Combination of behaviorally sub-effective doses of glutamate NMDA and dopamine D1 receptor antagonists impairs executive function

Assessment of executive function in a rodent model of Type 1 diabetes

This study examined the impact of Type 1 Diabetes Mellitus (T1DM) on executive function using a series of operant conditioning-based tasks in rats. Sprague Dawley rats were randomized to either non-diabetic (n = 12; 6 male) or diabetic (n = 14; 6 male) groups. Diabetes was induced using multiple low-dose streptozotocin injections. All diabetic rodents were insulin-treated using subcutaneous insulin pellet implants (9-15 mM). At week 14 of the study, rats were placed on a food restricted diet to induce 5-10 % weight loss. Rodents were familiarized and their set-shifting ability was tested on a series of tasks that required continuous adjustments to novel stimulus-reward paradigms in order to receive food rewards. Results showed no differences in the number of trials, nor number and type of errors made to successfully complete each task between groups. Therefore, we report no differences in executive function, or more specifically set-shifting abilities between non-diabetic and diabetic rodents that receive insulin.

Differential Plasticity in Auditory and Prefrontal Cortices, and Cognitive-Behavioral Deficits Following Noise-Induced Hearing Loss

Excessive exposure to loud noise causes hearing loss and neural plasticity throughout the auditory pathway. Recent studies have identified that non-auditory regions, such as the hippocampus, are also susceptible to noise exposure; however, the electrophysiological and behavioral consequences of noise-induced hearing loss on the prefrontal cortex (PFC) are unclear. Using chronically-implanted electrodes in awake rats, we investigated neural plasticity in the auditory and prefrontal cortices in the days following noise exposure via metrics associated with spontaneous neural oscillations and the 40-Hz auditory steady-state response (ASSR). Noise exposure did not alter the profile of spontaneous oscillations in either of the cortices, yet it caused a differential plasticity in the sound-evoked activity, which was characterized by enhanced event-related potentials (ERPs) in the auditory cortex (i.e., central gain), and decreased inter-trial coherence (ITC) of the 40-Hz ASSR within the PFC. Moreover, phase synchrony between auditory and prefrontal cortices was decreased post-exposure, suggesting a reduction in functional connectivity. Cognitive-behavioral testing using the Morris water maze and a series of lever-pressing tasks revealed that noise exposure impaired spatial learning and reference memory, as well as stimulus-response habit learning, whereas cognitive flexibility tasks requiring set-shifting and reversal learning appeared unaffected. Collectively, our findings identify the complex and region-specific cortical plasticity associated with noise-induced hearing loss, and highlight the varying degrees of susceptibility of non-auditory, cognitive tasks of learning, memory and executive function to noise exposure.

Dopamine Receptor Subtypes, Physiology and Pharmacology: New Ligands and Concepts in Schizophrenia

Dopamine receptors are widely distributed within the brain where they play critical modulator roles on motor functions, motivation and drive, as well as cognition. The identification of five genes coding for different dopamine receptor subtypes, pharmacologically grouped as D1- (D1 and D5) or D2-like (D2S, D2L, D3, and D4) has allowed the demonstration of differential receptor function in specific neurocircuits. Recent observation on dopamine receptor signaling point at dopamine-glutamate-NMDA neurobiology as the most relevant in schizophrenia and for the development of new therapies. Progress in the chemistry of D1- and D2-like receptor ligands (agonists, antagonists, and partial agonists) has provided more selective compounds possibly able to target the dopamine receptors homo and heterodimers and address different schizophrenia symptoms. Moreover, an extensive evaluation of the functional effect of these agents on dopamine receptor coupling and intracellular signaling highlights important differences that could also result in highly differentiated clinical pharmacology. The review summarizes the recent advances in the field, addressing the relevance of emerging new targets in schizophrenia in particular in relation to the dopamine - glutamate NMDA systems interactions.

Δ-9-Tetrahydrocannabinol and Cannabidiol produce dissociable effects on prefrontal cortical executive function and regulation of affective behaviors

The use of cannabis for therapeutic and recreational purposes is growing exponentially. Nevertheless, substantial questions remain concerning the potential cognitive and affective side-effects associated with cannabis exposure. In particular, the effects of specific marijuana-derived phytocannabinoids on neural regions such as the prefrontal cortex (PFC) are of concern, given the role of the PFC in both executive cognitive function and affective processing. The main biologically active phytocannabinoids, ∆-9-tetrahydrocannabinol (THC) and cannabidiol (CBD), interact with multiple neurotransmitter systems important for these processes directly within the PFC. Considerable evidence has demonstrated that acute or chronic THC exposure may induce psychotomimetic effects, whereas CBD has been shown to produce potentially therapeutic effects for both psychosis and/or anxiety-related symptoms. Using an integrative combination of cognitive and affective behavioral pharmacological assays in rats, we report that acute intra-PFC infusions of THC produce anxiogenic effects while producing no impairments in executive function. In contrast, acute infusions of intra-PFC CBD impaired attentional set-shifting and spatial working memory, without interfering with anxiety or sociability behaviors. In contrast, intra-PFC CBD reversed the cognitive impairments induced by acute glutamatergic antagonism within the PFC, and blocked the anxiogenic properties of THC, suggesting that the therapeutic properties of CBD within the PFC may be present only during pathologically aberrant states within the PFC. Interestingly, the effects of PFC THC vs. CBD were found to be mediated through dissociable CB1 vs. 5-HT_1A-dependent receptor signaling mechanisms, directly in the PFC.

Infusions of Nerve Growth Factor Into the Developing Frontal Cortex Leads to Deficits in Behavioral Flexibility and Increased Perseverance

In the pursuit of further establishing a neurodevelopmental animal model to investigate the mechanisms underlying impaired executive function, a core and severely debilitating symptom of schizophrenia, we sought to characterize the deficits in behavioral flexibility in adult rats following neonatal infusions of nerve growth factor (NGF) into the medial part of the developing frontal cortex. Our previous studies using this neonatal frontal cortical lesion model have shown that it leads to adult-onset positive and negative symptom-like features, and several neuropathological abnormalities of schizophrenia. In the present study, we used operant conditioning-based paradigms to investigate set-shifting ability and reversal learning performance in adult rats that received infusions of NGF into the developing frontal cortex on post-natal day 1. NGF-infusion caused apoptosis of cells in the subplate layer. Adult rats that received neonatal infusions of NGF showed decreased grey matter thickness, and decreased levels of parvalbumin in prelimbic and infralimbic areas of the medial prefrontal cortex (mPFC). NGF-treated rats had difficulty completing the set-shifting and reversal learning tasks due to increased perseverance (ie, a failure to disengage from the previously-learned strategy once the rule contingencies were changed) compared to the control group. Collectively, these results identify the crucial role of the frontal cortical subplate layer in the structural and functional development of the mPFC relevant to schizophrenia. Furthermore, the present findings substantially advance the face and construct validity of this putative preclinical model of schizophrenia based on developmental disruption of the frontal cortical subplate.

A Novel Dopamine Transporter Inhibitor CE-123 Improves Cognitive Flexibility and Maintains Impulsivity in Healthy Male Rats

Reduced cognitive abilities are often characterized by an impairment of flexibility, i.e., the ability to switch from learned rules or categories that were important in certain contexts to different new modalities that rule the task. Drugs targeting the dopamine transporter (DAT) are widely used for their potential to enhance cognitive abilities. However, commercially available drugs are of limited specificity for DAT, blocking also noradrenaline and serotonine transporters, that can lead to unwanted side effects in healthy subjects. Therefore, we tested a newly synthetized compound (CE-123) with higher specificity for DAT in male rats in an attentional set-shifting task (ASST), that proves for cognitive flexibility and a 5-choice serial-reaction time task (5-CSRTT) assessing visuospatial attention and impulsivity. Treated rats at a dose of 0.3 and 1.0 but not 0.1 mg/kg bodyweight showed reduced extra-dimensional shifts in the ASST compared to controls indicating increased cognitive flexibility. Rats treated with R-Modafinil, a commercially available DAT inhibitor at a dose of 10 mg/kg bodyweight showed increased premature responses, an indicator of increased impulsivity, during a 10 s but not a 2.5, 5, or 7.5 s intertrial interval when compared to vehicle-treated rats in the 5-CSRTT. This was not found in rats treated with CE-123 at the same dose as for R-Modafinil. Visuospatial attention, except premature responses, did not differ between R-Modafinil and CE-123-treated rats and their respective controls. Thus, CE-123 increased cognitive flexibility with diminished impulsivity.

Behavioural inflexibility in a comorbid rat model of striatal ischemic injury and mutant hAPP overexpression

Alzheimer disease (AD) and stroke coexist and interact; yet how they interact is not sufficiently understood. Both AD and basal ganglia stroke can impair behavioural flexibility, which can be reliably modeled in rats using an established operant based set-shifting test. Transgenic Fischer 344-APP21 rats (TgF344) overexpress pathogenic human amyloid precursor protein (hAPP) but do not spontaneously develop overt pathology, hence TgF344 rats can be used to model the effect of vascular injury in the prodromal stages of Alzheimer disease. We demonstrate that the injection of endothelin-1 (ET1) into the dorsal striatum of TgF344 rats (Tg-ET1) produced an exacerbation of behavioural inflexibility with a behavioural phenotype that was distinct from saline-injected wildtype & TgF344 rats as well as ET1-injected wildtype rats (Wt-ET1). In addition to profiling the types of errors made, interpolative modeling using logistic exposure-response regression provided an informative analysis of the timing and efficiency of behavioural flexibility. During set-shifting, Tg-ET1 committed fewer perseverative errors than Wt-ET1. However, Tg-ET1 committed significantly more regressive errors and had a less efficient strategy change than all other groups. Thus, behavioural flexibility was more vulnerable to striatal ischemic injury in TgF344 rats.