Research report
Prodynorphin knockout mice demonstrate diminished age-associated impairment in spatial water maze performance

https://doi.org/10.1016/j.bbr.2005.02.010Get rights and content

Abstract

Dynorphins, endogenous κ-opioid agonists widely expressed in the central nervous system, have been reported to increase following diverse pathophysiological processes, including excitotoxicity, chronic inflammation, and traumatic injury. These peptides have been implicated in cognitive impairment, especially that associated with aging. To determine whether absence of dynorphin confers any beneficial effect on spatial learning and memory, knockout mice lacking the coding exons of the gene encoding its precursor prodynorphin (Pdyn) were tested in a water maze task. Learning and memory assessment using a 3-day water maze protocol demonstrated that aged Pdyn knockout mice (13–17 months) perform comparatively better than similarly aged wild-type (WT) mice, based on acquisition and retention probe trial indices. There was no genotype effect on performance in the cued version of the swim task nor on average swim speed, suggesting the observed genotype effects are likely attributable to differences in cognitive rather than motor function. Young (3–6 months) mice performed significantly better than aged mice, but in young mice, no genotype difference was observed. To investigate the relationship between aging and brain dynorphin expression in mice, we examined dynorphin peptide levels at varying ages in hippocampus and frontal cortex of WT 129SvEv mice. Quantitative radioimmunoassay demonstrated that dynorphin A levels in frontal cortex, but not hippocampus, of 12- and 24-month mice were significantly elevated compared to 3-month mice. Although the underlying mechanisms have yet to be elucidated, the results suggest that chronic increases in endogenous dynorphin expression with age, especially in frontal cortex, may adversely affect learning and memory.

Introduction

The endogenous opioid peptide dynorphin, expressed widely in the central nervous system [17], [22], [46], [47] and particularly prominent in mossy fiber terminals of the hippocampal formation [26], [50], has been implicated in modulating learning and memory. Dynorphin peptides are pharmacological agonists at κ-opioid receptors [1], and to a lesser extent δ and μ receptors [13], but have been observed to interact with other non-opioid targets like the NMDA receptor [2], [3], [4], [40] and even the excitatory amino acids themselves [49]. The effects of dynorphins in learning and memory are supported by reports of the ability of endogenous dynorphin to inhibit neural plasticity by blocking long-term potentiation (LTP) at mossy fiber-CA3 pyramidal cell synapses, perforant path-granule cell synapses, and mossy fiber recurrent synapses in the dentate gyrus [42], [43], [45]. The mechanisms involved include presynaptic inhibition of glutamate release via κ-opioid receptors and postsynaptic modulation of LTP as a retrograde inhibitory neurotransmitter [44], [45], [48].

Behavioral studies have documented an inhibitory effect of this neuropeptide on learning and memory. In vivo approaches in rats have demonstrated impairment of spatial memory upon intrahippocampal injection of dynorphin A(1-8) [25] or dynorphin B [38]. Dynorphin was also found to dose-dependently impair retention in an inhibitory avoidance task in mice [18]. The findings that dynorphin is elevated in hippocampus and frontal cortex of aged rats, especially memory-impaired rats [12], [20], further suggested a possible involvement of dynorphins in age-related memory deficits. Of particular relevance to human disease, dynorphin A is markedly elevated in the frontal cortex of patients with Down syndrome or Alzheimer's disease (AD) [37] and in mice overexpressing human amyloid precursor protein [8]. Large increases in κ-opioid receptor levels have also been noted in brains of AD patients [16], and cognitive performance in AD patients can be improved by the nonselective opioid antagonist naloxone [36].

A definitive causative role of increases in dynorphin in producing memory deficits associated with normal aging has not been clearly established, and cellular genetic mechanisms related to its effect in the hippocampus and other parts of the brain have not been carefully explored. To investigate the mechanisms underlying age-dependent changes in spatial learning and memory and the potential role of dynorphin in mediating these changes, we examine in this study the effects of aging in knockout mice lacking the coding exons for the precursor prodynorphin (Pdyn) [39].

Section snippets

Animal subjects

All animals in this study were wild-type (WT) or Pdyn−/− mice that were maintained in an approved animal facility at the University of Kentucky. WT mice were derived from the 129SvEv-Tac line, and Pdyn−/− mice were originally obtained by targeted deletion of the coding exons of the prodynorphin gene and bred into a 129SvEv-Tac background as described [39]. The aged mice included in this analysis ranged from 13 to 17 months at the time of testing, averaging 15.8 ± 2.0 (mean ± S.D.) months for WT and

Water maze

A total of 18 aged Pdyn−/− mice, 17 aged WT mice, 11 young Pdyn−/− mice, and 13 young WT mice were initially tested on the cue discrimination task to assess visual acuity, sensorimotor function, and general motivation. Two animals, both aged Pdyn−/− mice, failed to consistently locate the visible platform and were removed from the study. Based on the remaining mice, all four groups showed significant learning effects across the four blocks of the cued task (Fig. 1), as determined by a

Discussion

The present study confirms a significant impairment with aging in the spatial water maze task [11], [24], [34]. The age-related impairment is clearly observable for the 13- to 17-month mice used in this study as demonstrated by the significant differences shown in the vast majority of the quantitative indicators of learning and memory. Although aging confers an approximately 15–20% decrease in swim speed in our study, motor activity by itself is unlikely to account for most of the

Acknowledgements

This work has been supported by NS 044157 and DAMD17-9919497 to G.B.; AG14979 and an Evelyn F. McKnight Brain Research Grant to T.C.F.; MH65055-01 and a Howard Hughes Medical Institute predoctoral fellowship to X.V.N.

References (50)

  • A. Heron et al.

    Prodynorphin mRNA expression in the rat dentate gyrus after cerebral ischemia

    Neuropeptides

    (1996)
  • J.M. Hiller et al.

    Selective changes in mu, delta and kappa opioid receptor binding in certain limbic regions of the brain in Alzheimer's disease patients

    Brain Res

    (1987)
  • H. Khachaturian et al.

    Dynorphin immunocytochemistry in the rat central nervous system

    Peptides

    (1982)
  • C.M. Kotz et al.

    Age-related changes in brain proDynorphin gene expression in the rat

    Neurobiol Aging

    (2004)
  • K.L. McDaniel et al.

    Microinjection of dynorphin into the hippocampus impairs spatial learning in rats

    Pharmacol Biochem Behav

    (1990)
  • I. Nylander et al.

    Levels of dynorphin peptides, substance P and CGRP in the spinal cord after subchronic administration of morphine in the rat

    Neuropharmacology

    (1991)
  • A.M. Owen et al.

    Visuo-spatial short-term recognition memory and learning after temporal lobe excisions, frontal lobe excisions or amygdalo-hippocampectomy in man

    Neuropsychologia

    (1995)
  • K. Ploj et al.

    Effects of neonatal handling on nociceptin/orphanin FQ and opioid peptide levels in female rats

    Pharmacol Biochem Behav

    (2001)
  • D. Risser et al.

    Endogenous opioids in frontal cortex of patients with Down syndrome

    Neurosci Lett

    (1996)
  • J. Sandin et al.

    Hippocampal dynorphin B injections impair spatial learning in rats: a kappa-opioid receptor-mediated effect

    Neuroscience

    (1998)
  • N. Sharifi et al.

    Generation of dynorphin knockout mice

    Brain Res Mol Brain Res

    (2001)
  • K. Tan-No et al.

    Cytotoxic effects of dynorphins through nonopioid intracellular mechanisms

    Exp Cell Res

    (2001)
  • S.J. Watson et al.

    Dynorphin is located throughout the CNS and is often co-localized with alpha-neo-endorphin

    Life Sci

    (1982)
  • N. Zamir et al.

    Ontogeny and regional distribution of proenkephalin- and prodynorphin-derived peptides and opioid receptors in rat hippocampus

    Neuroscience

    (1985)
  • C. Chavkin et al.

    Dynorphin is a specific endogenous ligand of the kappa opioid receptor

    Science

    (1982)
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