Orientation selectivity is reduced by monocular deprivation in combination with PKA inhibitors

GABAB receptor-dependent bidirectional regulation of critical period ocular dominance plasticity in cats

Gama amino butyric acid (GABA) inhibition plays an important role in the onset and offset of the critical period for ocular dominance (OD) plasticity in the primary visual cortex. Previous studies have focused on the involvement of GABA_A receptors, while the potential contribution of GABA_B receptors to OD plasticity has been neglected. In this study, the GABA_B receptor antagonist SCH50911 or agonist baclofen was infused into the primary visual cortex of cats concurrently with a period of monocular deprivation (MD). Using single-unit recordings we found that the OD shift induced by four days of MD during the critical period was impaired by infusion of the antagonist SCH50911, but enhanced by infusion of the agonist baclofen. In contrast, seven days of MD in adult cats did not induce any significant OD shift, even when combined with the infusion of SCH50911 or baclofen. Together, these findings indicate that an endogenous GABA_B receptor-mediated inhibition contributes to juvenile, but not adult, OD plasticity.

Experience-dependent regulation of functional maps and synaptic protein expression in the cat visual cortex

Although the basis of our knowledge of experience-dependent plasticity comes from studies on carnivores and primates, studies examining the physiological and molecular mechanisms that underlie development and plasticity have increasingly employed mice. We have used several common rearing paradigms, such as dark-rearing and monocular deprivation (MD), to examine the timing of the physiological and molecular changes to altered experience in the cat primary visual cortex. Dark-rearing from birth or for 1 week starting at 4 weeks of age produced a similar reduction in the amplitude of responses measured through intrinsic signal imaging and a reduction in orientation selectivity. One week of visual experience following dark-rearing until 4 weeks of age yielded normal responses in both amplitude and orientation selectivity. The depression of deprived-eye responses was similar in magnitude after 2 and 7 days of MD. In contrast, non-deprived-eye responses almost doubled in magnitude after 7 days compared with 2 days of MD. These changes in the functional properties of primary visual cortex neurons were mirrored by specific changes in synaptic protein expression. Changes in proteins such as the NR2A and NR2B subunits of the N-methyl-D-aspartate receptor, postsynaptic density protein 95, alpha-CA(2+) /calmodulin-dependent protein kinase II (αCaMKII), and GABA(A) α1a indicated that the levels of sensory activity regulated mechanisms associated with both excitatory (NR2A and NR2B) and inhibitory (GABA(A) α1a) transmission so as to maintain response homeostasis. Additionally, we found that MD regulated the AMPA receptor glutamate (GluR1) subunit as well as signalling molecules (αCaMKII and synaptic Ras GTPase activating protein, SynGAP) downstream of N-methyl-D-aspartate receptors. Proteins in a common signalling pathway appeared to have similar developmental expression profiles that were broadly similar between cats and rodents.

Mechanisms of sleep-dependent consolidation of cortical plasticity

Sleep is thought to consolidate changes in synaptic strength, but the underlying mechanisms are unknown. We investigated the cellular events involved in this process during ocular dominance plasticity (ODP)-a canonical form of in vivo cortical plasticity triggered by monocular deprivation (MD) and consolidated by sleep via undetermined, activity-dependent mechanisms. We find that sleep consolidates ODP primarily by strengthening cortical responses to nondeprived eye stimulation. Consolidation is inhibited by reversible, intracortical antagonism of NMDA receptors (NMDARs) or cAMP-dependent protein kinase (PKA) during post-MD sleep. Consolidation is also associated with sleep-dependent increases in the activity of remodeling neurons and in the phosphorylation of proteins required for potentiation of glutamatergic synapses. These findings demonstrate that synaptic strengthening via NMDAR and PKA activity is a key step in sleep-dependent consolidation of ODP.

Barrel map development relies on protein kinase A regulatory subunit II beta-mediated cAMP signaling

The cellular and molecular mechanisms mediating the activity-dependent development of brain circuitry are still incompletely understood. Here, we examine the role of cAMP-dependent protein kinase [protein kinase A (PKA)] signaling in cortical development and plasticity, focusing on its role in thalamocortical synapse and barrel map development. We provide direct evidence that PKA activity mediates barrel map formation using knock-out mice that lack type IIbeta regulatory subunits of PKA (PKARIIbeta). We show that PKARIIbeta-mediated PKA function is required for proper dendritogenesis and the organization of cortical layer IV neurons into barrels, but not for the development and plasticity of thalamocortical afferent clustering into a barrel pattern. We localize PKARIIbeta function to postsynaptic processes in barrel cortex and show that postsynaptic PKA targets, but not presynaptic PKA targets, have decreased phosphorylation in pkar2b knock-out (PKARIIbeta(-/-)) mice. We also show that long-term potentiation at TC synapses and the associated developmental increase in AMPA receptor function at these synapses, which normally occurs as barrels form, is absent in PKARIIbeta(-/-) mice. Together, these experiments support an activity-dependent model for barrel map development in which the selective addition and elimination of thalamocortical synapses based on Hebbian mechanisms for synapse formation is mediated by a cAMP/PKA-dependent pathway that relies on PKARIIbeta function.

Blockade of cyclic AMP-dependent protein kinase does not prevent the reverse ocular dominance shift in kitten visual cortex

Monocular deprivation (MD) during the critical period for the development of visual cortex causes a loss of binocular response of neurons and a shift to the open eye, a normal ocular dominance (OD) shift. However, when MD is combined with chronic inactivation of the visual cortex by muscimol, the OD distribution of the neurons shifts to the deprived eye (reverse OD shift). We have previously shown that the normal OD shift is abolished by chronic infusion of the protein kinase A (PKA) inhibitor, 8-chloroadenosine-3', 5'-cyclic monophosphorothioate, Rp-isomer (Rp-8-Cl-cAMPS), into kitten visual cortex. In this study, we investigated the effect of this inhibitor on the reverse OD shift. Combination of MD and muscimol infusion into the visual cortex of 6-wk-old kittens caused a reverse OD shift that was comparable to that seen in previous studies. However, a reverse OD shift was also seen with concurrent infusion of the PKA inhibitor with muscimol. The strongest OD shift was observed in layer IV regardless of the presence or absence of the PKA inhibitor. This suggests that the dissociation of pre- and postsynaptic activities, which occurs mainly at thalamocortical synapses, induces the reverse OD shift and that inhibition of PKA does not prevent it. Presumably, an inhibition of PKA has no effect in silent cortex. We conclude that 1) an activation of PKA is not required for the induction of the reverse OD shift, and 2) the intracellular signaling mechanism underlying MD-induced OD plasticity differs between normal and reverse OD shifts.

Molecular basis of plasticity in the visual cortex

Sensory experience is known to shape the maturation of cortical circuits during development. A paradigmatic example is the effect of monocular deprivation on ocular dominance of visual cortical neurons. Although visual cortical plasticity has been widely studied since its initial discovery by Hubel and Wiesel >40 years ago, the description of the underlying molecular mechanisms has lagged behind. Several new findings are now beginning to close this gap. Recent data deepen our knowledge of the factors involved in the intercellular communication and intracellular signaling that mediate experience-dependent plasticity in the developing visual cortex. In addition, new findings suggest a role for the extracellular matrix in inhibition of ocular-dominance plasticity in the adult visual cortex.