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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
  DAMGO binding with light and heavy membranes of rat brain The subcellular distribution of opioid receptors changes with age
  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
  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
  EM ICC with rat caudate-putamen nucleus Opioid receptor expression gradually increases from birth to adulthood and correlates with synapse formation
  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
  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
  1. IP, intraperitoneal