Νευροχημική και νευροβιολογική μελέτη της επίδρασης του εμπλουτισμένου περιβάλλοντος στη φυσιολογία της όρασης των επιμύων

Abstract

Environmental enrichment refers to the changes that happen in the centralnervous system of an animal which lives in bigger cages than typical labcages, with tunnels, running wheels and two floors. The purpose of this studywas to investigate the effects of environmental enrichment in the physiology ofthe visual system as well as in regions which contribute to the elaboration ofthe visual information. Sex differences were also taken into consideration. Ourgoal was achieved with the observation of developmental clues and the studyof behavioral and neurochemical parameters in male and female Wistar rats.The results of our study show that environmental enrichment accelerates theeye-opening procedure. Early enrichment decreases mobility duringprepuberty, without affecting it during adult life. On the contrary introducingenvironmental enrichment during adulthood increases mobility in both sexes.Early and late environmental enrichment also increases attention of maleadult animals. At the same time increased environmental stimuli give theability to male and female rats to identify more easily a newly presentedobject. It seems that enriched environment affects visual processing mostly infemale animals.For the first time in the present study we identified the involvement ofhistamine in the development and adaptation of the visual system. Histaminelevels in the optic chiasm but not the visual cortex are decreased in bothsexes through lifespan development. Histamine levels are also double in theoptic chiasm of males compared to females at all ages studied. Similar sexdifference was observed only during prepuberty in the visual cortex. Earlyenrichment decreased histamine levels in the optic chiasm of both sexesduring prepuberty, maintaining the sex difference observed in the basal levels.Histamine levels were comparable from prepuberty to adulthood upon rearing in richer environment. In contrast, introducing the enriched environment modelin adult animals increased histamine levels in the optic chiasm of female rats,indicating a sex difference in the adaptation of the central histaminergicsystem which interacts with the visual system.Neurochemical changes were also noted in the dopaminergic system of theretina and the visual cortex. Early enrichment does not affect dopaminergicactivity of the retina during development. On the contrary it inducedneurochemical changes in the adult female retina underlying adaptation inhigh illumination conditions, whereas late enrichment induced the sameneurochemical alterations in the retina of both male and female rats.Moreover the present study provides evidence that dopamine in not involvedin the development of the visual cortex, but is implicated in the function of thevisual cortex as well as visual processing.Enriched environment also ineracts with the serotonergic system of the retina.Serotonergic activity is increased by enhanced environmental stimuli possiblyleading to increased endogenous neuroprotection of the photosensitive layerof the eye. The present study also clarifies the interplay of plasticity andserotonergic activity in the visual cortex. Serotonergic activity is decreasedfrom prepuberty to adulthood. It seems that there is a range of serotonergicactivity within which it promotes plasticity. If serotonergic activity is increasedor decreased out of this range then plasticity is inhibited. It is also importantthat apart from the increase in neurotrophins levels observed in the visualcortex after environmental enrichment, plasticity is also facilitated throughserotonergic activity.Another important finding is that enriched environment alters neurochemistryin the prefrontal cortex which is important for stress response. For the firsttime in the present study we provide evidence that enriched environment(either from birth or during adulthood), leads to sex dependent activation ofstress mechanisms. The changes observed in the serotonergic activity of theprefrontal cortex are also sex dependent. It is possible that early environmental enrichment reduces the vulnerability to depression in adultfemale rats through increased serotonergic activity in the prefrontal cortex.Serotonergic activity of the hippocampus is also influenced by environmentalstimuli. Environmental enrichment either from birth or during adulthood affectsserotonergic activity of the hippocampus only in female animals. Thesechanges are capable of either to increase vulnerability to depression orattribute therapeutical advantages against this disease.In the present study the neurochemical profile of visual tissues in differentperiods of lifespan development and the neurochemical changesaccompanying the beneficial effects of environmental enrichment wereidentified. The results of the present raise the question of the role of histaminereceptors in visual perception and visual processing. The above resultsemphasize how crucial it is to map all the regions of the brain which interactwith the visual system and are involved in the function of vision. In additionthe present study shows how important it is to elucidate the role of enrichedenvironment in learning-memory and in the regulation of the sentiments.