Hormonal influences on seizure kindling: the effects of post-stimulation ACTH or cortisone injections

Stress and epilepsy: multiple models, multiple outcomes

Human studies show a link between stress and epilepsy, with stress causing an increase in seizure frequency and severity in patients with epilepsy. Many different animal model systems have been used to better understand this connection and the possible mechanisms involved. This review highlights the results of such studies relating stress and seizure susceptibility, with a focus on the hypothalamic-pituitary-adrenal axis and its relationship to seizure generation. The effects of hypothalamic-pituitary-adrenal axis mediators, acute stress, chronic stress, and early life stress on the seizure phenotype are summarized. Results suggest that stress has both anticonvulsive and proconvulsive properties, depending on the animal strain and the stress/seizure induction paradigm used. Attempts to interpret the stress-epilepsy literature must take these variables into account. The growing availability of genetically modified mice that carry either human epilepsy mutations or mutations in stress pathway genes now provide the opportunity to examine the relationship between stress and epilepsy more directly.

Infantile spasms: criteria for an animal model

Infantile spasms is an epilepsy syndrome with several distinctive features, including age specificity during infancy, characteristic semiology (epileptic spasms), specific electroencephalographic patterns (interictal hypsarrhythmia and ictal voltage suppression), and responsiveness to the adrenocorticotropic hormone (ACTH). There is no adequate animal model of infantile spasms, perhaps due to these clinically unique features, that is specific for the developing human brain. An informative animal model would provide insights into the pathophysiology of this syndrome and form the basis for the development of innovative therapies. This chapter considers criteria for an "ideal" animal model of infantile spasms, as well as "minimal" criteria that we consider essential to yield useful information. Two animal models of infantile spasms have been described in rodents: seizures induced by corticotropin-releasing factor and N-methyl-D-aspartic acid. Neither of these models conforms exactly to the human analog, but each possesses intriguing similarities that provide testable hypotheses for future investigations.

Adrenocorticotropic hormone immunoreactivity in the hippocampal formation of temporal lobe epilepsy patients

PURPOSE

We wished to identify immunocytochemically the distribution of proopiomelanocortin-related peptides in the hippocampal formation of patients with epilepsy.

METHODS

Surgical hippocampal specimens from temporal lobe epilepsy (TLE) patients and autopsy control tissue were examined immunocytochemically for ACTH, alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin.

RESULTS

There was a dense distribution of ACTH-immunoreactive neurons in the hippocampal formation of patients with mesial TLE syndrome (MTLE). These hippocampal specimens showed significant cell loss. ACTH-positive neurons were most prominent in the subiculum, with scattered ACTH-immunoreactive neuronal elements distributed in the cornu ammonis fields and hilus. Light ACTH immunoreactivity was detected in the tumor-related epileptic hippocampal specimens, which showed minimal cell loss. Although autopsy control tissue from the hypothalamus showed intense ACTH staining patterns in cells and fibers, there was little or no ACTH immunoreactivity in the autopsy hippocampal tissue. The expression of ACTH immunoreactive elements was correlated with patterns of cell loss. No alpha-MSH- or beta-endorphin-immunoreactive neurons were detected in any of the hippocampal specimens.

CONCLUSIONS

ACTH has anticonvulsant properties, and its novel expression in the glutamatergic subicular neurons, which provide the main outflow of the hippocampal formation, may represent an attempt by the damaged hippocampal circuit to restore the balance of excitatory/inhibitory neurotransmission in TLE.

Chemical models of epilepsy with some reference to their applicability in the development of anticonvulsants

This paper reviews chemical models of epilepsy and their relevance in the identification and characterization of anticonvulsants. For each convulsant we discuss possible modes of administration, clinical type(s) of seizures induced, proposed mechanism(s) of epileptogenesis and, where available, responsiveness of the induced seizures to anticonvulsants. The following compounds are reviewed: pentylenetetrazol, bicuculline, penicillin, picrotoxin, beta-carbolines, 3-mercaptopropionic acid, hydrazides, allylglycine; the glycine antagonist strychnine; gamma-hydroxybutyrate; excitatory amino acids (glutamate, aspartate, N-methyl-D-aspartate, quisqualate, kainate, quinolinic acid); monosubstituted guanidino compounds, metals (alumina, cobalt, zinc, iron); neuropeptides (opioid peptides, corticotropin releasing factor, somatostatin, vasopressin); cholinergic agents (acetylcholine, acetylcholinesterase inhibitors, pilocarpine); tetanus toxin; flurothyl; folates; homocysteine and colchicine. Although there are a multitude of chemical models of epilepsy, only a limited number are applied in the routine screening of potential anticonvulsants. Some chemical models have a predictive value with regard to the clinical profile of efficacy of the tested anticonvulsants. Some chemical models may contribute to a better understanding of possible mechanisms of epileptogenesis.

Some endorphin derivatives and hydrocortisone prevent EEG limbic seizures induced by corticotropin-releasing factor in rabbits

Corticotropin-releasing factor (CRF) injected into the cerebral ventricles of small mammals induces EEG limbic seizures, behavioral excitability, stereotyped behavior, and tardive enhancement of hippocampal theta voltage and frequency. Because we addressed this phenomenon when we explained the pathogenesis of infantile spasms in children, we wished to study the interference exerted by some gamma-endorphin fragments on EEG epileptiform and behavioral symptoms induced by CRF in the rabbit. Animals were implanted intracerebroventricularly (i.c.v.) with semichronic cortical and hippocampal electrodes, together with a cannula into the left lateral ventricle. When some gamma-endorphin derivatives (DT gamma E, DE gamma E) were injected intravenously (i.v.) for 4 days (or hydrocortisone once), they prevented the EEG ictal seizures induced in the hippocampus of rabbits by CRF injected i.c.v. Hydrocortisone and DE gamma E also prevented the appearance of scattered spiking and partially prevented tardive enhancement of theta voltage in the hippocampal EEG. Finally, DE gamma E also prevented stereotyped behavior and excitability induced by CRF. These results confirm the regulatory role exerted by CRF in limbic structure excitability and suggest that the above peptides may be involved in a regulatory feedback mechanism of CRF metabolism or activity. The possibility that these peptides may also have interesting antiepileptogenic properties should be considered.

The effect of hypophysectomy, ACTH fragments and thalamic lesions upon kindled epilepsy

Hypophysectomized rats showed aberrant and retarded rates of kindled epilepsy. In hypophysectomized rats administered adrenocorticotropic hormone (ACTH) subunits ACTH4-10 and ACTH1-24, the normal kindling pattern was restored. However, in hypophysectomized animals which also had lesions of the thalamus (nucleus parafascicularis), ACTH4-10 did not restore the normal pattern of kindling. There have been many conjectures that kindling may be a subcase of learning. These results are compatible with this hypothesis, since the same procedures act in an analogous fashion within avoidance conditioning paradigms.

Effects of ACTH on seizure susceptibility in the developing brain

Adrenocorticotropic hormone (ACTH) has been frequently used as an anticonvulsant drug in some childhood seizure disorders. Despite its widespread use, few studies have evaluated the effects of ACTH on seizure susceptibility in the developing animal or the long-term consequences of ACTH treatment on brain development. In this study, ACTH was given either for 2 days or 14 days prior to kindling in 15-, 22-, and 30-day-old rats. Morphological changes in the brain were studied using routine light microscopy and dendrite branch counting following Golgi staining. Both acute and chronic ACTH treatment inhibited the rate of kindling in all three age groups. There were no differences in brain morphology between the controls and the ACTH-treated rats killed shortly after kindling. Rats treated with ACTH and killed as adults, however, had significantly more dendrite branches than did controls. In the immature brain, ACTH treatment reduces seizure susceptibility and has no long-term deleterious effects on neuronal growth.

Plasma ACTH and corticosterone responses to limbic kindling in the rat

Changes in plasma ACTH and corticosterone concentrations were measured in individual cannulated rats at stages 1 and 5 of limbic kindling induced by electrical stimulation of the basolateral amygdala or the dorsal hippocampus. At both stages, a stimulation of either structure produced swift surges, first of ACTH and then of corticosterone. At stage 5 of hippocampal stimulation, ACTH baseline concentrations were four times higher than in the controls. The results are discussed in relation to the central control of the adrenocorticotropic system and to the neuroendocrine correlates of the kindling process.

Reduction of amygdaloid kindled seizures by an analog of ACTH/MSH

A behaviorally potent analog of ACTH/MSH(4-9), ORG-2766, markedly reduces both physiologic and behavioral components of convulsive seizures in an animal model of epilepsy--the amygdaloid kindled rat. We believe that such non-endocrine analogs of ACTH/MSH fragments may be clinically useful anticonvulsants, particularly in chronic applications, provided that their permeation of the blood-brain barrier can be improved.

Attenuation of epileptogenesis: proactive effect of a single epinephrine injection of amygdaloid kindling

Repeated daily electrical stimulation of the amygdala can lead to a progressive increase in brain and behavioral seizures. This phenomenon, termed kindling, has been viewed as a model for epileptogenesis. The results reported here demonstrate that a single systemic epinephrine injection can significantly retard such epileptogenesis for a period of at least several days. These findings suggest that peripheral catecholamines, responding either to stress near the time of seizure initiation or to treatments administered at that time, may be important in regulating the development of epileptic states. In addition, the results indicate that an acute episode of high plasma epinephrine levels may result in a durable modification of brain function.

Neuropeptides: a role as endogenous mediators or modulators of epileptic phenomena

As more small peptidergic components of the central nervous system are isolated, their role in disease states is being investigated. Several of these neuropeptides, especially the opioidlike peptides, adrenocorticotropic hormone, and some hypothalamic releasing factors, have been found to alter neuronal excitability. This finding has led to the proposal that these peptides may play a role in the pathogenesis of the epilepsies. We tested this hypothesis in a genetic model of epilepsy. At nontoxic doses, several exogenously administered peptides had anticonvulsant properties, while others were proconvulsant. The most potent anticonvulsant was the opioidlike peptide beta-endorphin. Its effect was similar to that of the opioid alkaloids. Using the potent antagonist naloxone hydrochloride to block possible endogenous opioid-like peptides, we found no effects on seizures in naive animals. Naloxone did alter postictal events, however, by partially blocking the postictal refractoriness to further seizures. We speculate that one possible role for the endogenous opioid peptides may be to limit the spread of seizures or to modulate postictal susceptibility to further seizures. Naloxone was effective in this model only after stressful situations occurred that modified the seizures and presumably induced a release of endogenous opioidlike peptides. Support for this hypothesis from other epilepsy models is discussed. Other peptidergic systems may also be active in various epileptic models, and the current understanding of their roles is reviewed.

Corticotropin releasing factor produces increases in brain excitability and convulsive seizures in rats

Corticotropin releasing factor (CRF) is a 41-residue peptide, capable of stimulating the secretion of corticotropin (ACTH)-like and beta-endorphin-like immunoreactivity from the adenohypophysis. Low doses of CRF (0.0015-0.15 nM) given intracerebroventricularly (i.c.v.) produced changes in electrographic activity suggestive of increased arousal. Higher doses of CRF (1.5-3.75 nM) induced, over a period of 3-7 h, electrographic and behavioral signs of seizure activity indistinguishable from those which occur following electrical 'kindling' of the amygdala.