Medium- and long-term alterations of brain A1 and A2A adenosine receptor characteristics following repeated seizures in developing rats

Changing effect of GABA B receptor antagonist CGP46381 after status epilepticus in immature rats

Possible proconvulsant action of GABA_B receptor antagonist CGP46381 was studied 3 and 13 days after status epilepticus elicited in 12-day-old rats. GABAA-dependent activity was tested by pentylenetetrazol administration and found different in 15-day-old rats after status epiolepticus but not in the older group. The interaction of the two GABAergic systems should be studied in detail.

Developmental increase in ecto-5'-nucleotidase activity overlaps with appearance of two immunologically distinct enzyme isoforms in rat hippocampal synaptic plasma membranes

Ecto-5'-nucleotidase (e-5NT), a glycosylphosphatidylinositol-linked membrane protein, catalyzes a conversion of AMP to adenosine, which influences nearly every aspect of brain physiology, including embryonic and postnatal brain development. The present study aimed to investigate a pattern of expression, activity and kinetic properties of e-5NT in the hippocampal formation and synaptic plasma membrane (SPM) preparations in rats at postnatal days (PDs) 7, 15, 20, 30 and 90. By combining gene expression analysis and enzyme histochemistry, we observed that e-5NT mRNA reached the adult level at PD20, while the enzyme activity continued to increase beyond this age. Further analysis revealed that hippocampal layers rich in synapses expressed the highest levels of e-5NT activity, while in layers populated with neuronal cell bodies, the enzyme activity was weak or absent. Therefore, activity and expression of e-5NT were analyzed in SPM preparations isolated from rats at different ages. The presence of two protein bands of about 65 and 68 kDa was determined by immunoblot analysis. The 65-kDa band was present at all ages, and its abundance increased from PD7 to PD20. The 68-kDa band appeared at PD15 and increased until PD30, coinciding with the increase of e-5NT activity, substrate affinity and enzymatic efficiency. Since distinct e-5NT isoforms may derive from different patterns of the enzyme protein N-glycosylation, we speculate that long-term regulation of e-5NT activity in adulthood may be effectuated at posttranslational level and without overall change in the gene and protein expression.

Modulatory role of adenosine and its receptors in epilepsy: possible therapeutic approaches

Adenosine is considered to be the brain's endogenous anticonvulsant as many studies have showed and it is responsible for seizure arrest and postictal refractoriness. Alterations in the adenosinergic system (adenosine and its receptors) have been referred by many previous studies indicating that deficiencies or modifications in the function of this purinergic system may contribute to epileptogenesis. Due to this emerging implication of adenosine in the managing of seizures, a new field of adenosine-based therapies has been introduced including adenosine itself, adenosine receptor agonists and antagonists and adenosine kinase inhibitors. The method with the least side effects (heart rate, blood pressure, temperature or even sedation) is being quested including intracerebral implantation of adenosine releasing cells or devices.

Different roles of adenosine A1, A2A and A3 receptors in controlling kainate-induced toxicity in cortical cultured neurons

Adenosine is a neuromodulator that can control brain damage through activation of A(1), A(2A) and A(3) receptors, which are located in both neurons and other brain cells. We took advantage of cultured neurons to investigate the role of neuronal adenosine receptors in the control of neurotoxicity caused by kainate and cyclothiazide. Both A(1), A(2A) and A(3) receptors were immunocytochemically identified in cortical neurons. Activation of A(1) receptors with 100 nM CPA did not modify the extent of neuronal death whereas the A(1) receptor antagonist, DPCPX (50 nM), attenuated neurotoxicity by 28 +/- 5%, and effect similar to that resulting from the removal of endogenous adenosine with 2U/ml of adenosine deaminase (27 +/- 3% attenuation of neurotoxicity). In the presence of adenosine deaminase, DPCPX had no further effect and CPA now exacerbated neurotoxicity by 42 +/- 4%. Activation of A(2A) receptor with 30 nM CGS21680 attenuated neurotoxicity by 40 +/- 8%, an effect prevented by the A(2A) receptor antagonists, SCH58261 (50 nM) or ZM241385 (50 nM), which by themselves were devoid of effect. Finally, neither A(3) receptor activation with Cl-IB-MECA (100-500 nM) nor blockade with MRS1191 (5 microM) modified neurotoxicity. These results show that A(1) receptor activation enhances and A(2A) receptor activation attenuates neurotoxicity in cultured cortical neurons, indicating that these two neuronal adenosine receptors directly control neurodegeneration. Interestingly, the control by adenosine of neurotoxicity in cultured neurons is similar to that observed in vivo in newborn animals and is the opposite of what is observed in adult brain preparations where A(1) receptor activation and A(2A) receptor blockade are neuroprotective.

Suppression of generalized seizures activity by intrathalamic 2-chloroadenosine application

In the present study, we investigated the effects of micro-injecting 2-chloroadenosine (2-CADO; an adenosine receptor agonist) into the thalamus alone and with theophylline (a nonspecific adenosine receptor antagonist) pretreatment on pentylenetetrazol (PTZ)-induced tonic-clonic seizures in male Wistar albino rats. Following intrathalamic 2-CADO injection alone or theophylline pretreatment, 50 mg kg(-1) PTZ was given ip after 1 and 24 hrs. The duration of epileptic seizure activity was recorded by cortical electroencephalogram (EEG), and seizure severity was behaviorally scored. Intrathalamic 2-CADO administration induced significant decreases in both seizure duration and seizure severity scores at 1 and 24 hrs, but the effects were more abundant on the seizures induced after 24 hrs. On the other hand, pretreatment with theophylline prevented the inhibitor effect of 2-CADO on seizure activity and increased both seizure duration and seizure scores. Present results suggest that the activation of adenosine receptors in the thalamus may represent another anticonvulsant/modulatory site of adenosine action during the course of the PTZ-induced generalized tonic-clonic seizures and provide additional data for the involvement of the adenosinergic system in the generalized seizures model.

Neuroprotection by adenosine in the brain: From A(1) receptor activation to A (2A) receptor blockade

Adenosine is a neuromodulator that operates via the most abundant inhibitory adenosine A(1) receptors (A(1)Rs) and the less abundant, but widespread, facilitatory A(2A)Rs. It is commonly assumed that A(1)Rs play a key role in neuroprotection since they decrease glutamate release and hyperpolarize neurons. In fact, A(1)R activation at the onset of neuronal injury attenuates brain damage, whereas its blockade exacerbates damage in adult animals. However, there is a down-regulation of central A(1)Rs in chronic noxious situations. In contrast, A(2A)Rs are up-regulated in noxious brain conditions and their blockade confers robust brain neuroprotection in adult animals. The brain neuroprotective effect of A(2A)R antagonists is maintained in chronic noxious brain conditions without observable peripheral effects, thus justifying the interest of A(2A)R antagonists as novel protective agents in neurodegenerative diseases such as Parkinson's and Alzheimer's disease, ischemic brain damage and epilepsy. The greater interest of A(2A)R blockade compared to A(1)R activation does not mean that A(1)R activation is irrelevant for a neuroprotective strategy. In fact, it is proposed that coupling A(2A)R antagonists with strategies aimed at bursting the levels of extracellular adenosine (by inhibiting adenosine kinase) to activate A(1)Rs might constitute the more robust brain neuroprotective strategy based on the adenosine neuromodulatory system. This strategy should be useful in adult animals and especially in the elderly (where brain pathologies are prevalent) but is not valid for fetus or newborns where the impact of adenosine receptors on brain damage is different.

Changes in purine levels and adenosine receptors in kindled seizures in the rat

Adenosine is an inhibitory modulator of neuronal activity and its possible involvement in seizures is of interest. We have examined changes in adenosine, its metabolites and receptors in brains of hippocampus-kindled rats, a model of partial epilepsy. Purine levels were measured by in vivo microdialysis and showed a small increase in adenosine and a dramatic increase in its metabolites after kindled seizures. Adenosine A1 receptor binding using [H]DPCPX was unaltered after seizures, whereas A1 agonist stimulated binding of GTP[gamma-S] and A1 mRNA expression increased in the CA3 and other regions. Striatal adenosine A2A mRNA and receptor binding with [H]SCH-58261 decreased. These findings indicate that kindled seizures increase adenosine release and metabolism and induces adaptive changes in adenosine receptors.

Recurrent Bicuculline-Induced Seizures in Rat Pups Cause Long-Term Motor Deficits and Increase Vulnerability to a Subsequent Insult

Recurrent neonatal seizures are associated with a high risk of neurological sequelae. The major concern is whether recurrent neonatal seizures induce adverse effects on long-term cognition and/or motor performance. Rats were treated with intraperitoneal (ip) bicuculline for 3 consecutive days, starting from Postnatal Day 5 (P5). Kainic acid (KA, 4 mg/kg ip) was injected at P53 to investigate the susceptibility to a second insult, and then cognitive function was tested using the Morris water maze, and motor performance using the Rotarod test, in adulthood (P60). Finally, histological assessments of brains were performed. The rats treated with bicuculline had no deficits in cognition function and pathology findings, but had worse motor performance and increased susceptibility to later KA challenge. Our findings indicate that recurrent bicuculline-induced seizures in the developing brain result in long-term motor deficits and increase the risk of subsequent cognitive damage in response to a second insult.

Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Adenosine exerts two parallel modulatory roles in the CNS, acting as a homeostatic modulator and also as a neuromodulator at the synaptic level. We will present evidence to suggest that these two different modulatory roles are fulfilled by extracellular adenosine originated from different metabolic sources, and involve receptors with different sub-cellular localisation. It is widely accepted that adenosine is an inhibitory modulator in the CNS, a notion that stems from the preponderant role of inhibitory adenosine A(1) receptors in defining the homeostatic modulatory role of adenosine. However, we will review recent data that suggests that the synaptically localised neuromodulatory role of adenosine depend on a balanced activation of inhibitory A(1) receptors and mostly facilitatory A(2A) receptors. This balanced activation of A(1) and A(2A) adenosine receptors depends not only on the transient levels of extracellular adenosine, but also on the direct interaction between A(1) and A(2A) receptors, which control each other's action.

The postictal state: a neglected entity in the management of epilepsy

Some of the disability deriving from epilepsy derives from the postictal state (PS). The PS may be complicated by impaired cognition, headache, injuries, or secondary medical conditions. Postictal depression is common, postictal psychosis relatively rare, but both add to the morbidity of seizures. The mechanisms of the PS are poorly understood. Alteration of cerebral blood flow both results from and contributes to the PS. Many neurotransmitters or neuromodulators are involved in the physiology of the PS. Response to glutamate may partially desensitize after a seizure. Endogenous opiates and adenosine serve as natural antiepileptic medications in some circumstances. Nitric oxide has numerous effects on brain excitability, and may be particularly important in regulating postictal cerebral blood flow. Just as the pathophysiology of seizures is complicated, so is that of the PS multifactorial. As a practical issue, it would be very useful to have medications that reduce the morbidity of the PS.