Learning by subtraction: Hippocampal activity and effects of ethanol during the acquisition and performance of response sequences

Understanding ethanol's acute effects on medial prefrontal cortex neural activity using state-space approaches

Acute ethanol (EtOH) intoxication results in several maladaptive behaviors that may be attributable, in part, to the effects of EtOH on neural activity in medial prefrontal cortex (mPFC). The acute effects of EtOH on mPFC function have been largely described as inhibitory. However, translating these observations on function into a mechanism capable of delineating acute EtOH's effects on behavior has proven difficult. This review highlights the role of acute EtOH on electrophysiological measurements of mPFC function and proposes that interpreting these changes through the lens of dynamical systems theory is critical to understand the mechanisms that mediate the effects of EtOH intoxication on behavior. Specifically, the present review posits that the effects of EtOH on mPFC N-methyl-d-aspartate (NMDA) receptors are critical for the expression of impaired behavior following EtOH consumption. This hypothesis is based on the observation that recurrent activity in cortical networks is supported by NMDA receptors, and, when disrupted, may lead to impairments in cognitive function. To evaluate this hypothesis, we discuss the representation of mPFC neural activity in low-dimensional, dynamic state spaces. This approach has proven useful for identifying the underlying computations necessary for the production of behavior. Ultimately, we hypothesize that EtOH-related alterations to NMDA receptor function produces alterations that can be effectively conceptualized as impairments in attractor dynamics and provides insight into how acute EtOH disrupts forms of cognition that rely on mPFC function. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

Ethanol Alters Variability, But Not Rate, of Firing in Medial Prefrontal Cortex Neurons of Awake-Behaving Rats

BACKGROUND

The medial prefrontal cortex (mPFC) is a brain region involved in the evaluation and selection of motivationally relevant outcomes. Neural activity in mPFC is altered following acute ethanol (EtOH) use and, in rodent models, doses as low as 0.75 g/kg yield cognitive deficits. Deficits in decision making following acute EtOH are thought to be mediated, at least in part, by decreases in mPFC firing rates (FRs). However, the data leading to this conclusion have been generated exclusively in anesthetized rodents. The present study characterizes the effects of acute EtOH injections on mPFC neural activity in awake-behaving rodents.

METHODS

Awake-behaving and anesthetized in vivo electrophysiological recordings were performed. We utilized 3 groups: the first received 2 saline injections, the second received a saline injection followed by 1.0 g/kg EtOH, and the last received saline followed by 2 g/kg EtOH. One week later, an anesthetized recording occurred where a saline injection was followed by an injection of 1.0 g/kg EtOH.

RESULTS

The anesthetized condition showed robust decreases in neural activity and differences in up-down states (UDS) dynamics. In the awake-behaving condition, FRs were grouped according to behavioral state: moving, not-moving, and sleep. The differences in median FRs were found for each treatment and behavioral state combination. A FR decrease was only found in the 2.0 g/kg EtOH treatment during not-moving states. However, robust decreases in FR variability were found across behavioral state in both the 1.0 and 2.0 g/kg EtOH treatment. Sleep was separately analyzed. EtOH modulated the UDS during sleep producing decreases in FRs.

CONCLUSIONS

In conclusion, the changes in neural activity following EtOH administration in anesthetized animals are not conserved in awake-behaving animals. The most prominent difference following EtOH was a decrease in FR variability suggesting that acute EtOH may be affecting decision making via this mechanism.

Sevoflurane exposure has minimal effect on cognitive function and does not alter microglial activation in adult monkeys

Postoperative Cognitive Dysfunction (POCD) is a complication that has been observed in a subset of adult and elderly individuals after general anesthesia and surgery. Although the pathogenesis of POCD is largely unknown, a growing body of preclinical research suggests that POCD may be caused by general anesthesia. A significant amount of research has examined the effects of general anesthesia on neurocognitive function in rodents, yet no studies have assessed the adverse effects of general anesthesia on brain function in adult nonhuman primates. Thus, this study sought to determine the effects of an extended exposure to sevoflurane anesthesia on cognitive function and neural inflammation in adult rhesus macaques. Five adult rhesus macaques (16-17 years of age) were exposed to sevoflurane anesthesia for 8 h and, and micro-positron emission tomography (PET)/computed tomography (CT) imaging and a battery of operant tasks were used to assess the effects of anesthesia exposure on ¹⁸F-labeled fluoroethoxybenzyl-N-(4-phenoxypyridin-3-yl) acetamide ([¹⁸F]-FEPPA) uptake, a biomarker of microglia activation, and aspects of complex cognitive function. Exposure to sevoflurane anesthesia for 8 h did not increase [¹⁸F]-FEPPA uptake in the adult monkey brain. Sevoflurane anesthesia significantly decreased accuracy (mean difference = 22.79) on a learning acquisition task 6 days after exposure [t(3) = 6.92, p =  0.006], but this effect did not persist when measured 1 week and 2 weeks after additional exposures. Further, sevoflurane anesthesia had no impact on performance in 4 additional cognitive tasks. These data suggest that exposure to anesthesia alone may not be sufficient to cause persistent POCD in adult populations.

Acute Low Alcohol Disrupts Hippocampus-Striatum Neural Correlate of Learning Strategy by Inhibition of PKA/CREB Pathway in Rats

The hippocampus and striatum guide place-strategy and response-strategy learning, respectively, and they have dissociable roles in memory systems, which could compensate in case of temporary or permanent damage. Although acute alcohol (AA) treatment had been shown to have adverse effects on hippocampal function, whether it causes the functional compensation and the underlying mechanisms is unknown. In this study, rats treated with a low dose of AA avoided a hippocampus-dependent spatial strategy, instead preferring a striatum-dependent response strategy. Consistently, the learning-induced increase in hippocampal, but not striatal, pCREB was rendered less pronounced due to diminished activity of pPKA, but not pERK or pCaMKII. As rats approached the turn-decision area, Sp-cAMP, a PKA activator, was found to mitigate the inhibitory effect of AA on intra- and cross-structure synchronized neuronal oscillations, and rescue response-strategy bias and spatial learning deficits. Our study provides strong evidence of the critical link between neural couplings and strategy selection. Moreover, the PKA/CREB-signaling pathway is involved in the suppressive effect of AA on neural correlates of place-learning strategy. The novel important evidence provided here shows the functional couplings between the hippocampus and striatum in spatial learning processing and suggests possible avenues for therapeutic intervention.

Chronic Binge Alcohol Administration Dysregulates Hippocampal Genes Involved in Immunity and Neurogenesis in Simian Immunodeficiency Virus-Infected Macaques

Alcohol use disorders (AUD) exacerbate neurocognitive dysfunction in Human Immunodeficiency Virus (HIV+) patients. We have shown that chronic binge alcohol (CBA) administration (13–14 g EtOH/kg/wk) prior to and during simian immunodeficiency virus (SIV) infection in rhesus macaques unmasks learning deficits in operant learning and memory tasks. The underlying mechanisms of neurocognitive alterations due to alcohol and SIV are not known. This exploratory study examined the CBA-induced differential expression of hippocampal genes in SIV-infected (CBA/SIV+; n = 2) macaques in contrast to those of sucrose administered, SIV-infected (SUC/SIV+; n = 2) macaques. Transcriptomes of hippocampal samples dissected from brains obtained at necropsy (16 months post-SIV inoculation) were analyzed to determine differentially expressed genes. MetaCore from Thomson Reuters revealed enrichment of genes involved in inflammation, immune responses, and neurodevelopment. Functional relevance of these alterations was examined in vitro by exposing murine neural progenitor cells (NPCs) to ethanol (EtOH) and HIV trans-activator of transcription (Tat) protein. EtOH impaired NPC differentiation as indicated by decreased βIII tubulin expression. These findings suggest a role for neuroinflammation and neurogenesis in CBA/SIV neuropathogenesis and warrant further investigation of their potential contribution to CBA-mediated neurobehavioral deficits.