P2X7 receptors modulate acquisition of cue fear extinction and contextual background memory generalization in male mice.

The purinergic P2X7 receptors (P2X7R) are activated by adenosine triphosphate (ATP) in several brain regions, particularly those involved with emotional control and the regulation of fear-related memories. Here, we investigate the role of P2X7R in fear learning memory, specifically in the acquisition and consolidation phases of the cued fear conditioning paradigm. C57Bl/6 wildtype (WT) male mice that received a single i.p. injection of the selective P2X7R antagonist A438079 prior the conditioning session showed generalization of cued fear memory and impaired fear extinction recall in the test session, while those treated prior the extinction session exhibited a similar behavior profile accompanied by resistance in the extinction learning. However, no effects were observed when this drug was administered immediately after the conditioning, extinction, or before the test session. Our results with P2X7R knockout (P2X7 KO) mice showed a behavioral profile that mirrored the collective effects observed across all pharmacological treatment conditions. This suggests that the P2X7R KO model effectively replicates the behavioral changes induced by the pharmacological interventions, demonstrating that we have successfully isolated the role of P2X7R in the fear and extinction phases of memory. These findings highlight the role of P2X7R in the acquisition and recall of extinction memory and supports P2X7R as a promising candidate for controlling abnormal fear processing, with potential applications for stress exposure-related disorders such as post-traumatic stress disorder (PTSD).

The influence of paraventricular nucleus of the hypothalamus soluble guanylate cyclase on autonomic and neuroendocrine responses to acute restraint stress in rats.

The paraventricular nucleus of the hypothalamus (PVN) regulates physiological and behavioural responses evoked by stressful stimuli, but the local neurochemical and signalling mechanisms involved are not completely understood. The soluble guanylate cyclase (sGC) within the PVN is implicated in autonomic and cardiovascular control in rodents under resting conditions. However, the involvement of PVN sGC-mediated signalling in stress responses is unknown. Therefore, we investigated the role of sGC within the PVN in cardiovascular, autonomic, neuroendocrine, and local neuronal responses to acute restraint stress in rats. Bilateral microinjection of the selective sGC inhibitor ODQ (1 nmol/100 nl) into the PVN reduced both the increased arterial pressure and the drop in cutaneous tail temperature evoked by restraint stress, while the tachycardia was enhanced. Intra-PVN injection of ODQ did not alter the number of Fos-immunoreactive neurons in either the dorsal cap parvocellular (PaDC), ventromedial (PaV), medial parvocellular (PaMP), or lateral magnocelllular (PaLM) portions of the PVN following acute restraint stress. Local microinjection of ODQ into the PVN did not affect the restraint-induced increases in plasma corticosterone concentration. Taken together, these findings suggest that sGC-mediated signalling in the PVN plays a key role in acute stress-induced pressor responses and sympathetically mediated cutaneous vasoconstriction, whereas the tachycardiac response is inhibited. Absence of an effect of ODQ on corticosterone and PVN neuronal activation in and the PaV and PaMP suggests that PVN sGC is not involved in restraint-evoked hypothalamus-pituitary-adrenal (HPA) axis activation and further indicates that autonomic and neuroendocrine responses are dissociable at the level of the PVN.

Pioglitazone attenuates behavioral and electrophysiological dysfunctions induced by two-hit model of schizophrenia in adult rodent offspring.

Maternal infection and stress exposure, especially during childhood and adolescence, have been implicated as risk factors for schizophrenia. Both insults induce an exacerbated inflammatory response, which could mediate disturbance of neurodevelopmental processes and, ultimately, malfunctioning of neural systems observed in this disorder. Thus, anti-inflammatory drugs, such as PPARγ agonists, may potentially be used to prevent the development of schizophrenia. Microglia culture was prepared from the offspring of saline or poly(I:C)-injected mice. The cells were pretreated with pioglitazone and then, stimulated by LPS. Proinflammatory mediators and phagocytic activity were measured. Also, pregnant rats were injected with saline or poly(I:C) on GD17. The offspring were subjected to footshock during adolescence and subsequently injected with pioglitazone or vehicle. At adulthood, behavior and dopaminergic activity were evaluated. Pioglitazone reduced proinflammatory mediators induced by poly(I:C) microglia stimulated by LPS without affecting their decreased phagocytic activity. The PPARγ agonist also prevented the emergence of social and cognitive impairments, as well as attenuated the increased number of spontaneously active dopamine neurons in the VTA, observed in both males and females from poly(I:C) and stress group. Therefore, pioglitazone could potentially prevent the emergence of the schizophrenia-like alterations induced by the two-hit model via reduction of microglial activation.

Fluoxetine and Ketamine Enhance Extinction Memory and Brain Plasticity by Triggering the p75 Neurotrophin Receptor Proteolytic Pathway.

BACKGROUND: Diverse antidepressants were recently described to bind to TrkB (tyrosine kinase B) and drive a positive allosteric modulation of endogenous BDNF (brain-derived neurotrophic factor). Although neurotrophins such as BDNF can bind to p75NTR (p75 neurotrophin receptor), their precursors are the high-affinity p75NTR ligands. While part of an unrelated receptor family capable of inducing completely opposite physiological changes, TrkB and p75NTR feature a crosslike conformation dimer and carry a cholesterol-recognition amino acid consensus in the transmembrane domain. As such qualities were found to be crucial for antidepressants to bind to TrkB and drive behavioral and neuroplasticity effects, we hypothesized that their effects might also depend on p75NTR. METHODS: Enzyme-linked immunosorbent assay-based binding and nuclear magnetic resonance spectroscopy were performed to assess whether antidepressants would bind to p75NTR. HEK293T cells and a variety of in vitro assays were used to investigate whether fluoxetine (FLX) or ketamine (KET) would trigger any α- and γ-secretase-dependent p75NTR proteolysis and lead to p75NTR nuclear localization. Ocular dominance shift was performed with male and female p75NTR knockout mice to study the effects of KET and FLX on brain plasticity, in addition to pharmacological interventions to verify how p75NTR signaling is important for the effects of KET and FLX in enhancing extinction memory in male wild-type mice and rats. RESULTS: Antidepressants were found to bind to p75NTR. FLX and KET triggered the p75NTR proteolytic pathway and induced p75NTR-dependent behavioral/neuroplasticity changes. CONCLUSIONS: We hypothesize that antidepressants co-opt both BDNF/TrkB and proBDNF/p75NTR systems to induce a more efficient activity-dependent synaptic competition, thereby boosting the brain's ability for remodeling.

Functional lateralization in the medial prefrontal cortex control of contextual conditioned emotional responses in rats.

A functional lateralization has been reported in control of emotional responses by the medial prefrontal cortex (mPFC). However, a hemisphere asymmetry in involvement of the mPFC in expression of fear conditioning responses has never been reported. Therefore, we investigated whether control by mPFC of freezing and cardiovascular responses during re-exposure to an aversively conditioned context is lateralized. For this, rats had guide cannulas directed to the mPFC implanted bilaterally or unilaterally in the right or left hemispheres. Vehicle or the non-selective synaptic inhibitor CoCl2 was microinjected into the mPFC 10 min before re-exposure to a chamber where the animals had previously received footshocks. A catheter was implanted into the femoral artery before the fear retrieval test for cardiovascular recordings. We observed that bilateral microinjection of CoCl2 into the mPFC reduced both the freezing behavior (enhancing locomotion and rearing) and arterial pressure and heart rate increases during re-exposure to the aversively conditioned context. Unilateral microinjection of CoCl2 into the right hemisphere of the mPFC also decreased the freezing behavior (enhancing locomotion and rearing), but without affecting the cardiovascular changes. Conversely, unilateral synaptic inhibition in the left mPFC did not affect either behavioral or cardiovascular responses during fear retrieval test. Taken together, these results suggest that the right hemisphere of the mPFC is necessary and sufficient for expression of freezing behavior to contextual fear conditioning. However, the control of cardiovascular responses and freezing behavior during fear retrieval test is somehow dissociated in the mPFC, being the former bilaterally processed.

Neuroprotective Profile of Triazole Grandisin Analogue against Amyloid-Beta Oligomer-Induced Cognitive Impairment.

Alzheimer's disease (AD) is a neurodegenerative disorder caused by accumulation of amyloid-ß oligomers (AßO) in the brain, neuroinflammation, oxidative stress, and cognitive decline. Grandisin, a tetrahydrofuran neolignan, exhibits relevant anti-inflammatory and antioxidant properties. Interestingly, grandisin-based compounds were shown to prevent AßO-induced neuronal death in vitro. However, no study has assessed the effect of these compounds on the AD animal model. This study focuses on a triazole grandisin analogue (TGA) synthesized using simplification and bioisosteric drug design, which resulted in improved potency and solubility compared with the parent compound. This study aimed to investigate the possible in vivo effects of TGA against AßO-induced AD. Male C57/Bl6 mice underwent stereotaxic intracerebroventricular AßO (90 µM) or vehicle injections. 24 h after surgery, animals received intraperitoneal treatment with TGA (1 mg/kg) or vehicle, administered on a 14 day schedule. One day after treatment completion, a novel object recognition task (NORT) was performed. Memantine (10 mg/kg) was administered as a positive control. NORT retention sessions were performed on days 8 and 16 after AßO injection. Immediately after retention sessions, animals were euthanized for cortex and hippocampus collection. Specimens were subjected to oxidative stress and cytokine analyses. TGA reduced the level of cortex/hippocampus lipoperoxidation and prevented cognitive impairment in AßO-injected mice. Additionally, TGA reduced tumor necrosis factor (TNF) and interferon-γ (IFN-γ) levels in the hippocampus. By contrast, memantine failed to prevent cortex/hippocampus lipid peroxidation, recognition memory decline, and AßO-induced increases in TNF and IFN-γ levels in the hippocampus. Thus, memantine was unable to avoid the AßO-induced persistent cognitive impairment. The results showed that TGA may prevent memory impairment by exerting antioxidant and anti-inflammatory effects in AßO-injected mice. Moreover, TGA exhibited a persistent neuroprotective effect compared to memantine, reflecting an innovative profile of this promising agent against neurodegenerative diseases, such as AD.

Contextual fear expression engages a complex set of interactions between ventromedial prefrontal cortex cholinergic, glutamatergic, nitrergic and cannabinergic signaling.

Rats re-exposed to an environment previously associated with the onset of shocks evoke a set of conditioned defensive responses in preparation to an eventual flight or fight reaction. Ventromedial prefrontal cortex (vmPFC) is mutually important for controlling the behavioral/physiological consequences of stress exposure and the one's ability to satisfactorily undergo spatial navigation. While cholinergic, cannabinergic and glutamatergic/nitrergic neurotransmissions within the vmPFC are shown as important for modulating both behavioral and autonomic defensive responses, there is a gap on how these systems would interact to ultimately coordinate such conditioned reactions. Then, males Wistar rats had guide cannulas bilaterally implanted to allow drugs to be administered in vmPFC 10 min before their re-exposure to the conditioning chamber where three shocks were delivered at the intensity of 0.85 mA for 2 s two days ago. A femoral catheter was implanted for cardiovascular recordings the day before fear retrieval test. It was found that the increment of freezing behavior and autonomic responses induced by vmPFC infusion of neostigmine (acetylcholinesterase inhibitor) were prevented by prior infusion of a transient receptor potential vanilloid type 1 (TRPV1) antagonist, N-methyl-d-aspartate receptor antagonist, neuronal nitric oxide synthase inhibitor, nitric oxide scavenger and soluble guanylate cyclase inhibitor. A type 3 muscarinic receptor antagonist was unable to prevent the boosting in conditioned responses triggered by a TRPV1 agonist and a cannabinoid receptors type 1 antagonist. Altogether, our results suggest that expression of contextual conditioned responses involves a complex set of signaling steps comprising different but complementary neurotransmitter pathways.

Bed nucleus of the stria terminalis CB1 receptors and the FAAH enzyme modulate anxiety behavior depending on previous stress exposure.

The endocannabinoid (eCB) anandamide (AEA) is synthesized on-demand in the post-synaptic terminal and can act on presynaptic cannabinoid type 1 (CB1) receptors, decreasing the release of neurotransmitters, including glutamate. AEA action is ended through enzymatic hydrolysis via FAAH (fatty acid amid hydrolase) in the post-synaptic neuron. eCB system molecules are widely expressed in brain areas involved in the modulation of fear and anxiety responses, including the Bed Nucleus of the Stria Terminalis (BNST), which is involved in the integration of autonomic, neuroendocrine, and behavioral regulation. The presence of the CB1 and FAAH was described in the BNST; however, their role in the modulation of defensive reactions is not fully comprehended. In the present work we aimed at investigating the role of AEA and CB1 receptors in the BNST in modulating anxiety-related behaviors. Adult male Wistar rats received local BNST injections of the CB1 receptor antagonist AM251 (0.1-0.6 nmol) and/or the FAAH inhibitor (URB597; 0.001-0.1 nmol) and were evaluated in the elevated plus maze (EPM) test, with or without previous acute restraint stress (2 h) exposure, or in the contextual fear conditioning. We observed that although AM251 and URB597 had no effects on the EPM, they increased and decreased, respectively, the conditioned fear response. Supporting a possible influence of stress in these differences, URB597 was able to prevent the restraint stress-induced anxiogenic effect in the EPM. The present data, therefore, suggest that eCB signaling in the BNST is recruited during more aversive situations to counteract the stress effect.

The medial prefrontal cortex and the cardiac baroreflex activity: physiological and pathological implications.

The cardiac baroreflex is an autonomic neural mechanism involved in the modulation of the cardiovascular system. It influences the heart rate and peripheral vascular resistance to preserve arterial blood pressure within a narrow variation range. This mechanism is mainly controlled by medullary nuclei located in the brain stem. However, supramedullary areas, such as the ventral portion of medial prefrontal cortex (vMPFC), are also involved. Particularly, the glutamatergic NMDA/NO pathway in the vMPFC can facilitate baroreflex bradycardic and tachycardic responses. In addition, cannabinoid receptors in this same area can reduce or increase those cardiac responses, possibly through alteration in glutamate release. This vMPFC network has been associated to cardiovascular responses during stressful situations. Recent results showed an involvement of glutamatergic, nitrergic, and endocannabinoid systems in the blood pressure and heart rate increases in animals after aversive conditioning. Consequently, baroreflex could be modified by the vMPFC neurotransmission during stressful situations, allowing necessary cardiovascular adjustments. Remarkably, some mental, neurological and neurodegenerative disorders can involve damage in the vMPFC, such as posttraumatic stress disorder, major depressive disorder, Alzheimer's disease, and neuropathic pain. These pathologies are also associated with alterations in glutamate/NO release and endocannabinoid functions along with baroreflex impairment. Thus, the vMPFC seems to play a crucial role on the baroreflex control, either during pathological or physiological stress-related responses. The study of baroreflex mechanism under such pathological view may be helpful to establish causality mechanisms for the autonomic and cardiovascular imbalance found in those conditions. It can explain in the future the reasons of the high cardiovascular risk some neurological and neurodegenerative disease patients undergo. Additionally, the present work offers insights on the possible contributions of vMPFC dysfunction on baroreflex alterations, which, in turn, may raise questions in what extent other brain areas may play a role in autonomic deregulation under such pathological situations.

The interaction between hippocampal cholinergic and nitrergic neurotransmission coordinates NMDA-dependent behavior and autonomic changes induced by contextual fear retrieval.

RATIONALE: Re-exposing an animal to an environment previously paired with an aversive stimulus evokes large alterations in behavioral and cardiovascular parameters. Dorsal hippocampus (dHC) receives important cholinergic inputs from the basal forebrain, and respective acetylcholine (ACh) levels are described to influence defensive behavior. Activation of muscarinic M1 and M3 receptors facilitates autonomic and behavioral responses along threats. Evidence show activation of cholinergic receptors promoting formation of nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) in dHC. Altogether, the action of ACh and NO on conditioned responses appears to converge within dHC. OBJECTIVES: As answer about how ACh and NO interact to modulate defensive responses has so far been barely addressed, we aimed to shed additional light on this topic. METHODS: Male Wistar rats had guide cannula implanted into the dHC before being submitted to the contextual fear conditioning (3footshocks/085 mA/2 s). A catheter was implanted in the femoral artery the next day for cardiovascular recordings. Drugs were delivered into dHC 10 min before contextual re-exposure, which occurred 48 h after the conditioning procedure. RESULTS: Neostigmine (Neo) amplified the retrieval of conditioned responses. Neo effects (1 nmol) were prevented by the prior infusion of a M1-M3 antagonist (fumarate), a neuronal nitric oxide synthase inhibitor (NPLA), a NO scavenger (cPTIO), a guanylyl cyclase inhibitor (ODQ), and a NMDA antagonist (AP-7). Pretreatment with a selective M1 antagonist (pirenzepine) only prevented the increase in autonomic responses induced by Neo. CONCLUSION: The results show that modulation in the retrieval of contextual fear responses involves coordination of the dHC M1-M3/NO/cGMP/NMDA pathway.

Dorsal hippocampal muscarinic cholinergic receptors orchestrate behavioral and autonomic changes induced by contextual fear retrieval.

Re-exposure of rats to a previously fear-conditioned environment arouses great alterations in behavioral and cardiovascular parameters. Pieces of works provide putative evidence for the contribution of the dorsal hippocampus (dHC) to contextual conditioning. dHC gathers massive cholinergic inputs from the basal forebrain, and dHC acetylcholine (ACh) is often described as triggering the retrieval of defensive behavior. ACh acts partially through muscarinic receptors (mAChRs) M1R and M3R subtypes. Hence, activation of mAChRs facilitates autonomic and behavioral responses associated with threats and dangers. Therefore, this study explored the likely involvement of M1R and M3R in rat dHC to establish the behavioral and autonomic changes associated with contextual fear retrieval. Male Wistar rats had stainless steel guide cannula implanted into the dHC before being submitted to contextual fear conditioning (6 footshocks, 1.5 mA, 3 s). A catheter placed within the femoral artery allowed autonomic recordings. A variety of drugs were delivered into the dHC 10 min before contextual re-exposure. The choline reuptake inhibitor hemicholinium induced a decrease of the fear conditioned responses, while did not modify it in non-conditioned animals. The non-selective mAChR antagonist atropine also reduced the fear-conditioned responses, as did the selective M1/M3 mAChRs antagonist fumarate. On the other hand, the M1 selective mAChR antagonist pirenzepine inhibited all the autonomic fear responses without affecting animal freezing. These findings support that cholinergic neurotransmission present in the dHC acts through mAChRs to coordinate the expression of fear evoked by contextual conditioning.

Distinct sex-dependent behavioral responses induced by two positive allosteric modulators of alpha 5 subunit-containing GABAA receptors.

Dysregulation of GABAergic neurotransmission has long been implicated in several psychiatric disorders, including schizophrenia, depression, and anxiety disorders. Alpha 5 subunit-containing GABAA receptors (α5-GABAAR), which are expressed mainly by pyramidal neurons in the hippocampus, have been proposed as a potential target to treat these psychiatric disorders. Here, we evaluated the effects produced by GL-II-73 and SH-053-2'F-R-CH3 (1, 5, and 10 mg/kg), two positive allosteric modulators of α5-GABAAR in behavioral tests sensitive to drugs with anxiolytic, antidepressant, and antipsychotic properties in male and female C57BL/6 mice. In both males and females, GL-II-73 produced an anxiolytic-like effect in the elevated plus-maze (EPM) and novelty-suppressed feeding and a rapid and sustained antidepressant-like effect in the forced swim test. GL-II-73 also induced antipsychotic-like effects in males indicated by attenuating MK-801-induced hyperlocomotion and prepulse inhibition (PPI) disruption. However, GL-II-73 per se increased locomotor activity and impaired fear memory extinction in males and females and PPI in males. On the other hand, SH-053-2'F-R-CH3 induced anxiolytic-like effects in the EPM and facilitated fear memory extinction in males. Contrary to GL-II-73, SH-053-2'F-R-CH3 attenuated MK-801-induced hyperlocomotion and PPI disruption in females but not in males. Neither of these drugs induced rewarding effects or impaired motor coordination. These findings suggest that GL-II-73 and SH-053-2'F-R-CH3 cause distinct sex-dependent behavioral responses and support continued preclinical research on the potential of positive allosteric modulators of α5-GABAAR for the treatment of psychiatric disorders.

Ventromedial prefrontal cortex CRF1 receptors modulate the tachycardic activity of baroreflex.

Ventral medial prefrontal cortex (vMPFC) glutamatergic neurotransmission has a facilitatory role on cardiac baroreflex activity which is mediated by NMDA receptors activation. Corticotrophin releasing factor receptors type1 and 2 (CRF1 and CRF2), present in the vMPFC, are colocalized in neurons containing glutamate vesicles, suggesting that such receptors may be involved in glutamate release in this cortical area. Therefore, our hypothesis is that the CRF1 and CRF2 receptors can modulate the baroreflex bradycardic and tachycardic responses. In order to prove this assumption, male Wistar rats had bilateral stainless steel guide cannula implanted into the vMPFC, and baroreflex was activated by intravenous infusion of phenylephrine or sodium nitroprusside through a vein catheter. A second catheter was implanted into the femoral artery for cardiovascular measurements. The CRF1 receptor antagonist administration in either infralimbic cortex (IL) or prelimbic cortex (PL), vMPFC regions, was unable to change the bradycardic responses but increased the slope of the baroreflex tachycardic activity. Microinjection of the CRF2 receptor antagonist into the IL and PL did not alter ether bradycardic nor tachycardic baroreflex responses. The administration of the non-selective CRF receptors agonist, urocortin in these areas, did not modify bradycardic responses but decreased tachycardia slope of the baroreflex. CRF1 receptor antagonist administration prior to non-selective CRF agonist in vMPFC prevented the tachycardic responses reduction. However, CRF2 receptor antagonism could not prevent the effect of CRF receptors agonist. These results suggest that IL and PL CRF1 but not CRF2 receptors have an inhibitory role on the baroreflex tachycardic activity. Furthermore, they have no influence on baroreflex bradycardic activity.

κ-Opioid receptors in the medial amygdaloid nucleus modulate autonomic and neuroendocrine responses to acute stress.

The medial amygdaloid nucleus (MeA) is a key neural structure in triggering physiologic and behavioral control during aversive situations. However, MeA role during stress exposure has not yet been fully elucidated. Thus, in the present study, we investigated the involvement of the MeA opioid neurotransmission in the modulation of autonomic, neuroendocrine and behavioral responses evoked by acute restraint stress (RS). The bilateral microinjection of naloxone (non-selective opioid antagonist) into the MeA potentiated RS-evoked autonomic responses and increased plasma corticosterone levels, in a dose-dependent manner. However, no effects were observed in RS-evoked increases on plasma oxytocin levels and anxiogenic-like behavior. Similar to naloxone, MeA pretreatment with the selective κ-opioid antagonist (nor-BNI) also enhanced heart rate and corticosterone increases induced by RS, whereas treatment with selective µ- or δ-opioid antagonists did not affect the physiologic and behavioral responses caused by RS. The present results showed MeA κ-opioid receptors modulate heart rate and corticosterone increases evoked by acute RS, reinforcing the idea of an inhibitory role exerted by MeA during aversive situations .

Bed nucleus of the stria terminalis modulates baroreflex cardiac activity: an interaction between alpha-1 receptors and NMDA/nitric oxide pathway.

The bed nucleus of the stria terminalis (BNST) is a forebrain structure, involved in the modulation of neuroendocrine, cardiovascular and autonomic responses. One of the responses is baroreflex activity, which consists in a neural mechanism responsible for keeping the blood pressure within a narrow range of variation. It has been reported that blockade of BNST α1-adrenoceptors increased the bradycardic component of baroreflex. In addition, such receptors are able to modulate glutamate release in this structure. Interestingly, BNST NMDA receptor antagonism and neuronal nitric oxide synthase (nNOS) inhibition led to the same effect of the α1-adrenoceptors blockade on baroreflex bradycardic response. Therefore, the hypothesis of the present study is that BNST noradrenergic transmission interacts with NMDA/NO pathway through α1 adrenoceptors to modulate the baroreflex activity. Male Wistar rats had stainless steel guide cannulas bilaterally implanted in the BNST. Subsequently, a catheter was inserted into the femoral artery for cardiovascular recordings, and into the femoral vein for assessing baroreflex activation. Injection of the noradrenaline reuptake inhibitor reboxetine in the BNST did not modify the tachycardic, but significantly decreased the bradycardic component of baroreflex. Administration of an α1, but not an α2 antagonist into the BNST prior to reboxetine prevented this effect. Likewise, previous injection of NMDA/NO pathway blockers inhibited the effect of reboxetine on bradycardic response. In conclusion, it was demonstrated for the first time the existence of an interaction between BNST noradrenergic, glutamatergic and nitrergic neurotransmissions in the modulation of bradycardic baroreflex response.

A single dose of the organophosphate triazophos induces fear extinction deficits accompanied by hippocampal acetylcholinesterase inhibition.

Acute organophosphate (OP) poisoning, particularly by suicide attempts, generates high mortality and morbidity. Few studies have systematically addressed the consequences of acute OP intoxication on cognition and memory of survivors. Preclinical evidence suggests that acute OP-induced effects are associated with inhibiting the brain acetylcholinesterase (AChE) enzyme. The OP triazophos has been used worldwide, although its effects on mnemonic processing are yet to be investigated. Based on the above, the present study investigated whether acute triazophos intoxication interferes with the expression and extinction of contextual fear memory in rats. Hippocampal and amygdalar AChE activity and plasma butyrylcholinesterase (BChE) were measured at the end of the experiment to confirm the cholinergic overstimulation. Independent cohorts of animals intoxicated with triazophos were evaluated in the novel object recognition (NOR) test, a less aversive associative memory task. At the dose of 15 mg/kg, triazophos administered immediately after contextual fear conditioning impaired the extinction but not the expression of freezing behavior. Triazophos poisoning induced no changes in the discrimination index in the NOR test. Triazophos inhibited the AChE activity in a time- and brain region-dependent manner. Our findings suggest that fear memory extinction deficits induced by acute triazophos intoxication are accompanied by hippocampal AChE inhibition. The deficient fear extinction associated with acute OP poisoning may represent a behavioral and biochemical phenotype helpful to study mechanisms of neurotoxicity and treatment approach of OP suicide survivors.

Nitric oxide in the insular cortex modulates baroreflex responses in a cGMP-independent pathway.

Insular cortex is a brain structure involved in the modulation of autonomic activity and cardiovascular function. The nitric oxide/cyclic guanosine-3',5'-monophosphate pathway is a prominent signaling mechanism in the central nervous system, controlling behavioral and physiological responses. Nevertheless, despite evidence regarding the presence of nitric oxide-synthesizing neurons in the insular cortex, its role in the control of autonomic and cardiovascular function has never been reported. Thus, the present study aimed to investigate the involvement of nitric oxide/cyclic guanosine-3',5'-monophosphate pathway mediated by neuronal nitric oxide synthase (nNOS) activation within the insular cortex in the modulation of baroreflex responses in unanesthetized rats. For this, we evaluated the effect of bilateral microinjection of either the nitric oxide scavenger carboxy-PTIO, the selective neuronal nitric oxide synthase inhibitor Nω-Propyl-l-arginine or the soluble guanylate cyclase inhibitor ODQ into the insular cortex on the bradycardia evoked by blood pressure increases in response to intravenous infusion of phenylephrine, and the tachycardia caused by blood pressure decreases evoked by intravenous infusion of sodium nitroprusside. Bilateral microinjection of either NPLA or carboxy-PTIO into the insular cortex increased the reflex bradycardic response, whereas the reflex tachycardia was decreased by these treatments. Bilateral microinjection of the soluble guanylate cyclase inhibitor into the insular cortex did not affect any parameter of baroreflex function evaluated. Overall, our findings provide evidence that insular cortex nitrergic signaling, acting via neuronal nitric oxide synthase, plays a prominent role in control of baroreflex function. However, control of reflex responses seems to be independent of soluble guanylate cyclase activation.

Differential modulation of the contextual conditioned emotional response by CB1 and TRPV1 receptors in the ventromedial prefrontal cortex: Possible involvement of NMDA/nitric oxide-related mechanisms.

BACKGROUND: Blockade of cannabinoid CB1 or vanilloid TRPV1 receptors in the ventromedial prefrontal cortex of rats respectively increases or decreases the conditioned emotional response during re-exposure to a context previously paired with footshocks. Although these mechanisms are unknown, they may involve local modulation of glutamatergic and nitrergic signaling. AIM: We investigated whether these mechanisms are involved in the reported effects of CB1 and TRPV1 modulation in the ventromedial prefrontal cortex. METHODS: Freezing behavior and autonomic parameters were recorded during the conditioned response expression. RESULTS: The CB1 receptors antagonist NIDA, or the TRPV1 agonist capsaicin (CPS) in the ventromedial prefrontal cortex increased the conditioned emotional response expression, and these effects were prevented by TRPV1 and CB1 antagonism, respectively. The increased conditioned emotional response evoked by NIDA and CPS were prevented by an NMDA antagonist or a neuronal nitric oxide synthase inhibitor. A nitric oxide scavenger or a soluble guanylate cyclase inhibitor prevented only the NIDA effects and the CPS effect was prevented by a non-selective antioxidant drug, as nitric oxide can also induce reactive oxygen species production. CONCLUSION: Our results suggest that CB1 and TRPV1 receptors in the ventromedial prefrontal cortex differently modulate the expression of conditioned emotional response through glutamatergic and nitrergic mechanisms, although different pathways may be involved.

Dorsal hippocampus cholinergic and nitrergic neurotransmission modulates the cardiac baroreflex function in rats.

Hippocampus is a limbic structure involved in the baroreflex and chemoreflex control that receives extensive cholinergic input from basal forebrain. Hippocampal muscarinic receptors activation by acetylcholine might evoke nitric oxide synthesis, which is an important neuromodulator of cardiovascular responses. Thus, we hypothesize that cholinergic and nitrergic neurotransmission within the DH modulates the baroreflex and chemoreflex function. We have used vasoactive drugs (phenylephrine and sodium nitroprusside), and potassium cyanide infused peripherally to induce, respectively, baroreflex or chemoreflex responses in awake animals. Bilateral injection into the DH of the acetylcholinesterase inhibitor (neostigmine) reduced baroreflex responses. Meanwhile, the non-selective muscarinic receptor antagonist (atropine) or the M1-selective muscarinic receptor antagonist increased baroreflex responses (pirenzepine). Furthermore, the neuronal nitric oxide synthase inhibitor (N-propyl) or the intracellular NO scavenger (carboxy-PTIO) increased baroreflex responses, as well as the selective inhibitor of NO-sensitive guanylyl cyclase (ODQ), increased the baroreflex responses. Besides, bilateral administration of an ineffective dose of a neuronal nitric oxide synthase inhibitor abolished the reduction in the baroreflex responses evoked by an acetylcholinesterase inhibitor. On the other hand, we have demonstrated that hippocampal cholinergic neurotransmission did not influence the chemoreflex function. Taken together, our findings suggest that nNOS-derived nitric oxide in the DH participates in acetylcholine-evoked baroreflex responses.

Acute restraint stress increases blood pressure and oxidative stress in the cardiorenal system of rats: a role for AT1 receptors.

We evaluate whether acute restraint stress may affect the oxidative state of the cardiorenal system and the possible contribution of angiotensin II/AT1 receptors in such response. Male Wistar rats were restrained for 60 min within wire mesh chambers. Some rats were treated with losartan (selective AT1 receptor antagonist, 10 mg/kg, p.o., gavage) 30 min before being stressed. Biochemical analyses were conducted after the 60-min period of restraint. Treatment with losartan prevented the increase in mean arterial pressure (MAP), but not heart rate (HR) induced by acute stress. Phenylephrine-induced contraction of endothelium-intact aortas was not affected by acute stress. Losartan prevented the increase in both superoxide anion (O2•-) and hydrogen peroxide (H2O2) levels induced by acute stress in the aorta and renal cortex. Similarly, the augmented activity of superoxide dismutase (SOD) induced by acute stress in the aorta and renal cortex was prevented by losartan. Enhanced levels of O2•- and thiobarbituric acid reactive species (TBARS) were detected in the left ventricle (LV) of stressed rats, but losartan did not prevent these responses. Similarly, losartan did not inhibited stress-induced decrease in the concentration of nitrate/nitrite (NOx) and H2O2 in the left ventricle. Stress increased ROS generation and affected the enzymatic antioxidant system in the cardiorenal system. In addition to its well-known cardiovascular changes during acute stress, angiotensin II also induces ROS generation in the cardiorenal system in a tissue-specific manner. The increase in oxidative stress mediated by angiotensin II/AT1 receptors could be one mechanism by which acute stress predisposes to cardiorenal dysfunctions.