Evidence for active synapse formation or altered postsynaptic metabolism in visual cortex of rats reared in complex environments.

Abstract

Animals placed in complex environments develop greater numbers of visual cortex synapses per neuron than animals housed in standard cages. Increased numbers of synapses could theoretically arise from (i) active formation of new synapses, or (ii) selective stabilization of constitutively produced synapses. The postsynaptic location of polyribosomal aggregates appears to be an indicator of newly forming synapses. In developmental synaptogenesis and adult reactive (to injury) synaptogenesis, polyribosomes are more frequently found at spine synapses and are more likely to appear in the spine head and stem. In the visual cortex of rats from complex environments, there was a greater frequency of spine synapses associated with polyribosomes, relative to rats from individual or group cages. Furthermore, a greater percentage of these spines had polyribosomes in the head and stem region. This suggests that synapses in this region may be actively induced by neural activity arising from the complex environment experience.The complex, or "enriched," laboratory environment, while quite different from the feral norm, has provided considerable evidence regarding possible forms of neuronal plasticity. Rats placed at weaning or in adulthood in these environments develop a thicker heavier occipital cortex with larger neuronal nuclei, more glial cells, and greater RNA content and enzyme activity per neuron, relative to rats housed in simple cages (1). Occipital cortex neurons have larger dendritic fields (2-6), greater density of postsynaptic spines on dendrites (7), and larger synaptic contacts in some layers (8), and synapses show additional morphological differences (9, 10) in the rats from a complex environment. Larger dendritic fields indicate more synapses, and this has been confirmed by electron microscopic analysis of the number of synapses per neuron (11)(12)(13) (26,27), connections may be overproduced, such that only a subset, activated by or appropriate to experience, is preserved. In adults, overproduction might take the form of constitutive (i.e., continuous, without regulation) generation of potentially transient synapses, selective retention of which depends on their appropriateness to ongoing neural activity (26-29). Second, synapses might be actively induced by experience-related neural activity. In this case, synaptogenesis would be local and temporally discrete, rather than widespread and continuous, although again only a subset of synapses produced in response to an experience might survive to encode it (30).These two hypothetical mechanisms predict different immediate consequences of a to-be-encoded event on synaptogenesis. The overproduction or constitutive generation model implies that exposure to such an event should not affect the rate of synapse production, but only the rate of synapse survival, whereas the induction model predicts that synaptogenesis will increase over baseline levels in response to novel experiences. Recently, a possible morphological indicant of newly forming synapses, the pres...

Activity-induced changes in synaptic release sites at the crayfish neuromuscular junction

“…For example, changes in numbers of synapses following conditioning (Artyukhina and Ryabinina, 1978;Wenzel et al, 1980) or in learning situations (Greenough et al, 1985;Black et al, 1990) have been reported in the mammalian brain. Perforated synapses (Jones and Calverley,199 1) have been correlated with functional changes linked to memory (Greenough et al, 1978;Geinisman et al, 1986) and kindling (Geinisman et al, 1989(Geinisman et al, , 1992Gomez et al, 1990).…”

Enhancement of auditory fear conditioning after housing in a complex environment is attenuated by prior treatment with amphetamine

“…The first evidence for structural changes in the brain as a result of living in a complex environment was provided by Rosenzweig and colleagues in the 1960s, who reported that animals living in a complex environment showed increased cortical weight compared with animals in standard laboratory cages (Rosenzweig et al 1962). Since that time, there have been many studies documenting a number of brain changes in rats living in a complex environment, including increases in cerebral volume, dendritic spine density and number, long-term potentiation (LTP), resistance to neural insult, and synaptic connectivity Greenough et al 1985Greenough et al , 1986Green and Greenough 1986; Kolb and Gibb 1991). Although these changes have been found in the cerebral cortex and other brain regions, the focus in many of these studies has been on the hippocampus.…”

Changes in Structural and Functional Plasticity of the Brain Induced by Environmental Enrichment

“…Таким об-разом, факторы ОС, вероятнее всего, избирательно дей-ствуют на разные компоненты программы нейрогенеза, а также дополнительно дифференцированно стимулиру-ют глио-, синапто-и ангиогенез, как это было предло-жено W. Greenough и соавт. [33,34]. Эти изменения впол-не ожидаемы, поскольку все события, инициируемые действием факторов ОС, как правило, ассоциированы с усилением нейрогенеза (обучение, запоминание, рас-познавание нового, высокая двигательная активность) и функциональной активности клеток в отдельных реги-онах мозга, что, в свою очередь, сопряжено с модуляцией энергетического метаболизма, нейрон-астроглиального метаболического сопряжения и локального кровотока.…”

“…Эти изменения впол-не ожидаемы, поскольку все события, инициируемые действием факторов ОС, как правило, ассоциированы с усилением нейрогенеза (обучение, запоминание, рас-познавание нового, высокая двигательная активность) и функциональной активности клеток в отдельных реги-онах мозга, что, в свою очередь, сопряжено с модуляцией энергетического метаболизма, нейрон-астроглиального метаболического сопряжения и локального кровотока. Ранние эксперименты показали, что обогащение увеличивает ветвление нейритов и формирование си-напсов в коре [12,33], и у животных, содержащихся в условиях ОС, регистрируется более высокий порядок дендритных ветвлений, чем в контрольной группе [33]. В последующих исследованиях установили, что обогаще-ние среды уменьшает плотность нейронов, но увеличи-вает соотношение синапс/нейрон, диаметр синаптиче-ских дисков и отверстия субсинаптических пластин [33].…”