Maria Cristina Tomasini

Differential enhancement of dialysate serotonin levels in distinct brain regions of the awake rat by modafinil: Possible relevance for wakefulness and depression

The present in vivo microdialysis study evaluates the possible existence of a differential regulation of serotonergic transmission by the antinarcoleptic drug modafinil [(diphenyl‐methyl)‐sulfinyl‐2‐acetamide; Modiodal] among various brain regions of the awake rat. The results show that, in the cerebral cortex, the central amygdala, and the dorsal raphe nucleus, modafinil in the dose range of 10–100 mg/kg i.p. dose‐dependently increases dialysate serotonin (5‐HT) levels. In other brain areas, such as the medial preoptic area and the posterior hypothalamus, the modafinil‐induced increase in dialysate 5‐HT levels is observed only at tenfold higher doses (100 mg/kg), 10–30 mg/kg being ineffective. Together these data suggest that, in the frontal cortex, the amygdala, and the dorsal raphe, modafinil is more potent in enhancing extracellular 5‐HT levels and presumably 5‐HT transmission than in the medial preoptic area and the posterior hypothalamus. In view of the role of ascending 5‐HT pathways in arousal and depression, it seems likely that the antinarcoleptic drug modafinil may also have an antidepressant potential in addition to its wakefulness‐promoting action, both actions involving enhancement of 5‐HT neurotransmission.

Cannabinoid receptor agonist WIN 55,212-2 inhibits rat cortical dialysate gamma-aminobutyric acid levels.

The effects of the cannabinoid receptor agonist WIN 55,212‐2 (0.1–5 mg/kg i.p.) on endogenous extracellular γ‐aminobutyric acid (GABA) levels in the cerebral cortex of the awake rat was investigated by using microdialysis. WIN 55,212‐2 (1 and 5 mg/kg i.p.) was associated with a concentration‐dependent decrease in dialysate GABA levels (–16% ± 4% and –26% ± 4% of basal values, respectively). The WIN 55,212‐2 (5 mg/kg i.p.) induced‐inhibition was counteracted by a dose (0.1 mg/kg i.p.) of the CB1 receptor antagonist SR141716A, which by itself was without effect on cortical GABA levels. These findings suggest that cannabinoids decrease cortical GABA levels in vivo, an action that might underlie some of the cognitive and behavioral effects of acute exposure to marijuana. J. Neurosci. Res. 66:298–302, 2001.

Nigral neurotensin receptor regulation of nigral glutamate and nigroventral thalamic GABA transmission: a dual-probe microdialysis study in intact conscious rat brain.

Dual-probe microdialysis in the awake rat was employed to investigate the effects of intranigral perfusion with the tridecapeptide neurotensin on local dialysate glutamate and GABA levels in the substantia nigra pars reticulata and on dialysate GABA levels in the ventral thalamus. Intranigral neurotensin (10-300nM, 60min) dose-dependently increased (+29+/-3% and +46+/-3% vs basal for the 100 and 300nM concentrations, respectively) local dialysate glutamate levels, while the highest 300nM concentration of the peptide exerted a long-lasting and prolonged reduction in both local and ventral thalamic (-20+/-4% and -22+/-2%, respectively) GABA levels. Intranigral perfusion with the inactive neurotensin fragment neurotensin(1-7) (10-300nM, 60min) was without effect. Furthermore, the non-peptide neurotensin receptor antagonist SR 48692 (0.2mg/kg) and tetrodotoxin (1microM) fully counteracted the intranigral neurotensin (300nM)-induced increase in local glutamate. SR 48692 (0.2mg/kg) also counteracted the decreases in nigral and ventral thalamic GABA release induced by the peptide. In addition, intranigral perfusion with the dopamine D(2) receptor antagonist raclopride (1microM) fully antagonized the neurotensin (300nM)-induced decreases in nigral and ventral thalamic GABA levels. The ability of nigral neurotensin receptor activation to differently influence glutamate and GABA levels, whereby it increases nigral glutamate and decreases both nigral and ventral thalamic GABA levels, suggests the involvement of neurotensin receptor in the regulation of basal ganglia output at the level of the nigra.

Acute exposure to methylmercury at two developmental windows: Focus on neurobehavioral and neurochemical effects in rat offspring

The neurobehavioral and neurochemical effects produced by prenatal methylmercury exposure (8 mg/kg, gestational-days 8 or 15), were investigated in rats. On postnatal day 40, animals exposed to methylmercury and tested in the open field arena, showed a reduction in the number of rearings, whereas the number of crossings and resting time was not altered with respect to the age-matched control rats. The methylmercury-exposed groups showed a lower level of exploratory behavior as well as an impairment in habituation and working memory when subjected to the novel object exploration task. The neophobia displayed by methylmercury-exposed rats is unlikely to be attributed to a higher degree of anxiety. Prenatal methylmercury exposure did not affect motor coordination or motor learning in 40-day-old rats subjected to the balance task on a rotating rod, and it did not impair the onset of reflexive behavior in pups screened for righting reflex, cliff aversion and negative geotaxis. In cortical cell cultures from pups exposed to methylmercury during gestation, basal extracellular glutamate levels were higher, whereas the KCl-evoked extracellular glutamate levels were lower than that measured in cultures from rats born to control mothers. In addition, a higher responsiveness of glutamate release to N-methyl-D-aspartic acid receptor activation was evident in cortical cell cultures from pups born from methylmercury-treated dams than in cultures obtained from control rats. The present results suggest that acute maternal methylmercury exposure induces, in rat offspring, subtle changes in short-term memory as well as in exploratory behavior. These impairments seem to be associated to alterations of cortical glutamatergic signaling.

Experimental studies and theoretical aspects on A2A/D2 receptor interactions in a model of Parkinson's disease. Relevance for L-dopa induced dyskinesias

Dual probe microdialysis was used to study A2A/D2 receptor interactions in the striato-pallidal GABA pathway in a model of Parkinsons Disease. The A2A agonist CGS21680 and/or the D2-like agonist quinpirole were perfused via reverse microdialysis into the DA denervated striatum and the effects on globus pallidus (GP) extracellular GABA levels were evaluated. CGS21680 alone produced in the DA denervated striatum a transient rise of GP GABA levels. Quinpirole perfused alone into the DA denervated striatum reduced GP GABA levels, which was not only counteracted by coperfused CGS21680, but led to an enhancement of the GABA levels, which was larger than that seen with CGS21680 alone. These results may reflect existence not only of antagonistic A2A/D2 interactions but also of the appearance of D2/A2A interactions increasing the A2A signaling at the level of the adenylate cyclase. Such actions diminish the therapeutic efficacy of L-dopa and D2 agonists. L-dopa induced dyskinesias could be caused by changes in the balance of A2A/D2 heteromers vs A2A homomers expressed at the surface membrane, where A2A homomers dominate with abnormal increases in A2A signaling. This may lead to stabilization of abnormal receptor mosaics (high order hetero-oligomers) leading to formation of abnormal motor programs contributing to dyskinesia development.

A new Multi-charged C60 Derivative: Synthesis and Biological Properties

A new water-soluble multi-charged monoadduct fullero[60]pyrrolidine derivative with three ethylene glycol chains and three ammonium groups has been synthesized by means of two alternative synthetic pathways. Increasing the concentration of this C60 derivative did not show a significant modification of concentration of superoxide anion radical O2·−, generated by the xanthine/xanthine oxidase system. Moreover, this C60 derivative was ineffective for neuroprotection in quantitative assessment of a neuronal injury considered to occur via radicals. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Neurotensin increases endogenous glutamate release in rat cortical slices

In the present study, the effects of the tridecapeptide neurotensin [NT(1-13)] and its fragments, NT(1-7) and NT(8-13), on endogenous glutamate release from rat cortical slices, were evaluated. NT(1-13) (100-1000 nM) slightly increased spontaneous glutamate release, while it was ineffective at 1 and 10 nM concentrations. Neither the biologically active NT fragment NT(8-13) nor the inactive one NT(1-7) affected basal glutamate release. NT(1-13) (1-1000 nM) enhanced potassium (35 mM)-evoked glutamate release displaying a bell-shaped concentration response curve. In addition NT(8-13) (10 nM) increased K+-evoked-glutamate release similarly to the parent peptide (10 nM), while the biologically inactive fragment NT(1-7) (10-100 nM) was ineffective. The effects of NT(1-13) and NT(8-13) were fully counteracted by the selective neurotensin receptor antagonist SR48692 (100 nM). These findings suggest that NT plays a role in regulating cortical glutamate transmission.

Kynurenic acid, by targeting α7 nicotinic acetylcholine receptors, modulates extracellular GABA levels in the rat striatum in vivo

Kynurenic acid (KYNA) is an astrocyte‐derived non‐competitive antagonist of the α7 nicotinic acetylcholine receptor (α7nAChR) and inhibits the NMDA receptor (NMDAR) competitively. The main aim of the present study was to examine the possible effects of KYNA (30 – 1000 nm), applied locally by reverse dialysis for 2 h, on extracellular GABA levels in the rat striatum. KYNA concentration‐dependently reduced GABA levels, with 300 nm KYNA causing a maximal reduction to ~60% of baseline concentrations. The effect of KYNA (100 nm) was prevented by co‐application of galantamine (5 μm), an agonist at a site of the α7nAChR that is very similar to that targeted by KYNA. Infusion of 7‐chlorokynurenic acid (100 nm), an NMDAR antagonist acting selectively at the glycineB site of the receptor, affected neither basal GABA levels nor the KYNA‐induced reduction in GABA. Inhibition of endogenous KYNA formation by reverse dialysis of (S)‐4‐(ethylsulfonyl)benzoylalanine (ESBA; 1 mm) increased extracellular GABA levels, reaching a peak of 156% of baseline levels after 1 h. Co‐infusion of 100 nm KYNA abolished the effect of ESBA. Qualitatively and quantitatively similar, bi‐directional effects of KYNA on extracellular glutamate were observed in the same microdialysis samples. Taken together, the present findings suggest that fluctuations in endogenous KYNA levels, by modulating α7nAChR function, control extracellular GABA levels in the rat striatum. This effect may be relevant for a number of physiological and pathological processes involving the basal ganglia.

Differential effects of acute and short-term lithium administration on dialysate glutamate and GABA levels in the frontal cortex of the conscious rat.

In the present study, we employed in vivo microdialysis in the frontal cortex of the awake rat to investigate the effects of acute and short‐term (twice daily, 3 days) lithium chloride administration (1, 2, and 4 meq/kg, s.c.) on local dialysate glutamate and GABA levels. Acute lithium (1 meq/kg) failed to influence cortical glutamate levels while the higher (2 and 4 meq/kg) doses increased (+38 ± 6% of basal levels) and reduced (‐27 ± 4%) cortical glutamate levels, respectively. Cortical GABA levels were affected by acute lithium only at the highest 4 meq/kg dose (+62 ± 6%). Furthermore, these effects were prevented by tetrodotoxin (1 μM) and low‐calcium (0.2 mM) medium perfusion. Following short‐term administration, lithium increased (+58 ± 4%) cortical dialysate glutamate levels at the 1 meq/kg dose, was ineffective at 2 meq/kg, while the effect of the 4 meq/kg dose was similar to that observed after acute administration. Interestingly, intracortical perfusion with the GABAB receptor antagonist CGP 35348 (100 μM) reversed the acute lithium (4 meq/kg)‐induced decrease in glutamate levels. Taken together, these findings indicate a differential dose and duration dependent effect of lithium on cortical dialysate glutamate levels involving both a direct enhancement and an indirect inhibition that is mediated via an activation of local GABAB receptor. These findings may be relevant for the therapeutic effects of the drug. Synapse 38:355–362, 2000.

Developmental exposure to methylmercury elicits early cell death in the cerebral cortex and long-term memory deficits in the rat

Experiments were performed to assess the neurotoxic effects induced by prenatal acute treatment with methylmercury on cortical neurons. To this purpose, primary neuronal cultures were obtained from cerebral cortex of neonatal rats born to dams treated with methylmercury (4 and 8 mg/kg by gavage) on gestational day 15, the developmental stage critical for cortical neuron proliferation. Prenatal exposure to methylmercury 8 mg/kg significantly reduced cell viability and caused either apoptotic or necrotic neuronal death. Moreover, this exposure level resulted in abnormal neurite outgrowth and retraction or collapse of some neurites, caused by a dissolution of microtubules. The severe and early cortical neuron damage induced by methylmercury 8 mg/kg treatment correlated with long term memory impairment, since adult rats (90 days of age) born to dams treated with this dose level showed a significant deficit in the retention performance when subjected to a passive avoidance task. Prenatal exposure to methylmercury 4 mg/kg significantly increased the neuronal vulnerability to a neurotoxic insult. This was determined by measuring the increment of chromatin condensation induced by glutamate, at a concentration (30 μM) able to induce an excitotoxic damage. This exposure level eliciting apoptotic death did not result in cognitive dysfunctions. In conclusion, the methylmercury‐induced disruption of glutamate pathway during critical windows of brain development may interfere with cell fate and proliferation resulting in a more or less severe cortical lesions associated or not with loss of function later in life, depending on the exposure levels. Therefore, the early biochemical effects and long‐term behavioral changes elicited by high methylmercury levels suggest that the developing brain is impaired in its ability to recover following toxic insult, and the initial effects on cortical neurons may lead to permanent cognitive dysfunctions.