Effect of an adenosine A1 agonist injected into substantia nigra on kindling of epileptic seizures and convulsion duration

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.

Anticonvulsant effect of bilateral injection of N6-cyclohexyladenosine into the CA1 region of the hippocampus in amygdala-kindled rats

In this study the role of adenosine A(1) receptors of CA1 region of the hippocampus on amygdala-kindled seizures was investigated in rats. Results obtained showed that in kindled animals, bilateral injection of N(6)-cyclohexyladenosine (CHA), an adenosine A(1) receptor agonist, at doses of 0.1, 1 and 10 microM into the CA1 region of the hippocampus significantly decreased the afterdischarge duration and stage 5 seizure duration and increased the latency to stage 4 seizure, but there were no changes in seizure stage. Also, bilateral injection of 1,3-dimethyl-8-cyclopenthylxanthine (CPT), an adenosine A(1) receptor antagonist, at doses of 0.5 and 1 microM into the CA1 region of the hippocampus could not produce any changes in the seizure parameters. Intrahippocampal pretreatment of CPT (1 microM) before CHA (0.1 and 1 microM), reduced the effects of CHA on seizure parameters significantly. Thus, it may be suggested that CA1 region of the hippocampus plays an important role in spreading seizure spikes from the amygdala to other brain regions and activation of adenosine A(1) receptors in this region, participates in anticonvulsant effects of adenosine agonists.

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.

Intraperitoneal and intraamygdala N(6)-cyclohexyladenosine suppress hippocampal kindled seizures in rats

Effects of intraperitoneal and intraamygdala N(6)-cyclohexyladenosine (CHA), a selective adenosine A(1) receptor agonist, and 1,3-dimethyl-8-cyclopentylxanthine (CPT), a selective adenosine A(1) receptor antagonist, were examined in fully hippocampal kindled rats. Intraperitoneal administration of CHA (0. 25, 0.5 and 1 mg/kg) decreased hippocampal secondary afterdischarge duration (SAD) and amygdala afterdischarge duration (ADD). Only the 1 mg/kg dose induced a significant increase in latency to stage 4. Intraperitoneal administration of CPT (0.25, 0.5 and 1 mg/kg) induced a significant increase in stage 5 duration, hippocampal SAD and ADD. Pretreatment of animals with CPT (1 mg/kg), antagonized effects of CHA on seizure parameters. Intraamygdala microinfusion (1 microl over 2 min) of CHA (5 nM-1 mM) significantly reduced hippocampal SAD and amygdala ADD. These effects were antagonized by intraamygdala CPT (1 microM). Results obtained suggest that in hippocampal kindled rats, amygdala may be regarded as a relay point for AD propagation specially in recruit activity of the hippocampus.

Strain differences in pentylenetetrazol-kindling development and subsequent potentiation effects

Rats from two different strains, i.e. Wistar rats and Lister hooded rats, were investigated for their ability to acquire the kindling syndrome. After having received 13 kindling stimulations (injection of pentylenetetrazol), the animals were tested for subsequent alterations in induction and maintenance of hippocampal long-term potentiation (LTP) and, moreover in glutamate binding. It was found that rats from both strains did not differ in the response to the initial injection of pentylenetetrazol (PTZ) and the amplitude of the population spike. This suggests that some aspects of basic central excitability are equivalent. Wistar rats acquired the kindling syndrome rapidly whereas seizure outcome was poor in Lister rats. As regards hippocampal LTP, the population spike was only dramatically increased in Wistar rats after kindling completion. Glutamate binding was not altered in animals from the Lister strain. The results suggest that changes in glutamate binding and the increase in the population spike are characteristic consequences of kindling.

Neonatal caffeine exposure alters seizure susceptibility in rats in an age-related manner

Early developmental exposure to caffeine in rats results in decreased susceptibility to certain chemically-induced seizures in the adult. To determine whether this effect first appears in adulthood or is present during preceding developmental stages, we exposed neonatal rats to caffeine and determined seizure thresholds in animals 28, 42 and 70-90 days of age. Rats were unhandled or received either vehicle (water) or caffeine (15-20 mg/kg/day) by gavage (0.05 ml/10 g) over postnatal days 2-6. At 28, 42, or 70-90 days of age, rats were infused intravenously with picrotoxin (PIC), bicuculline (BIC), pentylenetetrazol (PTZ), caffeine (CAFF), strychnine (STR), or kainic acid (KA). Seizure thresholds for each compound were analyzed as a function of neonatal treatment, sex, and age. At 28 days, neonatally caffeine-exposed rats had a higher seizure threshold only for PTZ (P < 0.03). At 42 days, neonatally caffeine-exposed rats had higher seizure thresholds for PIC (P < 0.0007) and PTZ (P < 0.0001) than did controls. These results at 28 and 42 days are compared with previously reported data that demonstrated that in adulthood, rats neonatally exposed to caffeine have higher thresholds for seizure induction with CAFF, PTZ, and KA. Thus, early developmental exposure to caffeine results in decreases in seizure susceptibility that are agent specific and may result in a delay in the decrease in seizure threshold that occurs for many agents between late juvenile ages and adulthood.

Kindled epileptic seizures, postictal refractoriness, status epilepticus, and electrical self-stimulation

A single stimulus applied once daily to the limbic system commonly leads to convulsive seizures yet seizures are relatively infrequent during intracranial self-stimulation (ICSS), a procedure that involves many hundreds of similar stimuli. The present study examined the possible role of electrode site, interstimulus interval, afterdischarge and reinforcement thresholds and postictal refractoriness in accounting for this paradox. Electrode location was an overriding factor: seizures were never seen with hypothalamic implants posterior to the level of the ventromedial nucleus but were elicited by the majority of more rostral reward sites. Frequent repeated stimulation by ICSS did not in itself prevent subsequent kindling or reverse the effects of earlier kindling; on the contrary, seizures induced by ICSS showed a progressive increase in severity similar to the progression produced by conventional kindling. Individual convulsive seizures, as in previous studies, conferred transient protection against further seizures whether from ICSS or from kindling. More prolonged protection occassionally developed after repeated convulsive seizures: protection was accompanied by continuous EEG slow-waves corresponding in presentation to clinical petit mal status. Prolonged resistance to seizures has also been reported after tonic-clonic status epilepticus causing temporal lobe damage. The relative infrequency of seizures during ICSS ordinarily appears to depend on the siting of the electrodes, on distinct short- and long-term postictal refractory states, and on the rat learning to restrict stimulus input to subseizural levels.

Autoradiographic analysis of serotonin receptors and transporter in kindled rat brain

While serotonin (5-HT) has been shown to be anticonvulsant in several types of experimentally induced seizures, 5-HT receptor binding has not been investigated in the kindling model of epilepsy. The present study examined the effects of amygdala kindling on two 5-HT receptor subtypes and on the 5-HT transporter in rat brain. Kindling induced a persistent bilateral increase in 5-HT1A binding in the dentate gyrus, while 5-HT1B receptors increased only in a delayed fashion. Binding to the 5-HT transporter was transiently decreased in dentate gyrus. In cerebral cortex, binding of the three ligands was unchanged. Alterations in 5-HT receptors and the 5-HT transporter may endogenously modulate kindled seizures. Additionally, autoradiography of adenosine A1 receptors revealed no change for these receptors in any brain region.