Recruitment of functional GABA(A) receptors to postsynaptic domains by insulin

GABA(A) receptor cell surface number and subunit stability are regulated by the ubiquitin-like protein Plic-1

Controlling the number of functional gamma-aminobutyric acid A (GABA(A)) receptors in neuronal membranes is a crucial factor for the efficacy of inhibitory neurotransmission. Here we describe the direct interaction of GABA(A) receptors with the ubiquitin-like protein Plic-1. Furthermore, Plic-1 is enriched at inhibitory synapses and is associated with subsynaptic membranes. Functionally, Plic-1 facilitates GABA(A) receptor cell surface expression without affecting the rate of receptor internalization. Plic-1 also enhances the stability of intracellular GABA(A) receptor subunits, increasing the number of receptors available for insertion into the plasma membrane. Our study identifies a previously unknown role for Plic-1, a modulation of GABA(A) receptor cell surface number, which suggests that Plic-1 facilitates accumulation of these receptors in dendritic membranes.

Dopamine D1 receptor-dependent trafficking of striatal NMDA glutamate receptors to the postsynaptic membrane

Recent work has shown substantial alterations in NMDA receptor subunit expression, assembly, and phosphorylation in the dopamine-depleted striatum of a rodent 6-hydroxydopamine model of Parkinson's disease. These modifications are hypothesized to result from the trafficking of NMDA receptors between subcellular compartments. Here we show that in rat striatal tissues the NR2A and NR2B subunits in the synaptosomal membrane, and not those in the light membrane and synaptic vesicle-enriched compartments, are tyrosine phosphorylated. The dopamine D1 receptor agonist SKF-82958 produces (1) an increase in NR1, NR2A, and NR2B proteins in the synaptosomal membrane fraction; (2) a decrease in NR1, NR2A, and NR2B proteins in the light membrane and synaptic vesicle-enriched fractions; and (3) an increase in the tyrosine phosphorylation of NR2A and NR2B in the synaptosomal membrane compartment. The protein phosphatase inhibitor pervanadate reproduces the alterations in subcellular distribution and phosphorylation, whereas the effects of the dopamine D1 receptor agonist are blocked by genistein, a protein tyrosine kinase inhibitor. Dopamine D1 receptor agonist treatment does not change the subcellular distribution of the AMPA receptor subunits GluR1 or GluR2/3 in the striatum and has no effect on cortical or cerebellar NMDA receptor subunits. These data reveal a rapid dopamine D1 receptor- and tyrosine kinase-dependent trafficking of striatal NMDA receptors between intracellular and postsynaptic sites. The subcellular trafficking of striatal NMDA receptors may play a significant role both in the pathogenesis of Parkinson's disease and in the development of adverse effects of chronic dopaminergic therapy in parkinsonian patients.

Strain-dependent differences in calcium signaling predict excitotoxicity in murine hippocampal neurons

Commonly used inbred murine strains differ substantially in their vulnerability to excitotoxic insults. We investigated whether differences in dendritic Ca(2+) signaling could underlie the differential vulnerability of C57BL/6 (resistant to kainate excitotoxicity) and C57BL/10 strains (vulnerable). A striking difference was found in fine dendrite Ca(2+) responses after kainate exposure. Ca(2+) signals in distal dendrites were large in C57BL/10 neurons, and, if a threshold concentration of approximately 1.5 microm was reached, a region of sustained high Ca(2+) was established in the distal dendritic tree. This region then served as an initiation site for a degenerative cascade, producing high Ca(2+) levels that slowly spread to involve the entire neuron and led to cell death. Dendritic Ca(2+) signals in C57BL/6 neurons were much smaller and did not trigger these propagating secondary responses. Strain differences in dendritic Ca(2+) signaling were also evident after tetanic stimulation of Schaffer collaterals. Ca(2+) responses were much larger and peaked earlier in distal dendrites of C57BL/10 compared with those in C57BL/6. Neurons from both strains had similar membrane properties and responded to kainate with intense action potential firing. Degenerative Ca(2+) responses were seen in both strains if soma Ca(2+) could be sustained above 1.5 microm. The early phases of secondary Ca(2+) responses were attributable to Ca(2+) influx and were abolished rapidly by buffered zero Ca(2+) saline. Taken together, these data indicate that the substantial difference in Ca(2+) signals in fine distal dendrites and in the initiation of spreading secondary responses may underlie the selective vulnerability of these neurons to excitotoxic insults.

Neurotransmitter receptor trafficking and the regulation of synaptic strength

Modulation of the strength of synapses is thought to be one of the mechanisms that underlies learning and memory and is also likely to be important in processes of neuropathology and drug tolerance. This review focuses on the emerging role of postsynaptic neurotransmitter receptor trafficking as an essential mechanism underlying the dynamic regulation of synaptic strength.

The subcellular distribution of GABARAP and its ability to interact with NSF suggest a role for this protein in the intracellular transport of GABA(A) receptors

GABA(A) receptors the major sites of fast synaptic inhibition in the brain are composed predominately of alpha, beta, and gamma2 subunits. The receptor gamma2 subunit interacts with a 17-kDa microtubule associated protein GABARAP, but the significance of this interaction remains unknown. Here we demonstrate that GABARAP, which immunoprecipitates with GABA(A) receptors, is not found at significant levels within inhibitory synapses, but is enriched within the Golgi apparatus and postsynaptic cisternae. We also demonstrate that GABARAP binds directly to N-ethylmaleimide-sensitive factor (NSF), a protein critical for intracellular membrane trafficking events. NSF and GABARAP complexes could be detected in neurons and these two proteins also colocalize within intracellular membrane compartments. Together our observations suggest that GABARAP may play a role in intracellular GABA(A) receptor transport but not synaptic anchoring, via its ability to interact with NSF. GABARAP may therefore have an important role in the production of GABAergic synapses.

Suppression of neuronal hyperexcitability and associated delayed neuronal death by adenoviral expression of GABA(C) receptors

The excessive neuronal excitation underlying several clinically important diseases is often treated with GABA allosteric modulators in an attempt to enhance inhibition. An alternative strategy would be to enhance directly the sensitivity of postsynaptic neurons to GABA. The GABA(C) receptor, normally found only in the retina, is more sensitive to GABA and demonstrates little desensitization compared with the GABA(A) receptor. We constructed an adenovirus vector that expressed cDNA for both the GABA(C) receptor rho(1) subunit and a green fluorescent protein (GFP) reporter and used it to transduce cultured hippocampal neurons. Transduced neurons were identified by fluorescence, double immunocytochemistry proved colocalization of the rho(1) protein and the reporter, Western blot verified the expected molecular masses, and electrophysiological and pharmacological properties confirmed the presence of functional GABA(C) receptors. rho(1)-GFP transduction resulted in an increased density of GABA(A) receptors as well as expression of novel GABA(C) receptors. This effect was not reproduced by addition of TTX or Mg(2+) to the culture medium to reduce action potentials or synaptic activity. In a model of neuronal hyperexcitability induced by chronic blockade of glutamate receptors, expression of GABA(C) receptors abolished the hyperactivity and the consequent delayed neuronal death. Adenovirus-mediated neuronal GABA(C) receptor engineering, via its dual mechanism of inhibition, may offer a way of inhibiting only those hyperexcitable neurons responsible for clinical problems, avoiding the generalized nervous system depression associated with pharmacological therapy.

Stimulation of beta-amyloid precursor protein trafficking by insulin reduces intraneuronal beta-amyloid and requires mitogen-activated protein kinase signaling

Alzheimer's Disease (AD) is characterized by cerebral accumulation of beta-amyloid peptides (Abeta), which are proteolytically derived from beta-amyloid precursor protein (betaAPP). betaAPP metabolism is highly regulated via various signal transduction systems, e.g., several serine/threonine kinases and phosphatases. Several growth factors known to act via receptor tyrosine kinases also have been demonstrated to regulate sbetaAPP secretion. Among these receptors, insulin and insulin-like growth factor-1 receptors are highly expressed in brain, especially in hippocampus and cortex. Emerging evidence indicates that insulin has important functions in brain regions involved in learning and memory. Here we present evidence that insulin significantly reduces intracellular accumulation of Abeta and that it does so by accelerating betaAPP/Abeta trafficking from the trans-Golgi network, a major cellular site for Abeta generation, to the plasma membrane. Furthermore, insulin increases the extracellular level of Abeta both by promoting its secretion and by inhibiting its degradation via insulin-degrading enzyme. The action of insulin on betaAPP metabolism is mediated via a receptor tyrosine kinase/mitogen-activated protein (MAP) kinase kinase pathway. The results suggest cell biological and signal transduction mechanisms by which insulin modulates betaAPP and Abeta trafficking in neuronal cultures.

Cognitive effects of insulin in the central nervous system

Evidence has been accumulating recently that the hormone insulin may modulate cognitive activity by acting in the central nervous system. Initially derived from the observation that insulin and insulin receptors are found in specific brain areas, this evidence also includes cognitive assessments of humans in insulin-deficient and insulin-resistant disease states and experimental manipulation of rodent models. Additional support is derived from in vivo and in vitro systems that are used to investigate the neurophysiological basis of learning and memory. This article is a brief review of the literature that suggests a connection between insulin and memory and draws together some of the findings relevant to possible physiological mechanisms for this cognitive effect.

Confocal microscopic study of GABA(A) receptors in Xenopus oocytes after rat brain mRNA injection: modulation by tyrosine kinase activity

The expression of GABA(A) receptors in Xenopus oocytes injected with rat brain mRNA was studied by immunocytochemistry and evaluation of the distribution of fluorescent probes at the confocal microscope. The beta(2/3) subunit distributed exclusively on the membrane at the animal pole of the oocytes. Treatment of oocytes for 20 min with the protein tyrosine kinase inhibitor genistein, 200 microM, resulted in a lower presence of GABA(A) receptors on the membrane. The inactive genistein analogue daidzein, 200 microM, had no effect even with a 30 min treatment. Alkaline phosphatase but not a protein tyrosine phosphatase, when injected into oocytes, reduced GABA(A) receptor membrane expression. The data indicate that protein tyrosine phosphorylation modulates the expression on the plasma membrane of presynthesized GABA(A) receptors.

Role of insulin and insulin receptor in learning and memory

As one of the most extensively studied protein hormones, insulin and its receptor have been known to play key roles in a variety of important biological functions. Until recent years, the functions of insulin and insulin receptor (IR) in the central nervous system (CNS) have largely remained unclear. IR is abundantly expressed in several specific brain regions that govern fundamental behaviors such as food intake, reproduction and high cognition. The IR from the periphery and CNS exhibit differences in both structure and function. In addition to that from the peripheral system, locally synthesized insulin in the brain has also been identified. Accumulated evidence has demonstrated that insulin/IR plays important roles in associative learning, as suggested by results from both interventive and correlative studies. Interruption of insulin production and IR activity causes deficits in learning and memory formation. Abnormal insulin/IR levels and activities are seen in Alzheimer's dementia, whereas administration of insulin significantly improves the cognitive performance of these patients. The synaptic bases for the action of insulin/IR include modifying neurotransmitter release processes at various types of presynaptic terminals and modulating the activities of both excitatory and inhibitory postsynaptic receptors such as NMDA and GABA receptors, respectively. At the molecular level, insulin/IR participates in regulation of learning and memory via activation of specific signaling pathways, one of which is shown to be associated with the formation of long-term memory and is composed of intracellular molecules including the shc, Grb-r/SOS, Ras/Raf, and MEK/MAP kinases. Cross-talk with another IR pathway involving IRS1, PI3 kinase, and protein kinase C, as well as with the non-receptor tyrosine kinase pp60c-src, may also be associated with memory processing.

Effect of neuropeptide-EI on the binding of [3H]SCH 23390 to the dopamine D1 receptor in rat striatal membranes

We have previously demonstrated that neuropeptide-EI, at high doses, stimulates the production of cAMP, in caudate putamen, through the activation of adenylate cyclase coupled to specific D1 receptors. The aim of the present work was to find evidences for a probable interaction between this neuropeptide and the dopamine D1 receptor in the mammalian central nervous system. The present data show that neuropeptide-EI, at high concentrations, affected both the maximum binding and the apparent affinity of [n-methyl-3H] (R)-(+)-8 chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol hemimaleate to the dopamine D1 receptor in a concentration-dependent manner.

Modulation of spontaneous quantal release of neurotransmitters in the hippocampus

Presynaptic action potentials trigger the exocytosis of neurotransmitters. However, even in the absence of depolarisation-dependent Ca2+ entry nearby release sites, spontaneous vesicular release still occurs. Even though this happens at low rate, such spontaneous release may play a trophic role in maintaining the shape of dendritic structures. Like evoked responses, action potential-independent release is subject to modulation. This review describes some of the regulatory factors that rapidly and presynaptically regulate the ongoing Ca2+-independent release of neurotransmitters in the hippocampus. For instance, the electrical activity of the nerve ending, neurotransmitters, hypertonic solutions, neurotoxins, polycations, neurotrophic factors, immunoglobulins, cyclothiazide and psychotropic drugs can all modify the rate of spontaneous release. This can be achieved through various mechanisms that can be Ca2+-dependent or Ca2+-independent, protein kinase-dependent or independent. Since action potential-independent release contributes to the maintenance of dendritic structures, neuromodulators are likely to influence the density and/or length of dendritic spines, which in turn may modulate information processing in the central nervous system (CNS).

Constructing inhibitory synapses

Control of nerve-cell excitability is crucial for normal brain function. Two main groups of inhibitory neurotransmitter receptors--GABA(A) and glycine receptors--fulfil a significant part of this role. To mediate fast synaptic inhibition effectively, these receptors need to be localized and affixed opposite nerve terminals that release the appropriate neurotransmitter at multiple sites on postsynaptic neurons. But for this to occur, neurons require intracellular anchoring molecules, as well as mechanisms that ensure the efficient turnover and transport of mature, functional inhibitory synaptic receptor proteins. This review describes the dynamic regulation of synaptic GABA(A) and glycine receptors and discusses recent advances in this rapidly evolving field.

BDNF reduces miniature inhibitory postsynaptic currents by rapid downregulation of GABA(A) receptor surface expression

Changes in neurotransmitter receptor density at the synapse have been proposed as a mechanism underlying synaptic plasticity. Neurotrophic factors are known to influence synaptic strength rapidly. In the present study, we found that brain-derived neurotrophic factor (BDNF) acts postsynaptically to reduce gamma-aminobutyric acid (GABA)-ergic function. Using primary cultures of rat hippocampal neurons, we investigated the effects of BDNF on GABAergic miniature inhibitory postsynaptic currents (mIPSCs) and on the localization of GABAA receptors. Application of BDNF (100 ng/mL) led within minutes to a marked reduction (33.5%) of mIPSC amplitudes in 50% of neurons, recorded in the whole-cell patch-clamp mode, leaving frequency and decay kinetics unaffected. This effect was blocked by the protein kinase inhibitor K252a, which binds with high affinity to trkB receptors. Immunofluorescence staining with an antibody against trkB revealed that about 70% of cultured hippocampal pyramidal cells express trkB. In dual labelling experiments, use of neurobiotin injections to label the recorded cells revealed that all cells responsive to BDNF were immunopositive for trkB. Treatment of cultures with BDNF reduced the immunoreactivity for the GABAA receptor subunits-alpha2, -beta2,3 and -gamma2 in the majority of neurons. This effect was detectable after 15 min and lasted at least 12 h. Neurotrophin-4 (NT-4), but not neurotrophin-3 (NT-3), also reduced GABAA receptor immunoreactivity, supporting the proposal that this effect is mediated by trkB. Altogether the results suggest that exposure to BDNF induces a rapid reduction in postsynaptic GABAA receptor number that is responsible for the decline in GABAergic mIPSC amplitudes.

Insulin promotes rapid delivery of N-methyl-D- aspartate receptors to the cell surface by exocytosis

Insulin potentiates N-methyl-d-aspartate receptors (NMDARs) in neurons and Xenopus oocytes expressing recombinant NMDARs. The present study shows that insulin induced (i) an increase in channel number times open probability (nP(o)) in outside-out patches excised from Xenopus oocytes, with no change in mean open time, unitary conductance, or reversal potential, indicating an increase in n and/or P(o); (ii) an increase in charge transfer during block of NMDA-elicited currents by the open channel blocker MK-801, indicating increased number of functional NMDARs in the cell membrane with no change in P(o); and (iii) increased NR1 surface expression, as indicated by Western blot analysis of surface proteins. Botulinum neurotoxin A greatly reduced insulin potentiation, indicating that insertion of new receptors occurs via SNARE-dependent exocytosis. Thus, insulin potentiation occurs via delivery of new channels to the plasma membrane. NMDARs assembled from mutant subunits lacking all known sites of tyrosine and serine/threonine phosphorylation in their carboxyl-terminal tails exhibited robust insulin potentiation, suggesting that insulin potentiation does not require direct phosphorylation of NMDAR subunits. Because insulin and insulin receptors are localized to glutamatergic synapses in the hippocampus, insulin-regulated trafficking of NMDARs may play a role in synaptic transmission and plasticity, including long-term potentiation.

Constitutive endocytosis of GABAA receptors by an association with the adaptin AP2 complex modulates inhibitory synaptic currents in hippocampal neurons

Type A GABA receptors (GABA(A)) mediate the majority of fast synaptic inhibition in the brain and are believed to be predominantly composed of alpha, beta, and gamma subunits. Although changes in cell surface GABA(A) receptor number have been postulated to be of importance in modulating inhibitory synaptic transmission, little is currently known on the mechanism used by neurons to modify surface receptor levels at inhibitory synapses. To address this issue, we have studied the cell surface expression and maintenance of GABA(A) receptors. Here we show that constitutive internalization of GABA(A) receptors in hippocampal neurons and recombinant receptors expressed in A293 cells is mediated by clathrin-dependent endocytosis. Furthermore, we identify an interaction between the GABA(A) receptor beta and gamma subunits with the adaptin complex AP2, which is critical for the recruitment of integral membrane proteins into clathrin-coated pits. GABA(A) receptors also colocalize with AP2 in cultured hippocampal neurons. Finally, blocking clathrin-dependant endocytosis with a peptide that disrupts the association between amphiphysin and dynamin causes a large sustained increase in the amplitude of miniature IPSCs in cultured hippocampal neurons. These results suggest that GABA(A) receptors cycle between the synaptic membrane and intracellular sites, and their association with AP2 followed by recruitment into clathrin-coated pits represents an important mechanism in the postsynaptic modulation of inhibitory synaptic transmission.

Interaction of the Unc-51-like kinase and microtubule-associated protein light chain 3 related proteins in the brain: possible role of vesicular transport in axonal elongation

We identified two mammalian ULK1 (Unc-51-like kinase involved in neurite extension) binding proteins by yeast two-hybrid screening. Both proteins showed high structural similarity to microtubule-associated protein (MAP) light chain 3 (LC3). One is identical to the Golgi-associated ATPase Enhancer of 16 kDa (GATE-16), an essential factor for intra-Golgi transport [39]. The other is identical to the gamma 2-subunit of GABA-A receptor associated protein (GABARAP) which has a possible role in receptor transport [46]. Using the yeast two-hybrid system and the in vitro GST pull-down assay, we found that the N-terminal proline/serine rich (PS) domain of ULK1 (amino acid 287-416) is required for ULK1-GATE-16 and ULK1-GABARAP protein interactions. However, the kinase activity of ULK1 affected neither ULK1-GATE-16 nor ULK1-GABARAP interaction. Immunohistochemical analysis using ULK1 and GABARAP antibodies showed that the ULK1 and the GABARAP proteins co-localized to many kind of neurons such as pyramidal cells of the hippocampus, mitral cells of the olfactory bulb, and Purkinje cells of the cerebellum. In HeLa cells, endogenous ULK1 and tagged GABARAP showed punctate structures in the cytosol, and were colocalized. These results suggest that the interaction of ULK1 and GABARAP is important to vesicle transport and axonal elongation in mammalian neurons.

Inhibition-based rhythms: experimental and mathematical observations on network dynamics

An increasingly large body of data exists which demonstrates that oscillations of frequency 12-80 Hz are a consequence of, or are inextricably linked to, the behaviour of inhibitory interneurons in the central nervous system. This frequency range covers the EEG bands beta 1 (12-20 Hz), beta 2 (20-30 Hz) and gamma (30-80 Hz). The pharmacological profile of both spontaneous and sensory-evoked EEG potentials reveals a very strong influence on these rhythms by drugs which have direct effects on GABA(A) receptor-mediated synaptic transmission (general anaesthetics, sedative/hypnotics) or indirect effects on inhibitory neuronal function (opiates, ketamine). In addition, a number of experimental models of, in particular, gamma-frequency oscillations, have revealed both common denominators for oscillation generation and function, and subtle differences in network dynamics between the different frequency ranges. Powerful computer and mathematical modelling techniques based around both clinical and experimental observations have recently provided invaluable insight into the behaviour of large networks of interconnected neurons. In particular, the mechanistic profile of oscillations generated as an emergent property of such networks, and the mathematical derivation of this complex phenomenon have much to contribute to our understanding of how and why neurons oscillate. This review will provide the reader with a brief outline of the basic properties of inhibition-based oscillations in the CNS by combining research from laboratory models, large-scale neuronal network simulations, and mathematical analysis.

Role of L- and N-type calcium channels in the pathophysiology of traumatic spinal cord white matter injury

Recent work has suggested a potential role for voltage-gated Ca(2+) channels in the pathophysiology of anoxic central nervous system white matter injury. To examine the relevance of these findings to neurotrauma, we conducted electrophysiological studies with inorganic Ca(2+) channels blockers and L- and N-subtype-specific calcium channel antagonists in an in vitro model of spinal cord injury. Confocal immunohistochemistry was used to examine for localization of L- and N-type calcium channels in spinal cord white matter tracts. A 30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber and injured with a modified clip (2g closing force) for 15s. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential at two points with glass microelectrodes. The compound action potential decreased to 71.4+/-2.0% of control (P<0. 05) after spinal cord injury. Removal of extracellular Ca(2+) promoted significantly greater recovery of compound action potential amplitude (86.3+/-7.6% of control; P< 0.05) after injury. Partial blockade of voltage-gated Ca(2+) channels with cobalt (20 microM) or cadmium (200 microM) conferred improvement in compound action potential amplitude. Application of the L-type Ca(2+) channel blockers diltiazem (50 microM) or verapamil (90 microM), and the N-type antagonist omega-conotoxin GVIA (1 microM), significantly enhanced the recovery of compound action potential amplitude postinjury. Co-application of the L-type antagonist diltiazem with the N-type blocker omega-conotoxin GVIA showed significantly greater (P<0.05) improvement in compound action potential amplitude than application of either drug alone. Confocal immunohistochemistry with double labelling for glial fibrillary acidic protein, GalC and NF200 demonstrated L- and N-type Ca(2+) channels on astrocytes and oligodendrocytes, but not axons, in spinal cord white matter. In conclusion, the injurious effects of Ca(2+) in traumatic central nervous system white matter injury appear to be partially mediated by voltage-gated Ca(2+) channels. The presence of L- and N-type Ca(2+) channels on periaxonal astrocytes and oligodendrocytes suggests a role for these cells in post-traumatic axonal conduction failure.

Regulation of somatodendritic GABAA receptor channels in rat hippocampal neurons: evidence for a role of the small GTPase Rac1

The role of the cytoskeleton in the activity of GABA(A) receptors was investigated in cultured hippocampal neurons. Receptor currents were measured with the whole-cell patch-clamp technique during repetitive stimulation with 1 microm muscimol. After destruction of the microtubular system with nocodazol, muscimol-induced currents showed a rundown by 78%. A similar rundown was observed when actin fibers were destroyed with latrunculin B or C2 toxin of Clostridium botulinum. Because the small GTPases of the Rho family RhoA, Rac1, and Cdc42 are known to control the organization of actin fibers, we investigated their possible involvement. Inactivation of the GTPases with clostridial toxins, as well as intracellular application of recombinant Rho GTPases, indicated that active Rac1 was necessary for full GABA(A) receptor activity. Immunocytochemical labeling of the receptors showed that the disappearance of receptor clusters in the somatic membrane as induced by muscimol stimulation was enhanced by Rac1 inactivation. It is suggested that Rac1 participates in the regulation of GABA(A) receptor clustering and/or recycling.

Postnatal maturation of gephyrin/glycine receptor clusters on developing Renshaw cells

Adult mammalian Renshaw cells express large and complex postsynaptic gephyrin/glycine receptor clusters on their surface. Larger gephyrin clusters correlate with more "efficacious" inhibitory synapses, in terms of larger postsynaptic quantal size amplitudes, in part because they likely contain more postsynaptic receptors (Lim et al. [1999] J. Physiol. (Lond.) 516:505-512; Oleskevich et al. [1999] J. Neurophysiology 82:312-319). Here, we studied the postnatal development of the gephyrin/glycine receptor cluster size on Renshaw cells. Renshaw cells were identified by their calbindin immunoreactivity, location and morphology, and presence of cholinergic input. The populations of clusters over developing Renshaw cells immunoreactive to gephyrin or glycine receptor alpha1 subunits were comparable in number, size, and complexity and displayed a high degree of colocalization (>90%) at all ages. Quantitative morphologic analysis was performed on gephyrin-immunoreactive clusters. In neonatal animals, Renshaw cells expressed small punctate gephyrin-immunoreactive clusters (mean cluster size +/- SD = 0.19 +/- 0.19 microm(2)at 2 days; 0.22 +/- 0. 19 microm(2)at 5 days). By 10 and 15 days of age, Renshaw cells exhibited gephyrin-immunoreactive clusters that were larger and more complex (0.32 +/- 0.19 microm(2) at 10 days; 0.41 +/- 0.32 microm(2) at 15 days). Cluster growth reached a plateau in 25- and 60-day-old Renshaw cells (0.45 +/- 0.43 microm(2); 0.56 +/- 0.55 microm(2), respectively). By using electron microscopy, we confirmed that gephyrin-immunoreactive clusters were located at postsynaptic sites at both early and late postnatal ages on Renshaw cells. The potential significance of this gephyrin/glycine receptor cluster size maturation that sets Renshaw cells apart from other interneurons is discussed.

Intrarenal dopamine: a key signal in the interactive regulation of sodium metabolism

The kidney regulates sodium metabolism with extraordinary precision and sensitivity. This is accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between anti-natriuretic and natriuretic factors. Dopamine, produced in renal proximal tubule cells, plays a central role in this interactive network. Natriuretic hormones that are released from extrarenal sources, such as atrial natriuretic peptide, mediate some of their effects via renal dopamine receptors. On the level of the tubules, dopamine acts by opposing the effects of anti-natriuretic factors, such as angiotensin II and alpha-adrenergic receptors. Sodium retention leads to an increase in renal dopamine tonus, and the natriuretic effects of dopamine are more prominent under this condition. Inhibition or down-regulation of dopamine receptors significantly attenuates the natriuretic response to salt loading. Renal dopamine is modulated by the supply of filtered L-DOPA and the metabolism of dopamine via catechol-O-methyldopamine. The importance of dopamine as a natriuretic hormone is reflected by its capacity to inhibit the majority of renal tubule sodium transporters. Notably, the activity of Na+, K+ ATPase is inhibited in most tubule segments by dopamine. Recent studies have elucidated many of the signaling pathways for renal dopamine receptors. Novel principles for homologous and heterologous sensitization of dopamine receptors have been detected that may explain some of the interaction between dopamine and other first messengers that modulate renal tubule sodium transport. A broad understanding of the renal dopamine system has become increasingly important, since there is now strong evidence from both clinical and experimental studies that dysregulation of the renal dopamine system plays a role in many forms of multigenetic hypertension.

Role of brain insulin receptor in control of body weight and reproduction

Insulin receptors (IRs) and insulin signaling proteins are widely distributed throughout the central nervous system (CNS). To study the physiological role of insulin signaling in the brain, we created mice with a neuron-specific disruption of the IR gene (NIRKO mice). Inactivation of the IR had no impact on brain development or neuronal survival. However, female NIRKO mice showed increased food intake, and both male and female mice developed diet-sensitive obesity with increases in body fat and plasma leptin levels, mild insulin resistance, elevated plasma insulin levels, and hypertriglyceridemia. NIRKO mice also exhibited impaired spermatogenesis and ovarian follicle maturation because of hypothalamic dysregulation of luteinizing hormone. Thus, IR signaling in the CNS plays an important role in regulation of energy disposal, fuel metabolism, and reproduction.

Clustering of inhibitory neurotransmitter receptors at developing postsynaptic sites: the membrane activation model

Recent studies indicate an important role of cytoskeleton-associated and lipid-anchored proteins in the formation of inhibitory postsynaptic membrane specializations. Membrane apposition of the tubulin-binding protein gephyrin is essential for the recruitment of inhibitory glycine receptors and GABAA receptors to developing postsynaptic sites. Newly disclosed interactions between gephyrin, exchange factors for G proteins of the Rho and Rac families, the translational regulator RAFT1, and actin-binding proteins like profilin might integrate activity-dependent and trophic-factor-mediated signals at developing postsynaptic sites. A model of inhibitory neurotransmitter receptor clustering, is proposed, in which this process is initiated by receptor-driven activation of phosphatidylinositol 3-kinase.

Mechanisms underlying LTP of inhibitory synaptic transmission in the deep cerebellar nuclei

Whole-cell recordings were used to investigate long-term potentiation of inhibitory synaptic currents (IPSCs) in neurons of deep cerebellar nuclei (DCN) in slices. IPSCs were evoked by electrical stimulation of the white matter surrounding the DCN in the presence of non-N-methyl-D-aspartate (non-NMDA) glutamate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (20 microM). High-frequency stimulation induced a long-term potentation (LTP) of the IPSC amplitude without changing its reversal potential, rise time, and decay-time constant. This LTP did not require the activation of postsynaptic gamma-aminobutyric acid-A (GABA(A)) receptors but depended on the activation of NMDA receptors. LTP of IPSCs in DCN neurons could also be induced by voltage-depolarizing pulses in postsynaptic neurons and appeared to depend on an increase in intracellular calcium as the LTP was blocked when the cells were loaded with a calcium chelator, 1,2-bis-(2-amino-phenoxy)-N,N,N', N'-tetraacetic acid (BAPTA, 10 mM). LTP of IPSCs was accompanied by an increase in the frequency of spontaneous IPSCs and miniature IPSCs (recorded in the presence of tetrodotoxin 1 microM), but there was no significant change in their amplitude. In addition, during the LTP, the amplitude of response to exogenously applied GABA(A) receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride was increased. Intracellular application of tetanus toxin, a powerful blocker of exocytosis, in DCN neuron prevented the induction of LTP of IPSCs. Our results suggest that the induction of LTP of IPSCs in the DCN neurons likely involves a postsynaptic locus. Plasticity of inhibitory synaptic transmission in DCN neurons may play a crucial role in cerebellar control of motor coordination and learning.

Functional regulation of gamma-aminobutyric acid transporters by direct tyrosine phosphorylation

Tyrosine phosphorylation regulates multiple cell signaling pathways and functionally modulates a number of ion channels and receptors. Neurotransmitter transporters, which act to clear transmitter from the synaptic cleft, are regulated by multiple second messenger pathways that exert their effects, at least in part, by causing a redistribution of the transporter protein to or from the cell surface. To test the hypothesis that tyrosine phosphorylation affects transporter function and to determine its mechanism of action, we examined the regulation of the rat brain gamma-aminobutyric acid (GABA) transporter GAT1 expressed endogenously in hippocampal neurons and expressed heterologously in Chinese hamster ovary cells. Inhibitors of tyrosine kinases decreased GABA uptake; inhibitors of tyrosine phosphatases increased GABA uptake. The decrease in uptake seen with tyrosine kinase inhibitors was correlated with a decrease in tyrosine phosphorylation of GAT1 and resulted in a redistribution of the transporter from the cell surface to intracellular locations. A mutant GAT1 construct that was refractory to tyrosine phosphorylation could not be regulated by tyrosine kinase inhibitors. Activators of protein kinase C, which are known to cause a redistribution of GAT1 from the cell surface, were additive to the effects of tyrosine kinase inhibitors suggesting that multiple signaling pathways control transporter redistribution. Application of brain-derived neurotrophic factor, which activates receptor tyrosine kinases, up-regulated GAT1 function suggesting one potential trigger for the cellular regulation of GAT1 signaling by tyrosine phosphorylation. These data support the hypothesis that transporter expression and function is controlled by the interplay of multiple cell signaling cascades.

The effect of topical insulin on the release of excitotoxic and other amino acids from the rat cerebral cortex during streptozotocin-induced hyperglycemic ischemia

Insulin has been demonstrated to be neuroprotective in brain and spinal cord ischemia. The mechanism of neuroprotection may involve alterations in metabolism, protein synthesis or uptake of GABA by astrocytes. Conversely, hyperglycemia increases the extent of neurologic damage observed during ischemia/reperfusion. Diabetic patients are 2-4 times more likely to suffer a stroke as normoglycemic patients and they also have worsened neurologic outcome. Determining if insulin, which many diabetics already use as therapy, can be neuroprotective, would be a possible means of alleviating the detrimental outcome from diabetic stroke. This study looked at the relationship between topically administered insulin (1 mIU insulin/ml and 100 mIU insulin/ml) during a four vessel occlusion model of global ischemia and the release of amino acids, especially glutamate, from the cortex in streptozotocin (STZ)-treated rats. The rats were utilized either 5-7 days (ASTZ) or 4-6 weeks (CSTZ) after a single STZ injection. In the ASTZ animals both doses of insulin increased the amount of the excitotoxic amino acids, aspartate and glutamate, released during reperfusion and the higher dose also increased the levels of taurine and GABA during reperfusion. In the CSTZ animals, both doses of insulin increased the amount of excitotoxic amino acids during reperfusion and the lower dose increased GABA levels released during reperfusion. The differences between the ACTZ and CSTZ animals may be due to metabolic differences in the utilization of glucose. Insulin may act as a neuroprotectant by increasing extracellular GABA resulting in neuroinhibition.

Mechanisms involved in tetanus-induced potentiation of fast IPSCs in rat hippocampal CA1 neurons

In the present study, possible mechanisms involved in the tetanus-induced potentiation of gamma-aminobutyric acid-A (GABA-A) receptor-mediated inhibitory postsynaptic currents (IPSCs) were investigated using the whole cell voltage-clamp technique on CA1 neurons in rat hippocampal slices. Stimulations (100 Hz) of the stratum radiatum, while voltage-clamping the membrane potential of neurons, induces a long-term potentiation (LTP) of evoked fast IPSCs while increasing the number but not the amplitude of spontaneous IPSCs (sIPSCs). The potentiation of fast IPSCs was input specific. During the period of IPSC potentiation, postsynaptic responses produced by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride and baclofen, GABA-A and GABA-B agonists respectively, were not significantly different from control. CGP 36742, a GABA-B antagonist, blocked the induction of tetanus-induced potentiation of evoked and spontaneous IPSCs, while GTPgammaS, an activator of G proteins, substitution for GTP in the postsynaptic recording electrode did not occlude potentiation. Since GABA-B receptors work through G proteins, our results suggest that pre- but not postsynaptic GABA-B receptors are involved in the potentiation of fast IPSCs. A tetanus delivered when GABA-A responses were completely blocked by bicuculline suggests that GABA-A receptor activation during tetanus is not essential for the induction of potentiation. Rp-cAMPs, an antagonist of protein kinase A (PKA) activation, blocks the induction of potentiation of fast IPSCs. Forskolin, an activator of PKA, increases baseline evoked IPSCs as well as the number of sIPSCs, and a tetanic stimulation during this enhancement uncovers a long-term depression of the evoked IPSC. Sulfhydryl alkylating agents, N-ethylmaleimide and p-chloromercuribenzoic acid, which have been found to presynaptically increase GABA release and have been suggested to have effects on proteins involved in transmitter release processes occurring in nerve terminals, occlude tetanus-induced potentiation of evoked and spontaneous IPSCs. Taken together our results suggest that LTP of IPSCs originates from a presynaptic site and that GABA-B receptor activation, cyclic AMP/PKA activation and sulfhydryl-alkylation are involved. Plasticity of IPSCs as observed in this study would have significant implications for network behavior in the hippocampus.

Regulation of neuronal function by protein trafficking: a role for the endosomal pathway

Protein trafficking plays a central role in many aspects of neuronal function, from the release of neurotransmitters by exocytosis and the recycling of synaptic vesicle proteins to the regulation of receptor signalling. Synaptic function can be significantly modified on a short time scale by alterations in the levels of receptors, ion channels and transporters both pre- and postsynaptically. In many cases, these alterations appear to be mediated by acute changes in the rates at which the proteins are endocytosed from and exocytosed to the cell surface from intracellular pools. While our current understanding of the signalling mechanisms and the intracellular pathways responsible for these acute changes is still in its infancy, intriguing details are beginning to emerge from a number of systems.

GABAergic mIPSCs in rat cerebellar Purkinje cells are modulated by TrkB and mGluR1-mediated stimulation of Src

Whilst protein tyrosine kinase (PTK) activity can modulate expressed GABAA receptors in cell culture, the physiological consequences on synaptic GABAA receptors are unknown. This was examined using whole-cell recording of bicuculline-sensitive mIPSCs in Purkinje cells (PCs) in cerebellar slices. Postsynaptic application of a peptide activator of the non-receptor PTK Src (Src-peptide) enhanced mIPSC amplitudes by 39 % in the presence of brain-derived neurotrophic factor (BDNF) only; neurotrophin-3 (NT-3) was ineffective in this regard. Thus Src and TrkB (the receptor for BDNF) can physiologically interact to modulate synaptic GABAA receptors. In the presence of BDNF, pharmacological activation of metabotrophic glutamate receptor subtype 1 (mGluR1) by (S)-3, 5-dihydrophenylglycine (3,5-DHPG) also lead to a 32 % enhancement of mIPSCs. This enhancement was blocked by intracellular dialysis of PCs with PP1, a selective inhibitor of Src. It is concluded that, whilst GABAA receptors are not constitutively regulated by endogenous PTK activity in PCs, co-activation of TrkB by BDNF and Src by mGluR1 is required to modulate GABAergic synapses in PCs.

Localization of the A kinase anchoring protein AKAP79 in the human hippocampus

The phosphorylation state of the proteins, regulated by phosphatases and kinases, plays an important role in signal transduction and long-term changes in neuronal excitability. In neurons, cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and calcineurin (CN) are attached to a scaffold protein, A kinase anchoring protein (AKAP), thought to anchor these three enzymes to specific sites of action. However, the localization of AKAP, and the predicted sites of linked phosphatase and kinase activities, are still unknown at the fine structural level. In the present study, we investigated the distribution of AKAP79 in the hippocampus from postmortem human brains and lobectomy samples from patients with intractable epilepsy, using preembedding immunoperoxidase and immunogold histochemical methods. AKAP79 was found in the CA1, presubicular and subicular regions, mostly in pyramidal cell dendrites, whereas pyramidal cells in the CA3, CA2 regions and dentate granule cells were negative both in postmortem and in surgical samples. In some epileptic cases, the dentate molecular layer and hilar interneurons also became immunoreactive. At the subcellular level, AKAP79 immunoreactivity was present in postsynaptic profiles near, but not attached to, the postsynaptic density of asymmetrical (presumed excitatory) synapses. We conclude that the spatial selectivity for the action of certain kinases and phosphatases regulating various ligand- and voltage-gated channels may be ensured by the selective presence of their anchoring protein, AKAP79, at the majority of glutamatergic synapses in the CA1, but not in the CA2/CA3 regions, suggesting profound differences in signal transduction and long-term synaptic plasticity between these regions of the human hippocampus.

Regulation of AMPA receptor-mediated synaptic transmission by clathrin-dependent receptor internalization

Redistribution of postsynaptic AMPA- (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-) subtype glutamate receptors may regulate synaptic strength at glutamatergic synapses, but the mediation of the redistribution is poorly understood. We show that AMPA receptors underwent clathrin-dependent endocytosis, which was accelerated by insulin in a GluR2 subunit-dependent manner. Insulin-stimulated endocytosis rapidly decreased AMPA receptor numbers in the plasma membrane, resulting in long-term depression (LTD) of AMPA receptor-mediated synaptic transmission in hippocampal CA1 neurons. Moreover, insulin-induced LTD and low-frequency stimulation-(LFS-) induced homosynaptic CA1 LTD were found to be mutually occlusive and were both blocked by inhibiting postsynaptic clathrin-mediated endocytosis. Thus, controlling postsynaptic receptor numbers through endocytosis may be an important mechanism underlying synaptic plasticity in the mammalian CNS.

Intracellular trafficking of GABA(A) receptors

Some of the mechanisms that control the intracellular trafficking of GABA(A) receptors have recently been described. Following the synthesis of alpha, beta, and gamma subunits in the endoplasmic reticulum, ternary receptor complexes assemble slowly and are inefficiently inserted into surface membranes of heterologous cells. While beta3, beta4, and gamma2S subunits appear to contain polypeptide sequences that alone are sufficient for surface targeting, these sequences are neither conserved nor essential for surface expression of heteromeric GABA(A) receptors formed from alpha1beta or alpha1betagamma subunits. At the neuronal surface, native GABA(A) receptor clustering and synaptic targeting require a gamma2 subunit and the participation of gephyrin, a clustering protein for glycine receptors. A linker protein, such as the GABA(A) receptor associated protein (GABARAP), may be necessary for the formation of GABA(A) receptor aggregates containing gephyrin. A substantial fraction of surface receptors are sequestered by endocytosis, another process which apparently requires a GABA(A) receptor gamma2 subunit. In heterologous cells, constitutive endocytosis seems to predominate while, in cortical neurons, internalization is evoked when receptors are occupied by GABA(A) agonists. After constitutive endocytosis, receptors are relatively stable and can be rapidly recycled to the cell surface, a process that may be regulated by protein kinase C. On the other hand, a portion of the intracellular GABA(A) receptors derived from ligand-dependent endocytosis is apparently degraded. The clustering of GABA(A) receptors at synapses and at coated pits are two mechanisms that may compete for a pool of diffusable receptors, providing a model for plasticity at inhibitory synapses.

Direct protein-protein coupling enables cross-talk between dopamine D5 and gamma-aminobutyric acid A receptors

GABA(A) (gamma-aminobutyric-acid A) and dopamine D1 and D5 receptors represent two structurally and functionally divergent families of neurotransmitter receptors. The former comprises a class of multi-subunit ligand-gated channels mediating fast interneuronal synaptic transmission, whereas the latter belongs to the seven-transmembrane-domain single-polypeptide receptor superfamily that exerts its biological effects, including the modulation of GABA(A) receptor function, through the activation of second-messenger signalling cascades by G proteins. Here we show that GABA(A)-ligand-gated channels complex selectively with D5 receptors through the direct binding of the D5 carboxy-terminal domain with the second intracellular loop of the GABA(A) gamma2(short) receptor subunit. This physical association enables mutually inhibitory functional interactions between these receptor systems. The data highlight a previously unknown signal transduction mechanism whereby subtype-selective G-protein-coupled receptors dynamically regulate synaptic strength independently of classically defined second-messenger systems, and provide a heuristic framework in which to view these receptor systems in the maintenance of psychomotor disease states.

Cell surface stability of gamma-aminobutyric acid type A receptors. Dependence on protein kinase C activity and subunit composition

Type A gamma-aminobutyric acid receptors (GABA(A)), the major sites of fast synaptic inhibition in the brain, are believed to be composed predominantly of alpha, beta, and gamma subunits. Although cell surface expression is essential for GABA(A) receptor function, little is known regarding its regulation. To address this issue, the membrane stability of recombinant alpha(1)beta(2) or alpha(1)beta(2)gamma(2) receptors was analyzed in human embryonic kidney cells. Alpha(1)beta(2)gamma(2) but not alpha(1)beta(2) receptors were found to recycle constitutively between the cell surface and a microtubule-dependent, perinuclear endosomal compartment. Similar GABA(A) receptor endocytosis was also seen in cultured hippocampal and cortical neurons. GABA(A) receptor surface levels were reduced upon protein kinase C (PKC) activation. Like basal endocytosis, this response required the gamma(2) subunit but not receptor phosphorylation. Although inhibiting PKC activity did not block alpha(1)beta(2)gamma(2) receptor endocytosis, it did prevent receptor down-regulation, suggesting that PKC activity may block alpha(1)beta(2)gamma(2) receptor recycling to the cell surface. In agreement with this observation, blocking recycling from endosomes with wortmannin selectively reduced surface levels of gamma(2)-containing receptors. Together, our results demonstrate that the surface stability of GABA(A) receptors can be dynamically and specifically regulated, enabling neurons to modulate cell surface receptor number upon the appropriate cues.

Mechanisms underlying the anti-epileptic efficacy of the ketogenic diet

The clinical efficacy of the ketogenic diet (KD) has now been well-documented. However, the underlying bases of KD antiepileptic efficacy are still a matter of speculation. A number of suggestions regarding underlying mechanisms have been offered, but all require rigorous testing. Development of appropriate animal model systems, and clear statement of experimentally testable hypotheses, are needed. Among the general hypotheses of interest are the following: (1) the KD alters the nature, and/or degree, of energy metabolism in the brain -- therefore altering brain excitability; (2) the KD leads to changes in cell (neuronal and perhaps glial) properties, which decrease excitability and dampen epileptiform discharge; (3) the KD induces changes in neurotransmitter function and synaptic transmission -- thus altering inhibitory-excitatory balance and discouraging hyper-synchronization; (4) the KD is associated with changes in a variety of circulating factors which act as neuromodulators that can regulate CNS excitability; and (5) the KD gives rise to alterations in brain extracellular milieu, which serve to depress excitability and synchrony. An understanding of the mechanism underlying KD antiepileptic efficacy will help us not only to optimize the clinical use of the ketogenic diet, but also to develop novel antiepileptic treatments.

Endocrine regulation of neurotransmitter transporters

Aminergic signalling in the CNS is terminated by clearance of neurotransmitters from the synapse via high affinity transporter molecules in the presynaptic membrane. Relatively recent sequence identification of these molecules has now permitted the initiation of studies of regulation of transporter function at the cellular and systems levels. In vitro studies provide evidence that the transporters for dopamine, serotonin, and gamma-aminobutyric acid (GABA) may be substrates for regulation by protein kinase C and protein kinase A signalling. Changes in energy balance and metabolic status, such as starvation, result in major shifts in hormonal output. It is now recognized that metabolic hormones such as insulin or the adrenal steroids can have significant acute and chronic effects on several aspects of CNS function. Data from this laboratory and others now provide evidence that insulin and adrenal and gonadal steroid hormones may regulate the synthesis and activity of the transporters. Future studies should permit elucidation of the cellular basis for endocrine regulation of neurotransmitter clearance, and thus, the role of endocrines in the maintenance of normal CNS aminergic signalling. The potential relevance of transporter regulation for the ketogenic diet is discussed.

Platelet-derived growth factor receptor-induced feed-forward inhibition of excitatory transmission between hippocampal pyramidal neurons

Growth factor receptors provide a major mechanism for the activation of the nonreceptor tyrosine kinase c-Src, and this kinase in turn up-regulates the activity of N-methyl-D-aspartate (NMDA) receptors in CA1 hippocampal neurons (1). Unexpectedly, applications of platelet-derived growth factor (PDGF)-BB to cultured and isolated CA1 hippocampal neurons depressed NMDA-evoked currents. The PDGF-induced depression was blocked by a PDGF-selective tyrosine kinase inhibitor, by a selective inhibitor of phospholipase C-gamma, and by blocking the intracellular release of Ca(2+). Inhibitors of cAMP-dependent protein kinase (PKA) also eliminated the PDGF-induced depression, whereas a phosphodiesterase inhibitor enhanced it. The NMDA receptor-mediated component of excitatory synaptic currents was also inhibited by PDGF, and this inhibition was prevented by co-application of a PKA inhibitor. Src inhibitors also prevented this depression. In recordings from inside-out patches, the catalytic fragment of PKA did not itself alter NMDA single channel activity, but it blocked the up-regulation of these channels by a Src activator peptide. Thus, PDGF receptors depress NMDA channels through a Ca(2+)- and PKA-dependent inhibition of their modulation by c-Src.

Insulin modulation of cloned mouse NMDA receptor currents in Xenopus oocytes

Modulation of recombinant N-methyl-D-aspartate receptor (NMDAR) currents by insulin was studied using the Xenopus oocyte expression system. Insulin (0.8 microM, 10 min) regulated NMDAR currents in a subunit-specific manner. Currents from epsilon1/zeta1, epsilon2/zeta1, and epsilon4/zeta1 receptors were variably potentiated, whereas currents from epsilon3/zeta1 receptors were not. Protein tyrosine kinases (PTKs) and protein kinase C were found to be involved in insulin-mediated modulation in an NMDAR subtype-specific way. Pretreatment with a specific PTK inhibitor, lavendustin A, attenuated and blocked the insulin effect on epsilon2/zeta1 and epsilon4/zeta1, respectively. Preincubation with selective protein kinase C inhibitors, staurosporine or calphostin C, depressed the response of epsilon1/zeta1 and epsilon2/zeta1 receptors to insulin. Basal regulation of NMDAR currents by endogenous PTKs and protein tyrosine phosphatases (PTPs) was also investigated. Of the four receptor subtypes, only epsilon1/zeta1 receptor currents were affected by basal PTK inhibition via lavendustin A, whereas PTP inhibition by phenylarsine oxide or orthovanadate enhanced currents from epsilon1/zeta1 and epsilon2/zeta1 receptors. Surprisingly, a stimulatory PTP modulation was observed for epsilon4/zeta1. As NMDAR subunits are differentially expressed in the brain, the observed subtype-specific modulations of NMDAR currents by insulin, PTKs, and PTPs may provide important insights into certain NMDAR-dependent physiological and pathological processes.

Segmental specification of GABAergic inhibition during development of hindbrain neural networks

A primordial rhythm-generating neural network emerges during the segmental period of vertebrate hindbrain development, suggesting a common genetic basis to both the structure and network activity of the region. We show here that segmentation influenced a postsegmental developmental step by which a GABAergic rhythm generator was incorporated into the primordial network and increased rhythm frequency to near mature values. This process depended on specifications in r3 and r5 that controlled, on the basis of a two-segment repeat, later maturation of GABAergic inhibition.

The insulin receptor tyrosine kinase substrate p58/53 and the insulin receptor are components of CNS synapses

The synapse is the primary locus of cell-cell communication in the nervous system. It is now clear that the synapse incorporates diverse cell signaling modalities in addition to classical neurotransmission. Here we show that two components of the insulin pathway are localized at CNS synapses, where they are components of the postsynaptic density (PSD). An immunochemical screen revealed that polypeptides of 58 and 53 kDa (p58/53) were highly enriched in PSD fractions from rat cerebral cortex, hippocampus, and cerebellum. These polypeptides were purified and microsequenced, revealing that p58/53 is identical to the insulin receptor tyrosine kinase substrate p58/53 (IRSp53). Our analysis of IRSp58/53 mRNA suggests that within rat brain there is one coding region for IRSp58 and IRSp53; we find no evidence of alternative splicing. We demonstrate that IRSp58/53 is expressed in the synapse-rich molecular layer of the cerebellum and is highly concentrated at the synapses of cultured hippocampal neurons, where it co-localizes with the insulin receptor. Together, these data suggest that insulin signaling may play a role at CNS synapses.

Febrile seizures in the developing brain result in persistent modification of neuronal excitability in limbic circuits

Febrile (fever-induced) seizures affect 3-5% of infants and young children. Despite the high incidence of febrile seizures, their contribution to the development of epilepsy later in life has remained controversial. Combining a new rat model of complex febrile seizures and patch clamp techniques, we determined that hyperthermia-induced seizures in the immature rat cause a selective presynaptic increase in inhibitory synaptic transmission in the hippocampus that lasts into adulthood. The long-lasting nature of these potent alterations in synaptic communication after febrile seizures does not support the prevalent view of the 'benign' nature of early-life febrile convulsions.

Evidence for phosphorylation-dependent internalization of recombinant human rho1 GABAC receptors

1. Recombinant wild-type or mutant human rho1 GABA receptors were expressed in human embryonic kidney (HEK) 293 or monkey COS-7 cells and studied using the patch clamp technique. 2. Standard whole-cell recordings with 4 mM Mg-ATP in the patch pipette induced a time-dependent decrease in the GABA-activated current (IGABA) amplitude that was not the result of a decrease in GABA sensitivity. In contrast, IGABA remained stable when recordings were obtained using the perforated patch configuration or with standard whole-cell recording and no Mg-ATP in the patch pipette. 3. The inhibitors of serine/threonine protein kinases KN-62 (20 microM) or staurosporine (20 nM) prevented the time-dependent decrease in the amplitude of IGABA seen in the presence of ATP. Alkaline phosphatase (220 U ml-1), when added to the patch pipette in the absence of ATP, induced a transient potentiation of IGABA. Although the protein kinase C (PKC) activator 4beta-phorbol 12-myristate, 13-acetate (PMA) did not reduce the amplitude of IGABA, inclusion of the catalytic domain of PKC in the recording pipette accelerated the time-dependent decrease in current amplitude. These data suggest that phosphorylation is involved in the regulation of the amplitude of IGABA. 4. Mutation of the three PKC consensus sequences of the rho1 receptor had no significant effect on the decline in IGABA, indicating that direct phosphorylation of these putative sites on the rho1 receptor does not underlie the time-dependent decrease in amplitude. 5. In COS-7 cells transfected with wild-type rho1 receptors, the amplitude of IGABA had completely recovered to the original value when the same cells were repatched after 30-40 min, indicating that the decline in IGABA was a reversible process. 6. The inhibitor of actin filament formation cytochalasin B, when added to the patch pipette in the absence of ATP, induced a time-dependent inactivation suggesting that the actin cytoskeleton may play a role in the regulation of the amplitude. 7. Coincident with the decrease in the amplitude of IGABA, the cell capacitance significantly decreased in the presence of ATP in the patch pipette. This decrease in capacitance was not observed in the absence of Mg-ATP. The decrease in the membrane surface area suggests that receptor internalization could be a potential mechanism for the observed inactivation. 8. At 32 C, compared with 22 C, the rate and magnitude of the decline was increased dramatically. In contrast, at 16 C, no significant change in IGABA was observed over the 20 min recording time. This marked temperature sensitivity is consistent with receptor internalization as a mechanism for the time-dependent decline in IGABA. 9. The specificity of the decrease in IGABA was assessed by coexpressing the voltage-dependent potassium channel Kv1.4 along with the rho1 receptor in HEK293 cells. The amplitude of the potassium current (IKv1.4) exhibited very little decrement in comparison to IGABA suggesting that the putative GABA receptor internalization was not the consequence of a non-specific membrane retrieval.

Receptor recruitment: a mechanism for interactions between G protein-coupled receptors

There is a great deal of evidence for synergistic interactions between G protein-coupled signal transduction pathways in various tissues. As two specific examples, the potent effects of the biogenic amines norepinephrine and dopamine on sodium transporters and natriuresis can be modulated by neuropeptide Y and atrial natriuretic peptide, respectively. Here, we report, using a renal epithelial cell line, that both types of modulation involve recruitment of receptors from the interior of the cell to the plasma membrane. The results indicate that recruitment of G protein-coupled receptors may be a ubiquitous mechanism for receptor sensitization and may play a role in the modulation of signal transduction comparable to that of the well established phenomenon of receptor endocytosis and desensitization.

Neuroprotection at Drosophila synapses conferred by prior heat shock

Synapses are critical sites of information transfer in the nervous system, and it is important that their functionality be maintained under stressful conditions to prevent communication breakdown. Here we show that synaptic transmission at the Drosophila larval neuromuscular junction is protected by prior exposure to heat shock that strongly induces expression of heat shock proteins, in particular hsp70. Using a macropatch electrode to record synaptic activity at individual, visualized boutons, we found that prior heat shock sustains synaptic performance at high test temperatures through pre- and postsynaptic alterations. After heat shock, nerve impulses release more quantal units at high temperatures and exhibit fewer failures of release (presynaptic modification), whereas the amplitude of quantal currents remains more constant than does that in nonheat-shocked controls (postsynaptic modification). The time course of these physiological changes is similar to that of elevated hsp70. Thus, stress-induced neuroprotective mechanisms maintain function at synapses by modifying their properties.

Mapping quantitative trait loci for seizure response to a GABAA receptor inverse agonist in mice

To define the genetic contributions affecting individual differences in seizure threshold, a beta carboline [methyl-beta-carboline-3-carboxylate (beta-CCM)]-induced model of generalized seizures was genetically dissected in mice. beta-CCM is a GABAA receptor inverse agonist and convulsant. By measuring the latency to generalized seizures after beta-CCM administration to A/J and C57BL6/J mice and their progeny, we estimated a heritability of 0.28 +/- 0.10. A genome wide screen in an F2 population of these parental strains (n = 273) mapped quantitative trait loci (QTLs) on proximal chromosome 7 [logarithm of the likelihood for linkage (LOD) = 3.71] and distal chromosome 10 (LOD = 4.29) for seizure susceptibility, explaining approximately 22 and 25%, respectively, of the genetic variance for this seizure trait. The best fitting logistic regression model suggests that the A/J allele at each locus increases the likelihood of seizures approximately threefold. In a subsequent backcross population (n = 223), we mapped QTLs on distal chromosome 4 (LOD = 2.88) and confirmed the distal chromosome 10 QTLs (LOD = 4.36). In the backcross, the C57BL/6J allele of the chromosome 10 QTL decreases the risk of seizures approximately twofold. These QTLs may ultimately lead to the identification of genes influencing individual differences in seizure threshold in mice and the discovery of novel anticonvulsant agents. The colocalization on distal chromosome 10 of a beta-CCM susceptibility QTL and a QTL for open field ambulation and vertical movement suggests the existence of a single, pleiotropic locus, which we have named Exq1.

Activity-dependent regulation of GABAA receptors

GABAA receptor heterogeneity is based on the combinatorial assembly of a large family of subunits into distinct receptor subtypes. A neuron-specific expression pattern of receptor subtypes has been demonstrated in adult rat brain, which can be reproduced in vitro in primary neuron cultures. This suggests that genetic programs established during ontogeny govern the expression of gamma-aminobutyric acid (GABAA) receptor subtypes. Activity-dependent mechanisms nevertheless modulate on a short-term basis the cell surface expression of GABAA receptors, as demonstrated in cultured hippocampal neurons upon blockade of synaptic transmission or application of brain-derived neurotrophic factor. Preliminary evidence points to changes in protein phosphorylation as a mechanism underlying short-term activity-dependent regulation of GABAA receptors. In vivo, chronic pharmacological modulation of neuronal activity during development, while having marked effects on the rate of cortical growth, failed to influence the expression of GABAA receptor subtypes, suggesting that additional factors are involved.

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

Insulin causes a transient tyrosine phosphorylation of NR2A and NR2B NMDA receptor subunits in rat hippocampus

NMDA receptors play a critical role in various aspects of CNS function. Hence, it is important to identify mechanisms that regulate NMDA receptor activity. We have shown previously that insulin rapidly potentiates NMDA receptor activity in both native and recombinant expression systems. Here we report that insulin causes a transient phosphorylation of NR2A and NR2B NMDA receptor subunits on tyrosine residues. Rat hippocampal slices were exposed to 1 microM insulin for 20 and 60 min and then solubilized. NR2A and NR2B subunits were immunoprecipitated and probed for tyrosine phosphorylation. Insulin incubation of hippocampal slices for 20 min elicited an increase in tyrosine phosphorylation to 176 +/- 16% (NR2A) and 203 +/- 15% (NR2B) of control levels. In contrast, 60 min of insulin incubation did not alter NR2 tyrosine phosphorylation levels (NR2A: 85 +/- 13% of control; NR2B: 93 +/- 10% of control). Although the consequence of insulin-stimulated tyrosine phosphorylation is unknown, it is possible that this site(s) is responsible for insulin potentiation of NMDA receptor activity. This possibility is consistent with our earlier finding that insulin potentiates hippocampal NMDA receptor activity after 20 min, but not after 60 min, of insulin exposure.

GABA(A) receptors mediate inhibition of T cell responses

We describe the presence of functional GABA(A) receptors on T cells. GABA inhibited anti-CD3 and antigen-specific T cell proliferation in vitro in a dose-dependent manner that was 1) mimicked by the GABA(A) receptor agonist muscimol (but not the GABA(B) receptor agonist baclofen), 2) blocked by GABA(A) receptor antagonists and a GABA(A) receptor Cl- channel blocker (picrotoxin) and 3) enhanced by pentobarbital. These data suggest that GABA(A) receptors mediate this immune inhibition and that these receptors can be modulated in a similar fashion to their neuronal counterparts. Finally, GABA inhibited DTH responses in vivo. Thus, pharmacological modulation of GABA(A) receptors may provide new approaches to modulate T cell responses in inflammation and autoimmune disease.