Gaboxadol, a selective extrasynaptic GABA(A) agonist, does not generalise to other sleep-enhancing drugs: a rat drug discrimination study

Comparing the discriminative stimulus effects of modulators of GABAA receptors containing α4-δ subunits with those of gaboxadol in rats

RATIONALE

Gaboxadol is a selective agonist at γ-aminobutyric acidA (GABAA) receptors that contain α4-δ subunits, and it produces anxiolytic and sedative effects. Although adverse effects preclude its clinical use, its mechanism of action suggests that those receptors might provide novel therapeutic targets, particularly for modulators of those GABAA receptor subtypes, by retaining therapeutic effects of gaboxadol and not adverse effects.

OBJECTIVES

The current study compared discriminative stimulus effects of gaboxadol with those of modulators acting at GABAA receptors containing α4-δ subunits.

MATERIALS

Eight rats discriminated 5.6 mg/kg gaboxadol from vehicle while responding under a fixed - ratio 10 schedule for food. Modulators acting at GABAA receptors containing α4-δ subunits (pregnanolone, ethanol, and flumazenil) and receptors that do not contain those subunits (midazolam) were studied alone; pregnanolone and ethanol were also combined with gaboxadol. In addition, gaboxadol was studied in separate groups discriminating 0.32 mg/kg midazolam, 3.2 mg/kg pregnanolone, or 0.75 g/kg ethanol from vehicle.

RESULTS

Gaboxadol produced ≥80 % gaboxadol-lever responding and did not alter rates. No other drug produced, on average, ≥80 % drug-lever responding up to doses that decreased rates, although 1.78 mg/kg midazolam produced 32 % gaboxadol-lever responding. Ethanol and pregnanolone did not enhance the effects of gaboxadol. Rats discriminating midazolam, pregnanolone, or ethanol from vehicle responded predominantly on the vehicle lever after receiving gaboxadol.

CONCLUSIONS

Drugs that modulate GABAA receptors containing α4-δ subunits neither mimicked nor enhanced the discriminative stimulus effects of gaboxadol, indicating that at least some effects of gaboxadol are not shared with modulators of that GABAA receptor subtype.

Anesthetics discriminate between tonic and phasic gamma-aminobutyric acid receptors on hippocampal CA1 neurons

BACKGROUND

Anesthesia is produced by a depression of neuronal signaling in the central nervous system (CNS); however, the mechanism(s) of action underlying this depression remain unclear. Recent studies have indicated that anesthetics can enhance inhibition of CNS neurons by increasing current flow through tonic gamma-aminobutyric acid (GABA(A)) receptor gated chloride channels in their membranes. Enhanced tonic inhibition would contribute to CNS depression produced by anesthetics, but it remains to be determined to what extent anesthetic actions at these receptors contribute to CNS depression. In the present study, we compared and contrasted the involvement of tonic versus synaptic GABA(A) receptors in the functional depression of CNS neurons produced by isoflurane and thiopental.

METHODS

In rat hippocampal slices, whole cell patch clamp recordings were used to study anesthetic effects on CA1 neuron intrinsic excitability, and population spike recordings were used to investigate effects on synaptically evoked discharge. These responses were chosen to test whether anesthetic effects on GABA receptors alter single neuron discharge and/or circuit level synaptic functioning. Phasic (synaptic) GABA receptors were selectively blocked using the GABA(A) antagonist gabazine and tonic responses were blocked using the chloride channel blocker picrotoxin.

RESULTS

Clinically relevant and equi-effective concentrations of thiopental and isoflurane depressed CA1 neuron synaptically evoked discharge. This depression was partially reversed by blocking synaptic GABA(A) receptors with gabazine (20 microM). The thiopental-induced depression was reversed by approximately 60%, but the isoflurane-induced depression was reversed by only approximately 20%. Blocking tonic GABA(A) receptors with the addition of 100 microM picrotoxin produced an additional 40% reversal of the thiopental-induced depression, but no additional reversal was seen for isoflurane-depressed responses. In response to direct DC current injection, CA1 neuron discharge was depressed by thiopental and membrane conductance was increased. Both of these effects were reversed by picrotoxin, but not by gabazine. Isoflurane, in contrast, neither depressed current-evoked discharge, nor altered the membrane conductance of CA1 neurons.

CONCLUSIONS

These results indicate that general anesthetics discriminate between synaptic and tonic GABA(A) receptors. Effects on both phasic and tonic receptors combined to depress circuit responses produced by thiopental, whereas only effects on synaptic GABA receptors appeared to play an important role for isoflurane. Together with the other known sites of action for these two anesthetics, our results support a multisite, agent-specific mechanism for anesthetic actions.

Discriminative stimulus effects of tiagabine and related GABAergic drugs in rats

RATIONALE

Tiagabine is an anticonvulsant drug which may also have sleep-enhancing properties. It acts by inhibiting reuptake at the gamma-aminobutyric acid (GABA) transporter (GAT-1).

OBJECTIVES

The aim of the study was to determine whether tiagabine acted as a discriminative stimulus and, if so, whether other GABAergic compounds would generalise to it.

MATERIALS AND METHODS

Rats were trained to discriminate tiagabine (30 mg/kg p.o.) from vehicle, and generalisation to drugs that modulate GABA was assessed.

RESULTS

Gaboxadol (5-20 mg/kg p.o.), a selective extrasynaptic GABA A agonist, generalised to tiagabine, although the extent of the generalisation was inconclusive. Indiplon (1 mg/kg p.o.), a benzodiazepine-like hypnotic, also partially generalised to tiagabine, although zolpidem and S-zopiclone did not. Baclofen, a GABA B receptor agonist, and gabapentin, which increases synaptic GABA, did not generalise to tiagabine. (+)-Bicuculline (3 mg/kg i.p.), a GABA A receptor antagonist, blocked the tiagabine cue, but the less brain-penetrant salt form, bicuculline methochloride, had no effect.

CONCLUSIONS

These data suggest that tiagabine generates a discriminative stimulus in rats, and provides a central GABA-mediated cue, but is distinct from the other GABAergic compounds tested.