A role for hippocampal adult neurogenesis in shifting attention toward novel stimuli.

New granule neurons are born in the dentate gyrus region of the hippocampus throughout life. Behavioral effects of slowing or stopping this ongoing neurogenesis are generally observed only in complex cognitive tasks involving high levels of cue or memory interference or in tests of emotion presented after stress exposure. Here, we tested the role of new neurons in naïve rats in a simple, one-trial orienting task previously shown to be affected by hippocampal lesions. Using a pharmacogenetic method to inhibit adult neurogenesis, we found that loss of new neurons decreased orienting toward a novel auditory cue. Rats lacking new neurons showed this change in orienting only when they were drinking from a water bottle and not when they were exploring an empty arena, suggesting that the deficit is not in the ability to orient to a novel sound but in shifting of attention toward a second stimulus. Orienting was reduced to the same extent after 4 or 8 weeks of neurogenesis reduction but was not detectably altered after 2 or 3 weeks of treatment, suggesting that new neurons must mature for approximately a month before functioning in this behavior. These findings demonstrate that adult-born neurons affect behavior in a simple attention reorienting task in naïve animals with no prior stress or task-related learning.

The role of the ventral dentate gyrus in anxiety-based behaviors.

Dorsoventral lesion studies of the hippocampus (HPP) indicate that the dorsal axis is important for spatial processing and the ventral axis is important in anxiety and olfactory processes. There is some evidence that ventral CA3 and ventral CA1 subregions are important for cued retrieval in fear conditioning, which supports a ventral-anxiety relationship. However, the role of the ventral dentate gyrus (DG) in anxiety-based behaviors is less understood. Therefore, we used elevated plus and open field mazes to investigate the role of the ventral DG in the ability to modify behavior in potentially dangerous conditions and to clarify a few previous reports that ventral HPP lesions may induce hyperactivity. Rats with ventral DG lesions spent significantly more time in the open arms of the elevated plus maze and inner zone of the open field test than did controls and rats with dorsal DG lesions. Locomotor measures indicate that all rats traveled at similar rates in enclosed arms, as well as in open arms of the elevated plus maze and all groups traveled at similar rates in the open field test, which indicates that differences in exploration were not likely due to hyperactivity. The present study findings indicate that the ventral DG plays an important role in anxiety-based behaviors, such as preference for safer environments and the ability to modify exploratory behavior when in potentially dangerous environments and that the dorsal DG is not importantly involved in anxiety.

The role of the ventral dentate gyrus in olfactory pattern separation.

Dorsoventral lesion studies of the hippocampus have indicated that the dorsal axis of the hippocampus is important for spatial processing and the ventral axis of the hippocampus is important for olfactory learning and memory and anxiety. There is some evidence to suggest that the ventral CA3 and ventral CA1 conduct parallel processes for pattern completion and temporal processing, respectively. Studies have indicated that the dorsal dentate gyrus (DG) is importantly involved in processes reflecting underlying pattern separation activity for spatial information. However, the ventral DG is less understood. The current study investigated the less-understood role of the ventral DG in olfactory pattern separation. A series of odor stimuli that varied on only one level, number of carbon chains (methyl groups), was used in a matching-to-sample paradigm in order to investigate ventral DG involvement in working memory for similar and less similar odors. Rats with ventral DG lesions were impaired at delays of 60 sec, but not at delays of 15 sec. A memory-based pattern separation effect was observed performance was poorest with only one carbon chain separation between trial odors and was highest for trials with four separations. The present study indicates that the ventral DG plays an important role in olfactory learning and memory processes for highly similar odors. The results also indicate a role for the ventral DG in pattern separation for odor information, which may have further implications for parallel processing across the dorsoventral axis for the DG in spatial (dorsal) and olfactory (ventral) pattern separation.