AMPAR signaling mediating GABA(A)R delta subunit up-regulation in cultured mouse cerebellar granule cells

Integration and regulation of glomerular inhibition in the cerebellar granular layer circuit

Inhibitory synapses can be organized in different ways and be regulated by a multitude of mechanisms. One of the best known examples is provided by the inhibitory synapses formed by Golgi cells onto granule cells in the cerebellar glomeruli. These synapses are GABAergic and inhibit granule cells through two main mechanisms, phasic and tonic. The former is based on vesicular neurotransmitter release, the latter on the establishment of tonic γ-aminobutyric acid (GABA) levels determined by spillover and regulation of GABA uptake. The mechanisms of post-synaptic integration have been clarified to a considerable extent and have been shown to differentially involve α1 and α6 subunit-containing GABA-A receptors. Here, after reviewing the basic mechanisms of GABAergic transmission in the cerebellar glomeruli, we examine how inhibition controls signal transfer at the mossy fiber-granule cell relay. First of all, we consider how vesicular release impacts on signal timing and how tonic GABA levels control neurotransmission gain. Then, we analyze the integration of these inhibitory mechanisms within the granular layer network. Interestingly, it turns out that glomerular inhibition is just one element in a large integrated signaling system controlled at various levels by metabotropic receptors. GABA-B receptor activation by ambient GABA regulates glutamate release from mossy fibers through a pre-synaptic cross-talk mechanisms, GABA release through pre-synaptic auto-receptors, and granule cell input resistance through post-synaptic receptor activation and inhibition of a K inward-rectifier current. Metabotropic glutamate receptors (mGluRs) control GABA release from Golgi cell terminals and Golgi cell input resistance and autorhythmic firing. This complex set of mechanisms implements both homeostatic and winner-take-all processes, providing the basis for fine-tuning inhibitory neurotransmission and for optimizing signal transfer through the cerebellar cortex.

Metabotropic regulation of extrasynaptic GABAA receptors

A large body of work now shows the importance of GABA_A receptor-mediated tonic inhibition in regulating CNS function. However, outside of pathological conditions, there is relatively little evidence that the magnitude of tonic inhibition is itself under regulation. Here we review the mechanisms by which tonic inhibition is known to be modulated, and outline the potential behavioral consequences of this modulation. Specifically, we address the ability of protein kinase A and C to phosphorylate the extrasynaptic receptors responsible for the tonic GABA_A current, and how G-protein coupled receptors can regulate tonic inhibition through these effectors. We then speculate about the possible functional consequences of regulating the magnitude of the tonic GABA_A current.

Regulation of neuronal activation by Alpha2A adrenergic receptor agonist

Stress factors induce neuronal activation in brain areas that are related to anxiety and fear. High doses of caffeine induce neuronal activation with Ca2+ influx followed by expression of the immediate early gene c-fos. In the present study, we investigated c-Fos protein expression in stress-responsive brain areas induced by caffeine, as well as the role of alpha2A receptor in the regulation of neuronal activation. Immunohistochemical analysis showed that an acute effect of caffeine induced c-Fos protein expression in the hippocampus, the bed nucleus of stria terminalis (BNST), the lateral septum, the basolateral and central amygdala, the paraventricular hypothalamic nucleus (PVN), the locus coeruleus, and the lateral parabrachial nucleus (LPBN). However, c-Fos expression was attenuated after repeated treatment of caffeine, spaced 24 h apart, compared to a single acute effect. Alpha2A receptor activation with the agonist guanfacine attenuated the acute effect of caffeine in terms of c-Fos expression in neurons in the CA1-CA3 areas of hippocampus, the locus coeruleus and the LPBN as compared with effect of caffeine alone, whereas the number of c-Fos expressing neurons increased in the lateral septum, the dorsal BNST, the central amygdala, and the PVN, areas that are densely innervated by noradrenergic neurons. Guanfacine alone induced c-Fos protein expression in neurons in the central amygdala, the dorsal BNST, the PVN, the LPBN, and the caudal nucleus of the solitary tract. Guanfacine alone also induced phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in neurons expressing c-Fos in the dorsal BNST, the central amygdala, and the LPBN. These results suggest that alpha2A receptor activation modulates synaptic transmission in neuronal circuits that are correlated with stress in vivo.