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mise à jour du
16 janvier 2003
Brain Research
1989;495:20-30  
cas cliniques
Effects of ageing on the behavioural responses to dopamine agonists: decreased yawning and locomotion, but increased stereotypy
A.J. Stoessl, M.T. Martin-Iverson, T.M. Barth, C.T. Dourish and S.D. Iversen Merck Sharp and Dohme Research Laboratories, Neuroscience Research Centre, Harlow, Essex (U. K.)

Chat-logomini

 
Introduction : It is well established that ageing is associated with changes in striatal dopamine metabolism as well as the density of dopamine receptors measured by radioligand binding. The functional significance of these changes is less well defined, however. Thus, most investigators would agree that there is a decrease in the density of 3H spiperone binding in the striatum of aged animals as weil as a decrease in doparnine-stimulated adenylate cyclase. However, loss of D2 binding sites is not associated with decreased D2-mediated inhibition of adenylate cyclase . In view of this seeming paradox, Missale and co-workers suggested that decreases in spiperone binding may reflect the loss of presynaptic dopamine receptors located on nigrostriatal nerve terminals.
 
Aged animals with unilateral 6-hydroxydopamine (6-OHDA) lesions have been reported to show decreased rotation following contralateral striatal dopamine infusion or systemic apomorphine. On the other hand, both unchanged and decreased amphetamine-induced turning have been observed in such animals; there was no age-relaied decrement in turning induced by the dopamine agonist lergotrile. Relatively few studies have examined the effects of ageing on the behavioural responses to apomorphine in otherwise intact animals. In general, an increased duration of stereotyped behaviour following apomorphine bas been noted, possibly reflecting differences in drug pharmacokinetics.
 
We have examined the effects of ageing on the motor behaviours elicited by a range of doses of the classical D1/D2 dopamine agonist apomorphine in the rat. In view of the known loss of nigrostriatal dopamine neurons and decreases in striatal dopamine that occur with ageing, we predicted a decrement in apomorphine-induced yawning and penile grooming, responses that are thought to result from stimulation of autoreceptors located on dopamine nerve terminals. The effect of age on the response to stimulation of postsynaptic dopamine receptors is more difficult to predict.
 
On the one hand, loss of presynaptic input might bc associated with denervation-induced supersensitivity of postsynaptic receptors, and increases in the behavioural responses mediated by these receptors. On the other hand, the results of receptor binding studies would predict decreased responsiveness to postsynaptic dopamine receptor stimulation in aged rats. In order to clarify this issue, stereotypic responses to apomorphine were examined. Sniffing and licking were measured, since they are consistently elicited, and readily quantified. Finally, in order to better appreciate whether the effects of ageing on the responses to apomorphine reflect alterations in D1 or D2 receptors, the locomotor stimulant effects of 14 days of continuous infusion of the selective D2 agonist (+)-4-propyl-9-hydroxynaphthoxazine (PHNO) were also investigated. This particular experimental paradigm was chosen in order to complement other work from our laboratory on the responses to continuous, selective D2 stimulation. Our observations suggest that there is an age-related decline in the functional response to stimulation of dopamine autoreceptors and postsynaptic D2 receptors, although postsynaptic responses to mixed D1/D2 stimulation are increased.
Discussion : The age-related impairment of spontaneous locomotor activity and the abnormal bracing responses are compatible with impaired dopaminergic function. Furthermore, the increased latency to turn towards a perioral stimulus is a classical sign of a contralateral nigrostriatal lesion. The increased latency to remove adhesive patches is also consistent with a dopaminergic deficit, but may also be seen with cortical lesions. The impairments seen on tests of strength and co-ordination are in keeping with deficits previously reported in aged animals.
Pharmacological stimulation of dopamine systems demonstrated an age-related decrement in yawning, but an increase in stereotyped sniffing and licking or gnawing induced by the mixed D1/D2 agonist apomorphine. A significant interaction between the effects of PHNO, a selective D2 agonist, and age on the circadian pattern of locomotor activity was also observed: PHNO increased both the amplitude (size of the circadian variation) and the mesor (average locomotor activity relative to which there is a 24-h oscillation) in locomotor activity for mature rats, and these actions of PHNO were greatly attenuated in aged rats. Stress reversed daytime tolerance to the effects of PHNO in mature rats, and this effect was also absent of the aged animais. Thus, the behavjoural effects of a 'postsynaptic' dose of a mixed D1/D2 agonist (apomorphine) were potentiated with age, while behavioural effects mediated by D2 receptors (either presynaptic autoreceptors activated by low doses of apomorphine, or postsynaptic D2 receptors activated by PHNO) were attenuated.
The increase in apomorphine-induced stereotyped sniffing, licking and gnawing in old animais is consistent with the results of others. One possible explanation for this phenomenon is an alteration in the pharmacokinetic handling of the drug. Campbell and colleagues demonstrated increased intensity and duration of stereotypy in old rats, associated with higher peak drug levels and slower elimination from the brain. Watanabe et al. found delayed onset and decreased intensity, but increased duration of stereotypy in mature versus young rats, as well as delayed peak levels and elimination of apomorphine from plasma and brain. Although we cannot entirely rule out such factors in the present study, the comparable body weights of our two groups would minimize any difference in fat stores. Furthermore, the time course of sniffing and licking/gnawing produced by 200 µglkg of apomorphine in the present study suggests increases in both the intensity and the duration of action of apomorphine in the aged animais.
The PHNO-induced increase in amplitude and mesor of the circadian rhythm of locomotor activity, and the reversal of daytime tolerance by stress in mature rats is similar to the effects of PHNO and stress produced in young rats. In contrast to the age-related increases in postsynaptic responses to apomorphine, the attenuation of these responses in old rats is consistent with reports of reduced density of D2 receptors.
Two possibilities might explain the differences between the effects of ageing on the responses to postsynaptic stimulation with apomorphine and PHNO. It is likely that agonist-induced locomotor activity is predominantly dependent upon stimulation of the mesolimbic dopamine system, as opposed to stereotypy, which appears to be striatally mediated. Thus, site-specific differences in the effects of age on D2 receptor density may explain potentiation of one response (stereotypy) while the other (locomotor stimulation) is depressed. On the other hand, receptor-specific differences in the effects of age could also be responsible for these differences. Ageing may be associated with alterations in the pattern of behavioural responses (increased vacuous chewing, but decreased intense grooming) to the selective D2 agonist, SKF 38393. This area is still controversial; decreases, no change and increases in D1 receptor density have ail been reported, although the activity of dopamine-stimulated adenylate cyclase seems to decline. The possibility that the increase in the stereotypy produced by apomorphine is due to increases in the biological activity of D1 receptors, or a shift in the balance between D1 and D2 receptors is a compelling explanation for the differences observed between responses to the two drugs.
Although some controversy remains, yawning induced by low doses of dopamine agonists is felt to reflect dopamine autoreceptor-mediated inhibition of striatal dopamine release and consequent disinhibition of striatal cholinergic interneurons. Loss of dopamine autoreceptors following 6-OHDA-induced destruction of nigrostriatal dopaminergic terminais attenuates apomorphine induced yawning . An age-related decrease in apomorphine-induced yawning is therefore compatible with a loss of these receptors and would be expected on the basis of the known loss of nigrostriatal dopamine neurons and the consequent decrease in striatal dopamine found in the present study and previously by others. An alternate explanation would be a loss of striatal cholinergic neurons with age. Age-related decreases in striatal cholinergic activity have been reported although, to our knowledge, there are no reports of agerelated decrements in the frequency of physostigymine-induced yawning. If this were the sole explanation for our findings, however, one would also expect to see a decrease in postsynaptically mediate apomorphine-induced behaviours (i.e. stereotypies) reflecting the loss of postsynaptic dopamine receptors, which are predominantly located on striatal cholinergic interneurones. In contrast, however, we observed an age-related increase in apomorphine induced stereotypy.
The dissociation observed between the effects of ageing on yawning and stereotypy following apomorphine may also suggest a differential localization or specificity of the receptors involved in these behaviours, and lends further support to the growing body of evidence that dopamine agonist-induced yawning is presynaptically mediated. It is of interest that Clark and Smith have recently reported a similar dissociation of ageing effects on pre- and postsynaptically mediated sexual responses to apomorphine in the rat.
Other potential explanations for the deperession in apomorphine-induced yawning in the aged animals should be examined. Altered pharmacokinetics do not adequately explain this finding, since the dose-response curve for yawning was of lower amplitude and not horizontally shifted. Similarly, behavioural response competition is not an important factor; although stereotyped sniffing, licking and gnawing were more pronounced in the older animals, these behaviours were not seen at the doses which elicited maximal yawning.
Another factor to be considered is the effect of dietary restriction on our findings. A mild (25 g/day) restriction was imposed on the aged animals used in this study, in order to prolong their natural life expectancy. Other investigators have demonstrated that a different form of restriction (ad libitum feeds but on alternate days only) retards the development of age-related decrements in dopamine receptor ligand binding and dopamine agonist-induced behavioural responses. Thus, the dietary manipulation employed would if anything have masked a true effect of ageing.
During the completion of this study, similar findings were reported by Ushijima et al.Their study was not entirely comparable, however. First, they were comparing young (2-month-old) with middle-aged (12-month-old) rats, as opposed to mature and senescent rats in our study. Secondly, because the weights of their two groups were quite different (270-300 g vs 600-650 g), differences in the behavioural responses to apomorphine might conceivably reflect differences in pharmacokinetics. Finally, Ushijima et al. examined the interactions between apomorphine and bromocriptine, and did not use a range of doses of apomorphine comparable to ours.
Further studies examining the effects of ageing on the behavioural responses to both low and high doses of selective D1 and D2 agonists are needed. A better appreciation of age-related changes in the functional effects of such agents may improve our understanding of the pathogenesis and treatment of disorders associated with dopaminergic dysfunction, including Parkinson's disease, schizophrenia and tardive dyskinesia.
 
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