Dustin J. Stairs

Neurobehavioral effects of environmental enrichment and drug abuse vulnerability.

Environmental enrichment during development produces a host of neurobehavioral effects in preclinical models. Early work demonstrated that enrichment enhances learning of a variety of behavioral tasks in rats and these changes are associated with neural changes in various cortical regions. In addition to promoting superior learning, more recent evidence suggests that environmental enrichment also has a protective effect in reducing drug abuse vulnerability. The current review describes some of the most important environment-dependent neural changes in reward-relevant brain structures and summarizes some of the key findings from the extensive literature showing how enrichment decreases the impact of drugs of abuse. Some critical neural mechanisms that may mediate the behavioral changes are postulated, along with some notes of caution about the limitations of the work cited.

Impulsive choice and environmental enrichment: Effects of d-amphetamine and methylphenidate

Individual differences in impulsive choice and rearing in differential environments are factors that predict vulnerability to drug abuse. The present study determined if rearing influences impulsive choice, and if d-amphetamine or methylphenidate alters impulsive choice in differentially reared rats. Male Sprague-Dawley rats were raised from 21 days of age in either an enriched condition (EC) or an isolated condition (IC) and were tested as young adults on an adjusting delay task. In this task, two levers were available and a response on one lever yielded one 45mg food pellet immediately, whereas a response on the other yielded three pellets after an adjusting delay. The delay was initially set at 6s, and it decreased or increased by 1s following responses on the immediate or delayed levers, respectively. A mean adjusted delay (MAD) was calculated upon completion of each daily session, and it served as the quantitative measure of impulsivity. Once MADs stabilized, rats were injected with saline, d-amphetamine (0.5, 1.0, or 2.0mg/kg, s.c.), or methylphenidate (2.5, 5.0, or 10.0mg/kg, s.c.) 15min prior to adjusting delay sessions. EC rats had higher baseline MADs (were less impulsive) than IC rats. Additionally, administration of d-amphetamine, but not methylphenidate, dose-dependently increased impulsive choice (decreased MADs) in EC rats. In IC rats, d-amphetamine and methylphenidate dose-dependently decreased impulsivity (increased MADs). These results indicate that rearing environment influences impulsive choice and moderates the effect of psychostimulants on impulsive choice. Specifically, psychostimulants may decrease environment-dependent impulsive choice in individuals with high levels of impulsivity (e.g., those with ADHD), whereas they may increase impulsive choice in individuals with low levels of impulsivity.

Deletion of Glutamate Delta-1 Receptor in Mouse Leads to Aberrant Emotional and Social Behaviors

The delta family of ionotropic glutamate receptors consists of glutamate δ1 (GluD1) and glutamate δ2 (GluD2) receptors. While the role of GluD2 in the regulation of cerebellar physiology is well understood, the function of GluD1 in the central nervous system remains elusive. We demonstrate for the first time that deletion of GluD1 leads to abnormal emotional and social behaviors. We found that GluD1 knockout mice (GluD1 KO) were hyperactive, manifested lower anxiety-like behavior, depression-like behavior in a forced swim test and robust aggression in the resident-intruder test. Chronic lithium rescued the depression-like behavior in GluD1 KO. GluD1 KO mice also manifested deficits in social interaction. In the sociability test, GluD1 KO mice spent more time interacting with an inanimate object compared to a conspecific mouse. D-Cycloserine (DCS) administration was able to rescue social interaction deficits observed in GluD1 KO mice. At a molecular level synaptoneurosome preparations revealed lower GluA1 and GluA2 subunit expression in the prefrontal cortex and higher GluA1, GluK2 and PSD95 expression in the amygdala of GluD1 KO. Moreover, DCS normalized the lower GluA1 expression in prefrontal cortex of GluD1 KO. We propose that deletion of GluD1 leads to aberrant circuitry in prefrontal cortex and amygdala owing to its potential role in presynaptic differentiation and synapse formation. Furthermore, these findings are in agreement with the human genetic studies suggesting a strong association of GRID1 gene with several neuropsychiatric disorders including schizophrenia, bipolar disorder, autism spectrum disorders and major depressive disorder.

Behavioral analysis of NR2C knockout mouse reveals deficit in acquisition of conditioned fear and working memory

N-methyl-D-aspartate (NMDA) receptors play an important role in excitatory neurotransmission and mediate synaptic plasticity associated with learning and memory. NMDA receptors are composed of two NR1 and two NR2 subunits and the identity of the NR2 subunit confers unique electrophysiologic and pharmacologic properties to the receptor. The precise role of NR2C-containing receptors in vivo is poorly understood. We have performed a battery of behavioral tests on NR2C knockout/nβ-galactosidase knock-in mice and found no difference in spontaneous activity, basal anxiety, forced-swim immobility, novel object recognition, pain sensitivity and reference memory in comparison to wildtype counterparts. However, NR2C knockout mice were found to exhibit deficits in fear acquisition and working memory compared to wildtype mice. Deficit in fear acquisition correlated with lack of fear conditioning-induced plasticity at the thalamo-amygdala synapse. These findings suggest a unique role of NR2C-containing receptors in associative and executive learning representing a novel therapeutic target for deficits in cognition.

GluN2C/GluN2D subunit-selective NMDA receptor potentiator CIQ reverses MK-801-induced impairment in prepulse inhibition and working memory in Y-maze test in mice.

Despite ample evidence supporting the N‐methyl‐d‐aspartate receptor (NMDAR) hypofunction hypothesis of schizophrenia, progress in the development of effective therapeutics based on this hypothesis has been limited. Facilitation of NMDA receptor function by co‐agonists (d‐serine or glycine) only partially alleviates the symptoms in schizophrenia; other means to facilitate NMDA receptors are required. NMDA receptor sub‐types differ in their subunit composition, with varied GluN2 subunits (GluN2A‐GluN2D) imparting different physiological, biochemical and pharmacological properties. CIQ is a positive allosteric modulator that is selective for GluN2C/GluN2D‐containing NMDA receptors (Mullasseril et al.).

Deletion of Glutamate Delta-1 Receptor in Mouse Leads to Enhanced Working Memory and Deficit in Fear Conditioning

Glutamate delta-1 (GluD1) receptors are expressed throughout the forebrain during development with high levels in the hippocampus during adulthood. We have recently shown that deletion of GluD1 receptor results in aberrant emotional and social behaviors such as hyperaggression and depression-like behaviors and social interaction deficits. Additionally, abnormal expression of synaptic proteins was observed in amygdala and prefrontal cortex of GluD1 knockout mice (GluD1 KO). However the role of GluD1 in learning and memory paradigms remains unknown. In the present study we evaluated GluD1 KO in learning and memory tests. In the eight-arm radial maze GluD1 KO mice committed fewer working memory errors compared to wildtype mice but had normal reference memory. Enhanced working memory in GluD1 KO was also evident by greater percent alternation in the spontaneous Y-maze test. No difference was observed in object recognition memory in the GluD1 KO mice. In the Morris water maze test GluD1 KO mice showed no difference in acquisition but had longer latency to find the platform in the reversal learning task. GluD1 KO mice showed a deficit in contextual and cue fear conditioning but had normal latent inhibition. The deficit in contextual fear conditioning was reversed by D-Cycloserine (DCS) treatment. GluD1 KO mice were also found to be more sensitive to foot-shock compared to wildtype. We further studied molecular changes in the hippocampus, where we found lower levels of GluA1, GluA2 and GluK2 subunits while a contrasting higher level of GluN2B in GluD1 KO. Additionally, we found higher postsynaptic density protein 95 (PSD95) and lower glutamate decarboxylase 67 (GAD67) expression in GluD1 KO. We propose that GluD1 is crucial for normal functioning of synapses and absence of GluD1 leads to specific abnormalities in learning and memory. These findings provide novel insights into the role of GluD1 receptors in the central nervous system.

Effect of D-cycloserine in conjunction with fear extinction training on extracellular signal-regulated kinase activation in the medial prefrontal cortex and amygdala in rat.

D‐cycloserine (DCS) is currently under clinical trials for a number of neuropsychiatric conditions and has been found to augment fear extinction in rodents and exposure therapy in humans. However, the molecular mechanism of DCS action in these multiple modalities remains unclear. Here, we describe the effect of DCS administration, alone or in conjunction with extinction training, on neuronal activity (c‐fos) and neuronal plasticity [phospho‐extracellular signal‐regulated kinase (pERK)] markers using immunohistochemistry. We found that intraperitoneal administration of DCS in untrained young rats (24–28 days old) increased c‐fos‐ and pERK‐stained neurons in both the prelimbic and infralimbic division of the medial prefrontal cortex (mPFC) and reduced pERK levels in the lateral nucleus of the central amygdala. Moreover, DCS administration significantly increased GluA1, GluN1, GluN2A, and GluN2B expression in the mPFC. In a separate set of animals, we found that DCS facilitated fear extinction and increased pERK levels in the infralimbic prefrontal cortex, prelimbic prefrontal cortex intercalated cells and lateral nucleus of the central amygdala, compared with saline control. In the synaptoneurosomal preparation, we found that extinction training increased iGluR protein expression in the mPFC, compared with context animals. No significant difference in protein expression was observed between extinction‐saline and extinction‐DCS groups in the mPFC. In contrast, in the amygdala DCS, the conjunction with extinction training led to an increase in iGluR subunit expression, compared with the extinction‐saline group. Our data suggest that the efficacy of DCS in neuropsychiatric disorders may be partly due to its ability to affect neuronal activity and signaling in the mPFC and amygdala subnuclei.

Essential role of GluD1 in dendritic spine development and GluN2B to GluN2A NMDAR subunit switch in the cortex and hippocampus reveals ability of GluN2B inhibition in correcting hyperconnectivity

The glutamate delta-1 (GluD1) receptor is highly expressed in the forebrain. We have previously shown that loss of GluD1 leads to social and cognitive deficits in mice, however, its role in synaptic development and neurotransmission remains poorly understood. Here we report that GluD1 is enriched in the medial prefrontal cortex (mPFC) and GluD1 knockout mice exhibit a higher dendritic spine number, greater excitatory neurotransmission as well as higher number of synapses in mPFC. In addition abnormalities in the LIMK1-cofilin signaling, which regulates spine dynamics, and a lower ratio of GluN2A/GluN2B expression was observed in the mPFC in GluD1 knockout mice. Analysis of the GluD1 knockout CA1 hippocampus similarly indicated the presence of higher spine number and synapses and altered LIMK1-cofilin signaling. We found that systemic administration of an N-methyl-d-aspartate (NMDA) receptor partial agonist d-cycloserine (DCS) at a high-dose, but not at a low-dose, and a GluN2B-selective inhibitor Ro-25-6981 partially normalized the abnormalities in LIMK1-cofilin signaling and reduced excess spine number in mPFC and hippocampus. The molecular effects of high-dose DCS and GluN2B inhibitor correlated with their ability to reduce the higher stereotyped behavior and depression-like behavior in GluD1 knockout mice. Together these findings demonstrate a critical requirement for GluD1 in normal spine development in the cortex and hippocampus. Moreover, these results identify inhibition of GluN2B-containing receptors as a mechanism for reducing excess dendritic spines and stereotyped behavior which may have therapeutic value in certain neurodevelopmental disorders such as autism.

Nicotine and cocaine self-administration using a multiple schedule of intravenous drug and sucrose reinforcement in rats.

There appears to be a relatively narrow range of contingencies in which intravenous (i.v) infusions of nicotine will maintain responding in rats. The schedule of reinforcement typically used when investigating i.v. nicotine self-administration is a simple fixed-ratio (FR) schedule. This study determined if responding in rats could be established using a multiple schedule of either i.v. cocaine or nicotine and sucrose reinforcement. Following training of individual components with each reinforcer, rats were placed on an FR15 60-s timeout multiple schedule of cocaine (0.3 mg/kg/infusion) and sucrose (45 mg pellets) reinforcement or an FR5 60-s timeout multiple schedule of nicotine (0.03 mg/kg/infusion) and sucrose (45 mg pellets) reinforcement. Both cocaine and nicotine maintained significant levels of responding under the multiple schedule. Pretreatment with the dopamine D1 antagonist SCH 23390 increased cocaine-maintained responding, but not sucrose responding. Acute pretreatment with the nicotinic antagonist mecamylamine or SCH 23390 specifically decreased nicotine self-administration. Extinction of the individual nicotine and sucrose components resulted in decreases in responding in each component under extinction. These results indicate that i.v. nicotine maintains responding under a multiple schedule. This procedure may be useful when studying the specificity of drug pretreatments on nicotine self-administration.

Effects of environmental enrichment on nicotine-induced sensitization and cross-sensitization to d-amphetamine in rats

INTRODUCTION Research indicates that adolescent nicotine exposure may predispose individuals to use other psychostimulants later in adulthood, offering support for the incentive-sensitization theory of addiction. Preclinical studies testing the incentive-sensitization theory show that repeated nicotine exposure in adolescent rats can lead to an increased sensitivity to the motor stimulant effects of nicotine and other psychostimulants in adulthood. Although previous nicotine exposure can increase sensitivity to stimulant drugs, rats raised in enriched conditions (EC) show, decreased sensitivity to psychostimulant drugs compared to rats raised in isolation conditions (IC). METHODS We examined whether nicotine sensitization or cross-sensitization to d-amphetamine induced by adolescent nicotine exposure is altered by exposure to environmental enrichment. Adolescent EC and IC male rats received subcutaneous (s.c.) injections of saline or 0.4mg/kg of nicotine once daily for seven days. Thirty-five days following the last nicotine injection EC and IC animals were challenged with saline, nicotine (0.2 or 0.4mg/kg) or d-amphetamine (0.5 or 1.0mg/kg). RESULTS EC rats failed to show nicotine sensitization at either nicotine dose tested while IC rats showed nicotine sensitization following the 0.4mg/kg nicotine dose. EC rats also failed to show nicotine-induced cross-sensitization to the 0.5mg/kg dose of d-amphetamine while IC rats displayed cross-sensitization. However, EC rats did exhibit nicotine-induced cross-sensitization to the 1.0mg/kg dose of d-amphetamine. CONCLUSION These findings indicate that environmental enrichment can alter the ability of adolescent nicotine exposure to induce sensitization and cross-sensitization in adulthood and may be used as a protectant factor against adolescent nicotine exposure.