D-2 agonists protect rodents against pilocarpine-induced convulsions by stimulating D-2 receptors in the striatum, but not in the substantia nigra

Utilizing GO/PEDOT:PSS/PtNPs-enhanced high-stability microelectrode arrays for investigating epilepsy-induced striatal electrophysiology alterations

The striatum plays a crucial role in studying epilepsy, as it is involved in seizure generation and modulation of brain activity. To explore the complex interplay between the striatum and epilepsy, we engineered advanced microelectrode arrays (MEAs) specifically designed for precise monitoring of striatal electrophysiological activities in rats. These observations were made during and following seizure induction, particularly three and 7 days post-initial modeling. The modification of graphene oxide (GO)/poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/platinu-m nanoparticles (PtNPs) demonstrated a marked reduction in impedance (10.5 ± 1.1 kΩ), and maintained exceptional stability, with impedance levels remaining consistently low (23 kΩ) even 14 days post-implantation. As seizure intensity escalated, we observed a corresponding increase in neuronal firing rates and local field potential power, with a notable shift towards higher frequency peaks and augmented inter-channel correlation. Significantly, during the grand mal seizures, theta and alpha bands became the dominant frequencies in the local field potential. Compared to the normal group, the spike firing rates on day 3 and 7 post-modeling were significantly higher, accompanied by a decreased firing interval. Power in both delta and theta bands exhibited an increasing trend, correlating with the duration of epilepsy. These findings offer valuable insights into the dynamic processes of striatal neural activity during the initial and latent phases of temporal lobe epilepsy and contribute to our understanding of the neural mechanisms underpinning epilepsy.

Bypassing the Blood-Brain Barrier: Direct Intracranial Drug Delivery in Epilepsies

Epilepsies are common chronic neurological diseases characterized by recurrent unprovoked seizures of central origin. The mainstay of treatment involves symptomatic suppression of seizures with systemically applied antiseizure drugs (ASDs). Systemic pharmacotherapies for epilepsies are facing two main challenges. First, adverse effects from (often life-long) systemic drug treatment are common, and second, about one-third of patients with epilepsy have seizures refractory to systemic pharmacotherapy. Especially the drug resistance in epilepsies remains an unmet clinical need despite the recent introduction of new ASDs. Apart from other hypotheses, epilepsy-induced alterations of the blood-brain barrier (BBB) are thought to prevent ASDs from entering the brain parenchyma in necessary amounts, thereby being involved in causing drug-resistant epilepsy. Although an invasive procedure, bypassing the BBB by targeted intracranial drug delivery is an attractive approach to circumvent BBB-associated drug resistance mechanisms and to lower the risk of systemic and neurologic adverse effects. Additionally, it offers the possibility of reaching higher local drug concentrations in appropriate target regions while minimizing them in other brain or peripheral areas, as well as using otherwise toxic drugs not suitable for systemic administration. In our review, we give an overview of experimental and clinical studies conducted on direct intracranial drug delivery in epilepsies. We also discuss challenges associated with intracranial pharmacotherapy for epilepsies.

Deep brain stimulation of the anterior nuclei of the thalamus relieves basal ganglia dysfunction in monkeys with temporal lobe epilepsy

Aims

Deep brain stimulation of the anterior nuclei of the thalamus (ANT‐DBS) is effective in temporal lobe epilepsy (TLE). Previous studies have shown that the basal ganglia are involved in seizure propagation in TLE, but the effects of ANT‐DBS on the basal ganglia have not been clarified.

Methods

ANT‐DBS was applied to monkeys with kainic acid–induced TLE using a robot‐assisted system. Behavior was monitored continuously. Immunofluorescence analysis and Western blotting were used to estimate protein expression levels in the basal ganglia and the effects of ANT stimulation.

Results

The seizure frequency decreased after ANT‐DBS. D1 and D2 receptor levels in the putamen and caudate were significantly higher in the ANT‐DBS group than in the epilepsy (EP) model. Neuronal loss and apoptosis were less severe in the ANT‐DBS group. Glutamate receptor 1 (GluR1) in the nucleus accumbens (NAc) shell and globus pallidus internus (GPi) increased in the EP group but decreased after ANT‐DBS. γ‐Aminobutyric acid receptor A (GABA_A‐R) decreased and glutamate decarboxylase 67 (GAD67) increased in the GPi of the EP group, whereas the reverse tendencies were observed after ANT‐DBS.

Conclusion

ANT‐DBS exerts neuroprotective effects on the caudate and putamen, enhances D1 and D2 receptor expression, and downregulates GPi overactivation, which enhanced the antiepileptic function of the basal ganglia.

The prescribable drugs with efficacy in experimental epilepsies (PDE3) database for drug repurposing research in epilepsy

OBJECTIVE

Current antiepileptic drugs (AEDs) have several shortcomings. For example, they fail to control seizures in 30% of patients. Hence, there is a need to identify new AEDs. Drug repurposing is the discovery of new indications for approved drugs. This drug "recycling" offers the potential of significant savings in the time and cost of drug development. Many drugs licensed for other indications exhibit antiepileptic efficacy in animal models. Our aim was to create a database of "prescribable" drugs, approved for other conditions, with published evidence of efficacy in animal models of epilepsy, and to collate data that would assist in choosing the most promising candidates for drug repurposing.

METHODS

The database was created by the following: (1) computational literature-mining using novel software that identifies Medline abstracts containing the name of a prescribable drug, a rodent model of epilepsy, and a phrase indicating seizure reduction; then (2) crowdsourced manual curation of the identified abstracts.

RESULTS

The final database includes 173 drugs and 500 abstracts. It is made freely available at www.liverpool.ac.uk/D3RE/PDE3. The database is reliable: 94% of the included drugs have corroborative evidence of efficacy in animal models (for example, evidence from multiple independent studies). The database includes many drugs that are appealing candidates for repurposing, as they are widely accepted by prescribers and patients-the database includes half of the 20 most commonly prescribed drugs in England-and they target many proteins involved in epilepsy but not targeted by current AEDs. It is important to note that the drugs are of potential relevance to human epilepsy-the database is highly enriched with drugs that target proteins of known causal human epilepsy genes (Fisher's exact test P-value < 3 × 10^-5 ). We present data to help prioritize the most promising candidates for repurposing from the database.

SIGNIFICANCE

The PDE3 database is an important new resource for drug repurposing research in epilepsy.

Reduced D2/D3 Receptor Binding of Extrastriatal and Striatal Regions in Temporal Lobe Epilepsy

Objective

Dopamine is an endogenous neuromodulator in cortical circuits and the basal ganglia. In animal models of temporal lobe epilepsy (TLE), seizure threshold is modulated to some extent by dopamine, with D1-receptors having a pro- and D2-receptors an anticonvulsant effect. We aimed to extend our previously reported results on decreased D2/D3 receptor binding in the lateral epileptogenic temporal lobe and to correlate them with demographic and seizure variables to gain a more comprehensive understanding of the underlying involvement of the dopaminergic system in the epileptogenesis of TLE.

Methods

To quantify D2/D3 receptor binding, we studied 21 patients with TLE and hippocampal sclerosis (13 left- and eight right-sided) and 18 controls using PET with the high-affinity dopamine D2/D3-receptor ligand ¹⁸F-Fallypride to image striatal and extrastriatal binding. TLE was defined by interictal and ictal video-EEG, MRI and ¹⁸F-Fluorodeoxyglucose PET. Voxel-based statistical and regions-of-interest analyses were performed.

Results

¹⁸F-Fallypride binding potential was significantly reduced in the affected temporal lobe and bilateral putamen. A positive correlation between age at onset of epilepsy and [¹⁸F]FP BP_nd (binding potential non-displaceable) in temporal regions on the epileptogenic side was found, as well as a negative correlation between epilepsy duration and [¹⁸F]FP BP_nd in the temporal pole on the epileptogenic side and a positive correlation between the estimated number of lifetime GTCS and [¹⁸F]FP BP_nd in the hippocampus on the epileptogenic side.

Significance

The areas of reduced D2/D3 receptor availability correspond to “the irritative zone” surrounding the epileptogenic area. Moreover, reduced D2/D3 receptor availability was detectable in the basal ganglia, which are suspected to be involved in a control circuit for epileptic seizures. The correlational analysis additionally suggests that increased epilepsy duration leads to increasing impairment of the dopaminergic system.

Changes on D2-like receptor induced Gi protein activation and hippocampal dopamine release in kindled rats

The present study aimed to characterize dopamine release in the hippocampus and D2-like receptor-induced Gi protein activation in several brain areas of fully kindled rats. During the interictal period, kindled rats showed lower extracellular levels of dopamine when compared with those obtained in the control group under basal conditions, a situation that was not modified when an afterdischarge was evoked. Hippocampal perfusion of sulpiride, a D2 receptor antagonist, enhanced dopamine release, which was more evident when an afterdischarge was induced in kindled rats. In addition, sulpiride perfusion was associated with longer seizure duration. Functional autoradiography experiments revealed increased [(35)S]GTPγS incorporation as a consequence of D2-like receptor activation in different brain areas of fully kindled animals, including the ventral hippocampus. The present study reveals that hippocampal kindling is associated with alterations in dopamine release and D2-like receptor-induced neurotransmission.

Large differences in blood measures, tissue weights, and focal areas of damage 1 year after postseizure treatment with acepromazine or ketamine

Approximately 1 year after rats were seized as young adults with lithium (3 mEq/kg) and pilocarpine (30 mg/kg) and given acepromazine or ketamine, 18 blood measures, wet tissue weights, and detailed damage scores for 107 brain structures were completed. Compared with normal and ketamine-treated rats, acepromazine-treated seized rats (total n=54) had lighter pancreata and spleens and elevated aspartate aminotransferase and alanine aminotransferase blood levels. Even though average damage did not differ, the mosaic of brain damage completely discriminated the two seized groups. Differential effects of postseizure treatment on functions of the thyroid, pancreas, and spleen were indicated. Ketamine-treated seized rats were healthier than acepromazine-treated seized rats or normal rats. This experiment demonstrates the importance of whole-organism assessment and that the single administration of a specific drug after onset of status epilepticus can produce marked differences in the evolution of brain damage and its influence on specific organs for the rest of the animal's life.

The significance of increase in striatal D(2) receptors in epileptic EL mice

The present study systematically and quantitatively analyzed the immunohistochemical distribution of various substances involved in synthesis, binding, and transport of dopamine in the forebrain of epileptic mice (EL mouse strain) using a brain mapping analyzer. A reduction in serum calcium levels decreases calcium/calmodulin-dependent-dopamine synthesis in the brain and subsequently increases susceptibility to epileptic convulsions and induces abnormal behavior in EL mice. The immunohistochemical levels of D(2) receptors in the medial area of the neostriatum were significantly higher in EL mice than in ddY mice (mother strain of EL mice), while there were no differences in the levels of tyrosine hydroxylase, calcium/calmodulin-dependent protein kinase II, calmodulin, D(1) receptors, and dopamine transporters. Together with our previous findings, the results suggest that the decrease in serum calcium levels and subsequent decrease in brain dopamine synthesis comprise the primary physiologic disorder in EL mice, and convulsions or increased D(2) receptors are secondarily-induced phenomena to improve or compensate for the principal disorder.

The role of catecholamines in seizure susceptibility: new results using genetically engineered mice

The catecholamines norepinephrine and dopamine are abundant in the CNS, and modulate neuronal excitability via G-protein-coupled receptor signaling. This review covers the history of research concerning the role of catecholamines in modulating seizure susceptibility in animal models of epilepsy. Traditionally, most work on this topic has been anatomical, pharmacological, or physiological in nature. However, the recent advances in transgenic and knockout mouse technology provide new tools to study catecholamines and their roles in seizure susceptibility. New results from genetically engineered mice with altered catecholamine signaling, as well as possibilities for future experiments, are discussed.

Inhibitory role of the zona incerta in the pilocarpine model of epilepsy

Recent experiments have suggested that the zona incerta might be regarded as a highly sensitive structure for seizure induction. This sensitivity has been linked to this structure's abundant expression of cholinergic receptors. Here we have decided to investigate the participation of the GABAergic system of the zona incerta, one of its major neurotransmitters with widespread projections to the neocortex, in the pilocarpine (Pilo) model of epilepsy. Stereotaxic administration of a GABA(A) agonist (muscimol), antagonist (bicuculline) or saline (controls) bilaterally into the zona incerta of adult male Wistar rats was performed 30 min prior to the systemic injection of pilocarpine. Animals were electroencephalographically and behaviorally monitored for seizure activity. Administration of muscimol had a pro-convulsant effect characterised by a higher percentage of animals developing SE with a shorter latency. Conversely, administration of bicuculline had a dose dependent anticonvulsant effect, with no animals displaying SE. Our results contribute to the further characterisation of the regulatory role of the zona incerta in seizure-related phenomena, suggesting that its modulation might be a relevant target for anticonvulsant strategies.

Up-regulation of D3 dopaminergic receptor mRNA in the core of the nucleus accumbens accompanies the development of seizures in a genetic model of absence-epilepsy in the rat

The basal ganglia system is thought to play a key role in the control of absence-seizures and there is ample evidence that epileptic seizures modify brain dopamine function. We recently reported that local injections of dopamine D1 or D2 agonists in the core of the nucleus accumbens suppressed absence-seizures in a spontaneous, genetic rodent model of absence-epilepsy whereas injections of D1 or D2 antagonists had aggravating effects. These findings raised the possibility that the dopaminergic system may be altered in absence-epilepsy prone rats. Therefore, we studied by in situ hybridization histochemistry the expression of pre- and postsynaptic components of the dopaminergic system in this strain of rats. When compared to non-epileptic control rats, epileptic rats displayed no change in the expression of mRNAs coding for the neuronal dopaminergic markers (tyrosine hydroxylase, membraneous and vesicular dopamine transporters). In addition, there was no difference between the two strains concerning the expression of the dopamine receptor transcripts D1, D2 and D5. In adult absence-epilepsy prone rat with an overt epileptic phenotype, however, an elevated level of D3 mRNA expression was observed in neurons of the core of the nucleus accumbens (+23% increase in silver grain density compared to non-epileptic control rats). D3 transcripts were not increased in juvenile epileptic rats without seizures. These findings suggests that up-regulation of D3 receptor mRNA is part of the epileptic phenotype in absence-epilepsy prone rats. Its localization in the core of the nucleus accumbens bears close resemblance to the dopamine-sensitive antiepileptic sites in ventral striatum and further support the involvement of ventral structures of the basal ganglia system in the control of absence-seizures.

Dopamine in the striatum modulates seizures in a genetic model of absence epilepsy in the rat

Inhibition of the substantia nigra pars reticulata has been shown to suppress seizures in different animal models of epilepsy. The striatum is the main input of the substantia nigra pars reticulata. The aim of the present study was to examine the role of dopaminergic neurotransmission within the striatum in the control of absence seizures in a genetic model in the rat. Injections of mixed dopaminergic D1/D2 or of selective D1 or D2 agonists or antagonists in the dorsal parts of the striatum led to suppression of absence seizures associated with strong behavioral and electroencephalographic side-effects. When injected in the ventral part of the striatum (i.e. the nucleus accumbens core), all these agonists and antagonists respectively decreased and increased absence seizures without behavioral or electroencephalographic side-effects. Combined injections of low doses of a D1 and a D2 agonist in the core of the nucleus accumbens had an additive effect in absence seizures suppression. Furthermore, combined injections of low doses of a GABA(A) agonist and a N-methyl-D-aspartate antagonist in the substantia nigra also had cumulative effects in absence seizures suppression. These results show that dopamine neurotransmission in the core of the nucleus accumbens is critical in the control of absence seizures. The modulatory and additive effects on absence seizures of dopaminergic neurotransmission through both the D1 and D2 receptors in the core of the nucleus accumbens further suggest that ventral pathways of the basal ganglia system are involved in the modulation of absence seizures.

Changes in striatal electroencephalography and neurochemistry induced by kainic acid seizures are modified by dopamine receptor antagonists

We investigated the involvement of striatal dopamine release in electrographic and motor seizure activity evoked by kainic acid in the guinea pig. The involvement of the dopamine receptor subtypes was studied by systemic administration of the dopamine D(1) receptor antagonist, R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390; 0.5 mg kg(-1)), or the dopamine D(2) antagonist, (5-aminosulphonyl)-N-[(1-ethyl-2-pyrrolidinyl)-methyl]-2-methoxybenzamide (sulpiride, 30 mg kg(-1)). Microdialysis and high performance liquid chromatography were used to monitor changes in extracellular levels of striatal dopamine and its metabolites, glutamate, aspartate and gamma-amino-butyric acid (GABA). These data were correlated with changes in the striatal and cortical electroencephalographs and clinical signs. We found that, although neither dopamine receptor antagonist inhibited behavioural seizure activity, blockade of the dopamine D(1)-like receptor with SCH 23390 significantly reduced both the 'power' of the electrical seizure activity and the associated change in extracellular striatal concentration of glutamate, whilst increasing the extracellular striatal concentration of GABA. In contrast, blockade of the dopamine D(2)-like receptor with sulpiride significantly increased the extracellular, striatal content of glutamate and the dopamine metabolites. These results confirm previous evidence in other models of chemically-evoked seizures that antagonism of the dopamine D(1) receptor tends to reduce motor and electrographic seizure activity as well as excitatory amino-acid transmitter activity, while antagonism of the dopamine D(2) receptor has relatively less apparent effect.

SCH 23390: the first selective dopamine D1-like receptor antagonist

SCH 23390, the halobenzazepine (R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5- tetrahydro-1H-3-benzazepine, is a highly potent and selective dopamine D1-like receptor antagonist with a K(i) of 0.2 and 0.3 nM for the D1 and D5 dopamine receptor subtypes, respectively. In vitro, it also binds with high affinity to the 5-HT2 and 5-HT1C serotonin receptor subtypes. However, the doses required to induce a similar response in vivo are greater than 10-fold higher than those required to induce a D1-mediated response. Previous in vivo pharmacological studies with SCH 23390 have shown it to abolish generalized seizures evoked by the chemoconvulsants: pilocarpine and soman. These studies provide evidence of the potential importance of D1-like dopaminergic receptor mechanisms in facilitating the initiation and spread of seizures. The inference from a majority of studies is that the activation of dopamine D1 receptors facilitates seizure activity, whereas activation of D2 receptors may inhibit the development of seizures. SCH 23390 has also been used in studies of other neurological disorders in which the dopamine system has been implicated, such as psychosis and Parkinson's disease. Apart from the study of neurological disorders, SCH 23390 has been extensively used as a tool in the topographical determination of brain D1 receptors in rodents, nonhuman primates, and humans. In summary, SCH 23390 has been a major tool in gaining a better understanding of the role of the dopamine system, more specifically the D1 receptor, in neurological function and dysfunction.

The role of basal ganglia in the control of generalized absence seizures

During the last two decades, evidence has accumulated to demonstrate the existence, in the central nervous system, of an endogenous mechanism that exerts an inhibitory control over different forms of epileptic seizures. The substantia nigra and the superior colliculus have been described as key structures in this control circuit; inhibition of GABAergic neurons of the substantia nigra pars reticulata results in suppression of seizures in various animal models of epilepsy. The role in this control mechanism of the direct GABAergic projection from the striatum to the substantia nigra and of the indirect pathway, from the striatum through the globus pallidus and the subthalamic nucleus, was examined in a genetic model of absence seizures in the rat. In this model, pharmacological manipulations of both the direct and indirect pathways resulted in modulation of absence seizures. Activation of the direct pathway or inhibition of the indirect pathway suppressed absence seizures through disinhibition of neurons in the deep and intermediate layers of the superior colliculus. Dopamine D1 and D2 receptors in the nucleus accumbens, appear to be critical in these suppressive effects. Along with data from the literature, our results suggest that basal ganglia circuits play a major role in the modulation of absence seizures and provide a framework to understand the role of these circuits in the modulation of generalized seizures.

The anticonvulsant effect of deprenyl on pentylenetetrazol-induced seizures in Lewis rats

There is recent evidence that deprenyl may have anticonvulsant action in a rat kindling model of epilepsy as well as in a maximal electroshock model. We therefore investigated the effect of deprenyl on the brain sensitivity threshold to pentylenetetrazol (PTZ)-induced maximal seizures in Lewis rats, in a model that provides pharmacodynamic information free of pharmacokinetic interference. The novel finding of this investigation was the anticonvulsant effect of deprenyl following repetitive administration whereas a single deprenyl dose did not affect the PTZ concentrations required to induce maximal seizures. The data suggests that the mechanism of this effect is not associated with the dopaminergic activity of deprenyl since pretreatment with both bromocriptine (a dopamine D2 agonist) and haloperidol (dopamine antagonist) did not affect the seizure threshold, whereas levodopa caused a proconvulsant effect. It was also concluded that the mechanism is not related to changes in acetylcholine levels since prolonged pretreatment with deprenyl did not attenuate the brain sensitivity to pilocarpine-induced seizures. The fact that long term administration of deprenyl was needed to produce its anticonvulsant effect may indicate that the anticonvulsant effect of deprenyl may be due to changes in levels of certain endogenous compounds or down or up-regulation of relevant receptor/effector units.

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.

Susceptibility of Flinders sensitive and resistant rats to pharmacologically induced seizures

The Flinders sensitive (FSL) and Flinders resistant (FRL) line rats have been selectively bred for hyper- and hyposensitivity to the hypothermic effect of cholinergic agonists respectively. In this study, pilocarpine (250 mg/kg) and physostigmine (0.8 mg/kg) doses that are subconvulsant to outbread Sprague-Dawley rats were systemically injected to the FSL and FRL rats and a heterogenous F2 cross. All of the FRL rats developed severe motor limbic seizures in response to pilocarpine, while none of the FSL animals did. The F2 crosses showed intermediate reaction. The FRL rats were also more affected by physostigmine than the other two groups. However, the FSL rats were confirmed to be more sensitive to the hypothermic effects of pilocarpine (20 mg/kg) and physostigmine (0.6 mg/kg). Picrotoxin and kainic acid produced similar responses in the both lines, i.e., induced clonic convulsions in some animals when applied in subthreshold doses (2 and 10 mg/kg, respectively). Thus, the normally cholinergic-insensitive rats are more sensitive to the convulsant effects of high doses of cholinergic agonists, but this increased sensitivity does not extend to noncholinergic convulsants.

Effects of dopamine D3 receptor agonists on pilocarpine-induced limbic seizures in the rat

The discrete localization of D3 receptors in the nucleus accumbens and subjacent islands of Calleja bears a close resemblance to the dopamine-sensitive anticonvulsant site in the anteroventral striatum. To determine if these D3 receptors were capable of attenuating limbic motor seizures induced by pilocarpine, dopamine agonists with preferential or non-selective D3 affinity were injected stereotaxically into these limbic brain regions of the rat via indwelling cannulae prior to pilocarpine challenge. Reliable clonic seizures were obtained by administering the proconvulsive dopamine D1 agonist SKF 38393 (10 mg/kg i.p.) followed by a subconvulsant dose of pilocarpine (280-300 mg/kg i.p.). Bilateral intra-accumbens pretreatment with the D3 > D2 agonist RU 24213 (0.2 pmol-7 nmol) significantly delayed the onset of seizures, with a minimum effective dose of 2 pmol, without altering their frequency or severity. The more selective D3 agonist LY 171555 (0.2 pmol-7.8 nmol) was less potent, and only attenuated pilocarpine-induced seizures at a dose (500 pmol) that would have stimulated accumbens D2 receptors as well. Intra-accumbens injections of the highly potent and selective D3 agonist 7-OH-DPAT (20 pmol to 7 nmol) afforded no protection against pilocarpine-induced seizures. Apomorphine, a mixed D1/D2/D3 agonist, delayed seizure onset at 100-500 pmol, but not at higher doses. RU 24213, LY 171555 and 7-OH-DPAT were all modestly anticonvulsant when microinjected into the islands of Calleja at D2/D3 unselective doses. These data support the notion that dopamine systems limit seizure propagation through the limbic forebrain, but suggest this protective effect is mediated by D2 rather than D3 receptors.

Persistent increases in dopamine D2 receptor mRNA expression in basal ganglia following kindling

Amygdala kindling resulted in significant increases in the expression of D2 receptor mRNA in the nucleus accumbens and striatum 30 days following the last kindling stimulation. Densitometric analyses of tissue sections incubated in the presence of an oligonucleotide probe directed against D2 receptor cDNA indicated a 20-35% increase in D2 receptor mRNA in these regions following kindling. Kindling from the amygdala followed by piriform cortical kindling in the transfer paradigm (overkindling) resulted in significant further increases in D2 receptor mRNA expression in both the accumbens (150% increase) and striatum (120% increase). There were no observed hemispheric asymmetries in D2 receptor mRNA in either kindled or overkindled animals. The data indicate an enduring upregulation of extrapyramidal D2 receptor mRNA following the kindling process. How this change may relate to kindling-induced alterations in seizure susceptibility or behaviors mediated by limbic dopaminergic pathways are questions for future studies.

Disordered dopamine neurotransmission in the striatum of rats undergoing pilocarpine-induced generalized seizures, as revealed by microdialysis

In this study rats were fitted with a concentric dialysis probe in one striatum and extracellular concentrations of dopamine and HVA measured by reverse phase high performance liquid chromatography. Injections of saline or the D1 agonist SKF 38393 (30 mg/kg) did not affect the releases of these compounds. On the other hand, the D2 agonist LY 171555 (0.5 mg/kg) inhibited the release of both dopamine and HVA, whilst amphetamine (1 mg/kg) increased the output of dopamine but not HVA. Treatment with 200 mg/kg pilocarpine caused minimal signs of epileptic activity and did not affect striatal dopamine neurotransmission. Concomitant administration of SKF 38393 (30 mg/kg) to this dose of pilocarpine greatly facilitated the incidence and severity of motor seizures, which were accompanied by an irregular pattern of dopamine release and a significant rise in HVA overflow. Similar results were obtained with rats made to convulse with 400 mg/mg pilocarpine, and to a lesser extent if these animals were first pretreated with a protective dose of LY 171555 (0.5 mg/kg). It is concluded that dopamine neurotransmission in the striatum is disrupted in rats undergoing a pilocarpine-induced motor seizure, and that the extent of this disruption increases as the seizure becomes more severe. An irregular release of dopamine could signify a loss of sensorimotor control by the striatum, which might conceivably contribute to the intractability of the seizure. An increase in the dialysate concentrations of metabolite and not dopamine, is consistent with a heightened glutamate-stimulated release of dopamine from a discrete striatal pool, caused by the seizure spreading through the cortex and activating the cortico-striatal system.

Dopaminergic modulation of pilocarpine-induced motor seizures in the rat: the role of hippocampal D2 receptors

This study examined the role of hippocampal dopamine D2 receptors in the genesis of limbic seizures induced by muscarinic agonists in the rat. Pilocarpine, 600 mg/kg, elicited rapid and usually fatal convulsions. These were not affected by focal injections of saline (1 microliter) into both hippocampi. Pretreatment of the dorsal, but not the lateral hippocampus, with the D2 agonist trans-(+)-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-2H-pyrazolo-(3,4-g)quinol ine hydrochloride (LY 171555, 2 micrograms per side), did not alter the frequency of pilocarpine-induced convulsions, but significantly delayed their appearance and reduced their intensity. LY 171555 similarly increased the latency of seizures induced by focal hippocampal injection of carbachol (100 micrograms), without changing the frequency or the severity. The selective D2 antagonist raclopride, injected dorsally into both hippocampi dose-dependently facilitated motor seizures evoked by pilocarpine (100 mg/kg), the cholinomimetic at this dose being ineffective as a convulsant in saline-treated animals. Intrahippocampal administration of the D1 agonist 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrochloride (SKF 38393, 2 micrograms per side) did not facilitate pilocarpine seizures and did not potentiate the proconvulsant action of raclopride. These data demonstrate that activation of the dopaminergic system, via D2 receptors in the dorsal hippocampus, is capable of protecting the animal against limbic motor seizures arising from excessive muscarinic stimulation of the hippocampus. Since the blockade of D2 receptors in the hippocampus markedly lowered the seizure threshold to pilocarpine, this would suggest that the dopaminergic input to the hippocampus is normally tonically active and functions physiologically to prevent epileptogenesis.

Seizure promotion by D1 agonists does not correlate with other dopaminergic properties

A range of D1 receptor agonists were tested for their ability to facilitate limbic motor seizures induced by a subthreshold dose of the chemoconvulsant pilocarpine (100 mg/kg IP) in mice. ED50 values (mumol/kg) were calculated from log dose-probit analyses, giving relative proconvulsant potencies of SKF 82958 > CY 208-243 > SKF 77434 = SKF 75670 = SKF 80723 > SKF 38393. The compound SKF 82526, which poorly crosses the blood-brain barrier, did not lower the seizure threshold. Convulsions consisted of rearing and forepaw myoclonus, leading to status epilepticus at higher doses of the D1 agonists. No deaths were recorded. A maximum seizure incidence of 50% was obtained with SKF 75670, compared to 100% for the other compounds. Apart from SKF 82526, the D1 agonists all elicited behavioural signs of central D1 receptor stimulation, including motor restlessness, grooming and sniffing. There was no obvious relationship between the abilities of these D1 agonist drugs to promote epilepsy and their effects on unconditioned motor behaviour, or their affinities and efficacies at the striatal D1 receptor. It is concluded that a reduction of the seizure threshold is an inevitable consequence of central D1 receptor stimulation with existing D1 agonists.

Dopaminergic modulation of pilocarpine-induced motor seizures in the rat: the role of hippocampal dopamine D1 receptors

The present study addressed the role of dopamine D1 receptors in pilocarpine-induced motor seizures in rats. Bilateral pretreatment of the hippocampus with the D1 agonist SKF 38393 (0.1-5 micrograms) did not alter the animals' sensitivity to a threshold (200 mg/kg i.p.) or fully convulsant dose (600 mg/kg i.p.) of pilocarpine, as compared to hippocampal saline-treated controls. Similarly, direct injection of pilocarpine (200 micrograms per side) into both hippocampi elicited low level seizure activity that was not modified by SKF 38393, either coadministered (2 micrograms per side) or injected systemically (30 mg/kg i.p.). On the other hand, intrahippocampal microinjections of the D1 antagonist, SCH 23390 (2 micrograms per side), whilst unable to prevent epileptogenesis to 600 mg/kg pilocarpine, delayed the onset of seizures and reduced their severity. These results suggest that hippocampal dopamine lowers the seizure threshold by activating D1 receptors, an effect which is only disclosed by D1 receptor blockade and is not surmountable by additional D1 stimulation.

Anticonvulsant effect of striatal dopamine D2 receptor stimulation: dependence on cortical circuits?

In the pilocarpine model of epilepsy, dopamine can either inhibit (via D2 receptors) or facilitate (via D1 receptors) the spread of limbic motor seizures. The anticonvulsant action of D2 receptor activation has been localized to the anterior striatum, but disappears if excessive damage is caused to the overlying cerebral cortex. This study examines the possibility that the corticostriatal projection is involved in the anticonvulsant response to striatal D2 receptor stimulation, by comparing the seizure-protecting efficacy of intrastriatal trans-(+)-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-2H-pyrazolo-(3,4-g)quinol ine hydrochloride (LY 171555) in control rats, and in rats bearing discrete bilateral kainic acid lesions of the cerebral cortex. The results show that neurotoxin injection induces a punctate lesion of the primary motor area of the cortex in each hemisphere, with no injury to the underlying caudate-putamen, or to more distant structures such as the hippocampus. The lesion, however, was sufficient to abolish the protective effect of intrastriatal LY 171555 against pilocarpine challenge. To explain these findings, an interplay between nigrostriatal dopaminergic and corticostriatal glutamatergic neurons is proposed, in which the anticonvulsant tendency of the excitatory amino acid is accentuated by dopamine, probably by acting on D2 receptors which facilitate the release of glutamate from axon terminals.