Influences of pituitary-adrenal hormones on kindling

Stress regulation in drug-resistant epilepsy

The prevalence of psychological distress, especially depressive and anxiety disorders, is higher in epilepsy than in other chronic health conditions. These comorbid conditions contribute even more than epileptic seizures themselves to impaired quality of life in patients with epilepsy (PWE). The link between these comorbidities and epilepsy appears to have a neurobiological basis, which is at least partly mediated by stress through psychological and pathophysiological pathways. The impact of stress in PWE is also particularly important because it is the most frequently reported seizure trigger. It is therefore crucial for clinicians to take stress-related conditions and psychiatric comorbidities into account when managing PWE and to propose clinical support to enhance self-control of stress. Screening tools have been specially designed and validated in PWE for depressive disorders and anxiety disorders (e.g. NDDI-E, GAD-7). Other instruments are useful for measuring stress-related variables (e.g. SRRS, PSS, SCS, MHLCS, DSR-15, ERP-R, QOLIE-31) in order to help characterize the individual "stress profile" and thus orientate patients towards the most appropriate treatment. Management includes both pharmacological treatment and nonpharmacological methods for enhancing self-management of stress (e.g. mindfulness-based therapies, yoga, cognitive-behavioral therapies, biofeedback), which may not only protect against psychiatric comorbidities but also reduce seizure frequency.

Role of hormones and neurosteroids in epileptogenesis

This article describes the emerging evidence of hormonal influence on epileptogenesis, which is a process whereby a brain becomes progressively epileptic due to an initial precipitating event of diverse origin such as brain injury, stroke, infection, or prolonged seizures. The molecular mechanisms underlying the development of epilepsy are poorly understood. Neuroinflammation and neurodegeneration appear to trigger epileptogenesis. There is an intense search for drugs that truly prevent the development of epilepsy in people at risk. Hormones play an important role in children and adults with epilepsy. Corticosteroids, progesterone, estrogens, and neurosteroids have been shown to affect seizure activity in animal models and in clinical studies. However, the impact of hormones on epileptogenesis has not been investigated widely. There is emerging new evidence that progesterone, neurosteroids, and endogenous hormones may play a role in regulating the epileptogenesis. Corticosterone has excitatory effects and triggers epileptogenesis in animal models. Progesterone has disease-modifying activity in epileptogenic models. The antiepileptogenic effect of progesterone has been attributed to its conversion to neurosteroids, which binds to GABA-A receptors and enhances phasic and tonic inhibition in the brain. Neurosteroids are robust anticonvulsants. There is pilot evidence that neurosteroids may have antiepileptogenic properties. Future studies may generate new insight on the disease-modifying potential of hormonal agents and neurosteroids in epileptogenesis.

Brain Physiology and Pathophysiology in Mental Stress

Exposure to various forms of stress is a common daily occurrence in the lives of most individuals, with both positive and negative effects on brain function. The impact of stress is strongly influenced by the type and duration of the stressor. In its acute form, stress may be a necessary adaptive mechanism for survival and with only transient changes within the brain. However, severe and/or prolonged stress causes overactivation and dysregulation of the hypothalamic pituitary adrenal (HPA) axis thus inflicting detrimental changes in the brain structure and function. Therefore, chronic stress is often considered a negative modulator of the cognitive functions including the learning and memory processes. Exposure to long-lasting stress diminishes health and increases vulnerability to mental disorders. In addition, stress exacerbates functional changes associated with various brain disorders including Alzheimer’s disease and Parkinson’s disease. The primary purpose of this paper is to provide an overview for neuroscientists who are seeking a concise account of the effects of stress on learning and memory and associated signal transduction mechanisms. This review discusses chronic mental stress and its detrimental effects on various aspects of brain functions including learning and memory, synaptic plasticity, and cognition-related signaling enabled via key signal transduction molecules.

Early life stress enhancement of limbic epileptogenesis in adult rats: mechanistic insights

Background

Exposure to early postnatal stress is known to hasten the progression of kindling epileptogenesis in adult rats. Despite the significance of this for understanding mesial temporal lobe epilepsy (MTLE) and its associated psychopathology, research findings regarding underlying mechanisms are sparse. Of several possibilities, one important candidate mechanism is early life ‘programming’ of the hypothalamic-pituitary-adrenal (HPA) axis by postnatal stress. Elevated corticosterone (CORT) in turn has consequences for neurogenesis and cell death relevant to epileptogenesis. Here we tested the hypotheses that MS would augment seizure-related corticosterone (CORT) release and enhance neuroplastic changes in the hippocampus.

Methodology/Principal Findings

Eight-week old Wistar rats, previously exposed on postnatal days 2–14 to either maternal separation stress (MS) or control brief early handling (EH), underwent rapid amygdala kindling. We measured seizure-induced serum CORT levels and post-kindling neurogenesis (using BrdU). Three weeks post-kindling, rats were euthanized for histology of the hippocampal CA3c region (pyramidal cell counts) and dentate gyrus (DG) (to count BrdU-labelled cells and measure mossy fibre sprouting). As in our previous studies, rats exposed to MS had accelerated kindling rates in adulthood. Female MS rats had heightened CORT responses during and after kindling (p<0.05), with a similar trend in males. In both sexes total CA3c pyramidal cell numbers were reduced in MS vs. EH rats post-kindling (p = 0.002). Dentate granule cell neurogenesis in female rats was significantly increased post-kindling in MS vs. EH rats.

Conclusions/Significance

These data demonstrate that early life stress results in enduring enhancement of HPA axis responses to limbic seizures, with increased hippocampal CA3c cell loss and augmented neurogenesis, in a sex-dependent pattern. This implicates important candidate mechanisms through which early life stress may promote vulnerability to limbic epileptogenesis in rats as well as to human MTLE and its associated psychiatric disorders.

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.

Endogenous morphine and codeine. Possible role as endogenous anticonvulsants

Exogenously administered morphine can have both convulsive or anticonvulsive effects, depending on the dose and species. The levels of the endogenous opiate alkaloids morphine and codeine were significantly elevated in specific rat brain regions by the convulsive drug, pentylenetetrazole, as well as by the anticonvulsant drugs, carbamazepine and phenytoin. Morphine and codeine levels in peripheral tissues (heart, lung, spleen and adrenal) were unaffected by these drugs. Maximal increases in morphine levels were seen in the hypothalamus and striatum (2-10-fold), while lesser increases occurred in the midbrain and brain stem (2-4-fold). Codeine levels were also markedly increased in hypothalamus (5-10 fold), In contrast to morphine, codeine levels were also increased in the hippocampus (2-10-fold), but were unchanged in the striatum. These studies suggest that the endogenous alkaloids morphine and codeine are involved in the modulation of convulsions and that morphine and/or codeine may act as an endogenous anticonvulsant.

Amygdala kindling rate is altered in rats with a deficit in the responsiveness of the hypothalamo-pituitary-adrenal axis

Fisher and Lewis rats were amygdala kindled in the morning and in the evening. Fisher rats displayed a diurnal variation in the rate of kindling through stage 2 which did not exist in the Lewis rats. Lewis rats also take much longer to kindle through stage 2 and a subcutaneous corticosterone pellet accelerates this rate. These observations are consistent with the hypothesis that the hormones of the hypothalamo-pituitary-adrenal (HPA) axis, particularly the glucocorticoids, play a role in epileptogenesis.

The effect of adrenalectomy on the circadian variation in the rate of kindled seizure development

The hypothalamic-pituitary adrenal axis (HPAA) is activated during kindled seizures and the circadian changes in this axis may contribute to the circadian variation in the kindling rate. Changes in the rate of seizure development were examined in rats that were amygdala kindled in the a.m., at midday and in the p.m.. Sham operated control groups were compared to adrenalectomized groups. Adrenalectomy had no effect on the midday rate but abolished the normal circadian variation by accelerating the rate in the a.m. and decreasing the rate in the p.m.. These results suggest a complicated relationship of kindled seizure rate to the hormones of the HPAA axis.

Effects of corticosterone on shaking and seizure behavior induced by deep prepyriform cortex kindling

The influence of adrenocorticosteroids on seizures and wet dog shakes (WDS) induced by deep prepyriform cortex kindling was studied by bilateral adrenalectomy and corticosterone replacement. The rate of kindling, latency to the onset and duration of motor seizures were not significantly affected by adrenalectomy or corticosterone treatment. However, the number of WDS observed after stage 5 seizures was reduced in adrenalectomized animals and it was not restored until 3 h following corticosterone replacement. This delay in onset of action suggests that the effects of adrenocorticosteroids and/or ACTH on WDS may be mediated by an indirect mechanism.

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.

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.

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.

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

Repeated application of brain stimulation can lead to a progressively augmenting electrical and behavioral response-- a phenomenon termed seizure kindling. In this experiment, stimulation was delivered once per day, and was followed by peripheral (intraperitoneal) administration of ACTH or cortisone. An intermediate or a high dose of either hormone (0.3 IU or 3.0 IU of ACTH/animal, 10 mg or 25 mg cortisone/animal) delayed the completion of kindling if administered shortly after each kindling stimulation. Lower doses (0.03 IU of ACTH or 2 mg of cortisone) had no significant effects. The high dose of ACTH or cortisone was no longer effective if administration was delayed more than 4 h after stimulation. Peripherally administered ACTH and cortisone can influence processes initiated by the brain stimulation which presumably underlie the augmentation of response to successive stimulations. This time-limited action is analogous to the effects of these hormones on memory consolidation.