The regulation of hippocampal dynorphin by neural/neuroendocrine pathways: models for effects of aging on an opioid peptide system

Abstract

Previous research has demonstrated increased messenger RNA expression and peptide content in an opioid system localized to hippocampal dentate granule cells in aged rats. This altered regulation of dynorphin was correlated with the emergence of an age-related impairment in spatial learning. Considerable evidence exists for additional effects of aging on systems that provide input to the dynorphin-containing dentate granule cells. Such changes have been well documented for loss of perforant path innervation from entorhinal cortex, deterioration in septohippocampal cholinergic neurons, and high amounts of glucocorticoids that have, among their targets, receptors located in the dentate gyrus. Similar to the effects of aging on hippocampal dynorphin, age-related changes in each of these systems correlate with the severity of spatial learning impairment in aged rats. This raises the possibility that dysregulation of dynorphin in the aged brain is a reactive response to antecedant change(s) in this circuitry, a hypothesis that was examined by separately manipulating in young rats the three neural/neuroendocrine systems identified above. Of the three models examined only removal of the perforant path reproduced the effect of aging on dynorphin in the hippocampal formation. An immunotoxin was used in Experiment 1 to selectively remove septo-hippocampal cholinergic neurons in young rats. No alteration in hippocampal opioid peptides was produced by this treatment. Experiment 2 examined effects of exposure to excess corticosterone. Adrenalectomized rats exhibited a significant decrease in hippocampal dynorphin-A (1–8) content, which was reversed by corticosterone replacement at a concentration approximating normal basal levels. Dynorphin-A (1–8) content, however, was not reliably increased by exposure to excess corticosterone. In contrast, perforant path removal was found to reproduce the effect of aging on dynorphin content; either aspiration of the entorhinal cortex or knife-cut transections of the perforant path reliably increased hippocampal dynorphin content. These results support the conclusion that age-related deterioration in the septohippocampal cholinergic system and evaluated exposure to corticosterone are not sufficient to induce an elevation in hippocampal dynorphin content. Only removal of the perforant path innervation was found to reproduce the elevation in hippocampal dynorphin content observed in aged rats with hippocampal-dependent learning impairment.