Environmental enrichment enhances directional selectivity of primary auditory cortical neurons in rats

Postnatal exposure to an acoustically enriched environment alters the morphology of neurons in the adult rat auditory system

The structure of neurons in the central auditory system is vulnerable to various kinds of acoustic exposures during the critical postnatal developmental period. Here we explored long-term effects of exposure to an acoustically enriched environment (AEE) during the third and fourth weeks of the postnatal period in rat pups. AEE consisted of a spectrally and temporally modulated sound of moderate intensity, reinforced by a behavioral paradigm. At the age of 3-6 months, a Golgi-Cox staining was used to evaluate the morphology of neurons in the inferior colliculus (IC), the medial geniculate body (MGB), and the auditory cortex (AC). Compared to controls, rats exposed to AEE showed an increased mean dendritic length and volume and the soma surface in the external cortex and the central nucleus of the IC. The spine density increased in both the ventral and dorsal divisions of the MGB. In the AC, the total length and volume of the basal dendritic segments of pyramidal neurons and the number and density of spines on these dendrites increased significantly. No differences were found on apical dendrites. We also found an elevated number of spines and spine density in non-pyramidal neurons. These results show that exposure to AEE during the critical developmental period can induce permanent changes in the structure of neurons in the central auditory system. These changes represent morphological correlates of the functional plasticity, such as an improvement in frequency tuning and synchronization with temporal parameters of acoustical stimuli.

Specific synaptopathies diversify brain responses and hearing disorders: you lose the gain from early life

Before hearing onset, inner hair cell (IHC) maturation proceeds under the influence of spontaneous Ca(2+) action potentials (APs). The temporal signature of the IHC Ca(2+) AP is modified through an efferent cholinergic feedback from the medial olivocochlear bundle (MOC) and drives the IHC pre- and post-synapse phenotype towards low spontaneous (spike) rate (SR), high-threshold characteristics. With sensory experience, the IHC pre- and post-synapse phenotype matures towards the instruction of low-SR, high-threshold and of high-SR, low-threshold auditory fiber characteristics. Corticosteroid feedback together with local brain-derived nerve growth factor (BDNF) and catecholaminergic neurotransmitters (dopamine) might be essential for this developmental step. In this review, we address the question of whether the control of low-SR and high-SR fiber characteristics is linked to various degrees of vulnerability of auditory fibers in the mature system. In particular, we examine several IHC synaptopathies in the context of various hearing disorders and exemplified shortfalls before and after hearing onset.

Cellular and molecular mechanisms of immunomodulation in the brain through environmental enrichment

Recent studies on environmental enrichment (EE) have shown cytokines, cellular immune components [e.g., T lymphocytes, natural killer (NK) cells], and glial cells in causal relationship to EE in bringing out changes to neurobiology and behavior. The purpose of this review is to evaluate these neuroimmune mechanisms associated with neurobiological and behavioral changes in response to different EE methods. We systematically reviewed common research databases. After applying all inclusion and exclusion criteria, 328 articles remained for this review. Physical exercise (PE), a form of EE, elicits anti-inflammatory and neuromodulatory effects through interaction with several immune pathways including interleukin (IL)-6 secretion from muscle fibers, reduced expression of Toll-like receptors on monocytes and macrophages, reduced secretion of adipokines, modulation of hippocampal T cells, priming of microglia, and upregulation of mitogen-activated protein kinase phosphatase-1 in central nervous system. In contrast, immunomodulatory roles of other enrichment methods are not studied extensively. Nonetheless, studies showing reduction in the expression of IL-1β and tumor necrosis factor-α in response to enrichment with novel objects and accessories suggest anti-inflammatory effects of novel environment. Likewise, social enrichment, though considered a necessity for healthy behavior, results in immunosuppression in socially defeated animals. This has been attributed to reduction in T lymphocytes, NK cells and IL-10 in subordinate animals. EE through sensory stimuli has been investigated to a lesser extent and the effect on immune factors has not been evaluated yet. Discovery of this multidimensional relationship between immune system, brain functioning, and EE has paved a way toward formulating environ-immuno therapies for treating psychiatric illnesses with minimal use of pharmacotherapy. While the immunomodulatory role of PE has been evaluated extensively, more research is required to investigate neuroimmune changes associated with other enrichment methods.

Environment and Brain Plasticity: Towards an Endogenous Pharmacotherapy

Brain plasticity refers to the remarkable property of cerebral neurons to change their structure and function in response to experience, a fundamental theoretical theme in the field of basic research and a major focus for neural rehabilitation following brain disease. While much of the early work on this topic was based on deprivation approaches relying on sensory experience reduction procedures, major advances have been recently obtained using the conceptually opposite paradigm of environmental enrichment, whereby an enhanced stimulation is provided at multiple cognitive, sensory, social, and motor levels. In this survey, we aim to review past and recent work concerning the influence exerted by the environment on brain plasticity processes, with special emphasis on the underlying cellular and molecular mechanisms and starting from experimental work on animal models to move to highly relevant work performed in humans. We will initiate introducing the concept of brain plasticity and describing classic paradigmatic examples to illustrate how changes at the level of neuronal properties can ultimately affect and direct key perceptual and behavioral outputs. Then, we describe the remarkable effects elicited by early stressful conditions, maternal care, and preweaning enrichment on central nervous system development, with a separate section focusing on neurodevelopmental disorders. A specific section is dedicated to the striking ability of environmental enrichment and physical exercise to empower adult brain plasticity. Finally, we analyze in the last section the ever-increasing available knowledge on the effects elicited by enriched living conditions on physiological and pathological aging brain processes.

The function of BDNF in the adult auditory system

The inner ear of vertebrates is specialized to perceive sound, gravity and movements. Each of the specialized sensory organs within the cochlea (sound) and vestibular system (gravity, head movements) transmits information to specific areas of the brain. During development, brain-derived neurotrophic factor (BDNF) orchestrates the survival and outgrowth of afferent fibers connecting the vestibular organ and those regions in the cochlea that map information for low frequency sound to central auditory nuclei and higher-auditory centers. The role of BDNF in the mature inner ear is less understood. This is mainly due to the fact that constitutive BDNF mutant mice are postnatally lethal. Only in the last few years has the improved technology of performing conditional cell specific deletion of BDNF in vivo allowed the study of the function of BDNF in the mature developed organ. This review provides an overview of the current knowledge of the expression pattern and function of BDNF in the peripheral and central auditory system from just prior to the first auditory experience onwards. A special focus will be put on the differential mechanisms in which BDNF drives refinement of auditory circuitries during the onset of sensory experience and in the adult brain. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Auditory discrimination training rescues developmentally degraded directional selectivity and restores mature expression of GABA(A) and AMPA receptor subunits in rat auditory cortex

Auditory frequency discrimination training can remediate deteriorated frequency representations and temporal information processing in the adult primary auditory cortex induced by early post-natal pulsed noise exposure. In this study, we investigated the neural mechanisms underlying the restoration of directional selectivity by auditory spatial discrimination training. Rats exposed to pulsed noise during a post-natal critical period demonstrated reduced auditory directional selectivity but could be successfully trained to identify a target sound stimulus at a specific azimuth angle using a reward-contingent auditory discrimination task (EXP rats). In contrast, rats passively exposed to the training procedure but no reward for correct identification of the azimuth angle (PNR rats) showed no improvement and behavioral performance remained significantly below EXP rats and control (CON) rats reared under a normal sonic environment. The expression levels of GABA(A) receptor subunits α1, α3, β2, and β3, and the AMPA GluR2 subunit were significantly altered in the auditory cortex of untrained noise-raised (NR and PNR) rats compared to age-matched CON rats, while trained noise-raised (EXP) rats exhibited levels of expression not significantly different from CON rats. Thus, reward-contingent sound-azimuth discrimination training may remediate directional selectivity by restoring the proper expression profile of neurotransmitter receptor subunits in the auditory cortex, allowing for normal spatial selectivity by cortical neurons. The development of auditory directional selectivity depends on the regulated expression of these excitatory and inhibitory neurotransmitter receptor subunits; early pulsed noise may disrupt the normal development of directional selectivity by interfering with receptor expression.

Developmentally degraded directional selectivity of the auditory cortex can be restored by auditory discrimination training in adults

Sound localization is one of the most important tasks performed by the auditory system. Studies have shown that intensive training can remediate deteriorated frequency representations and temporal information processing in the adult primary auditory cortex (A1) induced by early post-natal pulsed noise exposure. Here we demonstrate that intensive sound location discrimination training improved the dysfunctional sound azimuth selectivity degraded by early post-natal pulsed noise exposure. Rats exposed to pulsed white noise during a post-natal critical period were successfully trained to identify a target sound stimulus with specific azimuth angle that changed daily on a random schedule. Consistent with recovery of behavioral accuracy for sound-azimuth discriminations, we found that the average angular range (AR) of A1 neuronal azimuth selective curves in trained noise-raised rats was not significantly different from that measured in control rats, while the average AR of A1 neurons in untrained noise-raised rats was significantly higher, indicating that these neurons were less direction selective. Directional selectivity of A1 neurons was normalized by training, thus demonstrating the benefits of sensory discrimination training as a strategy for restoring auditory function in adult mammals damaged by sensory disruption during critical periods of cortical development.

Experience-induced plasticity of thalamocortical axons in both juveniles and adults

We examined the effect of sensory deprivation on thalamocortical (TC) projections to the rat primary somatosensory cortex at different postnatal ages ranging from P0 to P96. Rats had their whiskers clipped off with one or two vibrissae spared. TC axons innervating barrel cortex were specifically labeled by injecting virus expressing fluorescent proteins into the corresponding primary (VPM) and/or secondary (POm) thalamic nuclei. The density of VPM axons in deprived columns was ≈34% lower relative to spared columns with a concomitant decrease in bouton density, suggesting a deprivation-induced retraction of VPM axons. Axonal changes were reversible upon regrowth of the clipped whiskers and independent of age at deprivation, indicating the absence of a critical period for anatomical plasticity. The POm projection was not obviously altered by sensory deprivation. We suggest that retraction and regrowth of TC axons substantially contribute to long-term deprivation-dependent functional plasticity.