Electroencephalographic investigations in rabbits of drugs acting at GABA-benzodiazepine-barbiturate/picrotoxin receptors complex

In vivo pharmacological characterization of AC-3933, a benzodiazepine receptor partial inverse agonist for the treatment of Alzheimer's disease

GABAergic neurons are known to inhibit neural transduction and therefore negatively affect excitatory neural circuits in the brain. We have previously reported that 5-(3-methoxyphenyl)-3-(5-methyl-1,2,4-oxadiazol-3-yl)-1,6-naphthyridin-2(1H)-one (AC-3933), a partial inverse agonist for the benzodiazepine receptor (BzR), reverses GABAergic inhibitory effect on cholinergic neurons, and thus enhances acetylcholine release from these neurons in rat hippocampal slices. In this study, we evaluated AC-3933 potential for the treatment of Alzheimer's disease, a disorder characterized by progressive decline mainly in cholinergic function. Oral administration of AC-3933 (0.01-0.03mg/kg) resulted in the amelioration of scopolamine-induced amnesia, as well as a shift in electroencephalogram (EEG) relative power characteristic of pro-cognitive cholinergic activators, such as donepezil. In addition, treatment with AC-3933 even at the high dose of 100mg/kg p.o. produced no seizure or anxiety, two major adverse effects of BzR inverse agonists developed in the past. These findings indicate that AC-3933 with its low risk for side effects may be useful in the treatment of Alzheimer's disease.

Functional connectivity in the brain before and during intra-arterial amobarbital injection (Wada test)

We explored the effect of unilateral intracarotid sodium amobarbital injection during the Wada test (intra-arterial amobarbital procedure, IAP) on functional connectivity in the brain assessed by synchronization analysis of the EEG. Patients suffering from pharmaco-resistant epilepsy who were selected for epilepsy surgery and underwent a preoperative IAP to determine language dominance and contralateral memory capacity were eligible. All patients had brain abnormalities (mostly tumors) or mesial temporal sclerosis. Ipsilateral intrahemispheric, contralateral intrahemispheric, and interhemispheric synchronization likelihood (SL) was calculated in seven frequency bands before and during the IAP. Forty-two patients who underwent the IAP (34 left carotid injections, 32 right carotid injections) were included. In the delta and theta bands, SL increased over the hemisphere ipsilateral to injection, while contralateral and interhemispheric SL decreased. The SL increased in the beta band. In the gamma bands, differences between patients with right-sided and left-sided lesions were observed. When a left hemisphere lesion was present, SL increased after injection, while a more unequivocal pattern of change was present in patients with right hemisphere lesions. Our results indicate that amobarbital injection has effects on functional connectivity of both the anaesthetized and non-anaesthetized hemispheres. Synchronization consistently increases in the injected hemisphere. Functional connectivity in the contralateral hemisphere decreases in the lower frequency bands, while it tends to increase in the beta and gamma bands (depending on lesion lateralization). These results indicate that functional connectivity in both the injected as well as in the contralateral hemisphere is strongly influenced by the IAP.

Systemic administration of the benzodiazepine receptor partial inverse agonist FG-7142 disrupts corticolimbic network interactions

The medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) coordinate various stress responses. Although the effects of stressors on mPFC and BLA activity have been previously examined, it remains unclear to what extent stressors affect functional interactions between these regions. In vivo electrophysiology in the anesthetized rat was used to examine mPFC and BLA activity simultaneously in response to FG-7142, a benzodiazepine receptor partial inverse agonist that mimics various stress responses, in an attempt to model the effects of stressors on corticolimbic functional connectivity. Extracellular unit and local field potential (LFP) recordings, using multielectrode arrays positioned in mPFC and BLA, were conducted under basal conditions and in response to systemic FG-7142 administration. This drug increased mPFC and BLA unit firing at the lowest dose tested, whereas higher doses of FG-7142 decreased various burst firing parameters in both regions. Moreover, LFP power was attenuated at lower (<1 Hz) and potentiated at higher frequencies in mPFC (1-12 Hz) and BLA (4-8 Hz). Interestingly, FG-7142 diminished synchronized unit firing, both within and between mPFC and BLA. Finally, FG-7142 decreased LFP synchronization between these regions. In a separate group of animals, pretreatment with the selective benzodiazepine receptor antagonist flumazenil blocked the changes in burst firing, LFP power and synchronized activity induced by FG-7142, confirming direct benzodiazepine receptor-mediated effects. These results indicate that FG-7142 disrupts corticolimbic network interactions via benzodiazepine receptor partial inverse agonism. Perturbation of mPFC-BLA functional connectivity induced by FG-7142 may provide a useful model of corticolimbic dysfunction induced by stressors.

Physiological and pathological spindling phenomena have similar regional EEG power distributions

Sleep spindles in human and in rat are known to have a thalamocortical substrate. It has also been suggested that absence epilepsy spike and wave discharges may be generated by a similar mechanism. In addition, we have previously reported a possible thalamocortical origin of the EEG spindling rhythmic discharges associated with myoclonic jerks in the picrotoxin rat model of primary generalised epilepsy. To investigate whether pathological and physiological brain rhythms have common mechanisms of generation, we analysed four electroencephalographic (EEG) spindling activities in the rat. These were the non-convulsive spindle discharges induced by picrotoxin (picrotoxin spindles), naturally occurring absence epilepsy spike and wave discharges (absence spindles), spindle discharges during natural sleep (sleep spindles) and spindling activity that occurs under barbiturate anaesthesia (barbiturate spindles). We used power spectral analysis to define and compare the strength and brain distribution of EEG power during the spindling activities in 12 forebrain and 7 brainstem regions. There were brain-wide differences in power for each of the different spindle types with the pathological rhythms of the epilepsies containing more power than the physiological rhythms. There were also similar differences in the expression of spindles related to the region examined and no thalamic emphasis. These results provide evidence for a similar regional EEG power distribution for all four types of spindling activity and thus for the different spindles being expressions of a single phenomenon.

Changes in rectal temperature and ECoG spectral power of sensorimotor cortex elicited in conscious rabbits by i.c.v. injection of GABA, GABA(A) and GABA(B) agonists and antagonists

1. In order to ascertain whether both GABA(A) and GABA(B), or only GABA(B) receptors, directly modulate thermoregulation in conscious rabbits, GABA(A)/GABA(B) agonist and antagonist agents were injected intracerebroventricularly in conscious rabbits while monitoring changes in rectal temperature (RT), gross motor behaviour (GMB) and electrocorticogram (ECoG) power spectra (ps) from sensorimotor cortices. 2. GABA (48 micromol), nipecotic acid (50 nmol), THIP (60 nmol), muscimol (18 nmol) and baclofen (8 nmol) induced hypothermia (-deltaRTmax values of 1.70+/-0.1, 1.4+/-0.2, 1.0+/-0.4, 1.1+/-0.2 and 1.6+/-0.3 degrees C, respectively), accompanied by inhibition of GMB and ECoG synchronization. THIP increased ps at delta frequency band (1.1-3.3 Hz), while GABA, nipecotic acid, muscimol and baclofen did the same at both delta and (4.6-6.5 Hz) frequency bands. ECoG ps changes were concomitant or even preceded hypothermia. 3. Bicuculline (1.8 nmol) induced hyperthermia (deltaRTmax 1.2+/-0.5 degrees C) and slight excitation of GMB, while CGP35348 (1.2 micromol) did not affect RT nor GMB. Both compounds did not affect ECoG ps. 4. Bicuculline potentiated muscimol-induced hypothermia, inhibition of GMB and synchronization of ECoG, while CGP35348 fully antagonized these effects. 5. In conclusion, the present results, while confirming the prevailing role of GABA(B), also outline a direct involvement of GABA(A) receptors in the central mechanisms of thermoregulation. Ascending inhibition towards discrete cortical areas controlling muscular activity and thermogenesis may result from GABA receptor activation in neurones proximal to the ventricles, thus contributing to hypothermia, although hypothermia-induced reduction of neuronal activity of these cortical areas cannot be ruled out.

Frontal cortex leads other brain structures in generalised spike-and-wave spindles and seizure spikes induced by picrotoxin

Generaliszed spike-and-wave (SW) spindles (5-7 Hz) associated with myoclonic jerks precede the occurrence of regular spikes (2-3 Hz) associated with convulsive seizure induced by picrotoxin. SW spindles occur spontaneously in rodent and cat under some experimental conditions and are considered to be models of human generalised epilepsy. These spindles have been proposed as being led by a thalamic pacemaker. To examine this possibility in picrotoxin-induced SW spindles and seizure spikes, we recorded EEG using chronically implant unipolar electrodes during intravenous picrotoxin infusion in freely behaving rat. The 6 EEG signals were digitally sampled at 1000 Hz. Linear correlation, spectral, coherence and phase analyses were undertaken to determine time differences (TDs) between EEG channels and the brain structure leading seizure activity. One frontal cortex led all other structures during SW spindles. TD between SW spindles in the leading frontal cortex (Fr1) and the contralateral Fr1 was 3.6 + / - 0.5 msec. All ipsilateral structures (hippocampus, thalamus, amygdala, caudate nucleus and occipital cortex) were delayed by more than 3 msec from Fr1 (intralaminar thalamic nuclei - by 6.3 + / - 0.9 msec). TDs of SW spindles between subcortical regions were less than 1.5 msec. Similar relationships with slightly smaller TDs were found with spikes during convulsive seizure except TDs between frontal cortices did not significantly differ from zero. We suggest that seizure activity induced by picrotoxin is led by one Fr1 during SW spindles and by both frontal cortices working as one system during convulsive seizure.

Possible involvement of an endogenous benzodiazepine receptor ligand of the inverse agonist type in the regulation of rapid-eye movement (REM) sleep: an hypothesis

1. Rapid-eye movement (REM) sleep is a complex behavioral state characterized by desynchronized electroencephalographic (EEG) activity, postural atonia, rapid, saccadic movements of the eyes, and vivid dreaming. 2. A recently developed class of drugs, the inverse agonist beta-carboline-3-carboxylates, elicits a number of effects similar to the properties of REM sleep, such as desynchronized cortical EEG and penile erections. 3. The hypothesis is put forth that an endogenous beta-carboline-3-carboxylate exists which may initiate many aspects of REM sleep. 4. Clinical relevance of this hypothesis is discussed with regard to REM anxiety dreams, night terrors, narcolepsy, and depression.

Topographic electroencephalogram of propofol-induced conscious sedation

OBJECTIVES

To determine the effects of increasing doses of propofol that induce conscious sedation on the topographic electroencephalogram (EEG) of human volunteers and to test the hypothesis that more frontal brain areas are affected by low doses of propofol.

METHODS

The scalp EEG was recorded monopolarly from 16 different sites based on the 10-20 International System. Microcomputer-based hardware and RHYTHM 7.1 software were used to obtain quantitative power frequency topographic EEG data. A total of 10 normal adult volunteers were given incremental doses of propofol targeted to plasma concentrations of 0 to 1200 ng/ml.

RESULTS

Sedative concentrations of propofol produced a dramatic increase in beta 1, an increase in alpha 2 and beta 2, and an increase in delta activity at the largest concentration, with almost no change in theta activity. The increase in beta 1 activity had a linear correlation with plasma propofol levels (r = 0.9). Topographic mapping indicated that beta 1 activation was primarily in the frontal and central regions, with focal changes more in the left hemisphere.

CONCLUSIONS

Topographic brain EEG mapping techniques indicate that frontal brain beta 1 EEG activity may be useful as an objective brain index of propofol conscious sedation.

Anxiogenic effects of a benzodiazepine receptor partial inverse agonist, RO 19-4603, in a light/dark choice situation

In a light/dark choice procedure, the imidazothienodiazepinone RO 19-4603, given alone, induced a dose-dependent decrease in the time spent by mice in the lit box as well in the number of transitions between the two boxes. These data confirm the anxiogenic intrinsic properties of inverse agonists of the benzodiazepine receptor. Since RO 19-4603 also reversed the anxiolytic effects of ethanol and exhibited proconvulsant properties, it is suggested that the antagonistic action of this drug against ethanol could be due to an additive rather than an interactive process.

Involvement of the "peripheral" benzodiazepine receptor type (omega 3) in the tolerance to the electroencephalographic effects of benzodiazepines in rats: comparison of diazepam and clonazepam

Rapid tolerance to the sedative effect of large doses of diazepam (10 mg/kg IV), but not of large doses of clonazepam (2 mg/kg IV) occurs in rats after 5 days of treatment on a once-a-day regimen. Electroencephalographic (EEG) studies show that such behavioral tolerance is associated with a decreased induction of spindle bursts and with an increased induction of 20-30 Hz waves (beta-like activity). Administration of clonazepam plus the agonist of the "peripheral" benzodiazepine receptor type (omega 3) Ro 5-4864 (4 mg/kg IV) for 5 days induces signs of behavioral and EEG tolerance to sedative effects of the benzodiazepine agonist. In animals treated for 5 days with diazepam plus the omega 3 antagonist PK 11195 (5 mg/kg IV), no signs of EEG and behavioral tolerance are observed. These results suggest that omega 3 type activation influences the development of rapid tolerance to the sedative effect of diazepam in rats.

Relative efficacies of benzodiazepine receptor agonists in affecting red nucleus electrical activity in rabbits

The effects of benzodiazepine receptor agonists on the electrical activity of red nucleus (RN) and neocortex were studied in rabbits. Under basal conditions, 30-40 microV, 40-50 Hz waves were recorded in RN. An increase of the amplitude (Emax, 75-90 microV) was found after IV injection of flunitrazepam (ED50, 0.14 mg/kg), diazepam (ED50, 0.28 mg/kg), alpidem (ED50, 1.57 mg/kg) and zolpidem (ED50, 0.73 mg/kg). Clonazepam (ED50, 0.12 mg/kg) and Cl 218,872 (ED50, 0.63 mg/kg) were less effective. In contrast, 2-10-fold higher doses were required to induce a slight decrease of the frequency. At the level of the cortex all benzodiazepine agonists induced synchronization and spindles. The effects of diazepam (5 mg/kg IV) in both areas were antagonized by flumazenil (0.04 mg/kg IV) and bicuculline (0.2 mg/kg IV). Pentamethylentetrazole (10-30 mg/kg IV) selectively abated the effect at the level of the cortex, whereas both clonazepam (2 mg/kg IV) and beta-CCM (0.6 mg/kg IV) selectively suppressed only the effects on the RN. These results suggest that activation of benzodiazepine receptor mainly influences the RN waves amplitude. The efficacy in increasing the amplitude appears related to the reported relative efficacy of the compound in potentiating GABA responses. The possibility exists that these effects are dependent upon the partial or full agonist action of the drugs or upon their binding at distinct benzodiazepine receptor types.

Phaclofen antagonizes the antinociceptive but not the sedative effects of (-)-baclofen

1. Intraperitoneal (ip) injection of (-)-baclofen induced long-lasting antinociceptive and sedative effects in rats. 2. Phaclofen, the phosphonic derivative of baclofen, fully antagonized the antinociceptive effect of (-)-baclofen. When injected intracerebroventricularly (icv), but not ip, phaclofen antagonized in a dose-dependent fashion (50-200 micrograms) the delays in behavioral response induced by (-)-baclofen (2.5-10 mg/kg ip) in both hot plate and tail flick tests. 3. In addition phaclofen (100 micrograms icv) counteracted the loss of the righting reflex induced by (-)-baclofen (7.5-15 mg/kg ip). 4. In contrast, phaclofen (100-200 micrograms icv) counteracted only in part the sedative effect of (-)-baclofen. In rats pretreated with the antagonist (200 micrograms icv), the electrocorticographic hypersynchrony due to (-)-baclofen (5 mg/kg ip) is replaced by a synchronized pattern associated with behavioral sedation. 5. These data are consistent with the reported antagonism by phaclofen on the effects of (-)-baclofen. They also seem to indicate that in rats phaclofen-sensitive GABA-B receptors play an important role in the analgesic effects of baclofen, but only a minor role in the sedative effects of this drug.

Effect of GABAergic drugs on motor impairment from ethanol, barbital and lorazepam in rat lines selected for differential sensitivity to ethanol

The effect of GABAergic drugs on the motor-impairing effects of ethanol, barbital, and lorazepam were studied in the ethanol-sensitive ANT (Alcohol Nontolerant) and ethanol-insensitive AT (Alcohol Tolerant) rat lines, selected for differential ethanol-induced motor impairment on the tilting plane. The basic population from which these rat lines were derived, the mixed (M) line, was also included in the study. The ANT rats were more sensitive to the intoxicating effects of ethanol, barbital, and lorazepam than the AT and M rats at the dose ranges tested. Picrotoxin antagonized motor impairment from all three drugs. Flumazenil (Ro 15-1788) antagonized only the effects of lorazepam, and isoniazid did not modify motor impairment induced by any of the three drugs. These results confirm that the selection of AT and ANT lines has not been specific to ethanol, and that it has increased sensitivity to ethanol, barbital, and lorazepam in the ANT rats rather than decreasing it in the AT rats relative to the M rats. The finding that picrotoxin counteracted motor impairment from ethanol, barbital, and lorazepam support the view that the GABAA receptor complex is important in mediating the intoxicating effects of these drugs. These results also suggest that the genetically-determined difference in sensitivity to ethanol between the rat lines involves GABAergic mechanisms, but it remains to be determined whether any part of the GABAA receptor itself has been affected by the selection program.

Electroencephalographic evidence for a dose-related biphasic effect of morphine on bicuculline-induced seizures in the rat

The effects of systemic pretreatment with morphine sulfate (MS) on electroencephalographic (EEG) manifestations of seizures due to bicuculline (BIC) were studied in freely moving rats. Electrodes were placed on frontal and occipital cortices, into the hippocampus and into a region of the prepiriform cortex, area tempestas (AT). BIC (0.1-0.25 mg/kg i.v.) elicited dose-dependent changes in electrocortical activity, consisting of 3 stages: stage 1, slight increase in periods of desynchronization; stage 2, trains of 4-6 Hz spike-and-wave complexes and of 2-4 Hz slow waves; stage 3, 'grand mal' seizures. Isolated spikes and hypersynchronous high voltage spikes occurred in the hippocampus during stage 2 and stage 3, respectively. In the AT, EEG abnormalities were observed during stage 3 only. During the recovery period, cortical synchronization occurred associated with the presence of occasional synchronous spikes in both deep areas. After pretreatment with MS (2, 12, and 25 mg/kg), stage 3 EEG seizures occurred with doses of BIC (0.12 or 0.18 mg/kg i.v.) that in the absence of MS produced only stage 1 or 2. In contrast, after pretreatment with a low dose of MS(0.5 mg/kg) the seizures induced by a maximally effective dose of BIC (0.25 mg/kg) were markedly attenuated. Thus, the effect of MS on BIC-induced seizure activity appears to be biphasic, potentiating seizures at doses at or above 2 mg/kg and inhibiting seizures at lower doses.

Does RO 15-4513 reverse the anxiolytic effects of ethanol by its intrinsic properties?

In order to better understand the antagonistic effects of the partial inverse agonist of benzodiazepine receptors, RO 15-4513, against the disinhibitory action of ethanol, we examined the effects of RO 15-4513 at a dose (2.0 mg/kg) that did not alter locomotor activity, given alone or in combination with ethanol, on the behavior of mice confronted with the light/dark choice procedure and the staircase test. At this dose, RO 15-4513 given alone was found to have slight anxiogenic properties and when given in combination with ethanol, to completely reverse the disinhibitory effects of ethanol. Since we previously observed postictal depression after higher doses of RO 15-4513 given alone and antagonistic effects of these same doses on the action of ethanol, it can be suggested that the antagonistic effects of RO 15-4513 against ethanol are due to its anxiogenic or depressive properties depending on doses. However, this hypothesis can only be regarded as being in early stages of development at the present time since these results do not parallel with those of several other studies and the question whether the antagonistic action of RO 15-4513 against ethanol is additive or interactive remains open.

Neuropharmacology of several beta-carboline derivatives and their 9-acetylated esters. In vivo versus in vitro studies in the rabbit

The effects of 3-methoxycarbonyl- (beta-CCM, Ia), 3-ethoxycarbonyl- (beta-CCE, Ic), 3-propoxycarbonyl- (PrCC, Ie), 3-N-methylcarboxamido- (FG-7142, Ig) beta-carboline and 2-acetyl-3-methoxycarbonyl-1,2-dihydro-beta-carboline (IIa) as well as of their corresponding 9-acetyl derivatives (Ib, Id, If, Ih and IIb) have been studied in rabbits. In addition, the effects of 6,7-dimethoxy-4-ethyl-3-methoxycarbonyl-beta-carboline (DMCM) have also been studied. In in vitro studies, these drugs compete with 3H-diazepam to benzodiazepine (BDZ) receptor in membrane preparations from brain cortex. The values of IC50 are in the nanomolar range without significant differences between the acetyl derivatives and their congeners only compound If shows a 10-fold decrease of the binding capacity in respect to its congener Ie. In the presence of 10(-5) M GABA, a decrease in the binding capacity for DMCM, Ia, Ic and Ig and an increase for If are observed. In vivo studies show that DMCM, Ia, Ib, IIa and IIb elicit three dose-dependent stages of electrocortical changes (trains of slow waves, trains of spike-and-wave complexes and "grand-mal" seizures). Compounds Ic, Id and Ig elicit only the first two stages. Compound Ih elicits only the first stage. While compound Ie does not affect the EEG pattern, its 9-acetyl derivative If induces changes (cortical spindles and disruption of the hippocampal theta waves) characteristic of agonist ligands of BDZ receptor. These findings confirm that the efficacy of compounds DMCM, Ia, Ic, Id, Ig and Ih as inverse agonists of BDZ receptor in the EEG paradigm parallels the reduction of their apparent binding affinity in the presence of GABA. The 9-acetylated compounds may be more inverse agonist in vivo than predicted from the in vitro findings, due to hydrolysis in the plasma.

Modifications of brain electrical activity after activation of the benzodiazepine receptor types in rats and rabbits

The present study reports a comparative electroencephalographic (EEG) study of drugs belonging to different chemical classes which share the property to bind at benzodiazepine (BDZ) recognition sites. The EEG patterns are recorded from the neocortex of rats and rabbits as well as from dorsal hippocampus and red nucleus in rabbits after intravenous administration of diazepam (0.1-10 mg/kg), clonazepam (0.02-2.5 mg/kg), zopiclone (0.3-3 mg/kg), flunitrazepam (0.03-2.5 mg/kg), CGS 9896 (0.1-3 mg/kg), zolpidem (0.1-3 mg/kg) and Cl 218,872 (0.1-10 mg/kg). The most relevant differences are observed at the level of the neocortex. All drugs induced appearance of 7-12 Hz spindle bursts. On the contrary, the presence of 15-30 Hz waves (defined beta-like activity) mainly occurs after diazepam, clonazepam and zopiclone. Scarce beta-like activity is present after CGS 9896, zolpidem and Cl 218,872. According to the selectivity of these drugs for the various types of BDZ receptor, one can speculate that activation of BDZ2 is relevant for the appearance of the beta-like activity. Flunitrazepam, diazepam, and zolpidem increase the amplitude of the red nucleus waves. Such an effect is less marked after zopiclone and CGS 9896, whereas is almost absent after clonazepam and Cl 218,872. A reduction of the frequency is observed after flunitrazepam, diazepam, clonazepam, CGS 9896 and zolpidem, whereas it is almost absent after zopiclone and Cl 218,872. Finally, all drugs induce a reduction of the amplitude of the hippocampal theta rhythms, whereas after diazepam, flunitrazepam, zolpidem and CGS 9896 a slowing of the record also occurs.

Proconflict and electrocorticographic effects of drugs modulating GABAergic neurotransmission

Proconflict and electrocorticographic effects of drugs acting on the benzodiazepine (BDZ)/GABA/chloride-ionophore receptor complex were studied in rats in an attempt to correlate their anxiogenic and epileptogenic activities. Evidence for proconflict activity was assessed by means of an operant conflict procedure based on the simultaneous reward and punishment of a conditioned task, while epileptogenic properties were assessed by monitoring the electrocorticogram (ECoG) of free-moving rats. Pentylenetetrazole and picrotoxin, which act through a site on the chloride channel, and the benzodiazepine (BDZ) inverse agonist FG 7142 showed epileptogenic alterations in the ECoG at doses, respectively, 8, 2 and 3 times higher than those eliciting a significant proconflict effect. For the partial inverse agonist CGS 8216, a ratio of about 60 was found while the BDZ antagonist Ro 15-1788 showed neither epileptogenic nor proconflict activity, except at the highest tested dose for the latter effect (40 mg X kg-1 PO). Inhibition of GABA transmission may mediate both anxiogenic and epileptogenic actions, and a link between these properties may exist as a continuous spectrum of negative intrinsic efficacy at the central BDZ/GABA/chloride-ionophore receptor complex.