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Decrease of mu opioid receptors in the brain and in the hypothalamus of the aged male rat
Abstract:
Experiments have been designed in order to analyze whether the binding capability of mu opioid receptors in the brain of the male rat is modified by age. In a first experiment, the number of receptors (Bmax) and the constant of affinity (Ka) for the mu ligand 3H-dihydromorphine (3H-DHM) have been measured in the whole brain of male rats of 2, 15 and 22 months of age. In a second experiment the Bmax and the Ka for 3H-DHM have been evaluated in the hypothalamus of male rats of 2 and 22 months of age. In this experiment it was also investigated whether the administration of exogenous testosterone modifies the number and/or the affinity of the hypothalamic mu receptors. Serum levels of LH, FSH, prolactin and testosterone have been measured by specific RIAs. The results obtained show that: serum testosterone levels are significantly decreased in aged rats, while serum LH and FSH show only a small decline; serum prolactin is higher in old than in young animals; the number of mu receptors in the whole brain of 15 and 22 month old animals and in the hypothalamus of 22 month old rats is significantly lower than in the same tissues of young animals; the administration to old animals of testosterone, in doses able to bring back towards normal serum levels of testosterone, induces a decrease of LH and FSH, but has no effect on serum prolactin titers. Testosterone administration does not modify the number of hypothalamic mu opioid receptors, indicating that the decline of brain mu receptors in old animals is not the consequence of the physiological decline of testosterone secretion; in no instance the Ka for the mu ligand is significantly affected.
Insights
Aging male rats show a significant decrease in mu opioid receptors in the whole brain and hypothalamus. This decline is independent of age-related testosterone reduction.
Area of Science:
- Neuroscience
- Endocrinology
- Aging Research
Background:
- Mu opioid receptors play a crucial role in various physiological processes.
- Age-related changes in the brain and hormonal milieu are common.
- Understanding these changes is vital for addressing age-associated neurological conditions.
Purpose of the Study:
- To investigate the impact of aging on mu opioid receptor binding capability in male rats.
- To determine if age-related decline in testosterone affects mu opioid receptor levels.
- To examine the effects of exogenous testosterone administration on these receptors in aged rats.
Main Methods:
- Quantification of mu opioid receptor number (Bmax) and affinity (Ka) using 3H-dihydromorphine (3H-DHM) in whole brain and hypothalamus of rats aged 2, 15, and 22 months.
- Measurement of serum levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, and testosterone via radioimmunoassay (RIA).
- Administration of exogenous testosterone to aged rats to assess its effects on receptor binding and hormone levels.
Main Results:
- Aged rats (15 and 22 months) exhibited significantly lower mu opioid receptor numbers in the whole brain compared to young rats (2 months).
- Hypothalamic mu opioid receptor numbers were also significantly reduced in 22-month-old rats.
- Serum testosterone levels decreased with age, while prolactin increased; LH and FSH showed minor declines. Testosterone administration normalized testosterone levels but did not alter prolactin or mu opioid receptor numbers.
Conclusions:
- Aging leads to a significant reduction in mu opioid receptor density in the male rat brain, particularly in the hypothalamus.
- The observed decline in brain mu opioid receptors is not a consequence of age-related decreases in testosterone secretion.
- Opioid receptor affinity (Ka) remained unaffected by age or testosterone administration.