Abstract
Nervous systems respond with structural changes to environmental changes even in adulthood. In recent years, experience-dependent structural plasticity was shown not to be restricted to the cerebral cortex, as it also occurs at subcortical and even peripheral levels. We have previously shown that two populations of trigeminal nuclei neurons, trigeminothalamic barrelette neurons of the principal nucleus (Pr5), and intersubnuclear neurons in the caudal division of the spinal trigeminal nucleus (Sp5C) that project to Pr5 underwent morphometric and topological changes in their dendritic trees after a prolonged total or partial loss of afferent input from the vibrissae. Here we examined whether and what structural alterations could be elicited in the dendritic trees of the same cell populations in young adult rats after being exposed for 2 months to an enriched environment (EE), and how these changes evolved when animals were returned to standard housing for an additional 2 months. Neurons were retrogradely labeled with BDA delivered to, respectively, the ventral posteromedial thalamic nucleus or Pr5. Fully labeled cells were digitally reconstructed with Neurolucida and analyzed with NeuroExplorer. EE gave rise to increases in dendritic length, number of trees and branching nodes, spatial expansion of the trees, and dendritic spines, which were less pronounced in Sp5C than in Pr5 and differed between sides. In Pr5, these parameters returned, but only partially, to control values after EE withdrawal. These results underscore a ubiquity of experience-dependent changes that should not be overlooked when interpreting neuroplasticity and developing plasticity-based therapeutic strategies.
Highlights
Sustained alterations in sensory experience induce structural changes in the brain during development and at early maturational stages, and throughout life
The animals were divided into three groups: Control group (C; n = 23), rats subjected to minimal handling for routine cleaning; enriched environment group (E, n = 16), rats daily exposed to an EE for 2 months; and Enrichment Withdrawal group (EW; n = 5), animals that after 2 months of EE were returned for an additional 2 months to standard housing
The location of BDA injections and the retrogradely labeled neurons in control animals were described in detail in our previous studies (Negredo et al 2009; Martin et al 2014), and the new cases added here were consistent with those descriptions (Tables 1, 2)
Summary
Sustained alterations in sensory experience induce structural changes in the brain during development and at early maturational stages, and throughout life. This property started to be explored soon after Hebb formulated. Such effects include gains in cortical weight, thickness, volume and acetylcholinesterase activity (Diamond et al 1976; Machín et al 2004; Rosenzweig et al 1962), increased dendritic branching and length (Greenough et al 1973; Hickmott and Ethell 2006; Uylings et al 1978; Volkmar and Greenough 1972), synaptogenesis and addition of new spines (Globus et al 1973; Johansson and Belichenko 2002; Jung and Herms 2014; Landers et al 2011), expansion in number and volume of small blood vessels (Black et al 1991; Ekstrand et al 2008; He et al 2017), increased volume and fiber length of subcortical white matter (Yang et al 2015), glial plasticity (Ehninger and Kempermann 2003; Keiner et al 2017; Sirevaag and Greenough 1991; Viola et al 2009), and lasting and dynamic changes in genes encoding a variety of transcription factors (Valles et al 2011)
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