Table 4 Effect of age on the expression and functional properties of multiple opioid receptors
From: Molecular and cellular mechanisms of the age-dependency of opioid analgesia and tolerance
Opioid receptor type | Animal models and experimental means | Observed age-dependent expression and activities of opioid receptors |
|---|---|---|
μ | DAMGO and dihydromorphine binding assays in brains of mice of various ages | Bmax values and selectivity for -selective opioid ligands change as a function of age |
| Â | DAMGO binding with light and heavy membranes of rat brain | The subcellular distribution of opioid receptors changes with age |
| Â | DAMGO binding in the spinal cord of rats of different ages | The Kd value for DAMGO is significantly higher in the aged rats than in the young and mature rats, indicating a decreased affinity of spinal opioid receptors for DAMGO |
|  | Effect of opioid agonists on warm water–stimulated tail-withdrawal in young (3 months) and old (24 months) male rats | Old male rats are more sensitive to the antinociceptive effects of opioids than young ones; the age-related differences in opioid sensitivity are most apparent when lower-efficacy opioids and higher nociceptive intensities are tested |
| Â | EM ICC with rat caudate-putamen nucleus | The developmental expression of opioid receptors parallels asymmetric synapse formation |
δ | Quantitative autoradiography with opioid receptor binding in guinea pig brain | With age, opioid receptor density decreases in the globus pallidus and increases in the neocortex |
| Â | EM ICC with rat caudate-putamen nucleus | Opioid receptor expression gradually increases from birth to adulthood and correlates with synapse formation |
| Â | Agonists DSLET and DPDPE used to stimulate high-affinity GTPase activity in young (4 weeks) and old (16 weeks) guinea pig striatal membranes | Agonists can stimulate high-affinity GTPase activity in striatal membranes from old guinea pigs but not from young ones, indicating age-dependent opioid receptor-G protein functional coupling |
κ | Quantitative autoradiography with κ opioid receptor binding in guinea pig brain | Expression of opioid receptors decreases with age |
|  | IP injection of selective κ opioid agonist U50,488 H in young (6–8 weeks) and old (21–22 months) mice | Qualitative sex differences in opioid analgesia in the mice are dependent on age |
| Â | Tested effect of opioid agonists on the warm water-stimulated tail-withdrawal in young (3 months) and old (21 months) male rats | Aged male rats are more sensitive than young ones to the antinociceptive effects of opioid agonists |
ORL1 | In situ hybridization and autoradiography with human, rat, and mouse brains | Differential expression of ORL1 found in developing and adult brains |