Increasing brain expression of the endogenous opioid peptide may contribute to the cognitive impairment triggered by Aβ production, according to a new study from the lab of Lennart Mucke and colleagues at the Gladstone Institute of Neurological Disease at the University of California at San Francisco.
The results show that the elevation of enkephalins in the hippocampus of two mouse models of AD correlated with behavioral changes in the mice. An increase in enkephalins was also observed in the dentate gyrus of the hippocampus in brain tissue from AD patients. In the animals, Aβ-related deficits in learning and memory were partially reversed by an inhibitor of the μ-opioid receptor where enkephalins act. Published in the May 7 Journal of Neuroscience, the work suggests that enkephalin production, triggered by neuronal overactivity in response to Aβ, may contribute to the cognitive impairments of AD, and that blocking enkephalin effects may present a novel therapeutic target.
The enkephalins are a pair of neuromodulatory peptides that bind to opioid receptors and affect synaptic plasticity, learning, and memory. Enkephalins are cleaved from a larger precursor, preproenkephalin, and stored in vesicles for co-release with neurotransmitters after synaptic stimulation. Some disturbances in the enkephalin system have been reported in AD, notably decreases in μ- and δ-opioid receptors in people with AD, and elevated enkephalin peptides in a mouse model of AD (Diez et al., 2003).
In the new work, lead author William Meilandt and colleagues looked further into changes in enkephalin levels and their potential role in cognitive impairment. The researchers found that enkephalin immunoreactivity and precursor mRNA are both elevated in the hippocampus of AD model mice and humans with AD. In mice, the elevation correlated with loss of the neuronal protein calbindin, and with two behavioral measures, poorer performance in the Morris water maze and hyperactivity in an open field. Enkephalin immunoreactivity was elevated in very young mice, before plaque formation. Furthermore, genetic manipulations that caused increased Aβ-induced neuronal overexcitation (overexpression of the tyrosine kinase fyn (ref) also increased enkephalin levels. Decreasing Aβ-induced neurotoxic effects by lowering tau expression (lowered) decreased enkephalin levels.
These results are consistent with enkephalin elevation occurring as a result of Aβ’s neurostimulatory effects. But what are the ramifications? The peptides might contribute to behavioral deficits in the mice, or rise as part of a compensatory response. To sort that question out, the investigators treated 9-10-month-old AD mice with an irreversible μ-opioid receptor antagonist, β-funaltrexamine (β-FNA) for three weeks to block the effects of enkephalins. The result was a partial restoration of the animals’ performance in the Morris water maze. The mice improved their ability to learn in the visible platform portion of the test, but did not do any better at finding the hidden platform. More work is required to determine for sure if the behavioral effects stem from counteracting enkephalin effects in the hippocampus, or whether other actions of μ-opioid receptors might be involved.
The work suggests that blocking opioid receptors, and in particular the μ receptors, might benefit patients with AD. The authors cite a 25-year-old, double-blind placebo-controlled clinical study of the opioid antagonist naloxone that appeared promising (Reisberg et al., 1983). Three subsequent studies with the related antagonist naltrexone gave overall negative results, but the therapy seemed to benefit 10 percent of subjects in two studies. These two drugs, used mainly to treat opiate overdose or dependence, are both nonspecific antagonists. “It may be worth looking at effects of more specific opioid receptor antagonists, maybe aimed at the μ-opioid receptor that we targeted in our study,” Mucke told ARF. “Maybe the pharmacology of manipulating the opioid receptors in AD hasn’t been maxed out yet.”
The results tie into recent work from the same lab that found evidence of Aβ-induced spontaneous seizure activity in the APP mice (see ARF related news story). “Our idea is that enkephalins could be fueling a vicious cycle that starts with Aβ inducing aberrant excitatory activity, which is well known to increase enkephalin expression. The enkephalins then further enhance abnormal excitation by blocking inhibitory interneurons that normally keep this kind of activity in check,” Mucke says. The increased enkephalin expression would be a contributor to neural network dysfunction seen in people with AD. “That seems a plausible explanation to us, and one that we would like to test more directly in future studies.”
The increase in enkephalin reported in the transgenic mice seems to result from a boost in production. Mucke and colleagues report twofold increases in precursor mRNA in the AD mice. But could there be a role for enkephalin degradation, as well? One protease that breaks down these peptides, puromycin-sensitive aminopeptidase (PSA) has been implicated in neurodegeneration, and seems to dispose of remnants of polyglutamine expanded proteins and tau (see ARF related news story; Bhutani et al., 2007; Hui, 2007). Mucke says that neprilysin, an Aβ degrader, also chews up enkephalins, which may suggest links between Aβ and enkephalin degradation. None of this is exactly clear yet, but may gives researchers in the AD arena a new pathway to ponder.—Pat McCaffrey
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