Interactions of gamma-aminobutyric acid (GABA), pentobarbital, and homopantothenic acid (HOPA) on internally perfused frog sensory neurons

Comparative effects of pentobarbital on spontaneous and evoked transmitter release from inhibitory and excitatory nerve terminals in rat CA3 neurons

Pentobarbital (PB) modulates GABA(A) receptor-mediated postsynaptic responses through various mechanisms, and can directly activate the channel at higher doses. These channels exist both pre- and postsynaptically, and on the soma outside the synapse. PB also inhibits voltage-dependent Na⁺ and Ca²⁺ channels to decrease excitatory synaptic transmission. Just how these different sites of action combine to contribute to the overall effects of PB on inhibitory and excitatory synaptic transmission is less clear. To compare these pre- and postsynaptic actions of PB, we used a 'synaptic bouton' preparation of isolated rat hippocampal CA3 pyramidal neurons where we could measure in single neurons the effects of PB on spontaneous and single bouton evoked GABAergic inhibitory and glutamatergic excitatory postsynaptic currents (sIPSCs, sEPSCs, eIPSCs and eEPSCs), respectively. Low (sedative) concentrations (3-10 μM) of PB increased the frequency and amplitude of sIPSCs and sEPSCs, and also presynaptically increased the amplitude of both eIPSCs and eEPSCs. There was no change in current kinetics at this low concentration. At higher concentrations (30-300 μM), PB decreased the frequency, and increased the amplitude of sIPSCs, and presynaptically decreased the amplitude of eIPSCs. The current decay phase of sIPSCs and eIPSCs was increased. An increase in both frequency and amplitude was seen for sEPSCs, while the eIPSCs was also decreased by a bicuculline-sensitive presynaptic effect. The results confirm the multiple sites of action of PB on inhibitory and excitatory transmission and demonstrate that the most sensitive site of action is on transmitter release, via effects on presynaptic GABA(A) receptors. At low concentrations, however, both glutamate and GABA release is similarly enhanced, making the final effects on neuronal excitability difficult to predict and dependent on the particular systems involved and/or on subtle differences in susceptibility amongst individuals. At higher concentrations, release of both transmitters is decreased, while the postsynaptic effects to increase IPSPs and decrease EPSCs would be expected to both results in reduced neuronal excitability.

Pentobarbital produces activation and block of {alpha}1{beta}2{gamma}2S GABAA receptors in rapidly perfused whole cells and membrane patches: divergent results can be explained by pharmacokinetics

Millimolar concentrations of the barbiturate pentobarbital (PB) activate γ-aminobutyric acid (GABA) type A receptors (GABARs) and cause blockade reported by a paradoxical current increase or “tail” upon washout. To explore the mechanism of blockade, we investigated PB-triggered currents of recombinant α₁β₂γ_2S GABARs in whole cells and outside-out membrane patches using rapid perfusion. Whole cell currents showed characteristic bell-shaped concentration dependence where high concentrations triggered tail currents with peak amplitudes similar to those during PB application. Tail current time courses could not be described by multi-exponential functions at high concentrations (≥3,000 μM). Deactivation time course decayed over seconds and was slowed by increasing PB concentration and application time. In contrast, macropatch tail currents manifested eightfold greater relative amplitude, were described by multi-exponential functions, and had millisecond rise times; deactivation occurred over fractions of seconds and was insensitive to PB concentration and application time. A parsimonious gating model was constructed that accounts for macropatch results (“patch” model). Lipophilic drug molecules migrate slowly through cells due to avid partitioning into lipophilic subcellular compartments. Inclusion of such a pharmacokinetic compartment into the patch model introduced a slow kinetic component in the extracellular exchange time course, thereby providing recapitulation of divergent whole cell results. GABA co-application potentiated PB blockade. Overall, the results indicate that block is produced by PB concentrations sixfold lower than for activation involving at least three inhibitory PB binding sites, suggest a role of blocked channels in GABA-triggered activity at therapeutic PB concentrations, and raise an important technical question regarding the effective rate of exchange during rapid perfusion of whole cells with PB.

Sulfated and unsulfated steroids modulate gamma-aminobutyric acidA receptor function through distinct sites

Sulfated and unsulfated neurosteroids such as pregnenolone sulfate, dehydroepiandrosterone sulfate (DHEAS), pregnanolone, and allopregnanolone, modulate ionotropic amino acid neurotransmitter receptors, and may function as endogenous neuromodulators. The gamma-aminobutyric acid type A (GABAA) receptor exhibits both negative and positive modulation by neurosteroids, but the interaction between negative and positive modulators is not well-understood. For a number of neuroactive steroids, sulfation at C-3 reverses the direction of modulation from positive to negative, suggesting that sulfation could be an important control point for the activity of endogenous neurosteroids. Modulation by endogenous and synthetic steroids of the response to exogenous or synaptically released GABA was examined in primary chick spinal cord and rat hippocampal neurons, and in Xenopus laevis oocytes expressing alpha1beta2gamma2S GABAA receptors. Inhibitory activity is retained when hemisuccinate is substituted for sulfate at C-3, suggesting that it is the negative charge, rather than the sulfate group, that confers inhibitory efficacy. The interaction between steroid negative and positive modulators is not competitive, indicating that steroid negative and positive modulators act through distinct sites. Some steroids, such as 11-ketopregnenolone sulfate, appear to act at both negative and positive modulatory sites, as indicated by an 'off-response' upon washout. A similar off-response is also observed after co-application of the negative modulator DHEAS and the positive modulator allopregnanolone. The observation that simultaneous application of sulfated and unsulfated steroids, such as DHEAS and allopregnanolone, act at distinct sites implies that steroid negative and positive modulators can act independently or coordinately to regulate GABA-mediated inhibition in the central nervous system.

Modulation of the GABAA response in rat ventromedial hypothalamic neurons by pregnanolone

The effects of 5 beta,3 alpha-pregnanolone (PGN) on gamma-aminobutyric acid (GABA)-induced Cl- current in acutely dissociated rat ventromedial hypothalamic neurons were investigated using the nystatin perforated patch recording mode under voltage-clamp conditions. The PGN at concentrations between 10(-7) and 10(-5) M evoked an inward current at a holding potential (VH) of -40 mV. The reversal potential of the PGN-induced current was close to the equilibrium potential of Cl-. PGN potentiated the GABA-induced Cl- current in a concentration-dependent manner. The facilitatory effect was long lasting and disappeared slowly after being washed out. The effect of PGN on the GABA response was also affected by the pretreatment time of PGN. PGN shifted the concentration-response relationship for GABA to the left with a suppression of the maximal response to GABA, resulting from the rapid inactivation of the GABA response during PGN treatment. Facilitatory interactions were found to exist among GABA, pentobarbital, diazepam, and PGN. Three other PGN isomers were also effective in facilitating the GABA response. However, the isomers containing the 3 alpha-hydroxy configurations potentiated the GABA response much more potently and only these isomers exhibited inward currents themselves.

Potentiation by sevoflurane of the gamma-aminobutyric acid-induced chloride current in acutely dissociated CA1 pyramidal neurones from rat hippocampus

1. The effects of a new kind of volatile anaesthetic, sevoflurane (Sev), on gamma-aminobutyric acid (GABA)-gated chloride current (Icl) in single neurones dissociated from the rat hippocampal CA1 area were examined using the nystatin perforated patch recording configuration under the voltage-clamp condition. All drugs were applied with a rapid perfusion system, termed the "Y-tube' method. 2. When the concentrations were higher than 3 x 10(-4) M, Sev, itself, induced an inward current (ISev) at a holding potential (VH) of -40 mV. The concentration-response curve of ISev was bell-shaped, with a suppressed peak and plateau currents at high concentrations (above 2 x 10(-3) M). The reversal potential of ISev (ESev) was close to the theoretical Cl- equilibrium potential, indicating that ISev was carried mainly by Cl-. 3. ISev was reversibly blocked by bicuculline (Bic), an antagonist of the GABAA receptor, in a concentration-dependent manner with a half-inhibitory concentration (IC50) of 7.2 x 10(-7) M. But ISev was insensitive to strychnine (Str), an antagonist of the glycine receptor. 4. At low concentrations (between 3 x 10(-4) and 10(-3) M), Sev markedly enhanced the 10(-6) M GABA induced current (IGABA) but reduced the IGABA with accelerating desensitization accompanied by a "hump' current after washout at high concentrations (higher than 2 x 10(-3) M). 5. Sev, 10(-3) M potentiated the current induced by low concentrations of GABA (between 10(-7) and 3 x 10(-6) M) but reduced the current induced by high concentrations (higher than 10(-5) M) of GABA with a clear acceleration of IGABA desensitization. 6. Sev, like pentobarbitone (PB), pregnanolone (PGN) or diazepam (DZP), potentiated the 10(-6) M GABA-induced response without shifting the reversal potential of IGABA. 7. ISev was augmented by PB, PGN, or DZP at concentrations that maximally potentiated IGABA, suggesting that Sev enhanced IGABA at a binding site distinct from that for PB, PGN, or DZP. 8. It is concluded that Sev acts on the GABAA receptor complex mimicking the GABA-induced chloride current at high concentrations. At low concentrations, Sev enhances GABA-gated chloride current at a binding site independent of the allosteric modulator sites of barbiturates, benzodiazepines or neurosteroids. The reversible potentiation of the inhibitory GABAA receptor-mediated Cl- current may result in the depressing of postsynaptic excitability and may, at least in part, underlie the anaesthetic action of Sev.

Intracranial self-stimulation and locomotor traces as indicators for evaluating the homopantothenic acid

Our paper compares the effect of homopantothenic acid (HOPA) and haloperidol (HAL), i.e., representative neuroleptics, on intracranial self-stimulation (ICSS) and locomotor activity. These experiments showed that HOPA inhibited abnormal hyperactivity induced by methamphetamine (MAP) but did not inhibit ICSS which is considered with respect to physiological motivations. On the other hand, haloperidol (HAL) markedly inhibited both ICSS and abnormal hyperactivity induced by MAP. We propose that combined ICSS and locomotor traces can be chosen as indicators for evaluation and developing the new neuroleptics.