Stimulation of adenosine A1 receptors prevents the EEG arousal due to dopamine D1 receptor activation in rabbits.

The influence of adenosine A1 (N6-cyclopentyladenosine, CPA) and A2 (2-[4-(2-carboxylethyl)phenethylamino]-5'-N-ethylcarboxamido -adenosine hydrochloride, CGS 21680) receptor agonists on SKF 38393-induced electroencephalographic (EEG) arousal was studied in rabbits. While CPA (0.1 mg/kg i.v.) significantly prevented the EEG effects of SKF 38393, CGS 21680 (0.2 mg/kg i.v.) did not affect them. These results demonstrate that adenosine A1 receptors can modulate dopamine D1 receptor-induced EEG arousal and show, for the first time, that adenosine-dopamine interactions are involved in brain functions other than motor activity.

Glutamate-dependent mechanisms in the induction of a calcium long-term potentiation-like phenomenon.

The electric synaptic efficacy, in terms of extracellular electrical potentials, and the intracellular postsynaptic efficacy, in terms of inositol phosphate (IP) accumulation, were evaluated in rat hippocampal slices exposed for a brief period (10 min) to a high concentration of calcium (+2.7 mM). In addition, the effects of N-methyl-D-asparate (NMDA) ionotropic and metabotropic glutamate receptor (mGluR) antagonists on the induction and the establishment or maintenance of enhanced synaptic efficacy of CA1 pyramidal neurons due to high-calcium exposure were also tested. Elevation of the calcium concentration from 1.3-4 mM in the medium bathing hippocampal slices produced a long-lasting (80 over 90 min) increase in the slope of the CA1 somatic excitatory postsynaptic potential and the amplitude of the population spike (PS). Slice perfusion with NMDA antagonists cyclazocine and cis-4-phosphonomethyl-2-piperidine-carboxylic acid (CGS 19755) or with mGluR antagonists L-2-amino-3-phosphonopropionic acid (AP3) or alpha-methyl-4-carboxyphenyl-glycine (all 0.1 mM), during the 10-min period of exposure to high-calcium prevented the induction of such changes. By contrast, slice perfusion with the same concentration of CGS 19755 or L-AP3 did not affect the already established long-lasting increase in amplitude of CA1 PS induced by high-calcium. Moreover, high-calcium failed to produce any significant modification of the basal IP accumulation or of the IP accumulation elicited by mGluR agonist 1S,3R-trans-amino cyclo-pentane-1,3-dicarboxylic acid (ACPD). In conclusion, the results confirm that high-calcium induces a long-lasting increase in synaptic efficacy in rat hippocampal slices. Both NMDA ionotropic and mGluR receptors are involved in the induction, but not in the maintenance, of this phenomenon. In line with these data no modifications of basal or ACPD-induced phosphoinositide hydrolysis have been found during the maintenance stage.

Evidence for the occurrence of depressant EEG effects after stimulation of dopamine D3 receptors: a computerized study in rabbits.

The putative dopamine D3 receptor agonist, (+/-) 7-OH-di-n-propylaminotetralin (+/- 7-OH-DPAT), induced depressant effects on rabbit EEG at the dose of 1 mg/kg i.v. Bromocriptine, a preferential dopamine D2 receptor agonist, induced EEG activation at the dose of 0.5 mg/kg i.v. Although the lack of very selective ligands makes it difficult to discriminate between D2- and D3- dependent effects, these findings suggest that -unlike D2 receptors-dopamine D3 receptors may mediate depressant effects on the electrocorticogram.

Reduced hippocampal CA1 Ca(2+)-induced long-term potentiation is associated with age-dependent impairment of spatial learning.

Expression of Ca(2+)-induced CA1 long-term potentiation (LTP) was analysed in hippocampal slices obtained from (1) 3-month-old and (2) 18-20-month-old Sprague-Dawley rats selected for their performances in the Morris water maze task. In all slices, a transient (10 min) increase of extracellular Ca2+ concentration (4 mM) caused a long-lasting enhancement of potentials evoked by electrical stimulation of radiatum fibers. However, a significant difference was found in the degree of potentiation among groups. In particular, increases of the CA1 response amplitudes were significantly lower in old rats impaired in spatial learning than in young at 30 (P < 0.05), 60, 90 and 120 min (P < 0.01) after restoring the normal Ca2+ concentration. On the contrary, no differences were observed between young animals and the old ones with good performances in spatial learning. The data suggest that amplitude of CA1 Ca(2+)-induced LTP in old rats is related to spatial learning abilities.

The stimulation of cholecystokinin receptors in the rostral nucleus accumbens significantly antagonizes the EEG and behavioural effects induced by phencyclidine in rats.

The influence of cholecystokinin (CCK), bilaterally injected into the rostral nucleus accumbens, on the EEG and behavioural effects induced by phencyclidine (PCP) has been studied in rats. CCK (10 ng) significantly inhibited PCP-induced EEG effects (increase of spectral power with respect to pre-drug tracing; increase of relative power distribution in the slowest frequency bands), and behavioural effects (circling and ataxia). The inhibitory effects of CCK were completely antagonized by 1 ng PD 135-158, a selective CCKB receptor antagonist, but not by lorglumide (1 microgram), a selective CCKA receptor antagonist. Since the effects induced by PCP in rodents have been proposed to be an experimental correlate of the psychotic symptoms it induces in humans, these results indicate that CCK may act as a neuroleptic. They also suggest that CCKB receptors located in the rostral nucleus accumbens may be involved in the neuroleptic-like activity of CCK.

Microanatomical and electrophysiological changes of the rat dentate gyrus caused by lesions of the nucleus basalis magnocellularis.

The effect of unilateral or bilateral lesions of the nucleus basalis magnocellularis (NBM) on the dentate gyrus of the hippocampus were assessed using microanatomical and electrophysiological techniques. NBM is the main cholinergic basal forebrain nucleus that supplies the fronto-parietal cortex. Lesions were induced using the neurotoxin ibotenic acid or a radio-frequency system and did not affect glutamic acid decarboxylase activity both in the frontal cortex and in the hippocampus. At 4 weeks after lesioning, a loss of choline acetyltransferase (ChAT) activity and of ChAT-immunoreactive fibres was observed in the frontal cortex but not in the hippocampus and no changes in the density of granule neurons of the dentate gyrus or in the hippocampal long-term potentiation (LTP) were noticeable. At 8 weeks after lesioning the loss of both ChAT activity and of ChAT-immunoreactive fibres persisted in the frontal cortex of NBM-lesioned rats. Moreover, at this time a significant decrease in the density of granule neurons in the dentate gyrus accompanied by a reduced probability of dentate LTP induction were observed in both ibotenic acid- and radio-frequency-lesioned rats. These findings have shown that although NBM does not send direct cholinergic projections to the hippocampus, lesions of this cholinergic nucleus are accompanied by delayed neurodegenerative changes involving the dentate gyrus. This suggests the occurrence of indirect connections between NBM and the hippocampus, the functional relevance of which should be explored.

Influence of NMDA receptor ligands on thyrotropin-releasing hormone-induced scratching in rabbits.

The influence of intracaudate administration of N-methyl-D-aspartic acid (NMDA) and of the competitive NMDA receptor antagonist, DL-2-amino-5-phosphonovaleric acid (AP-5) was studied on thyrotropin-releasing hormone (TRH)-induced scratching in rabbits. NMDA (28 nmol) significantly increased the latency of TRH-induced scratching but did not modify the duration of this behaviour. Conversely, AP-5 (0.5 mumol) significantly potentiated scratching duration. Since TRH-induced scratching has been reported to be a dopamine-dependent behaviour, these results suggest that NMDA receptor ligands modulate dopaminergic neurotransmission.

Nucleus basalis magnocellularis lesions decrease histochemically reactive zinc stores in the rat brain: effect of choline alphoscerate treatment.

The effects of monolateral lesioning of the nucleus basalis magnocellularis (NBM) and of choline alphoscerate treatment on histochemically reactive vesicular zinc stores were assessed in the rat brain using the sulphide-silver histochemical technique. Histochemically reactive zinc stores are located primarily within association fibres of the neuropil of the cerebral cortex as well as in the mossy fibres of the hippocampus. The density of cortical and hippocampal sulphide-silver positive fibres, which might have a role in cognitive and mnemonic processes, parallels the density of zinc-containing presynaptic buttons. Unilateral lesions of NBM caused a remarkable decrease of sulphide-silver positive fibres from the 4th week after lesioning in the neuropil of the ipsilateral fronto-parietal cortex and from the 3rd week in the mossy fibres of the ipsilateral hippocampus. Treatment with choline alphoscerate, which is a precursor in the biosynthesis of brain phospholipids that increases the bioavailability of acetylcholine in the nervous tissue, restored, in part, the density and pattern of sulphide-silver positive fibres in the fronto-parietal cortex and in the hippocampus. The data suggest that, analogously to reports from Alzheimer's disease patients, lesions of the NBM cause a decrease of zinc stores in the rat brain. Choline alphoscerate treatment is able to counter the expression of this phenomenon which accompanies experimental lesions of the NBM.

Felbamate selectively blocks in vitro hippocampal kainate-induced irreversible electrical changes.

The influence of the anticonvulsant felbamate has been tested on in vitro excitotoxicity induced by treatment of hippocampal slices with elevated concentrations of NMDA, AMPA and kainic acid. For comparison, the effects of the glutamate antagonist 7-chlorokynurenic acid and of the anticonvulsants pentobarbitone and lamotrigine, were also studied. Slice perfusion with 50 microM NMDA or 25 microM AMPA or 12 microM kainic acid produced within 30 min a disappearance or a pronounced irreversible amplitude reduction of the CA1 electrical synaptic responses. Slice perfusion with 1.2-1.6 mM felbamate or 100 microM lamotrigine significantly decreased the incidence of the irreversible disappearance of the CA1 electrical response induced by kainic acid. On the contrary, slice perfusion with the same concentrations of the drugs did not affect the irreversible disappearance of the CA1 electrical response induced by NMDA or AMPA. By contrast, slice perfusion with 100 microM of 7-chlorokynurenic acid significantly prevented the neurotoxic effects induced by both NMDA and kainic acid, while 100 microM of pentobarbitone failed to affect kainic acid-induced neurotoxicity. The different profile of neuroprotection elicited by felbamate with respect to reference drugs indicates that a different mechanism of action than antagonism of NMDA response or potentiation of GABA response underlies the neuroprotectant effects of felbamate.

Felbamate displays in vitro antiepileptic effects as a broad spectrum excitatory amino acid receptor antagonist.

The in vitro antiepileptic activity of the novel anticonvulsant drug felbamate was tested in rat hippocampal slices on the CA1 epileptiform bursting induced by different chemical epileptogenic agents. The effects of felbamate were compared with those of the anticonvulsant drugs diphenylhydantoin and pentobarbitone and with the effects of excitatory amino acid antagonists acting at both N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Like the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), felbamate at a minimum effective concentration of 1 mM induced a significant (P < 0.01) reduction of the duration of the CA1 epileptiform bursting due to the K+ channel blocker, 4-aminopyridine, and the excitatory amino acids, kainate and quisqualate. Like the NMDA receptor antagonist ketamine, felbamate (1.6 mM) significantly (P < 0.01) decreased the duration of the CA1 epileptiform bursting caused by 'Mg(2+)-free' solutions. Conversely, felbamate (1.6 mM), CNQX (100 microM) and ketamine (100 microM) failed to affect the epileptiform bursting induced by the GABA antagonist penicillin. Pentobarbitone (100 microM) significantly (P < 0.01) decreased the duration of the CA1 epileptiform bursting caused by 'Mg(2+)-free' solutions, 4-aminopyridine or penicillin, while diphenylhydantoin (up to concentrations of 100 microM) failed to have an effect. The results indicate that felbamate displays a unique profile of in vitro antiepileptic effects as a broad spectrum antagonist of excitatory amino acid transmission.

Postsynaptic antagonistic interaction between adenosine A1 and dopamine D1 receptors.

Behavioural and biochemical evidence for the existence of a powerful specific postsynaptic interaction between adenosine A1 and dopamine D1 receptors in the mammalian brain was found. Behavioural data showed that A1 receptor stimulation induced a decrease in the D1-induced motor activation in reserpinized mice and a decrease in the D1-dependent oral dyskinesia in rabbits. Biochemical data suggested that A1 receptor stimulation could produce a GTP-independent uncoupling of the rat striatal D1 receptor to the G protein. The A1-D1 receptor-receptor interaction might represent an important additional mechanism of action responsible for the motor depressant effects of adenosine agonists and for the motor stimulant effects of adenosine antagonists, like the methylxanthines caffeine and theophylline.

Selective reduction of hippocampal dentate frequency-potentiation in striatally lesioned rats with impaired place learning.

The induction of hippocampal frequency-potentiation (i.e. post-tetanic potentiation (PTP) and long-term potentiation (LTP) was analyzed in rat hippocampal slices obtained from animals showing impaired place learning in the Morris water maze as a consequence of bilateral striatal injection of quinolinic acid. Vehicle-injected animals, showing normal performances in the Morris water maze, behaved as controls. After the application of an electrical tetanus (1 s, 100 Hz, 50 microA) in the stratum radiatum, no significant differences were found in the percent of induction of both PTP and LTP in the CA1 area of hippocampal slices obtained from lesioned and sham-operated rats. After the application of an electrical tetanus (1 s, 100 Hz 50 microA) in the stratum moleculare, a significant difference was found in the percent of dentate PTP induction in hippocampal slices obtained from lesioned and sham-operated rats. Specifically, dentate PTP induction was significantly (P < 0.01) higher in slices obtained from sham-operated rats with a good performance in the Morris water maze than in slices obtained from striatally lesioned rats, which had shown poor performance in the Morris water maze. On the contrary, no significant differences were found in the percent of dentate LTP in hippocampal slices obtained from rats of the two groups. The data demonstrate that the impairment of the place learning in striatally lesioned rats is associated with a selective reduction of hippocampal dentate frequency-potentiation.

CGS 21680 antagonizes motor hyperactivity in a rat model of Huntington's disease.

The influence of CGS 21680 (2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido-adenos ine), an adenosine A2 receptor agonist, was tested in an animal model of Huntington's disease. Male Wistar rats received bilateral intrastriatal injections of quinolinic acid and then, 1 and 2 weeks later, they were treated with intrastriatal CGS 21680 (3 micrograms/2 microliters) or saline. While quinolinic acid-lesioned rats not treated with CGS 21680 showed the typical motor hyperresponsiveness to d-amphetamine (1 mg/kg i.p.), the intrastriatal injection of CGS 21680 completely prevented this effect.

Age and strain differences in rat place learning and hippocampal dentate gyrus frequency-potentiation.

Induction of post-tetanic potentiation (PTP) and long-term potentiation (LTP) was analyzed in hippocampal slices obtained from (i) young (6 months old) rats of different strains (Sprague-Dawley, SD; spontaneously hypertensive rats, SHR; and Wistar-Kyoto, WKY), and (ii) from aged (20-24 months old) SD and Fischer 344 (F 344) rats, each group showing a different performance in the Morris maze test. After the application of an electrical tetanus (1 s, 100 Hz, 50 microA) in the stratum moleculare, a significant difference was found in the percent of induction of the dentate PTP in hippocampal slices obtained from rats of different strains and ages. In particular, the induction of the dentate PTP was significantly (P < 0.01) higher in slices obtained from young SD or spontaneously SHR rats, having the better performance in the Morris maze than in slices obtained from old SD or F 344 rats or young WKY rats which had poorer performances in the Morris maze. On the contrary, no significant differences were found in the percent of induction of the LTP in the dentate area of hippocampal slices obtained from rats of different strains and ages. Moreover, after the application of an electrical tetanus (1 s, 100 Hz, 50 microA) in the stratum radiatum, no significant differences were found in the percent of induction of both PTP and LTP in the CA1 area of hippocampal slices obtained from rats of different strains and ages.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral and electrophysiological correlates of the quinolinic acid rat model of Huntington's disease in rats.

The influence of bilateral intrastriatal injection of quinolinic acid (QA, 300 nmol) was studied in male Wistar rats. Behavioral and electrophysiological experiments were conducted in 15 lesioned plus 15 vehicle-injected (control) animals. With respect to control animals, QA-lesioned rats showed marked, statistically significant alterations from both the behavioral (greater motor activation in response to d-amphetamine, place-learning deficit in the Morris water maze), and the electroencephalographic (reduced voltage amplitude and EEG power at the level of frontal cortex) points of view. In addition, a significant loss in body weight and a marked striatal gliosis (GFAP staining) were observed in lesioned rats. Conversely, QA-lesioned rats did not show modifications in posttetanic potentiation (P.T.P.) or long-term potentiation (L.T.P.) in CA1 hippocampal area. The present results confirm that QA lesions of rat striatum may be regarded as a suitable model of Huntington's disease (HD).

Effects of calcium antagonists on hypoxic and NMDA injury in rat hippocampal slices.

The effects of various calcium antagonists, acting at the different neuronal calcium channels, were studied towards two models of in vitro neuronal injury in rat hippocampal slices. In particular, the influence of the drugs were tested on the electrical failure induced by treatment of hippocampal slices with hypoxia or high concentrations of the excitatory amino acid N-methyl-D-aspartate (NMDA). The L-type calcium antagonists, nifedipine (100 microM) and diltiazem (100 microM) or the T-type calcium antagonist amiloride (100 microM) failed to significantly affect the recovery from the CA1 electrical failure induced by both hypoxia or NMDA (50 microM). The N-type calcium antagonists, omega-conotoxin GVIA (0.5 microM) and neomycin (300 microM) significantly (P < 0.01) increased the probability of the recovery of the CA1 population spike after hypoxia but not after NMDA (50 microM). The glutamate antagonist dizocilipine (50 microM), tested for comparison, significantly (P < 0.01) increased the probability of the recovery of the CA1 population spike after hypoxia and NMDA (50 microM). The results suggest an involvement of calcium channels especially of N-type in the genesis of hypoxic but not NMDA neuronal injury.

Influence of adenosinergic drugs on the epileptiform and neurotoxic effects of N-methyl-d-aspartate: comparison with the effects of MK801.

In the present paper, the influence of the adenosine receptor agonist N6-(l-2-phenylisopropyl)adenosine (L-PIA) and of the adenosine receptor antagonist caffeine on the epileptiform and neurotoxic effects of N-methyl-d-aspartate (NMDA) has been tested in rat hippocampal slices. Slice superfusion with 1 microM NMDA changed within 30 min the control CA1 field potentials into an epileptiform bursting in all experiments. Slice superfusion with 0.5-1 microM L-PIA or 50-100 microM caffeine plus 1 microM NMDA inhibited or potentiated, respectively, the CA1 epileptiform bursting duration with respect to a slice superfusion with 1 microM NMDA alone. Slice superfusion with 50-100 microM NMDA induced within a few minutes the appearance of short-lived (1-2 min) additional epileptiform population spikes, followed by an irreversible disappearance of the CA1 population spike. Slice superfusion with 50 microM of the NMDA antagonist dizocilpine (MK801) plus 50-100 microM NMDA prevented in all experiments the irreversible disappearance of the CA1 population spike with respect to a slice superfusion with 50-100 microM alone. Neither in a slice superfusion with 0.5-1 microM L-PIA plus 50-100 microM NMDA nor in a slice superfusion with 50-100 microM caffeine plus 50-100 microM NMDA did this effect occur. The results demonstrate that adenosine receptor ligands modulate the epileptiform but not the neurotoxic effects of NMDA.

Selective reduction of hippocampal dentate frequency potentiation in aged rats with impaired place learning.

Induction of posttetanic potentiation (PTP) and long-term potentiation (LTP) was analyzed in hippocampal slices obtained from a) young 6-month-old Sprague-Dawley (SD) rats, all of them performing well in the Morris Maze, and b) aged SD 20-month-old and Fischer 344 24-month-old rats showing different degrees of ability in the same test. After the application of an electrical tetanus 1 s, 100 Hz, 50 microA in the stratum radiatum, no significant differences were found in the percent of induction of both PTP and LTP in the CA1 area of hippocampal slices obtained from rats of different strains and ages. After the application of an electrical tetanus 1 s, 100 Hz, 50 microA in the stratum moleculare, a significant difference was found in the percent of dentate PTP induction in hippocampal slices obtained from rats of different ages. Specifically, dentate PTP induction was significantly (p < 0.01) higher in slices obtained from young SD rats, and from old SD rats with a better performance in the Morris maze, escape latency less than 10 s and 150 cm, than in slices obtained from old SD or Fischer 344 rats that had shown poor performance in the Morris Maze. On the contrary, no significant differences were found in the percent of dentate LTP in hippocampal slices obtained from rats of different strains and ages. The data demonstrate that the induction of hippocampal dentate high-frequency PTP is selectively reduced in old rats with impaired Morris Maze performance.

Effects of non-opioid antitussives on hypoxia-induced electrical changes in rat hippocampal slices: a comparative study with anticonvulsant drugs.

1. The effects of the non-opioid antitussives caramiphen and carbetapentane and of the anticonvulsants 5,5-diphenylhydantoin and MK 801 were tested towards hypoxia-induced electrical changes in rat hippocampal slices. 2. The incidence of appearance of hypoxia-induced epileptiform bursting was significantly decreased (P < 0.05) by carbetapentane (50-100 microM), caramiphen (50-100 microM), 5,5-diphenylhydantoin (25-50 microM), and the glutamate antagonist dizocilpine (MK 801, 25-50 microM). 3. The incidence of reappearance of the CA1 population spike after hypoxia was significantly increased (P < 0.05) by carbentapentane (50-100 microM), caramiphen (50-100 microM), 5,5-diphenylhydantoin (25-50 microM), and MK 801 (25-50 microM). 4. The results suggest a useful role for non-opioid antitussives and some anticonvulsants in the treatment of hypoxia-induced functional disturbances.

Muscarinic cholinoceptor subtypes in the rat frontoparietal cortex after ipsilateral lesions of the nucleus basalis magnocellularis.

Muscarinic cholinoceptor subtypes (M1 and M2) were studied in membrane particles of the rat frontoparietal cortex 72 h and 1, 2, 3, and 4 weeks after ipsilateral lesioning of the nucleus basalis magnocellularis (NBM). The affinity of the ligand used to characterize muscarinic cholinoceptors, 3H-quinuclidinyl benzilate did not significantly change in lesioned compared with sham-operated rats as well as the density of high affinity (M1) sites. Low affinity muscarinic cholinoceptors (M2 sites) were significantly decreased in NBM-lesioned rats 72 h and 1 week after lesioning. The density of M2 sites did not significantly differ in lesioned rats 2 or 3 weeks after NBM lesioning, but increased, in comparison with sham-operation 4 weeks after NBM lesioning. These findings suggest that frontoparietal M2 muscarinic cholinoceptors, which probably have a presynaptic localization, are sensitive to NBM lesions. Their changes at different times after NBM lesioning suggest the occurrence of loss, compensation and upregulation of cholinergic projections arising to the neocortex from the NBM.