GABAB receptor transduction mechanisms, and cross-talk between protein kinases A and C, in GABAergic terminals synapsing onto neurons of the rat nucleus basalis of Meynert

Activating Transcription Factor 4 (ATF4) Regulates Neuronal Activity by Controlling GABABR Trafficking

Activating Transcription Factor 4 (ATF4) has been postulated as a key regulator of learning and memory. We previously reported that specific hippocampal ATF4 downregulation causes deficits in synaptic plasticity and memory and reduction of glutamatergic functionality. Here we extend our studies to address ATF4's role in neuronal excitability. We find that long-term ATF4 knockdown in cultured rat hippocampal neurons significantly increases the frequency of spontaneous action potentials. This effect is associated with decreased functionality of metabotropic GABA_B receptors (GABA_BRs). Knocking down ATF4 results in significant reduction of GABA_BR-induced GIRK currents and increased mIPSC frequency. Furthermore, reducing ATF4 significantly decreases expression of membrane-exposed, but not total, GABA_BR 1a and 1b subunits, indicating that ATF4 regulates GABA_BR trafficking. In contrast, ATF4 knockdown has no effect on surface expression of GABA_BR2s, several GABA_BR-coupled ion channels or β2 and γ2 GABA_ARs. Pharmacologic manipulations confirmed the relationship between GABA_BR functionality and action potential frequency in our cultures. Specifically, the effects of ATF4 downregulation cited above are fully rescued by transcriptionally active, but not by transcriptionally inactive, shRNA-resistant, ATF4. We previously reported that ATF4 promotes stabilization of the actin-regulatory protein Cdc42 by a transcription-dependent mechanism. To test the hypothesis that this action underlies the mechanism by which ATF4 loss affects neuronal firing rates and GABA_BR trafficking, we downregulated Cdc42 and found that this phenocopies the effects of ATF4 knockdown on these properties. In conclusion, our data favor a model in which ATF4, by regulating Cdc42 expression, affects trafficking of GABA_BRs, which in turn modulates the excitability properties of neurons.SIGNIFICANCE STATEMENT GABA_B receptors (GABA_BRs), the metabotropic receptors for the inhibitory neurotransmitter GABA, have crucial roles in controlling the firing rate of neurons. Deficits in trafficking/functionality of GABA_BRs have been linked to a variety of neurological and psychiatric conditions, including epilepsy, anxiety, depression, schizophrenia, addiction, and pain. Here we show that GABA_BRs trafficking is influenced by Activating Transcription Factor 4 (ATF4), a protein that has a pivotal role in hippocampal memory processes. We found that ATF4 downregulation in hippocampal neurons reduces membrane-bound GABA_BR levels and thereby increases intrinsic excitability. These effects are mediated by loss of the small GTPase Cdc42 following ATF4 downregulation. These findings reveal a critical role for ATF4 in regulating the modulation of neuronal excitability by GABA_BRs.

Loss of constitutive functional γ-aminobutyric acid type A-B receptor crosstalk in layer 5 pyramidal neurons of human epileptic temporal cortex

OBJECTIVE

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in adult central nervous system, and profound alterations of GABA receptor functions are linked to temporal lobe epilepsy (TLE). Here we describe the functional relationships between GABA receptors type B (GABA_B R) and type A (GABA_A R) in human temporal cortex and how TLE affects this aspect of GABAergic signaling.

METHODS

Miniature inhibitory postsynaptic currents (mIPSCs) were recorded by patch-clamp techniques from human L5 pyramidal neurons in slices from temporal cortex tissue obtained from surgery.

RESULTS

We describe a constitutive functional crosstalk between GABA_B Rs and GABA_A Rs in human temporal layer 5 pyramidal neurons, which is lost in epileptic tissues. The activation of GABA_B Rs by baclofen, in addition to the expected reduction of mIPSC frequency, produced, in cortex of nonepileptic patients, the prolongation of mIPSC rise and decay times, thus increasing the inhibitory net charge associated with a single synaptic event. Block of K⁺ channels did not prevent the increase of decay time and charge. Protein kinase A (PKA) blocker KT5720 and pertussis toxin inhibited the action of baclofen, whereas 8Br-cAMP mimicked the GABA_B R action. The same GABA_B R-mediated modulation of GABA_A Rs was observed in pyramidal neurons of rat temporal cortex, with both PKA and PKC involved in the process. In cortices from TLE patients and epileptic rats, baclofen lost its ability to modulate mIPSCs.

SIGNIFICANCE

Our results highlight the association of TLE with functional changes of GABAergic signaling that may be related to seizure propagation, and suggest that the selective activation of a definite subset of nonpresynaptic GABA_B Rs may be therapeutically useful in TLE.

GABAB receptor attenuation of GABAA currents in neurons of the mammalian central nervous system

Ionotropic receptors are tightly regulated by second messenger systems and are often present along with their metabotropic counterparts on a neuron's plasma membrane. This leads to the hypothesis that the two receptor subtypes can interact, and indeed this has been observed in excitatory glutamate and inhibitory GABA receptors. In both systems the metabotropic pathway augments the ionotropic receptor response. However, we have found that the metabotropic GABA_B receptor can suppress the ionotropic GABA_A receptor current, in both the in vitro mouse retina and in human amygdala membrane fractions. Expression of amygdala membrane microdomains in Xenopus oocytes by microtransplantation produced functional ionotropic and metabotropic GABA receptors. Most GABA_A receptors had properties of α‐subunit containing receptors, with ~5% having ρ‐subunit properties. Only GABA_A receptors with α‐subunit‐like properties were regulated by GABA_B receptors. In mouse retinal ganglion cells, where only α‐subunit‐containing GABA_A receptors are expressed, GABA_B receptors suppressed GABA_A receptor currents. This suppression was blocked by GABA_B receptor antagonists, G‐protein inhibitors, and GABA_B receptor antibodies. Based on the kinetic differences between metabotropic and ionotropic receptors, their interaction would suppress repeated, rapid GABAergic inhibition.

Automatic positioning device for cutting three-dimensional tissue in living or fixed samples. Proof of concept

The study and analysis of tissues has always been an important part of the subject in biology. For this reason, obtaining specimens of tissue has been vital to morphological and functionality research. Historically, the main tools used to obtain slices of tissue have been microtomes and vibratomes. However, they are largely unsatisfactory. This is because it is impossible to obtain a full, three-dimensional structure of a tissue sample with these devices. This paper presents an automatic positioning device for a three-dimensional cut in living or fixed tissue samples, which can be applied mainly in histology, anatomy, biochemistry and pharmacology. The system consists of a platform on which the tissue samples can be deposited, plus two containers. An electromechanical system with motors and gears gives the platform the ability to change the orientation of a sample. These orientation changes were tested with movement sensors to ensure that accurate changes were made. This device paves the way for researchers to make cuts in the sample tissue along different planes and in different directions by maximizing the surface of the tract that appears in a slice.

Modulation of learning and memory by natural polyamines

Spermine and spermidine are natural polyamines that are produced mainly via decarboxylation of l-ornithine and the sequential transfer of aminopropyl groups from S-adenosylmethionine to putrescine by spermidine synthase and spermine synthase. Spermine and spermidine interact with intracellular and extracellular acidic residues of different nature, including nucleic acids, phospholipids, acidic proteins, carboxyl- and sulfate-containing polysaccharides. Therefore, multiple actions have been suggested for these polycations, including modulation of the activity of ionic channels, protein synthesis, protein kinases, and cell proliferation/death, within others. In this review we summarize these neurochemical/neurophysiological/morphological findings, particularly those that have been implicated in the improving and deleterious effects of spermine and spermidine on learning and memory of naïve animals in shock-motivated and nonshock-motivated tasks, from a historical perspective. The interaction with the opioid system, the facilitation and disruption of morphine-induced reward and the effect of polyamines and putative polyamine antagonists on animal models of cognitive diseases, such as Alzheimer's, Huntington, acute neuroinflammation and brain trauma are also reviewed and discussed. The increased production of polyamines in Alzheimer's disease and the biphasic nature of the effects of polyamines on memory and on the NMDA receptor are also considered. In light of the current literature on polyamines, which include the description of an inborn error of the metabolism characterized by mild-to moderate mental retardation and polyamine metabolism alterations in suicide completers, we can anticipate that polyamine targets may be important for the development of novel strategies and approaches for understanding the etiopathogenesis of important central disorders and their pharmacological treatment.

Modulation by the GABA(B) receptor siRNA of ethanol-mediated PKA-α, CaMKII, and p-CREB intracellular signaling in prenatal rat hippocampal neurons

Fetal alcohol syndrome (FAS) is a developmental neuropathology resulting from in utero exposure to ethanol; many of ethanol's effects are likely to be mediated by the neurotransmitter γ-aminobutyric acid (GABA). We studied modulation of the neurotransmitter receptor GABA(B)R and its capacity for intracellular signal transduction under conditions of ethanol treatment (ET) and RNA interference to investigate a potential role for GABA signaling in FAS. ET increased GABA(B1)R protein levels, but decreased protein kinase A-α (PKA-α), calcium/calmodulin-dependent protein kinase II (CaMKII) and phosphorylation of cAMP-response element binding protein (p-CREB), in cultured hippocampal neurons harvested at gestation day 17.5. To elucidate GABA(B1)R response to ethanol, we observed the effects of a GABA(B)R agonist and antagonist in pharmacotherapy for ethanol abuse. Baclofen increased GABA(B)R, CaMKII and p-CREB levels, whereas phaclofen decreased GABA(B)R, CaMKII and p-CREB levels except PKA-α. Furthermore, when GABA(B1)R was knocked down by siRNA treatment, CaMKII and p-CREB levels were reduced upon ET. We speculate that stimulation of GABA(B1)R activity by ET can modulate CaMKII and p-CREB signaling to detrimental effect on fetal brain development.

Ethanol-enhanced GABA release: a focus on G protein-coupled receptors

While research on the actions of ethanol at the GABAergic synapse has focused on postsynaptic mechanisms, recent data have demonstrated that ethanol also facilitates GABA release from presynaptic terminals in many, but not all, brain regions. The ability of ethanol to increase GABA release can be regulated by different G protein-coupled receptors (GPCRs), such as the cannabinoid-1 receptor, corticotropin-releasing factor 1 receptor, GABA(B) receptor, and the 5-hydroxytryptamine 2C receptor. The intracellular messengers linked to these GPCRs, including the calcium that is released from internal stores, also play a role in ethanol-enhanced GABA release. Hypotheses are proposed to explain how ethanol interacts with the GPCR pathways to increase GABA release and how this interaction contributes to the brain region specificity of ethanol-enhanced GABA release. Defining the mechanism of ethanol-facilitated GABA release will further our understanding of the GABAergic profile of ethanol and increase our knowledge of how GABAergic neurotransmission may contribute to the intoxicating effects of alcohol and to alcohol dependence.

Neuroprotective effect of vitamin C against the ethanol and nicotine modulation of GABA(B) receptor and PKA-alpha expression in prenatal rat brain

Prenatal ethanol exposure has various deleterious effects on neuronal development and can induce various defects in developing brain, resulting in fetal alcohol syndrome (FAS). gamma-Aminobutyric acid (GABA(B)) receptor (R) is known to play an important role during the development of the central nervous system (CNS). Our study was designed to investigate the effect of ethanol (100 mM), nicotine (50 microM) (for 30 min and 1 h), vitamin C (vitC, 0.5 mM), ethanol plus vitC, and nicotine plus vitC on expression level of GABA(B1), GABA(B2)R, and protein kinase A-alpha (PKA) in prenatal rat cortical and hippocampal neurons at gestational days (GD) 17.5. The results showed that, upon ethanol and nicotine exposure, GABA(B1) and GABA(B2)R protein expression increased significantly in the cortex and hippocampus for a short (30 min) and long term (1 h), whereas only GABA(B2)R subunit was decreased upon nicotine exposure for a long term in the cortex. Furthermore, PKA expression in cortex and hippocampus increased with ethanol exposure during short term, whereas long-term exposure results increased in cortex and decreased in hippocampus. Moreover, the cotreatment of vitC with ethanol and nicotine showed significantly decreased expression of GABA(B1), GABA(B2)R, and PKA in cortex and hippocampus for a long-term exposure. Mitochondrial membrane potential, Fluoro-jade-B, and propidium iodide staining were used to elucidate possible neurodegeneration. Our results suggest the involvement of GABA(B)R and PKA in nicotine and ethanol-mediated neurodevelopmental defects and the potential use of vitC as a effective protective agent for FAS-related deficits.

Ethanol and PTZ effects on siRNA-mediated GABAB1 receptor: down regulation of intracellular signaling pathway in prenatal rat cortical and hippocampal neurons

GABA(B) receptors (R) are widely expressed and distributed in the nervous system, and have been implicated in variety of neurodegenerative and pathophysiological disorders. However, the exact molecular mechanism regarding responsibility of GABA(B1)R in downstream signaling pathway is not well understood. The present study was undertaken to explore the downstream signaling and role of GABA(B1)R upon acute ethanol and pentylenetetrazol (PTZ) exposure for (20 min) in cortical and hippocampal neuronal cell cultures by using GABA(B1)R RNA interference (i) (30 nM, 48 h) at gestational days 17.5. The results showed that GABA(B1)R and protein kinase A-alpha (PKA) showed decreased expression upon ethanol and PTZ exposure in cortical and hippocampal neurons during transfected and nontransfected conditions, whereas these effects could lead to significant changes in phosphorylation of cAMP-response element binding protein (p-CREB) expression where GABA(B1)R was knocked down. Furthermore, intracellular Ca(+2) concentrations were also reduced in some groups after transfection with GABA(B1)R RNAi. These results showed a critical role of GABA(B1)R upon ethanol and PTZ exposure by modulating downstream signaling pathway. Finally, these findings suggested that inhibition of GABA(B1)R results in the modulation of PKA, p-CREB pathway may play a role in long-term changes in the nervous system, and may be an underlying cause of ethanol's effects.

The role of protein kinase A in the ethanol-induced increase in spontaneous GABA release onto cerebellar Purkinje neurons

Ethanol increases miniature inhibitory postsynaptic current frequency and decreases the paired-pulse ratio, which suggests that ethanol increases both spontaneous and evoked GABA release, respectively. We have shown previously that ethanol increases GABA release at the rat interneuron-Purkinje cell synapse and that this ethanol effect involves calcium release from internal stores; however, further exploration of the mechanism responsible for ethanol-enhanced GABA release was needed. We found that a cannabinoid receptor 1 (CB1) agonist, WIN-55212, and a GABA(B) receptor agonist, baclofen, decreased baseline spontaneous GABA release and prevented ethanol from increasing spontaneous GABA release. The CB1 receptor and GABA(B) receptor are Galpha i-linked G protein-coupled receptors with common downstream messengers that include adenylate cyclase and protein kinase A (PKA). Adenylate cyclase and PKA antagonists blocked ethanol from increasing spontaneous GABA release, whereas a PKA antagonist limited to the postsynaptic neuron did not block ethanol from increasing spontaneous GABA release. These results suggest that presynaptic PKA plays an essential role in ethanol-enhanced spontaneous GABA release. Similar to ethanol, we found that the mechanism of the cannabinoid-mediated decrease in spontaneous GABA release involves internal calcium stores and PKA. A PKA antagonist decreased baseline spontaneous GABA release. This effect was reduced after incubating the slice with a calcium chelator, BAPTA-AM, but was unaffected when BAPTA was limited to the postsynaptic neuron. This suggests that the PKA antagonist is acting through a presynaptic, calcium-dependent mechanism to decrease spontaneous GABA release. Overall, these results suggest that PKA activation is necessary for ethanol to increase spontaneous GABA release.

D2-like dopamine receptor-mediated modulation of activity-dependent plasticity at GABAergic synapses in the subthalamic nucleus

Reciprocally connected glutamatergic subthalamic nucleus (STN) and GABAergic external globus pallidus (GP) neurons normally exhibit weakly correlated, irregular activity but following the depletion of dopamine in Parkinson's disease they express more highly correlated, rhythmic bursting activity. Patch clamp recording was used to test the hypothesis that dopaminergic modulation reduces the capability of GABAergic inputs to pattern 'pathological' activity in STN neurons. Electrically evoked GABA(A) receptor-mediated IPSCs exhibited activity-dependent plasticity in STN neurons, i.e. IPSCs evoked at frequencies between 1 and 50 Hz exhibited depression that increased with the frequency of activity. Dopamine, the D(2)-like dopamine receptor agonist quinpirole and external media containing a low [Ca(2+)] reduced both the magnitude of IPSCs evoked at 1-50 Hz and synaptic depression at 10-50 Hz. Dopamine/quinpirole also reduced the frequency but not the amplitude of miniature IPSCs recorded in the presence of tetrodotoxin. D(1)-like and D(4) agonists were ineffective and D(2/3) but not D4 receptor antagonists reversed the effects of dopamine or quinpirole. Together these data suggest that presynaptic D(2/3) dopamine receptors modulate the short-term dynamics of GABAergic transmission in the STN by lowering the initial probability of transmitter release. Simulated GABA(A) receptor-mediated synaptic conductances representative of control or modulated transmission were then generated in STN neurons using the dynamic clamp technique. Dopamine-modulated transmission was less effective at resetting autonomous activity or generating rebound burst firing than control transmission. The data therefore support the conclusion that dopamine acting at presynaptic D(2)-like receptors reduces the propensity for GABAergic transmission to generate correlated, bursting activity in STN neurons.

Activation of presynaptic kainate receptors suppresses GABAergic synaptic transmission in the rat globus pallidus

The globus pallidus (GP) plays a central integrative role in the basal ganglia circuitry. It receives strong GABAergic inputs from the striatum (Str) and significant glutamatergic afferents from the subthalamic nucleus (STN). The change in firing rate and pattern of GP neurons is a cardinal feature of Parkinson's disease pathophysiology. Kainate receptor (KAR) GluR6/7 subunit immunoreactivity is expressed presynaptically in GABAergic striatopallidal terminals which provides a substrate for regulation of GABAergic transmission in GP. To test this hypothesis, we recorded GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) in the GP following electrical stimulation of the Str. Following blockade of AMPA and N-methyl-d-aspartate receptors with selective antagonists, bath application of kainate (KA) (0.3-3 microM) reduced significantly the amplitude of evoked IPSCs. This inhibition was associated with a significant increase in paired-pulse facilitation ratio and a reduction of the frequency, but not amplitude, of miniature inhibitory postsynaptic currents (mIPSCs), suggesting a presynaptic site of KA action. The KA effects on striatopallidal GABAergic transmission were blocked by the G-protein inhibitor, N-ethylmaleimide (NEM), or protein kinase C (PKC) inhibitor calphostin C. Our results demonstrate that KAR activation inhibits GABAergic transmission through a presynaptic G protein-coupled, PKC-dependent metabotropic mechanism in the rat GP. These findings open up the possibility for the development of KA-mediated pharmacotherapies aimed at decreasing the excessive and abnormally regulated inhibition of GP neurons in Parkinson's disease.

Ethanol's effect on intracellular signal pathways in prenatal rat cortical neurons is GABAB1 dependent

To confirm the modulation role of GABA(B) on ethanol' effects, we studied the effects of ethanol on the neuronal intracellular signals, protein kinase A (PKA) and cAMP-response element binding protein (CREB), by using a system where GABA(B1) receptors were specifically knocked down in the in vitro cultivated cortical neurons. The results showed that the PKA alpha subunit was increased with ethanol treatment, and could be further increased by administering baclofen and phaclofen. By contrast, baclofen and/or phaclofen could decrease ethanol's up-regulation effects on PKA alpha subunit expression in primary cultured cortical neurons in which the GABA(B1) receptor was specifically knocked down using GABA(B1) receptor RNA interference. Furthermore, these effects could lead to changes of phospho (p)-CREB expression, which showed the same expression pattern as PKA. Finally, we observed changes of GABA(B1), PKA, and p-CREB distribution within the same neuronal cells. These results showed that the GABA(B) receptors are critical to ethanol's cellular effects, which occur via modulating the PKA and CREB transcription pathway, and may be an underlying cause of ethanol's effects.

Regulation by nicotine of Gpr51 and Ntrk2 expression in various rat brain regions

Our previous genetic studies demonstrated that variants of the gamma-Aminobutyric acid B receptor subunit 2 (GPR51) and neurotrophic tyrosine kinase receptor type 2 (NTRK2) genes are significantly associated with nicotine dependence (ND) in smokers. However, whether such genetic associations lead to changes in the expression of the two genes in response to nicotine remains undetermined. In this study, we investigated the regulatory effect of nicotine on the expression of Gpr51 and Ntrk2 in seven rat brain regions during the administration of nicotine in a daily dose of 3.15 mg/kg for 7 days. With quantitative real-time RT-PCR, we found that nicotine increased the mRNA of Gpr51 by 70, 78, and 32% in the amygdala, striatum, and prefrontal cortex (PFC), respectively, but decreased by 54% in the nucleus accumbens (NA). The Gpr51 protein was upregulated by nicotine in the amygdala (26%), striatum (73%), PFC (28%), and medial basal hypothalamus (MBH; 19%) but downregulated in the NA (-72%). Similarly, the mRNA level of Ntrk2 was enhanced by nicotine in the striatum (86%) and PFC (38%), but decreased in the NA (-46%) and ventral tegmental area (VTA; -49%). A significant change in protein expression was also obtained for Ntrk2 in the PFC (24%), MBH (33%), NA (-33%), and VTA (-70%). Interestingly, these two genes showed a closely coordinated expression pattern in response to nicotine in most of the brain regions examined. In summary, our results demonstrate that the expression of Gpr51 and Ntrk2 is significantly regulated by nicotine at both the mRNA and protein levels in various brain regions, which provides further evidence that these two genes are involved in the etiology of ND, as reported in our previous genetic association studies in humans.

Regulation of synaptic input to hypothalamic presympathetic neurons by GABA(B) receptors

The hypothalamic paraventricular (PVN) neurons projecting to the spinal cord and brainstem play an important role in the control of homeostasis and the sympathetic nervous system. Although GABA(B) receptors are present in the PVN, their function in the control of synaptic inputs to PVN presympathetic neurons is not clear. Using retrograde tracing and whole-cell patch-clamp recordings in rat brain slices, we determined the role of presynaptic GABA(B) receptors in regulation of glutamatergic and GABAergic inputs to spinally projecting PVN neurons. The GABA(B) receptor agonist baclofen (1-50 microM) dose-dependently decreased the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) and inhibitory postsynaptic currents (sIPSCs). The effect of baclofen on sEPSCs and sIPSCs was completely blocked by 10 microM CGP52432, a selective GABA(B) receptor antagonist. Baclofen also significantly reduced the frequency of both miniature excitatory and miniature inhibitory postsynaptic currents (mEPSCs and mIPSCs). Furthermore, uncoupling pertussis toxin-sensitive G(i/o) proteins with N-ethylmaleimide abolished baclofen-induced inhibition of mEPSCs and mIPSCs. However, the inhibitory effect of baclofen on the frequency of mIPSCs and mEPSCs persisted in the presence of either Cd2+, a voltage-gated Ca2+ channel blocker, or 4-aminopyridine, a blocker of voltage-gated K+ channels. Our results suggest that activation of presynaptic GABA(B) receptors inhibits synaptic GABA and glutamate release to PVN presympathetic neurons. This presynaptic action of GABA(B) receptors is mediated by the N-ethylmaleimide-sensitive G(i/o) proteins, but independent of voltage-gated Ca2+ and K+ channels.

Localization and function of pre- and postsynaptic kainate receptors in the rat globus pallidus

Kainate receptors (KARs) are widely expressed the basal ganglia. In this study, we used electron microscopic immunocytochemistry and whole-cell recording techniques to examine the localization and function of KARs in the rat globus pallidus (GP). Dendrites were the most common immunoreactive elements, while terminals forming symmetric or asymmetric synapses and unmyelinated axons comprised most of the presynaptic labeling. To determine whether synaptically released glutamate activates KARs, we recorded excitatory postsynaptic currents (EPSCs) in the GP following single-pulse stimulation of the internal capsule. 4-(8-Methyl-9H-1,3-dioxolo[4,5 h]{2,3}benzodiazepine-5-yl)-benzenamine hydrochloride (GYKI 52466, 100 microm), an alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonist, reduced but did not completely block evoked EPSCs. The remaining EPSC component was mediated through activation of KARs because it was abolished by 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX), an AMPA/KAR antagonist. The rise time (10-90%) and decay time constant (tau) for those EPSCs were longer than those of AMPA-mediated EPSCs recorded before GYKI 52466 application. KAR activation inhibited EPSCs. This inhibition was associated with a significant increase in paired-pulse facilitation ratio, suggesting a presynaptic action of KAR. KAR inhibition of EPSCs was blocked by the G-protein inhibitor, N-ethylmaleimide (NEM), or the protein kinase C (PKC) inhibitor calphostin C. Our results demonstrate that KAR activation has dual effects on glutamatergic transmission in the rat GP: (1) it mediates small-amplitude EPSCs; and (2) it reduces glutamatergic synaptic transmission through a presynaptic G-protein coupled, PKC-dependent, metabotropic mechanism. These findings provide evidence for the multifarious functions of KARs in regulating synaptic transmission, and open up the possibility for the development of pharmacotherapies to reduce the hyperactive subthalamofugal projection in Parkinson's disease.

GABA modulates Drosophila circadian clock neurons via GABAB receptors and decreases in calcium

Circadian clocks play vital roles in the control of daily rhythms in physiology and behavior of animals. In Drosophila, analysis of the molecular and behavioral rhythm has shown that the master clock neurons are entrained by sensory inputs and are synchronized with other clock neurons. However, little is known about the neuronal circuits of the Drosophila circadian system and the neurotransmitters that act on the clock neurons. Here, we provide evidence for a new neuronal input pathway to the master clock neurons, s-LN(v)s, in Drosophila that utilizes GABA as a slow inhibitory neurotransmitter. We monitored intracellular calcium levels in dissociated larval s-LN(v)s with the calcium-sensitive dye Fura-2. GABA decreased intracellular calcium in the s-LN(v)s and blocked spontaneous oscillations in calcium levels. The duration of this response was dose-dependent between 1 nM and 100 microM. The response to GABA was blocked by a metabotropic GABA(B) receptor (GABA(B)-R) antagonist, CGP54626, but not by an ionotropic receptor antagonist, picrotoxin. The GABA(B)-R agonist, 3-APMPA, produced a response similar to GABA. An antiserum against one of the Drosophila GABA(B)-Rs (GABA(B)-R2) labeled the dendritic regions of the s-LN(v)s in both adults and larvae, as well as the dissociated s-LN(v)s. We found that some GABAergic processes terminate at the dendrites of the LN(v)s, as revealed by GABA immunostaining and a GABA-specific GAL4 line (GAD1-gal4). Our results suggest that the s-LN(v)s receive slow inhibitory GABAergic inputs that decrease intracellular calcium of these clock neurons and block their calcium cycling. This response is mediated by postsynaptic GABA(B) receptors.

Cholecystokinin-2 receptors couple to cAMP–protein kinase A to depress excitatory synaptic currents in rat nucleus accumbens in vitro

We recently reported that the activation of cholecystokinin-2 receptors depress evoked excitatory postsynaptic currents (EPSCs) in nucleus accumbens (NAc) indirectly through γ-aminobutyric acid (GABA) acting on γ-aminobutyric acid-B (GABAB) receptors. Here, we determined the second messenger system that couples cholecystokinin-2 receptors to the observed synaptic depression. Using in vitro forebrain slices of rats and whole-cell patch recording, we tested the hypothesis that cholecystokinin-2 receptors are coupled to cAMP and protein kinase A signaling pathway. Cholecystokinin-8S induced inward currents and depressed evoked EPSCs. Forskolin, an activator of adenylyl cyclase and rolipram that is an inhibitor of phosphodiesterase type IV, independently increased EPSC amplitude and blocked the inward current and synaptic depression induced by cholecystokinin-8S. Furthermore, the membrane-permeable cAMP analog, 8-bromo-cAMP, blocked the cholecystokinin-8S effects. H89, a protein kinase A inhibitor, also blocked cholecystokinin-8S effects. However, depression of the evoked EPSC by baclofen, a GABABreceptor agonist, was not blocked by H89 or forskolin. These findings indicate that cholecystokinin-2, but not GABAB, receptors are coupled to the adenylyl cyclase – cAMP – protein kinase A signaling pathway in the NAc to induce inward currents and cause synaptic depression.

GABAB receptors in the medial septum/diagonal band slice from 16-25 day rat

GABA(B) receptors are believed to play a role in rhythmic activity in the mammalian brain. The aim of our study was to examine the presynaptic and postsynaptic locations of these receptors in the medial septal diagonal band area (MS/DB), an area known to pace the hippocampus theta rhythm. Whole-cell patch recordings were made from parasagittal MS/DB slices obtained from the 16-25 day rat. Neurons were classified into GABAergic and cholinergic subtypes according to previous electrophysiological criteria. Bath application of the GABA(B) receptor agonist baclofen in the presence of tetrodotoxin, and brief tetanic fiber stimulation in the presence of ionotropic receptor antagonists, provided evidence for the presence of postsynaptic GABA(B) receptor transmission to GABAergic but not cholinergic neurons. Bath application of baclofen, at concentrations too low to elicit postsynaptic activity in MS/DB neurons, significantly reduced the amplitudes of stimulus-evoked ionotropic receptor inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs) and the paired pulse depression of these evoked potentials. Baclofen also significantly reduced the frequencies but not the amplitudes of miniature inhibitory postsynaptic currents (IPSCs) and excitatory postsynaptic currents (EPSCs), indicating the presence of presynaptic GABA(B) receptors on GABAergic and glutamatergic terminals in the MS/DB. Baclofen, also at a concentration too low to elicit postsynaptic activity, reduced the frequencies and amplitudes of spontaneous IPSCs and EPSCs recorded in the presence of 200-400 nM kainate. Rhythmic compound IPSCs at theta frequencies were recorded under these conditions in some neurons, and these rhythmic compound IPSCs were disrupted by the activation but not by the inhibition of GABA(B) receptors. These results suggest that GABA(B) receptors modulate rather than generate rhythmic activity in the MS/DB, and that this modulatory effect occurs via receptors located on presynaptic terminals.

Chronic cocaine administration switches corticotropin-releasing factor2 receptor-mediated depression to facilitation of glutamatergic transmission in the lateral septum

Corticotropin-releasing factor (CRF) and urocortin (Ucn I) are endogenous members among a family of CRF-related peptides that activate two different and synaptically localized G-protein-coupled receptors, CRF1 and CRF2. These peptides and their receptors have been implicated in stress responses and stress with cocaine abuse. In this study, we observed significant alterations in excitatory transmission and CRF-related peptide regulation of excitatory transmission in the lateral septum mediolateral nucleus (LSMLN) after chronic cocaine administration. In brain slice recordings from the LSMLN of control (saline-treated) rats, glutamatergic synaptic transmission was facilitated by activation of CRF1 receptors with CRF but was depressed after activation of CRF2 receptors with Ucn I. After acute withdrawal from a chronic cocaine administration regimen, CRF1 activation remained facilitatory, but CRF2 activation facilitated rather than depressed LSMLN EPSCs. These alterations in CRF2 effects occurred through both presynaptic and postsynaptic mechanisms. In saline-treated rats, CRF1 and CRF2 coupled predominantly to protein kinase A signaling pathways, whereas after cocaine withdrawal, protein kinase C activity was more prominent and likely contributed to the CRF2-mediated presynaptic facilitation. Neither CRF nor Ucn I altered monosynaptic GABA(A)-mediated IPSCs before or after chronic cocaine administration, suggesting that loss of GABAA-mediated inhibition could not account for the facilitation. This switch in polarity of Ucn I-mediated neuromodulation, from a negative to positive regulation of excitatory glutamatergic transmission after chronic cocaine administration, could generate an imbalance in the brain reward circuitry associated with the LSMLN.

Calcitonin gene-related peptide receptor activation produces PKA- and PKC-dependent mechanical hyperalgesia and central sensitization

Calcitonin gene-related peptide (CGRP), acting through CGRP receptors, produces behavioral signs of mechanical hyperalgesia in rats and sensitization of wide dynamic range (WDR) neurons in the spinal cord dorsal horn. Although involvement of CGRP receptors in central sensitization has been confirmed, the second-messenger systems activated by CGRP receptor stimulation and involved in pain transmission are not clear. This study tested whether the hyperalgesia and sensitizing effects of CGRP receptor activation on WDR neurons are mediated by protein kinase A or C (PKA or PKC) signaling. Intrathecal injection of CGRP in rats produced mechanical hyperalgesia, as shown by paw withdrawal threshold tests. CGRP-induced hyperalgesia was attenuated significantly by the CGRP1 receptor antagonist, CGRP8-37. The effect was also attenuated significantly by a PKA inhibitor (H89) or a PKC inhibitor (chelerythrine chloride). Electrophysiological experiments demonstrated that superfusion of the spinal cord with CGRP-induced sensitization of spinal dorsal horn neurons. The CGRP effect could be blocked by CGRP8-37. Either a PKA or PKC inhibitor (H89 or chelerythrine) also attenuated this effect of CGRP. These results are consistent with the hypothesis that CGRP produces hyperalgesia by a direct action on CGRP1 receptors in the spinal cord dorsal horn and suggest that the effects of CGRP are mediated by both PKA and PKC second-messenger pathways.

Differential GABAB Receptor Modulation of Ethanol Effects on GABAA Synaptic Activity in Hippocampal CA1 Neurons

We tested the hypothesis that differential sensitivity to ethanol of synaptic GABA(A) somatic and dendritic inhibitory postsynaptic currents (IPSCs) in hippocampal CA1 pyramidal neurons could be due to differences in the extent of GABA(B) receptor activity at GABAergic synapses in these two hippocampal subfields. Our present results show that dendritic (distally evoked) GABA IPSCs contain a larger GABA(B) IPSC component of the total GABA IPSC than the somatic (proximally evoked) subfield. The inhibition of GABA(B) receptors by pretreatment of hippocampal slices with CGP-52432 [3[[(3,4-dichlorophenyl)methyl]amino]propyl](diethoxymethyl) phosphinic acid], a selective GABA(B) receptor antagonist, changes the basal ethanol-insensitive, distally evoked GABA(A) IPSCs to become more sensitive to ethanol. In addition, paired-pulse stimulation of the proximal and distal subfields of hippocampal pyramidal neurons shows that ethanol alone increases the probability of GABA release at proximal but not distal regions. Changes by ethanol on the probability of GABA release are only seen at distal locations during GABA(B) blockade. Finally, when the modulation of presynaptic GABA(B) receptors is minimized by the local application of 10 mM GABA directly onto somatic or dendritic GABAergic synaptic regions, postsynaptic GABA(B) receptors seem to exert significant negative (inhibiting) influence on the effects of ethanol on GABA(A) IPSCs in the distal subfields of CA1 pyramidal neurons. Together, our data suggest that differences in both presynaptic and postsynaptic GABA(B) receptor activity at these GABAergic synapses may modulate the differential ethanol sensitivity of proximal and distal GABA IPSCs(A) in hippocampal CA1 pyramidal neurons.

A study comparing the actions of gabapentin and pregabalin on the electrophysiological properties of cultured DRG neurones from neonatal rats

BACKGROUND

Gabapentin and pregabalin have wide-ranging therapeutic actions, and are structurally related to the inhibitory neurotransmitter GABA. Gabapentin, pregablin and GABA can all modulate voltage-activated Ca2+ channels. In this study we have used whole cell patch clamp recording and fura-2 Ca2+ imaging to characterise the actions of pregabalin on the electrophysiological properties of cultured dorsal root ganglion (DRG) neurones from neonatal rats. The aims of this study were to determine whether pregabalin and gabapentin had additive inhibitory effects on high voltage-activated Ca2+ channels, evaluate whether the actions of pregabalin were dependent on GABA receptors and characterise the actions of pregabalin on voltage-activated potassium currents.

RESULTS

Pregabalin (25 nM - 2.5 microM) inhibited 20-30% of the high voltage-activated Ca2+ current in cultured DRG neurones. The residual Ca2+ current recorded in the presence of pregabalin was sensitive to the L-type Ca2+ channel modulator, Bay K8644. Saturating concentrations of gabapentin failed to have additive effects when applied with pregabalin, indicating that these two compounds act on the same type(s) of voltage-activated Ca2+ channels but the majority of Ca2+ current was resistant to both drugs. The continual application of GABA, the GABAB receptor antagonist CGP52432, or intracellular photorelease of GTP-gamma-S had no effect on pregabalin-induced inhibition of Ca2+ currents. Although clear inhibition of Ca2+ influx was produced by pregabalin in a population of small neurones, a significant population of larger neurones showed enhanced Ca2+ influx in response to pregabalin. The enhanced Ca2+ influx evoked by pregabalin was mimicked by partial block of K+ conductances with tetraethylammonium. Pregabalin produced biphasic effects on voltage-activated K+ currents, the inhibitory effect of pregabalin was prevented with apamin. The delayed enhancement of K+ currents was attenuated by pertussis toxin and by intracellular application of a (Rp)-analogue of cAMP.

CONCLUSIONS

Pregabalin reduces excitatory properties of cultured DRG neurones by modulating voltage-activated Ca2+ and K+ channels. The pharmacological activity of pregabalin is similar but not identical to that of gabapentin. The actions of pregabalin may involve both extracellular and intracellular drug target sites and modulation of a variety of neuronal conductances, by direct interactions, and through intracellular signalling involving protein kinase A.

Mechanism of GABAB receptor-mediated inhibition of spontaneous GABA release onto cerebellar Purkinje cells

gamma-Aminobutyric acid (GABA)(B) receptor-mediated modulation of spontaneous GABA release onto Purkinje cells was investigated in cerebellar slices from 3- to 5-week-old mice. The GABA(B) receptor agonists baclofen and CGP 44533 each reduced the frequency of miniature inhibitory postsynaptic currents (mIPSCs), with no significant effect on mIPSC amplitude; together, consistent with a presynaptic site of action. The GABA(B) receptor antagonist CGP 55845 blocked baclofen-induced inhibition. The sulphydryl alkylating agent N-ethylmaleimide occluded baclofen effects, implicating G(i/o) subunits in mediating a GABA(B) G protein-coupled receptor pathway. Baclofen-induced inhibition persisted in the presence of Ba(2+), a blocker of K(+) channels, and Cd(2+), a blocker of Ca(2+) channel-mediated GABA release. Application of nominally Ca(2+)-free extracellular solutions reduced mIPSC frequency and amplitude; however, baclofen produced a significant inhibition in mIPSC frequency, further suggesting that this pathway was independent of Ca(2+) influx. Spontaneous GABA release was increased by the adenylate cyclase activator, forskolin, and the phorbol ester, phorbol 12,13-dibutyrate. However, baclofen-induced inhibition was not significantly changed in either condition. Baclofen action was also not affected by the adenylate cyclase inhibitor SQ 22536 or the protein kinase C inhibitor chelerythrine chloride. Baclofen still reduced mIPSC frequency in the presence of the polyvalent cation ruthenium red, which acts as a secretagogue here; however, baclofen-induced inhibition was reduced significantly. Furthermore, baclofen produced no clear inhibition during high-frequency mIPSCs bursts induced by the potent secretagogue alpha-Latrotoxin. Together, these results suggest that GABA(B) inhibition occurs downstream of Ca(2+) influx and may be mediated, in part, by an inhibition of the vesicular release mechanism.