactions were analysed for 20min, and the nociceptive threshold was then measured at 10-min intervals for 60min. Pretreatment of the ACC with LY235959 reduced both panic-like defensive behaviour and fear-induced antinociception evoked by PH GABAergic disinhibition. Our findings suggest that ACC NMDA receptor-signalled glutamatergic inputs play a relevant role in the organisation of anxiety- and panic attack-like behaviours and in fear-induced antinociception.

Cold-Induced Thermogenesis and Inflammation-Associated Cold-Seeking Behavior Are Represented by Different Dorsomedial Hypothalamic Sites: A Three-Dimensional Functional Topography Study in Conscious Rats

In the past, we showed that large electrolytic lesions of the dorsomedial hypothalamus (DMH) promoted hypothermia in cold-exposed restrained rats, but attenuated hypothermia in rats challenged with a high dose of bacterial lipopolysaccharide (LPS) in a thermogradient apparatus. The goal of this study was to identify the thermoeffector mechanisms and DMH representation of the two phenomena and thus to understand how the same lesion could produce two opposite effects on body temperature. We found that the permissive effect of large electrolytic DMH lesions on cold-induced hypothermia was due to suppressed thermogenesis. DMH-lesioned rats also could not develop fever autonomically: they did not increase thermogenesis in response to a low, pyrogenic dose of LPS (10 μg/kg, i.v.). In contrast, changes in thermogenesis were uninvolved in the attenuation of the hypothermic response to a high, shock-inducing dose of LPS (5000 μg/kg, i.v.); this attenuation was due to a blockade of cold-seeking behavior. To compile DMH maps for the autonomic cold defense and for the cold-seeking response to LPS, we studied rats with small thermal lesions in different parts of the DMH. Cold thermogenesis had the highest representation in the dorsal hypothalamic area. Cold seeking was represented by a site at the ventral border of the dorsomedial nucleus. Because LPS causes both fever and hypothermia, we originally thought that the DMH contained a single thermoregulatory site that worked as a fever-hypothermia switch. Instead, we have found two separate sites: one that drives thermogenesis and the other, previously unknown, that drives inflammation-associated cold seeking.SIGNIFICANCE STATEMENT Cold-seeking behavior is a life-saving response that occurs in severe systemic inflammation. We studied this behavior in rats with lesions in the dorsomedial hypothalamus (DMH) challenged with a shock-inducing dose of bacterial endotoxin. We built functional maps of the DMH and found the strongest representation of cold-seeking behavior at the ventral border of the dorsomedial nucleus. We also built maps for cold-induced thermogenesis in unanesthetized rats and found the dorsal hypothalamic area to be its main representation site. Our work identifies the neural substrate of cold-seeking behavior in systemic inflammation and expands the functional topography of the DMH, a structure that modulates autonomic, endocrine, and behavioral responses and is a potential therapeutic target in anxiety and panic disorders.

Cannabinoids and Pain: Sites and Mechanisms of Action

The endocannabinoid system, consisting of the cannabinoid₁ receptor (CB₁R) and cannabinoid₂ receptor (CB₂R), endogenous cannabinoid ligands (endocannabinoids), and metabolizing enzymes, is present throughout the pain pathways. Endocannabinoids, phytocannabinoids, and synthetic cannabinoid receptor agonists have antinociceptive effects in animal models of acute, inflammatory, and neuropathic pain. CB₁R and CB₂R located at peripheral, spinal, or supraspinal sites are important targets mediating these antinociceptive effects. The mechanisms underlying the analgesic effects of cannabinoids likely include inhibition of presynaptic neurotransmitter and neuropeptide release, modulation of postsynaptic neuronal excitability, activation of the descending inhibitory pain pathway, and reductions in neuroinflammatory signaling. Strategies to dissociate the psychoactive effects of cannabinoids from their analgesic effects have focused on peripherally restricted CB₁R agonists, CB₂R agonists, inhibitors of endocannabinoid catabolism or uptake, and modulation of other non-CB₁R/non-CB₂R targets of cannabinoids including TRPV1, GPR55, and PPARs. The large body of preclinical evidence in support of cannabinoids as potential analgesic agents is supported by clinical studies demonstrating their efficacy across a variety of pain disorders.

μ-Opioid and 5-HT1A receptors in the dorsomedial hypothalamus interact for the regulation of panic-related defensive responses

The dorsomedial hypothalamus (DMH) and the dorsal periaqueductal gray (DPAG) have been implicated in the genesis and regulation of panic-related defensive behaviors, such as escape. Previous results point to an interaction between serotonergic and opioidergic systems within the DPAG to inhibit escape, involving µ-opioid and 5-HT1A receptors (5-HT1AR). In the present study we explore this interaction in the DMH, using escape elicited by electrical stimulation of this area as a panic attack index. The obtained results show that intra-DMH administration of the non-selective opioid receptor antagonist naloxone (0.5 nmol) prevented the panicolytic-like effect of a local injection of serotonin (20 nmol). Pretreatment with the selective μ-opioid receptor (MOR) antagonist CTOP (1 nmol) blocked the panicolytic-like effect of the 5-HT1AR agonist 8-OHDPAT (8 nmol). Intra-DMH injection of the selective MOR agonist DAMGO (0.3 nmol) also inhibited escape behavior, and a previous injection of the 5-HT1AR antagonist WAY-100635 (0.37 nmol) counteracted this panicolytic-like effect. These results offer the first evidence that serotonergic and opioidergic systems work together within the DMH to inhibit panic-like behavior through an interaction between µ-opioid and 5-HT1A receptors, as previously described in the DPAG.

Unravelling cortico-hypothalamic pathways regulating unconditioned fear-induced antinociception and defensive behaviours

The medial prefrontal cortex can influence unconditioned fear-induced defensive mechanisms organised by diencephalic neurons that are under tonic GABAergic inhibition. The posterior hypothalamus (PH) is involved with anxiety- and panic attack-like responses. To understand this cortical mediation, our study characterised anterior cingulate cortex (ACC)-PH pathways and investigated the effect of ACC local inactivation with lidocaine. We also investigated the involvement of PH ionotropic glutamate receptors in the defensive behaviours and fear-induced antinociception by microinjecting NBQX (an AMPA/kainate receptor antagonist) and LY235959 (a NMDA receptor antagonist) into the PH. ACC pretreatment with lidocaine decreased the proaversive effect and antinociception evoked by GABA_A receptor blockade in the PH, which suggests that there may be descending excitatory pathways from this cortical region to the PH. Microinjections of both NBQX and LY235959 into the PH also attenuated defensive and antinociceptive responses. This suggests that the blockade of AMPA/kainate and NMDA receptors reduces the activity of glutamatergic efferent pathways. Both inputs from the ACC to the PH and glutamatergic hypothalamic short links disinhibited by intra-hypothalamic GABA_A receptors blockade are potentially implicated. Microinjection of a bidirectional neurotracer in the PH showed a Cg1-PH pathway and PH neuronal reciprocal connections with the periaqueductal grey matter. Microinjections of an antegrade neurotracer into the Cg1 showed axonal fibres and glutamatergic vesicle-immunoreactive terminal boutons surrounding both mediorostral-lateroposterior thalamic nucleus and PH neuronal perikarya. These data suggest a critical role played by ACC-PH glutamatergic pathways and AMPA/kainate and NMDA receptors in the panic attack-like reactions and antinociception organised by PH neurons.

5-Hydroxytryptamine 1A receptors in the dorsomedial hypothalamus connected to dorsal raphe nucleus inputs modulate defensive behaviours and mediate innate fear-induced antinociception

The dorsal raphe nucleus (DRN) is an important brainstem source of 5-hydroxytryptamine (5-HT), and 5-HT plays a key role in the regulation of panic attacks. The aim of the present study was to determine whether 5-HT1A receptor-containing neurons in the medial hypothalamus (MH) receive neural projections from DRN and to then determine the role of this neural substrate in defensive responses. The neurotracer biotinylated dextran amine (BDA) was iontophoretically microinjected into the DRN, and immunohistochemical approaches were then used to identify 5HT1A receptor-labelled neurons in the MH. Moreover, the effects of pre-treatment of the dorsomedial hypothalamus (DMH) with 8-OH-DPAT and WAY-100635, a 5-HT1A receptor agonist and antagonist, respectively, followed by local microinjections of bicuculline, a GABAA receptor antagonist, were investigated. We found that there are many projections from the DRN to the perifornical lateral hypothalamus (PeFLH) but also to DMH and ventromedial (VMH) nuclei, reaching 5HT1A receptor-labelled perikarya. DMH GABAA receptor blockade elicited defensive responses that were followed by antinociception. DMH treatment with 8-OH-DPAT decreased escape responses, which strongly suggests that the 5-HT1A receptor modulates the defensive responses. However, DMH treatment with WAY-100635 failed to alter bicuculline-induced defensive responses, suggesting that 5-HT exerts a phasic influence on 5-HT1A DMH neurons. The activation of the inhibitory 5-HT1A receptor had no effect on antinociception. However, blockade of the 5-HT1A receptor decreased fear-induced antinociception. The present data suggest that the ascending pathways from the DRN to the DMH modulate panic-like defensive behaviours and mediate antinociceptive phenomenon by recruiting 5-HT1A receptor in the MH.

Blockade of the dorsomedial hypothalamus and the perifornical area inhibits respiratory responses to arousing and stressful stimuli

The dorsomedial hypothalamus (DMH) and the perifornical area (DMH/PeF) is one of the key regions of central autonomic processing. Previous studies have established that this region contains neurons that may be involved in respiratory processing; however, this has never been tested in conscious animals. The aim of our study was to investigate the involvement of the DMH/PeF area in mediating respiratory responses to stressors of various intensities and duration. Adult male Wistar rats ( n = 8) received microinjections of GABAA agonist muscimol or saline into the DMH/PeF bilaterally and were subjected to a respiratory recording using whole body plethysmography. Presentation of acoustic stimuli (500-ms white noise) evoked transient responses in respiratory rate, proportional to the stimulus intensity, ranging from +44 ± 27 to +329 ± 31 cycles/min (cpm). Blockade of the DMH/PeF almost completely abolished respiratory rate and tidal volume responses to the 40- to 70-dB stimuli and also significantly attenuated responses to the 80- to 90-dB stimuli. Also, it significantly attenuated respiratory rate during the acclimatization period (novel environment stress). The light stimulus (30-s 2,000 lux) as well as 15-min restraint stress significantly elevated respiratory rate from 95 ± 4.0 to 236 ± 29 cpm and from 117 ± 5.2 to 189 ± 13 cpm, respectively; this response was abolished after the DMH/PeF blockade. We conclude that integrity of the DMH/PeF area is essential for generation of respiratory responses to both stressful and alerting stimuli.

Dissociation between the panicolytic effect of cannabidiol microinjected into the substantia nigra, pars reticulata, and fear-induced antinociception elicited by bicuculline administration in deep layers of the superior colliculus: The role of CB1-cannabinoid receptor in the ventral mesencephalon

Many studies suggest that the substantia nigra, pars reticulata (SNpr), a tegmental mesencephalic structure rich in γ-aminobutyric acid (GABA)- and cannabinoid receptor-containing neurons, is involved in the complex control of defensive responses through the neostriatum-nigral disinhibitory and nigro-tectal inhibitory GABAergic pathways during imminently dangerous situations. The aim of the present work was to investigate the role played by CB1-cannabinoid receptor of GABAergic pathways terminal boutons in the SNpr or of SNpr-endocannabinoid receptor-containing interneurons on the effect of intra-nigral microinjections of cannabidiol in the activity of nigro-tectal inhibitory pathways. GABAA receptor blockade in the deep layers of the superior colliculus (dlSC) elicited vigorous defensive behaviour. This explosive escape behaviour was followed by significant antinociception. Cannabidiol microinjection into the SNpr had a clear anti-aversive effect, decreasing the duration of defensive alertness, the frequency and duration of defensive immobility, and the frequency and duration of explosive escape behaviour, expressed by running and jumps, elicited by transitory GABAergic dysfunction in dlSC. However, the innate fear induced-antinociception was not significantly changed. The blockade of CB1 endocannabinoid receptor in the SNpr decreased the anti-aversive effect of canabidiol based on the frequency and duration of defensive immobility, the frequency of escape expressed by running, and both the frequency and duration of escape expressed by jumps. These findings suggest a CB1 mediated endocannabinoid signalling in cannabidiol modulation of panic-like defensive behaviour, but not of innate fear-induced antinociception evoked by GABAA receptor blockade with bicuculline microinjection into the superior colliculus, with a putative activity in nigro-collicular GABAergic pathways.

Emotional regulatory function of receptor interacting protein 140 revealed in the ventromedial hypothalamus

Receptor-interacting protein (RIP140) is a transcription co-regulator highly expressed in macrophages to regulate inflammatory and metabolic processes. However, its implication in neurological, cognitive and emotional conditions, and the cellular systems relevant to its biological activity within the central nervous system are currently less clear. A transgenic mouse line with macrophage-specific knockdown of RIP140 was generated (MΦRIPKD mice) and brain-region specific RIP140 knockdown efficiency evaluated. Mice were subjected to a battery of tests, designed to evaluate multiple behavioral domains at naïve or following site-specific RIP140 re-expression. Gene expression analysis assessed TNF-α, IL-1β, TGF-1β, IL1-RA and neuropeptide Y (NPY) expression, and in vitro studies examined the effects of macrophage's RIP140 on astrocytes' NPY production. We found that RIP140 expression was dramatically reduced in macrophages within the ventromedial hypothalamus (VMH) and the cingulate cortex of MΦRIPKD mice. These animals exhibited increased anxiety- and depressive-like behaviors. VMH-targeted RIP140 re-expression in MΦRIPKD mice reversed its depressive- but not its anxiety-like phenotype. Analysis of specific neurochemical changes revealed reduced astrocytic-NPY expression within the hypothalamus of MΦRIPKD mice, and in vitro analysis confirmed that conditioned medium of RIP140-silnenced macrophage culture could no longer stimulate NPY production from astrocytes. The current study revealed an emotional regulatory function of macrophage-derived RIP140 in the VMH, and secondary dysregulation of NPY within hypothalamic astrocyte population, which might be associated with the observed behavioral phenotype of MΦRIPKD mice. This study highlights RIP140 as a novel target for the development of potential therapeutic and intervention strategies for emotional regulation disorders.

A critical role for prefrontocortical endocannabinoid signaling in the regulation of stress and emotional behavior

The prefrontal cortex (PFC) provides executive control of the brain in humans and rodents, coordinating cognitive, emotional, and behavioral responses to threatening stimuli and subsequent feedback inhibition of the hypothalamic-pituitary-adrenal (HPA) axis. The endocannabinoid system has emerged as a fundamental regulator of HPA axis feedback inhibition and an important modulator of emotional behavior. However, the precise role of endocannabinoid signaling within the PFC with respect to stress coping and emotionality has only recently been investigated. This review discusses the current state of knowledge regarding the localization and function of the endocannabinoid system in the PFC, its sensitivity to stress and its role in modulating the neuroendocrine and behavioral responses to aversive stimuli. We propose a model whereby steady-state endocannabinoid signaling in the medial PFC indirectly regulates the outflow of pyramidal neurons by fine-tuning GABAergic inhibition. Local activation of this population of CB1 receptors increases the downstream targets of medial PFC activation, which include inhibitory interneurons in the basolateral amygdala, inhibitory relay neurons in the bed nucleus of the stria terminalis and monoamine cell bodies such as the dorsal raphe nucleus. This ultimately produces beneficial effects on emotionality (active coping responses to stress and reduced anxiety) and assists in constraining activation of the HPA axis. Under conditions of chronic stress, or in individuals suffering from mood disorders, this system may be uniquely recruited to help maintain appropriate function in the face of adversity, while breakdown of the endocannabinoid system in the medial PFC may be, in and of itself, sufficient to produce neuropsychiatric illness. Thus, we suggest that endocannabinoid signaling in the medial PFC may represent an attractive target for the treatment of stress-related disorders.

The role of 5-HT1A receptors in the anti-aversive effects of cannabidiol on panic attack-like behaviors evoked in the presence of the wild snake Epicrates cenchria crassus (Reptilia, Boidae)

The potential anxiolytic and antipanic properties of cannabidiol have been shown; however, its mechanism of action seems to recruit other receptors than those involved in the endocannabinoid-mediated system. It was recently shown that the model of panic-like behaviors elicited by the encounters between mice and snakes is a good tool to investigate innate fear-related responses, and cannabidiol causes a panicolytic-like effect in this model. The aim of the present study was to investigate the 5-hydroxytryptamine (5-HT) co-participation in the panicolytic-like effects of cannabidiol on the innate fear-related behaviors evoked by a prey versus predator interaction-based paradigm. Male Swiss mice were treated with intraperitoneal (i.p.) administrations of cannabidiol (3 mg/kg, i.p.) and its vehicle and the effects of the peripheral pre-treatment with increasing doses of the 5-HT1A receptor antagonist WAY-100635 (0.1, 0.3 and 0.9 mg/kg, i.p.) on instinctive fear-induced responses evoked by the presence of a wild snake were evaluated. The present results showed that the panicolytic-like effects of cannabidiol were blocked by the pre-treatment with WAY-100635 at different doses. These findings demonstrate that cannabidiol modulates the defensive behaviors evoked by the presence of threatening stimuli, and the effects of cannabidiol are at least partially dependent on the recruitment of 5-HT1A receptors.

The role of dorsomedial and ventrolateral columns of the periaqueductal gray matter and in situ 5-HT₂A and 5-HT₂C serotonergic receptors in post-ictal antinociception

The periaqueductal gray matter (PAG) consists in a brainstem structure rich in 5-hydroxytryptamine (5-HT) inputs related to the modulation of pain. The involvement of each of the serotonergic receptor subtypes found in PAG columns, such as the dorsomedial (dmPAG) and the ventrolateral (vlPAG) columns, regarding post-ictal antinociception have not been elucidated. The present work investigated the participation of the dmPAG and vlPAG columns in seizure-induced antinociception. Specifically, we studied the involvement of serotonergic neurotransmission in these columns on antinociceptive responses that follow tonic-clonic epileptic reactions induced by pentylenetetrazole (PTZ), an ionophore GABA-mediated Cl(-) influx antagonist. Microinjections of cobalt chloride (1.0 mM CoCl2 /0.2 µL) into the dmPAG and vlPAG caused an intermittent local synaptic inhibition and decreased post-ictal antinociception that had been recorded at various time points after seizures. Pretreatments of the dmPAG or the vlPAG columns with the nonselective serotonergic receptors antagonist methysergide (5.0 µg/0.2 µL) or intramesencephalic microinjections of ketanserin (5.0 µg/0.2 µL), a serotonergic antagonist with more affinity to 5-HT2A/2C receptors, decreased tonic-clonic seizure-induced antinociception. Both dmPAG and vlPAG treatment with either the 5-HT2A receptor selective antagonist R-96544 (10 nM/0.2 µL), or the 5-HT2C receptors selective antagonist RS-102221 (0.15 µg/0.2 µL) also decrease post-ictal antinociception. These findings suggest that serotonergic neurotransmission, which recruits both 5-HT2A and 5-HT2C serotonergic receptors in dmPAG and vlPAG columns, plays a critical role in the elaboration of post-ictal antinociception.