Acquisition of Pavlovian Fear Conditioning Using beta-Adrenoceptor Activation of the Dorsal Premammillary Nucleus as an Unconditioned Stimulus to Mimic Live Predator-Threat Exposure

In the present work, we sought to mimic the internal state changes in response to a predator threat by pharmacologically stimulating the brain circuit involved in mediating predator fear responses, and explored whether this stimulation would be a valuable unconditioned stimulus (US) in an olfactory fear conditioning paradigm (OFC). The dorsal premammillary nucleus (PMd) is a key brain structure in the neural processing of anti-predatory defensive behavior and has also been shown to mediate the acquisition and expression of anti-predatory contextual conditioning fear responses. Rats were conditioned by pairing the US, which was an intra-PMd microinjection of isoproterenol (ISO; beta-adrenoceptor agonist), with amyl acetate odor-the conditioned stimulus (CS). ISO (10 and 40 nmol) induced the acquisition of the OFC and the second-order association by activation of beta-1 receptors in the PMd. Furthermore, similar to what had been found for contextual conditioning to a predator threat, atenolol (beta-1 receptor antagonist) in the PMd also impaired the acquisition and expression of OFC promoted by ISO. Considering the strong glutamatergic projections from the PMd to the dorsal periaqueductal gray (dPAG), we tested how the glutamatergic blockade of the dPAG would interfere with the OFC induced by ISO. Accordingly, microinjections of NMDA receptor antagonist (AP5, 6 nmol) into the dPAG were able to block both the acquisition, and partially, the expression of the OFC. In conclusion, we have found that PMd beta-1 adrenergic stimulation is a good model to mimic predatory threat-induced internal state changes, and works as a US able to mobilize the same systems involved in the acquisition and expression of predator-related contextual conditioning. Neuropsychopharmacology (2011) 36, 926-939; doi:10.1038/npp.2010.231; published online 5 January 2011

A comparative study on the effects of the benzodiazepine midazolam and the dopamine agents, apomorphine and sulpiride, on rat behavior in the two-way avoidance test

In recent years. studies in behavioral pharmacology have shown the involvement of dopaminergic mechanisms in avoidance behavior as assessed by the two-way active avoidance test (CAR). Changes in dopaminergic transmission also occur in response to particularly threatening challenges. However, studies on the effects of benzodiazepine (BZD) drugs ill this test are still unclear. Given the interplay of dopamine and other neurotransmitters in the neurobiology of anxiety and schizophrenia the aim of this work was to evaluate the effects of systemic administration of midazolam, the dopaminergic agonist apomorphine, and the D(2) receptor antagonist sulpiride using the CAR, a test that shows good sensitivity to typical neuroleptic drugs. Whereas midazolam did not alter the avoidance response. apomorphine increased and sulpiride reduced them in this test. Escape was not affected by any drug treatments. Heightened avoidance was not associated with the increased motor activity caused by apomorphine. In contrast with the benzodiazepine midazolam, activation of post-synaptic D(2) receptors with apomorphine facilitates, whereas the D(2) receptor antagonism with sulpiride inhibited the acquisition of the avoidance behavior. Together, these results bring additional evidence for a role of D(2) mechanisms in the acquisition of the active avoidance. (C) 2009 Elsevier Inc. All rights reserved.

Role of glutamate NMDA receptors and nitric oxide located within the periaqueductal gray on defensive behaviors in mice confronted by predator

The midbrain periaqueductal gray (PAG) is part of the brain system involved in active defense reactions to threatening stimuli. Glutamate N-methyl-d-aspartate (NMDA) receptor activation within the dorsal column of the PAG (dPAG) leads to autonomic and behavioral responses characterized as the fear reaction. Nitric oxide (NO) has been proposed to be a mediator of the aversive action of glutamate, since the activation of NMDA receptors in the brain increases NO synthesis. We investigated the effects of intra-dPAG infusions of NMDA on defensive behaviors in mice pretreated with a neuronal nitric oxide synthase (nNOS) inhibitor [N omega-propyl-l-arginine (NPLA)], in the same midbrain site, during a confrontation with a predator in the rat exposure test (RET). Male Swiss mice received intra-dPAG injections of NPLA (0.1 or 0.4 nmol/0.1 mu l), and 10 min later, they were infused with NMDA (0.04 nmol/0.1 mu l) into the dPAG. After 10 min, each mouse was placed in the RET. NMDA treatment enhanced avoidance behavior from the predator and markedly increased freezing behavior. These proaversive effects of NMDA were prevented by prior injection of NPLA. Furthermore, defensive behaviors (e.g., avoidance, risk assessment, freezing) were consistently reduced by the highest dose of NPLA alone, suggesting an intrinsic effect of nitric oxide on defensive behavior in mice exposed to the RET. These findings suggest a potential role of glutamate NMDA receptors and NO in the dPAG in the regulation of defensive behaviors in mice during a confrontation with a predator in the RET.