After spatial exploration in rats, Arc mRNA is expressed in ∼2% of dentate gyrus (DG) granule cells, and this proportion of Arc-positive neurons remains stable for ∼8 h. This long-term presence of Arc mRNA following behavior is not observed in hippocampal CA1 pyramidal cells. We report here that in rats ∼50% of granule cells with cytoplasmic Arc mRNA, induced some hours previously during exploration, also show Arc expression in the nucleus. This suggests that recent transcription can occur long after the exploration behavior that elicited it. To confirm that the delayed nuclear Arc expression was indeed recent transcription, Actinomycin D was administered immediately after exploration. This treatment resulted in inhibition of recent Arc expression both when evaluated shortly after exploratory behavior as well as after longer time intervals. Together, these data demonstrate a unique kinetic profile for Arc transcription in hippocampal granule neurons following behavior that is not observed in other cell types. Among a number of possibilities, this sustained transcription may provide a mechanism that ensures that the synaptic connection weights in the sparse population of granule cells recruited during a given behavioral event are able to be modified.
"It has been shown that Arc is involved in the regulation of AMPA receptor trafficking and synaptic plasticity in different brain areas, and that it is implicated in memory mechanisms (Tzingounis and Nicoll, 2006). In the Dentate Gyrus, sustained Arc expression has been suggested to be important for synaptic plasticity and memory consolidation (Messaoudi et al., 2007; Ramirez-Amaya et al., 2013). Thus, in light of these data, it is possible that the restoration of LTD magnitude by L-DOPA treatment in the DG of treated hemiparkinsonian rats could be mediated by changes induced by L-DOPA on IEG expression . "
[Show abstract][Hide abstract] ABSTRACT: Parkinson’s disease (PD) patients exhibit motor and non-motor symptoms that severely affect quality of life. Cognitive alterations in PD subjects have been related to both structural and functional hippocampal changes. Here we investigated the effects of the 6-hydroxydopamine (6-OHDA) lesion in the Medial Forebrain Bundle (MFB) on the hippocampus focusing on the Dentate Gyrus (DG). In vivo microdialysis measurements revealed that the 6-OHDA injection disrupts both dopaminergic and noradrenergic transmission in rat DG. In vitro electrophysiological recordings showed that these neurochemical alterations were accompanied by impairment of long-term depression (LTD) at medial perforant path/DG synapses. Furthermore, this alteration was reversed by L-DOPA treatment. Notably, the therapeutic effect of L-DOPA on LTD was blocked by the antagonism of β-noradrenergic receptors, but not by D1 or D2 receptor antagonists. Thus, while the dopaminergic transmission does not seem to be implicated in this therapeutic effect of L-DOPA, the noradrenergic system plays a central role in the synaptic dysfunction of the DG in experimental PD. Our work provides new evidence on the role of catecholamines in DG synaptic plasticity and sheds light on the possible synaptic mechanisms underlying cognitive deficits in PD. Furthermore, our results indicate that L-DOPA exerts a therapeutic effect on the parkinsonian brain through different, coexistent, mechanisms.
[Show abstract][Hide abstract] ABSTRACT: For the past two decades an increasing number of studies have underlined the crucial role of the immediate-early gene Arc in plasticity processes thought to sustain memory function. Because of the high spatial and temporal resolution of this technique, the detection of Arc products appears to have become a new standard for the mapping of cognitive processes. To date, most Arc studies have focused on identifying the contribution of the hippocampal subfields CA1 and CA3 to spatial processes. In contrast, few have investigated their role in non-spatial memory, or the role of other medial temporal lobe (MTL) areas in spatial and non-spatial memory. This short review describes recent studies focusing on these issues. After a brief overview of Arc's functions, we report a set of studies that put to the test some well-accepted theories in recognition memory. First, we describe data indicating that the parahippocampal areas may not to be strictly segregated into a spatial and a non-spatial streams, as originally described. Second, we report findings revealing a functional segregation along the dorsoventral axis in CA1, but not in CA3. Finally, we bring evidence for a segregation of CA3 along the proximodistal axis and discuss the involvement of a proximal CA3-distal CA1 network during non-spatial memory. In summary, 'Arc imaging' appears to be a powerful tool to identify neural substrates of cognitive processes, not only in the hippocampus but also in the remaining of the MTL. Moreover, because of its fundamental role in synaptic processes, it offers a rare and exciting opportunity to further bridge plasticity processes and memory function.
Behavioural brain research 05/2013; 254. DOI:10.1016/j.bbr.2013.04.048 · 3.03 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Neonatal infection has enduring effects on the brain, both at the cellular and behavioral levels. We determined the effects of peripheral infection with Escherichia coli at postnatal day (P) 4 in rats on a water maze task in adulthood, and assessed neuronal activation in the dentate gyrus (DG) following the memory test. Rats were trained and tested on one of 3 distinct water maze task paradigms: 1) minimal training (18 trials/ 3 days), 2) extended training (50 trials/ 10 days) or 3) reversal training (extended training followed by 30 trials/ 3 days with a new platform location). Following a 48HR memory test, brains were harvested to assess neuronal activation using activity-regulated cytoskeleton-associated (Arc) protein in the DG. Following minimal training, rats treated neonatally with E. coli had improved performance and paradoxically reduced Arc expression during the memory test compared to control rats treated with PBS early in life. However, neonatally-infected rats did not differ from control rats in behavior or neuronal activation during the memory test following extended training. Furthermore, rats treated neonatally with E. coli were significantly impaired during the 48HR memory test for a reversal platform location, unlike controls. Specifically, whereas neonatally-infected rats were able to acquire the new location at the same rate as controls, they spent significantly less time in the target quadrant for the reversal platform during a memory test. However, neonatally-infected and control rats had similar levels of Arc expression following the 48HR memory test for reversal. Together, these data indicate that neonatal infection may improve the rate of acquisition on hippocampal-dependent tasks while impairing flexibility on the same tasks; in addition, network activation in the DG during learning may be predictive of future cognitive flexibility on a hippocampal-dependent task.
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