Late-adolescent onset of prefrontal endocannabinoid control of hippocampal and amygdalar inputs and its impact on trace-fear conditioning behavior.

Prefrontal cortex (PFC) maturation during adolescence is characterized by structural and functional changes, which involve the remodeling of GABA and glutamatergic synapses, as well as changes in the endocannabinoid system. Yet, the way PFC endocannabinoid signaling interacts with local GABA and glutamatergic function to impact its processing of afferent transmission during the adolescent transition to adulthood remains unknown. Here we combined PFC local field potential recordings with local manipulations of 2-AG and anandamide levels to assess how PFC endocannabinoid signaling is recruited to modulate ventral hippocampal and basolateral amygdalar inputs in vivo in adolescent and adult male rats. We found that the PFC endocannabinoid signaling does not fully emerge until late-adolescence/young adulthood. Once present, both 2-AG and anandamide can be recruited in the PFC to limit the impact of hippocampal drive through a CB1R-mediated mechanism whereas basolateral amygdalar inputs are only inhibited by 2-AG. Similarly, the behavioral effects of increasing 2-AG and anandamide in the PFC do not emerge until late-adolescence/young adulthood. Using a trace fear conditioning paradigm, we found that elevating PFC 2-AG levels preferentially reduced freezing behavior during acquisition without affecting its extinction. In contrast, increasing anandamide levels in the PFC selectively disrupted the extinction of trace fear memory without affecting its acquisition. Collectively, these results indicate a protracted recruitment of PFC endocannabinoid signaling, which becomes online in late adolescence/young adulthood as revealed by its impact on hippocampal and amygdalar-evoked local field potential responses and trace fear memory behavior.

Higher Neuronal Facilitation and Potentiation with APOE4 Suppressed by Angiotensin II.

Progressive hippocampal degeneration is a key component of Alzheimer's disease (AD) progression. Therefore, identifying how hippocampal neuronal function is modulated early in AD is an important approach to eventually prevent degeneration. AD-risk factors and signaling molecules likely modulate neuronal function, including APOE genotype and angiotensin II. Compared to APOE3, APOE4 increases AD risk up to 12-fold, and high levels of angiotensin II are hypothesized to disrupt neuronal function in AD. However, the extent that APOE and angiotensin II modulates the hippocampal neuronal phenotype in AD-relevant models is unknown. To address this issue, we used electrophysiological techniques to assess the impact of APOE genotype and angiotensin II on basal synaptic transmission, presynaptic, and post-synaptic activity in mice that express human APOE3 (E3FAD) or APOE4 (E4FAD) and overproduce Aß. We found that compared to E3FAD mice, E4FAD mice have lower synaptic activity, but higher levels of paired-pulse facilitation (PPF) and long-term potentiation (LTP) in the Schaffer Collateral Commissural Pathway (SCCP) of the hippocampus. We also found that exogenous angiotensin II has a profound inhibitory effect on hippocampal LTP in both E3FAD and E4FAD mice. Collectively, our data suggests that APOE4 and Aß are associated with a hippocampal phenotype comprised of lower basal activity and higher responses to high-frequency stimulation, the latter of which is suppressed by angiotensin II. These novel data suggest a potential mechanistic link between hippocampal activity, APOE4 genotype, and angiotensin II in AD.

Loss of Depalmitoylation Disrupts Homeostatic Plasticity of AMPARs in a Mouse Model of Infantile Neuronal Ceroid Lipofuscinosis.

Protein palmitoylation is the only reversible post-translational lipid modification. Palmitoylation is held in delicate balance by depalmitoylation to precisely regulate protein turnover. While over 20 palmitoylation enzymes are known, depalmitoylation is conducted by fewer enzymes. Of particular interest is the lack of the depalmitoylating enzyme palmitoyl-protein thioesterase 1 (PPT1) that causes the devastating pediatric neurodegenerative condition infantile neuronal ceroid lipofuscinosis (CLN1). While most of the research on Ppt1 function has centered on its role in the lysosome, recent findings demonstrated that many Ppt1 substrates are synaptic proteins, including the AMPA receptor (AMPAR) subunit GluA1. Still, the impact of Ppt1-mediated depalmitoylation on synaptic transmission and plasticity remains elusive. Thus, the goal of the present study was to use the Ppt1 -/- mouse model (both sexes) to determine whether Ppt1 regulates AMPAR-mediated synaptic transmission and plasticity, which are crucial for the maintenance of homeostatic adaptations in cortical circuits. Here, we found that basal excitatory transmission in the Ppt1 -/- visual cortex is developmentally regulated and that chemogenetic silencing of the Ppt1 -/- visual cortex excessively enhanced the synaptic expression of GluA1. Furthermore, triggering homeostatic plasticity in Ppt1 -/- primary neurons caused an exaggerated incorporation of GluA1-containing, calcium-permeable AMPARs, which correlated with increased GluA1 palmitoylation. Finally, Ca2+ imaging in awake Ppt1 -/- mice showed visual cortical neurons favor a state of synchronous firing. Collectively, our results elucidate a crucial role for Ppt1 in AMPAR trafficking and show that impeded proteostasis of palmitoylated synaptic proteins drives maladaptive homeostatic plasticity and abnormal recruitment of cortical activity in CLN1.SIGNIFICANCE STATEMENT Neuronal communication is orchestrated by the movement of receptors to and from the synaptic membrane. Protein palmitoylation is the only reversible post-translational lipid modification, a process that must be balanced precisely by depalmitoylation. The significance of depalmitoylation is evidenced by the discovery that mutation of the depalmitoylating enzyme palmitoyl-protein thioesterase 1 (Ppt1) causes severe pediatric neurodegeneration. In this study, we found that the equilibrium provided by Ppt1-mediated depalmitoylation is critical for AMPA receptor (AMPAR)-mediated plasticity and associated homeostatic adaptations of synaptic transmission in cortical circuits. This finding complements the recent explosion of palmitoylation research by emphasizing the necessity of balanced depalmitoylation.

Endothelial Cell APOE3 Regulates Neurovascular, Neuronal, and Behavioral Function.

BACKGROUND: Specialized brain endothelial cells and human APOE3 are independently important for neurovascular function, yet whether APOE3 expression by endothelial cells contributes to brain function is currently unknown. In the present study, we determined whether the loss of endothelial cell APOE3 impacts brain vascular and neural function. METHODS: We developed APOE3fl/fl/Cdh5(PAC)-CreERT2+/- (APOE3Cre+/-) and APOE3fl/fl/Cdh5(PAC)-CreERT2-/- (APOE3Cre-/-, control) mice and induced endothelial cell APOE3 knockdown with tamoxifen at ≈4 to 5 weeks of age. Neurovascular and neuronal function were evaluated by biochemistry, immunohistochemistry, behavioral testing, and electrophysiology at 9 months of age. RESULTS: We found that the loss of endothelial APOE3 expression was sufficient to cause neurovascular dysfunction including higher permeability and lower vessel coverage in tandem with deficits in spatial memory and fear memory extinction and a disruption of cortical excitatory/inhibitory balance. CONCLUSIONS: Our data collectively support the novel concept that endothelial APOE3 plays a critical role in the regulation of the neurovasculature, neural circuit function, and behavior.

Higher Neuronal Facilitation and Potentiation with APOE4 Suppressed by Angiotensin II.

Progressive hippocampal degeneration is a key component of Alzheimer's disease (AD) progression. Therefore, identifying how hippocampal neuronal function is modulated early in AD is an important approach to eventually prevent degeneration. AD-risk factors and signaling molecules likely modulate neuronal function, including APOE genotype and angiotensin II. Compared to APOE3 , APOE4 increases AD risk up to 12-fold, and high levels of angiotensin II are hypothesized to disrupt neuronal function in AD. However, the extent that APOE and angiotensin II modulates the hippocampal neuronal phenotype in AD-relevant models is unknown. To address this issue, we used electrophysiological techniques to assess the impact of APOE genotype and angiotensin II on basal synaptic transmission, presynaptic and post-synaptic activity in mice that express human APOE3 (E3FAD) or APOE4 (E4FAD) and overproduce Aß. We found that compared to E3FAD mice, E4FAD mice had lower basal synaptic activity, but higher levels of paired pulse facilitation (PPF) and Long-Term Potentiation (LTP) in the Schaffer Collateral Commissural Pathway (SCCP) of the hippocampus. We also found that exogenous angiotensin II has a profound inhibitory effect on hippocampal LTP in both E3FAD and E4FAD mice. Collectively, our data suggests that APOE4 and Aß are associated with a hippocampal phenotype comprised of lower basal activity and higher responses to high frequency stimulation, the latter of which is suppressed by angiotensin II. These novel data suggest a potential mechanistic link between hippocampal activity, APOE4 genotype and angiotensin II in AD.

Benzalkonium chloride, a common ophthalmic preservative, compromises rat corneal cold sensitive nerve activity.

PURPOSE: Corneal nerves comprise the densest sensory network in the body. Dysfunction of the corneal cold sensitive neurons (CSN) is implicated in ophthalmic disorders, including Dry Eye Disease, the most common ocular surface disorder. The preservative Benzalkonium chloride (BAK) and the mydriatic agent Phenylephrine hydrochloride (PHE) are considered to be inactive at the level of the CSNs. The purpose of this study is to test the impacts of continuous exposures to BAK or PHE at their clinically used concentrations on corneal nerve structure and function. METHODS: In vivo extracellular electrophysiology of the rat trigeminal ganglion was used to monitor CSN functional response to stimuli mimicking physiological states and stressors of the cornea. Corneal nerve structure was evaluated by immunostaining. RESULTS: Among the tested stimuli, cold probe receptive field stimulation and hyperosmolar stress were the most sensitive methods of detecting activity changes. CSN activity was attenuated after 30 min exposure to either PHE or BAK. After an hour-long washout period, BAK-treated neurons failed to recover activity while PHE-treated neurons showed signs of functional recovery. Intraepithelial nerve density was reduced and nerve fragmentation was increased in BAK-treated corneas, while PHE exposure left corneal nerves structurally intact. CONCLUSIONS: Our study suggests that prolonged ocular instillations of BAK or PHE alter CSN activity through two different processes - irreversible neuronal damage in the case of BAK vs. reversible attenuation in the case of PHE.

Maturation of Corticolimbic Functional Connectivity During Sensitive Periods of Brain Development.

The maturation of key corticolimbic structures and the prefrontal cortex during sensitive periods of brain development from early life through adolescence is crucial for the acquisition of a variety of cognitive and affective processes associated with adult behavior. In this chapter, we first review how key cellular and circuit level changes during adolescence dictate the development of the prefrontal cortex and its capacity to integrate contextual and emotional information from the ventral hippocampus and the amygdala. We further discuss how afferent transmission from ventral hippocampal and amygdala inputs displays unique age-dependent trajectories that directly impact prefrontal functional maturation through adolescence. We conclude by proposing that time-sensitive strengthening of specific corticolimbic synapses is a critical contributing factor for the protracted maturation of cognitive and emotional regulation by the prefrontal cortex.

Synaptic Integration of Subquantal Neurotransmission by Colocalized G Protein-Coupled Receptors in Presynaptic Terminals.

In presynaptic terminals, membrane-delimited Gi/o-mediated presynaptic inhibition is ubiquitous and acts via Gßγ to inhibit Ca2+ entry, or directly at SNARE complexes to inhibit Ca2+-dependent synaptotagmin-SNARE complex interactions. At CA1-subicular presynaptic terminals, 5-HT1B and GABAB receptors colocalize. GABAB receptors inhibit Ca2+ entry, whereas 5-HT1B receptors target SNARE complexes. We demonstrate in male and female rats that GABAB receptors alter Pr, whereas 5-HT1B receptors reduce evoked cleft glutamate concentrations, allowing differential inhibition of AMPAR and NMDAR EPSCs. This reduction in cleft glutamate concentration was confirmed by imaging glutamate release using a genetic sensor (iGluSnFR). Simulations of glutamate release and postsynaptic glutamate receptor currents were made. We tested effects of changes in vesicle numbers undergoing fusion at single synapses, relative placement of fusing vesicles and postsynaptic receptors, and the rate of release of glutamate from a fusion pore. Experimental effects of Pr changes, consistent with GABAB receptor effects, were straightforwardly represented by changes in numbers of synapses. The effects of 5-HT1B receptor-mediated inhibition are well fit by simulated modulation of the release rate of glutamate into the cleft. Colocalization of different actions of GPCRs provides synaptic integration within presynaptic terminals. Train-dependent presynaptic Ca2+ accumulation forces frequency-dependent recovery of neurotransmission during 5-HT1B receptor activation. This is consistent with competition between Ca2+-synaptotagmin and Gßγ at SNARE complexes. Thus, stimulus trains in 5-HT1B receptor agonist unveil dynamic synaptic modulation and a sophisticated hippocampal output filter that itself is modulated by colocalized GABAB receptors, which alter presynaptic Ca2+ In combination, these pathways allow complex presynaptic integration.SIGNIFICANCE STATEMENT Two G protein-coupled receptors colocalize at presynaptic sites, to mediate presynaptic modulation by Gßγ, but one (a GABAB receptor) inhibits Ca2+ entry whereas another (a 5-HT1B receptor) competes with Ca2+-synaptotagmin binding to the synaptic vesicle machinery. We have investigated downstream effects of signaling and integrative properties of these receptors. Their effects are profoundly different. GABAB receptors alter Pr leaving synaptic properties unchanged, whereas 5-HT1B receptors fundamentally change properties of synaptic transmission, modifying AMPAR but sparing NMDAR responses. Coactivation of these receptors allows synaptic integration because of convergence of GABAB receptor alteration on Ca2+ and the effect of this altered Ca2+ signal on 5-HT1B receptor signaling. This presynaptic convergence provides a novel form of synaptic integration.

GluN3-Containing NMDA Receptors in the Rat Nucleus Accumbens Core Contribute to Incubation of Cocaine Craving.

Cue-induced cocaine craving progressively intensifies (incubates) after withdrawal from cocaine self-administration in rats and humans. In rats, the expression of incubation ultimately depends on Ca2+-permeable AMPARs that accumulate in synapses onto medium spiny neurons (MSNs) in the NAc core. However, the delay in their accumulation (∼1 month after drug self-administration ceases) suggests earlier waves of plasticity. This prompted us to conduct the first study of NMDAR transmission in NAc core during incubation, focusing on the GluN3 subunit, which confers atypical properties when incorporated into NMDARs, including insensitivity to Mg2+ block and Ca2+ impermeability. Whole-cell patch-clamp recordings were conducted in MSNs of adult male rats 1-68 d after discontinuing extended-access saline or cocaine self-administration. NMDAR transmission was enhanced after 5 d of cocaine withdrawal, and this persisted for at least 68 d of withdrawal. The earliest functional alterations were mediated through increased contributions of GluN2B-containing NMDARs, followed by increased contributions of GluN3-containing NMDARs. As predicted by GluN3-NMDAR incorporation, fewer MSN spines exhibited NMDAR-mediated Ca2+ entry. GluN3A knockdown in NAc core was sufficient to prevent incubation of craving, consistent with biotinylation studies showing increased GluN3A surface expression, although array tomography studies suggested that adaptations involving GluN3B also occur. Collectively, our data show that a complex cascade of NMDAR and AMPAR plasticity occurs in NAc core, potentially through a homeostatic mechanism, leading to persistent increases in cocaine cue reactivity and relapse vulnerability. This is a remarkable example of experience-dependent glutamatergic plasticity evolving over a protracted window in the adult brain.SIGNIFICANCE STATEMENT "Incubation of craving" is an animal model for the persistence of vulnerability to cue-induced relapse after prolonged drug abstinence. Incubation also occurs in human drug users. AMPAR plasticity in medium spiny neurons (MSNs) of the NAc core is critical for incubation of cocaine craving but occurs only after a delay. Here we found that AMPAR plasticity is preceded by NMDAR plasticity that is essential for incubation and involves GluN3, an atypical NMDAR subunit that markedly alters NMDAR transmission. Together with AMPAR plasticity, this represents profound remodeling of excitatory synaptic transmission onto MSNs. Given the importance of MSNs for translating motivation into action, this plasticity may explain, at least in part, the profound shifts in motivated behavior that characterize addiction.

APOE4 Promotes Tonic-Clonic Seizures, an Effect Modified by Familial Alzheimer's Disease Mutations.

Seizures are emerging as a common symptom in Alzheimer's disease (AD) patients, often attributed to high levels of amyloid ß (Aß). However, the extent that AD disease risk factors modulate seizure activity in aging and AD-relevant contexts is unclear. APOE4 is the greatest genetic risk factor for AD and has been linked to seizures independent of AD and Aß. The goal of the present study was to evaluate the role of APOE genotype in modulating seizures in the absence and presence of high Aß levels in vivo. To achieve this goal, we utilized EFAD mice, which express human APOE3 or APOE4 in the absence (EFAD-) or presence (EFAD+) of familial AD mutations that result in Aß overproduction. When quantified during cage change day, we found that unlike APOE3, APOE4 is associated with tonic-clonic seizures. Interestingly, there were lower tonic-clonic seizures in E4FAD+ mice compared to E4FAD- mice. Restraint handing and auditory stimuli failed to recapitulate the tonic-clonic phenotype in EFAD mice that express APOE4. However, after chemical-induction with pentylenetetrazole, there was a higher incidence of tonic-clonic seizures with APOE4 compared to APOE3. Interestingly, the distribution of seizures to the tonic-clonic phenotype was higher with FAD mutations. These data support that APOE4 is associated with higher tonic-clonic seizures in vivo, and that FAD mutations impact tonic-clonic seizures in a paradigm dependent manner.

Developmental regulation of excitatory-inhibitory synaptic balance in the prefrontal cortex during adolescence.

The prefrontal cortex (PFC) is a cortical structure involved in a variety of complex functions in the cognitive and affective domains. The intrinsic function of the PFC is defined by the interaction of local glutamatergic and GABAergic neurons and their modulation by long-range inputs. The ensuing interactions generate a ratio of excitation and inhibition (E-I) in each output neuron, a balance which is refined during the adolescent to adult transition. In this short review, we aim to describe how an increase in GABAergic transmission during adolescence modifies the E-I ratio in adults. We further discuss how this new setpoint may change the dynamics of PFC networks observed during the transition to adulthood.

Dopaminergic Modulation of Forced Running Performance in Adolescent Rats: Role of Striatal D1 and Extra-striatal D2 Dopamine Receptors.

Improving exercise capacity during adolescence impacts positively on cognitive and motor functions. However, the neural mechanisms contributing to enhance physical performance during this sensitive period remain poorly understood. Such knowledge could help to optimize exercise programs and promote a healthy physical and cognitive development in youth athletes. The central dopamine system is of great interest because of its role in regulating motor behavior through the activation of D1 and D2 receptors. Thus, the aim of the present study is to determine whether D1 or D2 receptor signaling contributes to modulate the exercise capacity during adolescence and if this modulation takes place through the striatum. To test this, we used a rodent model of forced running wheel that we implemented recently to assess the exercise capacity. Briefly, rats were exposed to an 8-day period of habituation in the running wheel before assessing their locomotor performance in response to an incremental exercise test, in which the speed was gradually increased until exhaustion. We found that systemic administration of D1-like (SCH23390) and/or D2-like (raclopride) receptor antagonists prior to the incremental test reduced the duration of forced running in a dose-dependent manner. Similarly, locomotor activity in the open field was decreased by the dopamine antagonists. Interestingly, this was not the case following intrastriatal infusion of an effective dose of SCH23390, which decreased motor performance during the incremental test without disrupting the behavioral response in the open field. Surprisingly, intrastriatal delivery of raclopride failed to impact the duration of forced running. Altogether, these results indicate that the level of locomotor response to incremental loads of forced running in adolescent rats is dopamine dependent and mechanistically linked to the activation of striatal D1 and extra-striatal D2 receptors.

Prefrontal α7nAChR Signaling Differentially Modulates Afferent Drive and Trace Fear Conditioning Behavior in Adolescent and Adult Rats.

Increased level of kynurenic acid is thought to contribute to the development of cognitive deficits in schizophrenia through an α7nAChR-mediated mechanism in the prefrontal cortex (PFC). However, it remains unclear to what extent disruption of PFC α7nAChR signaling impacts afferent transmission and its modulation of behavior. Using male rats, we found that PFC infusion of methyllycaconitine (MLA; α7nAChR antagonist) shifts ventral hippocampal-induced local field potential (LFP) suppression to LFP facilitation, an effect only observed in adults. Hippocampal stimulation can also elicit a GluN2B-mediated LFP potentiation (when PFC GABAAR is blocked) that is insensitive to MLA. Conversely, PFC infusion of MLA diminished the gain of amygdalar transmission, which is already enabled by postnatal day (P)30. Behaviorally, the impact of prefrontal MLA on trace fear-conditioning and extinction was also age related. While freezing behavior during conditioning was reduced by MLA only in adults, it elicited opposite effects in adolescent and adult rats during extinction as revealed by the level of reduced and increased freezing response, respectively. We next asked whether the late-adolescent onset of α7nAChR modulation of hippocampal inputs contributes to the age-dependent effect of MLA during extinction. Data revealed that the increased freezing behavior elicited by MLA in adult rats could be driven by a dysregulation of the GluN2B transmission in the PFC. Collectively, these results indicate that distinct neural circuits are recruited during the extinction of trace fear memory in adolescents and adults, likely because of the late-adolescent maturation of the ventral hippocampal-PFC functional connectivity and its modulation by α7nAChR signaling.SIGNIFICANCE STATEMENT Abnormal elevation of the astrocyte-derived metabolite kynurenic acid in the prefrontal cortex (PFC) is thought to impair cognitive functions in schizophrenia through an α7nAChR-mediated mechanism. Here, we found that prefrontal α7nAChR signaling is recruited to control the gain of hippocampal and amygdalar afferent transmission in an input-specific, age-related manner during the adolescent transition to adulthood. Behaviorally, prefrontal α7nAChR modulation of trace fear memory was also age-related, likely because of the late-adolescent maturation of the ventral hippocampal pathway and its recruitment of PFC GABAergic transmission enabled by local α7nAChR signaling. Collectively, these results reveal that distinct α7nAChR-sensitive neural circuits contribute to regulate behavior responses in adolescents and adults, particularly those requiring proper integration of hippocampal and amygdalar inputs by the PFC.

Neural substrates underlying the negative impact of cannabinoid exposure during adolescence.

As cannabinoid use among the adolescent population becomes widespread with recent legalizations, understanding more about its effects on the developing brain becomes increasingly important. Adolescent cannabinoid use has been shown to elicit both short and long lasting effects on cortical function, in part due to its impact on maturing brain regions including the prefrontal cortex and associated inputs. This paper provides an overview of current state of knowledge on the lasting impact of repeated cannabinoid exposure on behavior and associated neural circuits in adolescents compared to other age groups. Data obtained from human and rodent literature are integrated to discuss potential neural mechanisms underpinning the enduring negative impact of cannabinoid exposure during this sensitive period of brain development.

MK-801 Exposure during Adolescence Elicits Enduring Disruption of Prefrontal E-I Balance and Its Control of Fear Extinction Behavior.

Understanding how disruption of prefrontal cortex (PFC) maturation during adolescence is crucial to reveal which neural processes could contribute to the onset of psychiatric disorders that display frontal cortical deficits. Of particular interest is the gain of GABAergic function in the PFC during adolescence and its susceptibility to the impact of transient blockade of NMDA receptor function. Here we assessed whether exposure to MK-801 during adolescence in male rats triggers a state of excitatory-inhibitory imbalance in the PFC that limits its functional capacity to regulate behavior in adulthood. Recordings from PFC brain slices revealed that MK-801 exposure during adolescence preferentially reduces the presynaptic functionality of GABAergic activity over that of excitatory synapses. As a result, an imbalance of excitatory-inhibitory synaptic activity emerges in the PFC that correlates linearly with the GABAergic deficit. Notably, the data also suggest that the diminished prefrontal GABAergic function could arise from a deficit in the recruitment of fast-spiking interneurons by excitatory inputs during adolescence. At the behavioral level, MK-801 exposure during adolescence did not disrupt the acquisition of trace fear conditioning, but markedly increased the level of freezing response during extinction testing. Infusion of the GABAA receptor-positive allosteric modulator Indiplon into the PFC before extinction testing reduced the level of freezing response in MK-801-treated rats to control levels. Collectively, the results indicate NMDA receptor signaling during adolescence enables the gain of prefrontal GABAergic function, which is required for maintaining proper excitatory-inhibitory balance in the PFC and its control of behavioral responses.SIGNIFICANCE STATEMENT A developmental disruption of prefrontal cortex maturation has been implicated in the pathophysiology of cognitive deficits in psychiatric disorders. Of particular interest is the susceptibility of the local GABAergic circuit to the impact of transient disruption of NMDA receptors. Here we found that NMDA receptor signaling is critical to enable the gain of prefrontal GABAergic transmission during adolescence for maintaining proper levels of excitatory-inhibitory balance in the PFC and its control of behavior.

Downregulation of parvalbumin expression in the prefrontal cortex during adolescence causes enduring prefrontal disinhibition in adulthood.

The expression of the calcium binding protein parvalbumin (PV) has been observed in several cortical regions during development in a temporal pattern consistent with increased afferent-dependent activity. In the prefrontal cortex (PFC), PV expression appears last and continues to substantially increase throughout adolescence, yet the significance of this increase remains unclear. Because of the expression of PV in fast-spiking GABAergic interneurons, we hypothesized that PV upregulation during adolescence is necessary to sustain the increase in GABAergic activity observed in the PFC during this period. To test this hypothesis, we utilized an RNAi strategy to directly downregulate PV levels in the PFC during adolescence and examined its impact on prefrontal GABAergic function, plasticity, and associated behaviors during adulthood. The data indicate that a mere 25% reduction of adult PV levels in the PFC was sufficient to reduce local GABAergic transmission onto pyramidal neurons, disrupt prefrontal excitatory-inhibitory balance, and alter processing of afferent information from the ventral hippocampus. Accordingly, these animals displayed an impairment in the level of extinction learning of a trace fear conditioning response, a behavioral paradigm that requires intact PFC-ventral hippocampus connectivity. These results indicate the PV upregulation observed in the PFC during adolescence is necessary for refinement of prefrontal GABAergic function, the absence of which results in immature afferent processing and a hypofunctional state. Importantly, these results suggest there is a critical window of plasticity during which PV upregulation supports the acquisition of mature GABAergic phenotype necessary to sustain adult PFC functions.

Inhibitory Control of Basolateral Amygdalar Transmission to the Prefrontal Cortex by Local Corticotrophin Type 2 Receptor.

BACKGROUND: Basolateral amygdalar projections to the prefrontal cortex play a key role in modulating behavioral responses to stress stimuli. Among the different neuromodulators known to impact basolateral amygdalar-prefrontal cortex transmission, the corticotrophin releasing factor (CRF) is of particular interest because of its role in modulating anxiety and stress-associated behaviors. While CRF type 1 receptor (CRFR1) has been involved in prefrontal cortex functioning, the participation of CRF type 2 receptor (CRFR2) in basolateral amygdalar-prefrontal cortex synaptic transmission remains unclear. METHODS: Immunofluorescence anatomical studies using rat prefrontal cortex synaptosomes devoid of postsynaptic elements were performed in rats with intra basolateral amygdalar injection of biotinylated dextran amine. In vivo microdialysis and local field potential recordings were used to measure glutamate extracellular levels and changes in long-term potentiation in prefrontal cortex induced by basolateral amygdalar stimulation in the absence or presence of CRF receptor antagonists. RESULTS: We found evidence for the presynaptic expression of CRFR2 protein and mRNA in prefrontal cortex synaptic terminals originated from basolateral amygdalar. By means of microdialysis and electrophysiological recordings in combination with an intra-prefrontal cortex infusion of the CRFR2 antagonist antisauvagine-30, we were able to determine that CRFR2 is functionally positioned to limit the strength of basolateral amygdalar transmission to the prefrontal cortex through presynaptic inhibition of glutamate release. CONCLUSIONS: Our study shows for the first time to our knowledge that CRFR2 is expressed in basolateral amygdalar afferents projecting to the prefrontal cortex and exerts an inhibitory control of prefrontal cortex responses to basolateral amygdalar inputs. Thus, changes in CRFR2 signaling are likely to disrupt the functional connectivity of the basolateral amygdalar-prefrontal cortex pathway and associated behavioral responses.

EGF Treatment Improves Motor Behavior and Cortical GABAergic Function in the R6/2 Mouse Model of Huntington's Disease.

Recent evidence indicates that disruption of epidermal growth factor (EGF) signaling by mutant huntingtin (polyQ-htt) may contribute to the onset of behavioral deficits observed in Huntington's disease (HD) through a variety of mechanisms, including cerebrovascular dysfunction. Yet, whether EGF signaling modulates the development of HD pathology and the associated behavioral impairments remain unclear. To gain insight on this issue, we used the R6/2 mouse model of HD to assess the impact of chronic EGF treatment on behavior, and cerebrovascular and cortical neuronal functions. We found that bi-weekly treatment with a low dose of EGF (300 µg/kg, i.p.) for 6 weeks was sufficient to effectively improve motor behavior in R6/2 mice and diminish mortality, compared to vehicle-treated littermates. These beneficial effects of EGF treatment were dissociated from changes in cerebrovascular leakiness, a result that was surprising given that EGF ameliorates this deficit in other neurodegenerative diseases. Rather, the beneficial effect of EGF on R6/2 mice behavior was concomitant with a marked amelioration of cortical GABAergic function. As GABAergic transmission in cortical circuits is disrupted in HD, these novel data suggest a potential mechanistic link between deficits in EGF signaling and GABAergic dysfunction in the progression of HD.

Emergence of Endocytosis-Dependent mGlu1 LTD at Nucleus Accumbens Synapses After Withdrawal From Cocaine Self-Administration.

Extended-access cocaine self-administration induces a progressive intensification of cue-induced drug craving during withdrawal termed "incubation of cocaine craving". Rats evaluated after >1 month of withdrawal (when incubation of craving is robust) display alterations in excitatory synapses onto medium spiny neurons (MSNs) of the nucleus accumbens (NAc), including elevated levels of Ca2+-permeable AMPA receptors (CP-AMPAR) and a transition from group I metabotropic glutamate receptor (mGluR) mGlu5- to mGlu1-mediated synaptic depression. It is important to further characterize the emergent form of mGlu1-mediated synaptic depression because it has been demonstrated that mGlu1 stimulation, by normalizing CP-AMPAR transmission, reduces cue-induced cocaine craving. In the present study, we conducted whole-cell patch-clamp recordings in NAc core MSNs, comparing rats that underwent >35 days of withdrawal from cocaine self-administration to control rats that had self-administered saline. Bath application of the nonselective group I mGluR agonist dihydroxyphenylglycine (DHPG) produced a transient mGlu5-mediated synaptic depression in saline controls, whereas a persistent mGlu1-mediated synaptic depression emerged in cocaine rats. This form of long-term depression (LTD) was abolished by the inclusion of dynamin inhibitory peptide (DIP) in the recording electrode, indicating that it is mediated by removal of CP-AMPARs through a dynamin-dependent endocytosis mechanism. We further showed that CP-AMPAR endocytosis is normally coupled to the PICK1-mediated insertion of Ca2+-impermeable AMPARs (CI-AMPAR). Interestingly, this coupling is not obligatory because disruption of PICK1-mediated CI-AMPAR insertion with pep2-EVKI spared mGlu1-mediated CP-AMPAR endocytosis. Collectively, these results reveal similarities but also differences from mGlu1-LTD observed in other brain regions, and further our understanding of a form of plasticity that may be targeted to reduce cue-induced craving for cocaine and methamphetamine.

Putative dopamine neurons in the ventral tegmental area enhance information coding in the prefrontal cortex.

It has been proposed that neuronal populations in the prefrontal cortex (PFC) robustly encode task-relevant information through an interplay with the ventral tegmental area (VTA). Yet, the precise computation underlying such functional interaction remains elusive. Here, we conducted simultaneous recordings of single-unit activity in PFC and VTA of rats performing a GO/NoGO task. We found that mutual information between stimuli and neural activity increases in the PFC as soon as stimuli are presented. Notably, it is the activity of putative dopamine neurons in the VTA that contributes critically to enhance information coding in the PFC. The higher the activity of these VTA neurons, the better the conditioned stimuli are encoded in the PFC.