Differential regulation of synapsin phosphorylation by monocular deprivation in juveniles and adults

Compensatory neuritogenesis of serotonergic afferents within the striatum of a transgenic rat model of Parkinson's disease

The majority of patients with Parkinson's disease (PD) suffer from L-DOPA-induced dyskinesia (LID). Besides a dysfunctional dopaminergic system, changes of the serotonergic network may be linked to this severe and adverse symptom. Particularly, serotonergic neurons have the potential to synthesize dopamine, likely associated with a disproportional dopamine release within the striatum. We hypothesized that the serotonergic system is adaptively altered in the striatum due to the reduced dopaminergic input. To answer this question, we analyzed a transgenic rat PD model ubiquitously expressing human α-synuclein using a bacterial artificial chromosome. Neurite analysis showed a profound loss of dopaminergic fibers by ~30-40% within the dorsal striatum paralleled by a ~50% reduction of dopaminergic neurons in the substantia nigra pars compacta. In contrast, serotonergic fibers showed an increased fiber density in the dorsal striatum by ~100%, while the number of serotonergic neurons within the raphe nuclei (RN) and its proximal neuritic processes were unaffected. Furthermore, both the dopaminergic and serotonergic fiber density remained unchanged in the neighboring motor cortex M1/M2. Interestingly, essential enzymes required for L-DOPA turnover and dopamine release were expressed in serotonergic neurons of the RN. In parallel, the serotonergic autoreceptor levels involved in a serotonergic negative feedback loop were reduced within the striatum, suggesting a dysfunctional neurotransmitter release. Overall, the increased serotonergic fiber density with its capacity for dopamine release within the striatum suggests a compensatory, site-specific serotonergic neuritogenesis. This maladaptive serotonergic plasticity may be linked to adverse symptoms such as LIDs in PD.

Neuregulin-Dependent Regulation of Fast-Spiking Interneuron Excitability Controls the Timing of the Critical Period

Maturation of excitatory drive onto fast-spiking interneurons (FS INs) in the visual cortex has been implicated in the control of the timing of the critical period for ocular dominance plasticity. However, the mechanisms that regulate the strength of these synapses over cortical development are not understood. Here we use a mouse model to show that neuregulin (NRG) and the receptor tyrosine kinase erbB4 regulate the timing of the critical period. NRG1 enhanced the strength of excitatory synapses onto FS INs, which inhibited ocular dominance plasticity during the critical period but rescued plasticity in transgenics with hypoexcitable FS INs. Blocking the effects of endogenous neuregulin via inhibition of erbBs rescued ocular dominance plasticity in postcritical period adults, allowing recovery from amblyopia induced by chronic monocular deprivation. Thus, the strength of excitation onto FS INs is a key determinant of critical period plasticity and is maintained at high levels by NRG-erbB4 signaling to constrain plasticity in adulthood.

SIGNIFICANCE STATEMENT

Despite decades of experimentation, the mechanisms by which critical periods of enhanced synaptic plasticity are initiated and terminated are not completely understood. Here we show that neuregulin (NRG) and the receptor tyrosine kinase erbB4 determine critical period timing by controlling the strength of excitatory synapses onto FS INs. NRG1 enhanced excitatory drive onto fast spiking interneurons, which inhibited ocular dominance plasticity in juveniles but rescued plasticity in transgenics with hypoexcitable FS INs. Blocking the effects of endogenous neuregulin via inhibition of erbBs rescued ocular dominance plasticity in adults, allowing recovery from amblyopia induced by chronic monocular deprivation. Thus, in contrast to prevailing views of the termination of the critical period, active maintenance of strong excitation onto FS INs constrains plasticity in adults.

Monocular deprivation delays the dynamic changes of phosphorylated synapsin Ia/b at site-1 in contralateral visual cortex of juvenile mice

Synapsins as a family of presynaptic terminal phosphoprotein participates in neuronal development, but their role in the synaptic plasticity of visual cortex is unclear. In this study, the impact of monocular deprivation (MD) on dynamic changes of isoform-specific protein expression and site 1 phosphorylation of synapsins in visual cortex of the postnatal mice were observed by using the technique of Western blot analysis. The results showed that the total (T-) protein levels of synapsins including the isoform of Ia/b, IIa/b and IIIa were about 21-26% of adult level in visual cortex of mice at postnatal 7 days (P7), and then the T-synapsin Ia/b and IIb could quickly reach adult level at P35. However, the T-synapsin IIa and IIIa increased more slowly (71-74% at P35), and then kept increasing in the visual cortex of mice at P60. Unlike to the changes of T-synapsins, the level of phosphorylated (P-) synapsin Ia/b (not IIa/b and IIIa) at site 1 increased with development to the highest level at P21, and then decreased rapidly to a low level in visual cortex of mice at P35-60. In addition, we found that the levels of P-synapsin Ia/b increased significantly in left visual cortex of P28 and P35 (not P21 and P42) mice with 1-week MD of right eye; and no significant changes of T-synapsins were observed in both left and right sides of visual cortex in P21-42 mice with MD treatment. These results suggested that the isoform-specific protein expression and site-1 phosphorylation of synapsins might play a different role in the synaptic plasticity of visual cortex, and MD delays the dynamic changes of phosphorylated synapsin Ia/b at site-1 in contralateral visual cortex of juvenile mice.