Involvement of nitric oxide-dependent pathways of dorsolateral periaqueductal gray in the effects of ethanol in rats submitted to the elevated plus-maze test

Further proof on the role of accumbal nNOS in cocaine-seeking behavior in rats

BACKGROUND

Cocaine use disorder (CUD) remains a severe health problem with no effective pharmacological therapy. One of the potential pharmacological strategies for CUD pharmacotherapy includes manipulations of the brain glutamatergic (Glu) system which is particularly involved in drug withdrawal and relapse. Previous research indicated a pivotal role of ionotropic N-methyl-D-aspartate (NMDA) receptors or metabotropic receptors' type 5 (mGlu5) receptors in controlling the reinstatement of cocaine. Stimulation of the above molecules results in the activation of the downstream signaling targets such as neuronal nitric oxide synthase (nNOS) and the release of nitric oxide.

METHODS

In this paper, we investigated the molecular changes in nNOS in the prefrontal cortex and nucleus accumbens following 3 and 10 days of cocaine abstinence as well as the effectiveness of nNOS blockade with the selective enzyme inhibitor N-ω-propyl-L-arginine hydrochloride (L-NPA) on cocaine seeking in male rats. The effect of L-NPA on locomotor activity in drug-naïve animals was investigated.

RESULTS

Ten-day (but not 3-day) cocaine abstinence from cocaine self-administration increased nNOS gene and protein expression in the nucleus accumbens, but not in the prefrontal cortex. L-NPA (0.5-5 mg/kg) administered peripherally did not change locomotor activity but attenuated the reinstatement induced with cocaine priming or the drug-associated conditioned cue.

CONCLUSIONS

Our findings support accumbal nNOS as an important molecular player for cocaine seeking while its inhibitors could be considered as anti-cocaine pharmacological tools in male rats.

Shared Dorsal Periaqueductal Gray Activation Patterns during Exposure to Innate and Conditioned Threats

The brainstem dorsal periaqueductal gray (dPAG) has been widely recognized as being a vital node orchestrating the responses to innate threats. Intriguingly, recent evidence also shows that the dPAG mediates defensive responses to fear conditioned contexts. However, it is unknown whether the dPAG displays independent or shared patterns of activation during exposure to innate and conditioned threats. It is also unclear how dPAG ensembles encode and predict diverse defensive behaviors. To address this question, we used miniaturized microscopes to obtain recordings of the same dPAG ensembles during exposure to a live predator and a fear conditioned context in male mice. dPAG ensembles encoded not only distance to threat, but also relevant features, such as predator speed and angular offset between mouse and threat. Furthermore, dPAG cells accurately encoded numerous defensive behaviors, including freezing, stretch-attend postures, and escape. Encoding of behaviors and of distance to threat occurred independently in dPAG cells. dPAG cells also displayed a shared representation to encode these behaviors and distance to threat across innate and conditioned threats. Last, we also show that escape could be predicted by dPAG activity several seconds in advance. Thus, dPAG activity dynamically tracks key kinematic and behavioral variables during exposure to threats, and exhibits similar patterns of activation during defensive behaviors elicited by innate or conditioned threats. These data indicate that a common pathway may be recruited by the dPAG during exposure to a wide variety of threat modalities.SIGNIFICANCE STATEMENT The dorsal periaqueductal gray (dPAG) is critical to generate defensive behaviors during encounters with threats of multiple modalities. Here we use longitudinal calcium transient recordings of dPAG ensembles in freely moving mice to show that this region uses shared patterns of activity to represent distance to an innate threat (a live predator) and a conditioned threat (a shock grid). We also show that dPAG neural activity can predict diverse defensive behaviors. These data indicate the dPAG uses conserved population-level activity patterns to encode and coordinate defensive behaviors during exposure to both innate and conditioned threats.

Additive interaction between scopolamine and nitric oxide agents on immobility in the forced swim test but not exploratory activity in the hole-board

RATIONALE

The muscarinic cholinergic antagonist scopolamine has received an attention due to its unique antidepressant effects. However, the considerable adverse effects on nervous system limit the use of scopolamine as a psychiatric drug.

OBJECTIVE

In order to overcome the limitations and increase the therapeutic effects of scopolamine, we decided to examine the effects of joint administration of sub-effective dose of scopolamine and the sub-effective dose of a nitric oxide (NO) precursor L-Arginine or a non-selective nitric oxide synthase (NOS) inhibitor L-NAME on depression- and anxiety-related behaviors in male NMRI mice.

METHODS

To this aim, animal behavior was assessed in the forced swim test (FST) and hole-board apparatus.

RESULTS

Scopolamine (0.05 mg/kg) significantly decreased immobility time in the FST, suggesting an antidepressant-like effect. Moreover, L-Arginine (50 mg/kg) produced an antidepressant-like response in the FST and decreased head-dip counts in the hole-board apparatus, indicating an anxiety-like effect. The same doses of scopolamine and L-Arginine decreased the locomotor activity in mice. Joint administration of sub-effective dose of scopolamine (0.01 mg/kg) with a low dose of L-Arginine (25 mg/kg) or L-NAME (1 mg/kg) induced a profound antidepressant-like effect in the FST. These drug combinations did not influence on anxiety-related behaviors. Meanwhile, L-NAME alone did not alter the performance of mice in the FST and hole-board. Isobolographic analysis revealed an additive effect for scopolamine and L-Arginine or L-NAME.

CONCLUSION

Data suggests that NO agents could positively impact the therapeutic profile of scopolamine, because they might be useful for inducing antidepressant-like effect associated to scopolamine.

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.

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 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.

Role of nitric oxide in brain regions related to defensive reactions

Nitric oxide synthase (NOS) positive neurons are located in most brain areas related to defensive reactions, including the dorsolateral periaqueductal grey (dlPAG). NOS inhibitors injected into this structure induce anxiolytic-like responses whereas NO donors promote flight reactions. Intra-dlPAG administration of carboxy-PTIO, a NO scavenger, or ODQ, a soluble guanylate cyclase inhibitor, produced anxiolytic-like effects on rats exposed to the elevated plus-maze (EPM). A double-staining experiment using NADPHd histochemistry and c-Fos immunohistochemistry in rats exposed to a cat or to the EPM showed increased activation of NO producing neurons in the dlPAG, paraventricular and lateral nuclei of hypothalamus and dorsal raphe nucleus. Cat exposure also increased activation of NOS neurons in the medial amygdala, dorsal pre-mammillary nucleus and bed nucleus of stria terminalis. Local infusion into the dlPAG of a glutamate NMDA-receptor antagonist (AP7) or a benzodiazepine agonist (midazolam) completely prevented the flight reactions induced by intra-dlPAG administration of SIN-1, a NO donor. The responses were also inhibited by the 5-HT2A/C agonist DOI but not by a 5-HT1A agonist. These results suggest a modulatory role for NO on brain areas related to defensive reactions, probably by interacting with glutamate, serotonin and/or GABA-mediated neurotransmission.