Pilocarpine-induced convulsions in rats: evidence for muscarinic receptor-mediated activation of locus coeruleus and norepinephrine release in cholinolytic seizure development

Frontier of epilepsy research - mTOR signaling pathway

Studies of epilepsy have mainly focused on the membrane proteins that control neuronal excitability. Recently, attention has been shifting to intracellular proteins and their interactions, signaling cascades and feedback regulation as they relate to epilepsy. The mTOR (mammalian target of rapamycin) signal transduction pathway, especially, has been suggested to play an important role in this regard. These pathways are involved in major physiological processes as well as in numerous pathological conditions. Here, involvement of the mTOR pathway in epilepsy will be reviewed by presenting; an overview of the pathway, a brief description of key signaling molecules, a summary of independent reports and possible implications of abnormalities of those molecules in epilepsy, a discussion of the lack of experimental data, and questions raised for the understanding its epileptogenic mechanism.

Noradrenergic induction of odor-specific neural habituation and olfactory memories

For many mammals, individual recognition of conspecifics relies on olfactory cues. Certain individual recognition memories are thought to be stored when conspecific odor cues coincide with surges of noradrenaline (NA) triggered by intensely arousing social events. Such familiar stimuli elicit reduced behavioral responses, a change likely related to NA-dependent plasticity in the olfactory bulb (OB). In addition to its role in these ethological memories, NA signaling in the OB appears to be relevant for the discrimination of more arbitrary odorants as well. Nonetheless, no NA-gated mechanism of long-term plasticity in the OB has ever been directly observed in vivo. Here, we report that NA release from locus ceruleus (LC), when coupled to odor presentation, acts locally in the main OB to cause a specific long-lasting suppression of responses to paired odors. These effects were observed for both food odors and urine, an important social recognition cue. Moreover, in subsequent behavioral tests, mice exhibited habituation to paired urine stimuli, suggesting that this LC-mediated olfactory neural plasticity, induced under anesthesia, can store an individual recognition memory that is observable after recovery.

The developmental decrease in REM sleep: the role of transmitters and electrical coupling

STUDY OBJECTIVES

This mini-review considers certain factors related to the developmental decrease in rapid eye movement (REM) sleep, which occurs in favor of additional waking time, and its relationship to developmental factors that may influence its potential role in brain development.

DESIGN

Specifically, we discuss some of the theories proposed for the occurrence of REM sleep and agree with the classic notion that REM sleep is, at the least, a mechanism that may play a role in the maturation of thalamocortical pathways. The developmental decrease in REM sleep occurs gradually from birth until close to puberty in the human, and in other mammals it is brief and coincides with eye and ear opening and the beginning of massive exogenous activation. Therefore, the purported role for REM sleep may change to involve a number of other functions with age.

MEASUREMENTS AND RESULTS

We describe recent findings showing that morphologic and physiologic properties as well as cholinergic, gamma amino-butyric acid, kainic acid, n-methyl-d-aspartic acid, noradrenergic, and serotonergic synaptic inputs to mesopontine cholinergic neurons, as well as the degree of electrical coupling between mostly noncholinergic mesopontine neurons and levels of the neuronal gap-junction protein connexin 36, change dramatically during this critical period in development. A novel mechanism for sleep-wake control based on well-known transmitter interactions, as well as electrical coupling, is described.

CONCLUSION

We hypothesize that a dysregulation of this process could result in life-long disturbances in arousal and REM sleep drive, leading to hypervigilance or hypovigilance such as that observed in a number of disorders that have a mostly postpubertal age of onset.

Study pharmacologic of the GABAergic and glutamatergic drugs on seizures and status epilepticus induced by pilocarpine in adult Wistar rats

This work was designed to study the influence of drugs during seizures and status epilepticus (SE) induced by pilocarpine and mortality in adult rats. Glutamate (10 and 20 mg/kg), N-methyl-d-aspartate (NMDA, 5 and 10 mg/kg), ketamine (1.5 and 2.0 mg/kg), gabapentin (200 and 250 mg/kg), phenobarbital (50 and 100 mg/kg) and vigabatrin (250 and 500 mg/kg) were administered intraperitoneally, 30 min prior to pilocarpine (400 mg/kg, i.p.). The animals were observed (24 h) to determine: number of peripheral cholinergic signs, tremors, stereotyped movements, seizures, SE, latency to first seizure and number of deaths after pilocarpine treatment. NMDA and glutamate had pro-convulsive effects in both doses tested. Smaller and higher doses of these drugs no protected and increased pilocarpine-induced seizures and/or mortality. Gabapentin, vigabatrin, phenobarbital and ketamine protected against seizures and increased the latency to first seizure. Thus, these results suggest that caution should be taken in the selection of pharmacotherapy and dosages for patients with seizures and SE because of the possibility of facility the convulsive process toxicity, SE and the mortality of adult animals in this seizures model that is similar temporal lobo epilepsy in humans.

Effect of gabaergic, glutamatergic, antipsychotic and antidepressant drugs on pilocarpine-induced seizures and status epilepticus

This work was designed to study the influence of drugs during seizures and status epilepticus (SE) induced by pilocarpine and mortality in adult rats. Fluoxetine (10 and 20 mg/kg), NMDA (N-methyl-D-aspartate, 10 and 20 mg/kg), amitriptyline (25 and 50 mg/kg), ketamine (0.5 and 1.0 mg/kg), gabapentin (100 and 150 mg/kg) and pimozide (10 and 20 mg/kg) were administered intraperitoneally, 30 min prior to pilocarpine (400mg/kg, s.c.). The animals were observed (24h) to determine: number of peripheral cholinergic signs, tremors, stereotyped movements, seizures, SE, latency to first seizure and number of deaths after pilocarpine treatment. Fluoxetine, amitriptyline, NMDA, and pimozide had proconvulsant effects in both doses tested. Smaller and higher doses of these drugs no protected and increased pilocarpine-induced seizures and/or mortality. Gabapentin and ketamine protected against seizures and reduced the latency to first seizure. Thus, these results suggest that caution should be taken in the selection of pharmacotherapy and dosages for patients with epilepsy because of the possibility of potentiating convulsive process toxicity.

Differential effects of unilateral olfactory deprivation on noradrenergic and cholinergic systems in the main olfactory bulb of the rat

The lack of environmental olfactory stimulation produced by sensory deprivation causes significant changes in the deprived olfactory bulb. Olfactory transmission in the main olfactory bulb (MOB) is strongly modulated by centrifugal systems. The present report examines the effects of unilateral deprivation on the noradrenergic and cholinergic centrifugal systems innervating the MOB. The morphology, distribution, and density of positive axons were studied in the MOBs of control and deprived rats, using dopamine-beta-hydroxylase (DBH)-immunohistochemistry and acetylcholinesterase (AChE) histochemistry in serial sections. Catecholamine content was compared among the different groups of MOBs (control, contralateral, and ipsilateral to the deprivation) using high-performance liquid chromatography analysis. Sensory deprivation revealed that the noradrenergic system developed adaptive plastic changes after olfactory deprivation, including important modifications in its fiber density and distribution, while no differences in cholinergic innervation were observed under the same conditions. The noradrenergic system underwent an important alteration in the glomerular layer, in which some glomeruli showed a dense noradrenergic innervation that was not detected in control animals. The DBH-positive glomeruli with the highest noradrenergic fiber density were compared with AChE-stained sections and it was observed that the strongly noradrenergic-innervated glomeruli were always atypical glomeruli (characterized by their strong degree of cholinergic innervation). In addition to the morphological findings, our biochemical data revealed that olfactory deprivation caused a decrease in the content of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid in the ipsilateral MOB in comparison to the contralateral and control MOBs, together with an increase in noradrenaline levels in both the ipsilateral and contralateral MOBs. Our results show that regulation of the noradrenergic centrifugal system in the MOB depends on environmental olfactory stimulation and that it is highly reactive to sensory deprivation. By contrast, the cholinergic system is fairly stable and does not exhibit clear changes after the loss of sensory inputs.

Pro-oxidant activity of zuclopenthixol in vivo: differential effect of the drug on brain oxidative status of scopolamine-treated rats

Several clinical studies implicated oxidative stress in the pathophysiology of both psychosis and dementia. As dementia is commonly associated with psychosis, antipsychotic medications are of importance in the pharmacotherapy of dementia particularly as a number of antipsychotics were reported to demonstrate neuronal pro-oxidant and/or antioxidant properties. Impairment of learning and memory, as the most characteristic manifestation of dementia, could be induced in experimental animals by acute administration of scopolamine (SCO) with a resultant elevation in brain oxidative status. This study investigated the potential pro-oxidant and/or antioxidant activity of the antipsychotic drug zuclopenthixol acetate, as its effect on brain oxidative status has yet to be evaluated. A 2 x 3 between-subjects factorial design was used to investigate the simultaneous and interactive effects of zuclopenthixol (0.7 and 1.4 mg/kg i.p.) and SCO on rat brain malondialdehyde, glutathione, glutathione peroxidase and superoxide dismutase levels/activities. Results revealed a significant pro-oxidant effect for both zuclopenthixol and SCO alone conditions. In addition, combined treatment of zuclopenthixol and SCO was found to be significantly different compared to either treatment conditions with regard to their effect on different brain oxidative stress indices. Such findings may have valuable implications in the pharmacotherapy of both psychosis and dementia.

Nerve agents: a comprehensive review

Nerve agents are perhaps the most feared of potential agents of chemical attack. The authors review the history, physical characteristics, pharmacology, clinical effects, and treatment of these agents.

[Acute alterations of neurotransmitters levels in striatum of young rat after pilocarpine-induced status epilepticus]

High doses of the muscarinic cholinergic agonist, pilocarpine, result in behavioural changes, seizures and status epilepticus in rats. The purpose of the present work is to invetigate the striatal neurotransmissors level in young rats after status epilepticus induced by pilocarpine. Wistar rats were treated with a single dose of pilocarpine (400mg/Kg; s.c.). Controls received saline. Young animals were closed observed for behavioural changes during 1 and 24h. In these periods, the animals that developed status epilepticus and didn't survive this acute phase of seizures had the brains removed and striatal neurotransmissors level determined by HPLC. The concentration of dopamine, serotonine, dihydroxyphenylacetic acid, 5-hydroxyindolacetic acid was reduced and an increase in 4-hydroxy-3-methoxy-phenylacetic acid was observed. These results suggest that cholinergic activation can interage with dopaminergic and serotonergic systems in acute phase of the convulsive process in immature striatum.

Locus coeruleus involvement in the effects of immobilization stress on the p13 midlatency auditory evoked potential in the rat

1. Adult male rats were prepared for recording midlatency auditory evoked responses from the vertex (Vx, P13 potential) and auditory cortex (ACx, P7 potential). 2. The P13 potential is the rodent equivalent of the human P1 or P50 potential, which exhibits decreased sensory gating in posttraumatic stress disorder. 3. Immobilization (IMB) stress for 60 min led to a significant decrease in P13 potential amplitude and sensory gating of the potential for the first 30-40 min of IMB. 4. The effects of IMB on the P13 potential were reduced by pre-treatment with the alpha-2 adrenergic receptor blocker yohimbine (YOH). 5. Injections of corticotropin releasing factor (CRF) into the locus coeruleus (LC), but not injections dorsal or ventral to the LC, induced a dose-dependent decrease in P13 potential amplitude and sensory gating. 6. The effects of CRF were blocked by cotreatment with the CRF receptor antagonist alpha-helical CRF (alpha-h CRF). 7. The effects of IMB on the P13 potential were mimicked by injections of the alpha-2 adrenergic receptor agonist dexmedetomidine (DEX) into the pedunculopontine nucleus (PPN). 8. The effects of DEX injections into PPN were reduced by pre-treatment with the alpha-2 adrenergic receptor blocker YOH. 9. The effects of IMB on P13 potential amplitude and sensory gating may be mediated in part via CRF activation of LC, which sends inhibitory alpha-2 adrenergic projections to PPN, a major source of the P13 potential.

Evidence for cholinergic regulation of basal norepinephrine release in the rat olfactory bulb

The effects of locally infused cholinergic agonists on extracellular levels of norepinephrine in the olfactory bulb of anesthetized rats were determined using in vivo microdialysis coupled with high-performance liquid chromatography and electrochemical detection. Using chronically implanted microdialysis probes, the basal norepinephrine level in the olfactory bulb was 0.55 pg/10 microl dialysate. Local infusion of K+ (30 mM) or the norepinephrine re-uptake inhibitor desipramine (1 microM) through the dialysis probe significantly increased basal norepinephrine levels. Focal activation of noradrenergic locus coeruleus neurons, the sole source of norepinephrine innervation of the olfactory bulb, increased norepinephrine levels by 247% of control. Local infusion of the acetylcholinesterase inhibitor soman (0.4 mM) into the olfactory bulb increased basal norepinephrine levels by 134% of control, suggesting that endogenously released acetylcholine modulates norepinephrine release. Intrabulbar infusion of acetylcholine (40 mM) or nicotine (40 mM) increased norepinephrine levels (317% and 178% of control, respectively), while infusion of the muscarinic receptor agonist pilocarpine (40 mM) reduced norepinephrine levels (54% of control). These results demonstrate that basal norepinephrine release in the olfactory bulb is potently modulated by stimulation of local cholinergic receptors. Nicotinic receptors stimulate, and muscarinic receptors inhibit, norepinephrine release from locus coeruleus terminals.

Pilocarpine treatments differentially affect alpha2-adrenoceptors which modulate the firing rate of locus coeruleus neurones and the synthesis and release of noradrenaline in rat brain

We have investigated the effect of treatments with the muscarinic acetylcholine receptor agonist, pilocarpine, on the sensitivity of central alpha2-adrenoceptors that regulate the firing activity of rat locus coeruleus, the tyrosine hydroxylase activity in the rat cortex, hippocampus and hypothalamus, and the K(+)-evoked release of [3H]noradrenaline from rat cortical and hippocampal synaptosomes. Short-term (4 days), but not acute, treatment with pilocarpine caused a small but statistically significant increase in the inhibitory effect of the alpha2-adrenoceptor agonist clonidine on the firing rate of locus coeruleus neurones, with a decrease in the ED50 of 290% (P<0.001). However, no change in the effect of clonidine on the locus coeruleus was observed after longer pilocarpine (11 days) treatment. In the rat cerebral cortex, but not in hippocampus or hypothalamus, chronic (19 days) treatment with pilocarpine caused a decrease in the inhibitory effect of clonidine on tyrosine hydroxylase activity (55%, P<0.05), but did not change the stimulatory effect of the alpha2-adrenoceptor antagonist idazoxan. Moreover, treatments (4, 11 and 19 days) with pilocarpine did not alter the inhibitory effect of clonidine [10(-8)-10(-5) M] on the K(+)-evoked release of [3H]noradrenaline from rat cortical and hippocampal synaptosomes. These results indicate that administration of pilocarpine slightly potentiates some but not all the functional responses mediated by brain presynaptic alpha2-adrenoceptors. In conclusion, these results do not support the hypothesis that chronic treatments with pilocarpine lead to a suitable model of alpha2-adrenoceptor supersensitivity.

Effects of soman-induced convulsions on phosphoinositide metabolism

Turnover of [3H]phosphoinositides (PI) was examined in brain slices from the hippocampus of rats undergoing soman-induced seizure activity. Hydrolysis of PI was determined by measuring the accumulation of [3H]inositol-1-phosphate (IP1). Incubation of hippocampal slices in the presence of carbachol or norepinephrine (NE) increased PI hydrolysis. Stimulated hydrolysis by NE, but not carbachol was significantly reduced in slices from soman-challenged rats undergoing convulsive activity. NE-stimulated PI hydrolysis was not reduced in slices from animals exposed to soman that did not exhibit convulsive activity. In rats surviving for 24 h, the response to NE was not different from control rats. In control slices, NE-stimulated hydrolysis of PI was potentiated by GABA. No potentiation by GABA was seen in slices from animals undergoing seizures. Uptake and incorporation of myo-[2-3H]inositol into phospholipids was reduced in slices from rats undergoing convulsions. Reduced IP1 production appeared to be owing, in part, to decreased synthesis of inositol lipids. These observations suggest that during soman-induced seizure activity, there is an apparent decrease in the response of the PI second messenger system to NE stimulation, and that this may contribute to the severity and duration of convulsions and brain damage resulting from exposure to soman and other anticholinesterase compounds.

Disorders of the reticular activating system

The organization of components of the reticular activating system and their role in sleep-wake mechanisms and arousal are described. A functional model is proposed based on known neuroanatomical and neurophysiological findings. The involvement of these elements of the reticular activating system in various neurological and psychiatric disorders is discussed. A series of hypotheses are advanced to account for the role of these nuclei in such diverse disorders as schizophrenia, post-traumatic stress disorder, REM behavior disorder, Parkinson's disease and narcolepsy. This line of reasoning suggests that, when neurological or psychiatric disorders manifest symptoms related to arousal and sleep-wake control, disturbances of elements of the reticular activating system must be considered responsible.

Scopolamine-induced convulsions in food given fasted mice: effects of physostigmine and MK-801

We recently reported that scopolamine pretreated mice fasted for 48 h developed clonic convulsions soon after they were allowed to eat a small amount of food for 30 s. The present experiments were performed to determine whether animals also develop convulsions when they were allowed to eat ad libitum and to find some evidence for the contribution of the cholinergic and/or glutamatergic systems in the underlying mechanism(s) of convulsions. Animals fasted for 48 h were treated with 3 mg/kg scopolamine or saline. Twenty minutes later, they were allowed to eat either ad libitum or a small portion of food for 30 s. Scopolamine pretreated animals after starting to eat ad libitum or a small amount in a restricted time developed convulsions in a few minutes, the incidence being 76 and 54%, respectively. Pretreatment of 0.17 mg/kg MK-801, the noncompetitive NMDA antagonist, decreased the incidence of scopolamine-induced convulsions (22%) without affecting latency to the onset of seizures. Pretreatment of 0.1 mg/kg physostigmine, the cholinesterase inhibitor, changed neither the incidence (90%) nor latency to the onset of scopolamine-induced convulsions.

Effect of pentylenetetrazol on the expression of tyrosine hydroxylase mRNA and norepinephrine and dopamine transporter mRNA

Seizure activity has been shown to have differential effects on the terminal content of the monoamines, norepinephrine (NE) and dopamine (DA). Induction of seizure activity reduces the terminal content of NE, while DA levels remain unchanged or slightly elevated. This study examined the effect of the chemoconvulsant pentylenetetrazol (PTZ) on the mRNA expression of regulatory proteins which maintain the terminal content of NE and DA (i.e., synthesis and re-uptake). The areas examined were the noradrenergic neurons of the locus coeruleus (LC) and dopaminergic neurons of the substantia nigra pars compacta/ventral tegmentum area (SNpc/VTA) in the rat. In the LC, PTZ increased mRNA expression of the immediate early gene, c-fos, and mRNA expression of the synthesizing enzyme, tyrosine hydroxylase (TH), and the re-uptake protein, norepinephrine transporter (NET). This effect on TH and NET was observed only 1 day after the administration of PTZ. In contrast, PTZ did not alter the expression of c-fos mRNA in the SNpc/VTA, but reduced the expression of the dopamine transporter (DAT) mRNA. This effect was observed only 1 day after the administration of PTZ. TH mRNA expression in dopaminergic neurons was elevated initially in a manner similar to that observed in the LC. However, the effect of PTZ on TH mRNA expression in dopaminergic neurons was more prolonged (still elevated 3 days later). These results indicate that the chemoconvulsant PTZ has differential effects on the mRNA expression of regulatory systems (TH and neurotransporter proteins) in noradrenergic and dopaminergic neurons.

Alterations in mRNA expression of systems that regulate neurotransmitter synaptic content in seizure-naive genetically epilepsy-prone rat (GEPR): transporter proteins and rate-limiting synthesizing enzymes for norepinephrine, dopamine and serotonin

Two models of genetically epilepsy-prone rat (GEPR) exist, the GEPR-3 and GEPR-9, GEPR-3 and GEPR-9 share a deficiency in presynaptic norepinephrine (NE) and serotonin (5HT) content in specific regions of the central nervous system (CNS). The presynaptic content of dopamine (DA) does not appear to be altered in either adult GEPR strain compared to Sprague-Dawley (SD) rats, the strain from which the GEPR was derived. Presynaptic content of monoamine neurotransmitters, such as NE, 5HT and DA, are maintained by several regulatory proteins which include: synthesis, re-uptake, release, degradation and vesicular transport. To further characterize the monoamine deficiency observed in the GEPR, the mRNA level of the rate limiting enzymes for the synthesis of NE, 5HT and DA and each of the neurotransporter proteins were measured in seizure-naive GEPR-3, GEPR-9 and SD rats. In the locus coeruleus (LC), the major noradrenergic locus, tyrosine hydroxylase (TH) mRNA level was significantly reduced only in GEPR-9 animals compared to SD rats and GEPR-3, while NE transporter (NET) mRNA was significantly elevated in GEPR-3 compared to SD rats and GEPR-9. TH and DA transporter (DAT) mRNA was measured in the dopaminergic neurons of the substantia nigra pars compacta (SNpc), ventral tegmental area (VTA) and zona incerta (ZI), DAT mRNA level was significantly reduced in all dopaminergic neurons in the GEPR-3 compared to SD rats and GEPR-9, while TH mRNA level was significantly elevated in the SNpc/VTA equally in GEPR-3 and GEPR-9 compared to SD rats. In the ZI, TH mRNA level was significantly reduced in GEPR-3 compared to SD rats and GEPR-9. In the dorsal raphe (DR), a major serotonergic locus, tryptophan hydroxylase (TRH) mRNA level was not significantly different from SD in either strain of GEPR; however, 5HT transporter (SERT) mRNA level was significantly reduced in GEPR-9 in the dorsal and lateral regions of the DR compared in SD rats and GEPR-3. These data indicate that two of the regulatory systems that maintain NE, 5HT and DA content are altered in a differential manner in seizure-naive GEPR-3 compared to seizure-naive GEPR-9, with GEPR-3 showing more alterations in dopaminergic neurons. It is uncertain at the present time how these alterations in mRNA level relate to the enhanced seizure susceptibility of these animals. It was apparent that a straightforward correlation between neurotransmitter loss to transcriptional changes in synthesizing enzymes mRNA or to re-uptake protein mRNA was not observed in noradrenergic and serotonergic neurons. Therefore, the decrease in presynaptic NE and 5HT tissue content in these animals may be due to posttranscriptional modification. In contrast, presynaptic DA tissue content which was unaltered in both strains of GEPR, shows an alteration in TH and DAT mRNA level compared to SD rats in all dopaminergic neurons examined. This indicates a possible involvement of DA in regulating the seizure susceptibility of these animals.

Activation of locus coeruleus enhances the responses of olfactory bulb mitral cells to weak olfactory nerve input

The main olfactory bulb (MOB) receives a dense projection from the pontine nucleus locus coeruleus (LC), the largest collection of norepinephrine (NE)-containing cells in the brain. LC is the sole source of NE innervation of MOB. Previous studies of the actions of exogenously applied NE on mitral cells, the principal output neurons of MOB, are contradictory. The effect of synaptically released NE on mitral cell activity is not known, nor is the influence of NE on responses of mitral cells to olfactory nerve inputs. The goal of the present study was to assess the influence of LC activation on spontaneous and olfactory nerve-evoked activity of mitral cells. In methoxyflurane-anesthetized rats, intracoerulear microinfusions of acetyicholine (ACh) (200 mM; 90-120 nl) evoked a four- to fivefold increase in LC neuronal discharge, and a transient EEG desynchronization and decrease in mitral cell discharge. LC activation increased excitatory responses of mitral cells evoked by weak (i.e., perithreshold) nasal epithelium shocks (1.0 Hz) in 17/18 cells (mean Increase = 67%). The discharge rate of mitral cells at the time that epithelium-evoked responses were increased did not differ significantly from pre-LC activation baseline values. Thus, changes in mitral baseline activity do not account for the increased response to epithelium stimulation. These findings suggest that increased activity in LC-NE projections to MOB may enhance detection of relatively weak odors.

The effect of the selective reversible acetylcholinesterase inhibitor E2020 on extracellular acetylcholine and biogenic amine levels in rat cortex

E2020 is a piperidine cholinesterase inhibitor (ChEI) which is structurally distinct from other compounds presently under study for treatment of Alzheimer's disease. We studied the effect of this compound on acetylcholine (ACh), norepinephrine (NE), dopamine (DA) and serotonin (5-HT; 5-hydroxytryptamine) by means of transcortical microdialysis in the cortex of awake rats with no ChEI in the probe. We also compared the inhibition of brain cholinesterase (ChE) by two different approaches. Following 0.5 and 2.0 mg/kg s.c. administration, the increase in ACh was 200% (30 min) and 2100% (1 hr), respectively. The maximal ChE inhibition at 30 min was 35.5% (2.0 mg/kg) and 15.6% (0.5 mg/kg) when measured as ACh hydrolysis in the diluted homogenate. After the 2.0 mg/kg dose, phosphorylation by DFP was completely blocked at 30 min. After 0.5 mg/kg, ChE phosphorylation was maximally inhibited at 30 min (56%) and declined thereafter to negligible levels by 3 hr. In addition, E2020 increased extracellular levels of catecholamines in cortex in agreement with our previous findings with carbamate ChEI. Following 2.0 mg/kg, both NE (100%) and DA (80%) were elevated, whereas after 0.5 mg/kg only NE (50%) was affected. Neither dose affected extracellular 5-HT. Thus, E2020, which inhibits brain ChE by a novel, reversible mechanism, elevates extracellular ACh in a comparable manner to other centrally-active ChEI, and this elevation of ACh is associated with stimulation of catecholamine release.

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

Regional changes in brain dopamine utilization during status epilepticus in the rat induced by systemic pilocarpine and intrahippocampal carbachol

Systemic administration of pilocarpine (400 mg/kg i.p.) or intrahippocampal injection of carbachol (100 micrograms/1 microliters) induced limbic motor seizures in rats, characterized by head weaving and paw treading, rearing and falling, and forepaw myoclonus, developing into status epilepticus. After being in status for 30 min, rats were killed and levels of dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined in eight brain regions by high performance liquid chromatography. Pilocarpine-induced seizures significantly elevated dopamine in the striatum, and in both dorsal and ventral aspects of the hippocampus, but did not affect dopamine in substantia nigra, nucleus accumbens, olfactory tubercle, cingulate cortex or amygdala. Metabolite levels were increased in striatum, substantia, nigra, nucleus accumbens and cingulate cortex, and fell in the hippocampus, but remained unchanged in the olfactory tubercle and amygdala. Intrahippocampal carbachol significantly raised the dopamine contents of striatum and nigra, and in both ventral and dorsal aspects of the hippocampus, but not elsewhere. DOPAC and/or HVA were elevated in all brain regions tested, save for amygdala and dorsal hippocampus. These changes translated into seizure-induced increases in dopamine utilization in the nucleus accumbens, olfactory tubercle and cingulate cortex, and to a fall in dopamine utilisation in the hippocampus, with no net change in amygdala. In addition pilocarpine (but not carbachol) increased dopamine utilization in the nigrostriatal axis, possibly through a seizure-unrelated mechanism. The relevance of these findings to seizure development are discussed.

Dendrites of locus coeruleus neurons extend preferentially into two pericoerulear zones

The intrinsic cytoarchitecture and neurochemical organization of the nucleus locus coeruleus have been characterized extensively, but there is little information about the organization of locus coeruleus neuronal processes extending outside of the nucleus proper. Light and electron microscopic immunocytochemical techniques were used to investigate the distribution of dopamine-beta-hydroxylase- or tyrosine-hydroxylase-labeled extranuclear processes in the rat pericoerulear region. The vast majority of these processes extended preferentially into two zones: (1) the pontine tegmentum medial and rostral to locus coeruleus, here termed the rostromedial pericoerulear region; and (2) a narrow region adjacent to the IVth ventricle caudomedial to locus coeruleus, designated here as the caudal juxtaependymal pericoerulear region. Far fewer labeled processes extended into the lateral and ventral pericoerulear regions. Seventy-seven percent of the labeled profiles in the pericoerulear region were dendrites. All labeled profiles in the rostromedial pericoerulear region and 94% of the labeled profiles in the caudal juxtaependymal zone were dendrites. By contrast, in the rostroventral pericoerulear region, 25% of the labeled profiles were axons. Locus coeruleus extranuclear dendrites were never presynaptic to other structures but were often contacted by several unlabeled presynaptic terminals. These results indicate that the dendrites of locus coeruleus neurons extend preferentially into two pericoerulear zones. Extranuclear dendrites in all pericoerulear regions receive extensive, nonnoradrenergic synaptic contacts. Thus, pericoerulear dendrites, particularly in the rostromedial and caudal juxtaependymal zones, are important sites for the integration of inputs to locus coeruleus neurons.

The pedunculopontine nucleus--auditory input, arousal and pathophysiology

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

Massive activation of c-fos in forebrain after mechanical stimulation of the locus coeruleus

Brief implantation of a 33-ga cannula in the locus coeruleus (LC) of the rat caused widespread and intense ipsilateral activation of c-fos throughout the forebrain. Areas showing heavy staining included the cingulate, piriform, parietal, frontal cortex, and the olfactory tubercle. Prior lesion of the LC with 6-hydroxydopamine (6-OHDA) abolished the response. It is concluded that the mechanical stimulation and/or trauma involved in the implantation of a cannula in the LC is sufficient to cause widespread activation of noradrenergic neurotransmission throughout the forebrain. The use of this procedure for drug delivery should therefore be reevaluated.