Effect of PKG activation on recombinant GABAA receptors

cGMP-dependent protein kinases and cGMP phosphodiesterases in nitric oxide and cGMP action

To date, studies suggest that biological signaling by nitric oxide (NO) is primarily mediated by cGMP, which is synthesized by NO-activated guanylyl cyclases and broken down by cyclic nucleotide phosphodiesterases (PDEs). Effects of cGMP occur through three main groups of cellular targets: cGMP-dependent protein kinases (PKGs), cGMP-gated cation channels, and PDEs. cGMP binding activates PKG, which phosphorylates serines and threonines on many cellular proteins, frequently resulting in changes in activity or function, subcellular localization, or regulatory features. The proteins that are so modified by PKG commonly regulate calcium homeostasis, calcium sensitivity of cellular proteins, platelet activation and adhesion, smooth muscle contraction, cardiac function, gene expression, feedback of the NO-signaling pathway, and other processes. Current therapies that have successfully targeted the NO-signaling pathway include nitrovasodilators (nitroglycerin), PDE5 inhibitors [sildenafil (Viagra and Revatio), vardenafil (Levitra), and tadalafil (Cialis and Adcirca)] for treatment of a number of vascular diseases including angina pectoris, erectile dysfunction, and pulmonary hypertension; the PDE3 inhibitors [cilostazol (Pletal) and milrinone (Primacor)] are used for treatment of intermittent claudication and acute heart failure, respectively. Potential for use of these medications in the treatment of other maladies continues to emerge.

Kindling-induced alterations in GABAAreceptor-mediated inhibition and neurosteroid activity in the rat piriform cortex

The piriform cortex makes strong interconnections with limbic structures (amygdala, entorhinal cortex and hippocampus) that are involved in memory processing. These connections have also been implicated in the development of temporal lobe epilepsy. However, little is known about how neurones in this region may change during seizure genesis. Here we tested the hypothesis that in the kindling model of temporal lobe epilepsy GABAA receptor-mediated inhibition is altered in the piriform cortex. To do this we performed whole-cell patch-clamp recordings in piriform cortex brain slices obtained from non-kindled and amygdala-kindled adult rats. We found that kindling coincided with an increase in the amplitude and duration of miniature inhibitory post-synaptic currents (mIPSCs) recorded from non-pyramidal neurones, whereas the mIPSCs occurring on pyramidal (excitatory) cells did not change. Non-stationary noise analysis of mIPSCs occurring on the non-pyramidal neurones showed that inferred unitary conductance of synaptic channels were the same before and after kindling, implying that the channel number increased significantly. Immunocytochemical analysis of the inhibitory innervation showed that it was also unaltered by seizure induction. We also found that the effect of the positive modulator tetrahydrodeoxycorticosterone was reduced on the pyramidal neurones after kindling. In contrast, the potentiating effects of tetrahydrodeoxycorticosterone on non-pyramidal cells were about the same after kindling as in control (sham) rats. These data indicate that amygdala kindling causes a shift in the inhibition 'balance' between the pyramidal and non-pyramidal cells, perhaps leading to the disinhibition of pyramidal cells.

Pancreatic two P domain K+ channels TALK-1 and TALK-2 are activated by nitric oxide and reactive oxygen species

This study firstly shows with in situ hybridization on human pancreas that TALK-1 and TALK-2, two members of the 2P domain potassium channel (K(2P)) family, are highly and specifically expressed in the exocrine pancreas and absent in Langherans islets. On the contrary, expression of TASK-2 in mouse pancreas is found both in the exocrine pancreas and in the Langherans islets. This study also shows that TALK-1 and TALK-2 channels, expressed in Xenopus oocytes, are strongly and specifically activated by nitric oxide (obtained with a mixture of sodium nitroprussate (SNP) and dithiothreitol (DTT)), superoxide anion (obtained with xanthine and xanthine oxidase) and singlet oxygen (obtained upon photoactivation of rose bengal, and with chloramine T). Other nitric oxide and reactive oxygen species (NOS and ROS) donors, as well as reducing conditions were found to be ineffective on TALK-1, TALK-2 and TASK-2 (sin-1, angeli's salt, SNP alone, tBHP, H(2)O(2), and DTT). These results suggest that, in the exocrine pancreas, specific members of the NOS and ROS families could act as endogenous modulators of TALK channels with a role in normal secretion as well as in disease states such as acute pancreatitis and apoptosis.

Endothelin-1 and -3 diminish neuronal NE release through an NO mechanism in rat anterior hypothalamus

The existence of endothelin binding sites on the catecholaminergic neurons of the hypothalamus suggests that endothelins (ETs) participate in the regulation of noradrenergic transmission modulating various hypothalamic-controlled processes such as blood pressure, cardiovascular activity, etc. The effects of ET-1 and ET-3 on the neuronal release of norepinephrine (NE) as well as the receptors and intracellular pathway involved were studied in the rat anterior hypothalamus. ET-1 (10 nM) and ET-3 (10 nM) diminished neuronal NE release and the effect blocked by the selective ET type B receptor antagonist BQ-788 (100 nM). N(omega)-nitro-L-arginine methyl ester (10 microM), methylene blue (10 microM), and KT5823 (2 microM), inhibitors of nitric oxide synthase activity, guanylate cyclase, and protein kinase G, respectively, prevented the inhibitory effects of both ETs on neuronal NE release. In addition, both ETs increased nitric oxide synthase activity. Furthermore, 100 microM picrotoxin, a GABA(A)-receptor antagonist, inhibited ET-1 and ET-3 response. Our results show that ET-1 as well as ET-3 has an inhibitory neuromodulatory effect on NE release in the anterior hypothalamus mediated by the ET type B receptor and the involvement of a nitric oxide-dependent pathway and GABA(A) receptors. ET-1 and ET-3 may thus diminish available NE in the synaptic gap leading to decreased noradrenergic activity.

Glycosylated and phosphorylated proteins--expression in yeast and oocytes of Xenopus: prospects and challenges--relevance to expression of thermostable proteins

Phosphorylation and glycosylation are important posttranslational events in the biosynthesis of proteins. The different degrees of phosphorylation and glycosylation of proteins have been an intriguing phenomenon. Advances in genetic engineering have made it possible to control the degree of glycosylation and phosphorylation of proteins. Structural biology of phosphorylated and glycosylated proteins has been advancing at a much slower pace due to difficulties in using high-resolution NMR studies in solution phase. Major difficulties have arisen from the inherent mobilities of phosphorylated and glycosylated side chains. This paper reviews molecular and structural biology of phosphorylated and glycosylated proteins expressed in eukaryotic expression systems which are especially suited for large-scale production of these proteins. In our laboratory, we have observed that eukaryotic expression systems are particularly suited for the expression of thermostable light-activated proteins, e.g., bacteriorhodopsins and plastocyanins.

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.

Guanylyl cyclase stimulatory coupling to K(Ca) channels

We coexpressed the human large-conductance, calcium-activated K (K(Ca)) channel (alpha- and beta-subunits) and rat atrial natriuretic peptide (ANP) receptor genes in Xenopus oocytes to examine the mechanism of guanylyl cyclase stimulatory coupling to the channel. Exposure of oocytes to ANP stimulated whole cell K(Ca) currents by 21 +/- 3% (at 60 mV), without altering current kinetics. Similarly, spermine NONOate, a nitric oxide donor, increased K(Ca) currents (20 +/- 4% at 60 mV) in oocytes expressing the channel subunits alone. Stimulation of K(Ca) currents by ANP was inhibited in a concentration-dependent manner by a peptide inhibitor of cGMP-dependent protein kinase (PKG). Receptor/channel stimulatory coupling was not completely abolished by mutating the cAMP-dependent protein kinase phosphorylation site on the alpha-subunit (S869; Nars M, Dhulipals PD, Wang YX, and Kotlikoff MI. J Biol Chem 273: 14920-14924, 1998) or by mutating a neighboring consensus PKG site (S855), but mutation of both residues virtually abolished coupling. Spermine NONOate also failed to stimulate channels expressed from the double mutant cRNAs. These data indicate that nitric oxide donors stimulate K(Ca) channels through cGMP-dependent phosphorylation and that two serine residues (855 and 869) underlie this stimulatory coupling.

Regulation of the GABA(A) receptor by nitric oxide in frog pituitary melanotrophs

Nitric oxide (NO) is implicated in the regulation of various endocrine functions, but the effect of NO on GABA(A) receptor transmission has never been reported in endocrine cells. In the present study, we have investigated the effects of various agents acting on the NO transduction pathway on GABA(A) receptor function in frog pituitary melanotrophs. Histochemical studies using the NADPH-diaphorase reaction and immunohistochemical labeling with antibodies against neuronal NO synthase (nNOS) revealed that nNOS is expressed in the intermediate lobe of the pituitary and in cultured melanotrophs. Whole-cell patch-clamp recordings showed that the specific substrate of NOS L-arginine (L-Arg, 10(-4) M) or the NO donor sodium nitroprusside (10(-5) M) provoked a long-lasting inhibition of the current evoked by GABA (5 x 10(-6) M). The NOS inhibitor L-nitroarginine (10(-5) M) produced a biphasic effect, i.e. a transient decrease followed by a delayed increase of the GABA-evoked current amplitude. Similarly, the specific nNOS inhibitor 7-nitroindazole and the specific inducible NOS (iNOS) inhibitor aminoguanidine (10(-5) M each) provoked a transient depression of the current followed by a sustained potentiation. Formation of cGMP in neurointermediate lobes was enhanced by L-Arg (10(-4) M) and by the calcium-releasing agent caffeine (10(-4) M), and inhibited by the calmodulin (CaM)/Ca2+ complex blocker W7 (10(-5) M). The GABA-evoked current was potentiated by the guanylyl cyclase inhibitor ODQ (10(-8)-10(-7) M) and inhibited by the protein kinase G (PKG) activator 8pCPT-cGMP (3 x 10(-7)-3 x 10(-5) M). The present data indicate that NO, produced by a CaM/Ca2+-dependent NOS in frog melanotrophs, exerts an autocrine inhibitory effect on the GABA-evoked current. The action of NO on the GABA(A) receptor function is mediated through activation of the cGMP/PKG pathway.

Cyclic GMP-dependent feedback inhibition of AMPA receptors is independent of PKG

In central neurons, the second messenger cGMP is believed to induce long-term changes in efficacy at glutamatergic synapses through activation of protein kinase G (PKG). Stimulating nitric oxide synthase, activating soluble guanylyl cyclase or elevating concentrations of intracellular cGMP depressed excitatory synaptic transmission in CA1 hippocampal neurons. Unexpectedly, intracellular cGMP depressed responses of AMPA receptors and inhibited excitatory postsynaptic currents in hippocampal neurons independently of phosphorylation. Our findings demonstrate that cGMP's modulation of excitatory transmission may involve a coupling of AMPA channel activity to levels of cGMP.

Protein kinase and phosphatase modulation of quail brain GABA(A) and non-NMDA receptors co-expressed in Xenopus oocytes

The GABA(A) receptor and the non-NMDA subtype of the ionotropic glutamate receptor were co-expressed in Xenopus oocytes by injection of quail brain mRNA. The oocytes were treated with various protein kinase (PK) and protein phosphatase (PP) activators and inhibitors and the effects on receptor functioning were monitored. Two phorbol esters, 4-beta-phorbol 12-myristate-13-acetate (PMA) and 4-beta-phorbol 12,13-dibutyrate (PDBu); the cGMP-dependent PK activators sodium nitroprusside (SNP) and S-nitrosoglutathione (SNOG); and the PP inhibitor okadaic acid (OA) reduced the amplitude of the GABA-induced currents, whilst the PK inhibitor staurosporine potentiated it. In addition, PMA, PDBu, SNP, and OA reduced the desensitization of the GABA-induced response. Identical treatments generally had similar but less pronounced effects on responses generated by kainate (KA) but the desensitization characteristic of the non-NMDA receptor was not affected. None of the treatments had any effect on the reversal potentials of the induced currents. Immunoblots revealed that the oocytes express endogenous PKG and guanylate cyclase. The results are discussed in terms of the molecular structures of GABA(A) and non-NMDA receptors and the potential functional consequences of phosphorylation/dephosphorylation.

Activation of protein kinase C by phorbol dibutyrate modulates GABAA receptor binding in rat brain slices

Effects of protein kinase C (PKC) activation on the function of the GABA/benzodiazepine receptor-chloride complex were analyzed by quantitative autoradiography using [3H]muscimol, [3H]flunitrazepam and [35S]TBPS in rat brain slices. The density of [3H]muscimol binding was highest in cerebellar granular layers and high in both the frontal cortex and thalamus, but binding levels in the hippocampus were low. After activation of PKC by 100 nM phorbol-12,13-dibutyrate (PDBu), [3H]muscimol binding was decreased in the frontal cortex, striatum and thalamus, but binding levels were not changed in the hippocampus or cerebellum. The density of [3H]flunitrazepam binding was high in the cortex, hippocampus and molecular layers of cerebellum but was low in thalamus. PDBu increased the [3H]flunitrazepam binding only in the striatum and in part of the cortex and thalamus after activation of PKC. After activation of PKC by PDBu [35S]TBPS binding was increased in most areas, but binding levels were not changed in the brainstem or cerebellum. The receptor binding was markedly decreased in almost all areas by the addition of 2.5 mM Mg2+. Elevated [35S]TBPS binding produced by PDBu was significantly inhibited by the addition of Mg2+. These results suggest that the activation of PKC potentiates benzodiazepine and TBPS binding, but decreases muscimol binding in a region-specific manner in the rat brain.

Differential regulation of synaptic GABAA receptors by cAMP-dependent protein kinase in mouse cerebellar and olfactory bulb neurones

1. It has been demonstrated that the regulation of recombinant GABAA receptors by phosphorylation depends on the subunit composition. Here we studied the regulation of synaptic GABAA receptor function by cAMP-dependent protein kinase (PKA) in neurones expressing distinct receptor subtypes. 2. Light microscopic immunocytochemistry revealed that granule cells of the olfactory bulb express only the beta3 as the beta subunit variant, whereas cerebellar stellate and basket cells express only the beta2 as the beta subunit. 3. In cerebellar interneurones, intracellular application of 20 microM microcystin, a protein phosphatase 1/2A inhibitor, prolonged (63 +/- 14 %; mean +/- s.e.m.) the decay time course of miniature IPSCs (mIPSCs) without significantly affecting their amplitude, rise time and frequency. The effect of microcystin could be blocked by co-applying PKA inhibitory peptide (PKA-I, 1 microM). 4. No significant changes in any of the mIPSC parameters could be detected after intracellular application of PKA-I alone or following the inhibition of calcineurin with FK506 (50 nM). 5. In granule cells of the olfactory bulb expressing the beta3 subunit fast and slowly rising mIPSCs were detected, resulting in a bimodal distribution of the 10-90 % rise times, suggesting two distinct populations of events. Fast rising mIPSCs (mIPSCFR) had a 10-90 % rise time of 410 +/- 50 micros, an amplitude of 68 +/- 6 pA, and a weighted decay time constant (tauw) of 15.8 +/- 2.9 ms. In contrast, slowly rising mIPSCs (mIPSCSR) displayed an approximately threefold slower rise time (1.15 +/- 0.12 ms), 57 % smaller amplitude (29 +/- 1.7 pA), but had a tauw (16.8 +/- 3.0 ms) similar to that of the fast events. 6. mIPSCs in olfactory granule cells were not affected by the intracellular perfusion of microcystin. In spite of this, intracellular administration of constitutively active PKA caused a small, gradual, but significant increase (18 +/- 5 %) in the amplitude of the events without changing their time course. 7. These findings demonstrate a cell-type-dependent regulation of synaptic inhibition by protein phosphorylation. Furthermore, our results show that the effect of PKA-mediated phosphorylation on synaptic inhibition depends upon the subunit composition of postsynaptic GABAA receptors.

A novel serine kinase with specificity for beta3-subunits is tightly associated with GABA(A) receptors

Tuning of gamma-aminobutyric acid type A (GABA(A)) receptor function via phosphorylation of the receptor potentially allows neurons to modulate their inhibitory input. Several kinases, both of the serine-threonine kinase and the tyrosine kinase families, have been proposed as candidates for such a modulatory role in vivo. However, no GABA(A) receptor-phosphorylating kinase physically associated with the receptor has been identified so far on a molecular level. In this study, we demonstrate a GABA(A) receptor-associated protein serine kinase phosphorylating specifically beta3-subunits of native GABA(A) receptors. The characteristics of this novel kinase clearly distinguish it from enzymatic activities that have been shown so far to phosphorylate the GABA(A) receptor. We putatively identify this protein kinase as the previously described GTAP34 (GABA(A) receptor-tubulin complex-associated protein of molecular mass 34 kDa). Using expressed recombinant fusion proteins, we identify serine 408 as a major target of the phosphorylation reaction, whereas serine 407 is not phosphorylated. This demonstrates the high specificity of the kinase. Phosphorylation of serine 408 is known to result in a decreased receptor function. The direct association of this kinase with the receptor indicates an important physiological role.

Phosphorylation of proteins in chick ciliary ganglion under conditions that induce long-lasting changes in synaptic transmission: phosphoprotein targets for nitric oxide action

Production of nitric oxide and the activation of protein kinases are required for long-term potentiation of synaptic transmission at the giant synapses in chicken ciliary ganglion. In the present study, we investigated the ability of nitric oxide to regulate the phosphorylation of endogenous proteins under conditions that induced long-term potentiation in intact ciliary ganglion and the protein kinases responsible for the phosphorylation of these proteins in lysed ciliary ganglion. Using Calcium Green-1 we showed that the nitric oxide donor sodium nitroprusside did not change the intraterminal Ca2+ dynamics in ciliary ganglion. Two dimensional phosphopeptide analysis of 32Pi-labelled intact ciliary ganglion showed that the sodium nitroprusside (300 microM) increased the phosphorylation of several phosphopeptides (P50a, P50b and P41) derived from proteins at 50,000 and 41,000 mol. wts which we have called nitric oxide-responsive phosphoproteins. A similar stimulation of phosphorylation was achieved by 8-bromo-cyclic AMP (100 microM), which also induced long-term potentiation, but not by phorbol dibutyrate (2 microM) that does not induce long-term potentiation in ciliary ganglion. When subcellular fractions from lysed ciliary ganglion were labelled in vitro by [gamma-32P]ATP in the presence of purified cGMP-dependent, cAMP-dependent or Ca2+-phospholipid-dependent protein kinases, we identified cyclic GMP-dependent protein kinase substrates that gave rise to phosphopeptides co-migrating with P50a, P50b and P41 from 32Pi-labelled intact ciliary ganglion. P50a and P41 were derived from soluble proteins while P50b was derived from a membrane-associated protein. The proteins giving rise to P50a, P50b and P41 were also substrates for cyclic AMP-dependent protein kinase, but not for calcium and phospholipid-dependent protein kinase in vitro, suggesting that nitric oxide-responsive phosphoproteins are convergence points in information processing in vivo and their phosphorylation might represent an important mechanism in nitric oxide-mediated synaptic plasticity in ciliary ganglion.

Inhibition of primate spinothalamic tract neurons by spinal glycine and GABA is modulated by guanosine 3',5'-cyclic monophosphate

Our recent work has suggested that the nitric oxide/guanosine 3', 5'-cyclic monophosphate (NO/cGMP) signal transduction system contributes to central sensitization of spinothalamic tract (STT) neurons in part by influencing the descending inhibition of nociception resulting from stimulation in the periaqueductal gray. This study was designed to examine further whether activation of the NO/cGMP cascade reduces the inhibition of the activity of STT neurons mediated by spinal inhibitory amino acid (IAA) receptors. Responses of STT cells to noxious cutaneous stimuli were inhibited by iontophoresis of glycine and GABA agonists in anesthetized monkeys. Administration of 8-bromoguanosine-3',5'-cyclophosphate sodium (8-bromo-cGMP), a membrane permeable analogue of cGMP, either by microdialysis or by iontophoresis reduced significantly the IAA-induced inhibition of wide dynamic range (WDR) STT cells in the deep layers of the dorsal horn. The reduction in inhibition lasted for up to 1-1.5 h after the cessation of drug infusion. In contrast, IAA-induced inhibition of WDR STT cells in the superficial dorsal horn and high-threshold (HT) cells in superficial or deep layers was not significantly changed during 8-bromo-cGMP infusion. Iontophoresis of 8-bromo-cGMP onto STT cells produced the same actions as produced by microdialysis of this agent, but the effect was not as long-lasting nor as potent. Finally, an attenuation of the IAA receptor-mediated inhibition of STT cells produced by iontophoretic release of a NO donor, 3-morpholinosydnonimine, could be blocked by pretreatment of the spinal cord with a guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. These results suggest that an increased spinal cGMP level contributes to the sensitization of WDR STT neurons in the deep dorsal horn in part by down-regulating spinal IAA receptors. However, no evidence is provided in this study that the NO/cGMP cascade regulates IAA receptors on HT and superficial WDR neurons. Combined with the preceding studies, our data support the view that NO and cGMP function in the same signal transduction cascade and play an important role in central sensitization.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Regulation of neuronal plasticity in the central nervous system by phosphorylation and dephosphorylation

Neuronal plasticity can be defined as adaptive changes in structure and function of the nervous system, an obvious example of which is the capacity to remember and learn. Long-term potentiation and long-term depression are the experimental models of memory in the central nervous system (CNS), and have been frequently utilized for the analysis of the molecular mechanisms of memory formation. Extensive studies have demonstrated that various kinases and phosphatases regulate neuronal plasticity by phosphorylating and dephosphorylating proteins essential to the basic processes of adaptive changes in the CNS. These proteins include receptors, ion channels, synaptic vesicle proteins, and nuclear proteins. Multifunctional kinases (cAMP-dependent protein kinase, Ca2+/phospholipid-dependent protein kinase, and Ca2+/calmodulin-dependent protein kinases) and phosphatases (calcineurin, protein phosphatases 1, and 2A) that specifically modulate the phosphorylation status of neuronal-signaling proteins have been shown to be required for neuronal plasticity. In general, kinases are involved in upregulation of the activity of target substrates, and phosphatases downregulate them. Although this rule is applicable in most of the cases studied, there are also a number of exceptions. A variety of regulation mechanisms via phosphorylation and dephosphorylation mediated by multiple kinases and phosphatases are discussed.

D5 dopamine receptors enhance Zn2+-sensitive GABA(A) currents in striatal cholinergic interneurons through a PKA/PP1 cascade

Cholinergic interneurons have been implicated in striatally mediated associative learning. In classical conditioning paradigms, conditioned stimuli trigger a transient suppression of neuronal activity that is dependent upon an intact dopaminergic innervation. Our hypothesis was that this suppression reflected dopaminergic enhancement of sensory-linked GABAergic input. As a test, the impact of dopamine on interneuronal GABA(A) receptor function was studied by combined patch-clamp recording and single-cell reverse transcription PCR. Activation of D5 dopamine receptors reversibly enhanced a Zn2+-sensitive component of GABA(A) currents. Although dependent upon protein kinase A (PKA) activation, the modulation was blocked by protein phosphatase 1 (PP1) inhibition, suggesting it was dependent upon dephosphorylation. These results establish a novel mechanism by which intrastriatally released dopamine mediates changes in GABAergic signaling that could underlie the initial stages of associative learning.

Shaping of IPSCs by endogenous calcineurin activity

Synaptic inhibition, mediated by GABAA receptors, regulates neuronal firing, influences coincidence detection (König et al., 1996), and can synchronize the output of neural circuits (Cobb et al., 1995). Although GABAA receptors can be modulated by phosphorylation, few studies have directly addressed the role of such modulation at synapses, where the nonequilibrium conditions of receptor activation are quite different from those often used to study GABAA receptors in vitro. Here we promoted endogenous phosphorylation by inhibiting specific phosphatases in rat hippocampal neurons and compared the effects on IPSCs with GABAA channel responses in outside-out patches. Brief and saturating GABA pulses (5 msec; 10 mM) activated patch currents resembling the IPSC. Inhibition of calcineurin (protein phosphatase 2B), but not phosphatases 1 or 2A, produced a similar shortening of IPSC and patch responses, as did nonspecific inhibition of dephosphorylation using ATPgammaS or high concentrations of intracellular phosphate. Calcineurin inhibition increased the microscopic ligand unbinding rate, which was measured using the competitive antagonist 2-(3-carboxypropyl)-3-amino-6-(4-methoxyphenyl)pyridazinium bromide, suggesting that the IPSC shortening was partly caused by destabilization of the ligand binding site. Calcineurin inhibition also increased the rate and extent of macroscopic receptor desensitization. These results show that endogenous regulation by kinases and calcineurin can produce substantial changes in the IPSC duration by altering the unbinding and gating kinetics of the GABAA receptor. Dynamic regulation of synaptic inhibition may thus allow for the tuning of circuit behavior at the level of individual inhibitory synapses.

Regulation of excitatory and inhibitory neurotransmitter-gated ion channels by protein phosphorylation

Phosphorylation of ligand-gated ion channels is recognised as a potentially important mechanism for short- and long-term modulation of ion-channel function. Following the discovery of numerous sites of phosphorylation on ligand-gated ion channel proteins, recent studies have demonstrated that neurotransmitter-induced activation of serine/threonine, tyrosine and other kinases can result in the modulation of glutamate, type A gamma-aminobutyric acid (GABAA) and glycine receptors. These findings may have important consequences for our understanding of synaptic transmission and neuronal excitability.