S. Sagratella

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)

Neuroprotective effects of allopregnenolone on hippocampal irreversible neurotoxicity in vitro

1. The effects of allopregnenolone, a neurosteroid, endowed with GABAmimetic properties, were tested towards two models of irreversible hippocampal neurotoxicity: i) the irreversible depression produced by hypoxia on the CA1 evoked field potentials in rat hippocampal slices, and ii) glutamate-induced irreversible changes in intracellular calcium concentration in primary hippocampal cell coltures. 2. In control conditions during the reoxygenation period after the application of 15 min of hypoxia, the CA1 evoked field potentials were irreversibly suppressed in almost the 50% of the experiments. In the remaining experiments there were a significative (p<0.01) irreversible reduction of the magnitude of the CA1 population spike with respect with the pre-hypoxia values. Allopregnenolone (50-75 microM) perfused 30 min before, during and 30 min after hypoxia produced a significative (p<0.05) decrease both in the hypoxia-induced irreversible suppression of the CA1 PS and both in the irreversible decrease of the CA1 PS at the end of reoxygenation. 3. The exposition of the primary hippocampal cultured cells to glutamate 0.5 mM for 10 min was followed by a sustained elevation of [Ca2+]i, that persisted at 70-80% of maximal increase for the rest of the experiment (60 min). When a pretreatment with 10-50 microM allopregnanolone preceded Glu 0.5 mM application, [Ca2+]i increased to a maximal value during the glutamate application, after which a fast decrease to 50% was observed, followed by a slow recovery within about 30 min. 4. The results showed that the neurosteroid allopregnenolone, endowed with GABAmimetic properties, ameliorated the functional correlates of irreversible hippocampal neurotoxicity.

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.

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.

Reduced hippocampal CA1 Ca2+-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.

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.

Dexamethasone-induced selective inhibition of the central μ opioid receptor: functional in vivo and in vitro evidence in rodents

1 Endogenous corticosteroids and opioids are involved in many functions of the organism, including analgesia, cerebral excitability, stress and others. Therefore, we considered it important to gain information on the functional interaction between corticosteroids and specific opioid receptor subpopulations. 2 We have found that systemic administration (i.p.) of the potent synthetic corticosteroid, dexamethasone, reduced the antinociception induced by the highly selective μ agonist, DAMGO or by less selective μ agonists morphine and β‐endorphin administered i.e.v., On the contrary dexamethasone exerted little or no influence on the antinociception induced by a δ1 agonist, DPDPE and a δ2 agonist deltorphin II. Dexamethasone potentiated the antinociception induced by the k agonist, U50,488. 3 In experiments performed in an in vitro model of cerebral excitability in the rat hippocampal slice, dexamethasone strongly prevented both the increase of the duration of the field potential recorded in CA1, and the appearance and number of additional population spikes induced by μ receptor agonists. 4 In both models pretreatment with cycloheximide, a protein synthesis inhibitor, prevented the antagonism by dexamethasone of responses to the μ opioid agonists. 5 Our data indicate that in the rodent brain there is an important functional interaction between the corticosteroid and the opioid systems at least at the μ receptor level, while δ and k receptors are modulated in different ways.

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 Huntingtons disease (HD).

Different capability of N-methyl-D-aspartate antagonists to affect locomotor/exploratory activity of mice in a computerized on-line open field test

The effects of the competitive N-methyl-D-aspartate (NMDA) antagonists CGS 19755 and CPP, and of the noncompetitive NMDA antagonists PCP, MK 801, and dextromethorphan (DM) have been studied on the locomotor/exploratory activity of mice in a computerized on-line open field test. CGS 19755 (12.5-25 mg/kg, IP) induced a dose-dependent decrease in the locomotor/exploratory activity of mice; CPP (25-50 mg/kg, IP) did not present such an effect. PCP (1.25-10 mg/kg, IP) induced a dose-dependent increase/decrease in the locomotor/exploratory activity of mice, and DM (25-50 mg/kg, IP) and MK 801 (0.125-0.250 mg/kg, IP) increased it. The data show that NMDA antagonists affect locomotor/exploratory activity of mice in different ways, inducing both potentiating and inhibitory effects.

Behavioural and electroencephalographic interactions between haloperidol and PCP/sigma ligands in the rat

Phencyclidine (PCP) and sigma ligands produce a typical excitatory behaviour in rats, characterized by circling and head- and body-weaving. Excitatory amino acid antagonists such as 2-amino 5-phosphonovaleric acid (AP5) or 3-(±)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) also produce a PCP-like excitatory behaviour in rats. In the present paper, the interactions between PCP/sigma drugs or excitatory amino acid receptor antagonists and haloperidol have been investigated in rats. In addition, the influence of two other butyrophenones having a different affinity for the sigma/haloperidol receptors, such as spiperone and 3-(4-(3(4-fluorobenzoyl)-propyl-piperazino-1-ylisoquinolino (HR 375), has been tested on the behavioural and EEG effects of PCP/sigma drugs and excitatory amino acid antagonists. PCP (2.5–5 mg/kg IP), (+) or (−) SKF 10,047 (1–15 mg/kg IP), (+) or (−) cyclazocine (2–8 mg/kg IP) and AP5 (0.5 µmol ICV) dose-dependently and significantly (P<0.01) antagonized the haloperidol-induced catalepsy in the horizontal bar and podium tests in rats. On the other hand, either haloperidol (1 mg/kg IP) or spiperone (1 mg/kg IP) reduced the head-weaving induced by (+) SKF 10,047, PCP, or AP5. On the contrary, HR 375 (6 mg/kg IP) was ineffective in blocking the excitatory effects of these drugs. In addition, either haloperidol (1 mg/kg IP) or spiperone (1 mg/kg IP), but not HR 375 (6 mg/kg IP) reduced the amplitude increase of the fast (20–30 Hz) frequency/low (30–50 µV) voltage background cortical activity elicited by PCP or (+) SKF 10,047. The results demonstrate an interaction between dopamine and excitatory amino acid receptors. At the same time, the data reveal the scarce relevance of the high affinity sigma/haloperidol receptors in the interference between PCP/sigma drugs and butyrophenones.