GABAA receptor in the Pedunculopontine tegmental (PPT) nucleus: Effects on cardiovascular system.

BACKGROUND: The pedunculopontine tegmental (PPT) nucleus is a heterogeneous nucleus with several functions including cardiovascular regulation. The presence of GABAA receptor has been shown in the PPT. Therefore, the cardiovascular effects of this receptor were examined. METHODS: Rats were divided into: Control; Muscimol; Bicuculline (BMI); Hexamethonium (Hexa)+BMI and Atropine+BMI groups. The femoral vein and artery were cannulated for drug administration and recording of cardiovascular parameters, respectively. Muscimol (a GABAA agonist; 1.5 and 2.5nmol), BMI (a GABAA antagonist; 0.1 and 0.2nmol) were stereotaxically microinjected into the PPT. To evaluate the peripheral cardiovascular mechanisms of GABAA receptors, Hexa (a ganglionic blocker; 10mg/kg) and atropine (a muscarinic receptor antagonist; 1mg/kg) were intravenously (iv) injected before BMI (0.2nmol). The average changes of mean arterial pressure (ΔMAP), systolic blood pressure (ΔSBP) and heart rate (ΔHR) in different intervals were calculated and compared both within and between case group and control group (repeated measures ANOVA). The peak changes in each group were also calculated and compared with those of the control group (independent sample t-test). RESULTS: Both doses of BMI significantly increased ΔMAP, ΔSBP and ΔHR compared to control, while the only higher dose of muscimol significantly decreased ΔSBP. Iv injection of Hexa significantly attenuated ΔMAP, ΔSBP and ΔHR responses induced by BMI but atropine did not affect. CONCLUSIONS: Our results demonstrate that GABAA receptor of the PPT has a tonic inhibitory effect on the cardiovascular system and its peripheral effect mostly is mediated by sympathetic system.

Chronic GABAergic blockade in the spinal cord in vivo induces motor alterations and neurodegeneration.

Inhibitory GABAergic and glycinergic neurotransmission in the spinal cord play a central role in the regulation of neuronal excitability, by maintaining a balance with the glutamate-mediated excitatory transmission. Glutamatergic agonists infusion in the spinal cord induce motor neuron death by excitotoxicity, leading to motor deficits and paralysis, but little is known on the effect of the blockade of inhibitory transmission. In this work we studied the effects of GABAergic and glycinergic blockade, by means of microdialysis perfusion (acute administration) and osmotic minipumps infusion (chronic administration) of GABA and glycine receptors antagonists directly in the lumbar spinal cord. We show that acute glycinergic blockade with strychnine or GABAergic blockade with bicuculline had no significant effects on motor activity and on motor neuron survival. However, chronic bicuculline infusion, but not strychnine, induced ipsilateral gait alterations, phalange flaccidity and significant motor neuron loss, and these effects were prevented by AMPA receptor blockade with CNQX but not by NMDA receptor blockade with MK801. In addition, we demonstrate that the chronic infusion of bicuculline enhanced the excitotoxic effect of AMPA, causing faster bilateral paralysis and increasing motor neuron loss. These findings indicate a relevant role of GABAergic inhibitory circuits in the regulation of motor neuron excitability and suggest that their alterations may be involved in the neurodegeneration processes characteristic of motor neuron diseases such as amyotrophic lateral sclerosis.

GABA(A) receptors in the central amygdala are involved in memory retention deficits induced by cannabinoids in rats.

The central nucleus of the amygdala (CeA) as the main output of amygdala plays an important role in memory processes. In this study we first evaluated the effects of intra-CeA administrations of different doses of a cannabinoid CB1 agonist, WIN55, 212-2, GABA(A) receptor agonist and antagonist, muscimol and bicuculline, alone on memory retention using passive avoidance learning (PAL) test in rats. Then we examined the effects of GABA(A) receptor agents on the responses induced by intra-CeA microinjection of different doses of WIN55, 212-2. We found that administration of WIN55, 212-2 (0.05, 0.1, 0.2 and 0.4µg/rat) immediately after training impaired memory retrieval in a dose-dependent fashion. Although pre-test intra-CeA administration of muscimol (125, 250 and 500ng/rat) alone had no effect on the step-through latency, its co-administration (125ng/rat) with different doses of WIN55, 212-2 potentiated the amnesic effects of any doses of WIN55, 212-2. The results also showed that pre-test intra-CeA administration of bicuculline (200, 400 and 800ng/rat) alone had no significant effect, but at dose of 200ng/rat disrupted post-training WIN55, 212-2-induced amnesia in the retention test. Furthermore, the additional effect of muscimol (125ng/rat) on memory impairment induced by WIN55, 212-2 (0.1µg/rat) was prevented by intra-CeA co-injection of bicuculline (200ng/rat). We indicated that stimulating or blocking GAGA(A) receptors in the CeA by muscimol and bicuculline interfere with WIN55, 212-2-induced deficits in memory retention in a PAL task and therefore suggests an interaction between cannabinergic and GABAergic systems of the CeA in memory process.

[Peculiarities of cerebrovascular effects of GABA conjugate with docosahexaenoyl dopamine under conditions of separate and combined vascular pathology of brain and heart].

A GABA conjugate with docosahexaenoyl dophamine (DHED) enhanced local cerebral blood flow in rats under conditions of global transient cerebral ischemia, experimental myocardial infarction, and combined vascular pathology of brain and heart. At the same time, the GABA-DHED conjugate did not influence brain hemoperfusion in intact animals. The cerebrovascular effect of this conjugate is determined by its direct action on the vascular tone, since no changes in blood pressure have been observed. Under conditions of the combined vascular pathology of brain and heart, the cerebrovascular effect of GABA-DHED conjugate is inhibited by bicuculline, which is evidence for the involvement of GABAergic mechanisms in the drug action upon cerebrovascular tone.

GABA(A) receptor blockade enhances memory consolidation by increasing hippocampal BDNF levels.

Memory consolidation is the process by which acquired information is converted to something concrete to be retrieved later. Here we examined a potential role for brain-derived neurotrophic factor (BDNF) in mediating the enhanced memory consolidation induced by the GABA(A) receptor antagonist, bicuculline methiodide. With the administration of an acquisition trial in naïve mice using a passive avoidance task, mature BDNF (mBDNF) levels were temporally changed in the hippocampal CA1 region, and the lowest levels were observed 9 h after the acquisition trial. In the passive avoidance task, bicuculline methiodide administration within 1 h of training but not after 3 h significantly increased latency time in the retention trial 24 h after the acquisition trial. Concomitantly, 1 h post-training administration of bicuculline methiodide, which enhanced memory consolidation, significantly increased mBDNF levels 9 h after training compared to those of the vehicle-treated control group. In addition, exogenous human recombinant BDNF (hrBDNF) administration 9 h after training into the hippocampal CA1 region facilitated memory consolidation confirming that the increase in mBDNF at around 9 h after training plays a key role in the enhancement of memory consolidation. Moreover, the increases in latency time and immediate early gene expressions by bicuculline methiodide or hrBDNF were significantly blocked by anisomycin, a protein synthesis inhibitor, K252a, a tyrosine receptor kinase (Trk) inhibitor, or anti-TrkB IgG. These findings suggest that the increase in the level of mBDNF and its function during a restricted time window after training are required for the enhancement of memory consolidation by GABA(A) receptor blockade.

GABAergic disinhibition induced pain hypersensitivity by upregulating NMDA receptor functions in spinal dorsal horn.

Intense noxious stimuli impair GABAergic inhibition in spinal dorsal horn, which has been proposed as a critical contributor to pathological pain. However, how the reduced inhibition exacerbates the transfer of nociceptive information at excitatory glutamatergic synapses is still poorly understood. The present study demonstrated that one of the striking consequences of GABAergic disinhibition was to enhance the function of N-methyl-D-aspartate subtype glutamate receptors (NMDARs), a well-characterized player in central sensitization. We found that intrathecal application of bicuculline, a GABA(A) receptor antagonist, to remove the inhibition readily elicited mechanical allodynia in naive mice, which could be dose-dependently attenuated by NMDARs antagonist D-APV. Biochemical analysis demonstrated that bicuculline did not affect the total expression levels of the obligatory NMDARs subunit NR1 and the regulatory subunit NR2A and NR2B. However, bicuculline promoted NR1 phosphorylation at Serine 897 (NR1-S897) by cAMP-dependent protein kinase (PKA). This PKA-mediated phosphorylation incorporated NR1 along with NR2B into synapses. When PKA inhibitor H-89 was intrathecally applied, it totally eliminated bicuculline-induced NMDARs phosphorylation, synaptic redistribution as well as pain sensitization. Importantly, the reduced inhibition also operated to enhance NMDARs functions after peripheral inflammation, because spinal injection of diazepam to rescue the inhibition in inflamed mice greatly depressed PKA phosphorylation of NR1-S897, reduced the synaptic concentration of NR1/NR2B and meanwhile, alleviated the inflammatory pain. These data suggested that removal of GABAergic inhibition allowed for PKA-mediated NMDARs phosphorylation and synaptic accumulation, thus exaggerating NMDARs-dependent nociceptive transmission and behavioral sensitization.

Anticonvulsant, anxiolytic, and non-sedating actions of imidazenil and other imidazo-benzodiazepine carboxamide derivatives.

Recent evidence suggests that alpha1-containing GABA(A) receptors mediate the sedative, amnestic, and to some extent the anticonvulsant actions of non-selective benzodiazepine (BZ) receptor ligands, such as diazepam (DZ). Anxiolytic and in part, anticonvulsant actions of BZ ligands are mediated by alpha2-, alpha3-, and alpha5-containing GABA(A) receptors. This has resulted in increasing interest in developing BZ ligands with selective actions at GABA(A) receptors, including alpha2-, alpha3-, and alpha5-subunits, but devoid of efficacy at alpha1-containing receptors. To refine their spectrum of pharmacological actions, efforts are being made to minimize unwanted effects such as sedation, amnesia, and tolerance liabilities. A prototype for such BZ ligands is imidazenil (IMD), an imidazo-benzodiazepine carboxylic acid derivative that elicits potent anticonvulsant and anxiolytic actions at doses virtually devoid of sedative, cardio-respiratory depressant and amnestic effects, and anticonvulsant tolerance liability. To define the pharmacological profile of IMD and its derivatives, we compared the anticonflict (anxiolytic), anti-proconflict (antipanic), anti-bicuculline (BIC), and maximal electroshock seizure (MES) effects, and the suppression of locomotor activity by imidazo-benzodiazepine carboxylic acid derivatives to those of DZ and bretazenil (BTZ). We report here that IMD and one of its derivatives (RO 25-2775) possess dose-dependent anticonflict, anti-proconflict, and anti-BIC actions but failed to suppress locomotor activity. Like DZ, the other IMD derivatives (enazenil, RO 25-2776, and RO 25-2847) not only elicit dose-dependent anticonflict, anti-proconflict, anti-BIC, anti-MES effects but also suppress locomotor activity. In contrast, none of the IMD derivatives studied shows any similarity to BTZ, which elicits anticonflict, anti-proconflict actions and suppresses locomotor activity but is virtually inactive against BIC-induced tonic-clonic convulsions.

[GABAergic mechanism of cerebrovasculareffect of mexidol].

Experiments on rats showed that mexidol significantly increases local cerebral blood flow in animals under conditions of global transient brain ischemia, whereas in intact rats this drug initially causes a decrease in the blood flow, followed by its recovery. Mechanism of the cerebrovascular effect of mexidol is determined by its action on GABA receptors of cerebral vessels, which confirmed the fact that the cerebrovascular effect of mexidol is absent in the presence of bicuculline.

Occurrence of bicuculline-, NMDA- and kainic acid-induced seizures in prenatally methamphetamine-exposed adult male rats.

Stimulant drugs are often associated with increased seizure susceptibility. Inhibitory gamma-aminobutyric acid (GABA) and excitatory N-methyl-D-aspartate (NMDA) systems play an important role in the effect of stimulants on epileptic seizures. No studies investigating the effect of prenatal methamphetamine (MA) exposure on seizures are available. In this study, bicuculline (GABAA receptor antagonist), NMDA (NMDA receptor agonist) and kainic acid (non-NMDA receptor agonist) were used to induce seizures in adult male rats. Three groups of animals were tested in each seizure test: prenatally MA- (5 mg/kg) exposed, prenatally saline-exposed, and absolute controls without any prenatal exposure. In bicuculline-induced seizures, the latency to onset of tonic-clonic seizures was shorter in MA-exposed rats than in controls, but it did not differ from saline-exposed rats. There were no differences in clonic seizure onset between groups. In NMDA-induced seizures, the latency to onset of clonic-tonic seizures was shorter in prenatally MA-exposed rats than in controls; however, the latency to onset of saline-exposed animals did not differ from either MA-exposed or from control rats. There were no differences in seizure susceptibility in kainic acid-induced clonic seizures. There were no differences in seizure incidences or stereotypical behavior in any seizure model. The question remains as to how much the present data demonstrate the effect of prenatal drug exposure on seizure susceptibility per se, and how much they may be explained by the effect of prenatal stress or by other mechanism(s).

GABAA receptors inhibit acetylcholine release in cat pontine reticular formation: implications for REM sleep regulation.

This study used in vivo microdialysis in cat (n=12) to test the hypothesis that gamma aminobutyric acid A (GABAA) receptors in the pontine reticular formation (PRF) inhibit acetylcholine (ACh) release. Animals were anesthetized with halothane to hold arousal state constant. Six concentrations of the GABAA receptor antagonist bicuculline (0.03, 0.1, 0.3, 1, 3, and 10 mM) were delivered to a dialysis probe in the PRF, and endogenously released ACh was collected simultaneously. Bicuculline caused a concentration dependent increase in ACh release (maximal increase=345%; EC50=1.3 mM; r2=0.997). Co-administration of the GABAA receptor agonist muscimol prevented the bicuculline-induced increase in ACh release. In a second series of experiments, the effects of bicuculline (0.1, 0.3, 1, and 3 mM) on ACh release were examined without the use of general anesthesia. States of wakefulness, rapid-eye-movement (REM) sleep, and non-REM sleep were identified polygraphically before and during dialysis delivery of bicuculline. Higher concentrations of bicuculline (1 and 3 mM) significantly increased ACh release during wakefulness (36%), completely suppressed non-REM sleep, and increased ACh release during REM sleep (143%). The finding that ACh release in the PRF is modulated by GABAA receptors is consistent with the interpretation that inhibition of GABAergic transmission in the PRF contributes to the generation of REM sleep, in part, by increasing pontine ACh release.

Effect of intravenous angiotensin II infusion on responses to hypothalamic PVN injection of bicuculline.

The hypothalamic paraventricular nucleus (PVN) plays an important role in the sympathoexcitatory response to elevated plasma angiotensin II (Ang II). However, the mechanism by which Ang II influences sympathetic activity is not fully understood. In this study, we tested the hypothesis that GABA(gamma-aminobutyric acid)-ergic function in the PVN is reduced by peripheral infusion of Ang II. To accomplish this, rats received either intravenous Ang II (12 ng/kg per minute) or vehicle (D5W) for 7 days, and renal sympathetic nerve activity (SNA), mean arterial pressure (MAP), and heart rate (HR) responses were recorded after unilateral PVN microinjection of the GABA-A receptor antagonist bicuculline methiodide (BMI, 0.1 nmol). Results indicate that in contrast to a significant increase in renal SNA, MAP, and HR observed in vehicle-infused rats (P<0.05), BMI injection into the PVN of Ang II-infused animals was without effect on all recorded variables. In a separate groups of animals, ganglionic blockade produced a significantly greater fall in MAP (P<0.01) in Ang II-infused rats than in vehicle-infused control rats, indicating that the contribution of SNA to the maintenance of blood pressure was elevated in the Ang II-infused group. Overall, these data indicate that cardiovascular and sympathoexcitatory responses to acute GABA-A receptor antagonism in the PVN are significantly blunted in rats after 7 days of intravenous infusion of Ang II. We conclude that an Ang II-induced reduction in GABAergic inhibition within the PVN may contribute to elevated SNA observed in this study.

Intracerebroventricular injection of the antibiotic cefoselis produces convulsion in mice via inhibition of GABA receptors.

A majority of beta-lactam antibiotics (e.g., cephalosporins and penicillins) have convulsive activity to a greater or lesser extent. (6R,7R)-3-[[3-Amino-2-(2-hydroxyethyl)-2H-pyrazol-1-ium-1-yl]methyl]-7-[(Z)-2-(2-aminothiazol-4-yl)-2-methoxyiminoacetylamino]-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate monosulfate (cefoselis), a newly developed injectable beta-lactam antibiotic with activity against methicillin-resistant Staphylococcus aureus (MRSA), might induce convulsions if cerebral concentrations become highly elevated. In the present study, we examined whether or not cefoselis had convulsive activity after direct brain administration, and we attempted to clarify the pharmacological mechanism of action. When cefoselis was injected into the lateral ventricle of the mouse brain at doses higher than 20 microg/animal, it produced convulsions dose-dependently. Cefoselis (50 microg/animal)-induced convulsions were prevented by pretreatment with 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), diazepam and phenobarbital (ED(50) values (mg/kg) of 0.78, 1.59 and 33.0, respectively), but not by carbamazepine or phenytoin. When the effects of these anticonvulsants on the convulsions induced by intracerebral injection of bicuculline methiodide (BMI) or N-methyl-D-aspartate (NMDA) were investigated, the inhibitory profile of anticonvulsants on cefoselis-induced convulsions was similar to those induced by BMI (125 ng/animal) but differed markedly in their inhibitory activity on NMDA (100 ng/animal)-induced convulsions, which were not inhibited by diazepam. These results suggest that cefoselis may be convulsive at higher concentrations through a mechanism involving inhibition of gamma-aminobutyric acid (GABA)(A) receptors.

Differential bicuculline-induced epileptogenesis in rat neonatal, juvenile and adult CA3 pyramidal neurons in vitro.

The GABA(A) receptor antagonist bicuculline methiodide (BMI, 10 microM) transformed the evoked synaptic responses, recorded intracellularly from the CA3 area of neonatal (postnatal days 3-7, P3-P7), juvenile (P8-P20) and adult hippocampal slices, into long-lasting paroxysmal depolarizations (PDs), with repetitive action potentials (APs). In the same preparation, GABA(A)-mediated fast-IPSPs were depolarizing at resting membrane potential (RMP), with a reversal potential shifting to a hyperpolarizing direction with age (n=15, P6-P17). BMI provoked also spontaneous PDs in juvenile (20/30) and adult (7/10) but not in neonatal (0/12) neurons. PDs were depressed by either the NMDA receptor antagonist CPP (10 microM) or the non-NMDA antagonist CNQX (10 microM), but were blocked only by the combination of the two (n=6), indicating that activation of either NMDA or non-NMDA receptors can independently sustain PDs in immature hippocampus. In conclusion, these findings show that endogenous GABA tonically inhibits CA3 synaptic responses in neonatal life despite the depolarizing nature of GABA(A)-mediated potentials. Moreover, they suggest that during the 1st postnatal week, disinhibition alone is not sufficient to provoke spontaneous epileptiform discharges in CA3 hippocampal area.

Abecarnil enhances recovery from diazepam tolerance.

Treatment with diazepam (25 mg/kg; p.o., twice-daily for 17 days) induced tolerance to the anticonvulsant effect of diazepam against bicuculline-induced convulsions in mice. Cross-tolerance was observed to the anticonvulsant action of clonazepam, imidazenil but not abecarnil. While substitution of clonazepam (12 mg/kg; p.o., twice-daily for 15 days) for diazepam did not affect tolerance to diazepam, substitution of imidazenil (17 mg/kg; p.o., twice-daily for 15 days) for diazepam significantly increased sensitivity to the anticonvulsant effect of diazepam, although tolerance was not abolished. Tolerance to diazepam progressively decreased either after suspension of diazepam administration or replacement treatment with abecarnil (20 mg/kg; p.o., twice-daily). Complete recovery of diazepam efficacy was detected after 8 and 15 days of administration of abecarnil and vehicle, respectively. Binding experiments using [3H]-flumazenil showed that Kd values did not differ among treatment groups. A significant decrease in Bmax (-42%) was observed in the cortex of diazepam-tolerant mice whether or not also treated with imidazenil and clonazepam. Conversely, chronically diazepam-treated mice, that further received abecarnil for either 8 or 15 days or vehicle for 15 days showed Bmax values similar to those of vehicle-treated mice never exposed to diazepam. Results suggest that repeated abecarnil administration to diazepam-tolerant mice can facilitate re-adaptation of receptors to the diazepam-free state. It is proposed that replacement therapy with abecarnil after long-term treatment with conventional benzodiazepines (BDZs) may provide a novel approach for reducing tolerance to their anticonvulsant effects.

Inhibition of GABA system involved in cyclosporine-induced convulsions.

In this study, we attempted to clarify the mechanisms mediating cyclosporine-evoked convulsions. Cyclosporine (50 mg/kg, i.p.) significantly enhanced the intensity of convulsions induced by bicuculline (GABA receptor antagonist), but not those induced by strychnine (glycine receptor antagonist), N-methyl-D-aspartic acid, quisqualic acid or kainic acid (glutamate receptor agonists). Bicuculline plus cyclosporine-induced convulsions were significantly suppressed by an activation of GABAergic transmission with diazepam, phenobarbital and valproate. The GABA turnover estimated by measuring aminooxyacetic acid-induced GABA accumulation in the mouse brain was significantly inhibited by cyclosporine (50 mg/kg, i.p.). When cultured rat cerebellar granule cells were exposed to 1 microM cyclosporine for 24 hr, the specific [3H]muscimol (10 nM) binding to intact granule cells decreased to 53% of vehicle controls. The present study provides the first evidence suggesting that cyclosporine inhibits GABAergic neural activity and binding properties of the GABAA receptor. These events are closely related to the occurrence of adverse central effects including tremors, convulsions, coma and encephalopathy under cyclosporine therapy.

Bicuculline increases Ca2+ transients in rat cerebellar granule cells through non-GABA(A) receptor associated mechanisms.

The effects of bicuculline methiodide (bicuculline) and gabazine, two GABA(A) antagonists, on cytosolic calcium increases (Ca2+ transients) in rat cerebellar granule cells were examined using Fluo-3-spectrofluorometry. Bicuculline (25 microM) markedly potentiated Ca2+ transients caused by KCl (25 mM) and by A23187 (4 microM) whereas gabazine (25 microM) had no effect. Calcium increases caused by glutamate (2 microM), N-methyl-D-aspartic acid (200 microM), trans-1-amino-cyclopentane-1,3 dicarboxylate (200 microM), thapsigargin (1 microM) or caffeine (5 mM) were not altered by bicuculline. Thapsigargin, which depletes intracellular Ca2+ stores, had no effect on either KCl- or A23187-induced Ca2+ transients, but completely blocked bicuculline-induced potentiation of Ca2+ increases. Our data suggest that bicuculline triggers calcium release when calcium entry is evoked by KCl or A23187 and that this effect is not mediated via GABA(A) receptor blockade.

Role of GABA receptors in the bronchial response: studies in sensitized guinea-pigs.

BACKGROUND: Gamma-aminobutyric acid (GABA), an important inhibitory neurotransmitter in the mammalian central nervous system, is also found in peripheral tissues, including the lung. GABA has recently been shown to modulate the contraction of airway smooth muscle. OBJECTIVE: We studied the effect of GABA on the contractile properties of tracheal smooth muscle by measuring the tension of the trachea isolated from non-sensitized and ovalbumin (OA)-sensitized guinea-pigs under isometric conditions. METHODS: Guinea-pigs were sensitized by intraperitoneal doses of OA to prepare a bronchial asthma model. Tracheal spiral rings were prepared from the OA-sensitized as well as normal, non-sensitized guinea-pigs. Using the tracheal preparations, the effects of GABA and GABAa and GABAb receptor agonists (muscimol and baclofen) and antagonists (bicuculline and saclofen) on the basal tone of the trachea and on tracheal contraction induced by electrical field stimulation (EFS) were determined. The effect of GABA on tracheal contraction induced by exogenous acetylcholine was also studied. RESULTS: GABA and GABA agonists and antagonists had no effect on the basal tone of normal guinea-pig tracheae. Both GABAa and GABAb receptor agonists, as well as GABA, suppressed EFS-induced contraction of normal guinea-pig tracheae in a reversible, dose-dependent manner. Moreover, this suppression was reserved to the control level by either GABAa and GABAb receptor antagonists. In tracheal spiral ring prepared from OA-sensitized guinea-pigs, GABA and baclofen caused a smaller reversible inhibition of EFS-induced contraction than in normal tracheal spiral ring, while muscimol inhibited EFS-induced tracheal contraction to a similar extent to that observed in normal tracheae. GABA had no effect on the tracheal contractile response to acetylcholine. CONCLUSION: The results suggest that there may be a biological mechanism mediated by prejunctional GABAb receptors which attenuates cholinergic contraction of airway smooth muscle and that dysfunction of the receptors may underlie the airway obstruction in asthmatics.

Inhibition by levetiracetam of a non-GABAA receptor-associated epileptiform effect of bicuculline in rat hippocampus.

1. Extracellular recording of field potentials, evoked by commissural stimulation in hippocampal area CA3 of anaesthetized rats, was performed in order to study the mode of action of the novel antiepileptic drug levetiracetam (ucb LO59). 2. The amplitude of orthodromic field population spike (PS2) markedly increased and repetitive population spikes appeared when the recording micropipette contained either bicuculline methiodide (BMI), or the specific GABAA antagonist gabazine (SR-95531). 3. BMI-induced increases in PS2 were reduced in a dose-dependent manner by 1 to 320 mumol kg-1 levetiracetam i.v., with a U-shape dose-response relationship. However, levetiracetam did not reduce the increases in PS2 produced by gabazine. 4. Clonazepam (1 mg kg-1, i.p.), carbamazepine (20 mg kg-1, i.p.) and valproate (200 mg kg-1, i.v.) were ineffective in preventing BMI-induced increases in PS2, while the calcium channel antagonist flunarizine, 50 mumol kg-1, i.p., reduced PS2 increments caused by BMI. The L-type calcium channel blocker nifedipine, 100 mumol kg-1, i.p., was without effect. Similar to levetiracetam, flunarizine did not reduce the increases in PS2 induced by gabazine. 5. These data suggest that the increased excitability of CA3 neurones, caused by BMI administered in situ, involves calcium-dependent processes not associated with blockade of GABAA receptors. The inhibition by levetiracetam of this calcium-dependent effect of BMI might contribute to the antiepileptic effects of the drug.

Propofol inhibits epileptiform activity in rat hippocampal slices.

Propofol (2,6 di-isopropylphenol) is an intravenous general anesthetic used widely in neuroanesthesia, as a sedative in intensive care units, and has successfully aborted refractory status epilepticus. We investigated the effects of propofol on epileptiform activity in rat hippocampal slices. Interictal epileptiform activity was produced by bath applying one of the following: picrotoxin (PTX; 10 and 50 microM), bicucculine methiodide (BMI; 10 and 50 microM), 4-aminopyridine (4-AP; 50 microM), 8.5 mM [K+]o or 0 [Mg2+]o artificial cerebrospinal fluid. Propofol was then added in increasing concentrations and the effect on the rate of extracellular field epileptiform discharges was measured. Ictal-like discharges (> 2 Hz for > 2 s) were produced by 7.5 mM [K+]o and pilocarpine (10 microM). Propofol (30 micrograms/ml, 168 microM) completely abolished discharges induced by 8.5 mM [K+]o and at 60 micrograms/ml (337 mM) completely suppressed discharges induced by 4-AP and 0 [Mg2+]o. Propofol was less effective in reducing discharges produced by GABAA/Cl- receptor complex antagonists. Propofol at a concentration of 300 micrograms/ml (1.7 mM) was needed to reduce BMI-induced (50 microM) discharges by 77% and only reduced PTX-induced (50 microM) discharges by 20%. Ictal-like discharges produced by pilocarpine were disrupted by low concentrations of propofol (3-10 micrograms/ml, 16.9-56.2 microM) and the duration of the ictal-like discharge period was significantly reduced. We found that propofol has significant in vitro antiepileptic effects. Additionally, propofol was less effective against GABAA antagonists suggesting that the GABAA receptor complex is the site of its action.

Bicuculline-induced circling from the rat superior colliculus is blocked by GABA microinjection into the deep cerebellar nuclei.

In a recent electrophysiological experiment, we showed the deep cerebellar nuclei to be a major source of excitatory input to the superior colliculus. Furthermore, target neurons in the colliculus were found, in every case, to receive convergent tonic inhibitory input from the substantia nigra pars reticulata. In the present study, we investigated these effects in the awake rat. We asked whether circling behaviour, induced by unilateral injection of a GABA antagonist into the lateral colliculus, could be suppressed by concurrent cerebellar inactivation. Rats were chronically implanted with bilateral guide cannulae located above the superior colliculus and deep cerebellar nuclei. Bicuculline methiodide (25 pmol) was microinjected unilaterally into intermediate layers of the colliculus at increasing depths until an optimal contralateral circling response was elicited. This behaviour was taken as the "baseline response" and was the first of three treatments. The second was an identical manipulation of the colliculus with a concurrent 200-nl microinjection of 1 M GABA into the contralateral deep cerebellar nuclei. The third was a repeat of BIC alone into the colliculus or, if rotation had been suppressed by more than 50% on test 2, the treatment was collicular BIC plus deep cerebellar saline. This latter treatment was used as a control for possible non-pharmacological injection effects. The effect of cerebellar GABA at 26 sites (17 within cerebellar nuclei and 9 outside) on BIC-induced rotation at 15 collicular sites was studied in ten animals. Only GABA injections at sites that fell within the cerebellar nuclei significantly reduced turning (P < 0.0001). A full behavioural analysis showed that this was a specific suppression of turning, not the result of general motor impairment. These results provide clear behavioral evidence that opposing, convergent influences from the basal ganglia and cerebellum interact in the lateral superior colliculus to control head and body movements. They furthermore suggest that the tonic deep cerebellar excitation of the superior colliculus could be the driving force in the expression of rotation induced by manipulations of the basal ganglia.