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.

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.

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.

A spherical treadmill system to train head-fixed adult rats.

BACKGROUND: While spherical treadmills are widely used in mouse models, there are only a few experimental setups suitable for adult rats, and none of them include head-fixation. NEW METHOD: We introduce a novel spherical treadmill apparatus for head-fixed rats that allows a wide repertory of natural responses. The rat is secured to a frame and placed on a freely rotating sphere. While being head-fixed, it can walk in any direction and perform different motor tasks. COMPARISON WITH EXISTING METHODS: Instead of being air-lifted, which is acceptable for light animals, the treadmill is sustained by three spherical bearings ensuring a smooth rotation in any direction. Movement detection is accomplished using a video camera that registers a dot pattern plotted on the sphere. RESULTS: Long Evans rats were trained to perform an auditory discrimination task in a Go/No-Go (walking/not-walking) paradigm. Animals were able to successfully discriminate between a 1 kHz and a 8 kHz auditory stimulus and execute the correct response, reaching the learning criterion (80% of correct responses) in approximately 20 training sessions. CONCLUSIONS: Our system broadens the possibilities of head-fixation experiments in adult rats making them compatible with spatial navigation on a spherical treadmill.

Dopamine bioavailability in the mPFC modulates operant learning performance in rats: an experimental study with a computational interpretation.

Dopamine encodes reward and its prediction in reinforcement learning. Catechol-O-methyltransferase (COMT) activity in the medial prefrontal cortex (mPFC) has been shown to influence cognitive abilities by modifying dopamine clearance. Nevertheless, it is unknown how COMT in the mPFC influences operant learning. Systemic entacapone (50mg/kg), as well as local entacapone (3 pg) and recombinant COMT (17 µg) in the mPFC were administered to male Long Evans rats prior to training in an operant conditioning task. We found that systemic and local administration of the COMT inhibitor entacapone significantly improves learning performance. Conversely, recombinant COMT administration totally impaired learning. These data have been interpreted through a computational model where the phasic firing of dopaminergic neurons was computed by means of a temporal difference algorithm and dopamine bioavailability in the mPFC was simulated with a gating window. The duration of this window was selected to simulate the effects of inhibited or enhanced COMT activity (by entacapone or recombinant COMT respectively). The model accounts for an improved performance reproducing the entacapone effects, and a detrimental impact on learning when the clearance is increased reproducing the recombinant COMT effects. The experimental and computational results show that learning performance can be deeply influenced by COMT manipulations in the mPFC.