Restriction and functional changes of dopamine release in rat striatum from young adult and old rats

Aging and sex differences in striatal dopaminergic function

In this report the potassium- (30 mM) and amphetamine- (10 microM) stimulated responses of dopamine (DA) and 3,4-dihydroxy phenylacetic acid (DOPAC) from superfused striatal tissue of female and male mice as sampled at 2, 6, 18 and 24 months of age were compared. When assessed relative to responses obtained from 2-month-old female mice, potassium-stimulated DA output of female mice was significantly decreased at 18 months of age and significantly increased at 24 months of age. In male mice, the only statistically significant change was an increase in potassium-stimulated DA in the 24 versus 2-month-old mice. In response to amphetamine-stimulation, DA responses from striatal tissue of 18-month-old females were significantly decreased and that of 24-month-old mice significantly increased relative to that of the 2-month-old females. In the case of male mice, amphetamine-stimulated DA responses of 6- and 18-month-old mice were significantly decreased compared with responses observed in the 2-month-old males. In addition, amphetamine-stimulated DA responses of the 24-month-old females were significantly greater than the 24-month-old males. In general, the response profiles for DOPAC to potassium and amphetamine stimulation were similar to that of DA for male, but not female, mice. These results demonstrate that sex differences in striatal dopaminergic function are differentially affected by age. Overall, striatal DA responsiveness of female mice shows more extreme age-related changes, particularly between the 2- and 6-month versus the 18- and 24-month-old mice and a discord between DA and DOPAC responses. Such extreme changes may be related to the presence (at 2 and 6 months) versus absence (at 18 and 24 months) of estrous cycles/gonadal steroid hormonal functions in female mice.

Effects of repeated injections of the neurotensin analog NT69L on dopamine release and uptake in rat striatum in vitro

The effect of five daily intraperitoneal (i.p.) injections of NT69L on in vitro dopamine release, uptake, and [(3)H]NT binding in rat striatal tissue was investigated. NT69L perfusion increased K(+)-evoked and electrically evoked [(3)H]DA release. NT receptor-1 antagonist SR48692 inhibited the stimulatory effect of NT69L on K+-evoked [(3)H]DA release, but not on electrical depolarization. Pretreatment with NT69L, in vivo, daily for 5 days, did not cause significant change in K(+) evoked [(3)H]DA release, but reduced electrically evoked [(3)H]DA release induced by NT69L perfusion. Repeated perfusion with NT69L in vitro caused marked reduction on K(+)-evoked [(3)H]DA release and no change in electrically evoked [(3)H]DA release. [(3)H]NT binding was not significantly changed by one injection but was decreased after five injections of NT69L. Desensitization to the effects of NT69L in vitro was different depending upon whether tissue was preexposed to the compound in vivo or in vitro. These results provide further proof for the involvement of different NT receptor subtypes in mediating the effect of NT69L on dopamine release evoked by K(+) or electrical depolarization.

Effect of methylphenidate on dopamine/DARPP signalling in adult, but not young, mice

Methylphenidate (MPH), a dopamine uptake inhibitor, is the most commonly prescribed drug for the treatment of attention-deficit/hyperactivity disorder (ADHD) in children. We examined the effect of MPH on dopamine- and cAMP-regulated phosphoprotein, Mr 32 kDa (DARPP-32) phosphorylation at Thr34 (PKA-site) and Thr75 (Cdk5-site) using neostriatal slices from young (14-15- and 21-22-day-old) and adult (6-8-week-old) mice. MPH increased DARPP-32 Thr34 phosphorylation and decreased Thr75 phosphorylation in slices from adult mice. The effect of MPH was blocked by a dopamine D1 antagonist, SCH23390. In slices from young mice, MPH did not affect DARPP-32 phosphorylation. As with MPH, cocaine stimulated DARPP-32 Thr34 phosphorylation in slices from adult, but not from young mice. In contrast, a dopamine D1 agonist, SKF81297, regulated DARPP-32 phosphorylation comparably in slices from young and adult mice, as did methamphetamine, a dopamine releaser. The results suggest that dopamine synthesis and the dopamine transporter are functional at dopaminergic terminals in young mice. In contrast, the lack of effect of MPH in young mice is likely attributable to immature development of the machinery that regulates vesicular dopamine release.

Enhanced effects of 6-hydroxydopamine on evoked overflow of striatal dopamine in aged rats

Nigrostriatal dopamine neurons degenerate during aging, and the excessive loss of dopamine neurons that occurs with Parkinson's disease is usually confined to older individuals. Although 6-hydroxydopamine lesioning of the nigrostriatal dopamine system is a common method for producing animal models of dopamine neuron degeneration, there have been relatively few studies that have examined the effects of 6-hydroxydopamine on the dopamine systems of aged animals. The present experiments were designed to determine if nigrostriatal dopamine neurons in aged rats are more sensitive to the neurotoxic effects of 6-hydroxydopamine than those of younger rats. Young (4-month-old), middle-aged (14-month-old) and aged (24-month-old) Fischer-344 rats were given a single injection of vehicle, 50 or 100 microg 6-hydroxydopamine into the right lateral ventricle. Three to four weeks later in vivo electrochemistry was used to measure potassium-evoked overflow of dopamine in the striatum. In the young rats the 50-microg dose had no significant effect on evoked overflow of dopamine in the striatum or on post-mortem levels of dopamine in the striatum or substantia nigra. The higher dose in the young animals diminished evoked overflow of dopamine as well as tissue levels of dopamine. In the aged rats both doses of 6-hydroxydopamine led to significant decreases in evoked overflow of striatal dopamine and in tissue levels of dopamine in the striatum and substantia nigra. These results suggest that dopamine neurons of aged Fischer-344 rats are more susceptible to the toxic effects of 6-hydroxydopamine than those of younger animals.

Plasticity of GABA(a) system during ageing: focus on vestibular compensation and possible pharmacological intervention

The lesion of the vestibular end organ evokes static and dynamic symptoms, which spontaneously regress during a complex process known as 'vestibular compensation'. Vestibular compensation is age-dependent and involves several transmitter-identified pathways in the central nervous system. In this paper we studied the time course of vestibular compensation in adult (3 months) and old (24 months) rats and correlated behavioral recovery with modifications of glutamic acid decarboxylase (GAD) mRNA expression and benzodiazepine receptor density in different brain areas. Compensation in adult rats was complete 28 days after hemilabyrinthectomy, whereas old rats still showed significant behavioral impairment. A higher GABAergic tone was found in old rats, as indicated by higher benzodiazepine receptor density in lateral vestibular nucleus and higher mRNA level for glutamic acid decarboxylase in cerebral cortex and medial vestibular nucleus. In adult, compensated rats, benzodiazepine receptor density in the vestibular nuclei was normal 28 days after lesion, whereas GAD mRNA level was higher in anterior cingulate cortex, only. On the contrary, these parameters were still altered in anterior cingulate and somatosensory cortex, basal ganglia, vestibular nuclei and cerebellum in old rats 28 days after vestibular lesion. We also evaluated the effect of the ergoline derivative nicergoline on behavioral and neurochemical correlates of vestibular compensation in old rats. Nicergoline treatment attenuated the severity of oculomotor and postural symptoms after vestibular lesion and reversed most of these age- and lesion-induced alterations in GAD mRNA expression. Thus, lesion-related alterations of the GABAergic transmission and behavioral profile after vestibular lesion are age-dependent.

Age-related changes in striatal function of freely-moving F344 rats

Multi-wire electrode arrays were used to record extracellular electrophysiological activity in striatal medium spiny-like neurons of freely-moving young (6-8 months) and aged (24-26 months) Fischer 344 rats. While overall basal firing rates did not differ between the two groups, d-amphetamine (5.0 mg/kg) increased firing rates more in the young rats. D-Amphetamine had heterogeneous effects on firing rates, however, exciting 63% of the neurons while inhibiting 37%. Neurons were classified according to their response to d-amphetamine (excited vs. inhibited) to examine age-related differences in firing rates and bursting activity. In the d-amphetamine-excited neurons, pre-drug intraburst firing rates were higher in the old rats. This effect was reversed by d-amphetamine. D-Amphetamine increased the percentage of spikes within bursts to a greater extent in the aged animals and decreased burst durations greater in the young group. In d-amphetamine-inhibited neurons, firing rates were diminished in the old rats more than they were in the young rats. These results demonstrate age-related alterations in striatal electrophysiological activity that may help explain motor deficits seen in senescence.

Recovery of high potassium-evoked dopamine release after depolarization challenge in the striatum of young and old male rats

The aim of this study was to assess the recovery of high potassium-evoked dopamine (DA) release after depolarization challenge in young (3-4 months) and old (21-25 months) male Wistar rats. Recovery of DA release was evaluated by comparison of the peak responses of DA release induced by two serial high potassium stimulations. Concentric microdialysis probes were stereotaxically implanted in the lateral striatum of rats, and microdialysis was commenced 24 h after surgery. Using a low flow rate of perfusion (1 microl/min), all rats received 2 x 20 min infusions of 100 mM potassium solution separated by either 60 or 140 min. No difference in the basal DA concentration or the potassium-evoked DA release or its recovery was seen between the two groups. Our results suggest that the vesicular DA store recovers rapidly after high potassium challenge in both young and old rats.

Decreased mRNA levels for exocytotic proteins in the pituitary of aged rats

Several protein components that are involved in the molecular regulation of transmitter release have been identified in neuronal, neuroendocrine and endocrine tissues. The expression of VAMP-2 (vesicle-associated membrane protein), munc-18 (mammalian homologue of unc-18) and SNAP-25 (synaptosomal-associated protein of 25 kDa) mRNA was studied in the rat anterior and intermediate pituitary gland of adult (2 months) and old (24 months) rats using in situ hybridization. In the pituitary anterior lobe of aged rats, there was a significant decrease in VAMP-2 (33%), munc-18 (17%) and SNAP-25 (20%) mRNA as compared to adult rats. In the intermediate lobe, there was a significant decrease in VAMP-2 (48%) and SNAP-25 (32%) mRNA of aged rats, whereas munc-18 mRNA levels were not significantly changed. Pituitaries from aged rats showed an increase in size which was paralleled by a significant decrease in the number of cells per unit area in the intermediate lobe, whereas the number was unaltered in the anterior lobe. The results suggest a genuine decrease in mRNA for exocytotic protein mRNA in the anterior pituitary, but that part of the decrease in the expression of VAMP-2 and SNAP-25 mRNA in the intermediate lobe can be explained by a decreased number of cells per unit area. The decline in anterior pituitary hormone secretion reported in aged rats appears to be parallelled by a down-regulation in mRNA levels for several proteins involved in the molecular regulation of exocytosis.

DA1 and DA2 receptor regulation in the striatum of young and old rats after peripheral vestibular lesion

Anatomical, lesion and functional studies have indicated that the mesostriatal dopaminergic (DAergic) system may serve as supravestibular center in posture and locomotion control. Nevertheless, no data are available on the involvement of DAergic systems during vestibular compensation. This study was designed for the analysis of DA1 and DA2 receptors in the striatum by means of quantitative receptor autoradiography 28 days after unilateral or bilateral lesion of the labyrinth in 3-month-old rats. Considering the severe decline of DA content and receptors in striatum and the difference in behavioral recovery after vestibular lesions in old age, we also analyzed 24-month-old, lesioned and unlesioned rats. In young rats, hemilabyrinthectomy caused a bilateral increase (20-30%) of DA1 receptors and a two-fold increase of DA2 receptors. In old-rats, we observed a similar modification of DA2 receptors, and a 50% increase in DA1 receptors. Bilabyrinthectomy did not modify DA1 receptor density and decreased DA2 receptor density in young animals, whereas it produced an increase in both DA1 and DA2 in old rats. This study provides evidence for the involvement of the DAergic system during vestibular compensation. Our results also indicate great biochemical plasticity of the remaining DA receptors in the striatum of old rats.