Vagus Nerve Stimulation as a Tool to Induce Plasticity in Pathways Relevant for Extinction Learning

Extinction describes the process of attenuating behavioral responses to neutral stimuli when they no longer provide the reinforcement that has been ...

The University of Texas at Dallas

This review discusses evidence that cells in the mesocortical dopamine (DA) system influence information processing in target areas across three distinct temporal domains.

The ability of the mesocortical dopamine system to operate in distinct temporal modes.

Cognitive functions supported by neurons in the prefrontal cortex (PFC) are disrupted by acute and chronic exposure to alcohol, yet little is known about the mechanisms that underlie these effects. In the present study, in vivo and in vitro electrophysiology was used to determine the effects of ethanol on neuronal firing and network patterns of persistent activity in PFC neurons. In vivo, ethanol (0.375-3.5 g/kg) dose-dependently reduced spike activity in the PFC measured with multielectrode extracellular recording in the anesthetized rat. In an in vitro coculture system containing slices of PFC, hippocampus, and ventral tegmental area (VTA), ethanol (25-100 mM) decreased persistent activity of PFC neurons, but had little effect on firing evoked by direct current injection. Persistent activity was often enhanced after ethanol washout and this effect was maintained in cultures lacking the VTA. A low concentration of the NMDA antagonist APV (5 microM) mimicked the inhibition of ethanol of persistent activity with no change in activity after washout. Ethanol inhibition of spontaneous and VTA-evoked persistent activity was enhanced by the D1 dopamine receptor antagonist SCH23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride]. The results of this study show that ethanol inhibits persistent activity and spike firing of PFC neurons and that the degree of ethanol inhibition may be influenced by D1 receptor tone. Ethanol-induced alterations in the activity of deep-layer cortical neurons may underlie some of the behavioral effects associated with ethanol intake.

Ethanol inhibits persistent activity in prefrontal cortical neurons.

In the primate dorsolateral prefrontal cortex (DLPFC), the density of excitatory synapses decreases by 40-50% during adolescence. Although such substantial circuit refinement might underlie the adolescence-related maturation of working memory performance, its functional significance remains poorly understood. The consequences of synaptic pruning may depend on the properties of the eliminated synapses. Are the synapses eliminated during adolescence functionally immature, as is the case during early brain development? Or do maturation-independent features tag synapses for pruning? We examined excitatory synaptic function in monkey DLPFC during postnatal development by studying properties that reflect synapse maturation in rat cortex. In 3-month-old (early postnatal) monkeys, excitatory inputs to layer 3 pyramidal neurons had immature properties, including higher release probability, lower alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/N-methyl-D-aspartate (NMDA) ratio, and longer duration of NMDA-mediated synaptic currents, associated with greater sensitivity to the NMDA receptor subunit B (NR2B) subunit-selective antagonist ifenprodil. In contrast, excitatory synaptic inputs in neurons from preadolescent (15 months old) and adult (42 or 84 months old) monkeys had similar functional properties. We therefore conclude that the contribution of functionally immature synapses decreases significantly before adolescence begins. Thus, remodeling of excitatory connectivity in the DLPFC during adolescence may occur in the absence of widespread maturational changes in synaptic strength.

Functional maturation of excitatory synapses in layer 3 pyramidal neurons during postnatal development of the primate prefrontal cortex.

The importance of dopamine (DA) for prefrontal cortical (PFC) cognitive functions is widely recognized, but its mechanisms of action remain controversial. DA is thought to increase signal gain in active networks according to an inverted U dose-response curve, and these effects may depend on both tonic and phasic release of DA from midbrain ventral tegmental area (VTA) neurons.

Dopamine modulates persistent synaptic activity and enhances the signal-to-noise ratio in the prefrontal cortex.

A functionally hypoactive prefrontal cortex (PFC) is thought to contribute to decreased cognitive inhibitory control over drug-seeking behavior in cocaine addicts. Alterations in PFC dopamine (DA) and γ-aminobutyric acid (GABA) transmission are involved in the development of behavioral sensitization to cocaine, and repeated exposure to cocaine decreases DA D2 receptor (D2R) function in the PFC. We used recordings in PFC slices from adult rats to investigate how repeated cocaine treatment followed by 2 weeks of withdrawal affects DA modulation of GABA transmission and interneuron firing. In agreement with previous results in drug-naïve animals we found that in saline-treated control animals DA (20 μM) modulated evoked inhibitory post-synaptic currents (eIPSCs) in a biphasic, time- and receptor-dependent manner. Activation of D2Rs transiently reduced, whereas D1 receptor activation persistently increased the amplitude of eIPSCs. In cocaine-sensitized animals the D2R-dependent modulation of eIPSCs was abolished and the time course of DA effects was altered. In both saline- and cocaine-treated animals the effects of DA on eIPSCs were paralleled by distinct changes in spontaneous IPSCs (sIPSCs). In cocaine-treated animals the alterations in DA modulation of eIPSCs and sIPSCs correlated with a lack of D2R-specific reduction in action potential-independent GABA release, which might normally oppose D1-dependent increases in GABA transmission. Recordings from interneurons furthermore show that D2R activation can increase current-evoked spike firing in saline, but not in cocaine-treated animals. Altered DA regulation of inhibition during cocaine withdrawal could disturb normal cortical processing and contribute to a hypoactive PFC.

Altered dopamine modulation of inhibition in the prefrontal cortex of cocaine-sensitized rats.

In the present study, we used a mouse model of chronic intermittent ethanol (CIE) exposure to examine how CIE alters the plasticity of the medial prefrontal cortex (mPFC). In acute slices obtained either immediately or 1-week after the last episode of alcohol exposure, voltage-clamp recording of excitatory post-synaptic currents (EPSCs) in mPFC layer V pyramidal neurons revealed that CIE exposure resulted in an increase in the NMDA/AMPA current ratio. This increase appeared to result from a selective increase in the NMDA component of the EPSC. Consistent with this, Western blot analysis of the postsynaptic density fraction showed that while there was no change in expression of the AMPA GluR1 subunit, NMDA NR1 and NRB subunits were significantly increased in CIE exposed mice when examined immediately after the last episode of alcohol exposure. Unexpectedly, this increase in NR1 and NR2B was no longer observed after 1-week of withdrawal in spite of a persistent increase in synaptic NMDA currents. Analysis of spines on the basal dendrites of layer V neurons revealed that while the total density of spines was not altered, there was a selective increase in the density of mushroom-type spines following CIE exposure. Examination of NMDA-receptor mediated spike-timing-dependent plasticity (STDP) showed that CIE exposure was associated with altered expression of long-term potentiation (LTP). Lastly, behavioral studies using an attentional set-shifting task that depends upon the mPFC for optimal performance revealed deficits in cognitive flexibility in CIE exposed mice when tested up to 1-week after the last episode of alcohol exposure. Taken together, these observations are consistent with those in human alcoholics showing protracted deficits in executive function, and suggest these deficits may be associated with alterations in synaptic plasticity in the mPFC.

Chronic alcohol exposure alters behavioral and synaptic plasticity of the rodent prefrontal cortex.

Fearful experiences can produce long-lasting and debilitating memories. Extinction of the fear response requires consolidation of new memories that compete with fearful associations. Subjects with posttraumatic stress disorder (PTSD) show impaired extinction of conditioned fear, which is associated with decreased ventromedial prefrontal cortex (vmPFC) control over amygdala activity. Vagus nerve stimulation (VNS) enhances memory consolidation in both rats and humans, and pairing VNS with exposure to conditioned cues enhances the consolidation of extinction learning in rats. Here we investigated whether pairing VNS with extinction learning facilitates plasticity between the infralimbic (IL) medial prefrontal cortex and the basolateral complex of the amygdala (BLA). Rats were trained on an auditory fear conditioning task, which was followed by a retention test and 1 day of extinction training. Vagus nerve stimulation or sham-stimulation was administered concurrently with exposure to the fear-conditioned stimulus and retention of fear conditioning was tested again 24 h later. Vagus nerve stimulation-treated rats demonstrated a significant reduction in freezing after a single extinction training session similar to animals that received 5× the number of extinction pairings. To study plasticity in the IL-BLA pathway, we recorded evoked field potentials (EFPs) in the BLA in anesthetized animals 24 h after retention testing. Brief burst stimulation in the IL produced LTD in the BLA field response in fear-conditioned and sham-treated animals. In contrast, the same stimulation resulted in potentiation of the IL-BLA pathway in the VNS-treated group. The present findings suggest that VNS promotes plasticity in the IL-BLA pathway to facilitate extinction of conditioned fear responses (CFRs).

Vagus nerve stimulation enhances extinction of conditioned fear and modulates plasticity in the pathway from the ventromedial prefrontal cortex to the amygdala.

The N-methyl-D-aspartic acid (NMDA)-hypofunction theory of schizophrenia suggests that schizophrenia is associated with a loss of NMDA receptors, specifically on corticolimbic parvalbumin (PV)-expressing GABAergic interneurons, leading to disinhibition of pyramidal cells and cortical desynchronization. However, the presumed changes in glutamatergic inputs onto PV interneurons have not been tested directly. We treated mice with the NMDAR antagonist ketamine during the second postnatal week and investigated persistent cellular changes in the adult medial prefrontal cortex (mPFC) using whole-cell patch-clamp recordings and immunohistochemistry. Parvalbumin expression in the mPFC was reduced in ketamine-treated (KET) mice, and γ-aminobutyric acid release onto pyramidal cells was reduced in layers 2/3, but not layer 5. Consistent with pyramidal cell disinhibition the frequency of spontaneous glutamatergic inputs onto PV cells was also increased in KET mice. Furthermore, developmental ketamine treatment resulted in an increased NMDA:AMPA ratio in evoked synaptic currents and larger amplitudes of spontaneous NMDAR currents, indicating a homeostatic upregulation of NMDARs in PV interneurons. This upregulation was specific to NR2B subunits, without concomitant alterations in currents through NR2A subunits. These changes altered synaptic integration at PV cells during trains of excitatory postsynaptic potentials. These changes likely impact synaptic coincidence detection and impair cortical network function in the NMDAR antagonism model of schizophrenia.

Ketamine Administration During the Second Postnatal Week Alters Synaptic Properties of Fast-Spiking Interneurons in the Medial Prefrontal Cortex of Adult Mice.

Chronic alcohol-induced cognitive impairments and maladaptive plasticity of glutamatergic synapses are well-documented. However, it is unknown if prolonged alcohol exposure affects dendritic signaling that may underlie hippocampal dysfunction in alcoholics. Back-propagation of action potentials (bAPs) into apical dendrites of hippocampal neurons provides distance-dependent signals that modulate dendritic and synaptic plasticity. The amplitude of bAPs decreases with distance from the soma that is thought to reflect an increase in the density of Kv4.2 channels toward distal dendrites.

Neuroplasticity of A-type potassium channel complexes induced by chronic alcohol exposure enhances dendritic calcium transients in hippocampus.

Dysfunctions in the GABAergic system are considered a core feature of schizophrenia. Pharmacological blockade of NMDA receptors (NMDAR), or their genetic ablation in parvalbumin (PV)-expressing GABAergic interneurons can induce schizophrenia-like behavior in animals. NMDAR-mediated currents shape the maturation of GABAergic interneurons during a critical period of development, making transient blockade of NMDARs during this period an attractive model for the developmental changes that occur in the course of schizophrenia's pathophysiology. Here, we examined whether developmental administration of the non-competitive NMDAR antagonist ketamine results in persistent deficits in PFC-dependent behaviors in adult animals. Mice received injections of ketamine (30mg/kg) on postnatal days (PND) 7, 9 and 11, and then tested on a battery of behavioral experiments aimed to mimic major symptoms of schizophrenia in adulthood (between PND 90 and 120). Ketamine treatment reduced the number of cells that expressed PV in the PFC by ∼60% as previously described. Ketamine affected performance in an attentional set-shifting task, impairing the ability of the animals to perform an extradimensional shift to acquire a new strategy. Ketamine-treated animals showed deficits in latent inhibition, novel-object recognition and social novelty detection compared to their SAL-treated littermates. These deficits were not a result of generalized anxiety, as both groups performed comparably on an elevated plus maze. Ketamine treatment did not cause changes in amphetamine-induced hyperlocomotion that are often taken as measures for the positive-like symptoms of the disorder. Thus, ketamine administration during development appears to be a useful model for inducing cognitive and negative symptoms of schizophrenia.

Ketamine administration during the second postnatal week induces enduring schizophrenia-like behavioral symptoms and reduces parvalbumin expression in the medial prefrontal cortex of adult mice.

The medial prefrontal cortex (mPFC) inhibits impulsive and compulsive behaviors that characterize drug abuse and dependence. Acamprosate is the leading medication approved for the maintenance of abstinence, shown to reduce craving and relapse in animal models and human alcoholics. Whether acamprosate can modulate executive functions that are impaired by chronic ethanol (EtOH) exposure is unknown. Here we explored the effects of acamprosate on an attentional set-shifting task and tested whether these behavioral effects are correlated with modulation of glutamatergic synaptic transmission and intrinsic excitability of mPFC neurons.

Sven Kroener

School of Behavioral and Brain Sciences

The University of Texas at Dallas

Vagus Nerve Stimulation as a Tool to Induce Plasticity in Pathways Relevant for Extinction Learning

Sven Kroener

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}School of Behavioral and Brain Sciences

The University of Texas at Dallas

Vagus Nerve Stimulation as a Tool to Induce Plasticity in Pathways Relevant for Extinction Learning

School of Behavioral and Brain Sciences