Recent evidence suggests that the hypocretin-orexin system participates in the regulation of reinforcement processes. The current studies examined the extent to which hypocretin neurotransmission regulates behavioral and neurochemical responses to cocaine, and behavioral responses to food reinforcement. These studies used a combination of fixed ratio, discrete trials, progressive ratio and threshold self-administration procedures to assess whether the hypocretin 1 receptor antagonist, SB-334867, reduces cocaine self-administration in rats. Progressive ratio sucrose self-administration procedures were also used to assess the extent to which SB-334867 reduces responding to a natural reinforcer in food-restricted and food-sated rats. Additionally, these studies used microdialysis and in vivo voltammetry in rats to examine whether SB-334867 attenuates the effects of cocaine on dopamine signaling within the nucleus accumbens core. Furthermore, in vitro voltammetry was used to examine whether hypocretin knockout mice display attenuated dopamine responses to cocaine. Results indicate that when SB-334867 was administered peripherally or within the ventral tegmental area, it reduced the motivation to self-administer cocaine and attenuated cocaine-induced enhancement of dopamine signaling. SB-334867 also reduced the motivation to self-administer sucrose in food-sated but not food-restricted rats. Finally, hypocretin knockout mice displayed altered baseline dopamine signaling and reduced dopamine responses to cocaine. Combined, these studies suggest that hypocretin neurotransmission participates in reinforcement processes, likely through modulation of the mesolimbic dopamine system. Additionally, the current observations suggest that the hypocretin system may provide a target for pharmacotherapies to treat cocaine addiction.

Insulin resistance and other features of the metabolic syndrome are increasingly recognized for their effects on cognitive health. To ascertain mechanisms by which this occurs, we fed mice a very high fat diet (60% kcal by fat) for 17 days or a moderate high fat diet (HFD, 45% kcal by fat) for 8 weeks and examined changes in brain insulin signaling responses, hippocampal synaptodendritic protein expression, and spatial working memory. Compared to normal control diet mice, cerebral cortex tissues of HFD mice were insulin-resistant as evidenced by failed activation of Akt, S6 and GSK3β with ex-vivo insulin stimulation. Importantly, we found that expression of brain IPMK, which is necessary for mTOR/Akt signaling, remained decreased in HFD mice upon activation of AMPK. HFD mouse hippocampus exhibited increased expression of serine-phosphorylated insulin receptor substrate 1 (IRS1-pS616), a marker of insulin resistance, as well as decreased expression of PSD-95, a scaffolding protein enriched in post-synaptic densities, and synaptopodin, an actin-associated protein enriched in spine apparatuses. Spatial working memory was impaired as assessed by decreased spontaneous alternation in a T-maze. These findings indicate that HFD is associated with telencephalic insulin resistance and deleterious effects on synaptic integrity and cognitive behaviors.

Stress hormones are thought to be involved in the etiology of depression, in part, because animal models show they cause morphological damage to the brain, an effect that can be reversed by chronic antidepressant treatment. The current study examined two mouse strains selected for naturalistic variation of tissue regeneration after injury for resistance to the effects of chronic corticosterone (CORT) exposure on cell proliferation and neurotrophin mobilization. The wound healer MRL/MpJ and control C57BL/6J mice were implanted subcutaneously with pellets that released CORT for 7 days. MRL/MpJ mice were resistant to reductions of hippocampal cell proliferation by chronic exposure to CORT when compared to vulnerable C57BL/6J mice. Chronic CORT exposure also reduced protein levels of brain derived neurotrophic factor (BDNF) in the hippocampus of C57BL/6J but not MRL/MpJ mice. CORT pellet exposure increased circulating levels of CORT in the plasma of both strains in a dose dependent manner although MRL/MpJ mice may have larger changes from baseline. The strains did not differ in circulating levels of corticosterone binding globulin (CBG). There were also no strain differences in CORT levels in the hippocampus, nor did CORT exposure alter glucocorticoid receptor or mineralocorticoid receptor expression in a strain dependent manner. Strain differences were found in the NMDA receptor, and BDNF I and IV promoter. Strain and CORT exposure interacted to alter tropomyosine-receptor-kinase B (TrkB) expression and this may be potential mechanism protecting MRL/MpJ mice. In addition, differences in the inflammatory response of matrix metalloproteinases (MMPs) may also contribute to these strain differences in resistance to the deleterious effects of CORT to the brain.

It is well established that the dopamine (DA) and serotonin (5-HT) systems have extensive and complex interactions. However, the effects of specific 5-HT receptor agonists on traditionally DA-related behaviors remain unclear. Our goal in these studies was to characterize the effects of 5-HT receptor agonists on measures of locomotor activity and vertical rearing. The SSRIs fluoxetine and citalopram produced significant decreases in locomotor activity and vertical rearing at the highest doses used with females significant more sensitive to citalopram. The 5-HT1A agonist 8-OH-DPAT and the 5-HT2C agonist MK 212 significantly decreased activity in both male and female mice, with females more sensitive to 8-OH-DPAT. In contrast, the 5-HT1B agonist RU 24969 and the 5-HT2A agonist DOI both increased activity, with DOI exhibiting differential effects with regard to sex. Finally, the 5-HT3 agonist SR 57227 produced significant locomotor increases only in female mice at the lowest dose. The results of these experiments define locomotor profiles of several 5-HT agonists in male and female C57BL/6J mice, providing a foundation for further explorations of 5-HT receptor effects on activity.

Vesicular monoamine transporters (VMAT) are involved in presynaptic storage and release of neurotransmitters. While it was thought initially that only VMAT2 is brain expressed and VMAT1 is present only in the periphery, recent data has challenged the exclusive expression of VMAT2 in brain. To further elucidate the role of VMAT1 brain expression and its potential role in neuropsychiatric disorders, we have investigated mice lacking VMAT1. Comparison of wildtype and knock-out (KO) mice using qPCR and immunohistochemistry documents the expression of VMAT1 in the brain. Deletion of VMAT1 leads to increased hippocampal apoptosis and reduced neurogenesis as assessed by caspase-3-labeling and BrdU-labeling. Behavioral data show that mice lacking VMAT1 have neurocognitive deficits. VMAT2 expression is not altered in VMAT1 KO mice, suggesting a distinct role of VMAT1. Our data support VMAT1 brain expression and suggest that VMAT1 plays a key role in survival of hippocampal neurons and thus might contribute to neurocognitive deficits observed in neuropsychiatric disorders.