Long-term memory, neurogenesis, and signal novelty

Attention to memory: orienting attention to sound object representations

Despite a growing acceptance that attention and memory interact, and that attention can be focused on an active internal mental representation (i.e., reflective attention), there has been a paucity of work focusing on reflective attention to 'sound objects' (i.e., mental representations of actual sound sources in the environment). Further research on the dynamic interactions between auditory attention and memory, as well as its degree of neuroplasticity, is important for understanding how sound objects are represented, maintained, and accessed in the brain. This knowledge can then guide the development of training programs to help individuals with attention and memory problems. This review article focuses on attention to memory with an emphasis on behavioral and neuroimaging studies that have begun to explore the mechanisms that mediate reflective attentional orienting in vision and more recently, in audition. Reflective attention refers to situations in which attention is oriented toward internal representations rather than focused on external stimuli. We propose four general principles underlying attention to short-term memory. Furthermore, we suggest that mechanisms involved in orienting attention to visual object representations may also apply for orienting attention to sound object representations.

Social novelty enhances brain cell proliferation, cell survival, and chirp production in an electric fish, Apteronotus leptorhynchus

For many animals, enriched environments and social interaction promote adult neurogenesis. However, in some cases, the effect is transient, and long-term environmental stimuli have little benefit for neurogenesis. In electric fish, Apteronotus leptorhynchus, fish housed in pairs for 7 days show higher density of newborn brain cells (cell addition) than isolated fish, but fish paired for 14 days have rates of cell addition similar to isolated controls. We examined whether introduction of social novelty can sustain elevated levels of cell addition and prevent long-term habituation to social interaction. We also monitored electrocommunication signals ("chirps") as a measure of the behavioral response to social novelty. We paired fish for 14 days with one continuous partner (no social novelty), two sequential partners changed after 7 days (low novelty) or seven sequential partners changed every 2 days (high novelty). On Day 11, we injected fish with BrdU, sacrificed fish 3 days later and quantified BrdU labeling in the diencephalic periventricular zone. Fish exposed to no novelty had BrdU labeling similar to isolated fish. Fish with low novelty showed small increases in BrdU labeling and those with high novelty had much greater BrdU labeling. Similarly, chirp rates were greater in fish with low novelty than with no novelty and greatest yet in fish with high novelty. By varying the timing of novelty relative to BrdU injection, we showed that social novelty promoted both proliferation and survival of newborn cells. These results indicated that brain cell proliferation and survival is influenced more by social change than simply the presence of social stimuli.

Developmental iodine deficiency and hypothyroidism impair neural development in rat hippocampus: involvement of doublecortin and NCAM-180

BACKGROUND

Developmental iodine deficiency results in inadequate thyroid hormone (TH), which damages the hippocampus. Here, we explored the roles of hippocampal doublecortin and neural cell adhesion molecule (NCAM)-180 in developmental iodine deficiency and hypothyroidism.

METHODS

Two developmental rat models were established with either an iodine-deficient diet, or propylthiouracil (PTU)-adulterated water (5 ppm or 15 ppm) to impair thyroid function, in pregnant rats from gestational day 6 until postnatal day (PN) 28. Silver-stained neurons and protein levels of doublecortin and NCAM-180 in several hippocampal subregions were assessed on PN14, PN21, PN28, and PN42.

RESULTS

The results show that nerve fibers in iodine-deficient and 15 ppm PTU-treated rats were injured on PN28 and PN42. Downregulation of doublecortin and upregulation of NCAM-180 were observed in iodine-deficient and 15 ppm PTU-treated rats from PN14 on. These alterations were irreversible by the restoration of serum TH concentrations on PN42.

CONCLUSION

Developmental iodine deficiency and hypothyroidism impair the expression of doublecortin and NCAM-180, leading to nerve fiber malfunction and thus impairments in hippocampal development.

Zinc deficiency reduces neurogenesis accompanied by neuronal apoptosis through caspase-dependent and -independent signaling pathways

Dietary zinc deficiency may affect zinc homeostasis in the brain and lead to reductions of neurogenesis and neuronal survival. However, the mechanisms responsible for the effects of zinc deficiency on hippocampal neurogenesis and neuronal death remain obscure. In the present study, young CD-1 mice were fed with zinc-deficient diet (0.85 ppm) for 5 weeks. The vesicular zinc was reduced at CA1 and CA3 regions of the hippocampus in zinc-deficient mice. The significant decreased zinc ions was associated with a reduction in proliferating cells labeled with bromo-deoxyuridine (BrdU) and immature neurons labeled with doublecortin (DCX) immunoreactivity in the dentate gyrus of the hippocampus. The processes of DCX-positive neurons were shortened, and flexuously went through into the granular cell layer in zinc-deficient hippocampus. There was also a conspicuous increase in the number of TUNEL-positive cells in the hippocampus after zinc-deficient diet treatment. Meanwhile, the apoptosis proteins, including Fas, Fas ligand (FasL), apoptosis inducing factor (AIF), and caspase-3, were significantly activated in zinc-deficient mouse hippocampus. These data suggest that chronic treatment with zinc-deficient diet results in reduction in hippocampal neurogenesis and increases neuronal apoptosis, indicating that zinc deficiency is associated with destroying structural plasticity in the hippocampus.

Any novelty in hippocampal formation and memory?

PURPOSE OF REVIEW

Novel events tend to attract attention and become more effectively encoded in memory than predictable events. The hippocampus and medial-temporal cortical regions, along with regions of the prefrontal cortex, have been associated with enhanced memory for novel events. This review provides an update on recent studies of hippocampal novelty responses in relation to memory.

RECENT FINDINGS

Several different types of novelty have been studied. Stimulus novelty effects have been observed as reduced neural activity in the medial-temporal and prefrontal regions when the same stimulus is repeatedly presented. Contextual novelty effects, the detection of a stimulus or event in an unexpected context, is impaired in patients with hippocampal damage. Single-trial analyses of brain activity show that the hippocampus rapidly habituates to contextually novel situations. Associative novelty, the detection of new arrangements of familiar stimuli, has also been related to the medial-temporal regions. A division of labour among the medial-temporal regions has been proposed such that associative novelty selectively engages the hippocampus, whereas stimulus novelty is mediated by the perirhinal cortex. However, a simple account of when the hippocampus versus other medial-temporal cortical regions is recruited awaits further studies. Increased dopaminergic and cholinergic neurotransmission may account for enhanced memory encoding of novel events, and relate to structural neuronal changes.

SUMMARY

Interindividual variability in the responsiveness of the hippocampal-novelty system may be genetically mediated, and personality factors can also play a role. A better understanding of such variability can have implications for interventions aimed at supporting memory and for the treatment of drug abuse.