Noradrenergic sub-sensitivity in the cerebral cortex of the tottering mouse, a spontaneously epileptic mutant

The glycogenolytic action of norepinephrine (NE) was examined in the tottering mouse, a spontaneously epileptic mutant which presents a noradrenergic hyperinnervation of various CNS areas, including the cerebral cortex. The potency and efficacy of NE in promoting glycogenolysis were markedly decreased in cerebral cortical slices prepared from homozygous tottering (tg/tg) when compared to control C57BL/6j (+/+) mice, indicating a sub-sensitive response to a cellular action of NE. The metabolic nature of this adaptive change suggests that an impaired capacity of NE in mobilizing energy substrates may be related to the expression of the epileptic symptomatology in this mutant.     

Increased methionine-enkephalin levels in genetically epileptic (tg/tg) mice.

Recent experimental data indicate that endogenous brain ligands for the opioid receptors such as enkephalins, beta-endorphin (beta-End) and dynorphin (Dyn) may be involved in both generalized and partial seizures. The "tottering" (tg/tg) mouse provides an electrophysiological representation of generalized spontaneous human epilepsy. These mice exhibit behavioral absence seizures with accompanying spike-wave discharges. Methionine-enkephalin (M-Enk), beta-End and Dyn levels in various regions of brain were measured by radioimmunoassay (RIA) in 15-18-week-old tg/tg and control (+/+) mice to elucidate the relation between seizures and the opioid system. beta-End and Dyn levels were similar in tg/tg and +/+ mice. However, M-Enk levels were significantly increased in the striatum, cortex, pons and medulla of the tg/tg mice. Our data suggest that in the tottering mouse model of generalized epilepsy there is an alteration of enkephalinergic pathways and not of the endorphinergic or dynorphinergic pathways.     

Development of the paramedian lobule of the cerebellum in wild-type and tottering mice.

The mutant mouse tottering, (tg/tg), and the compound heterozygote mouse (tg/tg1a) exhibit three neurological disorders: ataxia, petit mal-like absence seizures and myoclonic intermittent movement disorders which are independent of the absence seizures. The tottering mouse carries an autosomal recessive single gene mutation on chromosome 8, and behavioral symptoms are first observed in the 3rd to 4th week of age. Using an additional genetic marker, Oligosyndactyly (Os), it is possible to distinguish tg/tg and tg/tg1a mice from wild-type mice at birth; nonaffected heterozygous littermates carry the Os mutation while tottering and compound heterozygous mice do not carry the Os gene. Similar to neurons found elsewhere in the brain, cerebellar Purkinje cells in both the wild-type and mutant mice were found to decrease in diameter with maturation. Forebrain weight, hindbrain weight, Purkinje cell dimensions and the thickness of the molecular layer in the paramedian lobule of the cerebellum in mutant mice were found to be reduced in mutants after, but not prior to the onset of behavioral symptoms.     

Anticonvulsant sensitivity of absence seizures in the tottering mutant mouse

Homozygous tottering mice (tg, autosomal recessive) exhibit frequent spontaneous "absence" seizures accompanied by bilaterally synchronous spike-and-wave or polyspike electrocorticographic discharges. In adult tottering mice, the antiepileptic effects of a single dose of ethosuximide, diazepam, phenobarbital, or phenytoin were assessed using continuous electrocorticographic recording to monitor seizure incidence. The dose chosen for each drug was selected to correspond to an effective antiepileptic dose in standard murine models. Ethosuximide, 150 mg/kg, diazepam, 1.4 mg/kg, and phenobarbital, 25 mg/kg were effective against absence seizures. In contrast, phenytoin at 5, 10, 30, or 60 mg/kg produced no significant reduction in the incidence of absence seizures. These results suggest that absence seizures in the tottering mutant may represent a relatively specific pharmacological model for the identification of drugs effective for clinical absence epilepsy, and emphasize the potential value of this new model for the study of fundamental mechanisms of absence of epilepsy and the actions of antiepileptic drugs.     

Expression of tyrosine hydroxylase in cerebellar Purkinje neurons of the mutant tottering and leaner mouse.

In situ hybridization histochemistry, Northern blot analysis and immunohistochemistry were used to examine tyrosine hydroxylase (TH) mRNA concentrations and immunoreactivity in the locus coeruleus and cerebellum of the tottering (tg/tg), leaner (tgla/tgla), compound heterozygous (tg/tgla) and wild type control (+/+) mice, bred on a C57BL/6J background. Cerebellar Purkinje neurons, long considered to be GABAergic, showed high levels of TH mRNA in the caudal vermis and the lateral hemispheres of the cerebellum of tg/tg, tg/tgla, and tgla/tgla mice. Analysis of grain density over individual Purkinje cells showed significantly greater concentrations of TH mRNA in tg/tg, tg/tgla, and tgla/tgla mice as compared to +/+ wild type control mice. Comparison of adult (greater than or equal to 2 months) and young, pre-seizure (less than or equal to 3 weeks) mutant mice showed Purkinje cells densely labelled for TH mRNA at both ages, suggesting that TH gene expression in Purkinje cells is independent of the onset of seizures. Northern blot analysis confirmed the findings from the in situ hybridization studies, demonstrating a single band identical to TH mRNA. Immunohistochemistry confirmed the presence of TH protein in Purkinje cells of the caudal vermis and the lateral hemispheres of the cerebellum in both control and mutant mice. Quantitation of mRNA for TH and the coexisting neuropeptide, galanin, in the locus coeruleus detected no significant differences between adult tg/tg, tg/tgla and +/+ control mice. The present findings demonstrate that the classically GABAergic Purkinje cells in the cerebellum express low levels of TH, and that the mutant tottering and leaner strains of mice express extremely high levels of mRNA and protein for TH.     

Glutathione levels in specific brain regions of genetically epileptic (tg/tg) mice

The tottering (tg/tg) mouse is a genetic model of human generalized epilepsy; these mice exhibit spontaneous absence seizures accompanied by bilaterally synchronous spike-wave discharges (6). The mechanism(s) for seizure activity are unknown in these mice. Several recent studies have suggested that membrane lipid peroxidation may be causally involved in some forms of experimentally induced epilepsies (18). Since reduced glutathione (GSH) is the most important free radical scavenging compound in vivo that can prevent membrane lipid peroxidation, the objective of this study was to investigate GSH concentrations in specific central nervous system regions of genetically epileptic, tg/tg, mice as compared to age-matched controls. Three brain regions, cerebellum, hippocampus, and occipital cortex, were dissected, weighed and the concentrations of reduced and oxidized glutathione (GSH and GSSG, respectively) were measured in each of these tissues. GSH content was significantly lower in the occipital cortex of tg/tg mice compared to controls; no differences were observed in the other two brain regions examined. Total GSH content (GSH plus 2 x GSSG) paralleled GSH concentration differences. GSSG content from tg/tg mice was lower in the hippocampus and occipital cortex, compared to controls. This is the first report of an association between decreased central nervous system glutathione concentrations and seizure activity in animals exhibiting generalized seizures.     

Seizure susceptibility of neuropeptide-Y null mutant mice in amygdala kindling and chemical-induced seizure models

Neuropeptide Y (NPY) administered exogenously is anticonvulsant, and, NPY null mutant mice are more susceptible to kainate-induced seizures. In order to better understand the potential role of NPY in epileptogenesis, the present studies investigated the development of amygdala kindling, post-kindling seizure thresholds, and anticonvulsant effects of carbamazepine and levetiracetam in 129S6/SvEv NPY(+/+) and NPY(-/-) mice. In addition, susceptibility to pilocarpine- and kainate-induced seizures was compared in NPY(+/+) and (-/-) mice. The rate of amygdala kindling development did not differ in the NPY(-/-) and NPY(+/+) mice either when kindling stimuli were presented once daily for at least 20 days, or, 12 times daily for 2 days. However, during kindling development, the NPY(-/-) mice had higher seizure severity scores and longer afterdischarge durations than the NPY(+/+) mice. Post-kindling, the NPY(-/-) mice had markedly lower afterdischarge thresholds and longer afterdischarge durations than NPY (+/+) mice. Carbamazepine and levetiracetam increased the seizure thresholds of both NPY (-/-) and (+/+) mice. In addition, NPY (-/-) mice had lower thresholds for both kainate- and pilocarpine-induced seizures. The present results in amygdala kindling and chemical seizure models suggest that NPY may play a more prominent role in determining seizure thresholds and severity of seizures than in events leading to epileptogenesis. In addition, a lack of NPY does not appear to confer drug-resistance in that carbamazepine and levetiracetam were anticonvulsant in both wild type (WT) and NPY null mutant mice.     

Basal and drug-induced cAMP levels in cortical slices from the tottering mouse

Basal and drug-induced levels of cAMP were determined in cortical slices from mice which were homozygous for the tottering (tg/tg) gene defect as well as from co-isogenic controls (+/+). Basal levels of cAMP were 77 +/- 16% higher in tg/tg slices compared to the controls. This difference was abolished by exposure of the slices to propranolol, a beta-adrenergic receptor antagonist. Both isoproterenol and veratridine stimulated cAMP formation, but only small differences were observed in the cAMP levels in tg/tg and +/+ slices after this treatment. Of the veratridine-dependent increase in cAMP, approximately 40% was blocked by propranolol treatment of slices from both strains. The results suggest that a higher level of endogenous norepinephrine release in tottering mice contributes to an elevation of basal cAMP levels.     

Glutathione levels in specific brain regions of genetically epileptic (tg/tg) mice

The tottering (tg/tg) mouse is a genetic model of human generalized epilepsy; these mice exhibit spontaneous absence seizures accompanied by bilaterally synchronous spike-wave discharges (6). The mechanism(s) for seizure activity are unknown in these mice. Several recent studies have suggested that membrane lipid peroxidation may be causally involved in some forms of experimentally induced epilepsies (18). Since reduced glutathione (GSH) is the most important free radical scavenging compound in vivo that can prevent membrane lipid peroxidation, the objective of this study was to investigate GSH concentrations in specific central nervous system regions of genetically epileptic, tg/tg, mice as compared to age-matched controls. Three brain regions, cerebellum, hippocampus, and occipital cortex, were dissected, weighed and the concentrations of reduced and oxidized glutathione (GSH and GSSG, respectively) were measured in each of these tissues. GSH content was significantly lower in the occipital cortex of tg/tg mice compared to controls; no differences were observed in the other two brain regions examined. Total GSH content (GSH plus 2 × GSSG) paralleled GSH concentration differences. GSSG content from tg/tg mice was lower in the hippocampus and occipital cortex, compared to controls. This is the first report of an association between decreased central nervous system glutathione concentrations and seizure activity in animals exhibiting generalized seizures.     

Neurotransmitter Receptors in Adult Tottering (tg/tg) Mice

Neurotransmitter receptor binding was analyzed in adult tottering (tg/tg) and control wild-type mice. Saturation studies were performed to analyze the density of muscarinic cholinergic receptors in whole brain, cortical, and hippocampal homogenates of 8-9-week-old animals. Scatchard plot analysis was also performed to determine the density and affinity of alpha-adrenergic and beta-adrenergic receptors. No significant difference in Bmax or Kd values was identified between adult tottering and control mice in any of the tissue preparations. The amount of radioligand binding to 5-hydroxytryptamine 1A (5-HT1A), non-5-HT1A, 5-HT2, dopamine D2, and benzodiazepine receptors was also identical in tottering and control mice. These findings suggest that the epilepsy expressed by adult tottering mice does not result from alterations in the density or affinity of the neurotransmitter receptors studied.     

Caffeine Blocks Absence Seizures in the Tottering Mutant Mouse

The neurological tottering mutant mouse is characterized by frequent "absence" seizures accompanied by bilateral synchronous spike and wave EEG bursts. Under anesthesia, adult homozygous tottering mice were implanted with permanent epidural electrodes, and at least 7 days elapsed before electrocorticograms in unrestrained mice were scored for seizure incidence and duration. Caffeine (5, 10, 15 mg/kg, n = 8) injected intraperitoneally (i.p.) at the fourth hour of 8-h recording sessions significantly (p less than 0.001 for 10 and 15 mg) decreased seizure incidence as compared with control saline injections. Spike and wave bursts were eliminated during the 30 min after injection and reached 50% preinjection levels between the first and the second hour after injection. Another central nervous system (CNS) stimulating drug, amphetamine (1 mg/kg; n = 5), under identical conditions failed to decrease seizure incidence in this mutant.     

Tottering mouse motor dysfunction is abolished on the Purkinje cell degeneration (pcd) mutant background

Tottering (tg) mice inherit a recessive mutation of the calcium channel alpha 1A subunit gene, which encodes the pore-forming protein of P/Q-type voltage-sensitive calcium channels and is predominantly expressed in cerebellar granule and Purkinje neurons. The phenotypic consequences of the tottering mutation include ataxia, polyspike discharges, and an intermittent motor dysfunction best described as paroxysmal dystonia. These dystonic episodes induce c-fos mRNA expression in the cerebellar circuitry, including cerebellar granule and Purkinje neurons, deep cerebellar nuclei, and the postsynaptic targets of the deep nuclei. Cellular abnormalities associated with the mutation include hyperarborization of brainstem nucleus locus ceruleus axons and abnormal expression of L-type calcium channels in cerebellar Purkinje cells. Here, the role of these two distinct neural pathways in the expression of tottering mouse intermittent dystonia was assessed. Lesion of locus ceruleus axons with the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzyl-amine (DSP-4) did not affect the frequency of tottering mouse dystonic episodes. In contrast, removal of cerebellar Purkinje cells with the Purkinje cell degeneration (pcd) mutation by generation of tg/tg; pcd/pcd double mutant mice completely eliminated tottering mouse dystonia. Further, the c-fos expression pattern of tg/tg; pcd/pcd double mutants following restraint was indistinguishable from that of wild-type mice, suggesting that the pcd lesion eliminated an essential link in this abnormal neural network. These data suggest that the cerebellar cortex, where the mutant gene is abundantly expressed, contributes to the expression of tottering mouse dystonic episodes.     

Neuronal to oligodendroglial α-synuclein redistribution in a double transgenic model of multiple system atrophy

Multiple system atrophy is a sporadic, progressive, neurodegenerative disease characterized by an oligodendroglial accumulation of alpha-synuclein (α-syn). The mechanisms underlying the oligodendroglial accumulation of α-syn in the brains of patients with multiple system atrophy have attracted a great deal of interest, given the primarily neuronal role reported for this protein. We examined the interactions between neuronal and oligodendroglial α-syn in the progeny of crosses between parental transgenic (tg) mouse lines that express α-syn either under the oligodendroglial-specific myelin-basic protein promoter (MBP1-hα-syn tg) or under the neuronal platelet-derived growth factor promoter (PDGF-hα-syn tg). Our results demonstrate that progeny from the cross [hα-syn double (dbl) tg mice] displayed a robust redistribution of α-syn accumulation, with a relocalization from a neuronal or a mixed neuronal/oligodendroglial α-syn expression to a more oligodendroglial pattern in both the neocortex and the basal ganglia that closely resembled the parental MBP-hα-syn tg line. The hα-syn dbl tg mice also displayed motor deficits, concomitant with reduced levels of tyrosine hydroxylase and augmented neuropathological alterations in the basal ganglia. These results suggest that the central nervous system milieu in the hα-syn dbl tg mice favors an oligodendroglial accumulation of α-syn. This model represents an important tool to examine the interactions between neuronal and oligodendrocytic α-syn in diseases such as multiple system atrophy.     

A static magnetic field modulates severity of audiogenic seizures and anticonvulsant effects of phenytoin in DBA/2 mice

RATIONALE: In a search for potential supplements or alternatives to the pharmacological treatment of epilepsy, we examined the effects of static magnetic fields on audiogenic seizures of DBA/2 mice. METHODS: Two strains of DBA/2 mice were subjected to auditory stimulation that resulted sequentially in wild running, loss of righting, clonus, tonic hindlimb extension, and death in 80-95% of animals in different experiments. The incidence of seizure stages in groups of animals pretreated with a static magnetic field, phenytoin (PHT) or both was compared to the incidence in sham-exposed control mice. RESULTS: Depending on magnetic flux density and duration of exposure to the field, seizure severity decreased significantly, but not completely, in both strains. However, incidence of five seizure stages was reduced in one strain, with about half of the mice seizure free. Two seizure stages (tonic hindlimb extension and death) were reduced significantly in the other. Magnetic field pretreatment potentiated the effect of PHT. Clonic seizures refractory to PHT or magnetic field pretreatment in DBA/2J mice responded to pretreatment with a combination of PHT and the magnetic field. CONCLUSIONS: A static magnetic field had some anticonvulsant effects when employed alone. More robust effects were seen in combination with PHT. Further testing of magnetic fields for anticonvulsant effects and elucidation of mechanisms of action seem to be warranted.     

Decreased gene expression of calretinin and ryanodine receptor type 1 in tottering mice

Tottering mice are a spontaneously occurring animal model of human absence epilepsy. They carry a mutation in the P/Q-type calcium channel alpha1A subunit gene which is highly expressed by cerebellar Purkinje cells. In this study, we investigated the role of calretinin and ryanodine receptor type 1 (RyR1) gene expression in the cerebellum of tottering mice. Cerebellar tissue specimens from four experimental groups were processed for in situ hybridization histochemistry (ISHH): (1) wild-type (+/+); (2) heterozygous (tg/+) and two homozygous groups; either (3) without occurrence of an episode of paroxysmal dyskinesia (tg/tg-N); or (4) after an episode of paroxysmal dyskinesia (tg/tg-P) that lasted about 45 min on average. Quantitative analysis showed a statistically significant decrease (p = 0.0001, ANOVA) of calretinin gene expression at the level of the simple lobule of the cerebellum in both homozygous groups compared to the wild-type and heterozygous groups. RyR1 was decreased in the flocculus of the cerebellum in both the tg/tg-N and tg/tg-P groups compared to wild type (p = 0.0174, ANOVA). These results suggest that calretinin gene expression, as well as other genes involved in regulation of calcium homeostasis, such as RyR1, may play a role in the biochemical functional alterations present in tottering mice.     

Single gene defects in mice: the role of voltage-dependent calcium channels in absence models.

Nineteen genes encoding alpha1, beta, gamma, or alpha2delta voltage-dependent calcium channel subunits have been identified to date. Recent studies have found that three of these genes are mutated in mice with generalised cortical spike-wave discharges (models of human absence epilepsy), emphasising the importance of calcium channels in regulating the expression of this inherited seizure phenotype. The tottering (tg) locus encodes the calcium channel alpha1 subunit gene Cacna1a, lethargic (lh) encodes the beta subunit gene Cacnb4, and stargazer (stg) encodes the gamma subunit gene Cacng2. These calcium channel mutants should provide important insights into the basic mechanisms of neuronal synchronisation, and the genes may be considered candidates for involvement in similar human disorders. The mutant models offer an important opportunity to elucidate the molecular, developmental, and physiological mechanisms underlying one subtype of absence epilepsy. Since calcium channels are involved in numerous cellular functions, including proliferation and differentiation, membrane excitability, neurite outgrowth and synaptogenesis, signal transduction, and gene expression, their role in generating the absence epilepsy phenotype may be complex. A comparative analysis of channel function and neural excitability patterns in tottering, lethargic, and stargazer brain should be useful in identifying the common elements of calcium channel involvement in these absence models.     

Proepileptic phenotype of SV2A-deficient mice is associated with reduced anticonvulsant efficacy of levetiracetam

Purpose:   Synaptic vesicle protein 2A (SV2A) constitutes a distinct binding site for an antiepileptic drug levetiracetam (Keppra). In the present study we characterized SV2A (+/−) heterozygous mice in several seizure models and tested if the anticonvulsant efficacy of levetiracetam is reduced in these mice.
Methods:   Seizure thresholds of male SV2A (+/−) mice and their wild-type littermates were assessed in pilocarpine (i.p.), kainic acid (s.c.), pentylenetetrazol (i.v.), 6-Hz and maximal electroshock models. Kindling development was compared in amygdala and corneal kindling models. Ex vivo binding of levetiracetam to SV2A was also performed.
Results:   Long-term electroencephalography (EEG) monitoring and behavioral observations of SV2A (+/−) mice did not reveal any spontaneous seizure activity. However, a reduced seizure threshold of SV2A (+/−) mice was observed in pilocarpine, kainic acid, pentylenetetrazol, and 6-Hz models, but not in maximal electroshock seizure model. Accelerated epileptogenesis development was also demonstrated in amygdala and corneal kindling models. Anticonvulsant efficacy of levetiracetam, defined as its ability to increase seizure threshold for 6 Hz electrical stimulation, was significantly reduced (approx. 50%) in the SV2A (+/−) mice, consistently with reduced binding to SV2A in these mice. In contrast, valproate produced the same anticonvulsant effect in both SV2A (+/+) and SV2A (+/−) mice.
Discussion: The present results evidence that SV2A is involved in mediation of the in vivo anticonvulsant activity of levetiracetam, in accordance with its previously proposed mechanism of action. Furthermore, the present data also indicate that even partial SV2A deficiency may lead to increased seizure vulnerability and accelerated epileptogenesis.     

Long-term enhancement of postsynaptic excitability after brief exposure to Mg2(+)-free medium in normal and epileptic mice

Brief exposure to Mg2(+)-free medium (MFM) enhanced the population response of CA1 neurons to stratum radiatum stimulation in hippocampal slices from normal (+/?) and epileptic tottering (tg/tg) mice. The enhancement was maintained in both groups for at least 2 h following reperfusion with normal medium (NM). Excitability curves obtained from the extracellular records suggest that, while both synaptic activation and postsynaptic excitability are enhanced during MFM perfusion, only the latter enhancement is maintained at significant levels after reperfusion with NM. The long-term increase in postsynaptic excitability was comparable in strength to that produced by long-term potentiation (LTP) inducing tetanic stimuli, was accompanied by an increase in the slope of the population spike/field excitatory postsynaptic potential (PS/fEPSP) curve and did not appear to depend on the induction of epileptiform activity by MFM. Both the short- and the long-term effects of MFM on synaptic activation and postsynaptic excitability were qualitatively similar in normal and epileptic mice and any quantitative differences were not statistically significant. Thus, epileptogenesis in the tottering mutant may not involve a change in the NMDA receptor-mediated control of excitability, at least in the CA1 area of hippocampus.     

Reduced anticonvulsant efficacy of valproic acid in dopamine beta-hydroxylase knockout mice

Valproic acid (VPA) is a widely used treatment for both epilepsy and bipolar disorders, although its therapeutic mechanism of action is not fully understood. Because norepinephrine (NE) is implicated in seizure susceptibility and affective disorders, and given previous findings indicating that VPA can act on the NE system, it is possible that NE may mediate some of the therapeutic actions of VPA. To test this hypothesis, we measured flurothyl-induced seizure susceptibility and severity parameters after both acute and chronic VPA treatments in dopamine beta-hydroxylase knockout (Dbh -/-) mice that lack NE. We found that the protective effects of acute VPA on seizure susceptibility, as measured by latency to first myoclonic jerk, were attenuated in Dbh -/- mice. Further, while acute VPA reduced the number of control mice that progressed to tonic extension, VPA did not reduce seizure severity in Dbh -/- mice. The carryover anticonvulsant effects following cessation of chronic VPA treatment were similar in both genotypes. Therefore, we conclude that NE is involved in some of the anticonvulsant effects of VPA, especially the effect of acute VPA on seizure severity.     

Seizure prophylaxis in an animal model of epilepsy by dietary fluoxetine supplementation

Clinical and animal model evidence suggests that selective serotonin reuptake inhibitors (SSRIs) act as anticonvulsants. The present studies tested the possibility that the El mouse model of genetically predisposed/handling-triggered epilepsy would exhibit fewer seizures following SSRI treatment via dietary fluoxetine adulteration. In particular, potential bioenergetic and neural mechanisms for anticonvulsant efficacy of fluoxetine were explored using food intake/body weight monitoring and quantification of brain serotonin transporter protein. El mice consuming a chow diet ad libitum or yoked in quantity to fluoxetine diet intake exhibited seizure incidence of 40% in response to tail-suspension handling, whereas seizures were abolished (0%) among El mice consuming a fluoxetine-adultered diet over 7 days. A 3 day period of fluoxetine administration was insufficient to exert anticonvulsant efficacy and all treatment groups exhibited the same circadian locomotor activity patterns at the time of seizure susceptibility testing. Bioenergetic factors could not account for the anticonvulsant efficacy of fluoxetine since yoked diet controls with matched food intake, body weight change and blood glucose levels exhibited the same 40% seizure incidence as ad libitum chow controls. Importantly, the 7 day period of dietary fluoxetine exposure was effective in selectively reducing cell density in the parietal cortex and increasing serotonin transporter protein content in the nucleus accumbens. Taken together, these results suggest that dietary fluoxetine supplementation abolishes handling-induced seizure susceptibility in El mice via a neural remodeling mechanism independent of energy balance.