Amygdala adenosine A1 receptors have no anticonvulsant effect on piriform cortex-kindled seizures in rat

Adenosine is an endogenous anticonvulsant that exerts its effects through A1 receptors. As the piriform/amygdala is a critical circuit for limbic seizure propagation, in this study, the role of basolateral amygdala A1 receptors on piriform cortex (PC)-kindled seizures was investigated. Rats were kindled by daily electrical stimulation of PC. In fully kindled animals, bilateral intra-amygdala N6-cyclohexyladenosine (CHA; 10-500 micromol/L, a selective A1 receptor agonist) had no effect on kindled-seizure parameters. However, bilateral intra-amygdala 2% lidocaine (reversal neuronal inhibitor) reduced the kindled seizure severity. There was significant increase in stage 4 latency and decrease in stage 5 duration. Bilateral lesion of basolateral amygdala of kindled animals (by electrical DC current) reduced the kindled seizure severity more dramatically. Our results showed afterdischarge duration, stage 5 duration, and seizure duration were decreased and stage 4 latency increased significantly. In addition, daily intra-amygdala CHA had no significant effect on PC kindling acquisition. Therefore, it may be concluded that although the basolateral amygdala neuronal activity has a critical role in the propagation of epileptic seizures from PC, the amygdala A1 receptors have no role in this regard. On the other hand, amygdala A1 receptors have no anticonvulsant or antiepileptogenic effect on PC-kindled seizures.     

Modification of the anticonvulsant efficacy of diazepam by Ro-15-1788 in the kindled amygdaloid seizure model

The effects of various doses of diazepam and the new central benzodiazepine antagonist Ro-15-1788 were investigated in fully amygdaloid kindled rats. Diazepam had a pronounced dose-dependent anticonvulsant effect in this model. Ro-15-1788 dose-dependently reduced the behavioral ranks of the elicited kindled seizures to a maximum of 60% of control without consistently modifying the afterdischarge duration. No prestimulation convulsant effects were seen with Ro-15-1788. When 2 mg/kg i.p. of Ro-15-1788 was given after various doses of diazepam, the prestimulation sedation and ataxia anticonvulsant effects of diazepam (0.5-2.0 mg/kg) were attenuated by treatment with 2 mg/kg dose of Ro-15-1788. At the low dose of diazepam (0.25 mg/kg), increased reduction of behavioral rank and after discharge duration was seen after the 2 mg/kg dose of Ro-15-1788. Thus, Ro-15-1788 appears not to have proconvulsant properties in the kindled amygdaloid seizure model. Further, Ro-15-1788 appears to have some anticonvulsant properties of its own. Mixed agonist and antagonist effects were seen with Ro-15-1788 when given after various doses of diazepam in this model.     

Expression of A1 adenosine receptors in the developing avian retina: in vivo modulation by A2A receptors and endogenous adenosine

Little is known about the mechanisms that regulate the expression of adenosine receptors during CNS development. We demonstrate here that retinas from chick embryos injected in ovo with selective adenosine receptor ligands show changes in A1 receptor expression after 48 h. Exposure to A1 agonist N⁶‐cyclohexyladenosine (CHA) or antagonist 8‐Cyclopentyl‐1, 3‐dipropylxanthine (DPCPX) reduced or increased, respectively, A1 receptor protein and [³H]DPCPX binding, but together, CHA+DPCPX had no effect. Interestingly, treatment with A_2A agonist 3‐[4‐[2‐[[6‐amino‐9‐[(2R,3R,4S,5S)‐5‐(ethylcarbamoyl)‐3,4‐dihydroxy‐oxolan‐2‐yl]purin‐2‐yl]amino] ethyl]phenyl] propanoic acid (CGS21680) increased A1 receptor protein and [³H]DPCPX binding, and reduced A_2A receptors. The A_2A antagonists 7‐(2‐phenylethyl)‐5‐amino‐2‐(2‐furyl)‐pyrazolo‐[4,3‐e]‐1,2,4‐trizolo[1,5‐c] pyrimidine (SCH58261) and 4‐(2‐[7‐amino‐2‐[2‐furyl][1,2,4]triazolo[2,3‐a][1,3,5]triazo‐5‐yl‐amino]ethyl)phenol (ZM241385) had opposite effects on A1 receptor expression. Exposure to CGS21680 + CHA did not change A1 receptor levels, whereas CHA + ZM241385 or CGS21680 + DPCPX had no synergic effect. The blockade of adenosine transporter with S‐(4‐nitrobenzyl)‐6‐thioinosine (NBMPR) also reduced [³H]DPCPX binding, an effect blocked by DPCPX, but not enhanced by ZM241385. [³H]DPCPX binding kinetics showed that treatment with CHA reduced and CGS21680 increased the Bmax, but did not affect Kd values. CHA, DPCPX, CGS21680, and ZM241385 had no effect on A1 receptor mRNA. These data demonstrated an in vivo regulation of A1 receptor expression by endogenous adenosine or long‐term treatment with A1 and A_2A receptors modulators.     

Urethane anesthesia blocks the development and expression of kindled seizures

The effect of anesthetic and subanesthetic doses of urethane on the development of amygdala kindled seizures and on the expression of previously kindled seizures was studied in hooded rats. An anesthetic dose of urethane (1.5 g/kg) almost completely eliminated evoked afterdischarge and completely eliminated convulsive behavior in both groups. It also eliminated the seizure response to pentylenetetrazol. Subanesthetic doses of urethane (0.25 and 0.5 g/kg) strongly attenuated the expression of previously kindled seizures. These results suggest that urethane may not be an appropriate anesthetic for the study of epileptiform phenomena.     

Implication of adenosine A2A receptors in hypotension-induced vasodilation and cerebral blood flow autoregulation in rat pial arteries

This study aimed to evaluate the role for adenosine A2A receptors in the autoregulatory vasodilation to hypotension in relation with cerebral blood flow (CBF) autoregulation in rat pial arteries. Changes in pial artery diameters were observed directly through a closed cranial window. Vasodilation induced by adenosine was markedly suppressed by ZM 241385 (1 micromol/l, A2A antagonist) and alloxazine (1 micromol/l, A2B antagonist), but not by 8-cyclopentyltheophylline (CPT, 1 micromol/l, A1 antagonist). CGS-21680-induced vasodilation was more strongly inhibited by ZM 241385 (25.3-fold; P<0.05) than by alloxazine. In contrast, 5'-N-ethylcarboxamido-adenosine (NECA)-induced vasodilation was more prominently suppressed by alloxazine (12.0-fold; P<0.001) than by ZM 241385. The autoregulatory vasodilation in response to acute hypotension of the pial arteries was significantly suppressed by ZM 241385, but not by CPT and alloxazine. Consistent with this finding, the lower limit of CBF autoregulation significantly shifted to a higher blood pressure by 1 micromol/l of ZM 241385 (53.0+/-3.9 mm Hg to 69.2+/-2.9 mm Hg, P<0.01) and 10 micromol/l of glibenclamide (54.7+/-6.5 mm Hg to 77.9+/-4.2 mm Hg, P<0.001), but not by CPT and alloxazine. Thus, it is suggested that adenosine-induced vasodilation of the rat pial artery is mediated via activation of adenosine A2A and A2B receptors, but not by A1 subtype, and activation of adenosine A2A receptor preferentially contributes to the autoregulatory vasodilation via activation of ATP-sensitive K+ channels in response to hypotension and maintenance of CBF autoregulation.     

Amygdala kindled seizure stage is related to altered benzodiazepine binding site density

Benzodiazepine receptor binding was examined in rats at 3 stages of amygdaloid kindling (i.e., initial afterdischarge, Stage 3 and Stage 5) immediately or 24 hr after seizure. 3H-diazepam binding site density (Bmax) was significantly increased 24 hr after Stage 3 and Stage 5 kindled seizures in the hippocampus but not in the amygdala. There were no significant differences in the dissociation constants (KD) between kindled and control rats at any time point examined for either brain region. These results demonstrate that changes in benzodiazepine binding are observed with partial kindled seizures (i.e., Stage 3), indicating that generalized seizures are not prerequisite to increased benzodiazepine receptor site density.     

Anticonvulsant effects of magnesium sulfate in hippocampal-kindled rats

The objective of the present study was to determine whether magnesium sulfate has anticonvulsant actions in the hippocampal-kindled rat model of epilepsy. Fully kindled rats received acute intraperitoneal injections of magnesium sulfate (270 mg/kg), phenytoin (20 mg/kg) or saline in random order. Electrical seizure duration, behavioral seizure stage and duration of postictal EEG depression were examined 15, 30 and 60 min after injection. In an additional group of rats, kindled seizures were measured before and after chronic (2 h) intraperitoneal injections of magnesium sulfate versus saline. There was a significant decrease in electrical seizure duration (p<0.01) and behavioral seizure stage (p<0.01) with acute magnesium sulfate injections compared to saline injections. Phenytoin had no statistically significant effects on hippocampal-kindled seizures. Chronic magnesium sulfate treatment significantly reduced behavioral seizure stage at 2, 24, and 48 h postinjection (p<0.05), but did not affect seizure duration. There was a significant time by treatment effect for magnesium sulfate on postictal EEG depression (p<0.01). We conclude that in this model of hippocampal epilepsy-induced (kindled) rats, magnesium sulfate has significant anticonvulsant effects.     

A Transient Upregulation of Glutamine Synthetase in the Dentate Gyrus Is Involved in Epileptogenesis Induced by Amygdala Kindling in the Rat

Reduction of glutamine synthetase (GS) function is closely related to established epilepsy, but little is known regarding its role in epileptogenesis. The present study aimed to elucidate the functional changes of GS in the brain and its involvement in epileptogenesis using the amygdala kindling model of epilepsy induced by daily electrical stimulation of basolateral amygdala in rats. Both expression and activity of GS in the ipsilateral dentate gyrus (DG) were upregulated when kindled seizures progressed to stage 4. A single dose of L-methionine sulfoximine (MSO, in 2 µl), a selective GS inhibitor, was administered into the ipsilateral DG on the third day following the first stage 3 seizure (just before GS was upregulated). It was found that low doses of MSO (5 or 10 µg) significantly and dose-dependently reduced the severity of and susceptibility to evoked seizures, whereas MSO at a high dose (20 µg) aggravated kindled seizures. In animals that seizure acquisition had been successfully suppressed with 10 µg MSO, GS upregulation reoccurred when seizures re-progressed to stage 4 and re-administration of 10 µg MSO consistently reduced the seizures. GLN at a dose of 1.5 µg abolished the alleviative effect of 10 µg MSO and deleterious effect of 20 µg MSO on kindled seizures. Moreover, appropriate artificial microRNA interference (1 and 1.5×10⁶ TU/2 µl) of GS expression in the ipsilateral DG also inhibited seizure progression. In addition, a transient increase of GS expression and activity in the cortex was also observed during epileptogenesis evoked by pentylenetetrazole kindling. These results strongly suggest that a transient and region-specific upregulation of GS function occurs when epilepsy develops into a certain stage and eventually promotes the process of epileptogenesis. Inhibition of GS to an adequate degree and at an appropriate timing may be a potential therapeutic approach to interrupting epileptogenesis.     

Amygdala Kindling Modifies Extracellular Opioid Peptide Content in Rat Hippocampus Measured by Microdialysis

Abstract: Opioid peptide release in the hippocampus was shown to be increased immediately following amygdala kindling stimulation in freely moving rats using microdialysis combined with a universal opioid peptide radioimmunoassay (RIA). Extracellular opioid peptide levels were elevated (55% above basal levels) within the first 10 min after electrical stimulation-induced partial seizures in previously nonkindled animals. Fully kindled rats showed lower extracellular opioid peptide levels (40% reduction) during the interictal period [16 ± 2.1 days (mean ± SEM) after the last stage V seizure], in comparison with values obtained from the sham-kindled group under basal conditions. However, opioid peptide release in fully kindled rats increased above 152% of interictal levels within the first 20 min after onset of fully kindled seizures, attaining peak levels equal to that of the partial kindled group and returning to prestimulation conditions 40–60 min following the ictal events. The majority of the immunoreactive material recovered from the hippocampus within the first 20 min following partial and generalized kindled seizures coeluted with dynorphin-A (1–6), dynorphin-A (1–8), and Leu-enkephalin by HPLC/RIA analysis. It is proposed that the enhanced opioid peptide release in hippocampus induced by amygdala kindling stimulation might be associated with either enhanced excitability or seizure suppression as seizure susceptibility fluctuates. The reduced interictal opioid peptide levels may also underlie some interictal behavioral disturbances.     

Permissive role of adenosine A2A receptors on metabotropic glutamate receptor 5 (mGluR5)-mediated effects in the striatum

The metabotropic glutamate receptors 5 (mGlu5Rs) and the adenosine A2A receptors (A2ARs) have been reported to functionally interact in the striatum. The aim of the present work was to verify the hypothesis that the state of activation of A2A Rs could influence mGlu5R-mediated effects in the striatum. In electrophysiological experiments (extracellular recording in rat corticostriatal slices), the ability of the selective mGlu5R agonist CHPG to potentiate the reduction of the field potential amplitude induced by NMDA was prevented not only by the selective mGlu5R antagonist MPEP, but also by the selective A2AR antagonist ZM 241385. Analogously, the application of CHPG potentiated NMDA-induced toxicity (measured by LDH release) in cultured striatal neurons, an effect that was abolished by both MPEP and ZM 241385. Finally, the A2AR agonist CGS 21680 potentiated CHGP effects, an action that was reproduced and abolished, respectively, by forskolin (an activator of the cAMP/protein kinase A, PKA, pathway) and KT 5720 (a PKA inhibitor). The results indicate that A2ARs exert a permissive role on mGlu5R-induced effects in the striatum. Such an interaction may represent an additional target for the development of therapeutic strategies towards striatal disorders.     

Adenosine receptor-mediated modulation of dopamine release in the nucleus accumbens depends on glutamate neurotransmission and N-methyl-D-aspartate receptor stimulation

Adenosine, by acting on adenosine A(1) and A(2A) receptors, exerts opposite modulatory roles on striatal extracellular levels of glutamate and dopamine, with activation of A(1) inhibiting and activation of A(2A) receptors stimulating glutamate and dopamine release. Adenosine-mediated modulation of striatal dopaminergic neurotransmission could be secondary to changes in glutamate neurotransmission, in view of evidence for a preferential colocalization of A(1) and A(2A) receptors in glutamatergic nerve terminals. By using in vivo microdialysis techniques, local perfusion of NMDA (3, 10 microm), the selective A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 3, 10 microm), the selective A(1) receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT; 300, 1000 microm), or the non-selective A(1)-A(2A) receptor antagonist in vitro caffeine (300, 1000 microm) elicited significant increases in extracellular levels of dopamine in the shell of the nucleus accumbens (NAc). Significant glutamate release was also observed with local perfusion of CGS 21680, CPT and caffeine, but not NMDA. Co-perfusion with the competitive NMDA receptor antagonist dl-2-amino-5-phosphonovaleric acid (APV; 100 microm) counteracted dopamine release induced by NMDA, CGS 21680, CPT and caffeine. Co-perfusion with the selective A(2A) receptor antagonist MSX-3 (1 microm) counteracted dopamine and glutamate release induced by CGS 21680, CPT and caffeine and did not modify dopamine release induced by NMDA. These results indicate that modulation of dopamine release in the shell of the NAc by A(1) and A(2A) receptors is mostly secondary to their opposite modulatory role on glutamatergic neurotransmission and depends on stimulation of NMDA receptors. Furthermore, these results underscore the role of A(1) vs. A(2A) receptor antagonism in the central effects of caffeine.     

Histamine H1 receptor binding capacities in the amygdalas of the amygdaloid kindled rat

The histamine H1 receptor binding capacity of the amygdalas of amygdaloid kindled rats was studied. In the kindled nonstimulated amygdala, significant decreases in K(D) and B(max) values compared with those of control amygdala were found 1 week after the last kindled seizure. One month after the last kindled seizure, the decreased K(D) value was sustained in the kindled nonstimulated amygdala. This decreased Bmax value 1 week after the last kindled seizure in nonstimulated amygdala may partly and transiently contribute to kindled seizure susceptibility. The decreased K(D) value in nonstimulated amygdala observed until 1 month after the last kindled seizure indicates the long-lasting increment of binding affinity of the pyrilamine binding site of the histamine H1 receptor in the steady state of kindled seizure susceptibility.     

Caffeine-induced seizures: apparent proconvulsant action of N-ethyl carboxamidoadenosine (NECA)

The effect of adenosine agonist pretreatment on the seizure activity of caffeine was investigated in NIH Swiss mice. The seizure ED50 of caffeine alone was determined to be 223 mg/kg and this was reduced to 68 mg/kg following pretreatment with 0.30 mg/kg N-ethyl carboxamidoadenosine (NECA). Additionally, NECA dose-dependently increased the seizure potency of 100 mg/kg caffeine (a dose which is normally subconvulsant). A proconvulsant effect of NECA was also detected following intracerebroventricular administration of 2.5 ug NECA, however the same doses of N6-cyclohexyladenosine (CHA) and 2-chloroadenosine (2 C1-ADO) did not precipitate seizures. The data reveal proconvulsant actions of both peripherally and centrally administered NECA towards caffeine-induced seizures. Such actions need to be reconciled with the general anticonvulsant action of adenosine and adenosine agonists.     

Neuropeptide Y Y5 receptors inhibit kindling acquisition in rats

Neuropeptide Y inhibits neuronal excitability and seizures in various experimental models. This peptide delays kindling epileptogenesis but the receptors involved in this action are unknown. We have studied the role of Y5 receptors in kindling using the selective antagonist GW438014A (IC50=210 nM), a small heterocycle molecule that crosses the blood-brain barrier, and the selective peptide agonist Ala31Aib34 NPY (IC50=6.0 nM). Intraperitoneal injection of GW438014A (10 mg/kg), 30 min before the beginning of a rapid-kindling protocol, significantly accelerated the rate of kindling acquisition as compared to vehicle-injected rats. Thus, the number of electrical stimuli required to reach stages 3 and 4-5 of kindling were reduced by 50% and 25%, respectively. The average afterdischarge duration in the stimulated hippocampus was prolonged by 2-fold. Conversely, kindling rate was delayed by intracerebroventricular administration of 24 nmol Ala31Aib32 NPY. Thus, the number of stimuli necessary to reach stages 2 and 3 of kindling was increased by 3- and 4-fold, respectively. During the stimulation protocol (40 stimuli) none of the rats treated with the Y5 agonist showed stages 4-5 seizures. Twenty-four hours after the last kindling stimulation, thus during the re-test session, Y5 agonist- or antagonist-treated rats had stages 4-5 seizures as their controls. In rats treated with both the antagonist and the agonist, kindling rate was similar to vehicle-injected rats. These data indicate that Y5 receptors mediate inhibitory effects of NPY in kindling and display anticonvulsant rather then antiepileptogenic effects upon agonist stimulation.     

Modulation of leukotriene D4 attenuates the development of seizures in mice

The present study has been designed to pharmacologically investigate the effect of Montelukast sodium, a leukotriene D₄ receptor antagonist, and 1,2,3,4, tetrahydroisoquinoline, a leukotriene D₄ synthetic pathway inhibitor, on the pathophysiological progression of seizures using mouse models of kindled epilepsy and status epilepticus induced spontaneous recurrent seizures. Pentylenetetrazole (40 mg kg⁻¹) (PTZ) administration every second day for a period of 15 d was used to elicit chemically induced kindled seizure activity in mice. In a separate set of groups, fifty consecutive electroshocks were delivered to mice using corneal electrodes with continuously increasing intensity with an inter-shock interval of 40 s. Severity of kindled seizures was assessed in terms of a composite kindled seizure severity score (KSSS). Pilocarpine (100 mg kg⁻¹) was injected every twenty minutes until the onset of status epilepticus. A spontaneous recurrent seizure severity score (SRSSS) was recorded as a measure of quantitative assessment of the progressive development of spontaneous recurrent seizures induced after pilocarpine status epilepticus. Sub-acute PTZ administration and electroshock induced the development of severe form of kindled seizures in mice. Severity of kindled seizures was assessed in terms of a composite kindled seizure severity score. Further, pharmacological status epilepticus elicited a progressive evolution of spontaneous recurrent seizures in the animals. However, Montelukast sodium, a leukotriene D₄ receptor antagonist, as well as 1,2,3,4, tetrahydroisoquinoline, a leukotriene D₄ synthetic pathway inhibitor, markedly and dose dependently suppressed the development of kindled seizures as well as pilocarpine induced spontaneous recurrent seizures. Therefore, leukotriene D₄ may be implicated in the pathogenesis of seizures.     

Characterization of the locomotor depression produced by an A2-selective adenosine agonist

Adenosine analogs, such as N6-cyclohexyladenosine (CHA) that are selective for A1-adenosine receptors, and analogs, such as 5'-N-ethylcarboxamidoadenosine (NECA) that are active at both A1 and A2 receptors, cause a profound depression of locomotor activity in mice via a central mechanism. The depression is effectively reversed by non-selective adenosine antagonists such as theophylline. We report that 2-([2-aminoethylamino) carbonylethylphenylethylamino]-5'-N-ethylcarboxamidoadenosine (APEC), an amine derivative of the A2-selective agonist, CGS21680, is a potent locomotor depressant in mice. The in vivo pharmacology is consistent with A2-selectivity at a central site of action. Two parameters indicative of locomotor activity, horizontal activity and total distance travelled, were measured using a computerized activity monitor. From dose-response curves it was found that APEC (ED50 16 micrograms/kg) is more potent than CHA (ED50 60 micrograms/kg) and less potent than NECA (ED50 2 micrograms/kg). The locomotor depression by APEC was reversible by theophylline, but not by the A1-selective antagonists 8-cyclopentyltheophylline (CPT) and 8-cyclopentyl-1, 3-dipropyl-2-thioxanthine, nor by the peripheral antagonists 8-p-sulfophenyltheophylline (8-PST) and 1,3-dipropyl-8-p-sulfophenylxanthine. The locomotor activity depression elicited by NECA and CHA was reversed by A1-selective antagonists. These results suggest that the effects of APEC are due to stimulation of A2 adenosine receptors in the brain.     

Pharmacological inhibition of inducible nitric oxide synthase attenuates the development of seizures in mice

The present study has been designed to pharmacologically expound the significance of inducible nitric oxide synthase in the pathophysiological progression of seizures using mouse models of chemically induced kindled epilepsy and status epilepticus induced spontaneous recurrent seizures. Pentylenetetrazole (40 mg kg⁻¹) (PTZ) administration every second day for a period of 15 days was used to elicit kindled seizure activity in mice. Severity of kindled seizures was assessed in terms of a composite kindled seizure severity score (KSSS). Pilocarpine (100 mg kg⁻¹) was injected every 20 min until the onset of status epilepticus. A spontaneous recurrent seizure severity score (SRSSS) was recorded as a measure of quantitative assessment of the progressive development of spontaneous recurrent seizures induced after pilocarpine status epilepticus. Sub-acute PTZ administration induced the development of severe form of kindled seizures in mice. Further, pharmacological status epilepticus elicited a progressive evolution of spontaneous recurrent seizures in the animals. However, treatment of aminoguanidine, a relatively selective inhibitor of inducible nitric oxide synthase, markedly and dose dependently suppressed the development of both PTZ induced kindled seizures as well as pilocarpine induced spontaneous recurrent seizures. Therefore inducible nitric oxide synthase may be implicated in the development of seizures.     

In vivo modulation of extracellular hippocampal glutamate and GABA levels and limbic seizures by group I and II metabotropic glutamate receptor ligands

The effects of several metabotropic receptor (mGluR) ligands on baseline hippocampal glutamate and GABA overflow in conscious rats and the modulation of limbic seizure activity by these ligands were investigated. Intrahippocampal mGluR group I agonist perfusion via a microdialysis probe [1 mm (R,S)-3,5-dihydroxyphenylglycine] induced seizures and concomitant augmentations in amino acid dialysate levels. The mGlu1a receptor antagonist LY367385 (1 mm) decreased baseline glutamate but not GABA concentrations, suggesting that mGlu1a receptors, which regulate hippocampal glutamate levels, are tonically activated by endogenous glutamate. This decrease in glutamate may contribute to the reported LY367385-mediated anticonvulsant effect. The mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (50 mg/kg) also clearly abolished pilocarpine-induced seizures. Agonist-mediated actions at mGlu2/3 receptors by LY379268 (100 microm, 10 mg/kg intraperitoneally) decreased basal hippocampal GABA but not glutamate levels. This may partly explain the increased excitation following systemic LY379268 administration and the lack of complete anticonvulsant protection within our epilepsy model with the mGlu2/3 receptor agonist. Group II selective mGluR receptor blockade with LY341495 (1-10 microm) did not alter the rats' behaviour or hippocampal amino acid levels. These data provide a neurochemical basis for the full anticonvulsant effects of mGlu1a and mGlu5 antagonists and the partial effects observed with mGlu2/3 agonists in vivo.     

The role of peripheral adenosine receptors in glutamate-induced pain nociceptive behavior

The role of peripheral adenosine receptors in pain is a controversial issue and seems to be quite different from the roles of spinal and central adenosine receptors. The present study is aimed at clarifying the role of these receptors in peripheral nociception. To clarify this, studies were done on Swiss mice with adenosine receptor agonists and antagonists. Nociceptive behavior was induced by subcutaneous injection of glutamate (10 μmol) into the ventral surface of the hind paw of mice. Statistical analyses were performed by one-way ANOVA followed by the Student-Newman-Keuls post hoc test. Results showed that intraplantar (i.pl.) administration of N6-cyclohexyl-adenosine (CHA), an adenosine A1 receptor agonist, at 1 or 10 μg/paw significantly reduced glutamate-induced nociception (p<0.01 and p<0.001 vs. vehicle, respectively, n=8−10). In contrast, i.pl. injection of hydrochloride hydrate ({"type":"entrez-protein","attrs":{"text":"CGS21680","term_id":"878113053"}}CGS21680, an adenosine A2A receptor agonist) (1 μg/paw) induced a significant increase in glutamate-induced nociception compared to the vehicle (p<0.05, n=8), while 4-(-2-[7-amino-2-{2-furyl}{1,2,4}triazolo{2,3-a} {1,3,5}triazin-5-yl-amino]ethyl)phenol (ZM241385, an adenosine A2A receptor antagonist) (20 μg/paw) caused a significant reduction (p<0.05, n=7−8). There were no significant effects on i.pl. administration of four additional adenosine receptor drugs—8-cyclopentyl-1,3-dipropylxanthine (DPCPX, an A1 antagonist, 1–10 μg/paw), N(6)-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA, an A2B agonist, 1–100 μg/paw), alloxazine (an A2B antagonist, 0.1–3 μg/paw), and 2-hexyn-1-yl-N(6)-methyladenosine (HEMADO) (an A3 agonist, 1–100 μg/paw) (p>0.05 vs. vehicle for all tests). We also found that prior administration of DPCPX (3 μg/paw) significantly blocked the anti-nociceptive effect of CHA (1 μg/paw) (p<0.05, n=7–9). Similarly, ZM241385 (20 μg/paw) administered prior to {"type":"entrez-protein","attrs":{"text":"CGS21680","term_id":"878113053"}}CGS21680 (1 μg/paw) significantly blocked {"type":"entrez-protein","attrs":{"text":"CGS21680","term_id":"878113053"}}CGS21680-induced exacerbation of nociception (p<0.05, n=8). Finally, inosine (10 and 100 μg/paw), a novel endogenous adenosine A1 receptor agonist recently reported by our research group, was also able to reduce glutamate-induced nociception (p<0.001 vs. vehicle, n=7–8). Interestingly, as an A1 adenosine receptor agonist, the inosine effect was significantly blocked by the A1 antagonist DPCPX (3 μg/paw) (p<0.05, n=7−9) but not by the A2A antagonist ZM241385 (10 μg/paw, p>0.05). In summary, these results demonstrate for the first time that i.pl administration of inosine induces an anti-nociceptive effect, similar to that elicited by CHA and possibly mediated by peripheral adenosine A1 receptor activation. Moreover, our results suggest that peripheral adenosine A2A receptor activation presents a pro-nociceptive effect, exacerbating glutamate-induced nociception independent of inosine-induced anti-nociceptive effects.     

Lack of effect of focally administered prostaglandins on electrically kindled seizure activity.

Several previous studies have demonstrated increased synthesis of cerebral prostaglandins (PGs) following convulsive activity. In addition, it has been proposed that endogenous prostanoids have anticonvulsive properties and may act to attenuate or limit seizure activity in vivo. In this study we have used focal injections of prostaglandins (PGs) to examine their potential modulatory effects on electrically kindled seizure activity. We report that the intra-amygdaloid administration of PGD2, PGE2 or PGF2a, (1-10 micrograms) showed no significant effects on any of the kindled seizure parameters studied. The highest dose of PGF2a was ineffective at all pretreatment times between 2-30 mins. Our data is inconsistent with the view that PGs exert protective effects against seizure activity, at least within the amygdala against electrically kindled seizures.