Age-related changes of serotonin and its metabolites content in the visual system of the rat

Why are neurotransmitters neurotoxic? An evolutionary perspective

In the CNS, minor changes in the concentration of neurotransmitters such as glutamate or dopamine can lead to neurodegenerative diseases. We present an evolutionary perspective on the function of neurotransmitter toxicity in the CNS. We hypothesize that neurotransmitters are selected because of their toxicity, which serves as a test of neuron quality and facilitates the selection of neuronal pathways. This perspective may offer additional explanations for the reduction of neurotransmitter concentration in the CNS with age, and suggest an additional role for the blood-brain barrier. It may also suggest a connection between the specific toxicity of the neurotransmitters released in a specific region of the CNS, and elucidate their role as chemicals that are optimal for testing the quality of cells in that region.

Relation between RNA content and ageing in neurons of the dorsal lateral geniculate nucleus

We carried out a study to establish ribonucleic acid (RNA) content in the nucleus and cytoplasm of single neuronal cells from the dorsal lateral geniculate nucleus (dLGN) of 3-30 month-old rats. Mean RNA content was calculated as the product of RNA concentration and nuclear or cytoplasmic surface. The analysis of neuronal nuclei revealed no significant differences in RNA concentration, nuclear area, and RNA content from 3-18 months. However, a significant decrease in RNA concentration (18.73%) was found from the 18th-24th month, although no changes were observed in nuclear area and RNA content. The oldest rats, 24-30 months old, presented a significant increase in nuclear area and RNA content. As regards to the neuronal cytoplasm, no significant differences were found in any of the parameters at the ages from 3-18 months and 18-24 months. In contrast, a significant increase in RNA concentration (26.26%), cytoplasm area (18%), and RNA content (52%) takes place from the 24th-30th month. The increase in RNA content could be related to neuronal hypertrophy.

Increased expression of serotonin transporter messenger RNA in raphe neurons of the aged rat

The action of serotonin (5-hydroxytryptamine; 5HT) in the nervous system is terminated by reuptake of the neurotransmitter into the presynaptic nerve terminal via a plasma membrane-bound transporter protein. Recently, cDNAs encoding serotonin transporter protein were cloned. Using in situ hybridization we have studied the expression of serotonin transporter mRNA in the bulbospinal 5HT system of adult (2-3 months) and aged (30 months) rats. In adult rats, serotonin transporter mRNA labelled neurons were detected in the nucleus raphe obscurus, pallidus and magnus. In all aged rats, the optical density of the serotonin transporter mRNA labelling of individual cell profiles was increased by 20-30% (P < 0.001) when compared with adult rats. Five of the six aged rats studied had various degrees of hindlimb motor dysfunctions. In rats with the most pronounced signs of hindlimb motor dysfunction the number of labelled neurons was increased by 25-75% (P < 0.01) when compared with young adult rats. It is concluded that an increased serotonin transporter gene expression is present in the bulbospinal 5HT system of aged animals.

Ageing and monoamine turnover in the lateral geniculate nucleus and visual cortex of the rat

The effects of ageing on the turnover of dopamine, noradrenaline and serotonin in the lateral geniculate nucleus and the visual cortex were evaluated, using high performance liquid chromatography (HPLC) with electrochemical detection. Compared to adult animals, aged rats showed more changes in the visual cortex than in the lateral geniculate nucleus, with dopamine turnover decreased in both structures and noradrenaline turnover unaltered. Changes in serotonin turnover were witnessed only in the visual cortex. A decrease in the monoamine oxidase-A to -B ratio was also observed with increased age for both the lateral geniculate nucleus and visual cortex.

The influence of age on neurotransmitter turnover in the rat's superior colliculus

Measurements of the turnover of dopamine, noradrenaline and serotonin and their metabolites have been performed in the superior colliculus of adult and aged rats. The turnover of dopamine, noradrenaline and their metabolites after pargyline treatment was significantly lower in aged rats than in adults. On the contrary, the synthesis rate of serotonin (measured by accumulation of 5-hydroxytryptophan after decarboxylase blockade) and the turnover rate of serotonin (after pargyline treatment) did not change during aging. These findings suggest that aging has a different effect on catecholamines and serotonin turnover in the superior colliculus of the aged rats.

Age-related alterations in behavioral and cerebral metabolic responses to the serotonin agonist meta-chlorophenylpiperazine in rats

To determine the functional relevance of the age-related neurochemical changes that occur in brain serotonin systems during aging, we measured the effects of the serotonin receptor agonist meta-chlorophenylpiperazine (MCPP) on behavior and on regional cerebral metabolic rates for glucose (rCMRglc) in awake rats. rCMRglc was determined in 74 regions of Fischer-344 rats aged 3, 12 and 24 months, at 15 and 90 min after MCPP 2.5 mg/kg IP, using the quantitative, autoradiographic [14C]2-deoxy-D-glucose technique. The time-course of motor performance following MCPP was assessed with a rotating rod. MCPP impaired motor performance in all ages maximally at 15-30 min. Three-month-old rats recovered completely within 60 min, whereas 12-month-old animals exhibited partial recovery and 24-month-old rats did not recover by 120 min. At 15 min after MCPP, rCMRglc was reduced in 51 of the 74 studied regions (overall decrease, 20%) of 3-month-old rats, in 21 regions (13% decrease) of 12-month-old rats and in 14 regions (2% decrease) of 24-month-old animals. Similar MCPP brain concentrations were achieved at 15 min in rats of all ages. The results suggest that the functional integrity of serotonergic transmission is reduced in aged rats and that the dysregulation is presynaptic.

Histochemical study of RNA content of neurones in the dorsal lateral geniculate nucleus during postnatal development

Nuclear and cytoplasmic dLGN neurons were investigated by cytophotometric measurements of RNA. This study has been carried out in rats from birth to adulthood. In order to quantify the RNA content a cytophotometer was used. Extinction mean values were obtained which indicated RNA concentrations per surface unit. The nuclear and cytoplasmic surface were calculated simultaneously and from the product of the mean extinction and the surface the RNA total content was calculated. Our results have suggested that the changes are age-related. From day 1 to day 21 the neuronal size and RNA content increase; this may somehow be involved with the differentiation process. Around post-natal day 21 neuronal maturation may begin, reaching its optimal phase around day 42, on which the RNA concentration per surface unit, surface neuronal content and RNA total content are stable.

Effects of enucleation on postnatal development of catecholamines and serotonin metabolism in the superior colliculus of the rat

We have measured the level of catecholamines and serotonin and their principal metabolites, and the activities of tyrosine hydroxylase, tryptophan hydroxylase and monoamine oxidase-A and -B in the rat superior colliculus during postnatal development after bilateral removal of the eyes. The visual deprivation has different effects on the catecholamines and serotonin metabolism. The major changes in both amines were at day 15. Tyrosine hydroxylase and tryptophan hydroxylase activities increased during postnatal development but this increase was higher in enucleated compared with controls. An increase of the MAO-B to MAO-A ratio during postnatal development was found. The significance of these changes has been discussed.

Changes in monoamines and their metabolite levels in substantia nigra of aged rats

A decrease in the monoamine and monoamine metabolite contents in the substantia nigra of aged rats vs. controls has been found. Hence, it can be speculated that this decrease may play a role in the multiple alterations in dopaminergic functions observed in aged rats.

Age-related changes of catecholamines and their metabolites content in the visual system of the rat

The changes in the content of the catecholamines in each structure of the geniculate and extrageniculate visual system of the rat during the aging period (6-30 months) have been studied. Dopamine was found at lower levels than noradrenaline in all the structures. The dopamine and noradrenaline showed different developmental profiles. Dopamine and its metabolite levels decreased in the lateral geniculate and visual cortex and increased in superior colliculus and posterior thalamus. Noradrenaline and its metabolites increased in all structures during the aging period. However, 3-methoxy-4-hydroxyphenylglycol/noradrenaline and normetanephrine/noradrenaline ratios decreased in all structures except in superior colliculus. These results suggest age-related changes in the catecholamines in the visual system of the rat.