Transient elevation of amygdala alpha 2 adrenergic receptor binding sites during the early stages of amygdala kindling

Differential noradrenergic influence on seizure expression in genetically Fast and Slow kindling rat strains during massed trial stimulation of the amygdala

The involvement of alpha(2) noradrenergic receptors during amygdala 'massed' stimulation (MS) was examined in rats that were selectively bred to be seizure-prone (Fast) or seizure-resistant (Slow) to amygdala kindling. The selective alpha(2) noradrenergic agonist guanfacine, or the antagonist idazoxan, was intraperitoneally injected during the MS procedure to study subsequent changes in afterdischarge (AD) threshold, AD duration and behavioral seizure expression. These measurements were again assessed weekly for 2 weeks after the MS treatment. Daily kindling began immediately thereafter. Following 6 stage-5 once daily convulsive seizures, guanfacine or idazoxan were re-administered. With idazoxan, the Slow rats expressed greater numbers of convulsive seizures and longer AD durations compared to guanfacine or saline controls during MS treatment. This pro-convulsive property of idazoxan was absent in Fast rats. By contrast, Fast rats showed enhanced convulsive expression in the presence of guanfacine. In the fully kindled rat, idazoxan and guanfacine differentially impacted seizure duration and severity in the Slow rats, but again not in the Fast rats. These data suggest that some aspect(s) of the alpha(2) noradrenergic system in the Fast and Slow rats are dissimilar and the mechanisms by which these receptors govern seizure genesis and propagation may be genetically controlled and distinct.

Chronic imipramine treatment induces downregulation of alpha-2 receptors in rat's locus coeruleus and A2 region of the tractus solitarius

Imipramine is an effective antidepressant agent that blocks the reuptake of monoamines. In order to understand some of its basic mechanisms of action, we investigated the effects of chronic imipramine administration (10 mg/kg, i.p.; 21 days) on the alpha-2 receptor population of several brain sites. Alpha-2 receptor density was estimated by in vitro autoradiography using [3H]Idazoxan. The densitometric analysis revealed a decreased receptor density in the A2 region of the tractus solitarius (20%) and locus coeruleus (16%). No changes were observed in the amygdala, pyriform cortex, periacueductal gray and the bed nucleus of the stria terminalis. These results suggest that chronic imipramine treatment selectively modulates the alpha-2 receptor population localized in the brain stem norepinephrine-rich nuclei and not in the population present on limbic structures innervated by noradrenergic terminal projections. The possible physiological consequences of this selective modulation of alpha-2 receptors are discussed.

gamma-Hydroxybutyric acid induced spike and wave discharges in rats: relation to high-affinity [3H]gamma-hydroxybutyric acid binding sites in the thalamus and cortex

gamma-Hydroxybutyric acid is a naturally occurring compound which induces bilaterally synchronous spike and wave discharges in rats. The gamma-hydroxybutyric acid model of absence seizures simulates clinical absence seizures behaviorally as well as electrographically. The present study was undertaken in order to establish the role of the high-affinity gamma-hydroxybutyric acid binding sites in the generation of gamma-hydroxybutyric acid-induced spike and wave discharges. Spike and wave discharges induced by gamma-hydroxybutyric acid were recorded with the aid of bipolar depth electrodes implanted in discrete regions of thalamus, cortex and hippocampus. In the present study we found that ventroposterolateral, ventroposteromedial, medial and the reticular nuclei of the thalamus discharged synchronously with the cortical generation of spike and wave discharges. In the cortex, the superficial layers (I-IV) of frontoparietal cortex generated spike and wave discharges, whereas no spike and wave discharges were recorded from deeper layers (V-VI) of frontoparietal cortex. At the onset of spike and wave discharges induced by gamma-hydroxybutyric acid, a rapid but reversible upregulation of gamma-hydroxybutyric acid binding sites was observed. This increased [3H]gamma-hydroxybutyric acid binding was characterized by an increase in the number of gamma-hydroxybutyric acid sites with no significant change in their affinity for gamma-hydroxybutyric acid. Moreover, the change in [3H]gamma-hydroxybutyric acid binding was observed only in those thalamic structures and cortical layers which were found to be involved in the generation of spike and wave discharges induced by gamma-hydroxybutyric acid. The CA3 field or dorsal hippocampus possesses the highest density of [3H]gamma-hydroxybutyric acid binding sites of all brain regions. However, no significant change in [3H]gamma-hydroxybutyric acid binding was observed in this region nor was the CA3 field involved in the generation of spike and wave discharges during gamma-hydroxybutyric acid-induced absence-like seizures. These findings confirm that gamma-hydroxybutyric acid-induced absence-like seizures originate from thalamocortical pathways and that the onset of gamma-hydroxybutyric acid-induced spike and wave discharges is directly related to the regulation of gamma-hydroxybutyric acid binding sites in those regions which constitute the involved thalamocortical loop.

Intra-amygdaloid infusions of clonidine retard kindling

It has been reported previously that systemic administration of clonidine, an agonist of alpha-2 noradrenergic receptors, significantly retards amygdaloid kindling by delaying the emergence from partial seizure. We examined the effect of either systemic administration (i.p.) or intra-amygdaloid infusions of clonidine on the kindling of seizures with electrical stimulation of the amygdala. Rats received either low-frequency stimulation of the amygdala, to induce rapid kindling, or conventional high-frequency stimulation. Clonidine and electrical stimulation were administered once every 48 h. We observed a significant retardation of kindling in rats receiving i.p. injections of clonidine (0.1 mg/kg) or infusions of clonidine in concentrations of 10(-7)-10(-4) M, regardless of the stimulation frequency. The prophylactic effect was due to a delay in the progression out of partial seizure. The effect was specific to the amygdala/pyriform region, because infusions of clonidine dorsal to the amygdala were without effect. Intra-amygdaloid clonidine had little effect on established generalized seizures, suggesting that it was producing a genuine prophylactic effect against kindling. We conclude that the subpopulation of alpha-2 adrenoceptors in the amygdala/pyriform region contributes to the antiepileptogenic effect observed after systemic administration of clonidine.

Suppression of amygdala kindling with massed stimulation: effect of noradrenaline antagonists

Afterdischarge (AD) triggered by brief, daily stimulation of the amygdala progressively increases in complexity and duration and, over days, develops into generalized convulsions. This progression, called kindling, is delayed by noradrenaline (NA). When brief stimulation of the amygdala occurs too frequently (massed), there is a suppression of AD growth and little evidence of kindling. Previously we showed that depletion of NA before massed amygdala stimulation prevented the suppression of AD growth described above, and readily precipitated generalized seizures. In the present report, we examined the role of NA in maintaining this suppression of AD growth, after it was well established. We showed that suppression of AD development during the first 15 massed stimulations (interstimulus interval of 5 min) was reduced by subsequent injection of the NA alpha 2 antagonist, yohimbine, with most rats exhibiting occasional generalized convulsions. Conversely, rats exposed to the beta antagonist, propranolol, like controls, not only showed suppressed AD growth, but also elevated AD thresholds. Three weeks later, only a small positive transfer to daily kindling was observed in all groups. We conclude that alpha 2 NA receptors help maintain suppression of AD growth induced by massed stimulation of the amygdala, while beta receptors provide only a small proepileptic influence. These results and those from the 'rapid' kindling model (Lothman et al., Brain Research, 360 (1985) 83-91) are compared, and related to NA receptor subtype variations in the amygdala and hippocampus.

Autoradiographic changes in brain adenosine A1 receptors and their coupling to G proteins following seizures in the developing rat

In the central nervous system, adenosine has been shown to be a major regulator of neuronal activity in convulsive disorders, mainly via the A1 receptor subtype. In a previous work, we have shown that seizures lead to an age-dependent upregulation of cerebral adenosine A1 sites measured in isolated rat cerebral membranes. However, information concerning regional changes in the receptor density was so far lacking. In the present study, the effects of bicuculline-induced seizures were investigated by quantitative autoradiography of central adenosine A1 receptors in developing rats and in adults. Animals were sacrificed 30 min after an intraperitoneal injection of either saline or a convulsive dose of bicuculline. Adenosine A1 receptors in brain sections were labeled by [3H]N6-cyclohexyladenosine (CHA), a potent receptor agonist. Generalized seizures induced a widespread increase in CHA-specific binding, with a marked enhancement in structures that mediate seizure activity, such as substantia nigra, amygdala, septum and hippocampus. Moreover, the addition of guanylyl-5'-imidodiphosphate, a GTP analogue, to the incubation medium reduced CHA binding by the same order of magnitude whether rats were given saline or bicuculline, suggesting that additional adenosine A1 receptors are also functionally linked to G proteins. The age-related postictal increase in adenosine receptors might contribute to facilitate adenosine anticonvulsant effect, especially in newborns.