Differential expression of immediate early genes after hippocampal long-term potentiation in awake rats

BDNF restores the expression of Jun and Fos inducible transcription factors in the rat brain following repetitive electroconvulsive seizures

The expression of inducible transcription factors was studied following repetitive electroconvulsive seizures (ECS), c-Fos, c-Jun, JunB, and JunD immunoreactivities were investigated following a single (1 x ECS) or repetitive ECS evoked once per day for 4, 5, or 10 days (4 x ECS, 5 x ECS, or 10 x ECS). Animals were killed 3 or 12 h following the last ECS. Three hours after 1 x ECS, c-Fos was expressed throughout the cortex and hippocampus. After 5 x ECS and 10 x ECS, c-Fos was reexpressed in the CA4 area, but was completely absent in the other hippocampal areas and cortex. In these areas, c-Fos became only reinducible when the time lag between two ECS stimuli was 5 days. In contrast to c-Fos, intense JunB expression was inducible in the cortex and hippocampus, but not CA4 subfield, after 1 x ECS, 5 x ECS, and 10 x ECS. Repetitive ECS did not effect c-Jun and JunD expression. In a second model of systemic excitation of the brain, repetitive daily injection of kainic acid for 4 days completely failed to express c-Fos, c-Jun, and JunB after the last application whereas injection of kainic acid once per week did not alter the strong expressions compared to a single application of kainic acid. In order to study the maintenance of c-Fos expression during repetitive seizures, brain-derived neurotrophic factor (BDNF) was applied in parallel for 5 or 10 days via miniosmotic pumps and permanent cannula targeted at the hippocampus or the parietal cortex. Infusion of BDNF completely reinduced c-Fos expression during 5 x ECS or 10 x ECS in the cortex ipsilaterally to the cannula and, to a less extent, also increased the expression of c-Jun and JunB when compared to saline-treated controls. BDNF had no effect on the expression patterns in the hippocampus. ECS with or without BDNF infusion did not change the expression patterns of the constitutive transcription factors ATF-2, CREB, and SRF. These data demonstrate that various transcription factors substantially differ in their response to acute and chronic neural stimulation. Repetitive pathophysiological excitation decreases the transcriptional actions of neurons over days in the adult brain, and this decrement can be prevented by BDNF restoring the neuroplasticity at the level of gene transcription.

Basal expression, subcellular distribution, and up-regulation of the proto-oncogene c-JUN in the rat dentate gyrus after unilateral entorhinal cortex lesion

The expression of the transcription factor c-JUN was investigated in the rat fascia dentata under normal conditions and after entorhinal cortex lesion. As shown by immunocytochemistry and in situ hybridization histochemistry c-JUN and its messenger RNA are present in the principal cell layers of the dentate gyrus and Ammon's horn (except hippocampal region CA2). Pre-embedding immunogold electron microscopy revealed an almost exclusive nuclear localization of c-JUN, where it is associated with chromatin. In addition, double immunolabelling for c-JUN and parvalbumin demonstrated that c-JUN immunoreactivity is primarily found in principal neurons since GABAergic parvalbumin-positive interneurons did not express c-JUN. After unilateral electrolytic lesion of the entorhinal cortex c-JUN was strongly up-regulated in the ipsilateral dentate gyrus within 2 h postlesion. This up-regulation was also present in the contralateral fascia dentata 12 h after entorhinal cortex lesion and returned to control levels on both sides 24 h postlesion. The cellular distribution of c-JUN did not change after entorhinal cortex lesion: parvalbumin-positive interneurons never contained c-JUN. These results point to a specific role of c-JUN in the granule cells of the fascia dentata in the normal animal and in rats with entorhinal cortex lesions. The selective induction of c-JUN after entorhinal lesion could be one of the first molecular steps that regulate transneuronal changes within granule cells after their denervation. A different mechanism has to be assumed for GABAergic interneurons known to receive an entorhinal innervation as well.

Changes in protein synthesis and synthesis of the synaptic vesicle protein, synaptophysin, in entorhinal cortex following induction of long-term potentiation in dentate gyrus: an age-related study in the rat

We have examined protein synthesis in entorhinal cortex following induction of long-term potentiation (LTP) in perforant path-granule cell synapses. The data presented here indicate that there was an increase in [35S]methionine labelling of TCA-precipitated proteins and [35S]methionine labelling of synaptophysin in the ipsilateral entorhinal cortex 40 min after induction of LTP in dentate gyrus. Intraventricular injection of both the NMDA antagonist, D-amino-phosphonovalerate, and the protein synthesis inhibitor, anisomycin reduced protein synthesis though the decrease caused by anisomycin was much more profound. Both agents blocked induction of LTP and the increase in protein synthesis and synaptophysin synthesis which accompanied LTP. These data indicate a close coupling of increased protein synthesis in the entorhinal cortex and expression of LTP in the dentate gyrus. This coupling was further suggested by the absence of an LTP-associated increase in protein synthesis in aged animals, in which LTP was markedly attenuated. The possibility that these changes impact on morphological changes which accompany LTP is discussed.

Olfactory memory and maternal behaviour-induced changes in c-fos and zif/268 mRNA expression in the sheep brain

In sheep maternal behaviour and the formation of the selective olfactory, ewe/lamb bond are induced by feedback to the brain from stimulation of the vagina and cervix during parturition. In the present study, we have used in situ hybridization histochemistry to quantify changes in cellular expression of two immediately-early genes, c-fos and zif/268, in order to identify activated brain regions during the induction of maternal behaviour and olfactory bonding as well as regions where plastic changes are occurring during with the formation of the olfactory memory associated with bonding. Three different treatment groups were used. One group gave birth normally, became maternal and were allowed to interact with their lambs for 30 min. A second group received exogenous treatment with oestradiol and progesterone to induce lactation and then received a 5-min period of artificial stimulation of the vagina and cervix (VCS) which reliably induces maternal behaviour but could not interact with lambs. A final control group received exogenous hormone treatment but no VCS or interaction with lambs. Compared to the control group, post-partum animals and animals that had received VCS showed increased c-fos expression in a number of cortical regions (cingulate, entorhinal and somatosensory), the mediodorsal thalamic nucleus and the lateral habenula, the limbic system (bed nucleus of the stria terminalis, lateral septum, medial arnygdala, dentate gyrus and the CA3 region of the hippocampus) and the hypothalamus (medial preoptic area, mediobasal hypothalamus, paraventricular nucleus, supraoptic nucleus and periventricular complex). The group that gave birth and had contact with their lambs for 30 min had significantly enhanced c-fos mRNA expression in the cingulate cortex compared to those receiving VCS and additionally showed significantly increased c-fos mRNA expression in olfactory processing regions (olfactory bulb, piriform cortex and orbitofrontal cortex). Expression of zif/268 was significantly increased in the entorhinal cortex, orbitofrontal cortex and dentate gyrus of the parturition group compared to either the control or the VCS alone groups. These results show a clear differentiation between neural substrates controlling the expression of maternal behaviour and those involved in the olfactory memory process associated with selective recognition of offspring although at the level of the hippocampus and cingulate cortex there may be some degree of overlap. Alterations in zif/268 at tertiary processing sites for olfactory information (orbitofrontal cortex) and the entorhinal cortex and dentate gyrus may reflect plastic changes occurring during the early stages of olfactory memory formation.

TrkB expression in dentate granule cells is associated with a late phase of long-term potentiation

Recent studies have demonstrated that the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are induced in hippocampal neurons following the induction of long-term potentiation (LTP), a model of memory, and that BDNF and NT-3 (but not NGF) can induce LTP-like increases in synaptic efficacy. Receptors for these neurotrophins have been cloned and characterized and we investigated whether LTP alters the expression of two neurotrophin receptors, trkB (BDNF receptor) and trkC (NT-3 receptor) in dentate granule neurons of the hippocampus using in situ hybridization analysis. Results show that trkB is strongly induced in these neurons in an N-methyl-D-aspartate (NMDA) receptor-dependent manner. Moreover, the induction of trkB and trkC mRNAs was attenuated by sodium pentobarbital, which interferes with the durability of LTP. Low-frequency stimulation of the perforant path had no effect on trkB mRNA levels but significantly reduced trkC mRNA in dentate granule cells. Thus, both BDNF and its receptor trkB are induced in granule cells by stimulation that produces durable LTP, suggesting that this neurotrophin and its receptor play an important role in memory formation and may be suitable targets for the development of cognitive-enhancing drugs in the treatment of diseases, such as Alzheimer's.

Complex patterns of immediate early gene induction in rat brain following brightness discrimination training and pseudotraining

Following training of rats on a footshock-motivated brightness discrimination task in a Y-maze, different sets of transcription factor encoding immediate early genes (IEGs) were induced in anatomically distinct brain regions. As revealed by Northern analysis, mRNA levels of c-fos, jun-B and zif/268 increased in the hippocampus, while the expression of c-jun remained unchanged over a period of 7 h. In the cerebral cortex, c-jun was induced in addition to the other genes examined. In contrast, only c-fos, but not c-jun or zif/268 mRNAs were increased in the cerebellum. The induction of IEGs was rapid and transient, reaching maximal levels immediately after training and returning to basal levels within 2 h. Similar spatiotemporal expression patterns were observed in rats that received identical, but unpaired, stimuli in a pseudotraining procedure. Our results suggest that the initial prerequisites of learning, such as stimulus novelty, lead to an increased expression of IEG mRNAs after training and pseudotraining as an early necessary but not sufficient precondition for memory consolidation. Additional converging inputs might control at the transcriptional, translational or post-translational level the synthesis and biological effectiveness of proteins necessary to complete the formation of the memory trace in trained animals.

Effect of unilateral tympanotomy on auditory induced c-fos expression in cochlear nuclei

The immediate early gene, c-fos, signals expression of target genes. Three natural occurring physiological entities: (1) learning, (2) plasticity, and (3) stress are proposed to use c-fos gene expression to signal molecular changes in neurons. The objective of this study was to determine whether c-fos expression is predominately activated by stress or by novel events associated with learning and plasticity. The approach was to quantitate the number of neurons in cochlear nuclei which express Fos protein following short-term novel sound stimuli together with either uni- or bilateral tympanotomy so as to differentiate novel sound stimuli from stress activation. The results show that routinely experienced sounds do not elicit c-fos expression in medullary cochlear nuclei, but novel sounds produced a 25-fold increase in the number of active cells. Following unilateral tympanotomy with novel sound stimulation, only a small number of cells were activated, ipsilaterally, (partially deafened side) while contralaterally, there was a 30-fold increase. After normalization of the data for control values, the data clearly indicate that novelty of sound stimuli induce c-fos gene expression. Furthermore, bilateral tympanotomy (bilateral partial deafening) with sound stimulation activated both sides by 20-fold, indicating that the c-fos response followed the sound stimulation. The data allow us to conclude that stress generates only a small contribution to c-fos gene expression while novel stimuli are potent signals, strongly implicating c-fos in novelty induced adaptation processes involved in learning and plasticity.

Androgens selectively modulate C-fos messenger RNA induction in the rat hippocampus following novelty

We have previously shown that androgen receptors are found in high concentrations in hippocampal CA1 pyramidal cells. To begin to explore the possible roles for androgen receptors in this area of the brain, we studied the effects of endogenous and exogenous androgen on the behaviourally induced expression of cellular immediate early gene messenger RNAs. Adult male Fischer 344 rats were either gonadectomized, gonadectomized and given two Silastic capsules of dihydrotestosterone propionate at the time of surgery, or left intact. Three weeks later, animals were placed into a novel open field for 20 min. This behavioural paradigm caused region- and gene-specific increases of c-fos, jun-B, c-jun and zif268 messenger RNA in the hippocampus as determined by semi-quantitative in situ hybridization histochemistry. The removal of circulating androgen by gonadectomy potentiated, whereas dihydrotestosterone treatment of castrates attenuated, the behaviourally induced expression of c-fos messenger RNA in the CA1 region of the hippocampus. No changes in c-fos messenger RNA expression were detected in the CA3 or dentate gyrus regions where androgen receptor levels are low. Androgen status did not affect either the basal or stimulated expression of Jun-B, c-Jun or zif268 messenger RNA in any of the three cellular regions of the hippocampus examined. These results implicate androgen receptors in modulating the active response of hippocampal neurons to a behaviourally relevant stimulus. Since the products of cellular immediate genes can function to alter an array of downstream genes, the modulation of these genes in the hippocampus by gonadal hormones may have important ramifications for hippocampal function.

A defect in nurturing in mice lacking the immediate early gene fosB

Although expression of the Fos family of transcription factors is induced by environmental stimuli that trigger adaptive neuronal response, evidence that Fos family members mediate these responses is lacking. To address this issue, mice were generated with an inactivating mutation in the fosB gene. fosB mutant mice are profoundly deficient in their ability to nurture young animals but are normal with respect to other cognitive and sensory functions. The nurturing defect is likely due to the absence of FosB in the preoptic area, a region of the hypothalamus that is critical for nurturing. These observations suggest that a transcription factor controls a complex behavior by regulating a specific neuronal circuit and indicate that nurturing in mammals has a genetic component.

Stimulus-dependent, reciprocal up- and downregulation of glutamic acid decarboxylase and Ca2+/calmodulin-dependent protein kinase II gene expression in rat cerebral cortex

Long-train tetanic stimulation of the cerebral cortex induces long-term changes in the excitability of cortical neurons, while short-train electrical stimulation does not. In the present study, we show that both forms of stimulation when applied to rat motor cortex for 4 h enhance c-fos expression, but only tetanic stimulation, when imposed upon short-train stimulation, modulates gene expression for 67-kDa glutamic acid decarboxylase (GAD) and alpha Ca2+/calmodulin-dependent protein kinase II (CaMKII alpha). Gene expression for beta Ca2+/calmodulin-dependent protein kinase II is not affected by either stimulation mode. GAD messenger RNA (mRNA) is increased from 1 h after the end of tetanization to the longest poststimulus survival time investigated (14 h). CaMKII alpha mRNA is decreased 1-3 h after the end of tetanization but thereafter returns to prestimulus levels. These results imply not only that mechanisms underlying neocortical plasticity are stimulus-dependent but also that they involve reciprocal changes in molecules regulating the balance of excitation and inhibition.

A role for immediate-early transcription factors in learning and memory

This article summarizes recent studies from the long-term potentiation (LTP), long-term depression (LTD), and behavioral learning literature, indicating that immediate-early genes (IEGs) may play an important role in learning and memory. The LTP studies suggest that synaptic modifications occurring during NMDA-receptor-mediated hippocampal LTP and LTD are stabilized by the protein products of the krox family of IEGs (as well as by brain-derived neurotrophic factor, BDNF). Activation of muscarinic receptors also induces members of the krox as well as the fos and jun family (jun-B but not c-jun) IEGs in hippocampal neurons and this action may be involved in the facilitatory effects of muscarinic receptor activation on both hippocampal LTP and learning. The possible role of IEGs in the learning-enhancing effects of cholinergically mediated hippocampal theta is also discussed. Finally, I review a number of recent studies showing IEG expression in brain neurons after behavioral learning. Together these results suggest some role for select IEGs (e.g., Krox 24) in learning and memory, although definitive studies using antisense DNA technology are required to establish any causal links. In particular, IEGs may be critical components of the signal transduction cascade that links NMDA and muscarinic receptors to the neuronal genome and ultimately to the generation of permanent modifications in neuronal biochemistry that provides the substrate for learning.

Messengers from the synapses to the nucleus (MSNs) that establish late phase of long-term potentiation (LTP) and long-term memory

The late stage of long-term potentiation (LTP) and long-term memory is believed to be largely governed by altered gene expression for its generation and maintenance, while the early stages of LTP and memory are controlled mainly by the phosphorylation-dephosphorylation of the synaptic proteins. For the altered gene expression, synaptic information must be transmitted from the synaptic sites to the nucleus. This article describes the presence of specific messenger molecules that transmit synaptic information to the nucleus; these molecules are referred to as MSNs (Messengers from Synapse to the Nucleus). In addition, recent studies have indicated that certain transcription factors localize at postsynaptic sites as well as the nucleus, and may function as MSNs.

CaM kinase II in long-term potentiation

The observation that autophosphorylation converts CaM kinase II from the Ca(2+)-dependent form to the Ca(2+)-independent form has led to speculation that the formation of the Ca(2+)-independent form of the enzyme could encode frequency of synaptic usage and serve as a molecular explanation of "memory". In cultured rat hippocampal neurons, glutamate elevated the Ca(2+)-independent activity of CaM kinase II through autophosphorylation, and this response was blocked by an NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). In addition, we confirmed that high, but not low frequency stimulation, applied to two groups of CA1 afferents in the rat hippocampus, resulted in LTP induction with concomitant long-lasting increases in Ca(2+)-independent and total activities of CaM kinase II. In experiments with 32P-labeled hippocampal slices, the LTP induction in the CA1 region was associated with increases in autophosphorylation of both alpha and beta subunits of CaM kinase II 1 h after LTP induction. Significant increases in phosphorylation of endogenous CaM kinase II substrates, synapsin I and microtubule-associated protein 2 (MAP2), which are originally located in presynaptic and postsynaptic regions, respectively, were also observed in the same slice. All these changes were prevented when high frequency stimulation was applied in the presence of AP5 or a calmodulin antagonist, calmidazolium. Furthermore, in vitro phosphorylation of the AMPA receptor by CaM kinase II was reported in the postsynaptic density and infusion of the constitutively active CaM kinase II into the hippocampal neurons enhanced kainate-induced response. These results support the idea that CaM kinase II contributes to the induction of hippocampal LTP in both postsynaptic and presynaptic regions through phosphorylation of target proteins such as the AMPA receptor, MAP2 and synapsin I.

Influence of actinomycin D, a RNA synthesis inhibitor, on long-term potentiation in rat hippocampal neurons in vivo and in vitro

1. Hippocampal long-term potentiation (LTP) may serve as an elementary process underlying certain forms of learning and memory in vertebrates. As is the case with behavioural memory, hippocampal LTP in the rat CA1 region and in the dentate gyrus occurs in stages, which can be separated by an inhibitor of RNA synthesis. 2. Experiments have been performed in two brain regions, in the hippocampal CA1 region in vitro and in the dentate gyrus in vivo. 3. Maintenance of hippocampal LTP in the CA1 region in vitro was influenced by the RNA synthesis inhibitor actinomycin D from 4 h onwards. 4. The effect of actinomycin D on the time course of the population spike potentiation was more pronounced than the effect on the time course of the EPSP component, suggesting different mechanisms for the two forms of potentiation. 5. Intrahippocampal and intracerebroventricular injection of actinomycin D into rats prevented a late stage of LTP in the dentate gyrus in vivo measured as the population spike amplitude. 6. Since actinomycin D was only effective in influencing the maintenance of LTP when applied before tetanization, the requirement for transcription during LTP may have a critical time window. 7. Actinomycin D influenced the maintenance of LTP specifically, since the drug did not alter any potentials in control experiments after its removal or when it was administered shortly after tetanization. A second, structurally different RNA synthesis inhibitor, 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole, mimicked the effect of actinomycin D in vitro.

Differential expression of inducible transcription factors in basal ganglia neurons

The dopamine receptor antagonist, haloperidol, produced a time-dependent differential induction of inducible transcription factors (ITFs) in rat striatal neurons: Fos, Fos B, Jun B, Jun D, Krox 20, and Krox 24, but not c-Jun, were induced in the caudate putamen and nucleus accumbens with varying time courses. The induction of Fos by haloperidol was stronger in anterior versus posterior regions of the striatum. In contrast, induction of Fos by the muscarinic agonist pilocarpine was stronger in the posterior regions of the striatum suggesting that muscarinic receptors do not play a role in the induction of ITFs in striatal neurons by haloperidol. Although c-Jun was not induced in caudate neurons by haloperidol it was strongly induced in these neurons following prolonged seizure activity. The differential pattern of Jun protein expression suggests that haloperidol induces a specific transcriptional program in basal ganglia neurons. These effects of haloperidol may be involved in producing its extrapyramidal side effects.

Differential time courses of c-fos mRNA expression in hippocampal subfields following acquisition and recall testing in mice

Spatio-temporal patterns of c-fos mRNA expression were studied in the mouse brain following the partial acquisition of an appetitive conditioning task in a Skinner box. We used two experimental situations: during the initial acquisition of the task (acquisition paradigm) and during the retention test (recall paradigm). In both paradigms the in situ hybridization signal was exclusively located in the hippocampal formation and the posterior cingulate cortex. However, the time-dependent pattern of expression was quite different according to the experimental situation: mRNA levels peaked at 90 min post-test in both paradigms but returned to basal (control) level by 180 min in the acquisition group, while in CA3 and DG subfields, high levels of mRNA expression were maintained at 180 min in the recall group. Taken together these results suggest that the IEG c-fos is implicated in the different phases of post-acquisition memory processes and involve a differential spatio-temporal regulation of its expression in hippocampal subfields.

N-methyl-D-aspartate and nitric oxide regulate the expression of calcium/calmodulin-dependent kinase II in the hippocampal dentate gyrus

Injection of small volumes of N-methyl-D-aspartate (NMDA) or Sin-1 molsidomine (a nitric oxide releasing agent) onto the dendrites of granule cells in the hippocampal dentate gyrus leads to changes in the level of expression of a number of genes. There is a fall in prodynorphin mRNA levels with a corresponding increase in proenkephalin mRNA levels. Similar changes in opioid gene expression occur following the induction of long-term potentiation (LTP). We report here that at short time periods (1-6 h) after injections of NMDA or sin-1 molsidomine, there is an increase in the levels of the mRNA encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II (CaMKII alpha), consistent with a report of elevated CaMKII alpha mRNA in postsynaptic neurons in the CA1 region of the hippocampus following LTP induction [54]. However, we also report that 24 h after injection of NMDA or sin-1, there is a dramatic decrease in CaMKII alpha mRNA levels in the vicinity of the injection. This effect is specific for CaMKII alpha mRNA, in that many other mRNA species are not affected, and occurs in the dendritic population of CaMKII alpha mRNA as well as in the pool of mRNA in the granule cell bodies. The effect is blocked by an inhibitor of cGMP-dependent protein kinase. The biphasic regulation of CaMKII alpha mRNA may be of considerable functional importance for the long-term response of granule cells to local stimulation of NMDA receptors or NO release.

The role of inducible transcription factors in apoptotic nerve cell death

Recent studies have shown that certain types of nerve cell death in the brain occur by an apoptotic mechanism. Researchers have demonstrated that moderate hypoxic-ischemic (HI) episodes and status epilepticus (SE) can cause DNA fragmentation as well as other morphological features of apoptosis in neurons destined to die, whereas more severe HI episodes lead to neuronal necrosis and infarction. Although somewhat controversial, some studies have demonstrated that protein synthesis inhibition prevents HI-and SE-induced nerve cell death in the brain, suggesting that apoptotic nerve cell death in the adult brain is de novo protein synthesis-dependent (i.e., programmed). The identity of the proteins involved in HI-and SE-induced apoptosis in the adult brain is unclear, although based upon studies in cell culture, a number of potential cell death and anti-apoptosis genes have been identified. In addition, a number of studies have demonstrated that inducible transcription factors (ITFs) are expressed for prolonged periods in neurons undergoing apoptotic death following HI and SE. These results suggest that prolonged expression of ITFs (in particular c-jun) may form part of the biological cascade that induces apoptosis in adult neurons. These various studies are critically discussed and in particular the role of inducible transcription factors in neuronal apoptosis is evaluated.

Analysis of the decremental nature of LTP in the dentate gyrus

The persistence of long-term potentiation (LTP) in the dentate gyrus was compared for two tetanization protocols: 50 trains on one day, or 50 trains on 5 consecutive days. LTP induction was significantly greater in the 250 train condition, but the LTP decay rate over weeks was similar between conditions. The decay of LTP could not be accounted for by deterioration of the preparation. Successive days of stimulation caused repetitive induction of immediate early genes, but did not prolong LTP, suggesting that either the effects of gene expression on LTP stabilization had saturated, or that these genes play other roles in synaptic plasticity.

Basal expression of the inducible transcription factors c-Jun, JunB, JunD, c-Fos, FosB, and Krox-24 in the adult rat brain

Jun, Fos, and Krox proteins are inducible transcription factors contributing to the control of gene expression. The elucidation of their individual expression patterns in the nervous system provides new insights into the ability of neurons to react with changes of gene expression to external stimulation under physiological or pathological conditions. The expression of c-Jun, JunB, JunD, c-Fos, FosB, and Krox-24 was investigated in the brain of untreated male Sprague-Dawley and female BDIX rats by immunocytochemistry using specific antibodies. JunD immunoreactivity (IR) labeled the highest number of neurons, being present in almost all neurons of the brain. JunD was expressed at high levels in those areas that also exhibit c-Jun, JunB, c-Fos, and FosB-IR, such as locus coeruleus, periolivary nuclei (ncl.), pontine and central gray, lateral lemniscal ncl., inferior and superior colliculi, leaflet of geniculate ncl., midline nuclei of thalamus, dorsomedial and paraventricular ncl. of hypothalamus, ncl. supraopticus, dorsolateral part of caudate putamen and lateral septal ncl. In contrast to the high number of JunD-positive neurons, c-Jun, JunB, c-Fos, and FosB proteins were detected in rather low numbers of neurons in these brain areas; the rank of the number of immunopositive neurons was c-Fos > JunB > c-Jun > FosB. Particularly high levels of expression were observed for c-Jun in medullary motoneurons, medial geniculate ncl., arcuate ncl., and dentate gyrus, and for JunB in the CA-1 area of the hippocampus and islands of Calleja. The zinc finger protein Krox-24 was expressed in many neurons of these brain areas, with only discrete Jun- and Fos-IR; additionally, many intensely labeled nuclei were present in spinal ncl. of the trigeminal ventromedial ncl. of the hypothalamus and the CA-1 area of the hippocampus. In the cerebellum, nuclear labeling was detected only for c-Jun, JunD, and Krox-24 in granule cells. JunD-IR was also found in glial cells of gray matter and fiber tracts, whereas glial c-Jun-IR was observed only in fiber tracts. Apart from a weak JunD-IR, some areas did not express Jun, Fos, and Krox proteins such as cuneate and gracile ncl., venterobasal complex of thalamus, globus pallidum, and Purkinje cells of the cerebellum. Our data indicate that inducible transcription factors of the fos, jun, and krox gene families show patterns of individual expression in untreated animals, thereby reflecting different mechanisms and/or thresholds for induction under physiological conditions.

Block of c-Fos and JunB expression by antisense oligonucleotides inhibits light-induced phase shifts of the mammalian circadian clock

Light-induced phase shifts of circadian rhythmic locomotor activity are associated with the expression of c-Jun, JunB, c-Fos and FosB transcription factors in the rat suprachiasmatic nucleus, as shown in the present study. In order to explore the importance of c-Fos and JunB, the predominantly expressed AP-1 proteins for the phase-shifting effects of light, we blocked the expression of c-Fos and JunB in the suprachiasmatic nucleus of male rats, housed under constant darkness, by intracerebroventricular application of 2 microliters of 1 mM antisense phosphorothioate oligodeoxynucleotides (ASO) specifically directed against c-fos and junB mRNA. A light pulse (300 lux for 1 h) at circadian time 15 induced a significant phase shift (by 125 +/- 15 min) of the circadian locomotor activity rhythm, whereas application of ASO 6 h before the light pulse completely prevented this phase shift. Application of control nonsense oligodeoxynucleotides had no effect. ASO strongly reduced the light-induced expression of c-Fos and JunB proteins. In contrast, light pulses with or without the control nonsense oligodeoxynucleotides evoked strong nuclear c-Fos and JunB immunoreactivity in the rat suprachiasmatic nucleus. These results demonstrate for the first time that inducible transcription factors such as c-Fos and JunB are an essential part of fundamental biological processes in the adult mammalian nervous system, e.g. of light-induced phase shifts of the circadian pacemaker.

Pattern of Fos and Jun expression in the female rat forebrain after sexual behavior

Previous studies indicated that sexual behavior in female rats primed with estradiol and progesterone induced expression of the immediate early gene (IEG) c-Fos in various brain areas rich in estradiol receptors, including the medial preoptic area (MPA), the medial amygdala (AMe), and the ventromedial nucleus of the hypothalamus (VMN), and to a lesser extent areas with low densities of estradiol receptors, such as the caudate nucleus, the dentate gyrus and the cingulate cortex. The goal of the present experiment was to compare this pattern of expression with the distribution of other IEG products within the Jun family. The results indicate that in non-mated animals, Jun-B, c-Jun and Jun-D were differentially present in several forebrain areas. As previously reported for c-Fos, there was little effect of estradiol and progesterone treatment on the brain expression of these Jun proteins. The most striking result was that sexual behavior stimulated expression of Jun-B and c-Jun, but not Jun-D, in areas containing high densities of estradiol receptors. Specifically, after sexual behavior the MPA and the bed nucleus of the stria terminalis co-expressed c-Fos, Jun-B, c-Jun. c-Fos was co-induced with Jun-B in the VMN, and with c-Jun in the AMe. In contrast, there was no detectable increase in Jun-B, c-Jun or Jun-D in either the caudate nucleus, dentate gyrus or cingulate cortex after sexual behavior, although these regions expressed weak to moderate levels of either Jun-B, c-Jun, or Jun-D basally.(ABSTRACT TRUNCATED AT 250 WORDS)

Krox20 may play a key role in the stabilization of long-term potentiation

Long-term potentiation-inducing stimulation of the perforant path was followed in dentate gyrus granule cells by a dramatic increase of mRNA and protein for Krox20, a zinc-finger-containing transcription factor. Induction of Krox20 required stimulation sufficient to induce LTP and was prevented by NMDA antagonists CPP and MK-801, which block LTP induction. Krox20 protein increased within 20 min of tetanization, was maximal between 1 and 8 h, and was still significantly elevated at 24 h after LTP induction. This prolonged appearance is in striking contrast with the more transient induction of the related molecule, Krox24. The elevation in the mRNA for Krox20 and Krox24 was of similar duration, suggesting that the Krox20 protein has a greater stability and may play a key role in the stabilization of long-term potentiation.

Immediate early gene expression associated with the persistence of heterosynaptic long-term depression in the hippocampus

Long-term depression (LTD) of synaptic efficacy is likely to be as important in memory processing as the more well-known long-term potentiation (LTP). The case for LTD serving as a memory mechanism, however, requires that it be shown to persist across days or weeks at least. Here we examined the persistence of heterosynaptic LTD in the medial and lateral perforant path inputs to the dentate gyrus in awake rats and correlated this persistence with the degree of immediate early gene expression as assessed immunohistochemically. Rats were chronically implanted with separate stimulating electrodes in the medial and lateral perforant paths and an extracellular field potential recording electrode in the dentate hilus. After recovery from surgery, either the medial or the lateral perforant path was tetanized with 400-Hz trains, and homosynaptic LTP and heterosynaptic LTD were followed across time. Heterosynaptic LTD was shown to occur readily in awake animals and to persist across days or weeks, depending on the stimulation protocol. The persistence of LTD and LTP was highly correlated within animals. Additional animals, given the same tetanization protocols, showed that the greatest immediate early gene expression occurred following that protocol which consistently gave the longest-lasting LTP and LTD. These data support the proposed role of LTD in memory processing but question whether immediate early genes are important for the persistence of LTP, LTD, or both.

Spatial and temporal changes in signal transduction pathways during LTP

Following LTP induction in freely moving rats, in situ hybridization revealed discrete changes in the expression of one isoform in each of four families of serine/threonine kinases constitutively expressed in the dentate gyrus of the hippocampus. Expression of the alpha isoform of CaMKII showed a transient increase over the soma and a more persistent increase over the dendritic field of dentate granule cells. Of the PKC isoforms, only gamma PKC was up-regulated substantially 2 hr after LTP induction, declining to control levels 48 hr later. An increase in the expression of mRNA for ERK2 and raf-B was seen at 24 hr only. These results show that, during the maintenance phase of LTP in the hippocampus, there are selective increases in the expression of serine/threonine kinases and that these increases have specific and characteristic temporal and spatial profiles.

Plastic genes are in!

Even though the synthesis of new proteins is thought to be essential for long-term changes in synaptic plasticity, as well as for long-term memory, little is known about the identity of the required proteins. The hunt for these molecules is under way, however, and in the past year several groups of researchers have entered this fascinating search by introducing new approaches that have lead to the identification of several potential candidates, amongst which are trophic factors, kinases, ion channels, and proteases. The results will have much to say not only about the nature of memory, but also about the mechanisms of learning.

Brain-derived neurotrophic factor expression after long-term potentiation

Long-term potentiation (LTP) of perforant-path dentate granule cell synapses, in awake rats, was followed by a time-dependent expression of brain-derived neurotrophic factor (BDNF) mRNA in dentate granule cells. This BDNF expression was blocked by the N-methyl-D-aspartate (NMDA) antagonist dizocilpine maleate (MK-801), which also blocked LTP induction, and by sodium pentobarbital, which shortens LTP persistence. These results suggest that BDNF may participate in the NMDA-receptor mediated cascade of events that result in LTP stabilization.

Correlations between immediate early gene induction and the persistence of long-term potentiation

The duration of long-term potentiation in the dentate gyrus of awake rats was examined following systematic manipulation of the number of stimulus trains delivered. This was correlated with the induction of immediate early genes in separate groups of animals given identical stimulus regimes. Following 10 trains of stimulation, long-term potentiation decayed with a time constant of up to several days (long-term potentiation 2), and this correlated with the appearance of an increase in the messenger RNA and protein levels of zif/268. Increasing the number of stimulus trains resulted in a greater probability of eliciting long-term potentiation with a time constant of several weeks (long-term potentiation 3), as well as increasing the induction of zif/268, c-Jun, Jun-B, Jun-D and Fos-related proteins. When 10 trains were delivered repeatedly on up to five consecutive days, only the zif/268 protein levels showed associated changes. These data provide support for the hypothesis that long-term potentiation 3 involves mechanisms additional to those for long-term potentiation 2. One possible mechanism is altered gene expression, initiated by immediate early gene transcription factors such as zif/268 and possibly homo- or heterodimers of Fos and Jun family members, that then contributes to the stabilization or maintenance of long-term potentiation 3.

Is c-Jun involved in nerve cell death following status epilepticus and hypoxic-ischaemic brain injury?

Neurons undergoing delayed neuronal death produced by hypoxia-ischaemia (HI) or status epilepticus (SE) showed a massive expression of c-Jun in their nuclei 24 h after the insult. With SE there was also a weaker induction of c-Fos and Jun B in dying neurons. SE induced in the presence of the NMDA antagonist MK-801 produced no delayed c-Jun expression in the hippocampus and nerve cell death did not occur in this region, although there was a delayed c-jun expression in the amygdala/piriform region, and cell death occurred in this area. Activation of central muscarinic receptors with pilocarpine, or block of D2 dopamine receptors with haloperidol, treatments which do not cause neuronal damage, strongly induced Fos and Jun B in hippocampal and striatal neurons, but only induced c-Jun very weakly. Thus, c-Jun may participate in the genetic cascade of events that produce programmed cell death in neurons.