Cannabinoid modulation of predator fear: involvement of the dorsolateral periaqueductal gray.

The present study investigated the effects of systemic or intra-dorsolateral periaqueductal gray (dlPAG) administration of CB1 agonists on behavioural changes induced in rats by predator (a live cat) exposure, a model of panic responses. Since nitric oxide (NO) and cannabinoid neurotransmission are proposed to interact in the dlPAG to modulate defensive responses, we also investigated if NO is involved in the biphasic effects of anandamide (AEA) injected into the dlPAG. The results showed that systemic administration of WIN55,212-2 or intra-dlPAG AEA attenuated the defensive behaviours caused by cat exposure. Both compounds produced biphasic curves. The cannabinoid receptor type 1 (CB1) antagonist AM251 prevented the panicolytic effect of AEA whereas a neuronal NOS inhibitor turned the ineffective high dose of AEA into an effective one. These results suggest that modulation of the cannabinoid system could be a target in the treatment of panic disorders. However, the biphasic effects of these compounds could limit their therapeutic potential.

Antiaversive effects of cannabinoids: is the periaqueductal gray involved?

Cannabinoids play an important role in activity-dependent changes in synaptic activity and can interfere in several brain functions, including responses to aversive stimuli. The regions responsible for their effects, however, are still unclear. Cannabinoid type 1 (CB1) receptors are widely distributed in the central nervous system and are present in the periaqueductal gray (PAG), a midbrain structure closely involved in responses related to aversive states. Accordingly, exposure to stressful stimuli increases endocannabinoid (eCB) levels in the PAG, and local administration of CB1 agonists or drugs that facilitate eCB-mediated neurotransmission produces antinociceptive and antiaversive effects. To investigate if these drugs would also interfere in animal models that are sensitive to anxiolytic drugs, we verified the responses to intra-PAG injection of CB1 agonists in rats submitted to the elevated plus-maze, the Vogel punished licking test, or contextual aversive conditioning model. The drugs induced anxiolytic-like effects in all tests. The same was observed with the transient receptor potential vanilloid type 1 (TRPV1) antagonist capsazepine and with cannabidiol, a nonpsychotomimetic phytocannabinoid that produces anxiolytic-like effects after systemic administration in humans and laboratory animals. These results, therefore, suggest that the PAG could be an important site for the antiaversive effects of cannabinoids.

Activation of CB1 cannabinoid receptors in the dorsolateral periaqueductal gray reduces the expression of contextual fear conditioning in rats.

RATIONALE: Conditioned fear to context causes freezing and cardiovascular changes in rodents and has been used to measure anxiety. It also activates the dorsolateral column of the periaqueductal gray (dlPAG). Microinjections of cannabinoid agonists into the dlPAG produced anxiolytic-like effects in the elevated plus maze, but the effects of these treatments on fear conditioning remains unknown. OBJECTIVE: The objective of this study was to verify if intra-dlPAG injection of the CB1 cannabinoid receptor agonist anandamide (AEA) or the anandamide transport inhibitor AM404 would attenuate behavioral (freezing) and cardiovascular (increase of arterial pressure and heart rate) responses of rats submitted to a contextual fear-conditioning paradigm. MATERIALS AND METHODS: Male Wistar rats with cannulae aimed at the dlPAG were re-exposed to a chamber where they had received footshocks 48 h before. Fifteen minutes before the test, the animals received a first intra-dlPAG injection of vehicle or AM251, a CB1 receptor antagonist (100 pmol/200 nl), followed 5 min later by vehicle, AEA (5 pmol/200 nl) or AM404 (50 pmol/200 nl). Freezing and cardiovascular responses were recorded for 10 min. RESULTS: Freezing and cardiovascular responses were reduced by administration of either AEA or AM404 into the dlPAG before re-exposition to the aversively conditioned context. These effects were abolished when the animals were locally pretreated with AM251. The latter drug, even at a higher dose (300 pmol), was ineffective when administered alone into the dlPAG. CONCLUSION: The results suggest that facilitation of endocannabinoid-mediated neurotransmission in the dlPAG, through activation of local CB1 receptors, attenuates the expression of contextual fear responses.

Mice lacking interleukin-18 gene display behavioral changes in animal models of psychiatric disorders: Possible involvement of immunological mechanisms.

Preclinical and clinical evidence suggests pro-inflammatory cytokines might play an important role in the neurobiology of schizophrenia and stress-related psychiatric disorders. Interleukin-18 (IL-18) is a member of the IL-1 family of cytokines and it is widely expressed in brain regions involved in emotional regulation. Since IL-18 involvement in the neurobiology of mental illnesses, including schizophrenia, remains unknown, this work aimed at investigating the behavior of IL-18 null mice (KO) in different preclinical models: 1. the prepulse inhibition test (PPI), which provides an operational measure of sensorimotor gating and schizophrenic-like phenotypes; 2. amphetamine-induced hyperlocomotion, a model predictive of antipsychotic activity; 3. resident-intruder test, a model predictive of aggressive behavior. Furthermore, the animals were submitted to models used to assess depressive- and anxiety-like behavior. IL-18KO mice showed impaired baseline PPI response, which was attenuated by d-amphetamine at a dose that did not modify PPI response in wild-type (WT) mice, suggesting a hypodopaminergic prefrontal cortex function in those mice. d-Amphetamine, however, induced hyperlocomotion in IL-18KO mice compared to their WT counterparts, suggesting hyperdopaminergic activity in the midbrain. Moreover, IL-18KO mice presented increased basal levels of IL-1ß levels in the hippocampus and TNF-α in the prefrontal cortex, suggesting an overcompensation of IL-18 absence by increased levels of other proinflammatory cytokines. Although no alteration was observed in the forced swimming or in the elevated plus maze tests in naïve IL-18KO mice, these mice presented anxiogenic-like behavior after exposure to repeated forced swimming stress. In conclusion, deletion of the IL-18 gene resembled features similar to symptoms observed in schizophrenia (positive and cognitive symptoms, aggressive behavior), in addition to increased susceptibility to stress. The IL-18KO model, therefore, could provide new insights into how changes in brain immunological homeostasis induce behavioral changes related to psychiatric disorders, such as schizophrenia.

The Endocannabinoid System and Anxiety.

The medical properties of Cannabis sativa is known for centuries. Since the discovery and characterization of the endogenous cannabinoid system, several studies have evaluated how cannabinoid compounds and, particularly, how the modulation of the endocannabinoid (eCB) system influences a wide range of functions, from metabolic to mental disorders. Cannabinoids and eCB system often exert opposite effects on several functions, such as anxiety. Although the mechanisms are not completely understood, evidence points to different factors influencing those effects. In this chapter, the recent advances in research about the relationship between eCB system and anxiety disorders in humans, as well as in animal models, will be discussed. The recent data addressing modulation of the eCBs in specific brain areas, such as the medial prefrontal cortex, amygdaloid complex, bed nucleus of stria terminalis, hippocampus, and dorsal periaqueductal gray, will be summarized. Finally, data from animal models addressing the mechanisms through which the eCB system modulates anxiety-related behavior dependent on stressful situations, such as the involvement of different receptors, distinct eCBs, modulation of neurotransmitters release, HPA axis and immune system activation, and plastic mechanisms, will also be discussed.

Dorsolateral periaqueductal gray matter CB1 and TRPV1 receptors exert opposite modulation on expression of contextual fear conditioning.

Cannabinoid type 1 (CB1) and Transient Potential Vanilloid type 1 (TRPV1) receptors in the dorsolateral periaqueductal gray (dlPAG) matter are involved in the modulation of conditioned response. Both CB1 and TRPV1 receptors are related to glutamate release and nitric oxide (NO) synthesis. It was previously demonstrated that both NMDA glutamate receptors and NO are involved in the conditioned emotional response. Therefore, one aim of this work was to verify whether dlPAG CB1 and TRPV1 receptors modulate the expression of contextual conditioned emotional response. Moreover, we also investigated the involvement of NMDA receptors and the NO pathway in this response. Male Wistar rats with local dlPAG guide cannula were submitted to contextual fear conditioning. Following 24 h, a polyethylene catheter was implanted in the femoral artery for cardiovascular recordings. After an additional 24 h, drugs were administered in the dlPAG and freezing behavior and autonomic responses were recorded during chamber re-exposure. Both a CB1 antagonist (AM251) and a TRPV1 agonist (Capsaicin; CPS) increased the expression of a conditioned emotional response. This response was prevented by an NMDA antagonist, a preferential neuronal NO synthase inhibitor, an NO scavenger and a soluble guanylate cyclase inhibitor (sGC). Furthermore, pretreatment with a TRPV1 antagonist also prevented the increased conditioned emotional response induced by AM251. Considering that GABA can counterbalance glutamate effects, we also investigated whether GABAA receptors were involved in the effect of a higher dose of AM251. Pretreatment with a GABAA receptor antagonist caused an increased conditioned emotional response by AM251. Our results support the possibility that dlPAG CB1 and TRPV1 receptors are involved in the expression of conditioned emotional response through the NMDA/NO/sGC pathway. Moreover, the opposite effects exerted by GABA and glutamate could produce different outcomes of drugs modulating eCBs.

Nitric oxide in the prelimbic medial prefrontal cortex is involved in the anxiogenic-like effect induced by acute restraint stress in rats.

Neurons containing the neuronal nitric oxide synthase (nNOS) enzyme are located in brain areas related to defensive behavior, such as the ventromedial prefrontal cortex (vMPFC). Rats exposed to a live predator (a cat) present anxiety-like behavior and an increased number of nNOS-positive neurons in this brain area one-week later. Moreover, stress-related behavioral changes in rodents can be prevented by systemic or local vMPFC nNOS inhibition. In the present study we investigated if acute restraint stress (RS)-induced delayed (one-week) anxiogenic-like effect was associated with increased nNOS expression or activity in the vMPFC. Furthermore, we also tested if local pharmacological nNOS inhibition would prevent stress-induced behavioral changes. Male Wistar rats were submitted to RS for 3h and tested in the elevated plus maze (EPM) 24h or 7 days later. Two hours after the EPM test, their brains were removed, processed and nNOS expression in the vMPFC was evaluated by immunohistochemistry. Another group of animals was used for measuring NO metabolites (NOx; an indirect measure of NOS activity) immediately after the EPM test, 24h after RS. Independent groups had guide cannula implanted bilaterally into the prelimbic (PL) portion of vMPFC. Five to six days after surgery, the animals were submitted to RS and 24h later received local administration of the nNOS inhibitor, N-propyl-l-arginine (NPLA; 0.04 nmol). They were tested in the EPM 10 min later. RS-induced anxiogenic-like effect was accompanied by increased nNOS expression in the PL (p<0.05), but not in the infralimbic (IL) vMPFC, both 24h and 7 days after RS. Moreover, open-arm exploration of the EPM was negatively correlated with nNOS expression (p<0.05) and NOx levels (p<0.05) in the PL. The anxiogenic-like effect observed 24h after RS was prevented by NPLA (p<0.05). Our results suggest that RS-induced anxiogenic-like effect might depend on increased nNOS-mediated signaling in the PL MPFC.

Modulation of defensive behavior by Transient Receptor Potential Vanilloid Type-1 (TRPV1) channels.

The Transient Receptor Potential Vanilloid Type-1 (TRPV1) was first characterized in primary afferent fibers as a receptor for capsaicin (the pungent ingredient of chili peppers). Later on, this cation-permeable ion channel was also described in the central nervous system, where its main putative endogenous ligand is N-arachidonoyl ethanolamide (an endocannabinoid, also known as anandamide). Recent results employing genetic, pharmacological and histochemical techniques indicate that TRPV1 tonically modulate anxiety, fear and panic responses in brain regions related to defensive responses, such as the dorsal periaqueductal gray, the hippocampus and the medial prefrontal cortex. Genetic deletion or antagonism of this ion channel induces anxiolytic-like effects in several animal models. The main mechanism responsible for TRPV1-mediated effects on anxiety seems to involve facilitation of glutamatergic neurotransmission. In addition, there is evidence for interactions with other neurotransmitter systems, such as nitric oxide and endocannabinoids.

Complex interaction between anandamide and the nitrergic system in the dorsolateral periaqueductal gray to modulate anxiety-like behavior in rats.

Stimulation of cannabinoid CB1 receptors or inhibition of nitric oxide synthase (NOS) in the dorsolateral periaqueductal gray (dlPAG) decreases anxiety-like behavior. Moreover, activation of CB1 receptors attenuates flight responses induced by nitric oxide (NO) donors in the dlPAG, suggesting that endocannabinoids and NO could interact to control defensive responses such as anxiety-like behavior. To test this hypothesis male Wistar rats received intra-dlPAG microinjections of anandamide (AEA) or NO inhibitors and were tested in the elevated plus maze (EPM). Combined administration of low and ineffective doses of AEA and the NO scavenger (c-Ptio), the nNOS inhibitor (NPA) or the soluble guanylate cyclase inhibitor (ODQ) induced anxiolytic-like effects. The CB1 receptor antagonist AM251, but not the GABAA receptor antagonist bicuculline, attenuated the effect induced by AEA+c-Ptio combination. No effect, however, was found when anxiolytic doses of these same drugs were administered together. Combination of higher, ineffective doses of AEA and c-Ptio, NPA or ODQ was again anxiolytic. The effect of the former combination was prevented by low and ineffective doses of the GABAA receptor antagonist bicuculline or the GABA synthesis inhibitor L-allilglycine, suggesting that they depend on GABAA-mediated neurotransmission. AM251 was also able to attenuate this effect, indicating that in the presence of NO inhibition, the resultant anxiolytic-like effect could be due to AEA action on CB1 receptors. The present results suggest that the AEA and nitrergic systems exert a complex functional interaction in the dlPAG to modulate anxiety behavior, probably interfering, in addition to glutamate, also with GABAergic mechanisms.

Differential role of CB1 and TRPV1 receptors on anandamide modulation of defensive responses induced by nitric oxide in the dorsolateral periaqueductal gray.

CB1, TRPV1 and NO can regulate glutamate release and modify defensive behaviors in regions related to defensive behavior such as the dorsolateral periaqueductal gray (dlPAG). A possible interaction between the endocannabinoid and nitrergic systems in this area, however, has not been investigated yet. The objective of the present work was to verify if activation of CB1 or TRPV1 receptors could interfere in the flight responses induced in rats by the injection of SIN-1, an NO donor, into the dlPAG. The results showed that local administration of a low dose (5 pmol) of anandamide (AEA) attenuated the flight responses, measured by the total distance moved and maximum speed in an open arena, induced by intra-dlPAG microinjection of SIN-1 (150 nmol). URB597 (0.1 nmol), an inhibitor of anandamide metabolism, produced similar effects. When animals were locally treated with the CB1 receptor antagonist AM251 the effective AEA dose (5 pmol) increased, rather than decreased, the flight reactions induced by SIN1-1. Higher (50-200 nmol) doses of AEA were ineffective and even tended to potentiate the SIN-1 effect. The TRPV1 antagonist capsazepine (CPZ, 30 nmol) prevented SIN-1 effects and attenuated the potentiation of its effect by the higher (200 nmol) AEA dose. The results indicate that AEA can modulate in a dual way the pro-aversive effects of NO in the dlPAG by activating CB1 or TRPV1 receptors.

Neuroanatomical substrates involved in cannabinoid modulation of defensive responses.

Administration of Cannabis sativa derivatives causes anxiolytic or anxiogenic effects in humans and laboratory animals, depending on the specific compound and dosage used. In agreement with these findings, several studies in the last decade have indicated that the endocannabinoid system modulates neuronal activity in areas involved in defensive responses. The mechanisms of these effects, however, are still not clear. The present review summarizes recent data suggesting that they involve modulation of glutamate and GABA-mediated neurotransmission in brain sites such as the medial prefrontal cortex, amygdaloid complex, bed nucleus of the stria terminalis, hippocampus and dorsal periaqueductal gray. Moreover, we also discuss results indicating that, in these regions, the endocannabinoid system could be particularly engaged by highly stressful situations.

Fine-tuning of defensive behaviors in the dorsal periaqueductal gray by atypical neurotransmitters.

This paper presents an up-to-date review of the evidence indicating that atypical neurotransmitters such as nitric oxide (NO) and endocannabinoids (eCBs) play an important role in the regulation of aversive responses in the periaqueductal gray (PAG). Among the results supporting this role, several studies have shown that inhibitors of neuronal NO synthase or cannabinoid receptor type 1 (CB1) receptor agonists cause clear anxiolytic responses when injected into this region. The nitrergic and eCB systems can regulate the activity of classical neurotransmitters such as glutamate and γ-aminobutyric acid (GABA) that control PAG activity. We propose that they exert a 'fine-tuning' regulatory control of defensive responses in this area. This control, however, is probably complex, which may explain the usually bell-shaped dose-response curves observed with drugs that act on NO- or CB1-mediated neurotransmission. Even if the mechanisms responsible for this complex interaction are still poorly understood, they are beginning to be recognized. For example, activation of transient receptor potential vanilloid type-1 channel (TRPV1) receptors by anandamide seems to counteract the anxiolytic effects induced by CB1 receptor activation caused by this compound. Further studies, however, are needed to identify other mechanisms responsible for this fine-tuning effect.

Different role of the ventral medial prefrontal cortex on modulation of innate and associative learned fear.

Reversible inactivation of the ventral portion of medial prefrontal cortex (vMPFC) of the rat brain has been shown to induce anxiolytic-like effects in animal models based on associative learning. The role of this brain region in situations involving innate fear, however, is still poorly understood, with several contradictory results in the literature. The objective of the present work was to verify in male Wistar rats the effects of vMPFC administration of cobalt chloride (CoCl(2)), a selective inhibitor of synaptic activity, in rats submitted to two models based on innate fear, the elevated plus-maze (EPM) and light-dark box (LDB), comparing the results with those obtained in two models involving associative learning, the contextual fear conditioning (CFC) and Vogel conflict (VCT) tests. The results showed that, whereas CoCl(2) induced anxiolytic-like effects in the CFC and VCT tests, it enhanced anxiety in rats submitted to the EPM and LDB. Together these results indicate that the vMPFC plays an important but complex role in the modulation of defensive-related behaviors, which seems to depend on the nature of the anxiety/fear inducing stimuli.

Fine-tuning of defensive behaviors in the dorsal periaqueductal gray by atypical neurotransmitters

This paper presents an up-to-date review of the evidence indicating that atypical neurotransmitters such as nitric oxide (NO) and endocannabinoids (eCBs) play an important role in the regulation of aversive responses in the periaqueductal gray (PAG). Among the results supporting this role, several studies have shown that inhibitors of neuronal NO synthase or cannabinoid receptor type 1 (CB1) receptor agonists cause clear anxiolytic responses when injected into this region. The nitrergic and eCB systems can regulate the activity of classical neurotransmitters such as glutamate and γ-aminobutyric acid (GABA) that control PAG activity. We propose that they exert a ‘fine-tuning’ regulatory control of defensive responses in this area. This control, however, is probably complex, which may explain the usually bell-shaped dose-response curves observed with drugs that act on NO- or CB1-mediated neurotransmission. Even if the mechanisms responsible for this complex interaction are still poorly understood, they are beginning to be recognized. For example, activation of transient receptor potential vanilloid type-1 channel (TRPV1) receptors by anandamide seems to counteract the anxiolytic effects induced by CB1 receptor activation caused by this compound. Further studies, however, are needed to identify other mechanisms responsible for this fine-tuning effect.