Environmental Enrichment Rescues Endocannabinoid-Dependent Synaptic Plasticity Lost in Young Adult Male Mice after Ethanol Exposure during Adolescence

Binge drinking (BD) is a serious health concern in adolescents as high ethanol (EtOH) consumption can have cognitive sequelae later in life. Remarkably, an enriched environment (EE) in adulthood significantly recovers memory in mice after adolescent BD, and the endocannabinoid, 2-arachydonoyl-glycerol (2-AG), rescues synaptic plasticity and memory impaired in adult rodents upon adolescent EtOH intake. However, the mechanisms by which EE improves memory are unknown. We investigated this in adolescent male C57BL/6J mice exposed to a drinking in the dark (DID) procedure four days per week for a duration of 4 weeks. After DID, the mice were nurtured under an EE for 2 weeks and were subjected to the Barnes Maze Test performed the last 5 days of withdrawal. The EE rescued memory and restored the EtOH-disrupted endocannabinoid (eCB)-dependent excitatory long-term depression at the dentate medial perforant path synapses (MPP-LTD). This recovery was dependent on both the cannabinoid CB1 receptor and group I metabotropic glutamate receptors (mGluRs) and required 2-AG. Also, the EE had a positive effect on mice exposed to water through the transient receptor potential vanilloid 1 (TRPV1) and anandamide (AEA)-dependent MPP long-term potentiation (MPP-LTP). Taken together, EE positively impacts different forms of excitatory synaptic plasticity in water- and EtOH-exposed brains.

Cognitive and neurobehavioral benefits of an enriched environment on young adult mice after chronic ethanol consumption during adolescence

Binge drinking (BD) is a common pattern of ethanol (EtOH) consumption by adolescents. The brain effects of the acute EtOH exposure are well-studied; however, the long-lasting cognitive and neurobehavioral consequences of BD during adolescence are only beginning to be elucidated. Environmental enrichment (EE) has long been known for its benefits on the brain and may serve as a potential supportive therapy following EtOH exposure. In this study, we hypothesized that EE may have potential benefits on the cognitive deficits associated with BD EtOH consumption. Four-week-old C57BL/6J male mice were exposed to EtOH following an intermittent 4-day drinking-in-the-dark procedure for 4 weeks. Then they were exposed to EE during EtOH withdrawal for 2 weeks followed by a behavioral battery of tests including novel object recognition, novel location, object-in-place, rotarod, beam walking balance, tail suspension, light-dark box and open field that were run during early adulthood. Young adult mice exposed to EE significantly recovered recognition, spatial and associative memory as well as motor coordination skills and balance that were significantly impaired after adolescent EtOH drinking with respect to controls. No significant permanent anxiety or depressive-like behaviors were observed. Taken together, an EE exerts positive effects on the long-term negative cognitive deficits as a result of EtOH consumption during adolescence.

Localization of the cannabinoid type-1 receptor in subcellular astrocyte compartments of mutant mouse hippocampus

Astroglial type-1 cannabinoid (CB₁ ) receptors are involved in synaptic transmission, plasticity and behavior by interfering with the so-called tripartite synapse formed by pre- and post-synaptic neuronal elements and surrounding astrocyte processes. However, little is known concerning the subcellular distribution of astroglial CB₁ receptors. In particular, brain CB₁ receptors are mostly localized at cells' plasmalemma, but recent evidence indicates their functional presence in mitochondrial membranes. Whether CB₁ receptors are present in astroglial mitochondria has remained unknown. To investigate this issue, we included conditional knock-out mice lacking astroglial CB₁ receptor expression specifically in glial fibrillary acidic protein (GFAP)-containing astrocytes (GFAP-CB₁ -KO mice) and also generated genetic rescue mice to re-express CB₁ receptors exclusively in astrocytes (GFAP-CB₁ -RS). To better identify astroglial structures by immunoelectron microscopy, global CB₁ knock-out (CB₁ -KO) mice and wild-type (CB₁ -WT) littermates were intra-hippocampally injected with an adeno-associated virus expressing humanized renilla green fluorescent protein (hrGFP) under the control of human GFAP promoter to generate GFAPhrGFP-CB₁ -KO and -WT mice, respectively. Furthermore, double immunogold (for CB₁ ) and immunoperoxidase (for GFAP or hrGFP) revealed that CB₁ receptors are present in astroglial mitochondria from different hippocampal regions of CB₁ -WT, GFAP-CB₁ -RS and GFAPhrGFP-CB₁ -WT mice. Only non-specific gold particles were detected in mouse hippocampi lacking CB₁ receptors. Altogether, we demonstrated the existence of a precise molecular architecture of the CB₁ receptor in astrocytes that will have to be taken into account in evaluating the functional activity of cannabinergic signaling at the tripartite synapse.

Cannabinoid CB1 Receptors Are Localized in Striated Muscle Mitochondria and Regulate Mitochondrial Respiration

The cannabinoid type 1 (CB₁) receptor is widely distributed in the brain and peripheral organs where it regulates cellular functions and metabolism. In the brain, CB₁ is mainly localized on presynaptic axon terminals but is also found on mitochondria (mtCB₁), where it regulates cellular respiration and energy production. Likewise, CB₁ is localized on muscle mitochondria, but very little is known about it. The aim of this study was to further investigate in detail the distribution and functional role of mtCB₁ in three different striated muscles. Immunoelectron microscopy for CB₁ was used in skeletal muscles (gastrocnemius and rectus abdominis) and myocardium from wild-type and CB₁-KO mice. Functional assessments were performed in mitochondria purified from the heart of the mice and the mitochondrial oxygen consumption upon application of different acute delta-9-tetrahydrocannabinol (Δ⁹-THC) concentrations (100 nM or 200 nM) was monitored. About 26% of the mitochondrial profiles in gastrocnemius, 22% in the rectus abdominis and 17% in the myocardium expressed CB₁. Furthermore, the proportion of mtCB1 versus total CB₁ immunoparticles was about 60% in the gastrocnemius, 55% in the rectus abdominis and 78% in the myocardium. Importantly, the CB₁ immunolabeling pattern disappeared in muscles of CB₁-KO mice. Functionally, acute 100 nM or 200 nM THC treatment specifically decreased mitochondria coupled respiration between 12 and 15% in wild-type isolated mitochondria of myocardial muscles but no significant difference was noticed between THC treated and vehicle in mitochondria isolated from CB₁-KO heart. Furthermore, gene expression of key enzymes involved in pyruvate synthesis, tricarboxylic acid (TCA) cycle and mitochondrial respiratory chain was evaluated in the striated muscle of CB₁-WT and CB₁-KO. CB₁-KO showed an increase in the gene expression of Eno3, Pkm2, and Pdha1, suggesting an increased production of pyruvate. In contrast, no significant difference was observed in the Sdha and Cox4i1 expression, between CB₁-WT and CB₁-KO. In conclusion, CB₁ receptors in skeletal and myocardial muscles are predominantly localized in mitochondria. The activation of mtCB₁ receptors may participate in the mitochondrial regulation of the oxidative activity probably through the relevant enzymes implicated in the pyruvate metabolism, a main substrate for TCA activity.

Astroglial CB1 cannabinoid receptors regulate leptin signaling in mouse brain astrocytes

Type-1 cannabinoid (CB1) and leptin (ObR) receptors regulate metabolic and astroglial functions, but the potential links between the two systems in astrocytes were not investigated so far. Genetic and pharmacological manipulations of CB1 receptor expression and activity in cultured cortical and hypothalamic astrocytes demonstrated that cannabinoid signaling controls the levels of ObR expression. Lack of CB1 receptors also markedly impaired leptin-mediated activation of signal transducers and activators of transcription 3 and 5 (STAT3 and STAT5) in astrocytes. In particular, CB1 deletion determined a basal overactivation of STAT5, thereby leading to the downregulation of ObR expression, and leptin failed to regulate STAT5-dependent glycogen storage in the absence of CB1 receptors. These results show that CB1 receptors directly interfere with leptin signaling and its ability to regulate glycogen storage, thereby representing a novel mechanism linking endocannabinoid and leptin signaling in the regulation of brain energy storage and neuronal functions.

GABA release by hippocampal astrocytes

Astrocytes can directly influence neuronal activity through the release of various transmitters acting on membrane receptors expressed by neurons. However, in contrast to glutamate and ATP for instance, the release of GABA (γ-amino-butyric acid) by astrocytes is still poorly documented. Here, we used whole-cell recordings in rat acute brain slices and electron microscopy to test whether hippocampal astrocytes release the inhibitory transmitter GABA. We observed that slow transient inhibitory currents due to the activation of GABA_A receptors occur spontaneously in principal neurons of the three main hippocampal fields (CA1, CA3, and dentate gyrus). These currents share characteristics with the slow NMDA receptor-mediated currents previously shown to result from astrocytic glutamate release: they occur in the absence of synaptic transmission and have variable kinetics and amplitudes as well as low frequencies. Osmotic pressure reduction, known to enhance transmitter release from astrocytes, similarly increased the frequency of non-synaptic GABA and glutamate currents. Simultaneous occurrence of slow inhibitory and excitatory currents was extremely rare. Yet, electron microscopy examination of immunostained hippocampal sections shows that about 80% of hippocampal astrocytes [positive for glial fibrillary acidic protein (GFAP)] were immunostained for GABA. Our results provide quantitative characteristics of the astrocyte-to-neuron GABAergic signaling. They also suggest that all principal neurons of the hippocampal network are under a dual, excitatory and inhibitory, influence of astrocytes. The relevance of the astrocytic release of GABA, and glutamate, on the physiopathology of the hippocampus remains to be established.

Acute cannabinoids impair working memory through astroglial CB1 receptor modulation of hippocampal LTD

Impairment of working memory is one of the most important deleterious effects of marijuana intoxication in humans, but its underlying mechanisms are presently unknown. Here, we demonstrate that the impairment of spatial working memory (SWM) and in vivo long-term depression (LTD) of synaptic strength at hippocampal CA3-CA1 synapses, induced by an acute exposure of exogenous cannabinoids, is fully abolished in conditional mutant mice lacking type-1 cannabinoid receptors (CB(1)R) in brain astroglial cells but is conserved in mice lacking CB(1)R in glutamatergic or GABAergic neurons. Blockade of neuronal glutamate N-methyl-D-aspartate receptors (NMDAR) and of synaptic trafficking of glutamate α-amino-3-hydroxy-5-methyl-isoxazole propionic acid receptors (AMPAR) also abolishes cannabinoid effects on SWM and LTD induction and expression. We conclude that the impairment of working memory by marijuana and cannabinoids is due to the activation of astroglial CB(1)R and is associated with astroglia-dependent hippocampal LTD in vivo.

GABAergic and cortical and subcortical glutamatergic axon terminals contain CB1 cannabinoid receptors in the ventromedial nucleus of the hypothalamus

BACKGROUND

Type-1 cannabinoid receptors (CB(1)R) are enriched in the hypothalamus, particularly in the ventromedial hypothalamic nucleus (VMH) that participates in homeostatic and behavioral functions including food intake. Although CB(1)R activation modulates excitatory and inhibitory synaptic transmission in the brain, CB(1)R contribution to the molecular architecture of the excitatory and inhibitory synaptic terminals in the VMH is not known. Therefore, the aim of this study was to investigate the precise subcellular distribution of CB(1)R in the VMH to better understand the modulation exerted by the endocannabinoid system on the complex brain circuitries converging into this nucleus.

METHODOLOGY/PRINCIPAL FINDINGS

Light and electron microscopy techniques were used to analyze CB(1)R distribution in the VMH of CB(1)R-WT, CB(1)R-KO and conditional mutant mice bearing a selective deletion of CB(1)R in cortical glutamatergic (Glu-CB(1)R-KO) or GABAergic neurons (GABA-CB(1)R-KO). At light microscopy, CB(1)R immunolabeling was observed in the VMH of CB(1)R-WT and Glu-CB(1)R-KO animals, being remarkably reduced in GABA-CB(1)R-KO mice. In the electron microscope, CB(1)R appeared in membranes of both glutamatergic and GABAergic terminals/preterminals. There was no significant difference in the percentage of CB(1)R immunopositive profiles and CB(1)R density in terminals making asymmetric or symmetric synapses in CB(1)R-WT mice. Furthermore, the proportion of CB(1)R immunopositive terminals/preterminals in CB(1)R-WT and Glu-CB(1)R-KO mice was reduced in GABA-CB(1)R-KO mutants. CB(1)R density was similar in all animal conditions. Finally, the percentage of CB(1)R labeled boutons making asymmetric synapses slightly decreased in Glu-CB(1)R-KO mutants relative to CB(1)R-WT mice, indicating that CB(1)R was distributed in cortical and subcortical excitatory synaptic terminals.

CONCLUSIONS/SIGNIFICANCE

Our anatomical results support the idea that the VMH is a relevant hub candidate in the endocannabinoid-mediated modulation of the excitatory and inhibitory neurotransmission of cortical and subcortical pathways regulating essential hypothalamic functions for the individual's survival such as the feeding behavior.

Precise localization of the voltage-gated potassium channel subunits Kv3.1b and Kv3.3 revealed in the molecular layer of the rat cerebellar cortex by a pre-embedding immunogold method

A proper motor activity relies on a correct cerebellar function. The Kv3.1 and Kv3.3 voltage-gated potassium channels are key proteins involved in cerebellar function and dysfunction, as the lack of these causes severe motor deficits. Both channel subunits are coexpressed in granule cells and are rapidly activated at relatively positive potentials to support the generation of fast action potentials. However, the contribution of each subunit to the molecular architecture of the parallel fibers, the granule cell axons, is so far unknown. The goal of this study was to elucidate the relative distribution of Kv3.1b and Kv3.3 in specific compartments of the rat parallel fibers by using a pre-embedding immunocytochemical method for electron microscopy. Numerous Kv3.1b and Kv3.3 silver-intensified gold particles were associated with membranes of parallel fiber synaptic terminals and their intervaricose segments. Kv3.1b was found in about 85% of parallel fiber synaptic terminals and in about 47% of their intervaricose portions. However, only 28% of intervaricosities and 23% of parallel fiber presynaptic boutons were Kv3.3 immunopositive. The analysis also revealed that 54% of Purkinje cell dendritic spines localized Kv3.3. Although both potassium channel subunits share localization in the same presynaptic parallel fiber compartments, the present results with the method used indicate that there are a higher percentage of parallel fibers labeled for Kv3.1b than for Kv3.3, and that the labeling intensity for each subunit is higher in specific subcompartments analyzed than in others.

Expression of calcium-binding proteins in the proliferative zones around the corticostriatal junction of rabbits during pre- and postnatal development

Herein we asked whether cells expressing calcium-binding proteins around the corticostriatal junction are of pallial or subpallial origin. Brains of rabbit embryos between embryonic day E18 and E28 and postnatal day 0-P22 were immunoreacted with monoclonal antibodies raised against calretinin, calbindin and parvalbumin. At E18-E21, calbindin- and calretinin-immunoreactive cells were seen in distinct proliferative zones in the vicinity of the corticostriatal junction. Whereas calbindin-immunoreactive neurons were in the ventricular zone of the ventral pallium (the medial wall of the lateral ventricular angle), calretinin-immunoreactive cells were, nearby, in the subventricular zone of the subpallium at the lateral edge of the lateral ganglionic eminence. From E25 to P22, both calbindin- and calretinin-immunoreactive cells appeared in the pallial ventricular and subventricular zones around the lateral ventricular angle. Some of these cells resembled migratory neuroblasts. Parvalbumin-immunoreactive cells appeared at P5-P10, albeit they were almost negligible in the proliferative zones around the corticostriatal junction and the lateral ventricular angle. The results suggest that a number of the calbindin-expressing neurons that are generated in mid-gestation and postnatally are of pallial origin. They also indicate that only a few of the late-generated calretinin-immunoreactive cells may have a pallial source. The origin of the parvalbumin-immunoreactive cells was not ascertained in the present study.

Neuroepithelial origin of the insular and endopiriform parts of the claustrum

The lateral and ventral pallia have been proposed as the source of neurons for the insular and endopiriform claustra, respectively. However, this correlation is controversial. Here, we analysed this relationship by labelling radial glia in coronal slices of the telencephalon of paraformaldehyde-fixed rabbit embryos (E18-E28) and newborn rabbits with an anti-vimentin antibody or with the fluorescent dye DiI. The radial glia that crossed the claustrum was anchored to the neuroepithelium of the lateral ventricular angle (LVA) at all ages studied. The LVA was deep at E18, but it subsequently become shallower, because of the apposition of the portion of its walls proximal to the vertex of the LVA. At E18, the radial glia that crossed most of the insular claustrum extended from the lateral wall of the LVA (presumptive lateral pallium), and the radial glia that crossed either the most ventral part of the insular claustrum or the endopiriform claustrum proceeded from the medial wall of the LVA (presumptive ventral pallium). These results suggest that although the endopiriform claustrum originates from the ventral pallium, the insular claustrum originates from both the lateral and the ventral pallial portions.