D1 dopamine receptor mediation of social and nonsocial emotional reactivity in mice: effects of housing and strain difference in motor activity

Cross-generational impact of a male murine pheromone 2-sec-butyl-4,5- dihydrothiazole in female mice

The current understanding of the activity of mammalian pheromones is that endocrine and behavioural effects are limited to the exposed individuals. Here, we demonstrate that the nasal exposure of female mice to a male murine pheromone stimulates expansion of mammary glands, leading to prolonged nursing of pups. Subsequent behavioural testing of the pups from pheromone-exposed dams exhibited enhanced learning. Sialic acid components in the milk are known to be involved in brain development. We hypothesized that the offspring might have received more of this key nutrient that promotes brain development. The mRNA for polysialyltransferase, which produces polysialylated neural cell adhesion molecules related to brain development,was increased in the brain of offspring of pheromone-exposed dams at post-natal day 10, while it was not different at embryonic stages, indicating possible differential brain development during early post-natal life.

Stress-induced alterations in anxiety-like behavior and adaptations in plasticity in the bed nucleus of the stria terminalis

In vulnerable individuals, exposure to stressors can result in chronic disorders such as generalized anxiety disorder (GAD), major depressive disorder (MDD), and post-traumatic stress disorder (PTSD). The extended amygdala is critically implicated in mediating acute and chronic stress responsivity and anxiety-like behaviors. The bed nucleus of the stria terminalis (BNST), a subregion of the extended amygdala, serves as a relay of corticolimbic information to the paraventricular nucleus of the hypothalamus (PVN) to directly influence the stress response. To investigate the influence of the corticosteroid milieu and housing conditions on BNST function, adult C57Bl/6J were either acutely or chronically administered corticosterone (CORT, 25mg/kg in sesame oil) or vehicle (sesame oil) or were group housed or socially isolated for 1 day (acute) or 6-8 weeks (chronic). To ascertain whether these stressors could influence anxiety-like behavior, studies were performed using the novel open-field (NOF) and the elevated zero maze (EZM) tests. To investigate potential associated changes in plasticity, alterations in BNST function were assessed using ex vivo extracellular field potential recordings in the (dorsal-lateral) dlBNST and a high frequency stimulus protocol to induce long-term potentiation (LTP). Our results suggest that chronic CORT injections and chronic social isolation housing conditions lead to an increase in anxiety-like behavior on the EZM and NOF. Chronically stressed mice also displayed a parallel blunting of LTP in the dlBNST. Conversely, acute social isolation housing had no effect on anxiety-like behavior but still resulted in a blunting of LTP in the dlBNST. Collectively, our results suggest acute and chronic stressors can have a distinct profile on plasticity in the BNST that is not uniformly associated with an increase in anxiety-like behavior.

Dopamine activity in the lateral anterior hypothalamus modulates AAS-induced aggression through D2 but not D5 receptors

Treatment with anabolic-androgenic steroids (AAS) throughout adolescence facilitates offensive aggression in Syrian hamsters. In the anterior hypothalamus (AH), the dopaminergic neural system undergoes alterations after repeated exposure to AAS, producing elevated aggression. Previously, systemic administration of selective dopamine receptor antagonists has been shown to reduce aggression in various species and animal models. However, these reductions in aggression occur with concomitant alterations in general arousal and mobility. Therefore, to control for these systemic effects, the current studies utilized microinjection techniques to determine the effects of local antagonism of D2 and D5 receptors in the AH on adolescent AAS-induced aggression. Male Syrian hamsters were treated with AAS throughout adolescence and tested for aggression after local infusion of the D2 antagonist eticlopride, or the D5 antagonist SCH-23390, into the AH. Treatment with eticlopride showed dose-dependent suppression of aggressive behavior in the absence of changes in mobility. Conversely, while injection of SCH-23390 suppressed aggressive behavior, these reductions were met with alterations in social interest and locomotor behavior. To elucidate a plausible mechanism for the observed D5 receptor mediation of AAS-induced aggression, brains of AAS and sesame oil-treated animals were processed for double-label immunofluorescence of GAD₆₇ (a marker for GABA production) and D5 receptors in the lateral subdivision of the AH (LAH). Results indicate a sparse distribution of GAD₆₇ neurons colocalized with D5 receptors in the LAH. Together, these results indicate that D5 receptors in the LAH modulate non-GABAergic pathways that indirectly influence aggression control, while D2 receptors have a direct influence on AAS-induced aggression.

An anxiety-like phenotype in mice selectively bred for aggression

Using selective bi-directional breeding procedures, two different lines of mice were developed. The NC900 line is highly reactive and attacks their social partners without provocation, whereas aggression in NC100 animals is uncommon in social environments. The enhanced reactivity of NC900 mice suggests that emotionality may have been selected with aggression. As certain forms of anxiety promote exaggerated defensive responses, we tested NC900 mice for the presence of an anxiety-like phenotype. In the open field, light-dark exploration, and zero maze tests, NC900 mice displayed anxiety-like responses. These animals were less responsive to the anxiolytic actions of diazepam in the zero maze than NC100 animals; diazepam also reduced the reactivity and attack behaviors of NC900 mice. The NC900 mice had reduced diazepam-sensitive GABA(A) receptor binding in brain regions associated with aggression and anxiety. Importantly, there was a selective reduction in levels of the GABA(A) receptor alpha(2) subunit protein in NC900 frontal cortex and amygdala; no changes in alpha(1) or gamma(2) subunit proteins were observed. These findings suggest that reductions in the alpha(2) subunit protein in selected brain regions may underlie the anxiety and aggressive phenotype of NC900 mice. Since anxiety and aggression are comorbid in certain psychiatric conditions, such as borderline personality and posttraumatic stress disorder, investigations with NC900 mice may provide new insights into basic mechanisms that underlie these and related psychiatric conditions.

Dihydrexidine--the first full dopamine D1 receptor agonist

The functional role of dopamine D(1) receptors is still controversial. One reason for this controversy is that for a long time the only available agonists for in vivo characterization of dopamine D(1) receptors were benzazepines. Among them was the prototype dopamine D(1) receptor partial agonist, SKF 38393. The lack of a selective and fully efficacious dopamine D(1) receptor agonist hampered basic research on dopamine D(1) receptors and left the potential clinical utility of dopamine D(1) receptor agonists elusive. The research situation improved when the first potent full dopamine D(1) receptor agonist dihydrexidine, a phenanthridine, was introduced in the late 1980s. In contrast to SKF 38393, dihydrexidine was shown to stimulate cyclic AMP synthesis just as well or better than dopamine, and potently displaced [(3)H]SCH 23390 from rat and monkey striatal membranes. Also, dihydrexidine was the first dopamine D(1) receptor agonist that had potent antiparkinsonian activity in a primate model of Parkinson's disease. This finding suggested clinical utility for dopamine D(1) receptor agonists in Parkinson's disease and that this utility might be critically dependent on the intrinsic efficacy of the drug. Clinical utility for dopamine D(1) receptor agonists in other central nervous disorders might also be dependent on the intrinsic efficacy of the drug. However, even though studies with dihydrexidine as a pharmacological tool have pointed to the clinical use for dopamine D(1) receptor agonists, dihydrexidine's unfavorable pharmacokinetic profile and various adverse effects are likely to restrict or even preclude its use in humans. This review article provides an updated overview of the pharmacology of dihydrexidine and discusses possible clinical utility of dopamine D(1) receptor agonists in various central nervous system disorders.