High-affinity extrasynaptic GABA A receptors (GABA A Rs) are a prominent feature of cerebellar granule neurons and thalamic relay neurons. In both cell types, the presence of synaptic glomeruli would be expected to promote activation of these GABA A Rs, contributing to phasic spillover-mediated currents and tonic inhibition. However, the precise role of different receptor subtypes in these two phenomena is unclear. To address this question, we made recordings from neurons in acute brain slices from mice, and from tsA201 cells expressing recombinant GABA A Rs. We found that ␦ subunit-containing GABA A Rs of both cerebellar granule neurons and thalamic relay neurons of the lateral geniculate nucleus contributed to tonic conductance caused by ambient GABA but not to spillover-mediated currents. In the presence of a low "ambient" GABA concentration, recombinant "extrasynaptic" ␦ subunit-containing GABA A Rs exhibited profound desensitization, rendering them insensitive to brief synaptic-or spillover-like GABA transients. Together, our results demonstrate that phasic spillover and tonic inhibition reflect the activation of distinct receptor populations.

AMPA-type glutamate receptors are ligand-gated cation channels responsible for a majority of the fast excitatory synaptic transmission in the brain. Their behavior and calcium permeability depends critically on their subunit composition and the identity of associated auxiliary proteins. Calcium-permeable AMPA receptors (CP-AMPARs) contribute to various forms of synaptic plasticity, and their dysfunction underlies a number of serious neurological conditions. For CP-AMPARs, the prototypical transmembrane AMPAR regulatory protein stargazin, which acts as an auxiliary subunit, enhances receptor function by increasing single-channel conductance, slowing channel gating, increasing calcium permeability, and relieving the voltage-dependent block by endogenous intracellular polyamines. We find that, in contrast, GSG1L, a transmembrane auxiliary protein identified recently as being part of the AMPAR proteome, acts to reduce the weighted mean single-channel conductance and calcium permeability of recombinant CP-AMPARs, while increasing polyaminedependent rectification. To examine the effects of GSG1L on native AMPARs, we manipulated its expression in cerebellar and hippocampal neurons. Transfection of GSG1L into mouse cultured cerebellar stellate cells that lack this protein increased the inward rectification of mEPSCs. Conversely, shRNA-mediated knockdown of endogenous GSG1L in rat cultured hippocampal pyramidal neurons led to an increase in mEPSC amplitude and in the underlying weighted mean single-channel conductance, revealing that GSG1L acts to suppress current flow through native CP-AMPARs. Thus, our data suggest that GSG1L extends the functional repertoire of AMPAR auxiliary subunits, which can act not only to enhance but also diminish current flow through their associated AMPARs.

SummaryInhibitory synaptic transmission requires the targeting and stabilization of GABAA receptors (GABAARs) at synapses. The mechanisms responsible remain poorly understood, and roles for transmembrane accessory proteins have not been established. Using molecular, imaging, and electrophysiological approaches, we identify the tetraspanin LHFPL4 as a critical regulator of postsynaptic GABAAR clustering in hippocampal pyramidal neurons. LHFPL4 interacts tightly with GABAAR subunits and is selectively enriched at inhibitory synapses. In LHFPL4 knockout mice, there is a dramatic cell-type-specific reduction in GABAAR and gephyrin clusters and an accumulation of large intracellular gephyrin aggregates in vivo. While GABAARs are still trafficked to the neuronal surface in pyramidal neurons, they are no longer localized at synapses, resulting in a profound loss of fast inhibitory postsynaptic currents. Hippocampal interneuron currents remain unaffected. Our results establish LHFPL4 as a synapse-specific tetraspanin essential for inhibitory synapse function and provide fresh insights into the molecular make-up of inhibitory synapses.

High-affinity extrasynaptic GABA A receptors are persistently activated by the low ambient GABA levels that are known to be present in extracellular space. The resulting tonic conductance generates a form of shunting inhibition that is capable of altering cellular and network behavior. It has been suggested that this tonic inhibition will be enhanced by neurosteroids, antiepileptics, and sedative/ hypnotic drugs. However, we show that the ability of sedative/hypnotic drugs to enhance tonic inhibition in the mouse cerebellum will critically depend on ambient GABA levels. For example, we show that the intravenous anesthetic propofol enhances tonic inhibition only when ambient GABA levels are Ͻ100 nM. More surprisingly, the actions of the sleep-promoting drug 4,5,6,7-tetrahydroisothiazolo-[5,4-c]pyridin-3-ol (THIP) are attenuated at ambient GABA levels of just 20 nM. In contrast, our data suggest that neurosteroid enhancement of tonic inhibition will be greater at high ambient GABA concentrations. We present a model that takes into account realistic estimates of ambient GABA levels and predicted extrasynaptic GABA A receptor numbers when considering the ability of sedative/hypnotic drugs to enhance tonic inhibition. These issues will be important when considering drug strategies designed to target extrasynaptic GABA A receptors in the treatment of sleep disorders and other neurological conditions.

Understanding the neurobiology of the transition from initial drug use to excessive drug use has been a challenge in drug addiction. We examined the effect of chronic ‘binge’ escalating dose cocaine administration, which mimics human compulsive drug use, on behavioural responses and the dopaminergic system of mice and compared it with a chronic steady dose (3 × 15 mg/kg/day) ‘binge’ cocaine administration paradigm. Male C57BL/6J mice were injected with saline or cocaine in an escalating dose paradigm for 14 days. Locomotor and stereotypy activity were measured and quantitative autoradiographic mapping of D1 and D2 receptors, dopamine transporters and D2‐stimulated [35S]GTPγS binding was performed in the brains of mice treated with this escalating and steady dose paradigm. An initial sensitization to the locomotor effects of cocaine followed by a dose‐dependent increase in the duration of the locomotor effect of cocaine was observed in the escalating but not the steady dose paradigm. Sensitization to the stereotypy effect of cocaine and an increase in cocaine‐induced stereotypy score was observed from 3 × 20 to 3 × 25 mg/kg/day cocaine. There was a significant decrease in D2 receptor density, but an increase in D2‐stimulated G‐protein activity and dopamine transporter density in the striatum of cocaine‐treated mice, which was not observed in our steady dose paradigm. Our results document that chronic ‘binge’ escalating dose cocaine treatment triggers profound behavioural and neurochemical changes in the dopaminergic system, which might underlie the transition from drug use to compulsive drug use associated with addiction, which is a process of escalation.

H Hu um ma an n a al lv ve eo ol la ar r c ca ap pi il ll la ar ri ie es s u un nd de er rg go o a an ng gi io og ge en ne es si is s i in n p pu ul lm mo on na ar ry y v ve en no o--o oc cc cl lu us si iv ve e d di is se ea as se e ABSTRACT: The bronchial circulation undergoes angiogenesis in several pathological conditions, such as lung neoplasm and bronchiectasis, but whether the pulmonary circulation can do this has been questioned. A woman treated with mitomycin C and 5-fluorouracil developed progressive, fatal pulmonary hypertension over 5 months. In addition to light and transmission electron microscopic examination of her lung, her pulmonary vasculature was cast and the casts were studied with scanning electron microscopy.Light microscopy showed that she had pulmonary veno-occlusive disease and angiomatoid capillary growth in the alveolar walls. Transmission electron microscopy confirmed the presence of pulmonary hypertension and showed thickened endothelial basement membrane. Scanning electron microscopy of the cast blood vessels showed distortion and destruction of alveolar capillaries prohibiting the passage of erythrocytes. Large new capillaries developed on top of, and were connected to, the shrivelled capillaries that made up the alveolar wall. The new capillaries were larger and fewer, which reduced the alveolar-capillary interface. Arteries and veins were irregularly narrowed and the veins had broad muscularity. Oedema was present, and the pulmonary lymphatics were extensively cast, especially in the lobular septa, but the lymphatics had a normal appearance.It appears that this patient suffered extensive capillary damage and venous occlusion and that the response was extensive new capillary formation, sometimes in angiomatoid configurations, and hypertrophy of pulmonary veins and arteries. Casting the microvasculature and viewing it with scanning electron microscopy identified new alveolar capillaries in this patient with acquired pulmonary hypertension.

Inhibition of GABA A receptors by Cu 2ϩ has been appreciated for some time, but differences between synaptic and extrasynaptic GABA A receptors have not been explored. We show that Cu 2ϩ potently blocks steady-state GABA currents mediated by extrasynaptic ␦ subunitcontaining GABA A receptors (␦-GABA A Rs) with an IC 50 of 65 nM. This compares with an IC 50 of 85 M for synaptic ␥ subunit-containing GABA A Rs (␥-GABA A Rs). To test the significance of this subunit selectivity, we examined the blocking action of Cu 2ϩ on neurons of the mouse cerebellum and striatum, brain regions that are known to express both types of receptor. Cu 2ϩ was shown to significantly reduce tonic inhibition mediated by extrasynaptic ␦-GABA A Rs with little action on phasic inhibition mediated by conventional synaptic ␥-GABA A Rs. We speculate on the implications of these observations for conditions, such as Wilson's disease, that can involve raised Cu 2ϩ levels in the brain.

Most central nervous system aneurysms occur around the circle of Willis, and are congenital or arteriosclerotic in origin when in that location. Peripherally located aneurysms are either idiopathic or secondary to infection, tumor embolus (from choriocarcinoma and cardiac myxoma), Moyamoya disease, or trauma. The pathophysiologic features of these aneurysms are discussed.