An electrographic analysis of the synchronous discharge patterns of GEPR-9s generalized seizures.

Previous results from our Laboratory have shown a synchronous discharge pattern (less than 1 ms apart) in monopolar recordings from electrodes placed in the cortex, inferior colliculus, and medulla of seizing GEPR-9s. However, the wave morphology of the ictal EEG is quite different for electrodes placed in different anatomical structures. These results lead us to hypothesize that wave morphology was indicative of neural circuitry involved in the GEPR9 seizure and that volume conduction was accounting for synchronous epileptiform EEG pattern. We decided to approach the problem by using a set of two experiments. Experiment 1: Perform a complete precollicular transection in GEPR-9s before inducing seizure in order to observe changes in EEG morphology after forebrain circuitry removal. Experiment 2: A novel methodological approach using a three-dimensional bipolar array enabled the reconstruction of a vector indicative of to which direction is voltage increasing. Such time-varying vector is indicative of the source direction of the high-amplitude epileptiform EEG signal. By placing such an array of electrodes, used to record the 3 bipolar EEGs, in the forebrain, midbrain, and hindbrain, we were able to use a simple intersection method to infer source localization. Our results suggest that the slow wave component of the GEPR9 epileptiform ictal EEG pattern is associated with a midbrain-forebrain circuit while the spike component is associated with a midbrain-hindbrain substrate. These results are supported by experiment 1 in which only the spike component of EEG remained after the precollicular transection.

A comprehensive electrographic and behavioral analysis of generalized tonic-clonic seizures of GEPR-9s.

This study records noise-free intracerebral EEG of the genetically epilepsy prone rat (GEPR-9), along with behavioral correlates, during a seizure on unanesthetized freely behaving unrestrained animals. The GEPR-9 exhibits acoustically triggered generalized tonic-clonic seizures, and often times the EEG, recorded with conventional techniques, has resulted in data with imbedded movement artifact. For noise-free video-EEG recordings, we used a previously developed system that consists of a head connector with a FET preamplifier and battery, signal conditioning device (5000x gain, 1 Hz-100 Hz filters), A/D converter and video/PC-PC/video computer boards for recording image data. Each animal was implanted with three monopolar/referential electrodes chosen among the following areas: cortex, inferior colliculus, reticular formation and caudal medulla. The video-EEG data were quite similar for all recorded animals: (1) basal desynchronized EEG before sound stimulus; (2) increase in EEG frequency after stimulus and before seizure onset; (3) high-amplitude polyspikes during massive myoclonic thrusts with or without a very fast running episode; (4) an electrodecremental response during tonic extension; (5) wave and spike complex during forelimb and hindlimb tonic rigidity and posttonic clonus; (6) low-amplitude EEG during postictal depression. Time sequenced spectral analysis also highlights the epileptiform EEG pattern during seizure with high reproducibility between animals. While testing seizure naive GEPR-9s, there was a clear evolution from modest epileptiform EEG activity on the first acoustic stimulation to progressively higher amplitude, duration and frequency epileptiform EEG activity throughout seizure repetition.

Effect of increments in the concentration of dopamine in the central nervous system on audiogenic seizures in DBA/2J mice.

The effect on audiogenic seizures of drug-induced increments in biogenic amines in the brain was determined in DBA/2J mice. One group of mice was treated with L-dihydroxyphenylalanine (L-DOPA) which caused a large rise in levels of norepinephrine and dopamine in the central nervous system, but did not significantly alter the concentration of 5-hydroxytryptamine. This group of animals exhibited a dramatic reduction in the incidence of tonic extensor seizures. A second group of animals that had been pretreated with diethyldithiocarbamate, a dopamine-beta-hydroxylase inhibitor, was also given L-DOPA. In this group of mice, there was a highly significant rise in the concentration of dopamine in brain but no statistically-significant changes in levels of either norepinephrine or 5-hydroxytryptamine. These animals also had a dramatic decrease in the incidence of tonic extensor seizures. A third group of animals that received only diethyldithiocarbamate did not exhibit any statistically-significant changes in the incidence of seizure or in levels of biogenic amines. The drug-induced reduction in the incidence of seizure in the first two groups correlated with a large increase in levels of dopamine in brain. This reduction in seizures did not correlate with changes in levels of norepinephrine or 5-hydroxytryptamine in brain.

Abnormalities in 5-HT1A and 5-HT1B receptor binding in severe-seizure genetically epilepsy-prone rats (GEPR-9s).

The present study was designed to determine whether abnormalities in serotonin receptor binding co-exist with the presynaptic serotonergic deficits that have previously been identified in the genetically epilepsy-prone rat (GEPR) brain. In vitro binding of [3H]8-OH-DPAT (0.16-10.3 nM) to 5-HT1A receptor sites was found to be decreased in the hippocampus of severe seizure GEPRs (GEPR-9s) when compared to nonepileptic control rats, while no difference in [3H]8-OH-DPAT binding was observed in the GEPR-9 corpora quadrigemina or midbrain tegmentum. The decreased binding of [3H]8-OH-DPAT to hippocampal membranes was due to a decrease in Bmax (P < 0.001), rather than to a change in the Kd. Conversely, in vitro binding of [125I]cyanopindolol (2-400 pM) to 5-HT1B receptor sites was increased in the GEPR-9 hippocampus, corpora quadrigemina and midbrain tegmentum when compared to nonepileptic control rats. The increased binding of [125I]cyanopindolol in all three regions resulted from an increase in the Bmax (P < 0.05), rather than a change in the Kd. These finding suggest that in addition to the innate reduction in 5-HT presynaptic markers, GEPR-9s also exhibit abnormalities in the density of 5-HT1A and 5-HT1B receptors in some regions of the brain. Inasmuch as serotonin acts to attenuate audiogenic seizures in GEPRs, these abnormalities in 5-HT receptor binding may contribute to the seizure susceptibility exhibited by these animals.

Impairment in cochlear function produced by chloramphenicol and noise.

Ototoxic interaction between chloramphenicol and noise was studied in two separate investigations. In the first study, permanent ototoxicity was demonstrated in a group of rats which were subjected to short-duration, high-intensity noise and were then given chloramphenicol orally. The anatomical damage in this group was consistent with observed changes in cochlear round window recordings of cochlear microphonics at 4 kHz and of the N1 component of the eighth nerve action potential. In the second study, a temporary depression in the function of the cochlea was observed in rats subjected to the noise-chloramphenicol regimen used in the first study. Depressions in recordings of the round window similar to those in the first study were seen only during the first five days of recordings. After the fifth day, the recordings of the round window were normal, indicating recovery from a temporary shift in threshold produced by chloramphenicol and noise. Incidence of purulent otitis media was found in 57 and 0% of the animals in the first and second studies, respectively. The combination of chloramphenicol and noise appears to be responsible for the production of temporary cochlear deficits. The addition of the third variable, otitis media, appears to result in permanent impairment of the cochlea.

Electron microscopic immunocytochemical evidence that the calmodulin-dependent cyclic nucleotide phosphodiesterase is localized predominantly at postsynaptic sites in the rat brain.

The calmodulin-dependent cyclic nucleotide phosphodiesterase represents an important junction between the Ca2+ and the cyclic AMP/cyclic GMP second messenger systems. In brain it is a major cyclic nucleotide-degrading activity and is selectively expressed in the soma and dendrites of regional output neurons [Kincaid et al. (1987) Proc. natn. Acad. Sci. U.S.A. 84, 1118-1122]. In this study the subcellular localization of this enzyme in cerebral cortex, hippocampus and inferior colliculus of rat brain was analysed by electron microscopic immunocytochemical methods using affinity-purified antibodies. The immunoreactivity was found exclusively within neurons whereas glial cells were unstained; preabsorption of antibody with phosphodiesterase eliminated this reactivity, demonstrating the specificity of immunostaining. In the neuronal cell bodies, deposits of immunoreaction product occurred as sparse patches in the cytoplasm and were often associated with organelles such as mitochondria, Golgi-complex and endoplasmic reticulum; nuclei, however, were free from immunoreaction product. In the neuronal processes immunoreactivity was found within dendrites and dendritic spines, whereas the myelinated axons and axon terminals were immunonegative. The postsynaptic densities of asymmetric synapses were associated with especially high concentrations of immunoreaction product. However, the immunopositive synaptic profiles appeared to be quite selective, comprising only a small percentage of the total number of synapses in the neuropil. Our results indicate that the calmodulin-dependent cyclic nucleotide phosphodiesterase is concentrated at postsynaptic sites in specific classes of neurons. This finding supports other morphological evidence indicating a primary role for cyclic nucleotide action in postsynaptic and not presynaptic structures. Furthermore, since this enzyme is regulated by Ca2+, this interface between second messenger systems seems to play a significant role in the postsynaptic integration of Ca(2+)-mediated neuronal inputs.

Decrease in hippocampal [3H]vinylidene kainic acid binding in genetically epilepsy-prone rats.

Specific [3H]vinylidene kainic acid binding to the kainate-sensitive subtype of glutamate receptor was studied in brain of 31-day-old non-epileptic Sprague-Dawley control and two colonies of genetically epilepsy-prone rats using in vitro autoradiographic techniques. At 37.5 nM [3H]vinylidene kainic acid, specific [3H]vinylidene kainic acid binding was reduced significantly by 18 and 22% in dorsal and ventral hippocampal formation stratum lucidum of 31-day-old genetically epilepsy-prone-9 rats compared with non-epileptic controls. Hippocampal [3H]vinylidene kainic acid binding was reduced in genetically epilepsy-prone-3 rats by 15 and 18%, but these reductions were not statistically significant. Saturation of [3H]vinylidene kainic acid binding studies indicated that the total number of ventral hippocampal [3H]vinylidene kainic acid binding sites was decreased by 21% in genetically epilepsy-prone-3 rats and 28% in genetically epilepsy-prone-9 rats. The reduction in ventral hippocampal [3H]vinylidene kainic acid binding in genetically epilepsy-prone rats resembles the reduction in ventral hippocampal [3H]vinylidene kainic acid binding sites observed in perinatal hypothyroid rats. As genetically epilepsy-prone rats are hypothyroid during the neonatal period, the reduction in hippocampal [3H]vinylidene kainic acid binding in the genetically epilepsy-prone rats may be a consequence of a hypothyroid-induced defect in the development or maturation of the hippocampal mossy fiber projection in genetically epilepsy-prone rats. An alternative hypothesis is that the putative occurrence of spontaneous limbic seizures in genetically epilepsy-prone rats may lead secondarily to a reduction in hippocampal [3H]vinylidene kainic acid binding sites.

Comparative fos immunoreactivity in the brain after forebrain, brainstem, or combined seizures induced by electroshock, pentylenetetrazol, focally induced and audiogenic seizures in rats.

To help discern sites of focal activation during seizures of different phenotype, the numbers of Fos immunoreactive (FI) neurons in specific brain regions were analyzed following "brainstem-evoked," "forebrain-evoked" and forebrain/brainstem combination seizures induced by a variety of methods. First, pentylenetetrazol (PTZ, 50 mg/kg) induced forebrain-type seizures in some rats, or forebrain seizures that progressed to tonic/clonic brainstem-type seizures in other rats. Second, minimal electroshock induced forebrain seizures whereas maximal electroshock (MES) induced tonic brainstem-type seizures in rats. Third, forebrain seizures were induced in genetically epilepsy-prone rats (GEPRs) by microinfusion of bicuculline into the area tempestas (AT), while brainstem seizures in GEPRs were induced by audiogenic stimulation. A final set was included in which AT bicuculline-induced forebrain seizures in GEPRs were transiently interrupted by audiogenic seizures (AGS) in the same animals. These animals exhibited a sequence combination of forebrain clonic seizure, brainstem tonic seizure and back to forebrain clonic seizures. Irrespective of the methods of induction, clonic forebrain- and tonic/clonic brainstem-type seizures were associated with considerable Fos immunoreactivity in several forebrain structures. Tonic/clonic brainstem seizures, irrespective of the methods of induction, were also associated with FI in consistent brainstem regions. Thus, based on Fos numerical densities (FND, numbers of Fos-stained profiles), forebrain structures appear to be highly activated during both forebrain and brainstem seizures; however, facial and forelimb clonus characteristic of forebrain seizures are not observable during a brainstem seizure. This observation suggests that forebrain-seizure behaviors may be behaviorally masked during the more severe tonic brainstem seizures induced either by MES, PTZ or AGS in GEPRs. This suggestion was corroborated using the sequential seizure paradigm. Similar to findings using MES and PTZ, forebrain regions activated by AT bicuculline were similar to those activated by AGS in the GEPR. However, in the combination seizure group, those areas that showed increased FND in the forebrain showed even greater FND in the combination trial. Likewise, those areas of the brainstem showing FI in the AGS model, showed an even greater effect in the combination paradigm. Finally, the medial amygdala, ventral hypothalamus and cortices of the inferior colliculi showed markedly increased FND that appeared dependent upon activation of both forebrain and brainstem seizure activity in the same animal. These findings suggest these latter areas may be transitional areas between forebrain and brainstem seizure interactions. Collectively, these data illustrate a generally consistent pattern of forebrain Fos staining associated with forebrain-type seizures and a consistent pattern of brainstem Fos staining associated with brainstem-type seizures. Additionally, these data are consistent with a notion that separate seizure circuitries in the forebrain and brainstem mutually interact to facilitate one another, possibly through involvement of specific "transition mediating" nuclei.

Drug excretion in human breast milk: principles, pharmacokinetics and projected consequences.

The excretion of drugs in human breast milk is reviewed with regard to milk production, composition, feeding patterns and mechanisms of drug transfer into milk. Fundamental principles of breast milk excretion are used to construct a pharmacokinetic approach useful for the study of most drugs. An infant-modulated 3-compartment open model is proposed for drug distribution and elimination in the breast feeding woman. Milk/plasma drug concentration ratios are projected on the basis of pH partitioning. While some studies confirm these projections, other studies demonstrate a need to consider additional factors such as lipid solubility and protein binding characteristics of a drug in milk. Data are lacking for most drugs and hence dosing via milk or risk to the infant remains speculative. Very few pharmacokinetic studies of both milk and infant plasma were found. A review of selected drug classes cites available information as a basis for future studies. Few drugs are contraindicated in breast feeding women, but supportive data for either proscriptions or permissive statements are often lacking. A neglected but potentially serious infant risk--impaired behaviour and development--is discussed from the standpoint of emerging animal data. Conceptually valid and comprehensive studies on drug excretion in breast milk are needed if this valuable nutrient for infants is to be made available safely.

Further evidence of anticonvulsant role for 5-hydroxytryptamine in genetically epilepsy-prone rats.

1. This study was designed to evaluate further the role of 5-hydroxytryptamine (5-HT) in regulating susceptibility and/or intensity of audiogenic seizures in genetically epilepsy-prone rats. 2. The effects of sertraline, a highly selective and potent inhibitor of 5-HT uptake, on both the intensity of the audiogenic seizures and the extracellular concentrations of 5-HT in the thalamus were evaluated in severe seizure genetically epilepsy-prone rats. 3. Sertraline (7.5, 15 and 30 mg kg-1, i.p.) produced a dose-dependent reduction in the intensity of the audiogenic seizures. 4. Brain microdialysis studies showed that the same doses of sertraline also caused dose-dependent increases in the extracellular 5-HT concentration in the thalamus of the freely moving rats. 5. The peak anticonvulsant effect correlated temporally with the peak increases in the extracellular 5-HT concentration for this drug. 6. It is concluded that enhancement of 5-hydroxytryptaminergic transmission may contribute to the anticonvulsant effect of sertraline in severe seizure genetically epilepsy-prone rats. 7. The present results coupled with earlier investigations support the hypothesis that 5-HT plays an anticonvulsant role in genetically epilepsy-prone rats.

Scope and contribution of genetic models to an understanding of the epilepsies.

Studies of the genetic models of the epilepsies emphasize that some seizure disorders result from an aberrant "wiring diagram" coupled with abnormal activity of individual neurons. These defects cause the unique seizer-triggering mechanisms operative within the epileptic nervous system but which are inactive or do not exist in normal subjects. Moreover, causes of epilepsy reside not only within the brain area, wherein initial appearance of epileptic EEG discharge occurs, but also outside that region. Etiologically significant neurochemical dysfunctions may be common features of the epileptic condition in genetic models across species. Accordingly, genetically determined convulsive epileptogenesis in rats, baboons, and humans may result partially from noradrenergic and GABAergic deficits. In contrast, genetically derived absence seizures in the rat and perhaps also humans may occur in response to GABAergic excess. The unique features of the genetically epileptic animals emphasize their usefulness in developing novel drugs that selectively ameliorate seizure predisposition.

A noradrenergic and serotonergic hypothesis of the linkage between epilepsy and affective disorders.

Noradrenergic and/or serotonergic deficits, as well as other abnormalities, may contribute to predisposition to some epilepsies and depressions. Evidence for this hypothesis stems from several sources. Epidemiological investigations are intriguing but incomplete. Pharmacological studies show that noradrenergic and/or serotonergic transmission are both anticonvulsant and antidepressant. Therapeutically pertinent investigations show that antidepressant drugs have anticonvulsant properties, whereas antiepileptic drugs are effective in the management of affective disorders. Additional investigations demonstrate that seizures, whether spontaneously occurring or therapeutically induced, protect against depression. Through studies of innate pathophysiology, noradrenergic and serotonergic deficits have been identified in individuals with depression and in animal models of epilepsy, as well as in some humans with epilepsy. Vagal nerve stimulation, a treatment already known to be effective in the epilepsies, is presently under investigation for effectiveness in affective disorder. New evidence suggests that vagal nerve stimulation exerts at least some of its therapeutic effects through its capacity to increase noradrenergic and serotonergic transmission. Finally, emerging evidence supports the concept that some genetic mammalian models of the human epilepsies exhibit analogous manifestations of depression.

The combined EEG-intracerebral microdialysis technique: a new tool for neuropharmacological studies on freely behaving animals.

In this study we combined EEG and intracerebral microdialysis techniques in freely behaving rats. Various drugs were delivered into the hippocampus and cerebral cortex by means of microdialysis and, simultaneously, the EEG activity of the dialyzed area was monitored. The microdialysis procedure itself, when artificial cerebrospinal fluid was perfused, did not change the normal hippocampal or cortical EEG pattern. Drug inclusions into the microdialysis fluid, however, caused marked changes in the electrical activity of the dialyzed sites. In this report we present the following examples: (1) the dose-dependent spike-provoking effect of NMDA in hippocampus, (2) the potentiation of this NMDA effect in hippocampus by dibutyryl cyclic AMP, and (3) the EEG depressant effect of high concentration of K+ in the cerebral cortex. The artificial cerebrospinal fluid and drug solutions were alternated in the microdialysis system with a 2-way valve placed outside the test chamber. As a consequence, the drugs were delivered into the brain without interrupting the ongoing behavior, including sleep, of the examined animals. This study shows that the combined EEG-intracerebral microdialysis technique is a useful tool, with many unique advantages, for in vivo neuropharmacological studies.

Angular bundle kindling is accelerated in rats with a genetic predisposition to acoustic stimulus-induced seizures.

Limbic kindling was examined in genetically epilepsy-prone (GEPR) and non-epileptic control rats. The early stage of kindling development was accelerated in both groups of GEPR rats compared to controls. Later stages of kindling were accelerated in GEPR-9 but not GEPR-3 rats. These results indicate that GEPR rats have an enhanced susceptibility to limbic kindling and suggest that limbic brain alterations may contribute to acceleration of the early stage kindling development in GEPR rats.

Abnormalities in norepinephrine turnover rate in the central nervous system of the genetically epilepsy-prone rat.

Norepinephrine turnover rates were estimated in the hypothalamus-thalamus, midbrain, pons-medulla and telencephalon of genetically epilepsy-prone rats (GEPR). In each of these 4 brain areas the endogenous norepinephrine levels were significantly lower in the GEPR than in control animals. In the hypothalamus-thalamus, midbrain and telencephalon the calculated norepinephrine turnover rates were also significantly lower in GEPRs than in control. These studies confirm and extend earlier observations relating seizures in the GEPR to decrements in central nervous system noradrenergic function.

Noradrenergic mechanisms for the anticonvulsant effects of desipramine and yohimbine in genetically epilepsy-prone rats: studies with microdialysis.

A large body of evidence suggests that the seizure-prone state of genetically epilepsy-prone rats (GEPRs) results, in part, from deficits in central nervous system noradrenergic function. In order to link the synaptic concentration of norepinephrine (NE) to seizure behavior, we evaluated the effects of both desipramine and yohimbine on convulsions and on extracellular NE and serotonin (5-HT) concentrations in the thalamus of severe seizure GEPRs (GEPR-9s). Under anesthesia, guide cannulae were stereotaxically placed over thalami. After recovery from surgery, dialysis probes were inserted and the animals were placed individually into a plexiglass chamber where they were allowed to move about freely. Artificial CSF was perfused and samples were collected for analysis on HPLC with electrochemical detection. Either desipramine (10 and 20 mg/kg) or yohimbine (10 mg/kg) was administered i.p. after a stable baseline of NE or 5-HT was established. Significant increases in the extracellular NE concentration were seen after injection of both drugs. Temporal linkage exists between the maximum NE increase and the maximum decrease in audiogenic response score (ARS) for these two agents. No significant increases in the extracellular 5-HT concentration occurred after administration of either desipramine or yohimbine at a dose of 10 mg/kg. We conclude that these two drugs are effective anticonvulsants in GEPRs at least partially because they enhance noradrenergic transmission.

Dizocilpine (MK-801) increases not only dopamine but also serotonin and norepinephrine transmissions in the nucleus accumbens as measured by microdialysis in freely moving rats.

The extracellular concentrations of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in the nucleus accumbens (NACC) of freely moving rats were monitored simultaneously via intracerebral microdialysis. Local infusion of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine) (5-250 microM) produced significant increases in extracellular levels of DA, NE and 5-HT in a concentration-dependent fashion. Perfusion with tetrodotoxin (TTX, 1 microM) blocked the ability of focal MK-801 (50 microM) to increase DA, NE and 5-HT in the dialysate. Systemic administration of MK-801 (0.3 mg/kg, i.p.) also produced small, but statistically significant, increases in extracellular concentrations of DA, NE and 5-HT in the NACC. Our microdialysis results are consistent with the hypothesis that, in addition to dopaminergic, serotonergic and noradrenergic neurotransmissions in the NACC are involved in the mechanism by which MK-801 alters behavior in rats. Also, the present study gives further support to the concept that NMDA receptors within the NACC do not regulate DA release through direct excitatory control.

Anticonvulsant effect of enhancement of noradrenergic transmission in the superior colliculus in genetically epilepsy-prone rats (GEPRs): a microinjection study.

An expanding body of data has indicated that the seizure prone state in genetically epilepsy-prone rats (GEPRs) is partially caused by deficits in central nervous system noradrenergic transmission. Several lines of evidence suggest that the noradrenergic terminals in the superior colliculus (SC) may act as determinants of seizure predisposition in the GEPR. In order to assess the role of the noradrenergic transmission in the SC in the regulation of seizure severity, several drugs with different mechanisms of enhancing noradrenergic transmission were bilaterally microinfused into the SC of GEPR-9s (severe seizure GEPRs). The rats were tested for audiogenic seizure intensity at 0.25, 1, 2, 3, and 4 h after treatments. Bilateral infusion of vehicle produced no reduction in the severity of the audiogenic seizure. Desipramine (2, 4, 8 micrograms/side), nisoxetine (2, 4, 8 micrograms/side), and idazoxan (0.25, 1, 4 micrograms/side) all decreased the seizure severity in a dose-dependent fashion. Significant decreases in the seizure severity were also observed after administration of methoxamine (0.15 microgram/side) or phenylephrine (0.15 microgram/side). Pretreatment with prazosin (1 microgram/side) significantly diminished the anticonvulsant effectiveness of methoxamine and nisoxetine while prazosin, by itself, had no effects on the seizure intensity. These results suggest that noradrenergic transmission in the SC may be involved in the seizure regulation in GEPR-9s, and that this regulation may be mediated, at least in part, through alpha 1 receptors.

Evidence that a serotonergic mechanism is involved in the anticonvulsant effect of fluoxetine in genetically epilepsy-prone rats.

Fluoxetine (15 mg/kg i.p.) decreased the audiogenic seizure intensity in 33% of severe seizure genetically epilepsy-prone rats (GEPR-9s). 5-Hydroxytryptophan (5-HTP, 12.5 mg/kg i.p.) produced no anticonvulsant effect in GEPR-9s. When GEPR-9s were treated with a combination of these two drugs, the combination treatment decreased the audiogenic seizure intensity in 83% of the animals tested. Brain microdialysis studies showed that the same combination of 5-HTP and fluoxetine also produced a marked potentiation of the increase in the extracellular serotonin concentration in the thalamus of freely-moving GEPR-9s when compared with administration of either drug alone. A negative correlation between audiogenic seizure intensity and extracellular serotonin concentration existed after either fluoxetine alone or the combination treatment. No significant changes in extracellular norepinephrine concentrations were observed after the combination treatment. These results coupled with our earlier reports strongly suggest that a serotonergic mechanism is involved in the anticonvulsant effects of fluoxetine in GEPRs.

Carbamazepine increases extracellular serotonin concentration: lack of antagonism by tetrodotoxin or zero Ca2+.

Carbamazepine administration causes large increases in extracellular serotonin concentration and dose-related anticonvulsant effects in genetically epilepsy-prone rats (GEPRs). In order to determine the generality of the effect on serotonin, we determined the anticonvulsant ED50 for carbamazepine against maximal electroshock seizures in outbred, non-epileptic Sprague-Dawley rats. We then administered anticonvulsant carbamazepine doses to Sprague-Dawley rats and observed extracellular serotonin concentration in hippocampi by way of microdialysis. We found that administration of carbamazepine, either systemically or through the dialysis probe, resulted in significant and dose-related increases in extracellular serotonin concentration. Basal serotonin release was decreased by tetrodotoxin administration through the dialysis probe. Tetrodotoxin administration through the dialysis probe did not decrease the effect of systemically or focally administered carbamazepine on extracellular serotonin concentration. Similarly, elimination of Ca2+ from the dialysate did not alter the release of serotonin caused by carbamazepine. These findings suggest that the serotonin releasing effect of carbamazepine does not take place by exocytosis and does not require action potentials in the brain area in which the release takes place. Further they suggest that the effect is mediated by an action of carbamazepine directly on serotonergic nerve terminals.