Medial Nucleus Accumbens Projections to the Ventral Tegmental Area Control Food Consumption.

Decades of research have shown that the NAc is a critical region influencing addiction, mood, and food consumption through its effects on reinforcement learning, motivation, and hedonic experience. Pharmacological studies have demonstrated that inhibition of the NAc shell induces voracious feeding, leading to the hypothesis that the inhibitory projections that emerge from the NAc normally act to restrict feeding. While much of this work has focused on projections to the lateral hypothalamus, the role of NAc projections to the VTA in the control food intake has been largely unexplored. Using a retrograde viral labeling technique and real-time monitoring of neural activity with fiber photometry, we find that medial NAc shell projections to the VTA (mNAc→VTA) are inhibited during food-seeking and food consumption in male mice. We also demonstrate that this circuit bidirectionally controls feeding: optogenetic activation of NAc projections to the VTA inhibits food-seeking and food intake (in both sexes), while optogenetic inhibition of this circuit potentiates food-seeking behavior. Additionally, we show that activity of the NAc to VTA pathway is necessary for adaptive inhibition of food intake in response to external cues. These data provide new insight into NAc control over feeding in mice, and contribute to an emerging literature elucidating the role of inhibitory midbrain feedback within the mesolimbic circuit.SIGNIFICANCE STATEMENT The medial NAc has long been known to control consummatory behavior, with particular focus on accumbens projections to the lateral hypothalamus. Conversely, NAc projections to the VTA have mainly been studied in the context of drug reward. We show that NAc projections to the VTA bidirectionally control food intake, consistent with a permissive role in feeding. Additionally, we show that this circuit is normally inactivated during consumption and food-seeking. Together, these findings elucidate how mesolimbic circuits control food consumption.

Inhibitor of the tyrosine phosphatase STEP reverses cognitive deficits in a mouse model of Alzheimer's disease.

STEP (STriatal-Enriched protein tyrosine Phosphatase) is a neuron-specific phosphatase that regulates N-methyl-D-aspartate receptor (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking, as well as ERK1/2, p38, Fyn, and Pyk2 activity. STEP is overactive in several neuropsychiatric and neurodegenerative disorders, including Alzheimer's disease (AD). The increase in STEP activity likely disrupts synaptic function and contributes to the cognitive deficits in AD. AD mice lacking STEP have restored levels of glutamate receptors on synaptosomal membranes and improved cognitive function, results that suggest STEP as a novel therapeutic target for AD. Here we describe the first large-scale effort to identify and characterize small-molecule STEP inhibitors. We identified the benzopentathiepin 8-(trifluoromethyl)-1,2,3,4,5-benzopentathiepin-6-amine hydrochloride (known as TC-2153) as an inhibitor of STEP with an IC50 of 24.6 nM. TC-2153 represents a novel class of PTP inhibitors based upon a cyclic polysulfide pharmacophore that forms a reversible covalent bond with the catalytic cysteine in STEP. In cell-based secondary assays, TC-2153 increased tyrosine phosphorylation of STEP substrates ERK1/2, Pyk2, and GluN2B, and exhibited no toxicity in cortical cultures. Validation and specificity experiments performed in wild-type (WT) and STEP knockout (KO) cortical cells and in vivo in WT and STEP KO mice suggest specificity of inhibitors towards STEP compared to highly homologous tyrosine phosphatases. Furthermore, TC-2153 improved cognitive function in several cognitive tasks in 6- and 12-mo-old triple transgenic AD (3xTg-AD) mice, with no change in beta amyloid and phospho-tau levels.

Inhibition of the tyrosine phosphatase STEP61 restores BDNF expression and reverses motor and cognitive deficits in phencyclidine-treated mice.

Brain-derived neurotrophic factor (BDNF) and STriatal-Enriched protein tyrosine Phosphatase 61 (STEP61) have opposing functions in the brain, with BDNF supporting and STEP61 opposing synaptic strengthening. BDNF and STEP61 also exhibit an inverse pattern of expression in a number of brain disorders, including schizophrenia (SZ). NMDAR antagonists such as phencyclidine (PCP) elicit SZ-like symptoms in rodent models and unaffected individuals, and exacerbate psychotic episodes in SZ. Here we characterize the regulation of BDNF expression by STEP61, utilizing PCP-treated cortical culture and PCP-treated mice. PCP-treated cortical neurons showed both an increase in STEP61 levels and a decrease in BDNF expression. The reduction in BDNF expression was prevented by STEP61 knockdown or use of the STEP inhibitor, TC-2153. The PCP-induced increase in STEP61 expression was associated with the inhibition of CREB-dependent BDNF transcription. Similarly, both genetic and pharmacologic inhibition of STEP prevented the PCP-induced reduction in BDNF expression in vivo and normalized PCP-induced hyperlocomotion and cognitive deficits. These results suggest a mechanism by which STEP61 regulates BDNF expression, with implications for cognitive functioning in CNS disorders.

Striatal-enriched protein tyrosine phosphatase regulates the PTPα/Fyn signaling pathway.

The tyrosine kinase Fyn has two regulatory tyrosine residues that when phosphorylated either activate (Tyr(420)) or inhibit (Tyr(531)) Fyn activity. Within the central nervous system, two protein tyrosine phosphatases (PTPs) target these regulatory tyrosines in Fyn. PTPα dephosphorylates Tyr(531) and activates Fyn, while STEP (STriatal-Enriched protein tyrosine Phosphatase) dephosphorylates Tyr(420) and inactivates Fyn. Thus, PTPα and STEP have opposing functions in the regulation of Fyn; however, whether there is cross talk between these two PTPs remains unclear. Here, we used molecular techniques in primary neuronal cultures and in vivo to demonstrate that STEP negatively regulates PTPα by directly dephosphorylating PTPα at its regulatory Tyr(789). Dephosphorylation of Tyr(789) prevents the translocation of PTPα to synaptic membranes, blocking its ability to interact with and activate Fyn. Genetic or pharmacologic reduction in STEP61 activity increased the phosphorylation of PTPα at Tyr(789), as well as increased translocation of PTPα to synaptic membranes. Activation of PTPα and Fyn and trafficking of GluN2B to synaptic membranes are necessary for ethanol (EtOH) intake behaviors in rodents. We tested the functional significance of STEP61 in this signaling pathway by EtOH administration to primary cultures as well as in vivo, and demonstrated that the inactivation of STEP61 by EtOH leads to the activation of PTPα, its translocation to synaptic membranes, and the activation of Fyn. These findings indicate a novel mechanism by which STEP61 regulates PTPα and suggest that STEP and PTPα coordinate the regulation of Fyn. STEP61 , PTPα, Fyn, and NMDA receptor (NMDAR) have been implicated in ethanol intake behaviors in the dorsomedial striatum (DMS) in rodents. Here, we report that PTPα is a novel substrate for STEP61. Upon ethanol exposure, STEP61 is phosphorylated and inactivated by protein kinase A (PKA) signaling in the DMS. As a result of STEP61 inhibition, there is an increase in the phosphorylation of PTPα, which translocates to lipid rafts and activates Fyn and subsequent NMDAR signaling. The results demonstrate a synergistic regulation of Fyn-NMDAR signaling by STEP61 and PTPα, which may contribute to the regulation of ethanol-related behaviors. NMDA, N-methyl-D-aspartate; PTPα, receptor-type protein tyrosine phosphatase alpha; STEP, STriatal-Enriched protein tyrosine Phosphatase.

Aversive effects of ethanol in adolescent versus adult rats: potential causes and implication for future drinking.

BACKGROUND: Many people experiment with alcohol and other drugs of abuse during their teenage years. Epidemiological evidence suggests that younger initiates into drug taking are more likely to develop problematic drug seeking behavior, including binge and other high-intake behaviors. The level of drug intake for any individual depends on the balance of rewarding and aversive effects of the drug in that individual. Multiple rodent studies have demonstrated that aversive effects of drugs of abuse are reduced in adolescent compared to adult animals. In this study, we addressed 2 key questions: First, do reduced aversive effects of ethanol in younger rats correlate with increased ethanol consumption? Second, are the reduced aversive effects in adolescents attributable to reduced sensitivity to ethanol's physiologic effects? METHODS: Adolescent and adult rats were tested for ethanol conditioned taste aversion (CTA) followed by a voluntary drinking period, including postdeprivation consumption. Multivariate regression was used to assess correlations. In separate experiments, adolescent and adult rats were tested for their sensitivity to the hypothermic and sedative effects of ethanol, and for blood ethanol concentrations (BECs). RESULTS: We observed that in adolescent rats but not adults, taste aversion was inversely correlated with postdeprivation consumption. Adolescents also exhibited a greater increase in consumption after deprivation than adults. Furthermore, the age difference in ethanol CTA was not attributable to differences in hypothermia, sedation, or BECs. CONCLUSIONS: These results suggest that during adolescence, individuals that are insensitive to aversive effects are most likely to develop problem drinking behaviors. These results underscore the importance of the interaction between developmental stage and individual variation in sensitivity to alcohol.

Characterization of a semi-rapid method for assessing delay discounting in rodents.

Delay discounting is a key component of many psychiatric disorders, including drug addiction, compulsive gambling, ADHD, and obesity. However, its underlying mechanisms are not yet fully characterized. One impediment to full characterization of such mechanisms is the fact that rodent models of the task are often complicated and involve extended training of subjects, often requiring more than a month before a stable baseline is obtained. We have therefore characterized a version of the rodent delay discounting task which generates data more quickly than most other published versions. In this version of the task, learning of the operant response is established prior to introduction of the delay component, and delay is tested across subsequent daily sessions with a single delay length per day. We demonstrate here that this version generates a delay discounting curve similar to many published tasks, and is sensitive to changes in reward magnitude and to chronic treatment with cocaine. Furthermore, we present a detailed description of the within-session patterns of behavior in the task, which provides evidence of within-session learning and establishment of stable response patterns. This faster version of the delay discounting task will facilitate future studies involving pharmacological, electrophysiological, and other mechanistic studies of the underlying basis of this important disease process.

Individual differences in cocaine conditioned taste aversion are developmentally stable and independent of locomotor effects of cocaine.

Drugs of abuse induce complex motivational states in their users which have been shown to vary developmentally. In addition to developmental variation, interindividual variation in the rewarding and aversive effects of drugs of abuse is an important consideration. A rat model was used to assess whether the conditioned rewarding/aversive effects of cocaine were maintained as individuals matured from adolescence into adulthood. We tested rats in the cocaine conditioned taste aversion task as adolescents and again in adulthood. We observed a wide range of approach/avoidance behaviors in this task, and also observed that the relative interindividual differences in approach/avoidance are remarkably stable across the two developmental stages. Furthermore, we observed that these interindividual differences are not attributable to individual differences in cocaine-induced locomotor effects or individual differences in blood or brain cocaine levels. Taken together, these findings indicate that sensitivity to cocaine's motivational effects is stable across development and part of a unique neurological process.