The pharmacology of amino-acid responses in septal neurons

Actions of remimazolam on inhibitory transmission of rat spinal dorsal horn neurons

Remimazolam is an ultra-short benzodiazepine that acts on the benzodiazepine site of γ-aminobutyric acid (GABA) receptors in the brain and induces sedation. Although GABA receptors are found localized in the spinal dorsal horn, no previous studies have reported the analgesic effects or investigated the cellular mechanisms of remimazolam on the spinal dorsal horn. Behavioral measures, immunohistochemistry, and in vitro whole-cell patch-clamp recordings of dorsal horn neurons were used to assess synaptic transmission. Intrathecal injection of remimazolam induced behavioral analgesia in inflammatory pain-induced mechanical allodynia (six rats/dose; p < 0.05). Immunohistochemical staining revealed that remimazolam suppressed spinal phosphorylated extracellular signal-regulated kinase activation (five rats/group, p < 0.05). In vitro whole-cell patch-clamp analysis demonstrated that remimazolam increased the frequency of GABAergic miniature inhibitory post-synaptic currents, prolonged the decay time (six rats; p < 0.05), and enhanced GABA currents induced by exogenous GABA (seven rats; p < 0.01). However, remimazolam did not affect miniature excitatory post-synaptic currents or amplitude of monosynaptic excitatory post-synaptic currents evoked by Aδ- and C-fiber stimulation (seven rats; p > 0.05). This study suggests that remimazolam induces analgesia by enhancing GABAergic inhibitory transmission in the spinal dorsal horn, suggesting its potential utility as a spinal analgesic for inflammatory pain.

Cellular Mechanisms for Antinociception Produced by Oxytocin and Orexins in the Rat Spinal Lamina II-Comparison with Those of Other Endogenous Pain Modulators

Much evidence indicates that hypothalamus-derived neuropeptides, oxytocin, orexins A and B, inhibit nociceptive transmission in the rat spinal dorsal horn. In order to unveil cellular mechanisms for this antinociception, the effects of the neuropeptides on synaptic transmission were examined in spinal lamina II neurons that play a crucial role in antinociception produced by various analgesics by using the whole-cell patch-clamp technique and adult rat spinal cord slices. Oxytocin had no effect on glutamatergic excitatory transmission while producing a membrane depolarization, γ-aminobutyric acid (GABA)-ergic and glycinergic spontaneous inhibitory transmission enhancement. On the other hand, orexins A and B produced a membrane depolarization and/or a presynaptic spontaneous excitatory transmission enhancement. Like oxytocin, orexin A enhanced both GABAergic and glycinergic transmission, whereas orexin B facilitated glycinergic but not GABAergic transmission. These inhibitory transmission enhancements were due to action potential production. Oxytocin, orexins A and B activities were mediated by oxytocin, orexin-1 and orexin-2 receptors, respectively. This review article will mention cellular mechanisms for antinociception produced by oxytocin, orexins A and B, and discuss similarity and difference in antinociceptive mechanisms among the hypothalamic neuropeptides and other endogenous pain modulators (opioids, nociceptin, adenosine, adenosine 5’-triphosphate (ATP), noradrenaline, serotonin, dopamine, somatostatin, cannabinoids, galanin, substance P, bradykinin, neuropeptide Y and acetylcholine) exhibiting a change in membrane potential, excitatory or inhibitory transmission in the spinal lamina II neurons.

Hydrogen peroxide modulates synaptic transmission in ventral horn neurons of the rat spinal cord

KEY POINTS

Excessive production of reactive oxygen species (ROS) is implicated in many central nervous system disorders; however, the physiological role of ROS in spinal ventral horn (VH) neurons remains poorly understood. We investigated how pathological levels of H2O2, an abundant ROS, regulate synaptic transmission in VH neurons of rats using a whole-cell patch clamp approach. H2O2 increased the release of glutamate and GABA from presynaptic terminals. The increase in glutamate release involved N-type voltage-gated calcium channels (VGCCs), ryanodine receptors (RyRs), and inositol trisphosphate receptors (IP3 Rs); the increase in GABA release, which inhibited glutamatergic transmission, involved IP3 R. Inhibiting N-type VGCCs and RyRs attenuates excitotoxicity resulting from increased glutamatergic activity while preserving the neuroprotective effects of GABA, and may represent a novel strategy for treating H2O2-induced motor neuron disorders resulting from trauma or ischaemia-reperfusion injury. Excessive production of reactive oxygen species (ROS) is a critical component of the cellular and molecular pathophysiology of many central nervous system (CNS) disorders, including trauma, ischaemia-reperfusion injury, and neurodegenerative diseases. Hydrogen peroxide (H2O2), an abundant ROS, modulates synaptic transmission and contributes to neuronal damage in the CNS; however, the pathophysiological role of H2O2 in spinal cord ventral horn (VH) neurons remains poorly understood, despite reports that these neurons are highly vulnerable to oxidative stress and ischaemia. This was investigated in the present study using a whole-cell patch clamp approach in rats. We found that exogenous application of H2O2 increased the release of glutamate from excitatory presynaptic terminals and γ-aminobutyric acid (GABA) from inhibitory presynaptic terminals. The increase of glutamate release was induced in part by an increase in Ca(2+) influx through N-type voltage-gated calcium channels (VGCCs) as well as by ryanodine receptor (RyR)- and inositol trisphosphate receptor-mediated Ca(2+) release from the endoplasmic reticulum (ER). In inhibitory presynaptic neurons, increased IP3 R-mediated Ca(2+) release from the ER increased GABAergic transmission, which served to rescue VH neurons from excessive release of glutamate from presynaptic terminals. These findings indicate that inhibiting N-type VGCCs or RyRs may attenuate excitotoxicity resulting from increased glutamatergic activity while preserving the neuroprotective effects of GABA, and may therefore represent a novel and targeted strategy for preventing and treating H2O2-induced motor neuron disorders.

Novel mutation of GLRA1 in Omani families with hyperekplexia and mild mental retardation

Hyperekplexia is characterized by neonatal hypertonia and exaggerated startle reflex in response to loud noise or tactile stimuli. Mutations in patients with hyperekplexia were evident in several genes encoding proteins involved in glycinergic neurotransmission, i.e., glycine receptor α and β subunits, collybistin, gephyrin, and glycine transporter 2. We clinically and genetically characterized two large, unrelated consanguineous families with hyperekplexia. Affected members of the two families manifested hyperekplexia with mild mental retardation. Patients exhibited a novel homozygote c.593G>C missense mutation in GLRA1, resulting in amino acid substitution p.W170S in the corresponding mature glycine receptor α1 subunit. This mutation was absent in 400 randomly selected chromosomes in the same population. In conclusion, a novel p.W170S mutation in the extracellular ligand binding domain of glycine receptor α1 subunit was detected in patients with hyperekplexia and mild mental retardation.

Desipramine or glutamate antagonists synergized the antidepressant-like actions of intra-nucleus accumbens infusions of minocycline in male Wistar rats

Minocycline produces antidepressant-like actions in male rats tested in the forced swimming test (FST) and synergizes with several glutamate receptor antagonists. However, the limbic regions implicated in the antidepressant-like actions of minocycline are unknown. The objective of the present study was to test the potential antidepressant activity of nucleus accumbens infusions of minocycline alone or combined with antidepressant drugs or with several glutamate receptor antagonists, using the time-sampling method in the FST. The results show that intra-NAcc infusions of minocycline reduced immobility (1.0 microg, P<0.05; 1.5 microg, P<0.05) by increasing climbing (1.0 microg, P<0.05; 1.5 microg, P<0.05) in the FST. Likewise, systemic injections of desipramine (P<0.05), fluoxetine (P<0.05) or several glutamate receptor antagonists: EMQMCM (P<0.05), MTEP (P<0.05) or dizocilpine (P<0.05) combined with intra-nucleus accumbens infusions of vehicle produced antidepressant-like actions. The subthreshold dose of intra-nucleus accumbens infusions of minocycline combined with systemic injections of subthreshold doses of desipramine (P<0.05) or EMQMCM (P<0.05) or MTEP (P<0.05) or dizocilpine (P<0.05) produced antidepressant-like actions. It is concluded that intra-NAcc infusions of minocycline alone or combined with systemic injections of desipramine or with systemic injections of several glutamate receptor antagonists produced antidepressant-like actions in the FST.

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Potentiation, activation and blockade of GABAA receptors by etomidate in the rat sacral dorsal commissural neurons

Etomidate (ET), an imidazole general anesthetic, has been medically widely used. Recent evidence suggests that the inhibitory neurotransmitter GABA receptor may be one of the important molecular target(s) of general anesthetics. Up to date, little attention has been directed toward the sacral dorsal commissural nucleus (SDCN), which serves as a relay of sensory information from the pelvic viscera in the spinal cord. Therefore, the effect of ET on GABA(A) receptor function in neurons acutely dissociated from the SDCN was investigated using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. At a holding potential of -40 mV, ET (above 10 microM) induced an inward ET-activated current (I(ET)) with the EC(50) value of 33 +/- 3 microM, which was reversibly blocked by bicuculline and picrotoxin. The reversal potential of I(ET) was close to the Cl(-) equilibrium potential. ET also displayed a biphasic modulatory effect on GABA responses. At lower concentrations (0.1-100 microM), ET reversibly potentiated GABA (1 microM)-activated Cl(-) currents in a bell-shaped manner, with the maximal facilitative effect at 10 microM, whereas at concentrations >100 microM, the peak of the ET-induced current was suppressed in the absence or presence of GABA (1 microM). These results suggest that in SDCN, in addition to the potentiation of GABA(A) receptor-mediated responses at low concentrations and the direct activation of GABA(A) receptors at moderate concentrations as expected, ET produced a fast blocking action at high concentrations. The general anesthetic-induced effects in SDCN, at least the potentiation of GABA responses, may significantly contribute to anesthesia of pelvic viscera during the general anesthesia.

A Novel Hyperekplexia-causing Mutation in the Pre-transmembrane Segment 1 of the Human Glycine Receptor α1 Subunit Reduces Membrane Expression and Impairs Gating by Agonists

In this study, we have compared the functional consequences of three mutations (R218Q, V260M, and Q266H) in the alpha(1) subunit of the glycine receptor (GlyRA1) causing hyperekplexia, an inherited neurological channelopathy. In HEK-293 cells, the agonist EC(50s) for glycine-activated Cl(-) currents were increased from 26 microm in wtGlyRA1, to 5747, 135, and 129 microm in R218Q, V260M, and Q266H GlyRA1 channels, respectively. Cl(-) currents elicited by beta-alanine and taurine, which behave as agonists at wtGlyRA1, were decreased in V260M and Q266H mutant receptors and virtually abolished in GlyRA1 R218Q receptors. Gly-gated Cl(-) currents were similarly antagonized by low concentrations of strychnine in both wild-type (wt) and R218Q GlyRA1 channels, suggesting that the Arg-218 residue plays a crucial role in GlyRA1 channel gating, with only minor effects on the agonist/antagonist binding site, a hypothesis supported by our molecular model of the GlyRA1 subunit. The R218Q mutation, but not the V260M or the Q266H mutation, caused a marked decrease of receptor subunit expression both in total cell lysates and in isolated plasma membrane proteins. This decreased expression does not seem to explain the reduced agonist sensitivity of GlyRA1 R218Q channels since no difference in the apparent sensitivity to glycine or taurine was observed when wtGlyRA1 receptors were expressed at levels comparable with those of R218Q mutant receptors. In conclusion, multiple mechanisms may explain the dramatic decrease in GlyR function caused by the R218Q mutation, possibly providing the molecular basis for its association with a more severe clinical phenotype.

Activaton of GABAB receptor inhibits the excitability of rat small diameter trigeminal root ganglion neurons

A selective GABA(B) receptor agonist, baclofen, is known to suppress neuropathic pain. In the present study, we investigated the effect of baclofen on the excitability of trigeminal root ganglion (TRG) neurons by using the whole cell and perforated patch-clamp recording techniques. Under voltage-clamp (V(h)=-60 mV), voltage-dependent K(+) currents were recorded in the small diameter TRG neurons (<30 microm) and isolated by blocking Na(+) and Ca(2+) currents with appropriate ion replacement. Separation of the K(+) current components was achieved by the response to variation in the conditioning voltage. Two distinct K(+) current components, a transient (I(A)) and a sustained (I(k)), were identified. Baclofen significantly increased I(A) by 74.8% (50 microM) and in a dose-dependent manner (1-50 microM). Similarly, I(K) was also enhanced by baclofen administration (41.8%: 50 microM). The relative amplitude of potentiation of I(A) was significantly higher than that of I(K) (P<0.05). Baclofen-sensitive I(A) and I(K) currents were antagonized by K(+) channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). The augmentation of K(+) currents was antagonized by 3-amino-2-(4-chlorophenyl)-2-hydroxypropylsulfonic acid (saclofen; GABA(B) antagonist). In the current clamp mode, the resting membrane potential was -62+/-1.6 mV (n=24). Hyperpolarization of the membrane potential was elicited by baclofen (10-50 microM), and the response was associated with a decrease in the input resistance. Baclofen induced-hyperpolarization was blocked by saclofen (100 microM). In the presence of both 4-AP and TEA, no significant changes in membrane potential induced by baclofen application were observed. In the presence of BaCl(2), baclofen-evoked hyperpolarization with decreased resistance was observed. During application of baclofen, the firing rate of the action potentials by depolarizing step pulses was decreased. Application of baclofen reduced action potential duration evoked by a depolarization current pulse.These results indicated that activation of GABA(B) receptors inhibits the excitability of rat small diameter TRG neurons and this inhibitory action is mediated by potentiation of voltage-dependent K(+) currents. We therefore concluded that modification of nociceptive transmission in the trigeminal system by activation of GABA(B) receptors occurs at the level of small TRG neuron cell bodies and/or their primary afferent terminals, which are potential targets of analgesia by baclofen.

Intrinsic vesicular glutamate transporter 2-immunoreactive input to septohippocampal parvalbumin-containing neurons: Novel glutamatergic local circuit cells

Glutamatergic influence on the medial septum diagonal band of Broca complex (MSDB) is a crucial and powerful driver of hippocampal theta rhythm and associated memory processes, in the rat. The recent discovery of vesicular glutamate transporters (VGLUT) provided a specific marker for glutamatergic neuronal elements. Therefore, this study aimed to address two specific questions: (1) do glutamatergic axons innervate MSDB gamma-aminobutyric acid (GABA)ergic, parvalbumin (PV)-containing septohippocampal neurons that are known to have a great influence on the electric activity of the hippocampus; and (2) is the origin of these glutamatergic axons extrinsic and/or intrinsic to the septum. The results of the correlated light and electron microscopic double-labeling immunohistochemistry for VGLUT2 and PV, and single immunostaining for VGLUT2 in colchicine-treated animals, showed that (1) VGLUT2-containing boutons establish asymmetric synaptic contacts with PV-positive perikarya and dendrites; (2) a large population of VGLUT2-immunoreactive neurons is located primarily in the posterior division of the septum; and (3) following surgical fimbria/fornix transection and septal undercut, most VGLUT2-containing axons, including those terminating on MSDB PV cells, remains intact. The latter two observations suggest that the major portion of MSDB glutamate axons have an intraseptal origin and raise a novel functional aspect of glutamatergic cells as local circuit neurons. A constant impulse flow in the septohippocampal GABA pathway is essential for the generation of theta rhythm. Thus, the heavy glutamatergic innervation of these septohippocampal GABA cells establishes the morphological basis for the powerful glutamatergic influence upon theta rhythm and hippocampus-associated memory processes.

GABA-mediated inhibition of glutamate release during ischemia in substantia gelatinosa of the adult rat

An ischemia-induced change in glutamatergic transmission was investigated in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by use of the whole cell patch-clamp technique; the ischemia was simulated by superfusing an oxygen- and glucose-free medium (ISM). Following ISM superfusion, 21 of 37 SG neurons tested produced an outward current (23 +/- 4 pA at a holding potential of -70 mV), which was followed by a slow and subsequent rapid inward current; the remaining neurons had only inward currents. During such a change in holding currents, spontaneous excitatory postsynaptic currents (EPSCs) were remarkably decreased in a frequency with time (half-decay time of the frequency: about 65 s). The frequency of spontaneous EPSCs was reduced to 28 +/- 13% (n = 37) of the control level during the generation of the slow inward current (about 4 min after the beginning of ISM superfusion) without a change in the amplitude of spontaneous EPSCs. When ISM was superfused together with either bicuculline (10 microM) or CGP35348 (20 microM; GABA(A) and GABA(B) receptor antagonists, respectively), spontaneous EPSC frequency reduced by ISM recovered to the control level and then the frequency markedly increased [by 325 +/- 120% (n = 22) and 326 +/- 91% (n = 17), respectively, 4 min after ISM superfusion]; this alteration in the frequency was not accompanied by a change in spontaneous EPSC amplitude. Superfusing TTX (1 microM)-containing ISM resulted in a similar recovery of spontaneous EPSC frequency and following increase (by 328 +/- 26%, n = 12) in the frequency; strychnine (1 microM) did not affect ISM-induced changes in spontaneous EPSC frequency (n = 5). It is concluded that the ischemic simulation inhibits excitatory transmission to SG neurons, whose action is in part mediated by the activation of presynaptic GABA(A) and GABA(B) receptors, probably due to GABA released from interneurons as a result of an ischemia-induced increase in neuronal activities. This action might protect SG neurons from an excessive excitation mediated by L-glutamate during ischemia.

Kinetics of etomidate actions on GABA(A) receptors in the rat spinal dorsal horn neurons

Electrophysiological properties of etomidate (ET)-induced current (I(ET)) at different concentrations and effects of ET at clinically relevant concentrations (1-10 microM) on postsynaptic GABA(A) receptor function were investigated using whole-cell patch-clamp technique in mechanically dissociated rat spinal dorsal horn neurons. The results showed that ET actions were concentration-dependent: low concentrations (10 microM) of ET potentiated GABA-activated current (I(GABA)), slowed activation, desensitization and deactivation of GABA(A) receptors; moderate concentrations (10-1,000 microM) of ET directly activated and desensitized GABA(A) receptors; high concentrations (>1,000 microM) of ET produced an inhibitory effect on I(ET). In addition, ET prolonged the duration of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in the mechanically dissociated rat dorsal horn neurons. These results suggest that general anesthetics-induced changes at spinal level could significantly contribute to analgesia and general anesthesia.

Allosteric binding sites on cell-surface receptors: novel targets for drug discovery

Cell-surface receptors are the targets for more than 60% of current drugs. Traditionally, optimizing the interaction of lead molecules with the binding site for the endogenous agonist (orthosteric site) has been viewed as the best means of achieving selectivity of action. However, recent developments have highlighted the fact that drugs can interact with binding sites on the receptor molecule that are distinct from the orthosteric site, known as allosteric sites. Allosteric modulators could offer several advantages over orthosteric ligands, including greater selectivity and saturability of their effect.

Early postnatal ethanol intubation blunts GABA(A) receptor up-regulation and modifies 3alpha-hydroxy-5alpha-pregnan-20-one sensitivity in rat MS/DB neurons

Previously we found postnatal binge-like ethanol exposure using an artificial-rearing method in the rat delayed developmental up-regulation of GABA(A) receptors (GABA(A)Rs) in both medial septum/diagonal band (MS/DB) and cerebellar Purkinje neurons. In the present study, the impact of ethanol on developing GABA(A)Rs in MS/DB neurons was further tested under conditions not requiring anesthesia or maternal deprivation. Nursing rat pups received ethanol (4.5-5.25 g/kg/day) on postnatal days (PD) 4-9, which was administrated manually by oral intragastric intubation. This treatment caused dose-dependent blunting of peak GABA(A) receptor whole cell currents in acutely dissociated MS/DB cells on PD 12-15. The threshold with oral intubation was slightly higher than previously observed for artificial-rearing (4.9 vs. 4.5 g/kg/day). The previously observed reduced sensitivity of GABA(A)Rs to Zn(2+)-inhibition after ethanol was not found with the intubation model. In studies only carried out using the intubation method, 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha-OH-DHP) caused an allosteric concentration-dependent potentiation of currents activated by non-saturated concentrations of GABA. A bicuculline sensitive direct activation of GABA(A)Rs also occurred with higher concentrations of 3alpha-OH-DHP alone. Ethanol intubation up-regulated allosteric neurosteroid potentiation with low concentrations of GABA, but did not change direct agonist actions of 3alpha-OH-DHP. Finally, 3alpha-OH-DHP did not prime ethanol insensitive GABA(A)Rs to become sensitivity to acute ethanol potentiation. These results indicate ethanol consistently blunts postnatal GABA(A) receptor up-regulation across early postnatal binge-type ethanol exposure models and may increase positive modulation of GABA(A) receptors by endogenous neurosteroids.

Modification of ion channels and calcium homeostasis of basal forebrain neurons during aging

In this paper we review the last several years of work from our lab with attention to changes in the properties of basal forebrain neurons during aging. These neurons play a central role in behavioral functions, such as: attention, arousal, cognition and autonomic activity, and these functions can be adversely affected during aging. Therefore, it is fundamental to define the cellular mechanisms of aging in order to understand the basal forebrain and to correct deficits associated with aging. We have examined changes in the physiological properties of basal forebrain neurons during aging with whole-cell and single-channel patch-clamp, as well as, microfluorimetric measurements of intracellular calcium concentrations. These studies contribute to the understanding of integration within the basal forebrain and to the identification of age-related changes within central mammalian neurons. Although extensive functional/behavioral decline is often assumed to occur during aging, our data support an interpretation of compensatory increases in function for excitatory amino acid receptors, GABA(A) receptors, voltage-gated calcium currents and calcium homeostatic mechanisms. We believe that these changes occur to compensate for decrements accruing with age, such as decreased synaptic contacts, ion imbalances or neuronal loss. The basal forebrain must retain functionality into late aging if senescence is to be productive. Thus, it is critical to recognize the potential cellular and subcellular targets for therapeutic interventions intended to correct age-related behavioral deficits.

Baclofen inhibits more effectively C-afferent than Adelta-afferent glutamatergic transmission in substantia gelatinosa neurons of adult rat spinal cord slices

Although intrathecal administration of baclofen, a selective GABA(B)-receptor agonist, is known to have an antinociceptive effect on various pain models, the role of presynaptic GABA(B) receptors in antinociception is not well characterized. In the present study, the action of baclofen on primary afferent-evoked glutamatergic excitatory transmission was examined in substantia gelatinosa (SG) neurons of an adult rat spinal cord slice with an attached dorsal root, prepared from the lumbar segment, by use of the blind whole-cell patch-clamp technique. Under the condition where a postsynaptic action of baclofen was inhibited, baclofen (1 microM) reduced the amplitudes of excitatory postsynaptic currents (EPSCs; V(H)=-70 mV) which were monosynaptically evoked by stimulating primary-afferent C- and/or Adelta-fibers and which were remarkably depressed by CNQX (10 microM). The identification of the C-fiber or Adelta-fiber EPSC was based on antidromic action potentials recorded from neurons of isolated dorsal root ganglia. The C-fiber EPSC was depressed in peak amplitude by baclofen (1 microM) to a larger extent than the Adelta-fiber EPSC (20 and 45% of control, respectively). Each of the baclofen actions was suppressed by a selective GABA(B)-receptor antagonist, CGP 35348 (50 microM). Baclofen (1 microM) did not affect a response of SG neurons to bath-applied AMPA (10 microM). These results indicate that baclofen inhibits the release of L-glutamate from Adelta and C primary-afferent terminals in the SG through the activation of GABA(B) receptor; this action is more effective to C-fiber than Adelta-fiber transmission. Considering that the SG is the main part of termination of Adelta- and C-fibers transmitting nociceptive information, the present finding would account for at least a part of the inhibitory action of baclofen on pain transmission.

Presynaptic inhibition by baclofen of miniature EPSCs and IPSCs in substantia gelatinosa neurons of the adult rat spinal dorsal horn

Intrathecal administration of baclofen, a GABA(B)-receptor agonist, affects pain behavior induced by formalin in a biphasic manner; baclofen at low doses enhances pain while producing antinociception at high doses. This may be due to the fact that baclofen modulates each of excitatory and inhibitory transmission in the dorsal horn of the spinal cord with a distinct sensitivity, resulting in a biphasic action on pain transmission. To address this issue, we examined the actions of baclofen on miniature excitatory (glutamatergic) and inhibitory (GABAergic) postsynaptic currents (mEPSCs and mIPSCs, respectively) in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole-cell voltage-clamp technique. Baclofen reduced the frequency of both mEPSC and mIPSC without a change in their amplitudes. These actions were dose-dependent in a concentration range of 0.1-100 microM; the effective concentrations for the half-inhibition of mEPSC and mIPSC frequency were 4.44+/-0. 60 microM (n=7) and 4.31+/-0.77 microM (n=6), respectively. These results indicate that each of glutamatergic and GABAergic nerve terminals in the SG is endowed with the GABA(B) receptor, the activation of which depresses the release of neurotransmitter from the terminal; this provides a cellular basis for the modulation of pain by baclofen. It is suggested from a similar affinity for baclofen of the GABA(B) receptors on both terminals that the baclofen-induced biphasic action on pain behaviors cannot be accounted for by only its presynaptic actions in the SG and that other actions such as an inhibitory action of baclofen on postsynaptic neurons also have to be taken into consideration.

Intra-septal infusions of excitatory amino acid receptor antagonists have differential effects in two animal models of anxiety

Ablation or pharmacological inhibition of the septum produces anxiolytic-like effects in two animal models of anxiety: the elevated plus-maze and the shock-probe burying tests. Overall, these results suggest that the septum normally plays an excitatory role in the control of anxiety-related behaviors. The purpose of the present study was to examine this hypothesis in more detail, by testing the effects of intra-septal infusions of excitatory amino acid (EAA) receptor antagonists on rats' behavior in the shock-probe and plus-maze tests. We found that intra-septal infusions of the non-NMDA (N-methyl-D-aspartic acid) receptor antagonist CNQX (5 microg) suppressed open-arm avoidance in the plus-maze test, and reduced burying behavior in the shock-probe test, without affecting rats' normal avoidance of the shock-probe. In contrast, intra-septal infusions of the NMDA receptor antagonist AP-5 (5 microg) inhibited both shock-probe avoidance and burying behavior, without affecting rats' open-arm avoidance. These dissociations suggest that different EAA receptor types within the septum exert complex but distinct control of different anxiety-related behaviors.

Development of GABAA receptors on medial septum/diagonal band (MS/DB) neurons after postnatal ethanol exposure

The impact of 'binge-like' ethanol exposure on postnatal days (PD) 4-9 was examined on development of gamma-aminobutyric acid type A receptors (GABAAR) during the first month of life in the rat. Whole-cell patch-clamp recordings in acutely isolated medial septum/diagonal band (MS/DB) neurons were used to define effects of rapidly applied ethanol and other allosteric modulators on bicuculline-sensitive GABA currents. Three age groups were examined including 'pups' (PD 4-10), 'juveniles' (PD 11-16) and 'young adults' (PD 25-35). In untreated neurons, maximum responses to GABA and the apparent GABA EC50 increased approximately 2-fold during the first month of life. Potentiation of GABA responses by pentobarbital, midazolam, and loreclezole all increased with age, while Zn2+ inhibition declined. Initial inhibition by ethanol switched to potentiation of GABA responses during this time. In vivo, binge-like ethanol treatment (4.5 g kg-1 day-1 divided into two doses, 2 h apart on PD 4-9) reduced both the GABA maximal response and GABA EC50 measured on PD 11-16. These measures returned to control levels by PD 25-35. After binge-like postnatal ethanol exposure, age-dependent loss of Zn2+ inhibition of GABA responses was increased, while potentiating actions of in vitro ethanol were blocked. GABAAR modulation by other drugs was unaffected. These data suggest that early postnatal ethanol exposure disrupts the expected developmental pattern of GABAAR function in MS/DB neurons, an action that could contribute to neurobehavioral deficits associated with the fetal alcohol syndrome. Whether these changes are due to cellular damage, delayed gene expression or post-translational modification needs to be determined.

Action of furosemide on GABA- and glycine currents in rat septal cholinergic neurons in culture

The action of furosemide on GABA- and glycine-induced currents in rat septal cholinergic neurons in culture was examined by use of the whole-cell voltage-clamp technique. Furosemide inhibited the peak amplitude of either GABA or glycine current in a dose-dependent manner; its half-maximal concentration for them was some 2 and 1 mM, respectively. Furosemide at a concentration of 1 mM depressed the maximal peak amplitude of the dose-response curve for either GABA or glycine to about 70% of control. These results indicate that furosemide inhibits in a noncompetitive fashion GABA and glycine receptor-channels in septal neurons.