Supramammillary neurons projecting to the septum regulate dopamine and motivation for environmental interaction in mice.

The supramammillary region (SuM) is a posterior hypothalamic structure, known to regulate hippocampal theta oscillations and arousal. However, recent studies reported that the stimulation of SuM neurons with neuroactive chemicals, including substances of abuse, is reinforcing. We conducted experiments to elucidate how SuM neurons mediate such effects. Using optogenetics, we found that the excitation of SuM glutamatergic (GLU) neurons was reinforcing in mice; this effect was relayed by their projections to septal GLU neurons. SuM neurons were active during exploration and approach behavior and diminished activity during sucrose consumption. Consistently, inhibition of SuM neurons disrupted approach responses, but not sucrose consumption. Such functions are similar to those of mesolimbic dopamine neurons. Indeed, the stimulation of SuM-to-septum GLU neurons and septum-to-ventral tegmental area (VTA) GLU neurons activated mesolimbic dopamine neurons. We propose that the supramammillo-septo-VTA pathway regulates arousal that reinforces and energizes behavioral interaction with the environment.

Mouse Model of Chromosome 15q13.3 Microdeletion Syndrome Demonstrates Features Related to Autism Spectrum Disorder.

The chromosome 15q13.3 microdeletion is a pathogenic copy number variation conferring epilepsy, intellectual disability, schizophrenia, and autism spectrum disorder (ASD). We generated mice carrying a deletion of 1.2 Mb homologous to the 15q13.3 microdeletion in human patients. Here, we report that mice with a heterozygous deletion on a C57BL/6 background (D/+ mice) demonstrated phenotypes including enlarged/heavier brains (macrocephaly) with enlarged lateral ventricles, decreased social interactions, increased repetitive grooming behavior, reduced ultrasonic vocalizations, decreased auditory-evoked gamma band EEG, and reduced event-related potentials. D/+ mice had normal body weight, activity levels, sensory gating, and cognitive abilities and no signs of epilepsy/seizures. Our results demonstrate that D/+ mice represent ASD-related phenotypes associated with 15q13.3 microdeletion syndrome. Further investigations using this chromosome-engineered mouse model may uncover the common mechanism(s) underlying ASD and other neurodevelopmental/psychiatric disorders representing the 15q13.3 microdeletion syndrome, including epilepsy, intellectual disability, and schizophrenia. SIGNIFICANCE STATEMENT: Recently discovered pathologic copy number variations (CNVs) from patients with neurodevelopmental/psychiatric disorders show very strong penetrance and thus are excellent candidates for mouse models of disease that can mirror the human genetic conditions with high fidelity. A 15q13.3 microdeletion in humans results in a range of neurodevelopmental/psychiatric disorders, including epilepsy, intellectual disability, schizophrenia, and autism spectrum disorder (ASD). The disorders conferred by a 15q13.3 microdeletion also have overlapping genetic architectures and comorbidity in other patient populations such as those with epilepsy and schizophrenia/psychosis, as well as schizophrenia and ASD. We generated mice carrying a deletion of 1.2 Mb homologous to the 15q13.3 microdeletion in human patients, which allowed us to investigate the potential causes of neurodevelopmental/psychiatric disorders associated with the CNV.

Mice with subtle reduction of NMDA NR1 receptor subunit expression have a selective decrease in mismatch negativity: Implications for schizophrenia prodromal population.

Reductions in glutamate function are regarded as an important contributory factor in schizophrenia. However, there is a paucity of animal models characterized by developmental and sustained reductions in glutamate function. Pharmacological models using NMDA antagonists have been widely used but these typically produce only transient changes in behavior and brain function. Likewise, mice with homozygous constitutive reductions in glutamate receptor expression show stable brain and behavioral changes, but many of these phenotypes are more severe than the human disease. The current study examines a variety of schizophrenia-related EEG measures in mice with a heterozygous alteration of the NMDA receptor NR1 subunit gene (NR1) that is known to result in reduced NR1 receptor expression in the homozygous mouse (NR1-/-). (NR1+/-) mice showed a 30% reduction in NR1 receptor expression and were reared after weaning in either group or isolated conditions. Outcome measures include the response to paired white noise stimuli, escalating inter-stimulus intervals (ISIs) and deviance-related mismatch negativity (MMN). In contrast to what has been reported in (NR1-/-) mice and mice treated with NMDA antagonists, (NR1+/-) mice showed no change on obligatory Event Related Potential (ERP) measures including the murine P50 and N100 equivalents (P20 and N40), or measures of baseline or evoked gamma power. Alternatively, (NR1+/-) mice showed a marked reduction in response to a deviant auditory tone during MMN task. Data suggest that EEG response to deviant, rather than static, stimuli may be more sensitive for detecting subtle changes in glutamate function. Deficits in these heterozygous NR1 knockdown mice are consistent with data demonstrating MMN deficits among family members of schizophrenia patients and among prodromal patients. Therefore, the current study suggests that (NR1+/-) mice may be among the most sensitive models for increased vulnerability to schizophrenia.

Gastrin-releasing peptide contributes to the regulation of adult hippocampal neurogenesis and neuronal development.

In the postnatal hippocampus, newly generated neurons contribute to learning and memory. Disruptions in neurogenesis and neuronal development have been linked to cognitive impairment and are implicated in a broad variety of neurological and psychiatric disorders. To identify putative factors involved in this process, we examined hippocampal gene expression alterations in mice possessing a heterozygous knockout of the calcium/calmodulin-dependent protein kinase II alpha heterozygous knockout gene (CaMK2α-hKO), an established model of cognitive impairment that also displays altered neurogenesis and neuronal development. Using this approach, we identified gastrin-releasing peptide (GRP) as the most dysregulated gene. In wild-type mice, GRP labels NeuN-positive neurons, the lone exception being GRP-positive, NeuN-negative cells in the subgranular zone, suggesting GRP expression may be relevant to neurogenesis and/or neuronal development. Using a model of in vitro hippocampal neurogenesis, we determined that GRP signaling is essential for the continued survival and development of newborn neurons, both of which are blocked by transient knockdown of GRP's cognate receptor (GRPR). Furthermore, GRP appears to negatively regulate neurogenesis-associated proliferation in neural stem cells both in vitro and in vivo. Intracerebroventricular infusion of GRP resulted in a decrease in immature neuronal markers, increased cAMP response element-binding protein (CREB) phosphorylation, and decreased neurogenesis. Despite increased levels of GRP mRNA, CaMK2α-hKO mutant mice expressed reduced levels of GRP peptide. This lack of GRP may contribute to the elevated neurogenesis and impaired neuronal development, which are reversed following exogenous GRP infusion. Based on these findings, we hypothesize that GRP modulates neurogenesis and neuronal development and may contribute to hippocampus-associated cognitive impairment.

The immature dentate gyrus represents a shared phenotype of mouse models of epilepsy and psychiatric disease.

OBJECTIVES: There is accumulating evidence to suggest psychiatric disorders, such as bipolar disorder and schizophrenia, share common etiologies, pathophysiologies, genetics, and drug responses with many of the epilepsies. Here, we explored overlaps in cellular/molecular, electrophysiological, and behavioral phenotypes between putative mouse models of bipolar disorder/schizophrenia and epilepsy. We tested the hypothesis that an immature dentate gyrus (iDG), whose association with psychosis in patients has recently been reported, represents a common phenotype of both diseases. METHODS: Behaviors of calcium/calmodulin-dependent protein kinase II alpha (α-CaMKII) heterozygous knock-out (KO) mice, which are a representative bipolar disorder/schizophrenia model displaying iDG, and pilocarpine-treated mice, which are a representative epilepsy model, were tested followed by quantitative polymerase chain reaction (qPCR)/immunohistochemistry for mRNA/protein expression associated with an iDG phenotype. In vitro electrophysiology of dentate gyrus granule cells (DG GCs) was examined in pilocarpine-treated epileptic mice. RESULTS: The two disease models demonstrated similar behavioral deficits, such as hyperactivity, poor working memory performance, and social withdrawal. Significant reductions in mRNA expression and immunoreactivity of the mature neuronal marker calbindin and concomitant increases in mRNA expression and immunoreactivity of the immature neuronal marker calretinin represent iDG signatures that are present in both mice models. Electrophysiologically, we have confirmed that DG GCs from pilocarpine-treated mice represent an immature state. A significant decrease in hippocampal α-CaMKII protein levels was also found in both models. CONCLUSIONS: Our data have shown iDG signatures from mouse models of both bipolar disorder/schizophrenia and epilepsy. The evidence suggests that the iDG may, in part, be responsible for the abnormal behavioral phenotype, and that the underlying pathophysiologies in epilepsy and bipolar disorder/schizophrenia are strikingly similar.

Rewarding and incentive motivational effects of excitatory amino acid receptor antagonists into the median raphe and adjacent regions of the rat.

RATIONALE: The motivational process that regulates approach behavior toward salient distal stimuli (i.e., incentive motivation) plays a key role in voluntary behavior and motivational disorders such as addiction. This process may be mediated by many neurotransmitter systems and a network of many brain structures, including the median and dorsal raphe regions (MR and DR, respectively). OBJECTIVE: We sought to examine whether the blockade of excitatory amino acid receptors in the MR and DR is rewarding, using intracranial self-administration, and whether the self-administration effect can be explained by drug's effectiveness to enhance incentive motivation, using a visual sensation seeking procedure. RESULTS: Rats learned to self-administer the AMPA receptor antagonist ZK 200775 into the vicinity of the MR, DR, or medial oral pontine reticular regions, but not the ventral tegmental area. The NMDA receptor antagonist AP5 was also self-administered into the MR, while it was not readily self-administered into other regions. When ZK 200775 was noncontingently administered into the MR, rats markedly increased approach responses rewarded by brief illumination of a light stimulus. In addition, contingent administration of ZK 200775 into the MR induced a conditioning effect on approach responses. CONCLUSIONS: Rats self-administer excitatory amino acid receptor antagonists into the MR and adjacent regions. Self-administration effect of AMPA receptor antagonists into the MR can be largely explained by the manipulation's properties to invigorate ongoing approach behavior and induces conditioned approach. Glutamatergic afferents to the median raphe and adjacent regions appear to tonically suppress incentive-motivational processes.

SREB2/GPR85, a schizophrenia risk factor, negatively regulates hippocampal adult neurogenesis and neurogenesis-dependent learning and memory.

SREB2/GPR85, a member of the super-conserved receptor expressed in brain (SREB) family, is the most conserved G-protein-coupled receptor in vertebrate evolution. Previous human and mouse genetic studies have indicated a possible link between SREB2 and schizophrenia. SREB2 is robustly expressed in the hippocampal formation, especially in the dentate gyrus, a structure with an established involvement in psychiatric disorders and cognition. However, the function of SREB2 in the hippocampus remains elusive. Here we show that SREB2 regulates hippocampal adult neurogenesis, which impacts on cognitive function. Bromodeoxyuridine incorporation and immunohistochemistry were conducted in SREB2 transgenic (Tg, over-expression) and knockout (KO, null-mutant) mice to quantitatively assay adult neurogenesis and newborn neuron dendritic morphology. Cognitive responses associated with adult neurogenesis alteration were evaluated in SREB2 mutant mice. In SREB2 Tg mice, both new cell proliferation and new neuron survival were decreased in the dentate gyrus, whereas an enhancement of new neuron survival occurred in SREB2 KO mouse dentate gyrus. Doublecortin staining revealed dendritic morphology deficits of newly generated neurons in SREB2 Tg mice. In a spatial pattern separation task, SREB2 Tg mice displayed a decreased ability to discriminate spatial relationships, whereas SREB2 KO mice had enhanced abilities in this task. Additionally, SREB2 Tg and KO mice had reciprocal phenotypes in a Y-maze working memory task. Our results indicate that SREB2 is a negative regulator of adult neurogenesis and consequential cognitive functions. Inhibition of SREB2 function may be a novel approach to enhance hippocampal adult neurogenesis and cognitive abilities to ameliorate core symptoms of psychiatric patients.

Adult neurogenesis transiently generates oxidative stress.

An increasing body of evidence suggests that alterations in neurogenesis and oxidative stress are associated with a wide variety of CNS diseases, including Alzheimer's disease, schizophrenia and Parkinson's disease, as well as routine loss of function accompanying aging. Interestingly, the association between neurogenesis and the production of reactive oxidative species (ROS) remains largely unexamined. The adult CNS harbors two regions of persistent lifelong neurogenesis: the subventricular zone and the dentate gyrus (DG). These regions contain populations of quiescent neural stem cells (NSCs) that generate mature progeny via rapidly-dividing progenitor cells. We hypothesized that the energetic demands of highly proliferative progenitors generates localized oxidative stress that contributes to ROS-mediated damage within the neuropoietic microenvironment. In vivo examination of germinal niches in adult rodents revealed increases in oxidized DNA and lipid markers, particularly in the subgranular zone (SGZ) of the dentate gyrus. To further pinpoint the cell types responsible for oxidative stress, we employed an in vitro cell culture model allowing for the synchronous terminal differentiation of primary hippocampal NSCs. Inducing differentiation in primary NSCs resulted in an immediate increase in total mitochondria number and overall ROS production, suggesting oxidative stress is generated during a transient window of elevated neurogenesis accompanying normal neurogenesis. To confirm these findings in vivo, we identified a set of oxidation-responsive genes, which respond to antioxidant administration and are significantly elevated in genetic- and exercise-induced model of hyperactive hippocampal neurogenesis. While no direct evidence exists coupling neurogenesis-associated stress to CNS disease, our data suggest that oxidative stress is produced as a result of routine adult neurogenesis.

Synergistic interaction between baclofen administration into the median raphe nucleus and inconsequential visual stimuli on investigatory behavior of rats.

RATIONALE: Noncontingent administration of amphetamine into the ventral striatum or systemic nicotine increases responses rewarded by inconsequential visual stimuli. When these drugs are contingently administered, rats learn to self-administer them. We recently found that rats self-administer the GABA(B) receptor agonist baclofen into the median (MR) or dorsal (DR) raphe nuclei. OBJECTIVES: We examined whether noncontingent administration of baclofen into the MR or DR increases rats' investigatory behavior rewarded by a flash of light. RESULTS: Contingent presentations of a flash of light slightly increased lever presses. Whereas noncontingent administration of baclofen into the MR or DR did not reliably increase lever presses in the absence of visual stimulus reward, the same manipulation markedly increased lever presses rewarded by the visual stimulus. Heightened locomotor activity induced by intraperitoneal injections of amphetamine (3 mg/kg) failed to concur with increased lever pressing for the visual stimulus. These results indicate that the observed enhancement of visual stimulus seeking is distinct from an enhancement of general locomotor activity. Visual stimulus seeking decreased when baclofen was co-administered with the GABA(B) receptor antagonist, SCH 50911, confirming the involvement of local GABA(B) receptors. Seeking for visual stimulus also abated when baclofen administration was preceded by intraperitoneal injections of the dopamine antagonist, SCH 23390 (0.025 mg/kg), suggesting enhanced visual stimulus seeking depends on intact dopamine signals. CONCLUSIONS: Baclofen administration into the MR or DR increased investigatory behavior induced by visual stimuli. Stimulation of GABA(B) receptors in the MR and DR appears to disinhibit the motivational process involving stimulus-approach responses.

Administration of the GABAA receptor antagonist picrotoxin into rat supramammillary nucleus induces c-Fos in reward-related brain structures. Supramammillary picrotoxin and c-Fos expression.

BACKGROUND: Picrotoxin blocks GABAA receptors, whose activation typically inhibits neuronal firing activity. We recently found that rats learn to selectively self-administer picrotoxin or bicuculline, another GABAA receptor antagonist, into the supramammillary nucleus (SuM), a posterior hypothalamic structure localized anterior to the ventral tegmental area. Other drugs such as nicotine or the excitatory amino acid AMPA are also self-administered into the SuM. The SuM appears to be functionally linked with the mesolimbic dopamine system and is closely connected with other brain structures that are implicated in motivational processes, including the prefrontal cortex, septal area, preoptic area, lateral hypothalamic area and dorsal raphe nucleus. Here, we hypothesized that these brain structures are activated by picrotoxin injections into the SuM. RESULTS: Picrotoxin administration into the SuM markedly facilitated locomotion and rearing. Further, it increased c-Fos expression in this region, suggesting blockade of tonic inhibition and thus the disinhibition of local neurons. This manipulation also increased c-Fos expression in structures including the ventral tegmental area, medial shell of the nucleus accumbens, medial prefrontal cortex, septal area, preoptic area, lateral hypothalamic area and dorsal raphe nucleus. CONCLUSIONS: Picrotoxin administration into the SuM appears to disinhibit local neurons and recruits activation of brain structures associated with motivational processes, including the mesolimbic dopamine system, prefrontal cortex, septal area, preoptic area, lateral hypothalamic area and dorsal raphe nucleus. These regions may be involved in mediating positive motivational effects triggered by intra-SuM picrotoxin.

The GABAB receptor agonist baclofen administered into the median and dorsal raphe nuclei is rewarding as shown by intracranial self-administration and conditioned place preference in rats.

RATIONALE: The midbrain raphe regions have long been implicated in affective processes and disorders. There is increasing evidence to suggest that the median (MR) and dorsal raphe nuclei (DR) tonically inhibit reward-related processes. OBJECTIVES: Stimulation of GABAB receptors in the midbrain raphe nuclei is known to inhibit local neurons, especially serotonergic neurons. We sought to determine if injections of the GABAB receptor agonist baclofen into the MR or DR are rewarding, using intracranial self-administration and conditioned place preference. RESULTS: Rats quickly learned to lever press for infusions of baclofen (0.1­2.5 mM) into the MR, but not the ventral tegmental area or central linear nucleus. Rats increased lever pressing associated with intra-DR baclofen infusions, but not readily. Baclofen self-administration into the MR or DR was attenuated by coadministration of the GABAB receptor antagonist SCH 50911 (1 mM) or systemic pretreatment with the dopamine receptor antagonist SCH 23390 (0.025 mg/kg, i.p.). In addition, intra-DR and intra-MR injections of baclofen induced conditioned place preference; injection into DR was more effective. CONCLUSIONS: Baclofen injections into the midbrain raphe nuclei are rewarding. Baclofen was more readily self-administered into the MR than into the DR, while baclofen injections into the DR more readily induced conditioned place preference than those into the MR. These sites may be differentially involved in aspects of reward. These findings suggest that MR or DR neurons containing GABAB receptors are involved in tonic inhibitory control over reward processes.

Amphetamine administration into the ventral striatum facilitates behavioral interaction with unconditioned visual signals in rats.

BACKGROUND: Administration of psychomotor stimulants like amphetamine facilitates behavior in the presence of incentive distal stimuli, which have acquired the motivational properties of primary rewards through associative learning. This facilitation appears to be mediated by the mesolimbic dopamine system, which may also be involved in facilitating behavior in the presence of distal stimuli that have not been previously paired with primary rewards. However, it is unclear whether psychomotor stimulants facilitate behavioral interaction with unconditioned distal stimuli. PRINCIPAL FINDINGS: We found that noncontingent administration of amphetamine into subregions of the rat ventral striatum, particularly in the vicinity of the medial olfactory tubercle, facilitates lever pressing followed by visual signals that had not been paired with primary rewards. Noncontingent administration of amphetamine failed to facilitate lever pressing when it was followed by either tones or delayed presentation or absence of visual signals, suggesting that visual signals are key for enhanced behavioral interaction. Systemic administration of amphetamine markedly increased locomotor activity, but did not necessarily increase lever pressing rewarded by visual signals, suggesting that lever pressing is not a byproduct of heightened locomotor activity. Lever pressing facilitated by amphetamine was reduced by co-administration of the dopamine receptor antagonists SCH 23390 (D1 selective) or sulpiride (D2 selective). CONCLUSIONS: Our results suggest that amphetamine administration into the ventral striatum, particularly in the vicinity of the medial olfactory tubercle, activates dopaminergic mechanisms that strongly enhance behavioral interaction with unconditioned visual stimuli.

Intracranial self-administration of MDMA into the ventral striatum of the rat: differential roles of the nucleus accumbens shell, core, and olfactory tubercle.

RATIONALE: Behavioral and anatomical data suggest that the ventral striatum, consisting of the nucleus accumbens and olfactory tubercle, is functionally heterogeneous. Cocaine and D: -amphetamine appear to be more rewarding when administered into the medial olfactory tubercle or medial accumbens shell than into their lateral counterparts, including the accumbens core. OBJECTIVES: We sought to determine whether rats self-administer the popular recreational drug (+/-)-3,4-methylenedioxymethamphetamine (MDMA) into ventrostriatal subregions and whether the medial olfactory tubercle and medial accumbens shell mediate MDMA's positive reinforcing effects more effectively than their lateral counterparts. RESULTS: Rats receiving 30 mM MDMA into the medial olfactory tubercle, medial accumbens shell, or accumbens core, but not the lateral tubercle or lateral shell, showed higher self-administration rates than rats receiving vehicle. The medial shell supported more vigorous self-administration of MDMA at higher concentrations than the core or medial olfactory tubercle. In addition, intra-medial shell MDMA self-administration was disrupted by co-administration of the D1 or D2 receptor antagonists SCH 23390 (1-3 mM) or raclopride (3-10 mM). CONCLUSIONS: Our data suggest that the ventral striatum is functionally heterogeneous. The medial accumbens shell appears to be more important than other ventrostriatal subregions in mediating the positive reinforcing effects of MDMA via both D1- and D2-type receptors. Together with previous data, our data also suggest that unidentified actions of MDMA interfere with the positive reinforcing effects of dopamine in the medial olfactory tubercle.

Dual role of medial A10 dopamine neurons in affective encoding.

Increasing evidence suggests that the activation of medial A10 neurons mediates positive affective encoding. However, little is known about the functions of the inhibition of midbrain dopamine neurons. Here we show evidence suggesting that the inhibition of medial A10 neurons mediates a negative affective state, leading to negative affective encoding, whereas blunting the activation of medial A10 neurons disrupts positive affective encoding involving food reward. We used a microinjection procedure, in which the D(2) dopamine receptor agonist quinpirole was administered into the cell body region of the dopamine neurons, a procedure that reduces dopamine cell firing. Microinjections of quinpirole into the posteromedial ventral tegmental area, but not its more lateral counterparts, led to conditioned place aversion. Quinpirole administration to this site also decreased food intake and basal dopamine concentration in the ventromedial striatum, a major projection area of medial A10 neurons. In addition, moderate quinpirole doses that did not lead to conditioned place aversion or disrupt food intake abolished food-conditioned place preference, suggesting that blunting dopamine impulse activity in response to food reward disrupts positive affective encoding in associated external stimuli. Our data support the hypothesis that activation of medial A10 dopamine neurons mediates a positive affective state, leading to positive affective encoding, while their inhibition mediates a negative affective state, leading to negative affective encoding. Together with previous findings, we propose that medial A10 neurons are an important component of the mechanism via which animals learn to avoid negative incentive stimuli.

Differential noradrenergic influence on seizure expression in genetically Fast and Slow kindling rat strains during massed trial stimulation of the amygdala.

The involvement of alpha(2) noradrenergic receptors during amygdala 'massed' stimulation (MS) was examined in rats that were selectively bred to be seizure-prone (Fast) or seizure-resistant (Slow) to amygdala kindling. The selective alpha(2) noradrenergic agonist guanfacine, or the antagonist idazoxan, was intraperitoneally injected during the MS procedure to study subsequent changes in afterdischarge (AD) threshold, AD duration and behavioral seizure expression. These measurements were again assessed weekly for 2 weeks after the MS treatment. Daily kindling began immediately thereafter. Following 6 stage-5 once daily convulsive seizures, guanfacine or idazoxan were re-administered. With idazoxan, the Slow rats expressed greater numbers of convulsive seizures and longer AD durations compared to guanfacine or saline controls during MS treatment. This pro-convulsive property of idazoxan was absent in Fast rats. By contrast, Fast rats showed enhanced convulsive expression in the presence of guanfacine. In the fully kindled rat, idazoxan and guanfacine differentially impacted seizure duration and severity in the Slow rats, but again not in the Fast rats. These data suggest that some aspect(s) of the alpha(2) noradrenergic system in the Fast and Slow rats are dissimilar and the mechanisms by which these receptors govern seizure genesis and propagation may be genetically controlled and distinct.

Amygdala amino acid and monoamine levels in genetically Fast and Slow kindling rat strains during massed amygdala kindling: a microdialysis study.

We investigated the neurochemistry of epileptic seizures in rats selectively bred to be seizure-prone (Fast) vs. seizure-resistant (Slow) to amygdala kindling. Microdialysis was used to measure levels of amino acids [glutamate, aspartate and gamma-aminobutyric acid (GABA)] and monoamines (noradrenaline, dopamine and serotonin) during 'massed' stimulation (MS) (every 6 min) of the ipsilateral amygdala for a total of 40 stimulation trials. Behavioral seizure profiles together with their afterdischarge thresholds (ADTs) and associated durations were assessed during the procedure, and subsequently were redetermined 1, 7 and 14 days later. Then normal 'daily' kindling commenced and continued until the animal reached the fully kindled state. During MS, several generalized seizures were triggered in Fast rats that were associated with long afterdischarge (AD) durations and intermittent periods of elevated thresholds, but in Slow rats, most stimulations were associated with stable ADTs and short ADs. Progressively increasing extracellular glutamate and decreasing GABA was observed in Fast rats during the MS, whereas Slow rats showed levels similar to baseline values. Levels of noradrenaline and dopamine, but not of serotonin, were also increased in both strains throughout the MS treatment. In Fast rats, a dramatic lengthening of AD durations occurred 7 and 14 days following MS, as well as subsequent strong positive transfer to daily kindling, all of which were not seen in Slow rats. Together, these results show that repeated, closely spaced stimulations of the amygdala can differentially alter excitatory and/or inhibitory transmitter levels in a seizure network, and that sensitivity to this manipulation is genetically determined.

Changes in extracellular levels of amygdala amino acids in genetically fast and slow kindling rat strains.

A neurochemical basis for many of the epilepsies has long been suspected to result from an imbalance between excitatory and inhibitory neurotransmitter mechanisms. Data supporting changes in extrasynaptic amino acid levels during epileptogenesis, however, remain controversial. In the present study, we used in vivo microdialysis to measure the levels of extracellular GABA (gamma-aminobutyric acid) and glutamate during seizure development in rats with a genetic predisposition for (Fast), or against (Slow), amygdala kindling. Dialysates were collected from both amygdalae before, during, and up to 12 min after a threshold-triggered amygdala afterdischarge (AD). One hour later, samples were again collected from both amygdalae in response to a hippocampal threshold AD. Daily amygdala kindling commenced the next day but without dialysis. After the rats were fully kindled, the same protocol was again employed. Amino acid levels were not consistently increased above baseline with triggered seizures in either strain. Instead, before kindling, a focal seizure in the Slow rats was associated with a large decrease in GABA in the non-stimulated amygdala, while amino acid levels in the Fast rats remained near baseline in both amygdalae. Similar results were seen after kindling. By contrast, before and after kindling, hippocampal stimulation caused large decreases in all amino acid levels in both amygdalae in both strains. These data suggest that, in response to direct stimulation, extracellular amino acid concentrations remain stable in tissues associated with either greater natural (Fast) or induced (kindled Fast/Slow) excitability, but are lowered with indirect stimulation (hippocampus) and/or low excitability.