Showing papers by "Carol A. Barnes published in 1998"

Firing characteristics of deep layer neurons in prefrontal cortex in rats performing spatial working memory tasks.

Abstract: Single cells were recorded with 'tetrodes' in regions of the rat medial prefrontal cortex, including those which are targets of hippocampal afferents, while rats were performing three different behavioral tasks: (i) an eight-arm radial maze, spatial working memory task, (ii) a figure-eight track, delayed spatial alternation task, and (iii) a random food search task in a square chamber. Among 187 recorded units, very few exhibited any evidence of place-specific firing on any of the behavioral tasks, except to the extent that different spatial locations were related to distinct phases of the task. Furthermore, no prefrontal unit showed unambiguous spatially dependent delay activity that might mediate working memory for spatial locations. Rather, the cells exhibited diverse correlates that were generally associated with the behavioral requirements of performing the task. This included firing related to intertrial intervals, onset or end of trials, selection of specific arms on the eight-arm radial maze, delay periods, approach to or departure from goals, and selection of paths on the figure-eight track. Although a small number of cells showed similar behavioral correlates across tasks, the majority of cells showed no consistent correlate when recorded across multiple tasks. Furthermore, some units did not exhibit altered firing patterns in any of the three tasks, while others showed changes in firing that were not consistently related to specific behaviors or task components. These results are in agreement with previous lesion and behavioral studies in rats that suggest a prefrontal cortical role in encoding 'rules' (i.e. structural features) or behavioral sequences within a task but not in encoding allocentric spatial information. Given that the hippocampal projection to this cortical region is capable of undergoing LTP, our data lead to the hypothesis that the role of this projection is not to impose spatial representations upon prefrontal activity, but to provide a mechanism for learning the spatial context in which particular behaviors are appropriate.

Dynamic Regulation of RGS2 Suggests a Novel Mechanism in G-Protein Signaling and Neuronal Plasticity

Abstract: Long-term neuronal plasticity is known to be dependent on rapid de novo synthesis of mRNA and protein, and recent studies provide insight into the molecules involved in this response. Here, we demonstrate that mRNA encoding a member of the regulator of G-protein signaling (RGS) family, RGS2, is rapidly induced in neurons of the hippocampus, cortex, and striatum in response to stimuli that evoke plasticity. Although several members of the RGS family are expressed in brain with discrete neuronal localizations, RGS2 appears unique in that its expression is dynamically responsive to neuronal activity. In biochemical assays, RGS2 stimulates the GTPase activity of the alpha subunit of Gq and Gi1. The effect on Gi1 was observed only after reconstitution of the protein in phospholipid vesicles containing M2 muscarinic acetylcholine receptors. RGS2 also inhibits both Gq- and Gi-dependent responses in transfected cells. These studies suggest a novel mechanism linking neuronal activity and signal transduction.

Reactivation of neuronal ensembles in hippocampal dentate gyrus during sleep after spatial experience

Abstract: Patterns of neuronal activity recorded in CA1 of the hippocampus and in neocortex during waking-behavior, are reactivated during subsequent slow-wave sleep (SWS). It has been suggested that this reactivation may originate in the hippocampal CA3 region, where modifiable excitatory recurrent connections are abundant and where sharp-waves, in which the reactivation is most robust, appear to arise. The present experiment investigated whether ensemble firing patterns of granule cells in the fascia dentata (FD), an area ‘upstream’ from CA3, are also reactivated during sleep. Populations of FD granule cells were recorded from during spatial behavior and during prior and subsequent SWS. Firing rate correlations between cell-pairs with overlapping place fields were significantly enhanced during post behavioral sleep compared to pre behavioral sleep. Correlations between cells with non-overlapping place fields or which were silent during maze behavior, were not changed. Thus, reactivation of experience-specific correlation states also occurs in granule cells during sleep. Because these cells do not have excitatory interconnections, but form a major input to CA3 pyramidal cells, current models predicted that sleep reactivation would appear first in CA3. There are, however, both extensive polysynaptic excitatory interactions among granule cells and feedback from CA3 pyramidal cells. Granule cells also receive indirect input from neocortical regions known to undergo trace reactivation. Although a simple model for a CA3 origin of the reactivation phenomenon cannot be confirmed, the present results extend our understanding of the generality of this phenomenon.