Biochemical evidence of decreased muscarinic cholinergic neuronal communication following amygdala-kindled seizures

Update on the pathophysiology of the epilepsies

The pathophysiology of convulsive and non-convulsive epilepsies is discussed in its primary generalised forms. Focal, clinical and experimental epilepsies, with emphasis placed on the temporal lobe epilepsies (TLE) and their pathophysiologies are also reviewed. Neurotransmitters and neuromodulators and between them, the second messenger systems are considered in the generation, maintenance or inhibition of the epileptic discharge. Action mechanisms of the more classic antiepileptic drugs are briefly summarized along with the therapeutic strategies that might achieve the final control of abnormal discharges, including genetic control as a promising alternative in the current state of research. We emphasized the study of all type of glutamate and GABA receptors and their relation with mRNA editing in the brain. Some of the genetic studies which have been so fruitful during the last ten years and which have brought new insights regarding the understanding of epileptic syndromes are summarized in this article.

Muscarinic receptor loss and preservation of presynaptic cholinergic terminals in hippocampal sclerosis

PURPOSE

Prior single-photon emission tomography studies showed losses of muscarinic acetylcholine receptor (MAChR) binding in patients with refractory mesial temporal lobe epilepsy. Experimental animal studies demonstrated transient losses of MAChR due to electrically induced seizures originating in the amygdala. However, the relations between cholinergic synaptic markers, seizures, and underlying neuropathology in human temporal lobe epilepsy are unknown. We tested the hypotheses that human brain MAChR changes are attributable to hippocampal sclerosis (HS), and that HS resembles axon-sparing lesions in experimental animal models.

METHODS

We measured MAChR binding-site density, an intrinsic neuronal marker, within the hippocampal formation (HF) in anterior temporal lobectomy specimens from 10 patients with HS and in 10 autopsy controls. Binding-site density of the presynaptic vesicular acetylcholine transporter (VAChT) was measured as a marker of extrinsic cholinergic afferent integrity. MAChR and VAChT results were compared with neuronal cell counts to assess their relations to local neuronal losses.

RESULTS

Reduced MAChR binding-site density was demonstrated throughout the HF in the epilepsy specimens compared with autopsy controls and correlated in severity with reductions in cell counts in several HF regions. In contrast to MAChR, VAChT binding-site density was unchanged in the epilepsy specimens compared with autopsy controls.

CONCLUSIONS

Reduction in MAChR binding in HS is attributable to intrinsic neuronal losses. Sparing of afferent septal cholinergic terminals is consistent with the hypothesis that an excitotoxic mechanism may contribute to the development of HS and refractory partial epilepsy in humans.

An integrative analysis to sleep functions

Various studies suggest that some sleep functions, especially some slow wave sleep functions, are indispensable in mammals and related to brain regulation. It has been proposed that two of these functions are the adjustment of emotional balance and the processing of acquired emotional memories. During waking, the gradual accumulation of various randomly learned emotional memories in the limbic structures would inevitably imbalance and disorganize emotional behaviors. Although the emotional balance can be restored during waking by the ascending NA, DA, ACh and 5-HT systems, their roles in memory retention and emotional regulation may sometimes be dissociated and their adjustment of the emotional balance can only be a transient effect. On the other hand, the function of slow wave sleep for emotional adjustment can be long-lasting and is in agreement with its function on the processing of emotional memories. As a result, these sleep functions become indispensable in preventing the emotional imbalance inevitably caused by the accumulation of emotional memories. The effects of rapid eye movement sleep on memory and emotional regulation are just opposite to those of slow wave sleep. Low vigilance is required as premise for sleep to accomplish these indispensable functions.

Long-term increase in protein kinase C-gamma and muscarinic acetylcholine receptor expression in the cerebral cortex of amygdala-kindled rats--a quantitative immunocytochemical study

Kindling is an animal model for epilepsy in which repeated application of an electrical stimulus to brain pathways results in an epileptic focus. The animal holds a permanent state of hyperexcitability to the stimulus for the rest of its life. Understanding the cellular and molecular processes underlying hyperexcitability could provide insight into epileptogenesis. Furthermore, it could elucidate cellular and molecular bases of synaptic plasticity in the central nervous system. In the present study the long-term effect of a kindled focus in the amygdala on the gamma-isoform of protein kinase C and the muscarinic cholinergic receptor as cellular messengers was evaluated in the cerebral cortex of rats. Following an average of 10 bilaterally generalized seizures kindling stimulation was terminated and rats were left undisturbed for approximately three months. Brains were processed by immunocytochemistry using monoclonal antibodies against protein kinase C-gamma and muscarinic cholinergic receptor protein. Digital image analysis of sections through the entire forebrain revealed an increase in optical density of both protein kinase C-gamma and the muscarinic cholinergic receptor in the piriform and entorhinal cortex of the hemisphere contralateral to the stimulation site in kindled rats. However, on the ipsilateral side no change was observed in comparison with electrode implanted nonkindled control rats. The observed increase in expression of muscarinic cholinergic receptor protein and a component of the phosphoinositide second messenger system (protein kinase C-gamma) located in specific areas of the cerebral cortex in kindled rats could serve as a basis for the permanent state of hyperexcitability in these rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in acetylcholine-induced stimulation of inositol phospholipid hydrolysis in the dorsal hippocampus of kindled rats

Agonist-induced turnover or release of inositolphosphates (IP) was studied in the dorsal and the ventral hippocampus 24 h, 1 month, and 3 months after the last electrical stimulus of kindled rats. A significant increase in acetylcholine (ACh)-stimulated IP turnover was observed in dorsal, but not ventral, hippocampus 24 h and 1 month after the last electrical stimulus. However, this effect was not evident 3 months after kindling. The excitatory amino acids (quisqualic acid and ibotenic acid) at the concentrations used, however, failed to produce any change in receptor-stimulated release or turnover of IP. Thus the changes in ACh-induced IP release, although long-term, are not permanent and do not appear to be released to the neurobiological alterations associated with the long-term maintenance of the kindling phenomenon.

Mesolimbic dopaminergic supersensitivity following electrical kindling of the amygdala

Limbic seizures developed in rats following daily electrical stimulation of the basolateral nucleus of the amygdala. Animals were designated as "kindled" after five complete (stage 5) behavioral seizures were observed. A subgroup, designated as "superkindled," received three additional weeks of electrical stimulations. Kindled rats were significantly subsensitive to the stereotypy-inducing effects of apomorphine, a direct dopamine agonist, compared to controls. Superkindled rats were supersensitive to the effects of apomorphine. However, both kindled and superkindled rats demonstrated an increase in 3H-spiperone Bmax values, reflecting dopamine D2-receptor densities, in the nucleus accumbens ipsilateral to the stimulating electrode. The number of interictal spikes recorded from the stimulating amygdaloid electrode during the last week of kindling was correlated with changes in apomorphine sensitivity in individual animals.

Alterations in neurotransmitter amino acids in hippocampal kindled seizures

Rats were rapidly kindled with electrical stimuli to the hippocampus, and the concentrations of GABA and related amino acids were measured in several brain regions, both in the baseline kindled state and during active seizures. In the baseline kindled state, a consistent pattern was found throughout the hippocampus where GABA levels were depressed and glutamate and glutamine levels were elevated. During seizures GABA rose slightly while glutamate and glutamine increased to twice control values. These changes were seen to various degrees in other brain areas. Turnover in the GABA-glutamate-glutamine cycle was measured in kindled rats experiencing seizures and compared to control animals. During seizures there was an increased turnover of the excitatory compounds glutamate and aspartate relative to GABA. The data indicate that kindling produces a change of presynaptic GABA metabolism which results in diminished inhibition.

Status epilepticus facilitated by pilocarpine in amygdala-kindled rats

A novel model of status epilepticus based on the kindling model of epilepsy is described. The model involves the administration of a small dose of pilocarpine (20 mg/kg) to rats that have been previously kindled. Stimulation of these pretreated rats produces seizures which continue uninterrupted for approximately 4 h before spontaneous termination. The electroencephalographic discharge pattern showed characteristic changes in polarity and amplitude throughout the duration of status epilepticus. Behaviorally, the animals showed motor seizures which varied between stages I through IV, with evidence of extensive bilateral hemispheric involvement through much of the seizure episode. Animals that had been partially kindled to stage II seizures did not develop status epilepticus after stimulation when pretreated with pilocarpine, indicating that prior kindling is integral to the development of status epilepticus in this model. Administration of scopolamine was ineffective in terminating the condition when it had begun, suggesting that cholinergic stimulation is necessary for the initiation, but not the maintenance, of status epilepticus. This model holds promise for the study of status epilepticus because the condition develops in a seizure-prone (kindled) rat, and the seizures are self-sustaining, without the presence of exogenous chemicals or neurotoxins.

Cholinergic mechanisms in affective disorders. Future directions for investigation

Advances in clinical and basic research methodology combined with clearly articulated concepts create new opportunities for researching the roles of cholinergic mechanisms in the pathophysiology of affective disorders. Areas for study include: roles of cholinergic mechanisms in mediating effects of stress and cholinergic mechanisms linking the pathophysiologies of affective and panic disorders, use of pharmacologic agents to produce cholinergic system supersensitivity in modeling biologic aspects of affective illness, use of multigenerational intrapedigree studies of cholinergic markers associated with affective disease, research into the neurobiology of lithium and ECT as they pertain to muscarinic cholinergic mechanisms, study of the interrelationship of sodium, calcium and lithium ion metabolism and their relationship to cholinergic-monoaminergic interaction, the development of brain imaging strategies and techniques, e.g., positron emission tomography (PET), to measure changes in cholinergic receptor density and affinity as a function of clinical state, identification and validation of a peripheral model of the central muscarinic receptor, study of the pharmacology of abusable substances and its relationship to mechanisms regulating mood, affect, psychomotor function and other variables related to the affective disorders, and development of in vitro and in vivo models useful in studying the physiology and biochemistry of the interaction of cholinergic and monoaminergic neurons. These models may allow us to bridge the traditional cholinergic and monoamine hypotheses of affective disorders.

Endogenous anticonvulsant substances

Epileptic seizures will normally arrest abruptly and spontaneously, and the brain will remain refractory to further seizures for some time thereafter. This paper reviews the possible mechanisms underlying this seizure arrest and refractoriness. The data suggests that neuronal fatigue is not involved in either of these processes, whereas the role of ions and excitatory systems are unclear. Rather, seizure arrest and refractoriness may come about by the seizure-induced release and/or activity of multiple endogenous anticonvulsant substances. The spontaneous arrest of the seizure may involve the purine adenosine, in addition to other unknown mechanisms. Seizure refractoriness involves multiple systems, the most important of which, on the available evidence, are prostaglandins and opioid peptides and possibly benzodiazepine systems, although other neuropeptides and the purines may also be involved. The implications of these conclusions to anti-epileptic drug development and status epilepticus are discussed.

Covariation of depressive symptoms, parkinsonism, and post-dexamethasone plasma cortisol levels in a bipolar patient: simultaneous response to ECT and lithium carbonate

A patient presented with concurrent mood congruent delusions, parkinsonism, and elevated post-dexamethasone plasma cortisol levels. This triad could result from simultaneous development of cholinergic-monoaminergic dysfunction within critical limbic and extrapyramidal loci. The magnitude of each abnormality decreased in concert during a course of electroconvulsive therapy (ECT). Remaining abnormalities disappeared during treatment with lithium. Actions of ECT and lithium on muscarinic systems are reviewed, and a strategy for testing the hypothesis that dysfunction of cholinergic-monoaminergic mechanisms develops in parallel in different neural networks is considered.

Pathophysiology of "cholinoceptor supersensitivity" in affective disorders

Phenomenological and physiological variables demonstrate supersensitive changes to cholinergic challenge in affective disorder subjects. Theorists generally assume the primary defect is the postsynaptic muscarinic receptor. However, in addition to defectiveness or up-regulation of this receptor, the appearance of postsynaptic "cholinoceptor supersensitivity" can result from abnormal presynaptic mechanisms, membrane "pathology," derangement of intrasystolic mechanisms that amplify effects of receptor-agonist coupling, or aberrant cholinergic-monoaminergic interaction. This article discusses abnormalities of the postsynaptic receptor, regulation of postsynaptic receptor density, the presynaptic muscarinic receptor, and other mechanisms regulating the release of acetylcholine, membrane dynamics, and "cascade" mechanisms-specifically the phosphatidylinositol (PI) cycle, Ca2+ mobilization, and cyclic guanosine monophosphate (GMP) generation-as causes of cholinergic system "supersensitivity." It is suggested that an approach to the topic emphasizing site of abnormality will encourage greater clarity of thought in the study of the cholinergic component of the pathophysiology of affective illness.

Selective and reversible increase in the number of quisqualate-sensitive glutamate binding sites on hippocampal synaptic membranes after angular bundle kindling

The specific binding of L-[3H]glutamate to hippocampal synaptic membranes was examined in rats kindled by tetanic stimulation of the angular bundle. One day after the last of a minimum of 3 class 4 kindled seizures, the binding of L-[3H]glutamate to a quisqualate-sensitive site (GLU A) was about 40% greater than in electrode-implanted unstimulated controls. Saturation binding data indicated an increase in the maximum density of GLU A binding sites with no change in their affinity for L-glutamate. No such increase was detected 28 days after the last kindled seizure, although the animals were still kindled. Radioligand binding to a site that is much less sensitive to quisqualate (GLU B) was unaffected by kindling stimulation. These observations suggest that an increase in GLU A binding sites could be involved in the induction, but not in the maintenance, of the kindled state.

Kindled seizure-induced reduction of muscarinic cholinergic receptors in rat hippocampal formation: evidence for localization to dentate granule cells

The binding of [3H] quinuclidinyl benzilate ( [3H] QNB) to muscarinic cholinergic receptors in dentate gyrus of rat hippocampal formation was analyzed by membrane binding assay and in vitro autoradiography. The destruction of dentate granule cells, either by neonatal irradiation or colchicine injection, resulted in nearly complete elimination of [3H] QNB binding sites in the molecular and granule cell layers. By contrast, neither perforant path transection nor destruction of the septal-hippocampal cholinergic afferents caused a decline of [3H] QNB binding sites. Amygdala kindled seizures resulted in a 30% reduction of [3H] QNB binding sites which was distributed uniformly across the entire molecular and granule cell layers. Thus, most, if not all, of the muscarinic cholinergic receptors present in dentate gyrus appear to reside on the somata and dendritic trees of the dentate granule cells. We propose that this kindled seizure-induced decline of muscarinic receptors represents an endogenous compensatory mechanism designed to stabilize granule cell excitability.

Interaction between spontaneous and electrically induced convulsions and their short- and long-term effects in the abstinence after chronic barbital treatment in the rat

Male rats were treated with barbital supplied in their drinking water (daily dose around 200 mg/kg) for 50 weeks. When the treatment was stopped (day 0) spontaneous convulsions were monitored for the first 3 days of the abstinence. On day 3 a convulsion was induced by electricity in half of the rats (controls and barbital-treated) and 1 h later the sensitivity to hexobarbital was determined with a threshold test. Sensitivity to hexobarbital was then tested in the same manner at approximately weekly intervals for the first 110 days of the abstinence. On day 3 of the abstinence a tolerance to hexobarbital (48% increase in threshold above controls) and a reduced threshold to induce convulsions with electricity (-27% compared with controls) was seen in previously barbital-treated animals. Spontaneous or induced convulsions occurring prior to the hexobarbital threshold determination decreased the tolerance to the same extent (-22 to -28%). On day 28 rats with no convulsions up to day 3 had a marked renewal of tolerance to hexobarbital (29% increase above controls), while rats with convulsions recorded up to day 3 had less or no such tolerance. There was a positive correlation (r = 0.63) between the hexobarbital thresholds in barbital-treated rats recorded on day 3 and on day 28. Later in the abstinence, barbital-treated rats with convulsions prior to day 3 tended to have a hexobarbital threshold slightly but significantly elevated compared with the controls (10-15%). This change could be a sign of a long-lasting increased excitation.

Bidirectional transfer of electrical and carbachol kindling

In two experiments rats were kindled by electrical stimulation or the infusion of carbachol, and subsequently rekindled by carbachol or electrical stimulation, respectively, in the amygdala. The rats that previously had been kindled using one agent kindled significantly faster and displayed significantly stronger seizures when rekindled using the other agent. These results are consistent with the idea that cholinoceptive neurons may normally participate in electrical kindling of the amygdala although such participation does not appear to be crucial.

Neurochemical correlates of the kindling model of epilepsy

Repeated electrical stimulation of the brain can produce many epileptogenic effects including those which characterize the kindling model. Kindling stimulation, by definition, changes the brain is such a way that formerly subconvulsive stimuli can elicit electrographic and convulsive seizure activity. In addition, the kindled animal becomes more susceptible to many, but not all, other types of seizures. These facts suggest that kindling produces brain changes which may selectively model some types of epileptiform excitability. In order to understand the basis for such changes numerous neurochemical studies have been attempted in the last few years. Although many changes have been demonstrated to be produced by kindling, few studies have been designed to specifically examine the long-lasting (permanent) neurochemical correlates of kindling stimulation. In this review, neurochemical data relevant to kindling are presented and discussed in terms of their possible significance to the seizure susceptibility changes produced by kindling.

Intradentate colchicine retards the development of amygdala kindling

The mechanisms underlying the kindling model of epilepsy are unknown. Presumably, an altered network of neural circuits underlie amygdala kindling. Biochemical and radiohistochemical studies have pointed to the dentate granule cells (DGC) of the hippocampal formation as a member of this altered circuit. To test the role of these cells, colchicine, a neurotoxin of DGC, was directly injected into the dentate gyrus. Prior destruction of DGC retarded the development of amygdala kindling. Destruction of DGC after kindling was completed did not reverse the kindling effect. We conclude that DGC play a key role in the development, but not the permanence, of amygdala kindling. We propose a model whereby the greater the input to the hippocampal formation, the faster limbic kindling will proceed.

Seizures down-regulate muscarinic cholinergic receptors in hippocampal formation

Muscarinic cholinergic receptors (MCR) have been previously shown to decline in the hippocampal formation (HPF) of amygdala-kindled rats. Seizures have been proposed as the process responsible for this down-regulation. We now demonstrate similar down-regulation of MCR within HPF in 3 additional methods of inducing seizures: electroconvulsive shock, entorhinal kindling and entorhinal lesion. Two key parameters which causally link the MCR declines with seizures are their time course and reversal with anticonvulsants. The transient decline of MCR induced by entorhinal lesion-induced seizures parallels the time course established in amygdala kindling. Further, phenobarbital could block both these seizures and the MCR declines. Together, this supports the relationship of seizures causing the declines. We postulate that the MCR down-regulation represents an endogenous inhibitory response of neurons that are intensely and repeatedly depolarized during the seizures.