Changes in seizure latency correlate with alterations in A1 adenosine receptor binding during daily repeated pentylentetrazol-induced convulsions in different mouse brain areas

Pentylenetetrazole-Induced Kindling Mouse Model

Pentylenetetrazole (PTZ) is a GABA-A receptor antagonist. An intraperitoneal injection of PTZ into an animal induces an acute, severe seizure at a high dose, whereas sequential injections of a subconvulsive dose have been used for the development of chemical kindling, an epilepsy model. A single low-dose injection of PTZ induces a mild seizure without convulsion. However, repetitive low-dose injections of PTZ decrease the threshold to evoke a convulsive seizure. Finally, continuous low-dose administration of PTZ induces a severe tonic-clonic seizure. This method is simple and widely applicable to investigate the pathophysiology of epilepsy, which is defined as a chronic disease that involves repetitive seizures. This chemical kindling protocol causes repetitive seizures in animals. With this method, vulnerability to PTZ-mediated seizures or the degree of aggravation of epileptic seizures was estimated. These advantages have led to the use of this method for screening anti-epileptic drugs and epilepsy-related genes. In addition, this method has been used to investigate neuronal damage after epileptic seizures because the histological changes observed in the brains of epileptic patients also appear in the brains of chemical-kindled animals. Thus, this protocol is useful for conveniently producing animal models of epilepsy.

From unwitnessed fatality to witnessed rescue: Pharmacologic intervention in sudden unexpected death in epilepsy

The mechanisms of sudden unexpected death in epilepsy (SUDEP) have been difficult to define, as most cases occur unwitnessed, and physiologic recordings have been obtained in only a handful of cases. However, recent data obtained from human cases and experimental studies in animal models have brought us closer to identifying potential mechanisms. Theories of SUDEP should be able to explain how a seizure starting in the forebrain can sometimes lead to changes in brainstem cardiorespiratory control mechanisms. Herein we focus on three major themes of work on the causes of SUDEP. First, evidence is reviewed identifying postictal hypoventilation as a major contributor to the cause of death. Second, data are discussed that brainstem serotonin and adenosine pathways may be involved, as well as how they may contribute. Finally, parallels are drawn between SIDS and SUDEP, and we highlight similarities pointing to the possibility of shared pathophysiology involving combined failure of respiratory and cardiovascular control mechanisms. Knowledge about the causes of SUDEP may lead to potential pharmacologic approaches for prevention. We end by describing how translation of this work may result in future applications to clinical care.

Interleukin-6 upregulates neuronal adenosine A1 receptors: implications for neuromodulation and neuroprotection

The immunological response in the brain is crucial to overcome neuropathological events. Some inflammatory mediators, such as the immunoregulatory cytokine interleukin-6 (IL-6) affect neuromodulation and may also play protective roles against various noxious conditions. However, the fundamental mechanisms underlying the long-term effects of IL-6 in the brain remain unclear. We now report that IL-6 increases the expression and function of the neuronal adenosine A1 receptor, with relevant consequences to synaptic transmission and neuroprotection. IL-6-induced amplification of A1 receptor function enhances the responses to readily released adenosine during hypoxia, enables neuronal rescue from glutamate-induced death, and protects animals from chemically induced convulsing seizures. Taken together, these results suggest that IL-6 minimizes the consequences of excitotoxic episodes on brain function through the enhancement of endogenous adenosinergic signaling.

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.

Up-regulation of adenosine A1 receptor binding in pentylenetetrazol kindling in mice: effects of angiotensin IV

The effects of the hexapeptide angiotensin II (3-8) ANG IV, the selective A(1) receptor agonist cyclohexyladenosine (CHA) and the combination of ANG IV + CHA on pentylenetetrazol (PTZ)-generalized seizures; kindling development and maintenance were studied. By using in vitro quantitative receptor autoradiography, the regulation of adenosine A(1) receptor density at different time points during the kindling procedure and postkindling period was determined. ANG IV and CHA effectively reduced clonic seizures in PTZ-generalized seizure model, in PTZ-kindled mice as well as during kindling development and a week later by rechallenge with PTZ. Furthermore, coadministration of ANG IV and CHA had a strong anticonvulsant effect, both compounds acting synergistically. A significant increase of adenosine A(1) receptor density was detected in somatosensory cortex, hippocampus, amygdala and geniculate nuclei early in the kindling procedure (after the 3rd injection), which persisted at least 1 month after the end of kindling procedure. In addition, a delayed up-regulation of adenosine A(1) receptor binding was observed a week after kindling in the mamillary bodies and a month later in the motor cortex. The pretreatment with ANG IV caused a down-regulation of adenosine A(1) receptor density to the control level in most time points and brain areas. In conclusion, PTZ kindling-induced increase of adenosine A(1) receptor binding at different time points and in specific brain structures might represent an adaptive mechanism for coping with the hyperexcitability typical for this phenomenon. The antiepileptogenic effect of ANG IV could be realized partly through an adenosine-dependent mechanism.

Effects of chronic morphine and N(6)-cyclopentyl-adenosine administration on kainic acid-induced status epilepticus

The aim of the present study was to investigate if the upregulation of mu or A(1) receptors modifies the expression of the kainic acid (KA)-induced status epilepticus (SE). Male Wistar rats received one of the following treatments: saline solution (SS) (1 ml/kg, i.p. for 7 days); morphine (M) (20 mg/kg, i.p. for 7 days) or N(6)-cyclopentyl-adenosine (CPA) (1 mg/kg, i.p. for 9 days). Twenty-four hours after the last administration rats were sacrificed. Membranes were obtained mu and and A(1) receptor binding experiments were carried out. Furthermore, an injection of SS (1 ml/kg, i.p.) or KA (10 mg/kg, i.p.) was applied in rats pretreated chronically with M, CPA or SS, 48 h after the last administration. Seizure activity, death rate and a postictal explosive motor behavior were evaluated after KA administration. Chronic M administration increased mu receptor number in hippocampus (115%) and cortex (265%), whereas chronic CPA treatment enhanced A(1) receptor number in hippocampus (55%), amygdala (39%) and cortex (51%). The pretreatment with M facilitated the KA-induced SE and reduced the death rate as well as the postictal explosive motor behavior. The pretreatment with CPA delayed the SE presentation, increased the death rate and decreased the postictal explosive motor behavior. These findings suggest that upregulation of mu receptors enhances the KA seizures, whereas upregulation of A(1) receptors depresses these seizures.

Seizure activity produces differential changes in adenosine A1 receptors within rat hippocampus

Specific ligand binding to rat hippocampal adenosine A1 receptor after administration of the convulsant drug 3-mercaptopropionic acid (MP) was studied by means of a quantitative autoradiographic method. 2-Chloro-N6-[cyclopentyl-2,3,4,5-3H adenosine] ([3H]CCPA), a potent and selective A1 receptor ligand, was selected for binding studies. MP administration (150 mg/kg, i.p.), at seizure, caused significant increases in the following CA1 layers: pyramidal (45%), radiatum (18%) and lacunosum molecular (35%); in CA2 area, a significant decrease in stratum oriens (36%) and an increase in stratum radiatum (14%) and lacunosum molecular (33%) layers was observed. In CA3 area a rise in pyramidal (40%) and radiatum layers (26%), as well as in hillus (97%) was found. At postseizure, changes were restricted to CA1, CA2 and CA3 pyramidal layers and to CA1 lacunosum molecular layer, with increases ranging from 22 to 50%. These results show that [3H]CCPA binding is modified diversely in intrahippocampal layers and areas, thus indicating their dissimilar role in seizure activity.

Reduction of A1 adenosine receptors in rat hippocampus after kainic acid-induced limbic seizures

In a temporal lobe epilepsy (TLE) model induced by kainic acid (KA), we examined the effect of limbic seizures on A1 adenosine receptor distribution in hippocampus and cortex. By using quantitative autoradiography, we determined a progressive decrease in A1 receptor density in CA1 and CA3 regions of hippocampus, which coincided in time with the degenerating process of hippocampal pyramidal cells. This result indicates that a great amount of A1 receptors are located postsynaptically on pyramidal cell dendrites. No difference in A1 receptor density was observed in the inner compared to the outer molecular layer of dentate gyrus, or in the infrapyramidal band compared to the outer layer of stratum oriens of CA3. This could indicate that the newly sprouted mossy fiber glutamatergic terminals do not contain A1 receptors, thus lacking a restrain in the release of glutamate.

Upregulation of NMDA receptors in hippocampus and cortex in the pentylenetetrazol-induced "kindling" model of epilepsy

"Kindling" is a phenomenon of epileptogenesis, which has been widely used as an experimental model of temporal lobe epilepsy. At the present work we investigated the contribution of NMDA receptors in the Pentylenetetrazol-induced "kindling" model in the mouse brain, by using quantitative autoradiography and the radioactive ligands [3H]MK801 and [3H]L-glutamate (NMDA-sensitive component). One week after establishment of kindling, a small but significant increase in [3H]MK801 as well as NMDA-sensitive [3H]glutamate binding was seen, being restricted to the molecular layer (ML) of the dentate gyrus (DG) and the CA3 region of the hippocampus. These binding augmentations persisted one month after establishment of kindling. A significant increase of NMDA receptor binding was also observed in the cortex-somatosensory and temporal one week after acquisition of the kindled state. The upregulation of NMDA receptors seen in DG and CA3 region of the hippocampus could be associated with the kindling process of this model especially with its maintenance phase, since it persists at long term, is area-specific and consistent with electrophysiological data. The increase of NMDA receptors seen in the cortex of the kindled animals could underlie the hyperexcitability detected by electrophysiological studies in this area.

Medium- and long-term effects of repeated bicuculline-induced seizures in developing rats on local cerebral energy metabolism

To assess long-term metabolic consequences of recurrent ictal events arising during development, seizures were repeatedly generated in rats at different stages of cerebral maturation. Seizures were induced by i.p. injections of bicuculline for three consecutive days, starting from postnatal day 5 (P5), when the brain is very immature, or from P15, a period at which the brain is more structurally organized. Local cerebral metabolic rates for glucose were measured in 74 structures at P15, P25 and in adults (P60), by the autoradiographic method using 2-D-[14C]deoxyglucose. Repeated seizures in P5 to P7 pups led to a reduction (16-34%) of glucose consumption at P15, mainly significant in sensory, motor and functionally non-specific areas as well as in cerebellar nuclei. Selective decreases in metabolic activity were still recorded in adults, mostly in auditory system (20%) and cerebellar nuclei (27%). Seizures generated from P15 to P17 led to an overall mortality rate of 62% (versus 22% at P5 to P7). Surviving animals exhibited reduced metabolic rates for glucose (by 7-27%) at P25, significant in 23 structures, and depicting pronounced changes in limbic, hypothalamic, sensory and white matter areas, whereas brain functional activity finally returned to basal values at P60. Therefore, while younger rats seemed to better tolerate repeated bicuculline-induced seizures than older animals, the reverse was true for long-term metabolic effects, and the more immature the brain when seizures arise, the more persistent the functional consequences.

Lower density of A1 adenosine receptors in nucleus reticularis thalami in rats with genetic absence epilepsy

The possible involvement of the adenosinergic modulatory system in the pathogenesis of absence seizures was investigated in genetic absence epilepsy rats from Strasbourg (GAERS). Using in vitro quantitative autoradiography, the distribution of A1 adenosine receptors and adenosine uptake sites in the brain of GAERS was studied and compared to that of control animals. An area-specific lower density of A1 receptors (15% decrease) was detected in reticular (nRT) and anterior ventral (AV) thalamic nuclei as well as basal ganglia in the brains of GAERS animals compared with control animals. Since adenosine exerts an anti-oscillatory effect on the thalamic nuclei by suppressing (via A1 receptors) excitatory as well as inhibitory neurotransmitter release, the impairment in A1 receptor density seen here, especially in nRT, could be implicated in the thalamic rhythmicity underlying spike and wave discharges present in this absence epilepsy model.

Single and repeated episodes of ethanol withdrawal increase adenosine A1, but not A2A, receptor density in mouse brain

A history of multiple ethanol withdrawal experiences has been shown to exacerbate the severity of future withdrawal episodes, and this sensitization of the withdrawal response has been hypothesized to represent a 'kindling' phenomenon. Since adenosine functions as an inhibitory modulator of seizure activity and may interact with ethanol to influence neuronal excitability, the present study was conducted to examine the effects of single and repeated episodes of ethanol withdrawal on adenosine A1 and A2A receptors in adult C3H/He mice. Mice were chronically exposed to ethanol vapor in inhalation chambers and tested for withdrawal seizures following multiple withdrawal (MW) experience (four cycles of 16 h ethanol intoxication interrupted by 8 h periods of abstinence), single withdrawal experience following 16 h (SW) or 64 h (CE) continuous ethanol intoxication, or no ethanol exposure (controls). Separate groups of mice from each withdrawal condition were used to generate pooled cortical and striatal tissue for ligand saturation experiments using [3H]cyclohexyladenosine to label A1 receptors and [3H]CGS 21680 to label A2A receptors. Results indicated that withdrawal seizures were significantly more severe in mice with multiple withdrawal experience in comparison to animals that experienced only a single withdrawal episode, even when total amount of ethanol exposure was equated among groups. The density of A1 receptors in cerebral cortex was significantly increased over controls 8 h following final ethanol withdrawal by approximately 35% in SW and CE groups, with the largest increase observed in the MW group (56%). Withdrawal treatment groups did not differ in cortical A1 binding sites immediately upon withdrawal from ethanol, and no significant differences in binding of [3H]CGS 21680 to striatal A2A receptors were observed following ethanol withdrawal. Ethanol exposure and withdrawal did not significantly alter ligand affinity for either adenosine receptor. These results indicate that adenosine A1 receptors are selectively upregulated during ethanol withdrawal and that the degree of upregulation may be enhanced following multiple withdrawal episodes. Further, these observations suggest that the upregulation of brain A1 receptors during ethanol withdrawal may represent a compensatory inhibitory response to increased seizure severity associated with repeated episodes of ethanol withdrawal.

The effects of GR79236 on plasma fatty acid concentrations, heart rate and blood pressure in the conscious rat

GR79236 (N-[(1S,trans)-2-hydroxycyclopentyl]adenosine) is an orally active adenosine A1 receptor agonist, which decreases plasma non-esterified fatty acid levels in fasted rats. This study has quantified the effects of GR79236 on plasma non-esterified fatty acid levels, blood pressure and heart rate in conscious rats. Blood pressure and heart rate were continuously recorded in rats for 30 min before and 1 h after oral dosing with GR79236 (0.03-3 mg/kg) or vehicle. Plasma non-esterified fatty acid concentrations were determined in blood sampled before and 1 h after dosing. GR79236 produced dose-related reductions in plasma non-esterified fatty acid concentrations, with an almost maximal effect (63.2%) occurring after 1 mg/kg. This dose induced a small but significant decrease in heart rate (12.0%), and a non-significant decrease in mean blood pressure (6.3%). Thus, in the conscious rat, GR79236 can exert profound antilipolytic effects with minimal effects on blood pressure and heart rate.

Pentylenetetrazol-induced seizures decrease gamma-aminobutyric acid-mediated recurrent inhibition and enhance adenosine-mediated depression

To elucidate the consequences of convulsions, we examined biochemically and electrophysiologically the brains of mice that had sustained two complete tonic-clonic convulsions after administration of pentylenetetrazol (PTZ 50 mg/kg intraperitoneally, i.p.), 48 and 24 h before decapitation. Control mice were injected with saline. Input/output curves of the extracellular synaptic responses in the CA1 area of hippocampal slices showed that PTZ-induced seizures do not establish the persistent change in hippocampal excitability itself that can be detected in vitro. However, use of the paired-pulse stimulation paradigm showed that gamma-aminobutyric acid A (GABAA)-mediated recurrent inhibition was significantly weaker (by 19-25%) in the CA1 area of slices from PTZ-treated mice (PTZ slices) as compared with slices from control mice (control slices). The density of GABAA receptors (high-affinity component) was also lower in hippocampus (by 19%) and cortex (by 14%) of PTZ-treated mice. A GABA-related disinhibitory mechanism underlying PTZ seizures may thus persist for 1 day after the seizure, predisposing the brain to subsequent seizures. On the other hand, the depressant effect of a single dose of adenosine 10 microM on the CA1 synaptic response was stronger (by 35% on population spikes) and longer lasting in PTZ slices as compared with controls. This could be attributed to significantly higher adenosine A1 receptor density in hippocampus (Bmax of [3H]CHA was higher by 34%) as well as cortex and cerebellum of these animals. The phenomenon may reflect an adenosine A1-mediated adaptive mechanism that offers protection from subsequent seizures.

Involvement of adenosine-sensitive cyclic AMP-generating systems in cobalt-induced epileptic activity in the rat

An injection of cobalt chloride solution into the unilateral sensorimotor cortex of rats induced electrographic epileptic activity, which was followed by a peripheral motor disturbance. Brain slices were prepared from the cortical region including the injection site and from the other cortical regions of rats between 8 and 50 days after the injection. In the cortical slices, we examined cyclic AMP accumulations elicited by adenosine and its stable analogue 2-chloroadenosine. Adenosine and 2-chloroadenosine at their maximal dose increased cyclic AMP accumulation six- to 10-fold and 10-15-fold, respectively, and the elicitation was markedly inhibited by the adenosine antagonist 8-phenyltheophylline. The cyclic AMP accumulation was increased in the primary epileptic region of the cortex adjacent to the injection site of cobalt chloride solution, whereas it was unchanged in the other cortical regions. The increase in cyclic AMP accumulation was observed regardless of the presence or absence of the adenosine uptake inhibitor dipyridamole, the phosphodiesterase inhibitor DL-4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone, and adenosine deaminase. Such an increased accumulation of cyclic AMP in the primary epileptic cortex was detected as early as 8 days after the injection. The cyclic AMP accumulation continued to increase and reached a peak level 17-19 days after the injection, and it returned to the control levels after 40-50 days, in correspondence with the electrographic and behavioral findings. It is concluded that alterations in adenosine receptor-mediated generation of cyclic AMP in the primary epileptic cortex are closely associated with the central process of cobalt-induced epilepsy.(ABSTRACT TRUNCATED AT 250 WORDS)

Upregulation of A1 adenosine receptors in human temporal lobe epilepsy: a quantitative autoradiographic study

A significant increase of A1 adenosine receptor binding (48% increase of mean) was detected in human neocortex obtained from patients suffering from temporal lobe epilepsy as compared to control neocortex from non-epileptic patients. Such increase was equally distributed in the six cortical layers and reached similar levels in each of the five specimens tested independently of age, sex and pharmacological treatment of the patient. Since adenosine exerts a depressant effect on neocortical neurons in slices obtained from epileptic patients, this upregulation of A1 receptor binding may constitute a protective mechanism against subsequent seizures, which is exerted by elevating the depressant response of the brain to endogenous adenosine.

Effect of pentylentetrazol-induced seizures on A1 adenosine receptor regional density in the mouse brain: a quantitative autoradiographic study

Adenosine has been shown to be a major regulator of neuronal activity in convulsive disorders, exerting its anticonvulsant effect through central A1 adenosine receptors. The aim of the present study was to investigate the effect of generalized tonic-clonic seizures induced by pentylentetrazol on regional changes in A1 adenosine receptor density and distribution in the mouse brain by in vitro quantitative autoradiography. As radioligand the specific agonist of A1 receptors [3H]cyclohexyladenosine was used. After two consecutive (once daily) pentylentetrazol-induced convulsions a widespread upregulation of A1 receptor density was detected with a marked enhancement in structures that mediate seizure activity like hippocampus, mamillary bodies, septum, substantia nigra, thalamic nuclei and cerebral cortices. On the contrary, in basal ganglia a significant downregulation of A1 receptors was observed. These results indicate that: (i) the observed increases or decreases in A1 receptor density are organized in selective anatomical structures related to seizure development rather than uniform in the brain; and (ii) since the upregulation of A1 receptors is sufficient to enhance the physiological depressive response of adenosine, the overall evoked increases seen here may lead to a stronger inhibitory tone and accordingly to a more efficient anticonvulsant effect of endogenous adenosine.

Characterization of adenosine receptor-mediated generation of cyclic AMP in slices of rat cerebral cortex with chronic epileptic activity

Cyclic AMP accumulations elicited by adenosine analogues 2-chloroadenosine (2-CADO), R-N6-phenylisopropyladenosine (R-PIA), and N6-cyclohexyladenosine (CHA) were investigated in cortical slices of chronic iron-induced epileptic rats. Cyclic AMP accumulation was elicited 9- to 18-fold by 2-CADO and it was elicited 5- to 7-fold by either R-PIA or CHA; 2-CADO was more potent than R-PIA or CHA in eliciting cyclic AMP accumulation. The adenosine analogues elicited cyclic AMP accumulation in a dose-dependent manner, and the elicitation was inhibited by the adenosine antagonist 8-phenyltheophylline. The 2-CADO-elicited accumulation of cyclic AMP was greatly increased in the cortical region on the primary epileptic side, while the R-PIA- or CHA-elicited accumulation did not change in any cortical region. The deviation detected only in the 2-CADO-elicited accumulation of cyclic AMP may be due to the difference in relative potency for adenosine receptors of the adenosine analogues. The results suggest that adenosine receptor-mediated generation of cyclic AMP is altered in the primary region of iron-induced epileptic cortex, in which heterogeneous alterations in different adenosine receptor subtypes may occur in the epileptic process.

Long-term caffeine treatment leads to a decreased susceptibility to NMDA-induced clonic seizures in mice without changes in adenosine A1 receptor number

The effects of long-term caffeine treatment on N-methyl-D-aspartate (NMDA)-induced seizures in mice were studied. Caffeine was added (0.3 g/l) to drinking water for 14 days and the mice ingested 60-70 mg/kg/day. During the treatment, the plasma concentrations of methylxanthines (caffeine, theophylline and/or paraxanthine, theobromine) were measured. NMDA (150 mg/kg i.p.) was administered to control mice and to mice during and after the caffeine administration. A1 adenosine receptor density in the gyrus dentatus of hippocampus, measured by quantitative receptor autoradiography with [3H]cyclohexyl adenosine as the ligand, was not significantly altered after long-term caffeine treatment. NMDA-induced clonic seizures, wet dog shakes and mortality were significantly reduced at the end of long-term caffeine treatment but returned towards control at 1 and 2 days after withdrawal. At the end of caffeine treatment, tonic seizures were also absent. These results show that long-term treatment with caffeine in a dose that gives plasma levels of 6-10 microM decreases the effects of NMDA on e.g. seizure susceptibility, and that this effect cannot be ascribed to changes of A1 adenosine receptor density.

Circadian rhythm of cortical and striatal adenosine receptors

In order to investigate diurnal variations in binding parameters of A1 and A2 adenosine receptors, Kd and Bmax were calculated in mice that had been housed under controlled light-dark cycles for 4 weeks (light on from 7.00 to 19.00 h). A1 cortical receptors were labelled by N6-cyclohexyl-[3H]adenosine, and A1 and A2 striatal receptors by 5'-N-ethylcarboxamido[8-3H]adenosine. Significant differences were found for Bmax values measured at 3-h intervals across a 24-h period. Cyclic variations of the number of binding sites were shown, with a minimum number of A1 and A2 receptors during the light period and a maximum during the dark period. The amplitude for cortical A1 receptors, between 03.00 and 18.00 h, was 39%, while for striatal A1 receptors, between 03.00 and 15.00 h, was 92%. The amplitude for the A2 striatal receptors, between 03.00 and 15.00 h, was 147%. No substantial rhythm was found in the Kd values. These differences in the amplitude could suggest a different physiological modulation of the two adenosine receptor subtype moiety which could reflect a physiologically-relevant mechanism by which adenosine exerts its modulatory role in the central nervous system.