GABAA receptors control the excitability of neuronal populations

G-protein coupled receptors and synaptic plasticity in sleep deprivation

Insufficient sleep has been correlated to many physiological and psychoneurological disorders. Over the years, our understanding of the state of sleep has transcended from an inactive period of rest to a more active state involving important cellular and molecular processes. In addition, during sleep, electrophysiological changes also occur in pathways in specific regions of the mammalian central nervous system (CNS). Activity mediated synaptic plasticity in the CNS can lead to long-term and sometimes permanent strengthening and/or weakening synaptic strength affecting neuronal network behaviour. Memory consolidation and learning that take place during sleep cycles, can be affected by changes in synaptic plasticity during sleep disturbances. G-protein coupled receptors (GPCRs), with their versatile structural and functional attributes, can regulate synaptic plasticity in CNS and hence, may be potentially affected in sleep deprived conditions. In this review, we aim to discuss important functional changes that can take place in the CNS during sleep and sleep deprivation and how changes in GPCRs can lead to potential problems with therapeutics with pharmacological interventions.

Sleep-deprivation induces changes in GABA(B) and mGlu receptor expression and has consequences for synaptic long-term depression

Long term depression (LTD) in the CA1 region of the hippocampus, induced with a 20-Hz, 30 s tetanus to Schaffer collaterals, is enhanced in sleep-deprived (SD) rats. In the present study, we investigated the role of metabotropic glutamate receptors (mGluRs), γ-aminobutyric acid (GABA) B receptors (GABA(B)-Rs) and N-methyl-D-aspartic acid receptors (NMDARs) in the LTD of the population excitatory postsynaptic potential (pEPSP). The requirement of Ca(2+) from L- and T-type voltage-gated calcium channels (VGCCs) and intracellular stores was also studied. Results indicate that mGluRs, a release of Ca(2+) from intracellular stores and GABA(B)-Rs are required for LTD. Interestingly, while mGlu1Rs seem to be involved in both short-term depression and LTD, mGlu5Rs appear to participate mostly in LTD. CGP 55845, a GABA(B)-R antagonist, partially suppressed LTD in normally sleeping (NS) rats, while completely blocking LTD in SD rats. Moreover, GS-39783, a positive allosteric modulator for GABA(B)-R, suppressed the pEPSP in SD, but not NS rats. Since both mGluRs and GABA(B)-Rs seem to be involved in the LTD, especially in SD rats, we examined if the receptor expression pattern and/or dimerization changed, using immunohistochemical, co-localization and co-immunoprecipitation techniques. Sleep-deprivation induced an increase in the expression of GABA(B)-R1 and mGlu1αR in the CA1 region of the hippocampus. In addition, co-localization and heterodimerization between mGlu1αR/GABA(B)-R1 and mGlu1αR/GABA(B)-R2 is enhanced in SD rats. Taken together, our findings present a novel form of LTD sensitive to the activation of mGluRs and GABA(B)-Rs, and reveal, for the first time, that sleep-deprivation induces alterations in the expression and dimerization of these receptors.

Activity-mediated plasticity of GABA equilibrium potential in rat hippocampal CA1 neurons

The equilibrium potential (E(GABA)(-PSC)) for gamma-aminobutyric acid (GABA) A receptor mediated inhibitory postsynaptic currents (PSCs) in hippocampal CA1 pyramidal neurons shifts when theta-burst stimulation (four pulses at 100 Hz in each burst in a train consisting of five bursts with an inter-burst interval of 200 ms, the train repeated thrice at 30-s intervals) is applied to the input. E(GABA)(-PSC) is regulated by K(+)/Cl(-) co-transporter (KCC2). GABA(B) receptors are implicated in modulating KCC2 levels. In the current study, the involvement of KCC2, as well as GABA(B) receptors, in theta-burst-mediated shifts in E(GABA)(-PSC) was examined. Whole-cell patch recordings were made from hippocampal CA1 pyramidal neurons (from 9 to 12 days old rats), in a slice preparation. Glutamatergic excitatory postsynaptic currents were blocked with dl-2-amino-5-phosphonovaleric acid (50 microM) and 6,7-dinitroquinoxaline-2,3-dione (20 microM). The PSC and the E(GABA)(-PSC) were stable when stimulated at 0.05 Hz. However, both changed following a 30-min stimulation at 0.5 or 1 Hz. Furosemide (500 microM) and KCC2 anti-sense in the recording pipette but not bumetanide (20 or 100 microM) or KCC2 sense, blocked the changes, suggesting KCC2 involvement. Theta-burst stimulation induced a negative shift in E(GABA)(-PSC), which was prevented by KCC2 anti-sense; however, KCC2 sense had no effect. CGP55845 (2 microM), a GABA(B) antagonist, applied in the superfusing medium, or GDP-beta-S in the recording pipette, blocked the shift in E(GABA)(-PSC). These results indicate that activity-mediated plasticity in E(GABA)(-PSC) occurs in hippocampal CA1 pyramidal neurons and theta-burst-induced negative shift in E(GABA)(-PSC) requires KCC2, GABA(B) receptors and G-protein activation.

Calcineurin inhibitors cause an acceleration of the neurological phenotype in a mouse transgenic for the human Huntington's disease mutation

Calcineurin (CaN) is a Ca(2+)- and calmodulin-dependent protein serine-threonine phosphatase that is thought to play an important role in the neuronal response to changes in the intracellular Ca(2+) concentration. CaN has been implicated in numerous physiological processes including learning and memory. Decreases in CaN expression are thought to be responsible for some of the pathological features seen in brain ischemia, Down's syndrome and Alzheimer's disease. In this study, we examined the possibility of CaN playing a role in the progressive neurological phenotype of the R6/2 mouse of Huntington's disease. We studied the effects of the CaN inhibitors cyclosporin A and FK506 on the progressive neurological phenotype in the R6/2 mouse. We found that an immunosuppressive dose of both drugs dramatically accelerated the main features of the neurological phenotype in R6/2 mice. This was unlikely to be due solely to the immunosuppressive action of these drugs, since treatment with cyclophosphamide, an immunosuppressant drug with a mechanism of action that is not mediated via CaN, did not have deleterious effects on the R6/2 mouse. If anything, cyclophosphamide improved the neurological symptoms in the R6/2 mice. Together, our data suggest a central role for CaN in the deleterious phenotype of the R6/2 mouse. Treatments aimed at preventing the loss of CaN or stimulating its function may be beneficial in the treatment of HD.

Adenosine-mediated synaptic depression and EPSP/spike dissociation following high potassium-induced depolarization in rat hippocampal slices

Simultaneous recordings of orthodromic PS, fEPSP and antidromic PS revealed EPSP/spike (E-S) dissociation, indicating a conversion of input/output relations from early and brief excitability to a late and prolonged depression during the recovery from depolarization induced by high levels of potassium. E-S potentiation was partially attenuated by pre-treating the slices with BAPTA-AM and lidocaine and totally eliminated by a submaximal concentration of muscimol. The time lag for recovery was decreased by the GABA(A) antagonist and completely eliminated by the A(1) antagonist. From these observations, we conclude that Ca(2+) dependent inhibitory suppression is the main cause of a brief period of E-S potentiation, and accumulation of adenosine is the mechanism responsible for prolonged depression of synaptic transmission

Direct noncompetitive inhibition of 5-HT(3) receptor-mediated responses by forskolin and steroids

5-HT(3) receptors cloned from NCB-20 cells were expressed in Xenopus oocytes, and the effects of forskolin and steroids on the function of the receptors were investigated using the two-electrode voltage-clamp technique. Forskolin, 17-beta-estradiol, and progesterone inhibited the currents activated by 1 microM 5-HT in a reversible and concentration-dependent manner, with IC(50) values of 12, 33, and 89 microM, respectively. The inhibitory effects of forskolin and 17-beta-estradiol were independent of the membrane potential. Forskolin and 17-beta-estradiol significantly reduced the maximal amplitude of the 5-HT concentration-response curve (E(max)) without significantly affecting the EC(50), indicating that these compounds act as noncompetitive inhibitors of the 5-HT(3) receptor. The cAMP analogue, 8-Br-cAMP (0.2 mM), and the protein kinase A activator, Sp-cAMP (0.1 mM), did not affect the amplitude of 5-HT(3) receptor-mediated currents. The membrane-permeable protein kinase A inhibitor Rp-cAMP (0.1 mM) and the estrogen-receptor antagonist tamoxifen (1 microM) did not affect the inhibition of 5-HT-activated current. In addition, 5-HT(3) receptor-mediated currents were inhibited by both 1,9-dideoxy forskolin (30 microM), which does not activate adenylyl cyclase, and wForskolin (30 microM), a charged hydrophilic analogue of forskolin that is membrane impermeable. These results indicate that both forskolin and 17-beta-estradiol inhibit the function of the 5-HT(3) receptor in a noncompetitive manner and that this inhibition is independent of cAMP levels.

Prenatal morphine exposure alters susceptibility to bicuculline seizures in a sex- and age-specific manner

Bicuculline was used to investigate seizure susceptibility in pre- and peripubertal male and female rats exposed prenatally to morphine. Morphine-exposed males showed increased seizure susceptibility at prepubertal and decreased susceptibility at peripubertal ages. There was no difference in seizure susceptibility in morphine-exposed females at either age. Therefore, the present data suggest that males are more vulnerable than females to morphine-induced insults during prenatal brain development.

Muscarinic facilitation of the occurrence of depolarization-induced suppression of inhibition in rat hippocampus

Depolarization-induced suppression of inhibition is a transient decrease in GABAergic input to a hippocampal pyramidal cell following a brief depolarization of that cell. When recorded under whole-cell voltage clamp, monosynaptic, bicuculline-sensitive, GABA(A)-mediated currents are suppressed for a period lasting up to 1 min in response to a retrograde signal released by the pyramidal cell. The depolarization-induced suppression of inhibition process affects spontaneous, action-potential-dependent inhibitory postsynaptic currents, but suppression of these currents is seldom observed in the absence of carbachol, a cholinergic agonist. Because of the central roles played by cholinergic and GABAergic transmission in the regulation of hippocampal rhythmic activity, it will be important to understand the mechanism by which carbachol facilitates the appearance of depolarization-induced suppression of inhibition. As preliminary steps in the investigation of cholinergic actions on depolarization-induced suppression of inhibition, it is necessary to determine which cholinergic receptors are involved and the degree to which activation of these receptors is required for depolarization-induced suppression of inhibition. Nicotine did not mimic the effects of carbachol, and mecamylamine, a nicotinic receptor antagonist, did not block them. In contrast, the actions of carbachol were abolished by atropine and other muscarinic receptor antagonists. The actions of antagonists with relative selectivities for various subtypes of muscarinic receptors [4-diphenylacetoxy-N-methylpiperidine methiodide, pirenzepine, 11-([2-1-piperidinyl]acetyl)-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzod iaz epine-6-one] suggested that cholinergic facilitation of the occurrence of depolarization-induced suppression of inhibition is likely to be mediated through muscarinic receptors of the M1 or M3 rather than M2 subtype. Despite its potent facilitation of the occurrence of depolarization-induced suppression of inhibition, muscarinic stimulation was not required for expression of depolarization-induced suppression of inhibition. Occasionally, depolarization-induced suppression of inhibition of spontaneous inhibitory postsynaptic currents occurred in the absence of carbachol and could not be blocked by atropine, and hence was not likely to be mediated by endogenous acetylcholine. Also, depolarization-induced suppression of inhibition of monosynaptically evoked inhibitory postsynaptic currents occurred without carbachol perfusion, and this was also insensitive to atropine. Therefore, the mechanism of depolarization-induced suppression of inhibition is not dependent on muscarinic receptor activation. Nevertheless, in vivo, septal cholinergic input to the hippocampus may provide the necessary activation of interneurons to allow depolarization-induced suppression of inhibition to occur.

Effects of gonadal steroids on the GABA and glutamate contents of the early developing tilapia brain

The effects of gonadal steroids on the gamma-aminobutyric acid (GABA) and glutamate (Glu) contents of the early developing brain were investigated. Seven-day-old (7 days post-hatch) tilapia were divided into three groups which were continuously treated with 100 mg/kg diet 17beta-estradiol (E2), 100 mg/kg diet methyltestosterone (MT), and a normal diet, respectively. Until 10, 20, and 30 days old, the GABA and Glu contents of the brains were detected by HPLC-ECD. The brain GABA and Glu contents, before 30 days old, significantly increased with age. These results demonstrate that before 30 days old is a developing period of both GABA and Glu systems in the tilapia brain. During this period, both E2 and MT have a facilitative effect on the GABAergic and Gluergic system during a restricted effective period.

N- and L-type calcium channel involvement in depolarization-induced suppression of inhibition in rat hippocampal CA1 cells

1. We investigated depolarization-induced suppression of inhibition (DSI) under whole-cell voltage clamp in CA1 pyramidal neurons of rat hippocampal slices. DSI, a transient reduction in monosynaptic evoked GABAAergic IPSCs lasting for approximately 1 min, was induced by depolarizing the pyramidal cell to -10 or 0 mV for 1 or 2 s. 2. Raising extracellular Ca2+ concentration increased DSI, and varying the DSI-inducing voltage step showed that the voltage dependence of DSI was like that of high-voltage-activated Ca2+ channels. 3. The P- and Q-type Ca2+ channel blocker omega-agatoxin TK (200 nM and 1 microM) and the R- and T-type Ca2+ channel blocker Ni2+ (100 microM) reduced IPSCs without reducing DSI. 4. The specific N-type Ca2+ channel antagonist omega-conotoxin GVIA (250 nM) reduced IPSC amplitudes and almost completely abolished DSI. 5. Blocking L-type Ca2+ channels with nifedipine (10 microM) had no effect on IPSCs or DSI induced by our standard protocol, but reduced DSI induced by the unclamped Na+- and Ca2+-dependent spikes that occurred when 2(triethylamino)-N-(2,6-dimethylphenyl)acetamide (QX-314) was omitted from the recording pipette solution. 6. Although intracellular Ca2+ stores were not measured, DSI was not affected by cyclopiazonic acid (CPA, 20-40 microM), a blocker of Ca2+ uptake into intracellular stores. 7. We conclude that DSI is initiated by Ca2+ influx through N- and, under certain conditions, L-type Ca2+ channels.

Loss of perikaryal parvalbumin immunoreactivity from surviving GABAergic neurons in the CA1 field of epileptic gerbils

The Mongolian gerbil (Meriones unguiculatus) is known as a genetic model of epilepsy. Seizure behavior ranges from subtle events like arrest of motor activity and facial spasms to grand mal seizures followed by automatisms. Exploratory behavior in a stressful situation represents the most effective environment for provoking seizures in gerbils. Modifications of the inhibitory hippocampal circuits have been suggested as a cause of seizure susceptibility in the gerbil. This study presents a quantitative analysis of the hippocampal parvalbumin (PV)-immunoreactive and gamma-aminobutyric acid (GABA)-immunoreactive neurons in gerbils whose seizure sensitivity had been scored. PV is a cytosolic calcium-binding protein synthesized by a subpopulation of GABAergic neurons and thought to be responsible for the fast spiking capability of this subset of neurons. We show that the number of PV-immunoreactive neurons in the CA1 field of the gerbil hippocampus decreases in repeatedly seizing animals as compared to non-seizing controls. The lowest density of PV-immunoreactive neurons was observed 1 hour after the last generalized seizure. No changes in the density of GABA-immunoreactive neurons in field CA1 paralleled the obvious loss of perikaryal PV-immunoreactivity. The CA1 field represents the final output region to extrahippocampal brain areas, and its recruitment or not into seizure activity is crucial for the spreading of hippocampal discharges to the adjacent neocortex. A reduction of such a calcium-buffering system in the soma and dendrites may affect the spike characteristics of PV-containing GABAergic neurons and may alter their response to glutamatergic transmission. A reduced inhibitory control of pyramidal cells may ensue, facilitating neuronal excitability as a result.

Sex and the substantia nigra: administration, teaching, patient care, and research

The immature brain is most susceptible to the development of seizures. The substantia nigra may play a crucial role in the control of seizures as a function of age. In the adult substantia nigra pars reticulata (SNR), there are two regions that mediate opposing effects on seizures after infusions of GABA(A) agents. One region is located in the anterior SNR, and localized muscimol infusions mediate anticonvulsant effects. These anticonvulsant effects use a circuitry that may involve the ventromedial thalamic nucleus, the deep layer of the superior colliculus, or both. The second region is in the posterior SNR, and muscimol infusions produce proconvulsant effects, perhaps mediated by the striatum, the globus pallidus, the deep layer of the superior colliculus, or all three. In developing male rats, only the proconvulsant region is present up to the age of 21 days. In ongoing studies, it has been shown that, in the male rat, the transition from the immature to mature SNR-mediated seizure control occurs between the ages of 25 and 30 days, just before adolescence. In male rats castrated on the day of birth, the ensuing depletion of testosterone accelerates the development of the anterior SNR with its anticonvulsant features. Castration does not alter the development of the proconvulsant region. In the developing female SNR, muscimol infusions produce only anticonvulsant effects. The data indicate that gonadal hormones may have an important role in the maturation of systems involved in the containment of seizures.

GABA-ergic transmission in deep cerebellar nuclei

gamma-Aminobutyric acid (GABA) is the inhibitory transmitter released at Purkinje cell axon terminals in deep cerebellar nuclei (DCN). Neurons in DCN also receive excitatory glutamatergic inputs from the inferior olive. The output of DCN neurons, which depends on the balance between excitation and inhibition on these cells, is involved in cerebellar control of motor coordination. Plasticity of synaptic transmission observed in other areas of the mammalian central nervous system (CNS) has received wide attention. If GABA-ergic and/or glutamatergic synapses in DCN also undergo plasticity, it would have major implications for cerebellar function. In this review, literature evidence for GABA-ergic synaptic transmission in DCN as well as its plasticity are discussed. Studies indicate that fast inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in neurons of DCN are mediated by GABAA receptors. While GABAB receptors are present in DCN, they do not appear to be activated by Purkinje cell axons. The IPSPs undergo paired-pulse, as well as frequency-dependent, depressions. In addition, tetanic stimulation of inputs can induce a long-term depression (LTD) of the IPSPs and IPSCs. Excitatory synapses do not appear to undergo long-term potentiation or LTD. The LTD of the IPSP is not input-specific, as it can be induced heterosynaptically and is associated with a reduced response of DCN neurons to a GABAA receptor agonist. Postsynaptic Ca2+ and protein phosphatases appear to contribute to the LTD. The N-methyl-D-aspartate receptor-gated, as well as the voltage-gated Ca2+ channels are proposed to be sources of the Ca2+. It is suggested that LTD of GABA-ergic transmission, by regulating DCN output, can modulate cerebellar function.

Retrograde signalling in depolarization-induced suppression of inhibition in rat hippocampal CA1 cells

1. We have investigated the phenomenon of 'depolarization-induced suppression of inhibition' (DSI) using whole-cell voltage-clamp techniques in Ca1 pyramidal cells of rat hippocampal slices. DSI was induced by eliciting voltage-dependent calcium (Ca2+) currents with 1 s voltage steps of +60 to +90 mV from the holding potential. DSI was apparent as a reduction in synaptic GABAA responses for a period of about 1 min following the voltage step. 2. TTX-sensitive spontaneous IPSCs (sIPSCs) were susceptible to DSI, while TTX-resistant miniature inhibitory postsynaptic current (mIPSCs) were not. Miniature IPSCs are ordinarily infrequent and independent of external Ca2+ in the CA1 region. To increase the frequency of mIPSCs and to induce a population of Ca(2+)-sensitive mIPSCs, we increased the bath K+ concentration to 15 mM. The increased mIPSCs were also insensitive to DSI, however. 3. T whole-cell pipette-filling solution contained 5 mM 2(triethylamino-N-(2,6-dimethyl-phenyl)acetamide (QX-314) to block voltage-dependent Na+ currents and caesium to block K+ currents. Nevertheless, bath application of 50 microM 4-aminopyridine (4-AP) or 250 nM veratridine both clearly reduced DSI, evidently by acting at presynaptic sites. 4. The amplitudes of monosynaptically evoked IPSCs (elicited in the presence of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 50 microM 2-amino-5-phosphonovaleric acid (APV)) were dramatically reduced during the DSI period. Weak stimulation produced small IPSCs and occasional 'failures' of transmission during the control period. The percentage of failures increased markedly during the DSI period. Moderate-intensity stimulation produced larger IPSCs that were often composed of distinguishable multiquantal components. All-or-none failures of multiquantal IPSC components also occurred during DSI. 5. The degree of paired-pulse IPSC depression did not change during DSI, whereas it was decreased, as expected, by baclofen. 6. We conclude that the data represent novel evidence that DSI is mediated by a retrograde signalling process possibly involving presynaptic axonal conduction block.

Increased neuronal excitability during depolarization-induced suppression of inhibition in rat hippocampus

1. Depolarization-induced suppression of inhibition (DSI) is a form of plasticity of gamma-amino-butyric acid (GABAA)-mediated (henceforth 'GABAergic') responses in the CNS. We made whole-cell recordings from CA1 pyramidal neurons to investigate the effects of DSI on excitatory synaptic transmission in the hippocampal slice preparation. 2. Significant enhancement of the voltage-clamped excitatory postsynaptic current (EPSC) occurs during DSI of the temporally overlapping inhibitory postsynaptic current. With high levels of calcium chelators in the pipette solution, or bath application of bicuculline, EPSC enhancement is blocked, suggesting that it results from DSI and that the DSI process selectively affects GABAergic, but not glutamatergic, transmission. 3. The probability of synaptically evoked action potential firing is increased during DSI under current clamp. DSI could influence other excitatory phenomena as well.

Hormonal and neurotransmitter regulation of GnRH gene expression and related reproductive behaviors

Gonadotropin-releasing hormone (GnRH), having a highly conserved structure across mammalian species, plays a pivotal role in the control of the neuroendocrine events and the inherent sexual behaviors essential for reproductive function. Recent advances in molecular genetic technology have contributed greatly to the investigation of several aspects of GnRH physiology, particularly steroid hormone and neurotransmitter regulation of GnRH gene expression. Behavioral studies have focused on the actions of GnRH in steroid-sensitive brain regions to understand better its role in the facilitation of mating behavior. To date, however, there are no published reports which directly correlate GnRH gene expression and reproductive behavior. The intent of this article is to review the current understanding of the way in which changes in GnRH gene expression, and modifications of GnRH neuronal activity, may ultimately influence reproductive behavior.

Electrophysiological actions of hemoglobin on rat hippocampal CA1 pyramidal neurons

Hemoglobin, the oxygen-carrying component of red blood cells, can be released from erythrocytes in hemorrhagic stroke and intracranial bleeding associated with head injuries. Therefore, neurons may be exposed to this agent. In addition, hemoglobin can chelate nitric oxide (NO) and has been used in studying the role of NO in synaptic plasticity and excitotoxicity. However, the electrophysiological actions of hemoglobin on central neurons are not well characterized. In the present investigation, the electrophysiological actions of hemoglobin on CA1 pyramidal neurons in rat hippocampal slices were studied with conventional intracellular pointed microelectrode- as well as perforated patch-recordings. Superfusion of rat hippocampal slices with hemoglobin (0.05 or 0.1 mM for 10-15 min) induced a depolarization of CA1 neurons and suppressed the stratum radiatum stimulation-induced excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). The hemoglobin-induced depolarization as well as the suppression of the synaptic transients were present in slices pretreated with 0.1 or 0.5 mM of N omega-nitro-L-arginine, a nitric oxide synthase inhibitor, suggesting that hemoglobin has electrophysiological actions on hippocampal CA1 neurons that are independent of its NO scavenging property.

Retrograde signaling at GABAA-receptor synapses in the mammalian CNS

Recent studies have emphasized the role of signals that travel from a target cell, in a retrograde direction, to cells that synapse on the target and influence their output. While the focus of most research on this topic has been on long-lasting alterations at excitatory synapses, evidence that implicates retrograde transmission in the transient reduction of GABAA-receptor-mediated inhibition in hippocampus and cerebellum has begun to accumulate. Brief depolarizations of the postsynaptic principal cells lead to increases in the intracellular concentration of Ca2+, and a reduction in GABAA-receptor-mediated responses for 1-2 min. No concomitant reduction in postsynaptic GABAA-receptor responsiveness has been detected. Rather, release of GABA from inhibitory-interneuron terminals appears to be reduced. The properties of this 'depolarization-induced suppression of inhibition' might be appropriate for unique physiological roles.

Dehydroepiandrosterone sulfate suppresses hippocampal recurrent inhibition and synchronizes neuronal activity to theta rhythm

Several neurosteroids have proconvulsant and memory-enhancing properties and are potent modulators of the gamma-amino butyric acid (GABA) receptor/chloride-ionophore complex. The effects of in situ microelectrophoretic application of the natural sulfate ester of the neurosteroid dehydroepiandrosterone (DHEAS) on evoked field responses and single-unit activity were evaluated in the dentate gyrus and CA1 hippocampal subfield of halothane-anesthetized rats. The effects of endogenous stimulation of DHEAS by in situ micropressure application of Trilostane ((4 alpha, 5 alpha, 17 beta)-4,5-epoxy-3,17-dihydroxyandrost-2-ene-2- carbonitrile (WIN24540)), an inhibitor of 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta-HSD), the enzyme that metabolizes DHEAS, on evoked responses and cellular activity in the hippocampus were also investigated. In situ microelectrophoretic application of DHEAS or micropressure application of Trilostane into CA1 markedly increased population excitatory postsynaptic potential (pEPSP) slopes and population spike (PS) amplitudes. Neither DHEAS nor Trilostane altered dentate pEPSP slopes or PS amplitudes, but both increased the amplitude of a late component of the pEPSP. Both DHEAS and Trilostane abolished GABA-mediated paired-pulse inhibition in both the dentate and CA1. In addition, both DHEAS and Trilostane markedly increased the spontaneous firing rate of dentate hilar interneurons (INTs: 256% and 185%), CA1 pyramidal cells (PCs: 95% and 105%), and CA1 oriens/alveus (O/A) interneurons (179% and 85%) and synchronized their firing to hippocampal theta rhythm induced by tail-pinch. These findings indicate that exogenous application and endogenous stimulation of DHEAS modulates hippocampal GABA inhibition in a physiologically relevant manner possibly by entraining hippocampal neurons to theta rhythm.

Depolarization-induced suppression of GABAergic inhibition in rat hippocampal pyramidal cells: G protein involvement in a presynaptic mechanism

Following postsynaptic activation of a pyramidal cell, the degree of GABAergic synaptic inhibition that the cell receives is reduced dramatically for many seconds. Previously, we found that induction of depolarization-induced suppression of inhibition (DSI) required post-synaptic increases in intracellular [Ca2+], but absence of a decrease in responsiveness to iontophoretically applied GABA left the mechanism of DSI expression uncertain. We investigated DSI with whole-cell voltage-clamp recordings in rat hippocampal slices. Bath-applied carbachol was ordinarily used to increase the spontaneous action potential-induced IPSCs (sIPSCs) and enhance detectability of DSI; synaptically released ACh has the same effects. TTX-sensitive sIPSCs are markedly reduced by DSI, whereas TTX-insensitive miniature IPSC amplitudes do not change, suggesting that DSI represents a retrograde influence on presynaptic GABA release. A lag (approximately 1 s) prior to maximal DSI and prevention of DSI by pertussis toxin pointed to a G protein-linked second messenger that may be presynaptic, since perturbation of postsynaptic G protein function did not alter DSI.

Paradoxical enhancement by bicuculline of dentate granule cell IPSPs evoked by fimbria stimulation in rat hippocampal slices

Stimulation of the fimbria in rat hippocampal slices evoked an extracellular negativity in the granule cell layer and a small depolarization in granule cells at their resting potentials. The intracellular potentials appeared to be GABAA receptor-mediated IPSPs because they reversed at -69.1 +/- 1.0 mV (mean +/- S.E.M., n = 14) and were blocked by the GABAA receptor antagonist bicuculline (10-50 microM, n = 14). However, during the first few minutes of perfusion with bicuculline, IPSPs transiently and paradoxically increased in amplitude. As IPSPs increased, the reversal potential and latency to onset remained the same. These effects were reversible, and during the wash period IPSPs first increased and then stabilized at a smaller amplitude, similar to IPSPs evoked in control conditions. As the GABAA receptor-mediated IPSP decreased, it was followed by a second hyperpolarization. This late hyperpolarization appeared to be a GABAB receptor-mediated IPSP, because it reversed near the equilibrium potential for potassium (mean -81.8 +/- 2.3 mV, n = 12, [K+]o = 5 mM) and was blocked by the GABAB receptor antagonist 2-hydroxy saclofen (250-500 microM, n = 5). The results suggest that GABAA and GABAB receptor-mediated IPSPs evoked in granule cells by fimbria stimulation are normally inhibited by activation of GABAA receptors. The inhibition by GABAA receptors is strong enough that, in control conditions, the GABAA IPSPs are barely detectable and the GABAB IPSPs are undetectable.(ABSTRACT TRUNCATED AT 250 WORDS)

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional variation in gamma-aminobutyric acid turnover: effect of castration on gamma-aminobutyric acid turnover in microdissected brain regions of the male rat

This study compared the turnover of GABA neurons in different brain areas of the male rat and examined the effect of castration on GABA turnover in regions of the brain associated with the control of gonadotropin secretion. To estimate GABA turnover, GABA was quantified by HPLC in microdissected brain regions 0, 30, 60, 90, and 120 min after inhibition of GABA degradation by aminooxyacetic acid (100 mg/kg, i.p.). GABA accumulation was linear in all areas for 90 min (p < 0.01), and GABA turnover was estimated as the slope of the line formed by increased GABA concentration versus time, determined by linear regression. There was considerable regional variation both in the initial steady-state concentrations of GABA and in the rates of GABA turnover. Of 10 discrete brain structures, GABA turnover was highest in the medial preoptic nucleus and lowest in the caudate nucleus. Turnover times in the terminal fields of known GABAergic projection neurons ranged sevenfold, from 2.6 h in the substantia nigra to 0.4 h in the lateral vestibular nucleus. The effect of castration on GABA turnover in 13 microdissected brain regions was investigated by measuring regional GABA concentrations before and 30 min after injection of aminooxyacetic acid in intact rats or 2 or 6 days postcastration. Following castration, steady-state GABA concentrations were increased, and GABA turnover decreased in the diagonal band of Broca, the medial preoptic area, and the median eminence. GABA turnover increased in the medial septal nucleus and was unaffected in the cortex, striatum, and hindbrain. These results are consistent with the hypothesis that testosterone negative-feedback control of luteinizing hormone-releasing hormone involves steroid-sensitive GABAergic neurons in the rostral and medial basal hypothalamus.