Suppression of cAMP-dependent gene expression by cholecystokinin in the hippocampus.

Our previous study suggests that "the neuropeptidergic system" might promote a diversity of the mechanisms that regulate signal transmission in the hippocampus. Cholecystokinin (CCK) is the mostly expressed neuropeptide gene in the hippocampus. Here, we investigated whether CCK regulates immediate-early genes (Egr1/zif268 and Fos), critical indicators of cortical neuronal activity. We showed that CCK increased Egr1/zif268 promoter activity in a neuronal cell line, which is transfected with CCK(B) receptor. Unexpectedly, in living hippocampal slices, CCK significantly suppressed cAMP-induced expression of Egr1/zif268 and Fos through CCK(B) receptor activation. This suppression was involved in activating GABA(B) and cannabinoid 1 receptors. In addition to transient CCK modulation of action potentials on hippocampal principal neurons, we suggest that release of endogenous CCK might indirectly produce the suppression of cAMP-dependent gene expression in the hippocampus.

Hippocampal cells encode places by forming small anatomical clusters.

The hippocampus has been hypothesized to function as a "spatial" or "cognitive" map, however, the functional cellular organization of the spatial map remains a mystery. The majority of electrophysiological studies, thus far, have supported the view of a random-type organization in the hippocampus. However, using immediate early genes (IEGs) as an indicator of neuronal activity, we recently observed a cluster-type organization of hippocampal principal cells, whereby a small number ( approximately 4) of nearby cells were activated in rats exposed to a restricted part of an environment. To determine the fine structure of these clusters and to provide a 3D image of active hippocampal cells that encode for different parts of an environment, we established a functional mapping of IEGs zif268 and Homer1a, using in situ hybridization and 3D-reconstruction imaging methods. We found that, in rats exposed to the same location twice, there were significantly more double IEG-expressing cells, and the clusters of nearby cells were more "tightly" formed, in comparison to rats exposed to two different locations. We propose that spatial encoding recruits specific cell ensembles in the hippocampus and that with repeated exposure to the same place the ensembles become better organized to more accurately represent the "spatial map."

Changes in interneuronal phenotypes regulated by estradiol in the adult rat hippocampus: a potential role for neuropeptide Y.

Ovarian hormones regulate pyramidal cell synapse formation and excitability and interneuronal GABAergic tone in the CA1 region of the adult female rat hippocampus. The role of 17beta-estradiol in these effects is complex and appears to involve a subset of hippocampal interneurons, which express different calcium-binding protein and neuropeptide phenotypes and nuclear estrogen receptor alpha. We found that, in the hippocampus, nuclear estrogen receptor alpha-immunoreactive interneurons co-express neuropeptide Y, calbindin-D28k and calretinin but do not parvalbumin or cholecystokinin. Moreover, a proportion of neuropeptide Y-immunoreactive interneurons co-expresses calbindin-D28k and calretinin. This pattern is similar in the presence or absence of 17beta-estradiol treatment in ovariectomized rats. We then used immunohistochemistry and in situ hybridization to determine whether 17beta-estradiol treatment regulates expression of CA1 interneuronal phenotypic markers via nuclear estrogen receptor alpha activation. We found that 17beta-estradiol treatment of ovariectomized rats increased neuropeptide Y mRNA levels (25%) and the neuropeptide Y mRNA-associated grain density per cell (11%), as well as the number of neuropeptide Y-immunoreactive cells (11%), predominantly in the pyramidal cell layer (stratum pyramidale). Treatment with CI628, a selective estrogen response modulator that acts as an antagonist for nuclear estrogen receptor, blocked 17beta-estradiol-induced increase of neuropeptide Y mRNA levels. 17beta-Estradiol treatment did not alter the number of parvalbumin, calretinin, and cholecystokinin immunoreactive cells, nor mRNA levels for parvalbumin and cholecystokinin. Therefore, the present study has identified neuropeptide Y expression as the main interneuronal phenotype that co-expresses nuclear estrogen receptor alpha and shown that neuropeptide Y is responsive to 17beta-estradiol in CA1 pyramidal cell layer. We suggest that 17beta-estradiol may regulate neuropeptide Y expression mediated by nuclear estrogen receptor alpha-dependent activation in a subset of hippocampal interneurons, and we speculate that subsequent neuropeptide Y release may indirectly contribute to regulate glutamate-dependent neuronal activity in the adult rat hippocampus.