Kevin Fox

Mutant mice and neuroscience: Recommendations concerning genetic background

The following scientists made significant contributions to the recommendations in this article:

Anatomical pathways and molecular mechanisms for plasticity in the barrel cortex

The barrel cortex has yielded a wealth of information about cortical plasticity in recent years. Barrel cortex is one of the few cortical areas studied so far where plasticity can be examined from birth through to adulthood. This review looks at plasticity mechanisms in three periods of life: early post-natal development, adolescence and adulthood. Separate consideration is given to depression and potentiation mechanisms. Plasticity can be induced in barrel cortex by whisker deprivation. Single whisker experience leads to expansion of the area of cortex responding to the spared whisker. In early post-natal life, plasticity occurs in thalamocortical pathways, while later in adolescence, intracortical pathways become more important. Ablation of the spared whiskers barrel prevents expression of plasticity in the cortex. A row of lesions between the spared and an adjacent barrel prevents expression of plasticity in the adjacent barrel. This evidence, together with latency of response data and an analysis of pathways capable of inducing long-term potentiation (LTP) within barrel cortex, leads to the view that horizontal and/or diagonal pathways between barrels are responsible for plasticity expression. The mouse has become the most commonly mutated mammalian species and has a well-developed barrel cortex. Therefore, mutations can be used to study the role of particular molecules in experience-dependent plasticity of barrel cortex. Through this work, it has become clear that the major post-synaptic density protein, alpha-CaMKII, and its T286 autophosphorylation site are essential for experience-dependent plasticity. This points to a major role for excitatory transmission in cortical plasticity and raises the possibility that LTP like mechanisms are involved. Furthermore, transgenic mice carrying a reporter gene for CRE have provided evidence that CRE-mediated gene expression is also involved in barrel cortex plasticity. This view is supported by studies on alpha/delta CREB knockouts, and provides a starting point for studying the role of gene expression in experience-dependent cortical plasticity.

A Comparison of Experience-Dependent Plasticity in the Visual and Somatosensory Systems

In the visual and somatosensory systems, maturation of neuronal circuits continues for days to weeks after sensory stimulation occurs. Deprivation of sensory input at various stages of development can induce physiological, and often structural, changes that modify the circuitry of these sensory systems. Recent studies also reveal a surprising degree of plasticity in the mature visual and somatosensory pathways. Here, we compare and contrast the effects of sensory experience on the connectivity and function of these pathways and discuss what is known to date concerning the structural, physiological, and molecular mechanisms underlying their plasticity.

Reversing Neurodevelopmental Disorders in Adults

Abnormalities in brain development, thought to be irreversible in adults, have long been assumed to underlie the neurological and psychiatric symptoms associated with neurodevelopmental disorders. Surprisingly, a number of recent animal model studies of neurodevelopmental disorders demonstrate that reversing the underlying molecular deficits can result in substantial improvements in function even if treatments are started in adulthood. These findings mark a paradigmatic change in the way we understand and envision treating neurodevelopmental disorders.

Requirement for α-CaMKII in Experience-Dependent Plasticity of the Barrel Cortex

The mammalian sensory neocortex exhibits experience-dependent plasticity such that neurons modify their response properties according to changes in sensory experience. The synaptic plasticity mechanism of long-term potentiation requiring calcium-calmodulin-dependent kinase type II (CaMKII) could underlie experience-dependent plasticity. Plasticity in adult mice can be induced by changes in the patterns of tactile input to the barrel cortex. This response is strongly depressed in adult mice that lack the gene encoding α-CaMKII, although adolescent animals are unaffected. Thus, α-CaMKII is necessary either for the induction or for the expression of plasticity in adult mice.

The role of alpha-CaMKII autophosphorylation in neocortical experience-dependent plasticity

Calcium/calmodulin kinase type II (CaMKII) is a major postsynaptic density protein. CaMKII is postulated to act as a ‘molecular switch’, which, when triggered by a transient rise in calcium influx, becomes active for prolonged periods because of its ability to autophosphorylate. We studied experience-dependent plasticity in the barrel cortex of mice carrying a point mutation of the α-CaMKII gene (T286A), which abolishes this enzymes ability to autophosphorylate. Plasticity was prevented in adult and adolescent mice homozygous for the mutation, but was normal in heterozygotes and wild-type littermates. These results provide evidence that the molecular switch hypothesis is valid for neocortical experience-dependent plasticity.

Structural Plasticity Underlies Experience-Dependent Functional Plasticity of Cortical Circuits

The stabilization of new spines in the barrel cortex is enhanced after whisker trimming, but its relationship to experience-dependent plasticity is unclear. Here we show that in wild-type mice, whisker potentiation and spine stabilization are most pronounced for layer 5 neurons at the border between spared and deprived barrel columns. In homozygote αCaMKII-T286A mice, which lack experience-dependent potentiation of responses to spared whiskers, there is no increase in new spine stabilization at the border between barrel columns after whisker trimming. Our data provide a causal link between new spine synapses and plasticity of adult cortical circuits and suggest that αCaMKII autophosphorylation plays a role in the stabilization but not formation of new spines.

The critical period for long-term potentiation in primary sensory cortex

Long-term potentiation (LTP) is a synaptic process frequently proposed as a candidate mechanism underlying memory (Bliss and Collingridge, 1993). Under the correct conditions (which in almost all cases means during coincident activation of preand postsynaptic neurons), within a matter of seconds, the gain of synaptic transmission will increase and remain elevated for a period of hours to weeks. The rapidity of induction and longevity of expression characteristic of LTP coincide with the two prime temporal characteristics required of a memory mechanism. More direct support for the theory that LTP is a learning mechanism has come from experimental evidence that could link spatial memory to LTP (Davies et al., 1992). However, the problem that haunts LTP research (and indeed any other paradigm involving artificial stimulation) is that, while it demonstrates that synapses can behave in a given way, it does not demonstrate that they naturally do behave in that way. In particular, electrical stimulation is likely to produce a pattern of synaptic activation that natural stimuli would never produce: electrical stimuli produce more synchronous activation of inputs than natural stimuli, and inappropriate combinations of inputs are probably recruited, including the likelihood of activating fibers antidromically. In addition, inhibitory postsynaptic potentials appear to be activated powerfully by electrical stimuli, whereas the successful activation of a cell by a natural stimulus is partly dependent on its avoidance of inhibitory postsynpatic potentials. Despite these problems, several workers have sought evidence linking LTP to naturally occurring plasticity. In two recently published studies, correlative evidence has been found linking LTP to critical period plasticity in the primary somatosensory cortex (Crair and Malenka, 1995) and the primary visual cortex (Kirkwood et al., 1995). Cr i t ica l Per iod Plasticity Plasticity can be induced in sensory neocortex by simple manipulations of the natural sensory input. In the case of barrel cortex, plasticity can be induced by vibrissae deprivation, while monocular deprivation has a similar effect in the visual cortex. Both somatosensory and visual cortices show critical periods for the induction of plasticity early in postnatal life (Woolsey and Wan, 1976; Olsen and Freeman, 1980). In the case of the visual cortex, the critical period can be delayed by dark-rearing (Cynader and Mitchell, 1980). The new evidence presented by Crair and Malenka and Kirkwood et al. indicates that LTP also exhibits a critical period at particular synaptic locations within the cortex and therefore shares a common attribute with experience-dependent forms of plasticity. In approximate terms, the critical periods for LTP are similar, although not identical, to those for manipulations of the natural sensory input (see Figure 1A). Minireview

The role of nitric oxide and GluR1 in presynaptic and postsynaptic components of neocortical potentiation.

In this study, we investigated the mechanisms underlying synaptic plasticity at the layer IV to II/III pathway in barrel cortex of mice aged 6–13 weeks. This pathway is one of the likely candidates for expression of experience-dependent plasticity in the barrel cortex and may serve as a model for other IV to II/III synapses in the neocortex. We found that postsynaptic autocamtide-2-inhibitory peptide is sufficient to block long-term potentiation (LTP) (IC50 of 500 nm), implicating postsynaptic calcium/calmodulin-dependent kinase II in LTP induction. AMPA receptor subunit 1 (GluR1) knock-out mice also showed LTP in this pathway, but potentiation was predominantly presynaptic in origin as determined by paired-pulse analysis, coefficient of variation analysis, and quantal analysis, whereas wild types showed a mixed presynaptic and postsynaptic locus. Quantal analysis at this synapse was validated by measuring uniquantal events in the presence of strontium. The predominantly presynaptic LTP in the GluR1 knock-outs was blocked by postsynaptic antagonism of nitric oxide synthase (NOS), either with intracellular N-ω-nitro-l-arginine methyl ester or N-nitro-l-arginine, providing the first evidence for a retrograde transmitter role for NO at this synapse. Antagonism of NOS in wild types significantly reduced but did not eliminate LTP (group average reduction of 50%). The residual LTP formed a variable proportion of the total LTP in each cell and was found to be postsynaptic in origin. We found no evidence for silent synapses in this pathway at this age. Finally, application of NO via a donor induced potentiation in layer II/III cells and caused an increase in frequency but not amplitude of miniature EPSPs, again implicating NO in presynaptic plasticity.

The role of nitric oxide in pre-synaptic plasticity and homeostasis

Since the observation that nitric oxide (NO) can act as an intercellular messenger in the brain, the past 25 years have witnessed the steady accumulation of evidence that it acts pre-synaptically at both glutamatergic and GABAergic synapses to alter release-probability in synaptic plasticity. NO does so by acting on the synaptic machinery involved in transmitter release and, in a coordinated fashion, on vesicular recycling mechanisms. In this review, we examine the body of evidence for NO acting as a retrograde factor at synapses, and the evidence from in vivo and in vitro studies that specifically establish NOS1 (neuronal nitric oxide synthase) as the important isoform of NO synthase in this process. The NOS1 isoform is found at two very different locations and at two different spatial scales both in the cortex and hippocampus. On the one hand it is located diffusely in the cytoplasm of a small population of GABAergic neurons and on the other hand the alpha isoform is located discretely at the post-synaptic density (PSD) in spines of pyramidal cells. The present evidence is that the number of NOS1 molecules that exist at the PSD are so low that a spine can only give rise to modest concentrations of NO and therefore only exert a very local action. The NO receptor guanylate cyclase is located both pre- and post-synaptically and this suggests a role for NO in the coordination of local pre- and post-synaptic function during plasticity at individual synapses. Recent evidence shows that NOS1 is also located post-synaptic to GABAergic synapses and plays a pre-synaptic role in GABAergic plasticity as well as glutamatergic plasticity. Studies on the function of NO in plasticity at the cellular level are corroborated by evidence that NO is also involved in experience-dependent plasticity in the cerebral cortex.