Divergent projections of the paraventricular nucleus of the thalamus mediate the selection of passive and active defensive behaviors.

The appropriate selection of passive and active defensive behaviors in threatening situations is essential for survival. Previous studies have shown that passive defensive responses depend on activity of the central nucleus of the amygdala (CeA), whereas active ones primarily rely on the nucleus accumbens (NAc). However, the mechanisms underlying flexible switching between these two types of responses remain unknown. Here we show in mice that the paraventricular thalamus (PVT) mediates the selection of defensive behaviors through its interaction with the CeA and the NAc. We show that the PVT-CeA pathway drives conditioned freezing responses, whereas the PVT-NAc pathway is inhibited during freezing and, instead, signals active avoidance events. Optogenetic manipulations revealed that activity in the PVT-CeA or PVT-NAc pathway biases behavior toward the selection of passive or active defensive responses, respectively. These findings provide evidence that the PVT mediates flexible switching between opposing defensive behaviors.

Attenuated NMDAR signaling on fast-spiking interneurons in prefrontal cortex contributes to age-related decline of cognitive flexibility.

Ionotropic glutamate receptors of the NMDA and AMPA subtypes transduce excitatory signaling on neurons in the prefrontal cortex (PFC) in support of cognitive flexibility. Cognitive flexibility is reliably observed to decline at advanced ages, coinciding with changes in PFC glutamate receptor expression and neuronal physiology. However, the relationship between age-related impairment of cognitive flexibility and changes to excitatory signaling on distinct classes of PFC neurons is not known. In this study, one cohort of young adult (4 months) and aged (20 months) male F344 rats were characterized for cognitive flexibility on an operant set-shifting task. Expression of the essential NMDAR subunit, NR1, was correlated with individual differences in set-shifting abilities such that lower NR1 in the aged PFC was associated with worse set-shifting. In contrast, lower expression of two AMPAR subunits, GluR1 and GluR2, was not associated with set-shift abilities in aging. As NMDARs are expressed by both pyramidal cells and fast-spiking interneurons (FSI) in PFC, whole-cell patch clamp recordings were performed in a second cohort of age-matched rats to compare age-associated changes on these neuronal subtypes. Evoked excitatory postsynaptic currents were generated using a bipolar stimulator while AMPAR vs. NMDAR-mediated components were isolated using pharmacological tools. The results revealed a clear increase in AMPA/NMDA ratio in FSIs that was not present in pyramidal neurons. Together, these data indicate that loss of NMDARs on interneurons in PFC contributes to age-related impairment of cognitive flexibility.

The locus coeruleus drives disinhibition in the midline thalamus via a dopaminergic mechanism.

The paraventricular nucleus of the thalamus (PVT) is increasingly being recognized as a critical node linking stress detection to the emergence of adaptive behavioral responses to stress. However, despite growing evidence implicating the PVT in stress processing, the neural mechanisms by which stress impacts PVT neurocircuitry and promotes stressed states remain unknown. Here we show that stress exposure drives a rapid and persistent reduction of inhibitory transmission onto projection neurons of the posterior PVT (pPVT). This stress-induced disinhibition of the pPVT was associated with a locus coeruleus-mediated rise in the extracellular concentration of dopamine in the midline thalamus, required the function of dopamine D2 receptors on PVT neurons, and increased sensitivity to stress. Our findings define the locus coeruleus as an important modulator of PVT function: by controlling the inhibitory tone of the pPVT, it modulates the excitability of pPVT projection neurons and controls stress responsivity.

NR2A-Containing NMDARs in the Prefrontal Cortex Are Required for Working Memory and Associated with Age-Related Cognitive Decline.

Working memory, the ability to temporarily maintain representational knowledge, is a foundational cognitive process that can become compromised in aging and neuropsychiatric disease. NMDA receptor (NMDAR) activation in prefrontal cortex (PFC) is necessary for the pyramidal neuron activity believed to enable working memory; however, the distinct biophysical properties and localization of NMDARs containing NR2A and NR2B subunits suggest unique roles for NMDAR subtypes in PFC neural activity and working memory. Experiments herein show that working memory depends on NR2A- but not NR2B-NMDARs in PFC of rats and that NR2A-NMDARs mediate the majority of evoked NMDAR currents on layer 2/3 PFC pyramidal neurons. Moreover, attenuated expression of the NR2A but not the NR2B subunit in PFC associates with naturally occurring working memory impairment in aged rats. Finally, NMDAR currents and working memory are enhanced in aged rats by promoting activation of the NR2A-enriched synaptic pool of PFC NMDARs. These results implicate NR2A-NMDARs in normal working memory and suggest novel treatment strategies for improving working memory in cognitive disorders. SIGNIFICANCE STATEMENT: Working memory, the ability to hold information "in mind," requires persistent activity of pyramidal neurons in prefrontal cortex (PFC) mediated by NMDA receptor (NMDAR) activation. NMDAR loss in PFC may account for working memory impairments in aging and psychiatric disease. Our studies demonstrate that NMDARs containing the NR2A subunit, but not the NR2B subunit, are required for working memory and that loss of NR2A predicts severity of age-related working memory impairment. The importance of NR2A to working memory is likely due its abundant contribution to pyramidal neuron activity and location at synaptic sites in PFC. This information is useful in designing new therapies to treat working memory impairments by enhancing the function of NR2A-containing NMDARs.

Effects of acute administration of the GABA(B) receptor agonist baclofen on behavioral flexibility in rats.

RATIONALE: The ability to adjust response strategies when faced with changes in the environment is critical for normal adaptive behavior. Such behavioral flexibility is compromised by experimental disruption of cortical GABAergic signaling, as well as in conditions such as schizophrenia and normal aging that are characterized by cortical hyperexcitability. The current studies were designed to determine whether stimulation of GABAergic signaling using the GABA(B) receptor agonist baclofen can facilitate behavioral flexibility. METHODS: Male Fischer 344 rats were trained in a set-shifting task in which they learned to discriminate between two response levers to obtain a food reward. Correct levers were signaled in accordance with two distinct response rules (rule 1: correct lever signaled by a cue light; rule 2: correct lever signaled by its left/right position). The order of rule presentation varied, but they were always presented sequentially, with the trials and errors to reach criterion performance on the second (set shift) rule providing the measure of behavioral flexibility. Experiments determined the effects of the GABA(B) receptor agonist baclofen (intraperitoneal, 0, 1.0, 2.5, and 4.0 mg/kg) administered acutely before the shift to the second rule. RESULTS: Baclofen enhanced set-shifting performance. Control experiments demonstrated that this enhancement was not simply due to improved discrimination learning, nor was it due to impaired recall of the initial discrimination rule. CONCLUSIONS: The results demonstrate that baclofen can facilitate behavioral flexibility, suggesting that GABA(B) receptor agonists may have utility for treating behavioral dysfunction in neuropsychiatric disorders.

Affective and cognitive mechanisms of risky decision making.

The ability to make advantageous decisions under circumstances in which there is a risk of adverse consequences is an important component of adaptive behavior; however, extremes in risk taking (either high or low) can be maladaptive and are characteristic of a number of neuropsychiatric disorders. To better understand the contributions of various affective and cognitive factors to risky decision making, cohorts of male Long-Evans rats were trained in a "Risky Decision making Task" (RDT), in which they made discrete trial choices between a small, "safe" food reward and a large, "risky" food reward accompanied by varying probabilities of footshock. Experiment 1 evaluated the relative contributions of the affective stimuli (i.e., punishment vs. reward) to RDT performance by parametrically varying the magnitudes of the footshock and large reward. Varying the shock magnitude had a significant impact on choice of the large, "risky" reward, such that greater magnitudes were associated with reduced choice of the large reward. In contrast, varying the large, "risky" reward magnitude had minimal influence on reward choice. Experiment 2 compared individual variability in RDT performance with performance in an attentional set shifting task (assessing cognitive flexibility), a delayed response task (assessing working memory), and a delay discounting task (assessing impulsive choice). Rats characterized as risk averse in the RDT made more perseverative errors on the set shifting task than did their risk taking counterparts, whereas RDT performance was not related to working memory abilities or impulsive choice. In addition, rats that showed greater delay discounting (greater impulsive choice) showed corresponding poorer performance in the working memory task. Together, these results suggest that reward-related decision making under risk of punishment is more strongly influenced by the punishment than by the reward, and that risky and impulsive decision making are associated with distinct components of executive function.

Prefrontal cortical GABAergic dysfunction contributes to age-related working memory impairment.

Working memory functions supported by the prefrontal cortex decline in normal aging. Disruption of corticolimbic GABAergic inhibitory circuits can impair working memory in young subjects; however, relatively little is known regarding how aging impacts prefrontal cortical GABAergic signaling and whether such changes contribute to cognitive deficits. The current study used a rat model to evaluate the effects of aging on expression of prefrontal GABAergic synaptic proteins in relation to working memory decline, and to test whether pharmacological manipulations of prefrontal GABAergic signaling can improve working memory abilities in aged subjects. Results indicate that in aged medial prefrontal cortex (mPFC), expression of the vesicular GABA transporter VGAT was unchanged; however, there was a significant increase in expression of the GABA synthesizing enzyme GAD67, and a significant decrease in the primary neuronal GABA transporter GAT-1 and in both subunits of the GABA(B) receptor (GABA(B)R). Expression of VGAT, GAD67, and GAT-1 was not associated with working memory ability. In contrast, among aged rats, GABA(B)R expression was significantly and negatively associated with working memory performance, such that lower GABA(B)R expression predicted better working memory. Subsequent experiments showed that systemic administration of a GABA(B)R antagonist, CGP55845, dose-dependently enhanced working memory in aged rats. This enhancing effect of systemic CGP55845 was reproduced by direct intra-mPFC administration. Together, these data suggest that age-related dysregulation of GABAergic signaling in prefrontal cortex may play a causal role in impaired working memory and that targeting GABA(B)Rs may provide therapeutic benefit for age-related impairments in executive functions.

Adolescent risk taking, cocaine self-administration, and striatal dopamine signaling.

Poor decision making and elevated risk taking, particularly during adolescence, have been strongly linked to drug use; however the causal relationships among these factors are not well understood. To address these relationships, a rat model (the Risky Decision-making Task; RDT) was used to determine whether individual differences in risk taking during adolescence predict later propensity for cocaine self-administration and/or whether cocaine self-administration causes alterations in risk taking. In addition, the RDT was used to determine how risk taking is modulated by dopamine signaling, particularly in the striatum. Results from these experiments indicated that greater risk taking during adolescence predicted greater intake of cocaine during acquisition of self-administration in adulthood, and that adult cocaine self-administration in turn caused elevated risk taking that was present following 6 weeks of abstinence. Greater adolescent risk taking was associated with lower striatal D2 receptor mRNA expression, and pharmacological activation of D2/3 receptors in the ventral, but not dorsal, striatum induced a decrease in risk taking. These findings indicate that the relationship between elevated risk taking and cocaine self-administration is bi-directional, and that low striatal D2 receptor expression may represent a predisposing factor for both maladaptive decision making and cocaine use. Furthermore, these findings suggest that striatal D2 receptors represent a therapeutic target for attenuating maladaptive decision making when choices include risk of adverse consequences.

Distinct manifestations of executive dysfunction in aged rats.

Different components of executive function such as working memory, attention, and cognitive flexibility can be dissociated behaviorally and mechanistically; however, the within-subject influences of normal aging on different aspects of executive function remain ill-defined. To better define these relationships, young adult and aged male F344 rats were cross-characterized on an attentional set-shifting task that assesses cognitive flexibility and a delayed response task that assesses working memory. Across tasks, aged rats were impaired relative to young; however, there was significant variability in individual performance within the aged cohort. Notably, performance on the set-shifting task and performance at long delays on the delayed response task were inversely related among aged rats. Additional experiments showed no relationship between aged rats' performance on the set-shifting task and performance on a hippocampal-dependent spatial reference memory task. These data indicate that normal aging can produce distinct manifestations of executive dysfunction, and support the need to better understand the unique mechanisms contributing to different forms of prefrontal cortical-supported executive decline across the lifespan.