Abstract
Medium spiny neurons (MSNs) are the major striatal neuron and receive synaptic input from both glutamatergic and dopaminergic afferents. These synapses are made on MSN dendritic spines, which undergo density and morphology changes in association with numerous disease and experience-dependent states. Despite wide interest in the structure and function of mature MSNs, relatively little is known about MSN development. Furthermore, most in vitro studies of MSN development have been done in simple striatal cultures that lack any type of non-autologous synaptic input, leaving open the question of how MSN development is affected by a complex environment that includes other types of neurons, glia, and accompanying secreted and cell-associated cues. Here we characterize the development of MSNs in striatal-cortical co-culture, including quantitative morphological analysis of dendritic arborization and spine development, describing progressive changes in density and morphology of developing spines. Overall, MSN growth is much more robust in the striatal-cortical co-culture compared to striatal mono-culture. Inclusion of dopamine (DA) in the co-culture further enhances MSN dendritic arborization and spine density, but the effects of DA on dendritic branching are only significant at later times in development. In contrast, exogenous Brain Derived Neurotrophic Factor (BDNF) has only a minimal effect on MSN development in the co-culture, but significantly enhances MSN dendritic arborization in striatal mono-culture. Importantly, inhibition of NMDA receptors in the co-culture significantly enhances the effect of exogenous BDNF, suggesting that the efficacy of BDNF depends on the cellular environment. Combined, these studies identify specific periods of MSN development that may be particularly sensitive to perturbation by external factors and demonstrate the importance of studying MSN development in a complex signaling environment.
Highlights
Medium spiny neurons (MSNs) are the primary neuron type of the striatum and are morphologically identified by their cell body size, dendritic arborization pattern and high density of dendritic spines (Kemp, 1968; Graveland and DiFiglia, 1985; Rafols et al, 1989; Matamales et al, 2009)
At 7 days in vitro (DIV) MSNs had an average of 4.7 ± 0.2 dendrites and the highest level of dendrite complexity was within ∼20 μm of the soma, with an average of ∼6.1 ± 0.45 crossings corresponding to the primary dendrites and 1–2 branches
We characterized the developmental changes in MSN dendritic arborization, spine density and spine morphology in striatalcortical culture
Summary
Medium spiny neurons (MSNs) are the primary neuron type of the striatum and are morphologically identified by their cell body size, dendritic arborization pattern and high density of dendritic spines (Kemp, 1968; Graveland and DiFiglia, 1985; Rafols et al, 1989; Matamales et al, 2009). Alterations in MSN dendritic arborization are the primary morphological abnormality observed with deletions of genes associated with Tourette’s, OCD and autism (Shmelkov et al, 2010; Bacon and Rappold, 2012). Dopaminergic afferents arise from the substantia nigra pars compacta (SNc) and are believed to reach the striatum around embryonic day 14 (E14) in the rat (roughly equivalent to E12 in mouse; Voorn et al, 1986, 1988). Developmental alterations in signaling by DA, glutamate, or BDNF have been shown to affect MSN dendritic arborization in vivo (Baydyuk et al, 2011; Beutler et al, 2011; Cazorla et al, 2014); elucidation of the mechanisms involved and the interactions among these signaling pathways has been difficult in vivo
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