Adenosine A2A receptor antagonism and neuroprotection: mechanisms, lights, and shadows

Adenosine A2A receptor antagonists are regarded as potential neuroprotective drugs, although the mechanisms underlying their effects remain to be elucidated. In this review, quinolinic acid (QA)-induced striatal toxicity was used as a tool to investigate the mechanisms of the neuroprotective effects of A2A receptor antagonists. After having examined the effects of selective A2A receptor antagonists toward different mechanisms of QA toxicity, we conclude that (1) the effect elicited by A2A receptor blockade on QA-induced glutamate outflow may be one of the mechanisms of the neuroprotective activity of A2A receptor antagonists; (2) A2A receptor antagonists have a potentially worsening influence on QA-dependent NMDA receptor activation; and (3) the ability of A2A receptor antagonists to prevent QA-induced lipid peroxidation does not correlate with the neuroprotective effects. These results suggest that A2A receptor antagonists may have either potentially beneficial or detrimental influence in models of neurodegeneration that are mainly due to increased glutamate levels or enhanced sensitivity of NMDA receptors, respectively.

Neuroprotective effects of the mGlu5R antagonist MPEP towards quinolinic acid-induced striatal toxicity: involvement of pre- and post-synaptic mechanisms and lack of direct NMDA blocking activity

The aim of this work was to investigate the potential neuroprotective effects of the metabotropic glutamate receptor 5 (mGlu5R) antagonist 2-Methyl-6-(phenylethynyl)-pyridine (MPEP) towards quinolinic acid (QA)-induced striatal excitoxicity. Intrastriatal MPEP (5 nmol/0.5 micro L) significantly attenuated the body weight loss, the electroencephalographic alterations, the impairment in spatial memory and the striatal damage induced by bilateral striatal injection of QA (210 nmol/0.7 micro L). In a second set of experiments, we aimed to elucidate the mechanisms underlying the neuroprotective effects of MPEP. In microdialysis studies in naive rats MPEP (80-250 micro m through the dialysis probe) significantly reduced the increase in glutamate levels induced by 5 mm QA. In primary cultures of striatal neurons MPEP (50 micro m) reduced the toxicity induced by direct application of glutamate [measured as release of lactate dehydrogenase [LDH]). Finally, we found that 50 micro m MPEP was unable to directly block NMDA-induced effects (namely field potential reduction in corticostriatal slices, as well as LDH release and intracellular calcium increase in striatal neurons). We conclude that: (i) MPEP has neuroprotective effects towards QA-induced striatal excitotoxicity; (ii) both pre- and post-synaptic mechanisms are involved; (iii) the neuroprotective effects of MPEP do not appear to involve a direct blockade of NMDA receptors.

Metabotropic glutamate receptor 5 (mGluR5)-mediated phosphoinositide hydrolysis and NMDA-potentiating effects are blunted in the striatum of aged rats: a possible additional mechanism in striatal senescence

The aim of the present work was to verify whether an impairment of subtype 5 metabotropic glutamate receptor-mediated neurotransmission did occur in the aged striatum. To this end, the ability of the subtype 5 metabotropic glutamate receptor agonist, RS-2-chloro-5-hydroxyphenylglycine, to stimulate phosphoinositide hydrolysis and to potentiate N-methyl-d-aspartate-induced effects in striatal slices from young (3 months) and aged (24 months) rats was compared. The ability of RS-2-chloro-5-hydroxyphenylglycine to induce maximal phosphoinositide turnover and to potentiate N-methyl-d-aspartate effects was significantly reduced in slices from old vs. young rats. These changes were associated with a significant reduction in the expression of subtype 5 metabotropic glutamate receptor protein (-28.8%) and phospholipase C-beta1 (-55.8%) in old striata, while receptor messenger ribonucleic acid expression was unchanged. These results show that the signalling associated with subtype 5 metabotropic glutamate receptors undergoes significant age-related changes and that a reduced expression of subtype 5 metabotropic glutamate receptors and, more importantly, phospholipase C-beta1 may account for the functional decline of subtype 5 metabotropic glutamate receptors.

The neuroscience of drug addiction: Rome built in a day

The Second Conference on the Neuroscience of Drug Addiction was a one-day meeting held in Rome, Italy at the Istituto Superiore di Sanità on 27 September 2002. Molecular, behavioral, pharmacological and clinical aspects of drug addiction were covered.

Blockade of striatal adenosine A2A receptor reduces, through a presynaptic mechanism, quinolinic acid-induced excitotoxicity: possible relevance to neuroprotective interventions in neurodegenerative diseases of the striatum

The aim of the present study was to evaluate whether, and by means of which mechanisms, the adenosine A2A receptor antagonist SCH 58261 [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine] exerted neuroprotective effects in a rat model of Huntington's disease. In a first set of experiments, SCH 58261 (0.01 and 1 mg/kg) was administered intraperitoneally to Wistar rats 20 min before the bilateral striatal injection of quinolinic acid (QA) (300 nmol/1 microl). SCH 58261 (0.01 but not 1 mg/kg, i.p.) did reduce significantly the effects of QA on motor activity, electroencephalographic changes, and striatal gliosis. Because QA acts by both increasing glutamate outflow and directly stimulating NMDA receptors, a second set of experiments was performed to evaluate whether SCH 58261 acted by preventing the presynaptic and/or the postsynaptic effects of QA. In microdialysis experiments in naive rats, striatal perfusion with QA (5 mm) enhanced glutamate levels by approximately 500%. Such an effect of QA was completely antagonized by pretreatment with SCH 58261 (0.01 but not 1 mg/kg, i.p.). In primary striatal cultures, bath application of QA (900 microm) significantly increased intracellular calcium levels, an effect prevented by the NMDA receptor antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate]. In this model, bath application of SCH 58261 (15-200 nm) tended to potentiate QA-induced calcium increase. We conclude the following: (1) the adenosine A2A receptor antagonist SCH 58261 has neuroprotective effects, although only at low doses, in an excitotoxic rat model of HD, and (2) the inhibition of QA-evoked glutamate outflow seems to be the major mechanism underlying the neuroprotective effects of SCH 58261.

Modulation of neurosteroid synthesis/accumulation by L-ascorbic acid in rat brain tissue: inhibition by selected serotonin antagonists

We have investigated the possibility that the synthesis/accumulation of neurosteroids, i.e., brain-produced steroids putatively endowed with modulatory actions in the CNS, is regulated by monoaminergic receptor-mediated mechanisms. In minces of rat brain cortex, L-ascorbic acid concentration-dependently (0.07-1.0 mM) increases the levels of pregnenolone, allotetrahydrodeoxycorticosterone, and dehydroepiandrosterone. This effect of L-ascorbic acid is region-dependent: in hippocampus, progesterone and allopregnanolone are also increased, whereas dehydroepiandrosterone is unchanged, and in corpus striatum only progesterone is increased significantly. 5-Hydroxytryptamine (10 microM), 1-(3-chlorophenyl)piperazine (1.0 microM), and 5-methoxytryptamine (0.4 microM) mimic the effect of L-ascorbic acid, whereas a pretreatment with p-chlorophenylalanine (400 mg/kg i.p., 2 days) reduces the amplitude of the L-ascorbic acid effect on brain cortical neurosteroids. The effect of L-ascorbic acid is blocked by the nonselective serotonin antagonists methiothepin, clozapine, methysergide, and pizotifen, but not mesulergine, spiperone, MDL 72222, and DL-propranolol, nor by the catecholaminergic receptor antagonists prazosin and S(-)-sulpiride. L-Ascorbic acid is not additive with dibutyryl-cyclic AMP and, furthermore, the inhibition of adenylate cyclase by MDL 12330A, but not of phospholipase C by U-73122, markedly attenuates the L-ascorbic acid-induced increase of pregnenolone in rat brain cortical minces. Together these data suggest that L-ascorbic acid plays a role in the modulation of neurosteroidogenesis, presumably by favoring the activation of the purported serotonin type 6 receptor by endogenous serotonin.

Modulatory mechanisms of cyclic AMP-stimulated steroid content in rat brain cortex

The modulation of cyclic AMP dependent neurosteroidogenesis was studied in minces prepared from the cerebral cortex of adult rat. Forskolin or dibutyryl-cyclic AMP enhanced pregnenolone and progesterone production in a time and dose-dependent manner. The forskolin effect was mimicked by the cyclic AMP phosphodiesterase inhibitor isobutyl-methyl-xanthine, but not by the adenylate cyclase inactive forskolin analogue 1,9,dideoxy-forskolin. 4'-Chloro-diazepam, a high affinity ligand for the mitochondrial diazepam binding inhibitor (DBI) receptor, also elicited a time dependent increase in steroidogenesis. The forskolin and the 4'-chloro-diazepam stimulated pregnenolone increase was prevented by preexposing the rat brain cortical minces to 1-(2-chlorophenyl)-N-methyl-N-(1-methyl-propyl)-3-isoquinoline carboxamide (PK 11195), a high affinity ligand for the mitochondrial DBI receptor endowed with antagonistic properties. The protein synthesis inhibitor cycloheximide prevented the forskolin and 4'-chloro-diazepam stimulation of pregnenolone formation. In brain cortical minces of adrenalectomised/orchiectomised rats dibutyryl-cyclic AMP increased both pregnenolone and progesterone formation, while forskolin only increased progesterone. These data show that cyclic AMP enhances brain steroidogenesis by acting on a labile protein substrate which interacts with the mitochondrial DBI receptor.

Cyclic AMP-dependent increase of steroidogenesis in brain cortical minces

Rat brain cortical minces were incubated with forskolin and dibutyryl-cyclic AMP for 60 and 30 min, respectively. The concentrations of pregnenolone, progesterone and desoxycorticosterone in this preparation were significantly increased by both substances. The results indicate that, in brain tissue, steroidogenesis appears to be regulated by receptor transduction mechanisms that operate through adenylate cyclase.

Repeated administration of diazepam reduces [3H]Ro 5-4864 binding in cerebral cortex of rats

Binding of [3H]Ro 5-4864 to mitochondrial membrane preparations of rat cerebral cortex was measured after repeated (5 days) IV administration of diazepam (10 mg/kg daily) and clonazepam (2.5 mg/kg daily). The B(max) value for [3H]Ro 5-4864 was significantly reduced in rats treated with diazepam (-49%) but not in those treated with clonazepam. These findings suggest the involvement of peripheral-type binding sites in the development of rapid tolerance to the sedative effects of benzodiazepines. A downregulation of [3H]Ro 5-4864 (-65%) was also observed after repeated administration of Ro 5-4864 (4 mg/kg daily), thus confirming that this compound behaves as an agonist at its own recognition sites.

Antinociceptive action of opiates and opioid peptides after unilateral microinjection into area tempestas in rats

Area tempestas (AT) is a forebrain area involved in the genesis and generalization of clonic convulsions in rats. This study reports that upon AT application opiates and opioid peptides produce antinociceptive effects as measured with the hot-plate (HP) and tail-flick (TF) tests in rats. Unilateral infusion of mu and kappa agonists into AT at doses in the nanogram range delayed the latency to respond for the contralateral paws in the HP test (Emax, 67-91 degrees of analgesia), beginning 1 to 5 min after application. A smaller effect was observed after the Leu-enkephalin, [D-Ala2,D-Leu5][enkephalin and D-Pen2,D-Pen5]enkephalin. No effect was observed after Met-enkephalin. In the TF test, unilateral application of mu and kappa agonists in the nanogram range produced antinociception with Emax values of 43 to 62 degrees of analgesia, beginning 5 to 15 min after infusion. No significant effect was found after unilateral infusion of delta agonists. Infusion into AT (10 min before) of naltrexone (2-4 ng), ICI 154, 129 (1-10 ng) and WIN 44,441-3 (2-20 ng) antagonized the antinociception evoked by locally applied morphine (25 ng), [D-Pen2,D-Pen5]enkephalin (50 ng) and U 50,488 (100 ng), respectively. In addition, antinociception induced by systemic morphine (2.5 mg/kg sc) was antagonized by subsequent (23 min) unilateral application of naltrexone (15 ng). In the HP test, a reduction of the antinociceptive effect of morphine was obtained for both ipsilateral and contralateral paws after the antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of diazepam binding inhibitor in rat adrenal gland by adrenocorticotropin

Diazepam binding inhibitor (DBI) is a 9-kDa polypeptide that was initially isolated from rat brain and subsequently found to be present in several peripheral tissues. DBI is particularly abundant in steroidogenic tissues, such as the adrenal glands and testes, which also contain a high concentration of peripheral/mitochondrial benzodiazepine receptors (MBRs). Because occupancy of adrenal MBRs with DBI results in increased steroidogenesis, we have investigated the relation between ACTH, DBI, and the MBR in the rat adrenal glands. Evidence presented here indicates that both the amount of DBI and its rate of synthesis in the adrenal cortex are under the control of ACTH. Seven and 9 days after hypophysectomy, the amount of DBI-like immunoreactivity (DBI-LI) in rat adrenal glands decreased dramatically from approximately 80 to 15 ng/mg tissue. The administration of single dose of ACTH (ACTH residues 1-39; 200 mU/kg, iv) or repeated doses of ACTH-R (ACTH in saline containing 16% gelatin; 15 U/kg, sc, twice daily) reduced the decrease in adrenal DBI-LI caused by hypophysectomy. In hypophysectomized rats (7 days after hypophysectomy) the increases in both adrenal DBI-LI and plasma corticosterone induced by ACTH 1 h after a single injection (200 mU/kg, iv) were inhibited by injection of cycloheximide (40 mg/kg, ip) 10 min after ACTH. However, cycloheximide at this dose had no effect on the ACTH-induced increase in adrenal cAMP concentration or the number of affinity of MBRs for 4'-[3H]chlorodiazepam.

gamma-Aminobutyric acidA receptor heterogeneity in rat central nervous system: studies with clonazepam and other benzodiazepine ligands

The properties of [3H]clonazepam, [3H]diazepam and [3H]zolpidem (N,N,6[trimethyl-2-(4-methyl-phenyl)imidazo[1,2-a]pyridine-3-acetamide hemitratrate) binding to synaptic membranes of cerebellum, cortex, olfactory bulb, striatum and spinal cord of rat were compared to the binding properties of [3H]flunitrazepam, [3H]flumazenil and [3H]midazolam. In the cerebellar, cortical and olfactory bulb membranes, the density of high-affinity binding sites of all these tritiated benzodiazepine (BZ) ligands is almost identical. In contrast, in the striatum, the density of [3H]clonazepam and [3H]zolpidem binding sites is approximately 60 and 30%, respectively, of the density of [3H]diazepam, [3H]flunitrazepam or [3H]flumazenil sites. In spinal cord membranes, the number of high-affinity binding sites of [3H]clonazepam and [3H]zolpidem is less than 20% of the number of binding sites for [3H]diazepam, [3H]flunitrazepam, [3H]flumazenil and [3H]midazolam. Moreover, the displacement of [3H]flunitrazepam from spinal cord membranes by clonazepam and zolpidem was characterized by high IC50 values and Hill slopes significantly less than 1. Because [3H]BZ ligand binding in the spinal cord is enhanced by gamma-aminobutyric acid (GABA), these data suggest that different regions of the rat central nervous system may contain different GABA-BZ receptor subtypes. The different pharmacological properties of clonazepam, diazepam and zolpidem (i.e., regarding their ability to enhance bicuculline seizure threshold, to decrease locomotor activity, to induce ataxia or to elicit anticonflict action) further support the concept that in the rat central nervous system preferential occupancy of heterogeneous GABAA receptors by these drugs can be related to their effects on behavior.

Enhancement of pharmacologically induced bronchoconstriction by Ro 5-4864

Pretreatment with Ro 5-4864 enhanced the bronchoconstrictor responses to histamine or carbachol in the anaesthetized artificially ventilated guinea pig. This suggests the involvement of a peripheral-type benzodiazepine receptor as also indicated by the lack of effect of clonazepam. PK 11195 did not antagonize the Ro 5-4864-induced enhancement, but itself enhanced the bronchoconstrictor responses to histamine or carbachol. The data indicate that Ro 5-4864 and PK 11195 behave as metactoid sensitizers of histamine or carbachol.

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.

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.

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.

Electroencephalographic changes after short-term exposure to agonists of benzodiazepine receptors in the rat

Naive rats receiving IV diazepam (10 mg/kg), flunitrazepam (2.5 mg/kg) and clonazepam (2 mg/kg) show electroencephalographic (EEG) changes consisting of lengthening of the spindle bursts (7-12 Hz; 200-300 microV) and appearance of 15-30 Hz waves (beta-like activity). These EEG manifestations are associated with signs of behavioral sedation (crouched, eyes open and myorelaxation) and stimulation (gnawing, running, ear twitches and sometimes wet-dog shakes), respectively. Bursts of 2-4 Hz waves can be occasionally observed associated with either marked sedation (lying down, eyes closed and presence of righting reflex) or sleep (stretched in the side with absence of righting reflex). Measurements of the periods spent by the animals in the two EEG patterns within the first hour after intravenous injection show the large preponderance of the spindle bursts over the beta-like activity. After the triazolopyridazine Cl 218,872 (10 mg/kg) the beta-like activity is almost absent, and in no case loss of the righting reflex can be observed. These agonists of BDZ receptors have been injected at the above reported doses for 5 days, once-a-day. At the 5th day, animals receiving diazepam exhibit a preponderance of the EEG and behavioral activation within the first hour after injection. Rats receiving flunitrazepam show a significant increase of the periods of stimulation and a slight decrease of the periods of sedation. These phenomena of "habituation" are absent in animals treated with clonazepam and Cl 218,872.

SCH 23390 and its S-enantiomer stereoselectively prevent EEG and behavioral activation induced by dopamine agonists in the rabbit

The selective D-1 dopamine antagonist SCH 23390 (R-enantiomer) and its unselective S-enantiomer (SCH 23388) were compared for their ability to prevent EEG and behavioral activation induced by the dopamine receptor agonists SKF 38393, apomorphine and LY 171555 in the rabbit. SCH 23390, at very low doses (0.003 mg/kg IV), inhibited EEG responses elicited by SKF 38393 and apomorphine, while the S-enantiomer displayed similar effects at doses at least 300-fold higher (1-3 mg/kg IV). Both isomers were approximately equipotent in preventing behavioral excitation caused by the D-2 agonist LY 171555. The dose of SCH 23390 interacting with LY 171555 was at least 100-fold higher than that effective for D-1 mediated responses. Conversely, the doses of S-enantiomer which prevented the stimulating effects induced by the different dopamine agonists were similar. The data demonstrate the stereoselectivity of the R-isomer SCH 23390 for blockade of D-1 receptors in vivo and provide evidence for the sensitivity of the EEG models in studying D-1 mediated responses.

Inhibition of penicillin-induced EEG discharges by low doses of morphine or naloxone in the rabbit. Evidence for a possible non-opioid receptor-mediated mechanism at the sensorimotor cortex

In rabbits, pretreatment by intravenous (IV) and intracortical (IC) routes with low doses of morphine (250 micrograms/kg IV or 60 pmoles/rabbit IC) and naloxone (1-50 micrograms/kg IV or 0.3 pmoles/rabbit IC) antagonizes the EEG and behavioural seizures due to the IC injection of penicillin (150 Units) at the level of the sensorimotor cortex. Pretreatment with naloxone (20 micrograms/kg IV) did not alter the anticonvulsant effect of morphine (250 micrograms/kg IV). The similar anticonvulsant effect of the two drugs together with the absence of any antagonism by naloxone on the effect of morphine seem to suggest that both drugs act through a non-opioid receptor-mediated mechanism. Further, in light of the low effective doses of the drugs and of the absence of any additive effect after their combined administration, one might speculate that morphine and naloxone do not act through different pharmacological receptors. However, the presence of distinct EEG patterns with either morphine or naloxone, injected IC and IV, in animals fully protected against penicillin-induced seizures, does not seem to be in favour of the latter possibility.

The peripheral benzodiazepine receptor ligand Ro 5-4864 induces supraspinal convulsions in rabbits. Reversal by the central benzodiazepine antagonist Ro 15-1788

The peripheral BDZ receptor ligand Ro 5-4864 was administered to rabbits in doses ranging from 0.2 to 7 mg/kg IV. Changes in electrocortical activity appeared within 1 min after administration, characterized by trains of slow waves in the posterior sensorimotor and optic cortices (0.6-2 mg/kg) and by grand mal seizures (2-10 mg/kg). The low doses also induced alterations in the basic rhythms both of the hippocampus (reduced amplitude and spike-like waves) and of the nucleus ventralis of thalamus (trains of slow waves), not associated with observable behavioural changes. The paroxysmal EEG activity observed at higher doses of the drug was first recorded in the cortical areas and then spread to the subcortical structures. No change in electrical activity could be observed in the spinal cord. The paroxysmal activity was associated with tonic-clonic convulsions and scialorrea. The EEG and behavioural manifestations were inhibited by administration of Ro 15-1788. This drug at doses of 0.6 and 6 mg/kg antagonized the effects of Ro 5-4864 at doses of 0.6-5 mg/kg and 6-7 mg/kg, respectively. This effect began 1-3 min after administration of the antagonist, and led to EEG synchronization. These data suggest that in rabbits the convulsant effect of Ro 5-4864 is due to interference of the drug at the GABA-BDZ-picrotoxin receptor oligomeric complex. Such an effect seems to be mediated at least in part by central BDZ receptors.

Stimulation of dopamine D-1 receptors by SKF 38393 induces EEG desynchronization and behavioral arousal

The dopamine D-1 receptor agonist SKF 38393 dose-dependently (2.5-10 mg/kg) induced desynchronization of the electroencephalographic (EEG) activity and behavioral arousal in both rabbits and rats. Unlike apomorphine, SKF 38393 elicited no signs of stereotyped behavior in rabbits and minimal effects, such as episodes of grooming, in rats. The effects of SKF 38393 10 mg/kg on the EEG were prevented by the selective D-1 receptor antagonist SCH 23390 at a dose as low as 0.003 mg/kg, but not by the D-2 antagonist (-)-sulpiride (25-50 mg/kg). These data provide evidence of a role of D-1 receptors in the generation of EEG activity related to behavioral arousal. In addition, this model is a valuable tool to functionally evaluate the D-1 antagonistic properties of neuroleptics.

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

This paper describes the EEG profiles, observed in rabbits, of drugs which affect GABA synaptic activity at GBB complex. Drugs which enhance GABA synaptic activity induce sedation associated with EEG synchronization. However, muscimol, THIP, GHB and baclofen induce signs of CNS stimulation (light tremors of the forelimbs, chewing, light nystagmus and hyperpnea) associated with EEG spikes. Signs of light stimulation (chewing and jerks of the head) also occur after BDZs and barbiturates, and are associated with the presence of 12-24 and 20-25 Hz waves, respectively. Drugs which reduce GABA synaptic activity (bicuculline, inverse BDZ agonists, PTZ, picrotoxin and Ro 5-3663) induce three dose-dependent stages of EEG changes: trains of slow waves, trains of spike-and-wave complexes and paroxysmal activity in the rostral encephalic structures without apparent changes of the electrical activity in the spinal cord. The first two stages are associated with a behavioral state of alert and the third stage with tonico-clonic convulsions. Among the inverse BDZ agonists, DMCM and beta-CCM elicit all three stages, whereas FG 7142 and beta-CCE induce only the first two and CGS 8216 only the first. The BDZ antagonists Ro 15-1788 and Ro 15-3505 (0.2-30 mg/kg IV) do not significantly affect the EEG pattern. However, they selectively inhibit the effects of diazepam and of the inverse BDZ agonists. In both cases, the inhibition is observed with doses as low as 0.2 mg/kg IV and leads to an EEG desynchronization. The possible involvement of the modifications of GABA synaptic activity in the etiology of both petit mal and grand mal epilepsies is discussed.

Supraspinal convulsions induced by inverse benzodiazepine agonists in rabbits

The electroencephalographic (EEG) effects of inverse benzodiazepine (BDZ) agonists have been studied in rabbits after i.v. administration. A dose-dependent progression of three different stages of EEG changes have been observed with inverse BDZ agonists. At first, trains of slow waves in the occipital cortex occur, followed by trains of spike-and-wave complexes in the sensorimotor cortex. These two stages are superimposed on a desynchronized cortical activity, accompanied by an enhancement of the hippocampal theta rhythm. These EEG changes parallel a state of alertness. The third stage is characterized by generalized grand-mal seizures made up of high voltage spikes in the cortical and subcortical brain areas accompanied by generalized tonico-clonic convulsions. No modification of electrical activity is observed at the level of the spinal cord. Methyl-beta-carboline-3-carboxylate (beta-CCM) (at doses higher than 0.2 mg/kg) and 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) (at doses higher than 0.4 mg/kg) elicit all three stages, whereas ethyl-beta-carboline-3-carboxylate (beta-CCE) (0.2-2 mg/kg) and N-methyl-beta-carboline-3-carboxamide (2-20 mg/kg) only elicit the first two, and finally CGS 8216 only the first. The extent of the EEG progression by inverse BDZ agonists may therefore be used as an index of the efficacy of each compound. The BDZ antagonists Ro 15-1788 and Ro 15-3505 (0.3 mg/kg or higher), which do not change the EEG pattern, block the effects of the convulsant and subconvulsant doses of the inverse BDZ agonists, giving rise to a desynchronized EEG pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic administration of diazepam to rats causes changes in EEG patterns and in coupling between GABA receptors and benzodiazepine binding sites in vitro

The daily administration of diazepam (10 mg/kg i.v.) in rats elicited tolerance to the sedative effect within 2-3 days, parallel to the emerging of a stimulatory syndrome. In the EEG, the latter was accompanied by a progressive replacement of the trains of spindles with 25-30 Hz low amplitude waves. No clear signs of tolerance to the myorelaxant effect of diazepam were observed. After 10 days of treatment, the drug was discontinued. Thereafter, a single administration of diazepam elicited a stimulatory syndrome and EEG desynchronization throughout the 15 days of withdrawal. [3H]diazepam binding studies, performed in frozen-thawed membrane preparations incubated at 37 degrees C for 30 min, showed a decrease of KD values in tolerant rats up to 7 days after discontinuation of the drug. In parallel, a reduction in the ability of GABA to enhance [3H]diazepam binding, as well as to protect [3H]diazepam recognition sites from thermoinactivation at 60 degrees C, were found in tolerant rats up to 15 days after drug termination. These data suggest that a modification in the regulatory activity of GABA on benzodiazepine recognition sites might underlie the tolerance to the sedative effects and EEG changes of benzodiazepines.

Influence of morphine and cyclazocine on the cortical epileptic foci in rabbits

The effects of morphine, cyclazocine and naloxone on penicillin- and strychnine-induced epileptic foci were studied in rabbits. The intracortical injection of penicillin (75, 150 and 300 units) elicited isolated spikes followed by repeated ictal events. The application of strychnine (0.062 and 0.125%) over the cortical surface of one side induced appearance of ipsilateral spiking spreading to the contralateral cortex. Administration of morphine (0.25-0.75 mg/kg i.v.) or cyclazocine (0.05-3.0 mg/kg i.v.) inhibited the occurrence or the duration of the EEG and motor manifestations induced by penicillin (75 and 150 units) and strychnine (0.062 and 0.125%), while it did not influence the effect of 300 units of penicillin. High doses of morphine (up to 10 mg/kg i.v.) failed to affect the epileptic responses to penicillin and strychnine and at the same time significantly reduced the pO2 in arterial blood. Naloxone per se potentiated the effects of the lower doses of penicillin and strychnine. Only at very high doses (20 mg/kg i.v.) displayed a weak antagonism towards the anticonvulsant effect of the two opiates. A full antagonism is only observed towards the effect of cyclazocine (2 mg/kg i.v.) administered after penicillin. Present data provide additional evidence of the heterogeneity of regulation by opioids of convulsive phenomena. One can hypothesize that the anticonvulsant effect of the two opiate agonists is mediated by naloxone-insensitive opiate receptors, while the proconvulsant-convulsant effect of naloxone might be related to an inhibition of GABA and glycine-mediated transmission.

Convulsant and anticonvulsant effects of opioids: relationship to GABA-mediated transmission

The convulsant benzodiazepine Ro 5-3663, bicuculline and picrotoxin induced electroencephalographic (EEG) and behavioural convulsions. In rabbits, the EEG modifications consisted, with increasing doses, of three different patterns: slow waves in the optic lead, spike- and wave-complexes in the sensorimotor cortex, and grand-mal generalized seizures. These EEG effects were terminated by administration of diazepam (1 mg/kg) and morphine (0.25-1.0 mg/kg). Naloxone, in doses of 5-10 mg/kg, potentiated the effects of the three convulsant drugs. This potentiating phenomenon was also antagonized by the administration of diazepam and morphine. In membrane preparations, obtained from rat cortex, deprived of endogenous modulators of [3H]GABA binding, naloxone but not morphine, was able to inhibit [3H]GABA binding to its specific recognition sites. These data agree with previous findings indicating a GABA-antagonistic effect of naloxone, and support the hypothesis that the anticonvulsant effect of morphine might be, at least in part, due to an increase in GABAergic activity at the synaptic level.

Endogenous inhibitors of Na+-independent [3H]GABA binding to crude synaptic membranes

The characteristics of the Na+-independent high-affinity binding of [3H]GABA to various types of crude synaptic membranes (CSM) prepared from rat brain cortex were studied. In freshly prepared CSM the content of GABA was so high that the high-affinity [3H]GABA binding could not be determined. In contrast when the frozen-thawed CSM were incubated at 37 degrees for 30 min with or without Triton X-100 or phospholipase C and then washed repeatedly, there was a virtual disappearance of GABA from the supernatant extracts and the binding constants of [3H]GABA to CSM could be determined. Two apparent populations of [3H]GABA binding sites, one with a low- and the other with a high-affinity constant, were detected. The ratio of the number of high- to low-affinity binding sites varies with the method used to prepare the membranes. The lowest value of this ratio was observed with membranes incubated at 37 degrees for 30 min. However, when frozen-thawed CSM were treated with 0.05% Triton X-100 repeatedly, the ratio of the number of high- to low-affinity binding sites increased progressively. This increase in ratio is due to a selective increase in the number of the high-affinity sites without significant changes in the number of the low-affinity sites. The extent of the increase in the number of sites that bind [3H]GABA with high affinity after repeated Triton X-100 treatments was paralleled by a decrease of an endogenous protein which inhibits GABA binding. The reapplication of this endogenous material to membranes repeatedly treated with Triton X-100 reduces the number of high-affinity binding sites for [3H]GABA to values similar to those measured in membranes that were not treated with Triton X-100. The inhibitory preparation extracted from CSM incubated with Triton X-100 was shown to be free of GABA or phospholipids. The gel filtration chromatography reveals the presence of two molecular forms of the inhibitor; of these, the high-molecular-weight material fails to bind GABA, whereas the low-molecular-weight material appears to bind GABA. The high-molecular-weight endogenous inhibitor has been termed GABA modulin.

Characterization of benzodiazepine and gamma-aminobutyric recognition sites and their endogenous modulators

The solubilization and partial purification from rat brain cortex homogenates of [3H]gamma-aminobutyric acid (GABA) and [3H]diazepam recognition sites and of their endogenous modulators (GABA-modulin and an endogenous compound which specifically bound [3H]diazepam) are reported. A high percentage of GABA binding sites (virtually free of benzodiazepine binding sites) was solubilized from homogenates of rat brain cortex incubated at 0 degree C with 1% Triton X-100 and a mixture of protease inhibitors. A large proportion of benzodiazepine binding sites was solubilized in the absence of apparent GABA binding capacity by incubating crude synaptic membrane preparations at 37 degree C with 0.05% Triton X-100. The characteristics of these two solubilized binding sites resemble those of the membrane-bound binding sites. However, unlike the membrane-bound sites, solubilized GABA and benzodiazepine binding sites are insensitive to GABA stimulation, while solubilized GABA binding sites are no longer protected by the benzodiazepines against heat inactivation. These results indicate that GABA and benzodiazepine recognition sites reside in two different molecules which, when bound to membranes, can interact reciprocally and modulate their binding affinity for specific ligands.

Behavioral, electroencephalographic, and biochemical changes in porta-cava shunted rats

Behavioral, electroencephalographic, and biochemical alterations have been studied in rats with porta-caval shunt, up to 45--50 days after the operation. No behavioral or electroencephalographic changes have been observed, while modifications of various amino acids both in plasma and brain have been found. Among all the amino acids considered of particular significance are the plasma and brain increases of tyrosine, phenylalanine, and tryptophan, since they are the precursors of biogenic amines. In fact, the increase of 5-HIAA found in regions of CNS which are richest in serotoninergic synapsis could indicate an increased turnover of 5-HT. On the other hand no significant alterations of turnover of catecholamines have been found.

Role of the dopaminergic system in the cataleptogenic action of bulbocapnine

Time courses of the behavioural and biochemical effects of a cataleptogenic dose (50 mg kg-1 i.p.) of bulbocapnine have been studied in the rat. Catalepsy ensues immediately after administration of the drug and lasts 1 h. Concomitant with the onset of catalepsy there is a rise in HVA and DOPAC concentrations in whole and discrete parts of the brain (striatum, limbic system). Dopamine content does not change in whole brain but it decreases in the striatum and increases in the cortex and hippocampus. No significant effects on NA, 5-HT and 5 HIAA concentrations were observed.