Amygdala Kindling Potentiates Seizure-Stimulated Immediate-Early Gene Expression in Rat Cerebral Cortex

Kindling induces long-term adaptations in neuronal function that lead to a decreased threshold for induction of seizures. In the present study, the influence of amygdala kindling on levels of mRNA for the immediate-early genes (IEGs) c-fos, c-jun, and NGF1-A were examined both before and after an acute electroconvulsive seizure (ECS). Although amygdala kindling did not significantly influence resting levels of c-fos mRNA in cerebral cortex, ECS-stimulated levels of c-fos mRNA (examined 45 min after ECS) were approximately twofold greater in the cerebral cortex of kindled rats relative to sham-treated controls. The influence of kindling on IEG expression was dependent on the time course of kindling, as ECS-stimulated levels of c-fos mRNA were not significantly increased in stage 2 kindled animals. ECS-stimulated levels of c-jun and NGF1-A mRNA were also significantly increased in cerebral cortex of kindled rats relative to sham-treated controls. The influence of kindling on IEG expression was long-lasting because an acute ECS stimulus significantly elevated levels of c-fos and c-jun mRNA in the cerebral cortex of animals that were kindled 5 months previously. In contrast to these effects in cerebral cortex, kindling did not influence ECS-stimulated levels of c-fos mRNA in hippocampus. Finally, immunohistochemical studies revealed lamina-specific changes in the cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)     

Stress-induced activation of the immediate early gene Arc (activity-regulated cytoskeleton-associated protein) is restricted to telencephalic areas in the rat brain: relationship to c-fos mRNA

Arc is an effector immediate early gene whose expression is induced in situations of increased neuronal activity. However, there is no report on the influence of stress on Arc expression. Here, we compared the induction of both c-fos and Arc mRNAs in the brain of rats exposed to one of three different stressful situations: novel environment, forced swimming and immobilization. An absent or weak c-fos mRNA signal was observed in control rats, whereas those exposed to one of three stressors showed enhanced c-fos expression in a wide range of brain areas. Constitutive Arc expression was observed in some areas such as cortex, striatum, hippocampus, reticular thalamic nucleus and cerebellar cortex. In response to stressors, a strong induction of Arc was observed, but the pattern was different from that of c-fos. For instance, activation of Arc but not c-fos was observed in the nucleus accumbens after immobilization and in the hippocampus after novel environment. No Arc induction was observed in diencephalic and brainstem areas. The present data show that Arc has a neuroanatomically restricted pattern of induction in the brain after emotional stress. Telencephalic activation suggests that a more intense induction of synaptic plasticity is occurring in this area after exposure to emotional stressors.     

Differential expression of immediate early genes in the hippocampus and spinal cord

We have demonstrated that immediate early genes can be differentially activated within the central nervous system. We examined the effects of tetanic stimulation in the hippocampus and of noxious sensory stimulation of the spinal cord on the expression of eight immediate early genes. Induction of long-term potentiation (LTP) in the dentate gyrus resulted in an increase in mRNA and protein for NGFI-A (also termed Zif/268, Egr-1, or Krox 24), and less consistently for jun-B mRNA. No increase was seen for c-fos, NGFI-B, c-jun, jun-D, SRF, or PC4 mRNAs. Blockade of the NMDA receptor prevented the induction of both LTP and NGFI-A mRNA in the dentate gyrus. However, commissural stimulation, which prevented the induction of LTP, resulted in bilateral activation of all the genes examined, including NGFI-A. No change was seen in animals trained in a water maze. These results suggest that no simple relationship exists between LTP, spatial learning, and immediate early gene induction. Stimulation of sensory fibers resulted in an increase in mRNA for NGFI-A, c-fos, SRF, NGFI-B, and c-jun in spinal cord neurons. Blockade of the NMDA receptor had no effect on immediate early gene induction in the spinal cord.     

Repeated treatment with mirtazepine induces brain-derived neurotrophic factor gene expression in rats.

Recent studies indicate a role of the brain-derived neurotrophic factor (BDNF) in the pathophysiology of depression, as well as in the mechanism of action of antidepressant drugs (ADs). It has been shown that serum BDNF levels are decreased in depressed patients. Moreover, antidepressant treatment increases serum BDNF levels and it is positively correlated with medication response. In addition, repeated administration of ADs induces an increase in rat hippocampal or cortical BDNF gene expression. Since the most potent effect of ADs on BDNF gene expression was found after prolonged treatment, in the present study we investigated the influence of repeated treatment (twice daily for 14 days) of the new AD mirtazapine (5 or 10 mg/kg) on BDNF mRNA level (the Northern blot) in rat hippocampus and cerebral cortex. Imipramine was used as a reference compound. The experiment was carried out on male Wistar rats. The tissue for biochemical assays was collected 24 h after the last doses of mirtazapine and imipramine. We also studied the effect of repeated mirtazapine on the action of the 5-HT2A receptor agonist (+/-)DOI in the behavioral test (head twitches induced by (+/-)DOI) in rats. The obtained results showed that, like imipramine (10 mg/kg), mirtazapine (10 mg/kg) increased BDNF gene expression in both the examined brain regions: in the hippocampus by 24.0 and 26.5%, in the cerebral cortex by 29.9 and 41.5%, respectively, compared with the vehicle-treated control. Neither mirtazapine nor imipramine administered repeatedly at a lower dose (5 mg/kg) significantly changed BDNF mRNA levels in the hippocampus and cerebral cortex. Repeated treatment with mirtazapine (10, but not 5 mg/kg) inhibited the behavioral syndrome induced by (+/-)DOI. This study provides first conclusive evidence that repeated mirtazapine administration increases BDNF mRNA levels; moreover, it indicates that the enhancement of BDNF gene expression may be essential for the clinical effect of mirtazapine.     

Inducible brain COX-2 facilitates the recurrence of hippocampal seizures in mouse rapid kindling

Brain cyclooxygenase-2 (COX-2), the rate-limiting enzyme in prostaglandin synthesis, is rapidly and transiently induced by convulsions in hippocampal and cortical neurons. Therefore, we examined the effects of COX-2 on the 'rapid kindling' development in COX-2 knockout mice and in mice treated with nimesulide, a COX-2-selective inhibitor. Rapid kindling development was examined based on the incidence of hippocampal EEG seizures and behavioral seizures following repetitive electrical stimulation of the perforant path at an interval of 40 s, and on the total afterdischarge (AD) duration induced by 50 stimulations. In addition, we measured COX-2 mRNA expression by in situ hybridization and PGE2 concentration using enzyme immunoassay following rapid kindling stimulation. The results suggested that brain COX-2 mRNA levels were markedly increased in the hippocampal neurons and the concentration of PGE2 was elevated significantly, and that the incidence of AD and seizure behavior induction and the total AD duration were significantly decreased under conditions of COX-2 deficiency. Therefore, we concluded that inducible COX-2 facilitates the recurrence of hippocampal seizures.     

Amygdaloid kindling in alloxan-diabetic rats

Wistar rats, made diabetic by intravenous administration of alloxan, 40 mg/kg, were submitted to amygdala kindling. The EEG and behavioral responses elicited by stimulating the amygdala nuclei in these animals were compared with those observed in control rats. Alloxan-treated rats required more stimulation to kindle, had increased duration of afterdischarges (AD), presented intense interictal spiking, and exhibited greater number of wet-dog shakes than controls. Although the AD threshold was not different between control and experimental rats, the above results seem to indicate an increase in the local epileptic susceptibility represented by longer ADs. On the other hand, this increased local discharge seems to be unable to access the generalization mechanism, which can be verified by the increased kindling rate. Hyperosmolarity, pH alterations, or other generalized metabolic changes frequently associated with diabetes could be implicated in these results.     

The responsiveness of TrkB to BDNF and antidepressant drugs is differentially regulated during mouse development

Background

Previous studies suggest that the responsiveness of TrkB receptor to BDNF is developmentally regulated in rats. Antidepressant drugs (AD) have been shown to increase TrkB signalling in the adult rodent brain, and recent findings implicate a BDNF-independent mechanism behind this phenomenon. When administered during early postnatal life, ADs produce long-lasting biochemical and behavioural alterations that are observed in adult animals.

Methodology

We have here examined the responsiveness of brain TrkB receptors to BDNF and ADs during early postnatal life of mouse, measured as autophosphorylation of TrkB (pTrkB).

Principal Findings

We found that ADs fail to induce TrkB signalling before postnatal day 12 (P12) after which an adult response of TrkB to ADs was observed. Interestingly, there was a temporally inverse correlation between the appearance of the responsiveness of TrkB to systemic ADs and the marked developmental reduction of BDNF-induced TrkB in brain microslices ex vivo. Basal p-TrkB status in the brain of BDNF deficient mice was significantly reduced only during early postnatal period. Enhancing cAMP (cyclic adenosine monophosphate) signalling failed to facilitate TrkB responsiveness to BDNF. Reduced responsiveness of TrkB to BDNF was not produced by the developmental increase in the expression of dominant-negative truncated TrkB.T1 because this reduction was similarly observed in the brain microslices of trkB.T1 ^−/− mice. Moreover, postnatal AD administration produced long-lasting behavioural alterations observable in adult mice, but the responses were different when mice were treated during the time when ADs did not (P4-9) or did (P16-21) activate TrkB.

Conclusions

We have found that ADs induce the activation of TrkB only in mice older than 2 weeks and that responsiveness of brain microslices to BDNF is reduced during the same time period. Exposure to ADs before and after the age when ADs activate TrkB produces differential long-term behavioural responses in adult mice.     

The expression of the c-fos gene in the brain of mice in the dynamic acquisition of defensive behavioral habits

Levels of c-fos mRNA expression in mouse cerebral cortex and hippocampus at different stages of footshock escape and avoidance learning were studied by Northern hydridization. In the first series of experiments a mouse was presented with 30 electric footshock daily in a chamber where it could escape from the floor by jumping on the safe platform attached to the wall. A large increase in c-fos mRNA level in the cerebral cortex and hippocampus was observed during the first day of training. Mice that were trained for 9 consecutive days and acquired a footshock escape reaction showed no elevation of c-fos expression in the brain as compared to the quiet control group. In the second series of experiments the levels of c-fos expression were compared in individual mice trained to avoid the footshock by jumping on the platform in response to an auditory conditioned stimulus. Mice which acquired avoidance behavior more rapidly had lower c-fos mRNA levels than slow learners. There was no such to difference between the corresponding yoked control groups which consisted of animals matched the rapid and slow learners by the number of footshocks received. It is concluded that achievement of adaptive results in the course of learning leads to a suppression of further c-fos induction by motivational excitation.     

A burst of c-fos gene expression in the mouse occurs at birth.

Expression of the c-fos gene during murine perinatal development was studied. Before birth, all eight of the prenatal organs tested expressed undetectable or low levels of c-fos mRNA. On the day of birth, there occurred a 10- to 100-fold increase in the level of c-fos message in all of these organs. The expression was transient, in that 1 day after birth, the level of c-fos mRNA precipitously dropped. The c-fos gene expression at birth is unrelated to the expression of the c-myc gene and major histocompatibility complex class I genes, which display distinct kinetics during the perinatal development. The c-fos gene was also expressed locally and transiently in the gravid uterus 1 to 2 days prior to delivery. These results indicate that an event associated with birth induced c-fos gene expression in the mother and newborn.     

Convulsants induce interleukin-1 beta messenger RNA in rat brain.

The effects of systemic administration of kainic acid and pentylenetetrazol on interleukin-1 beta gene expression in the rat brain was studied. After the administration of kainic acid in a convulsive dose (10 mg/kg i.p.), Interleukin-1 beta mRNA was induced intensely in the cerebral cortex, thalamus and hypothalamus, moderately in the hippocampus and weakly in the striatum, but not in the midbrain, pons-medulla and cerebellum. Pentylenetetrazol induced Interleukin-1 beta mRNA in the cerebral cortex, hypothalamus, and hippocampus with a faster time-course than kainic acid. Diazepam suppressed both the convulsion and the induction of Interleukin-1 beta mRNA produced by kainic acid. Dexamethasone suppressed the induction of Interleukin-1 beta mRNA, but did neither the convulsion nor the induction of c-fos mRNA following the injection of kainic acid. These results provide the first evidence that intensive neuronal excitation induces Interleukin-1 beta mRNA in particular regions of the brain.     

Cell cycle dependent growth factor regulation of gene expression

The expression of the proto-oncogenes c-fos and c-myc is a rapid response of G0-arrested fibroblasts to serum and peptide growth factors; however, the role of the c-fos and c-myc gene products in subsequent cell cycle transit is not understood. We examined the expression of c-fos and c-myc mRNA in Balb/c 3T3 murine fibroblasts in response to platelet-derived growth factor (PDGF) and platelet-poor plasma, using arrest points associated with density dependent growth inhibition or metabolic inhibition to synchronize cells in S phase of the cell cycle. The expression of c-fos and c-myc mRNA in Balb/c 3T3 cells was differentially regulated with respect to growth factor dependence and cell cycle dependence. c-fos expression was induced in the presence of PDGF and was unaffected by plasma. The induction of c-fos expression in response to PDGF was cell cycle independent, occurring in cells transiting S phase and G2 as well as in G0 arrest. In contrast, c-myc expression was both growth factor and cell cycle dependent. In G0 arrested cells, c-myc expression was PDGF-dependent and plasma-independent, and PDGF was required for maintenance of elevated c-myc levels during G1 transit. In cells transiting S phase, c-myc mRNA was induced in response to PDGF, but was also plasma-dependent in S phase cells that had been "primed" by exposure to PDGF during S phase.