Age-related changes in central nervous system enkephalins and substance P

Decrease of mu opioid receptors in the brain and in the hypothalamus of the aged male rat

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.

Age dependence of somatostatin levels and somatostatin binding in the rat brain

1. Somatostatin-like immunoreactivity (SLI) and 125I-Tyrl-somatostatin binding were measured from the brains of rats aged 1, 8 and 18 months. 2. Somatostatin binding was reduced in the striatum, frontal cortex, hypothalamus and hippocampus of the 8-month-old rats compared to the 1-month-old group. 3. Somatostatin binding was reduced in the striatum, frontal cortex and hippocampus of the 18-month-old rats compared to the 1-month-old group. 4. The reduction (40%) was most striking in the frontal cortex. 5. In no area of the brain did changes in SLI differ significantly between the different age groups.

Age-related changes in neurotensin content and receptors in various rat brain areas

Neurotensin immunoreactivity (NT-IR) is significantly reduced in striatum, nucleus accumbens and hippocampus but not in frontal cortex, hypothalamus and septum of 24-month-old male Sprague Dawley rats. Neurotensin binding in cortex, striatum, hypothalamus and hippocampus shows a rather uniform decline with age. The changes in NT-IR content in striatum and nucleus accumbens are of particular interest in view of the existence of a functional relationship between dopamine and neurotensin.

Age-related decrements in morphine analgesia: a parametric analysis

Although age-related reductions in levels of opiate receptors and endogenous opioid peptides have been observed in rats, effects of aging upon basal pain thresholds and morphine analgesia have not indicated clear results. The present study evaluated the dose-dependent (1,2.5, 5 and 10 mg/kg, SC) and time-dependent (30, 60, 90, 180 min) properties of morphine analgesia on two nociceptive measures (tail-flick latencies and jump thresholds) across five age cohorts of rats (4, 9, 14, 19 and 24-months of age). To ascertain whether any changes were the result of specific alterations in pain inhibition or an overall shift in opiate responses, effects upon morphine-induced hyperthermia were also evaluated. Age-related effects upon morphine analgesia on the tail-flick test exhibited a biphasic pattern with the three older age groups displaying significant decreases in morphine analgesia 30 min after injection and significant increases in morphine analgesia 180 min after injection. Age-related effects upon morphine analgesia on the jump test revealed equivilent reductions in analgesic magnitude across doses for the three older cohorts. In contrast, morphine hyperthermia displayed small inconsistent changes across cohorts. The reductions in morphine analgesia in older animals could not be attributed to changes in either baseline pain thresholds or delayed peripheral absorption and/or clearance of the drug. Rather, the reductions in morphine analgesia in older animals complement the recent findings of similar age-related reductions in the analgesic responses induced by exposure to several environmental stressors.

Aging and the blood-brain barrier: changes in the carrier-mediated transport of peptides in rats

Age-related changes in the brain's saturable, specific, carrier-mediated transport system for the small, N-tyrosinated peptides Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and methionine-enkephalin (Met-Enk) were studied in Fischer 344 rats aged 4 and 26 months. These studies showed statistically significant differences between the two age groups for both the Tmax (transport maximum) [3.22 +/- 0.013 nmol/min/g (young rats, mean +/- S.E.M.) vs 2.41 +/- 0.009 nmol/min/g (age rats)] and T50 (the amount required to achieve 50% of that maximum) [84.9 +/- 1.0 nmol/g (young) vs 65.1 +/- 0.60 nmol/g (aged)]. The T50:Tmax ratio was nearly equal to the two groups: 26.4 (young) vs 26.9 (aged), consistent with the uncompetitive type of inhibition indicative of alterations in the substrate-carrier complex. In addition, blood concentrations of Tyr-MIF-1-like immunoactivity were nearly doubled in aged rats (3.24 +/- 0.373 vs 1.67 +/- 0.0904 pM/ml), while blood concentrations of Met-Enk-like immunoactivity and brain concentrations of immunoactive Tyr-MIF-1 and Met-Enk showed no statistically significant difference between age groups. Thus, a carrier-mediated system responsible for the transport of peptides across the blood-brain barrier undergoes changes with aging.

Multiple changes in the sensitivity of cingulate cortical neurones to putative neurotransmitters in ageing rats: substance P, acetylcholine and noradrenaline

The responsiveness of neurones in the anterior cingulate cortex to iontophoretically applied substance P(SP), acetylcholine (ACh), noradrenaline (NA) and GABA was compared in young (3-4 months) and old (24-30 months) rats. Neurones in the old rats were less sensitive to the depressant effects of NA but not GABA. These cells were also less sensitive to the excitatory actions of ACh but markedly more sensitive to those of SP. Such changes in responsiveness could be involved in the deficits in cerebral function which often occur in old age.

Age-related and diurnal changes in Met5-Enk-Arg6-Phe7 and Met5-enkephalin contents of pituitary and rat brain structures

The beta-endorphin, met5-enkephalin-arg6-phe7 (MEAP) and met5-enkephalin (ME) changes related to age and diurnal rhythms were studied in various regions of rat brain and in the pituitary by specific radioimmunoassays. The contents of MEAP, met5-enkephalin and beta-endorphin were higher in the pituitary of old rats (18 months old) than that of young rats (23 days old) while the content of these opioid peptides was higher in the hypothalamus of young rats than in that of old rats. Beta-endorphin was also higher in the striatum of 23 days old rats, but no age-associated changes were observed in the hippocampus, brain stem or cortex. In the diurnal rhythm study, it was found that in the hypothalamus and striatum of the adult rat (2-3 months old), both MEAP and ME contents were higher at mid-dark than at mid-light and that in the intermediate posterior lobe of the pituitary, the ME content was also higher at mid-dark.

Aging and daily rhythms of analgesia in mice: Effects of natural illumination and twilight

Day-night rhythms in aversive thresholds and morphine-induced analgesia were assessed in young (1-3 months) and old (22-30 months) male mice exposed to natural summer (43 degrees N lat.) lighting using a hot-plate technique. In both age groups peak aversive thresholds and morphine-induced analgesia were present at night, with significantly reduced response times in the day. Response times increased during the course of the day with maximum increases in aversive thresholds and analgesia occurring during the decreasing light levels of dusk. Maximum decreases in the nocturnal response times occurred during the increasing light levels of dawn. The old mice displayed significantly lower nocturnal aversive thresholds and morphine-induced analgesia, as well as less pronounced patterns of change in response times during the dawn and dusk twilight lighting transitions. No significant effects of age were evident during the day-time.

Immunoreactive-beta-endorphin and LHRH levels in the brains of aged male rats with impaired sex behavior

Levels of immunoreactive beta-endorphin and luteinizing hormone releasing hormone (LHRH) were measured in brain tissue of aged male Long-Evans rats. The animals were tested for sex behavior twice in one week at bimonthly intervals between the 7th and 27th month of life and were sacrificed along with a group of young (5-month old) sexually active rats. Thirty-one of the 89 rats which began the study remained healthy and tumor-free. By month 27, 21 of these had completely ceased to mate and 10 continued to show adequate sexual behavior. Diminished levels of beta-endorphin-like immunoreactivity were measured in the hypothalami and hindbrain of the old animals grouped together as compared to young animals and this reduction was shown to be significantly greater in hypothalamic tissue from the behaviorally inactive subgroup. Hypothalamic LHRH levels were not significantly altered by age in these animals. However, a marked reduction of LHRH content in the septal and midbrain regions of the aged-behaviorally inactive subgroup was evident when compared with the behaviorally active group. The data suggest that altered function of beta-endorphin and LHRH neurons of the aged brain may be involved in the behavioral deterioration observed in aged animals.

Daily rhythms of analgesia in mice: effects of age and photoperiod

Daily rhythms in response to aversive thermal stimulation and the analgesic effectiveness of morphine were assessed by the hot-plate method with young (1-2 months), mature (8-12 months) and old (20-30 months) mice exposed to various light-dark conditions (LD 12:12; 16:8; 8:16 h). The patterns of response after saline or morphine varied with the specific light-dark conditions examined, but routinely, there were increases in response latency from the early portions of the light phase to later time, and a further enhancement of the time to respond with onset of the dark phase. The dark-phase response declined abruptly with the start of the light-phase. Significant age-related declines were observed in the elevated, dark-phase morphine-analgesic and basal aversive responses, with no consistent effects of age evident in the light-phase.

The effects of aging on opioid modulation of feeding in rats

Feeding responses to naloxone and butorphanol tartrate were measured in Fisher-344 rats with ages of 2, 12, 22 and 28 months. The two younger groups were 10-100 times more sensitive than the older groups to the suppressive effects of naloxone on feeding. Additionally, the older rats were less responsive to the feeding enhancement following butorphanol injections. These results are consistent with reports of age-related changes in endogenous opioid systems.

Decreased content of met-enkephalin-like peptides in superior cervical and coeliac ganglia of aged rats

Enkephalin like peptides seem to have an important regulatory role at ganglia level. The aim of the present study is to investigate whether the content of enkephalin-like peptides in sympathetic ganglia is affected by the aging process. The results show that the enkephalin like peptides content is low in superior cervical and coeliac ganglia of aged rats (25 months). The age-related decrease of enkephalin content in these structures may be of importance in determining an altered sympathetic control during aging.

Ageing, opioid analgesia and the pineal gland

The effects of ageing on day-night rhythms of analgesia was examined with young (1-2 months), mature (8-12 months) and old (20-30 months) mice. Significant age-related declines were observed both in the absolute levels and diel rhythms of morphine analgesia, with the most pronounced changes occurring at night. Administration of the pineal hormone, melatonin, augmented day-time levels of analgesia in all age classes and reversed the age-related decline in nocturnal morphine analgesia in old mice. Inhibition of pineal function in young mice by either exposure to light pulses or treatment with benserazide mimicked the effects of ageing on nocturnal morphine analgesia. These findings suggest that the pineal gland and melatonin are involved in modulating diel rhythms of analgesia and have an influential role on age-related changes in opioid responses.

Age related changes of enkephalin in rat spinal cord

Met-enkephalin immunoreactive material content was found to be decreased in the cervical and thoracic segments of the spinal cord from rats aged 25 months as compared to young, 3-month-old, rats. No age-related variations were detectable at the lumbar level. Bio-Gel P 30 column chromatography of thoracic segment extracts indicates that the composition of the immunoreactive material is similar in the two age-groups investigated. At the thoracic level opiate receptor binding was also measured. Opiate receptor number is increased in the thoracic segments of the spinal cord from older rats. These age-related changes in immunoreactive Met-enkephalin content and opiate receptor number at spinal levels may contribute to determine an altered pain sensitivity during aging.

Changes of beta-endorphin and Met-enkephalin content in the hypothalamus-pituitary axis induced by aging

The amounts of beta-endorphin- and Met-enkephalin-immunoreactive material are higher in the pituitary of aged rats. However, the aging process decreases the content of beta-endorphin-, but does not affect that of Met-enkephalin-immunoreactive material, in hypothalamus. Thus, it seems that the regulatory mechanisms in the two areas are differentially affected by increasing age. On the other hand, the pituitary increase of these peptides is in line with the assumption that in the elderly the hormonal response to stress is impaired.

Neuropeptides, mental performance and aging

The presence of peptidergic neuronal networks in the brain and the modulating action of neuropeptides on brain functions as evidenced by their behavioral influence in particular support the concept that the brain like the peripheral endocrine glands is an endocrine target organ which is as sensitive to treatment with neuropeptides as the peripheral glands are to pituitary hormones. Animal and human data are reviewed showing that neuropeptides related to ACTH/MSH affect motivational and attentional processes and that those related to vasopressin are involved in memory processes. Since these functions decline during aging it is postulated that a decreased bioavailability of neuropeptides in brain of elderly people is associated with specific disturbances in mental performance. Thus, the decreased mental ability of the aged may be restored by treatment with neuropeptides particularly those with little, if any, peripheral, endocrine activity, like the ACTH neuropeptide Org 2766 and the vasopressin neuropeptide DGAVP.

A survey of substance P, somatostatin, and neurotensin levels in aging in the rat and human central nervous system

Levels of the neuropeptides substance P, somatostatin, and neurotensin were measured by radioimmunoassay in regions of the rat and human central nervous system (CNS) in aging. Somatostatin levels were significantly lower only in the corpus striatum of older rats. Substance P levels and neurotensin levels were generally stable with aging as were levels of somatostatin in regions other than the corpus striatum. In post-mortem human CNS tissues, no significant negative correlations of levels of the three peptides were observed with time to refrigeration or time to freezer for the samples. In the human CNS, there were no significant age-related alterations in substance P levels in frontal cortex, thalamus, hypothalamus, caudate nucleus, globus pallidus, or substantia nigra. There was a significant age-related decrease in substance P levels in the human putamen. This age-related decrease was not present in tissues from victims of Huntington's disease nor was there any striking difference in substance P levels as a function of duration of the disease. There were no significant age-related changes in somatostatin levels in human frontal cortex, caudate nucleus, putamen, medial globus pallidus, or substantia nigra. Among these same regions, there was a significant age-related decrease in neurotensin levels only in the pars compacta and pars reticulata of the human nigra. These, results implicate neuropeptides in aging processes in certain regions of the CNS. There are differences between rats and humans with respect to neuropeptides in the aging process in the CNS. Deterioration of some neuropeptide pathways in and to human basal ganglia may be involved in the suspected functional deterioration of parts of the extrapyramidal system in aging.