Effects of water immersion stress on convulsions induced by pentylenetetrazol

Hypothalamic-pituitary-adrenal axis targets for the treatment of epilepsy

Stress is a common seizure trigger in persons with epilepsy. The body's physiological response to stress is mediated by the hypothalamic-pituitary-adrenal (HPA) axis and involves a hormonal cascade that includes corticotropin releasing hormone (CRH), adrenocorticotropin releasing hormone (ACTH) and the release of cortisol (in humans and primates) or corticosterone (in rodents). The prolonged exposure to stress hormones may not only exacerbate pre-existing medical conditions including epilepsy, but may also increase the predisposition to psychiatric comorbidities. Hyperactivity of the HPA axis negatively impacts the structure and function of the temporal lobe of the brain, a region that is heavily involved in epilepsy and mood disorders like anxiety and depression. Seizures themselves damage temporal lobe structures, further disinhibiting the HPA axis, setting off a vicious cycle of neuronal damage and increasing susceptibility for subsequent seizures and psychiatric comorbidity. Treatments targeting the HPA axis may be beneficial both for epilepsy and for associated stress-related comorbidities such as anxiety or depression. This paper will highlight the evidence demonstrating dysfunction in the HPA axis associated with epilepsy which may contribute to the comorbidity of psychiatric disorders and epilepsy, and propose treatment strategies that may dually improve seizure control as well as alleviate stress related psychiatric comorbidities.

Stress and Seizures: Space, Time and Hippocampal Circuits

Stress is a major trigger of seizures in people with epilepsy. Exposure to stress results in the release of several stress mediators throughout the brain, including the hippocampus, a region sensitive to stress and prone to seizures. Stress mediators interact with their respective receptors to produce distinct effects on the excitability of hippocampal neurons and networks. Crucially, these stress mediators and their actions exhibit unique spatiotemporal profiles, generating a complex combinatorial output with time- and space-dependent effects on hippocampal network excitability and seizure generation.

Role of allopregnanolone biosynthesis in acute stress-induced anxiety-like behaviors in mice

The neurosteroid allopregnanolone (3α, 5α-tetra-hydroprogesterone: ALLO) elicits anxiolytic, anticonvulsant, and hypnotic anesthetic effects in vivo similar to those induced by other positive allosteric modulators of the GABA_A receptor. Endogenous ALLO has been shown to be rapidly elevated in the brain by acute stress paradigms, such as immobilization, in animal models. The present study was designed to ascertain the role of neurosteroid biosynthesis in the anxiety-like behavior induced by immobilization stress. Mice were exposed to an immobilization stressor for 2 h. After 24 h, the mice that had been immobilized did not behave significantly differently in the elevated plus maze (EPM) test and in the elevated open platform (EOP) test than the mice that had not been immobilized. In contrast, finasteride-pretreated immobilization stressed mice did behave significantly differently in the EPM and EOP tests. These findings suggest that ALLO biosynthesis contributes to stress resistance. Furthermore, the ALLO mimetic drug alfaxalone appeared to antagonize the effects of finasteride by significantly changing the behavior in the EPM test or in the EOP test in finasteride (10 mg kg^-1 )-pretreated immobilized mice. In addition, alfaxalone, unlike diazepam, did not affect the muscle tone of the mice, as measured by the grip strength test. These results suggest that alfaxalone is a promising anxiolytic candidate lacking benzodiazepine-like muscle-relaxant effects.

Stress, epilepsy, and psychiatric comorbidity: how can animal models inform the clinic?

Psychiatric complaints afflict many patients with epilepsy, and these contribute significantly to the impaired quality of life experienced by sufferers of this common group of neurological conditions. Psychiatric disorders in epilepsy patients are under-diagnosed and under-treated. Moreover, evidence suggests that the psychiatric disorders may act as risk factors for some types of epilepsy and exacerbate disease progression in established cases, promoting the case for a bidirectional relationship between epilepsy and psychopathology. While cause and effect relationships can be difficult to establish in human studies, appropriate animal models provide valuable tools with which to study the interactions between epilepsy and stress-related disorders. Indeed, many epilepsy models exhibit behavioral phenotypes which are reflective of psychiatric disorders, and, conversely, stressful environments appear to promote a vulnerability to developing epilepsy. This review summarizes this research area, exploring the behavioral phenotypes in animal models of epilepsy and then examining the influence of stressful environments on susceptibility to seizures and epilepsy. The ultimate goal of this line of research is to be able to translate these findings to humans. Understanding the relationships between epilepsy and associated psychiatric disorders will facilitate effective treatment of mood disorders in epilepsy, inform about the pathophysiology of each individually, and potentially open up novel therapeutic disease-modifying strategies for patients with epilepsy.

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.

Stress and GABA receptors

GABA(A) receptors are sensitive to subtle changes in the environment in both early-life and adulthood. These neurochemical responses to stress in adulthood are sex-dependent. Acute stress induces rapid changes in GABA(A) receptors in experimental animals, with the direction of the changes varying according to the sex of the animals and the stress-paradigm studied. These rapid alterations are of particular interest as they provide an example of fast neurotransmitter system plasticity that may be mediated by stress-induced increases in neurosteroids, perhaps via effects on phosphorylation and/or receptor trafficking. Interestingly, some studies have also provided evidence for long-lasting changes in GABA(A) receptors as a result of exposure to stressors in early-life. The short- and long-term stress sensitivity of the GABAergic system implicates GABA(A) receptors in the non-genetic etiology of psychiatric illnesses such as depression and schizophrenia in which stress may be an important factor.

Stress, the hippocampus, and epilepsy

Stress is among the most frequently self-reported precipitants of seizures in patients with epilepsy. This review considers how important stress mediators like corticotropin-releasing hormone, corticosteroids, and neurosteroids could contribute to this phenomenon. Cellular effects of stress mediators in the rodent hippocampus are highlighted. Overall, corticosterone--with other stress hormones--rapidly enhances CA1/CA3 hippocampal activity shortly after stress. At the same time, corticosterone starts gene-mediated events, which enhance calcium influx several hours later. This later effect serves to normalize activity but also imposes a risk for neuronal injury if and when neurons are concurrently strongly depolarized, for example, during epileptic activity. In the dentate gyrus, stress-induced elevations in corticosteroid level are less effective in changing membrane properties such as calcium influx; here, enhanced inhibitory tone mediated through neurosteroid effects on gamma-aminobutyric acid (GABA) receptors might dominate. Under conditions of repetitive stress (e.g., caused from experiencing repetitive and unpredictable seizures) and/or early life stress, hormonal influences on the inhibitory tone, however, are diminished; instead, enhanced calcium influx and increased excitation become more important. In agreement, perinatal stress and elevated steroid levels accelerate epileptogenesis and lower seizure threshold in various animal models for epilepsy. It will be interesting to examine how curtailing the effects of stress in adults, for example, by brief treatment with antiglucocorticoids, may be beneficial to the treatment of epilepsy.

Investigation of the anticonvulsive effect of acute immobilization stress in anxious Balb/cByJ mice using GABA A-related mechanistic probes

RATIONALE

A disordered regulation of neuroactive steroids release in response to acute stress could induce GABAergic dysfunctions underlying anxiety disorders.

OBJECTIVES

First, we conducted studies indicating that a short immobilization stress in anxious Balb/cByJ mice produced an anticonvulsive effect. Second, the effects of different positive allosteric modulators (etifoxine, progesterone, clonazepam, and allopregnanolone) of GABA A receptors were compared in a mouse model mimicking the disruption of the acute stress-induced neuroactive steroids release with finasteride (types I and II 5alpha-reductase inhibitor).

RESULTS

The acute stress-induced anticonvulsive effect, expressed by the threshold dose of t-butylbicyclophosphorothionate-producing clonic seizures, was time-dependent. The extent of the enhancement of acute stress-induced anticonvulsive effect was lowered in the presence of finasteride. The same effect was observed with PK11195, which behaves as an antagonist of the peripheral benzodiazepine receptor in the dose range used in this study. Picrotoxin reduced the acute stress anticonvulsive effect, proving that this effect operates through the GABA A receptor. Contrary to progesterone (up to 30 mg/kg), etifoxine (50 mg/kg), allopregnanolone (10 mg/kg), and clonazepam (10 microg/kg) inhibited the finasteride effect in stressed animals. The effect of etifoxine was blocked in the presence of finasteride and picrotoxin combined in stressed animals.

CONCLUSIONS

These findings support the hypothesis suggesting an involvement of neuroactive steroids in the anticonvulsive effect of restraint stress. The dual and complementary mechanisms of action of etifoxine (directly on the GABA A receptor and indirectly via the neuroactive steroids) may represent a therapeutic benefit in the treatment of various anxiety disorders with abnormal production of neuroactive steroids.

The role of 5-HT7 receptors in the control of seizures

Serotonin exerts its effects via at least 14 different receptor subtypes, but the role of only a few of them has been studied in relation to the control of seizures. A negative role of 5-HT(7) receptors has recently been proposed. To evaluate further in unstressed and stressed animals the possible role of this receptor subtype in the control of brain excitability, we treated mice with antagonists and agonists of these receptors prior to exposure to swim stress and the intravenous infusion of picrotoxin, a non-competitive GABA(A) receptor antagonist. In accordance with the previous studies, swim stress increased the doses of picrotoxin producing two convulsant signs (running/bouncing clonus, tonic hindlimb extension) and death, i.e., swim stress increased the seizure threshold for picrotoxin. SB-269970 (10-30 mg/kg ip), a selective antagonist of 5-HT(7) receptors, and ritanserin (1 mg/kg ip), a nonselective 5-HT (2/7) antagonist, failed to affect, while 5-carboxamidotryptamine (5-CT), a potent 5-HT (1/7) receptor agonist, increased in unstressed and swim-stressed mice the doses of picrotoxin producing convulsions and death. The anticonvulsant effect obtained with 5-CT 0.5 mg/kg was not greater than that obtained with 0.1 mg/kg. The 5-CT (0.1 mg/kg ip)-induced increase of the seizure threshold for picrotoxin in stressed mice was abolished with SB-269970 (10 mg/kg), but not with WAY-100635 (0.3 mg/kg), a selective antagonist of 5-HT(1A) receptors, suggesting that the anticonvulsant effect of 5-CT against convulsions produced by picrotoxin was achieved via 5-HT(7) receptors. The results suggest a positive role of 5-HT(7) receptors in the control of seizures.

Zimelidine decreases seizure susceptibility in stressed mice

To further evaluate whether selective serotonin reuptake inhibitors (SSRIs) have pro- or anticonvulsant properties and whether these properties will be modified by stress, we studied the effect of zimelidine on the convulsions produced by picrotoxin, a GABA(A) receptor antagonist, in unstressed and swim stressed mice. Zimelidine potentiated the ability of swim stress to enhance the threshold doses of intravenously administered picrotoxin producing convulsant signs and death, without having an effect in unstressed mice. The anticonvulsant effect of zimelidine was counteracted with mianserin, the antagonist of 5-HT(2A/2C), and diminished with WAY-100635, a selective antagonist of 5-HT(1A) receptors. In stressed mice, WAY-100635 prevented the anticonvulsant effect of 8-OH-DPAT, a 5-HT(1A) receptor agonist. SB-269970 and ketanserin, the antagonists of 5-HT(7) and 5-HT(2A) receptors, respectively, failed to reduce the effect of zimelidine. The results suggest the involvement of 5-HT(2C) and 5-HT(1A) receptors in the anticonvulsant effects of zimelidine and possibly other SSRIs in stress.

Stimulation of 5-HT 1A receptors increases the seizure threshold for picrotoxin in mice

To evaluate the possible role of 5-HT 1A and 5-HT 2A receptors in the anticonvulsant effect of swim stress, mice were pre-treated with agonists and antagonists of these receptors prior to exposure to stress and the intravenous infusion of picrotoxin. 8-OH-DPAT ((+/-)-8-hydroxy-2-(di-n-propylamino) tetralin) and WAY-100635 (a selective agonist and antagonist of 5-HT 1A receptors), DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) and ketanserin (a 5-HT 2A/2C receptor agonist and antagonist) were used. Results demonstrated that 1 and 3 mg/kg of 8-OH-DPAT increased the doses of picrotoxin producing running/bouncing clonus, tonic hindlimb extension and death in stressed and unstressed mice, respectively. Pre-treatment with WAY (0.3 mg/kg) prevented the effect of 8-OH-DPAT (3 mg/kg). DOI (2.5 mg/kg) and ketanserin (1 mg/kg) failed to affect the seizure threshold for picrotoxin. The results show that stimulation of 5-HT 1A receptors exerts anticonvulsant actions in stressed and unstressed mice, while stimulation of 5-HT 2A/2C receptors does not interfere with the effect of stress on picrotoxin-induced convulsions.

Alterations in GABAergic function following forced swimming stress

Forced swimming induces alterations in the GABA brain concentration and could change the sensitivity of the GABA/benzodiazepine receptor-chloride ionophore complex to benzodiazepines. This change in sensitivity could be explained by the allopregnanolone release that takes place during stress. The current study was carried out to determine whether forced swimming is able to modify the anti-anxiety effect of diazepam and to explore the possible relation of this change to allopregnanolone, the GABA concentration or/and the GABA/benzodiazepine receptor density. Unstressed and stressed mice, injected with the vehicle or diazepam, were evaluated in the exploratory behavior test. Diazepam induced clear anxiolytic actions at all doses in unstressed animals, but such an effect was not observed in stressed animals. The injection of allopregnanolone 24 h before the anxiety test blocked the effect of this benzodiazepine. Forced swimming decreased GABA concentrations in the hippocampus and the thalamus-hypothalamus region, besides decreasing the [(3)H]flunitrazepam labeling in both the hypothalamus and amygdala. These results show that forced swimming abolishes the anti-anxiety effect of diazepam.

Anticonvulsant effects of acute and repeated fluoxetine treatment in unstressed and stressed mice

Comorbidity of epilepsy and depression is not rare. Stress can affect both depression and seizures. Therefore, it is important to know whether an antidepressant drug has pro- or anticonvulsant properties and whether these properties will be modified by stress. We tested the effects of the antidepressant drug fluoxetine on the seizure threshold for picrotoxin in unstressed and swim-stressed mice. The mice were, prior to exposure to swim stress and the intravenous infusion of picrotoxin (a non-competitive GABA(A) receptor antagonist), pretreated with fluoxetine (a selective serotonin reuptake inhibitor), either acutely or repeatedly (5 days), and the latency to the onset of two convulsant signs and death was registered. The convulsant signs were running/bouncing clonus and tonic hindlimb extension. As expected, swim stress enhanced the seizure threshold for picrotoxin. Fluoxetine (20 mg/kg ip) given acutely increased in unstressed and swim-stressed mice the dose of picrotoxin producing tonic hindlimb extension and in unstressed mice the dose of picrotoxin producing death. Neither 10 nor 20 mg/kg of fluoxetine affected doses of picrotoxin needed to produce running bouncing/clonus. Repeated treatment with fluoxetine (20 mg/kg ip) enhanced significantly in unstressed and swim-stressed mice doses of picrotoxin needed to produce tonic hindlimb extension and death, and in stressed mice also the dose of picrotoxin producing running/bouncing clonus. The results demonstrate that the antidepressant drug fluoxetine, given acutely or repeatedly, shows anticonvulsant properties against convulsions induced in unstressed and swim-stressed mice by antagonist of GABA(A) receptors, picrotoxin. Swim stress failed to modify the anticonvulsant properties of fluoxetine.

α2-Adrenergic inhibition prevents the accompanied anticonvulsant effect of swim stress on behavioral convulsions induced by lithium and pilocarpine

There has been much debate regarding the potential influence of stress on epilepsy. Many studies have reported that stress can affect seizure susceptibility through eliciting either proconvulsant or anticonvulsant effects within the nervous system. In this study, we investigated the potential anticonvulsant effect of a 10-min swim stress on convulsions induced by a single systemic injection of lithium chloride followed 4 h later with pilocarpine. Rats pretreated with lithium chloride and exposed to a 10-min swim stressor prior to pilocarpine injection displayed a significant delay to seizure onset compared to unstressed rats or rats exposed to swim stress 10 min after lithium chloride, 2 h after lithium chloride, or immediately after pilocarpine injection. We then determined whether administration of a glucocorticoid antagonist (mifepristone; 10 or 50 mg/kg), an alpha(2)-adrenergic antagonist (yohimbine; 2 or 5 mg/kg), or a nonspecific opioid blocker (naloxone; 0.2 or 1 mg/kg) could prevent the anticonvulsant effect of swim stress. Only the high dose of yohimbine was capable of inhibiting the anticonvulsant effect of swim stress on lithium-pilocarpine seizures. Our findings highlight the importance of an endogenous noradrenergic-dependent anticonvulsant system in mediating the effects of swim stress on seizures. Further studies exploring the benefits of treatments with noradrenergic acting drugs in epilepsy is well warranted.

Cholera toxin B decreases bicuculline seizures in prenatally morphine- and saline-exposed male rats

Prenatal morphine exposure on gestation days 11-18 alters bicuculline-induced seizures in rats during development and in adulthood. Adult, morphine-exposed male progeny exhibit an increased latency to bicuculline seizures, which can be reversed by administration of the opioid receptor antagonist naloxone. In chronically morphine-treated adult mice, cholera toxin B (CTX-B) can reverse the effects of chronic morphine administration. Therefore, the present study investigated whether prenatally morphine-exposed rats show a similar response to CTX-B as chronically morphine-treated adult rodents. Prenatally morphine-, saline- and unexposed male progeny were tested for seizure susceptibility with a 7.5-mg/kg intraperitoneal injection of bicuculline in adulthood. CTX-B or saline was injected subcutaneously at 24, 12, and 0.5 h before bicuculline injection. CTX-B decreased the occurrence of bicuculline-induced seizures in both prenatally saline- and morphine-exposed but not unexposed rats. Furthermore, three, but not one, saline injections administered at 12-h intervals prior to bicuculline administration reversed the increase in seizure latency in prenatally morphine-exposed adult males, suggesting an altered responsiveness of the stress system. The present study demonstrates that CTX-B can decrease the occurrence of bicuculline seizures in prenatally stressed rats and that increased seizure latencies in prenatally morphine-exposed male rats may be related to stress responses.

Swim stress inhibits 5-HT2A receptor-mediated head twitch behaviour in mice

RATIONALE

Several studies have shown that swim stress lowers the convulsant potency of different convulsants. The involvement of alpha(2)-()adrenoceptors has been proposed. Drugs active at alpha(2)-adrenoceptors are known to modulate the head twitch response, the behaviour supposedly mediated by 5-HT(2A) receptors.

OBJECTIVES

We tested whether swim stress modulates head twitch behaviour in mice and whether alpha(2)-adrenoceptors interfere with this effect.

METHODS

The mice were stressed (10 min swimming at 18-19 degrees C), and the head twitch response was produced by 5-hydroxytryptophan (5-HTP, the precursor of serotonin) or by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI, a selective 5-HT(2) receptor agonist) administered IP before or after swimming. Yohimbine (a non-selective alpha(2)-adrenoceptor antagonist), idazoxan (a selective alpha(2)-adrenoceptor antagonist) and diazepam were also used.

RESULTS

Swim stress inhibited profoundly the 5-HTP-induced head twitch behaviour in mice. alpha(2)-Adrenoceptor antagonists and diazepam failed to counteract this effect. The head twitch behaviour produced by DOI given before or after stress was also inhibited. Repeatedly stressed mice had only a mild inhibition of the head twitch response.

CONCLUSIONS

The results demonstrate that swim stress inhibits, by an alpha(2)-adrenoceptor unrelated mechanism, 5-HT(2A) receptor-mediated head twitch behaviour in mice, suggesting that this effect and the swim stress-induced anticonvulsant effect are produced by two separate and independent mechanisms.

Neurosteroids and infantile spasms: the deoxycorticosterone hypothesis

Deoxycorticosterone (DOC) is a mineralocorticoid precursor that has anticonvulsant properties in animals and possibly also in humans. Studies indicate that the anticonvulsant activity of DOC requires its enzymatic conversion to 5 alpha,3 alpha-tetrahydrodeoxycorticosterone (THDOC), a neurosteroid that lacks classical hormonal properties but acts as a powerful positive allosteric modulator of GABAA receptors. DOC can be considered a stress hormone because its synthesis is under the control of ACTH. Therefore, stress-induced fluctuations in seizure susceptibility could in part result from alterations in DOC availability. Also, the therapeutic activity of ACTH in infantile spasms could partially relate to its stimulatory effects on the synthesis of DOC, which then undergoes biotransformation to neurosteroids. The recent demonstration that the synthetic neurosteroid analog ganaxolone reduces spasm frequency in children with intractable infantile spasms suggests that neurosteroid-related anticonvulsants may offer a potential new nonhormonal approach for the treatment of infantile spasms and other developmental epilepsies. In addition, it further confirms the utility of pharmacological enhancement of GABA-mediated inhibition in the control of infantile spasms.

Interaction of stress and noradrenergic drugs in the control of picrotoxin-induced seizures

To evaluate the possible role of noradrenergic system in the anticonvulsant effect of swim stress, the mice were prior to exposure to swim stress and the i.v. infusion of picrotoxin, pre-treated with desipramine (a noradrenaline reuptake inhibitor), N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, a neurotoxin which destructs noradrenergic axons) or alpha-methyl-p-tyrosine (alpha-MPT, an inhibitor of catecholamine synthesis) and the latency to the onset of two convulsant signs and death was registered. While in control unstressed animals desipramine (20 mg/kg i.p.) and alpha-MPT (200 mg/kg i.p.) failed to affect, DSP-4 (50 mg/kg i.p., given 3 weeks prior to experiment) tended to decrease the dose of picrotoxin needed to produce tonic hindlimb extension (THE) and death. Swim stress prolonged the latency, i.e. increased (64-116% above control) the dose of picrotoxin needed to produce convulsant signs and death. In swim stressed mice desipramine enhanced the doses of picrotoxin needed to produce running-bouncing clonus (RB clonus), THE and death. alpha-MPT and DSP-4 pre-treatment failed to prevent the anticonvulsant effect of stress. Moreover, the effect of stress was greater in DSP-4 pre-treated mice. Although further studies are needed, the results suggest that the integrity of noradrenergic system is not substantial for the anticonvulsant effect of stress.

Stress-induced deoxycorticosterone-derived neurosteroids modulate GABA(A) receptor function and seizure susceptibility

Stress affects seizure susceptibility in animals and humans, but the underlying mechanisms are obscure. Here, we provide evidence that GABA(A) receptor-modulating neurosteroids derived from deoxycorticosterone (DOC) play a role in stress-related changes in seizure control. DOC, an adrenal steroid whose synthesis is enhanced during stress, undergoes sequential metabolic reduction by 5alpha-reductase and 3alpha-hydroxysteroid oxidoreductase to form 5alpha-dihydrodeoxycorticosterone (DHDOC) and allotetrahydrodeoxycorticosterone (THDOC), a GABA(A) receptor-modulating neurosteroid with anticonvulsant properties. Acute swim stress in rats significantly elevated plasma THDOC concentrations and raised the pentylenetetrazol (PTZ) seizure threshold. Small systemic doses of DOC produced comparable increases in THDOC and PTZ seizure threshold. Pretreatment with finasteride, a 5alpha-reductase inhibitor that blocks the conversion of DOC to DHDOC, reversed the antiseizure effects of stress. DOC also elevated plasma THDOC levels and protected mice against PTZ, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate, picrotoxin, and amygdala-kindled seizures in mice (ED50 values, 84-97 mg/kg). Finasteride reversed the antiseizure activity of DOC (ED50, 7.2 mg/kg); partial antagonism was also obtained with indomethacin (100 mg/kg), an inhibitor of 3alpha-hydroxysteroid oxidoreductase. Finasteride had no effect on seizure protection by DHDOC and THDOC, whereas indomethacin partially reversed DHDOC but not THDOC. DHDOC, like THDOC, potentiated GABA-activated Cl- currents in cultured hippocampal neurons (< or =1 microm) and directly activated GABA(A) receptor currents (> or =1 microm), compatible with a role for DHDOC in the antiseizure activity of DOC. DOC is a mediator of the physiological effects of acute stress that could contribute to stress-induced changes in seizure susceptibility through its conversion to neurosteroids with modulatory actions on GABA(A) receptors including THDOC and possibly also DHDOC.

The involvement of endogenous opioids and nitricoxidergic pathway in the anticonvulsant effects of foot-shock stress in mice

The involvement of endogenous opioids and nitric oxide (NO) in the anticonvulsant effects of stress against pentylenetetrazole (PTZ)- or electroconvulsive shock-induced seizures was assessed in mice. The prolonged and intermittent foot-shock stress, which induced opioid-mediated analgesia, had significant protective effects against both seizure types which was reversible by naloxone (0.3, 1 or 2 mg/kg), while brief and continuous foot-shock did not alter the seizure susceptibility. Pre-treatment with non-specific nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME, 1, 2, 5, 10 or 30 mg/kg), but not with specific inducible NOS (iNOS) inhibitor, aminoguanidine (50 or 100 mg/kg), blocked the stress-induced anticonvulsant effects. The lower doses of naloxone (0.3 mg/kg) and L-NAME (2 mg/kg) showed additive effects in blocking the stress-induced anticonvulsant properties. L-arginine at a per se non-effective dose of 20 mg/kg potentiated the stress-induced anticonvulsant properties, an effect which was inhibited by L-NAME but not by aminoguanidine. Furthermore, a low dose of morphine (0.5 mg/kg) showed potentiation with stress in increasing PTZ seizure threshold. This potentiation was reversed by either naloxone or L-NAME at low doses but not by aminoguanidine. Taken together, these results show that NO synthesis, through constitutive but not iNOS, is involved in opioid-dependent stress-induced anticonvulsant effects against electrical and PTZ-induced convulsions.

Swim stress alters the behavioural response of mice to GABA-related and some GABA-unrelated convulsants

To elucidate the relationship between stress and seizures, the effect of a single swim stress on the convulsive signs and death produced by several GABA-related and GABA-unrelated convulsants, and the effect of repeated swim stress on picrotoxin-induced convulsions was studied. Mice were subjected to swim stress (10 min swimming at 18-19 degrees C), and the i.v. infusion of convulsants started 15 min thereafter. The latency to the onset of several convulsant signs and death was measured, and the doses of convulsants producing convulsions and death were calculated. Additional experiments included mice swimming at room temperature, and those which were stressed repeatedly (twice a day for four consecutive days, plus one stressful procedure on the fifth day). Swim stress increased the dose needed to produce convulsant signs and death after bicuculline, picrotoxin, pentylenetetrazole, strychnine and 4-aminopyridine, while kainic acid-induced convulsions were not affected. Using picrotoxin infusion, the effect of swimming in room temperature water was less than the effect of swimming in 18-19 degrees C water. In addition, the effect of repeated stress was less than the effect of acute stress on picrotoxin-induced convulsions. The results demonstrate that acute swim stress lowers the convulsive potency of GABA-related and some GABA-unrelated convulsants. Repeatedly stressed animals develop tolerance to anticonvulsive effect of swim stress.

Beta-1 adrenoceptor antagonists potentiate the anticonvulsive effect of swim stress in mice

To explore the possible involvement of beta adrenoceptor antagonists in the previously observed anticonvulsive effect of swim stress, the mice were, prior to administration of convulsants, pre-treated with propranolol (a non-selective beta adrenoceptor antagonist), betaxolol (a selective beta-1 adrenoreceptor antagonist), or ICI 118,551 (a selective beta-2 adrenoreceptor antagonist). In control unstressed animals, only propranolol [10 mg/kg, intraperitoneally (ip)] produced a significant change. It enhanced the threshold dose of picrotoxin producing tonic hindlimb extension. However, in swim-stressed animals, propranolol enhanced doses of picrotoxin producing tonic hindlimb extension and death, while betaxolol (20 mg/kg, i.p.) enhanced doses of picrotoxin producing running/bouncing clonus, tonic hindlimb extension and death. Pre-treatment with ICI 118,551 (4 mg/kg, i.p.) failed to affect doses of picrotoxin producing convulsions and death. The results demonstrate that blockade of beta-1 adrenoceptors potentiates the anticonvulsant effect of swim stress against convulsions produced by picrotoxin, a noncompetitive GABA(A) receptor antagonist.

Anticonvulsive effect of swim stress in mice

To explore the possible involvement of glucocorticoids in the previously observed anticonvulsive effect of swim stress, mice were, prior to administration of convulsants, subjected to treatments that diminish or enhance plasma corticosterone levels. Aminoglutethimide, the inhibitor of steroid synthesis, failed to modify convulsant doses of picrotoxin, but enhanced threshold doses of pentylenetetrazole producing myoclonus and death, both in unstressed and stressed animals. The same drug prevented the effect of stress on pentylenetetrazole-induced running bouncing clonus (RB clonus) and abolished the appearance of tonic hindlimb extension (THE). Doses of kainic acid producing convulsions and death were not affected by stress, but they were enhanced by aminoglutethimide. Corticosterone administration could not imitate the effect of swim stress. Finasteride, a 5 alpha-reductase inhibitor, did not interfere with the effect of stress on picrotoxin-induced convulsions. Swim stress failed to modify the binding of the convulsant t[3H]-butylbicycloorthobenzoate [3H]TBOB, to washed mouse forebrain membranes. The results confirmed an anticonvulsant effect of swim stress against convulsions produced by GABA-related convulsants, but they do not support the hypothesis suggesting the involvement of glucocorticoids or neurosteroids in this effect.

Alpha-adrenergic receptors mediate imipramine/alarm substance-induced reaction in rats

The mechanism of adverse imipramine-induced reactions (jitteriness, convulsions) was investigated by precipitating such reactions in rats with three injections (IP) of imipramine (5-40 mg/kg) at 24, 5, and 1 h before testing, and comparing their occurrence with comparable treatments using specific noradrenergic and serotonergic reuptake inhibitors [nortriptyline (10 or 30 mg/kg, IP), citalopram (0.5-5.0 mg/kg, IP)]. This initial study indicated that these reactions were mediated by imipramine's noradrenergic effects. Subsequent combinations of imipramine and an alpha 2 agonist (clonidine, 5 mg/kg) and antagonist (yohimbine, 2 mg/kg), and a beta-adrenergic antagonist (propranolol, 2 or 5 mg/kg) (all administered IP 0.5 h after the last injection of imipramine) suggested imipramine's adverse effects were mediated by alpha 2 receptors. The possible involvement of the locus ceruleus in these effects was considered.

Physiological correlates of the forced swim test in rats

Rats were tested in the forced swim test to evaluate the effects of duration of exposure (0, 5, 15, or 25 min), and water temperature (0, 35, 30, 25, or 20 degrees C), on a variety of physiological measures. Serum corticosterone, glucose, lactate, phosphorus levels, and the anion gap (a measure of acid-base status) were increased significantly, whereas carbon dioxide and potassium levels were consistently decreased by testing, as was the potassium/phosphorus ratio; creatinine, triglycerides, and magnesium were not altered significantly in any study. Effects on prolactin, amylase, lipase, cholesterol, alkaline phosphatase, sodium, and chloride were inconsistent. Levels of serum corticosterone were increased at each duration of testing, and the increments were significantly higher than the previous duration. Corticosterone levels were also increased in proportion to decreasing water temperature, but the increments were not significantly different from the previous or following temperatures. Glucose levels were increased at every duration and at every water temperature with the exception of the coldest water temperature. Lactate and phosphorus levels and the anion gap were all increased, whereas carbon dioxide levels decreased after 5 min of immersion. Potassium levels did not decrease until some time after 5 min of testing. Immobility times were marginally correlated with corticosterone levels (r = -0.38) but were highly correlated with serum carbon dioxide (r = 0.59), potassium (r = 0.67), and phosphorus levels (r = -0.73); the correlation between immobility times and the ratio of potassium/phosphorus was 0.82. The potassium/phosphorus ratio accounted for 67% of the variance in immobility. These high correlations were interpreted in terms of acid-base changes associated with testing.(ABSTRACT TRUNCATED AT 250 WORDS)