Functional Modifications of Acid-Sensing Ion Channels by Ligand-Gated Chloride Channels

Together, acid-sensing ion channels (ASICs) and epithelial sodium channels (ENaC) constitute the majority of voltage-independent sodium channels in mammals. ENaC is regulated by a chloride channel, the cystic fibrosis transmembrane conductance regulator (CFTR). Here we show that ASICs were reversibly inhibited by activation of GABAA receptors in murine hippocampal neurons. This inhibition of ASICs required opening of the chloride channels but occurred with both outward and inward GABAA receptor-mediated currents. Moreover, activation of the GABAA receptors modified the pharmacological features and kinetic properties of the ASIC currents, including the time course of activation, desensitization and deactivation. Modification of ASICs by open GABAA receptors was also observed in both nucleated patches and outside-out patches excised from hippocampal neurons. Interestingly, ASICs and GABAA receptors interacted to regulate synaptic plasticity in CA1 hippocampal slices. The activation of glycine receptors, which are similar to GABAA receptors, also modified ASICs in spinal neurons. We conclude that GABAA receptors and glycine receptors modify ASICs in neurons through mechanisms that require the opening of chloride channels

The Role of δ Subunit-containing γ-aminobutyric Acid Type A Receptors in Memory and Synaptic Plasticity

Background: Extrasynaptic γ-aminobutyric acid type A receptors that contain the δ subunit (δGABAA receptors) are highly expressed in the dentate gyrus (DG) of the hippocampus, where they generate a tonic conductance that regulates activity. GABAA receptor-dependent signaling regulates memory and neurogenesis in the adult DG; however, the role of δGABAA receptors in these processes is unclear. Accordingly, it was postulated that δGABAA receptors regulate memory and neurogenesis in the DG. Methods: A combination of genetic and pharmacologic techniques was employed. Memory in wild-type (WT) and δ subunit null (Gabrd–/–) mice was assessed using object-place recognition, novel object recognition, contextual discrimination, fear conditioning, fear extinction and water maze tasks. Long-term potentiation, a molecular correlate of memory, was examined using the in vitro hippocampal slice preparation. To ascertain the effects of enhanced δGABAA receptor activity, the receptor-preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP; 4 mg/kg) was applied either as a pre-treatment (2 weeks prior to testing) or an acute treatment (30 min prior to testing). Results: Gabrd–/– mice exhibited impaired object-place recognition, novel object recognition and contextual discrimination relative to WT mice. Further, Gabrd–/– mice exhibited impaired fear extinction, although fear acquisition was enhanced. Pre-treatment with THIP improved memory in WT but not Gabrd–/– mice. Consistent with these behavioural findings, neurogenesis was impaired in Gabrd–/– mice and enhanced in WT mice by pre-treatment with THIP. In contrast to the beneficial effects of pre-treatment with THIP, acute THIP impaired memory and long-term potentiation in WT mice. Conclusions: These results indicate that δGABAA receptors promote memory and neurogenesis under baseline conditions. These processes may also be enhanced by long-term activation of δGABAA receptors with selective drugs, provided that these drug are absent during testing. Further, these findings show that acute activation of δGABAA receptors impairs memory and long-term potentiation. Implications: δGABAA receptors may be a therapeutic target for the long-term treatment of memory dysfunction during aging, injury and disease. These findings also have clinical implications, as δGABAA receptors are molecular targets for therapeutic and recreational drugs. The acute amnestic effects of these compounds may be partially explained by δGABAA receptor activity.Ph

The Use of DREADDs to Deconstruct Behavior

A central goal in understanding brain function is to link specific cell populations to behavioral outputs. In recent years, the selective targeting of specific neural circuits has been made possible with the development of new experimental approaches, including chemogenetics. This technique allows for the control of molecularly-defined subsets of cells through engineered G protein-coupled receptors (GPCRs), which have the ability to activate or silence neuronal firing. Through chemogenetics, neural circuits are being linked to behavioral outputs at an unprecedented rate. Further, the coupling of chemogenetics with imaging techniques to monitor neural activity in freely-moving animals now makes it possible to deconstruct the complex whole-brain networks that are fundamental to behavioral states. In this review, we highlight a specific chemogenetic application known as DREADDs (Designer Receptors Exclusively Activated by Designer Drugs). DREADDs are used ubiquitously to modulate GPCR activity in vivo and have been widely applied in the basic sciences, particularly in the field of behavioral neuroscience. Here, we focus on the impact and utility of DREADD technology in dissecting the neural circuitry of various behaviors including memory, cognition, reward, feeding, anxiety and pain. By using DREADDs to monitor the electrophysiological, biochemical, and behavioral outputs of specific neuronal types, researchers can better understand the links between brain activity and behavior. Additionally, DREADDs are useful in studying the pathogenesis of disease and may ultimately have therapeutic potential

Comparative density of CCK- and PV-GABA cells within the cortex and hippocampus

Cholecystokinin (CCK)- and parvalbumin (PV)-expressing neurons constitute the two major populations of perisomatic GABAergic neurons in the cortex and the hippocampus. As CCK- and PV-GABA neurons differ in an array of morphological, biochemical and electrophysiological features, it has been proposed that they form distinct inhibitory ensembles which differentially contribute to network oscillations and behaviour. However, the relationship and balance between CCK- and PV-GABA neurons in the inhibitory networks of the brain is currently unclear as the distribution of these cells has never been compared on a large scale. Here, we systemically investigated the distribution of CCK- and PV-GABA cells across a wide number of discrete forebrain regions using an intersectional genetic approach. Our analysis revealed several novel trends in the distribution of these cells. While PV-GABA cells were more abundant overall, CCK-GABA cells outnumbered PV-GABA cells in several subregions of the hippocampus, medial prefrontal cortex and ventrolateral temporal cortex. Interestingly, CCK-GABA cells were relatively more abundant in secondary/association areas of the cortex (V2, S2, M2, and AudD/AudV) than they were in corresponding primary areas (V1, S1, M1 and Aud1). The reverse trend was observed for PV-GABA cells. Our findings suggest that the balance between CCK- and PV-GABA cells in a given cortical region is related to the type of processing that area performs; inhibitory networks in the secondary cortex tend to favour the inclusion of CCK-GABA cells more than networks in the primary cortex. The intersectional genetic labelling approach employed in the current study expands upon the ability to study molecularly defined subsets of GABAergic neurons. This technique can be applied to the investigation of neuropathologies which involve disruptions to the GABAergic system, including schizophrenia, stress, maternal immune activation and autism

Acutely increasing δGABAA receptor activity impairs memory and inhibits synaptic plasticity in the hippocampus

Extrasynaptic γ-aminobutyric acid type A (GABAA) receptors that contain the δ subunit (δGABAA receptors) are expressed in several brain regions including the dentate gyrus (DG) and CA1 subfields of the hippocampus. Drugs that increase δGABAA receptor activity have been proposed as treatments for a variety of disorders including insomnia, epilepsy and chronic pain. Also, long-term pretreatment with the δGABAA receptor–preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) enhances discrimination memory and increases neurogenesis in the DG. Despite the potential therapeutic benefits of such treatments, the effects of acutely increasing δGABAA receptor activity on memory behaviors remain unknown. Here, we studied the effects of THIP (4 mg/kg, i.p.) on memory performance in wild-type (WT) and δGABAA receptor null mutant (Gabrd–/–) mice. Additionally, the effects of THIP on long-term potentiation (LTP), a molecular correlate of memory, were studied within the DG and CA1 subfields of the hippocampus using electrophysiological recordings of field potentials in hippocampal slices. The results showed that THIP impaired performance in the Morris water maze, contextual fear conditioning and object recognition tasks in WT mice but not Gabrd–/– mice. Furthermore, THIP inhibited LTP in hippocampal slices from WT but not Gabrd–/– mice, an effect that was blocked by GABAA receptor antagonist bicuculline. Thus, acutely increasing δGABAA receptor activity impairs memory behaviors and inhibits synaptic plasticity. These results have important implications for the development of therapies aimed at increasing δGABAA receptor activity