. Our analysis revealed that LTP and LTD in layer IV principal cells is lost shortly after the eyes open, but persists in layers II/III beyond puberty. These results suggest that plasticity proceeds sequentially through cortical layers in a manner that parallels the flow of information during sensory processing.

Mutations in the postsynaptic scaffolding gene SHANK2 have recently been identified in individuals with autism spectrum disorder (ASD) and intellectual disability. However, the cellular and physiological consequences of these mutations in neurons remain unknown. We have analyzed the functional impact caused by two inherited and one de novo SHANK2 mutations from ASD individuals (L1008_P1009dup, T1127M, R462X). Although all three variants affect spine volume and have smaller SHANK2 cluster sizes, T1127M additionally fails to rescue spine volume in Shank2 knock-down neurons. R462X is not able to rescue spine volume and dendritic branching and lacks postsynaptic clustering, indicating the most severe dysfunction. To demonstrate that R462X when expressed in mouse can be linked to physiological effects, we analyzed synaptic transmission and behavior. Principal neurons of mice expressing rAAV-transduced SHANK2-R462X present a specific, long-lasting reduction in miniature postsynaptic AMPA receptor currents. This dominant negative effect translates into dose-dependent altered cognitive behavior of SHANK2-R462X-expressing mice, with an impact on the penetrance of ASD.

SUMMARY Neuromodulatory input, acting on G-protein coupled receptors, is essential for the induction of experience-dependent cortical plasticity. Here we report that G-coupled receptors in layer II/III of visual cortex control the polarity of synaptic plasticity through a pull-push regulation of LTP and LTD. In slices, receptors coupled to Gs promote LTP while suppressing LTD; conversely, receptors coupled to Gq11 promote LTD and suppress LTP. In vivo, the selective stimulation of Gs- or Gq11-coupled receptors brings the cortex into LTP-only or LTD-only states, which allows the potentiation or depression of targeted synapses with visual stimulation. The pull-push regulation of LTP/LTD occurs via direct control of the synaptic plasticity machinery and it is independent of changes in NMDAR activation or neuronal excitability. We propose these simple rules governing the pull-push control of LTP/LTD form a general metaplasticity mechanism that may contribute to neuromodulation of plasticity in other cortical circuits.

The primary motor cortex has an important role in the precise execution of learned motor responses. During motor learning, synaptic efficacy between sensory and primary motor cortical neurons is enhanced, possibly involving long-term potentiation and N-methyl-D-aspartate (NMDA)-specific glutamate receptor function. To investigate whether NMDA receptor in the primary motor cortex can act as a coincidence detector for activity-dependent changes in synaptic strength and associative learning, here we generate mice with deletion of the Grin1 gene, encoding the essential NMDA receptor subunit 1 (GluN1), specifically in the primary motor cortex. The loss of NMDA receptor function impairs primary motor cortex long-term potentiation in vivo. Importantly, it impairs the synaptic efficacy between the primary somatosensory and primary motor cortices and significantly reduces classically conditioned eyeblink responses. Furthermore, compared with wild-type littermates, mice lacking primary motor cortex show slower learning in Skinner-box tasks. Thus, primary motor cortex NMDA receptors are necessary for activity-dependent synaptic strengthening and associative learning.

Norepinephrine (NE) is synthesized in the Locus Coeruleus (LC) of the brainstem, from where it is released by axonal varicosities throughout the brain via volume transmission. A wealth of data from clinics and from animal models indicates that this catecholamine coordinates the activity of the central nervous system (CNS) and of the whole organism by modulating cell function in a vast number of brain areas in a coordinated manner. The ubiquity of NE receptors, the daunting number of cerebral areas regulated by the catecholamine, as well as the variety of cellular effects and of their timescales have contributed so far to defeat the attempts to integrate central adrenergic function into a unitary and coherent framework. Since three main families of NE receptors are represented—in order of decreasing affinity for the catecholamine—by: α2 adrenoceptors (α2Rs, high affinity), α1 adrenoceptors (α1Rs, intermediate affinity), and β adrenoceptors (βRs, low affinity), on a pharmacological basis, and on the ground of recent studies on cellular and systemic central noradrenergic effects, we propose that an increase in LC tonic activity promotes the emergence of four global states covering the whole spectrum of brain activation: (1) sleep: virtual absence of NE, (2) quiet wake: activation of α2Rs, (3) active wake/physiological stress: activation of α2- and α1-Rs, (4) distress: activation of α2-, α1-, and β-Rs. We postulate that excess intensity and/or duration of states (3) and (4) may lead to maladaptive plasticity, causing—in turn—a variety of neuropsychiatric illnesses including depression, schizophrenic psychoses, anxiety disorders, and attention deficit. The interplay between tonic and phasic LC activity identified in the LC in relationship with behavioral response is of critical importance in defining the short- and long-term biological mechanisms associated with the basic states postulated for the CNS. While the model has the potential to explain a large number of experimental and clinical findings, a major challenge will be to adapt this hypothesis to integrate the role of other neurotransmitters released during stress in a centralized fashion, like serotonin, acetylcholine, and histamine, as well as those released in a non-centralized fashion, like purines and cytokines.