Neuregulin-1/ErbB4 Signaling Regulates Visual Cortical Plasticity.

Experience alters cortical networks through neural plasticity mechanisms. During a developmental critical period, the most dramatic consequence of occluding vision through one eye (monocular deprivation) is a rapid loss of excitatory synaptic inputs to parvalbumin-expressing (PV) inhibitory neurons in visual cortex. Subsequent cortical disinhibition by reduced PV cell activity allows for excitatory ocular dominance plasticity. However, the molecular mechanisms underlying critical period synaptic plasticity are unclear. Here we show that brief monocular deprivation during the critical period downregulates neuregulin-1(NRG1)/ErbB4 signaling in PV neurons, causing retraction of excitatory inputs to PV neurons. Exogenous NRG1 rapidly restores excitatory inputs onto deprived PV cells through downstream PKC-dependent activation and AMPA receptor exocytosis, thus enhancing PV neuronal inhibition to excitatory neurons. NRG1 treatment prevents the loss of deprived eye visual cortical responsiveness in vivo. Our findings reveal molecular, cellular, and circuit mechanisms of NRG1/ErbB4 in regulating the initiation of critical period visual cortical plasticity.

Inhibitory Neuron Transplantation into Adult Visual Cortex Creates a New Critical Period that Rescues Impaired Vision.

The maturation of inhibitory circuits in juvenile visual cortex triggers a critical period in the development of the visual system. Although several manipulations of inhibition can alter the timing of the critical period, none have demonstrated the creation of a new critical period in adulthood. We developed a transplantation method to reactivate critical period plasticity in the adult visual cortex. Transplanted embryonic inhibitory neurons from the medial ganglionic eminence reinstate ocular dominance plasticity in adult recipients. Transplanted inhibitory cells develop cell-type-appropriate molecular characteristics and visually evoked responses. In adult mice impaired by deprivation during the juvenile critical period, transplantation also recovers both visual cortical responses and performance on a behavioral test of visual acuity. Plasticity and recovery are induced when the critical period would have occurred in the donor animal. These results reveal that the focal reactivation of visual cortical plasticity using inhibitory cell transplantation creates a new critical period that restores visual perception after childhood deprivation.

Permanent cerebral vessel occlusion via double ligature and transection.

Stroke is a leading cause of death, disability, and socioeconomic loss worldwide. The majority of all strokes result from an interruption in blood flow (ischemia). Middle cerebral artery (MCA) delivers a great majority of blood to the lateral surface of the cortex, is the most common site of human stroke, and ischemia within its territory can result in extensive dysfunction or death. Survivors of ischemic stroke often suffer loss or disruption of motor capabilities, sensory deficits, and infarct. In an effort to capture these key characteristics of stroke, and thereby develop effective treatment, a great deal of emphasis is placed upon animal models of ischemia in MCA. Here we present a method of permanently occluding a cortical surface blood vessel. We will present this method using an example of a relevant vessel occlusion that models the most common type, location, and outcome of human stroke, permanent middle cerebral artery occlusion (pMCAO). In this model, we surgically expose MCA in the adult rat and subsequently occlude via double ligature and transection of the vessel. This pMCAO blocks the proximal cortical branch of MCA, causing ischemia in all of MCA cortical territory, a large portion of the cortex. This method of occlusion can also be used to occlude more distal portions of cortical vessels in order to achieve more focal ischemia targeting a smaller region of cortex. The primary disadvantages of pMCAO are that the surgical procedure is somewhat invasive as a small craniotomy is required to access MCA, though this results in minimal tissue damage. The primary advantages of this model, however, are: the site of occlusion is well defined, the degree of blood flow reduction is consistent, functional and neurological impairment occurs rapidly, infarct size is consistent, and the high rate of survival allows for long-term chronic assessment.

A rat's whiskers point the way toward a novel stimulus-dependent, protective stroke therapy.

Stroke is the fourth leading cause of death in the United States and the leading cause of long-term disability. Ischemic stroke, due to an interruption in blood supply, is particularly prevalent; 87% of all strokes are ischemic. Unfortunately, current options for acute treatment are extremely limited and there is a great need for new treatment strategies. This review will discuss evidence that mild sensory stimulation can completely protect the jeopardized brain from an impending stroke in a rodent model. When delivered within the first 2 hours following ischemic onset, this stimulation results in complete protection, including a full reestablishment of cortical function, sensorimotor capabilities, and blood flow. Identical stimulation, however, initiated 3 hours following ischemic onset, results in an increase in damage compared with untreated animals. The protective effect is not specific to a single sensory modality, anesthesia, or age, and increasing evoked cortical activity by increasing stimulation accelerates recovery. Taken together, these findings demonstrate that cortical activity is a critical factor for protection and suggest a new, exciting potential avenue for the development of acute stroke treatment strategies that may produce a noninvasive, drug-free, equipment-free, and side effect-free means of protecting from ischemic stroke.

Sensory Stimulation-Based Complete Protection from Ischemic Stroke Remains Stable at 4 Months Post-Occlusion of MCA.

Previous research from our lab has shown that when using a rodent model of ischemic stroke (permanent middle cerebral artery occlusion), mild sensory stimulation, when delivered within two hours of ischemic onset, completely protects the cortex from impending ischemic stroke damage when assessed 24 hours post-occlusion. However, the long-term stability of this protection remains unclear. Using intrinsic signal optical imaging for assessment of cortical function, laser speckle imaging for assessment of blood flow, a battery of behavioral tests and cresyl violet for histological assessment, the present study examined whether this protection was long-lasting. When assessed 4 months post-occlusion (this length of time being equivalent to 10-15 years in humans), rats receiving sensory stimulation treatment immediately after ischemic onset exhibit normal neuronal and vascular function, and they are behaviorally and histologically equivalent to healthy controls (surgical shams). Thus, the complete neuroprotection due to cortical activation via sensory stimulation remains stable with time. These findings add support to the translational potential of this sensory stimulation-based treatment.

Mild sensory stimulation protects the aged rodent from cortical ischemic stroke after permanent middle cerebral artery occlusion.

BACKGROUND: Accumulated research has shown that the older adult brain is significantly more vulnerable to stroke than the young adult brain. Although recent evidence in young adult rats demonstrates that single-whisker stimulation can result in complete protection from ischemic damage after permanent middle cerebral artery occlusion (pMCAO), it remains unclear whether the same treatment would be effective in older animals. METHODS AND RESULTS: Aged rats (21 to 24 months of age) underwent pMCAO and subsequently were divided into "treated" and "untreated" groups. Treated aged rats received intermittent single-whisker stimulation during a 120-minute period immediately after pMCAO, whereas untreated aged rats did not. These animals were assessed using a battery of behavioral tests 1 week before and 1 week after pMCAO, after which their brains were stained for infarct. An additional treated aged group and a treated young adult group also were imaged with functional imaging. Results demonstrated that the recovery of treated aged animals was indistinguishable from that of the treated young adult animals. Treated aged rats had fully intact sensorimotor behavior and no infarct, whereas untreated aged rats were impaired and sustained cortical infarct. CONCLUSIONS: Taken together, our results confirm that single-whisker stimulation is protective in an aged rodent pMCAO model, despite age-associated stroke vulnerability. These findings further suggest potential for translation to the more clinically relevant older adult human population. (J Am Heart Assoc. 2012;1:e001255 doi: 10.1161/JAHA.112.001255.).

Mild sensory stimulation reestablishes cortical function during the acute phase of ischemia.

When delivered within 1 and in most cases 2 h of permanent middle cerebral artery occlusion (pMCAO), mild sensory stimulation (intermittent single whisker stimulation) was shown to be completely neuroprotective 24 h after pMCAO in a rodent model of ischemic stroke, according to assessment with multiple techniques (Lay et al., 2010). The acute effect of stimulation treatment on the ischemic cortex, however, has yet to be reported. Here we characterize cortical function and perfusion during the 120 min whisker stimulation period in four experimental groups with treatment initiated 0, 1, 2 (protected groups), or 3 h (unprotected group) post-pMCAO using multiple techniques. According to functional imaging, a gradual return of evoked whisker functional representation to baseline levels was initiated with treatment onset and completed within the treatment period. Evoked neuronal activity and reperfusion to the ischemic area also showed a gradual recovery in protected animals. Surprisingly, a similar recovery profile was observed in response to treatment in all protected animals, regardless of treatment onset time. Nonstimulated pMCAO control group data demonstrate that reperfusion is not spontaneous. This makes the complete protection observed in the majority of animals stimulated at 2 h post-pMCAO even more surprising, as these animals recovered despite having been in a severely ischemic state for two full hours. In summary, when delivered within a 2 h window post-pMCAO, whisker stimulation treatment initiated reperfusion and a gradual recovery of cortical function that was completed or nearly completed within the treatment period.

Amount but not pattern of protective sensory stimulation alters recovery after permanent middle cerebral artery occlusion.

BACKGROUND AND PURPOSE: Using a rodent model of ischemia (permanent middle cerebral artery occlusion), our laboratory previously demonstrated that 4.27 minutes of patterned single-whisker stimulation delivered over 120 minutes can fully protect from impending damage when initiated within 2 hours of permanent middle cerebral artery occlusion ("early"). When initiated 3 hours postpermanent middle cerebral artery occlusion ("late"), stimulation resulted in irreversible damage. Here we investigate the effect of altering pattern, distribution, or amount of stimulation in this model. METHODS: We assessed the cortex using functional imaging and histological analysis with altered stimulation treatment protocols. In 2 groups of animals we administered the same number of whisker deflections but in a random rather than patterned fashion distributed either over 120 minutes or condensed into 10 minutes postpermanent middle cerebral artery occlusion. We also tested increased (full-whisker array versus single-whisker) stimulation. RESULTS: Early random whisker stimulation (condensed or dispersed) resulted in protection equivalent to early patterned stimulation. Early full-whisker array patterned stimulation also resulted in complete protection but promoted faster recovery. Late full-whisker array patterned stimulation, however, resulted in loss of evoked function and infarct volumes larger than those sustained by single-whisker counterparts. CONCLUSIONS: When induced early on after ischemic insult, stimulus-evoked cortical activity, irrespective of the parameters of peripheral stimulation that induced it, seems to be the important variable for neuroprotection.

Mild sensory stimulation completely protects the adult rodent cortex from ischemic stroke.

Despite progress in reducing ischemic stroke damage, complete protection remains elusive. Here we demonstrate that, after permanent occlusion of a major cortical artery (middle cerebral artery; MCA), single whisker stimulation can induce complete protection of the adult rat cortex, but only if administered within a critical time window. Animals that receive early treatment are histologically and behaviorally equivalent to healthy controls and have normal neuronal function. Protection of the cortex clearly requires reperfusion to the ischemic area despite permanent occlusion. Using blood flow imaging and other techniques we found evidence of reversed blood flow into MCA branches from an alternate arterial source via collateral vessels (inter-arterial connections), a potential mechanism for reperfusion. These findings suggest that the cortex is capable of extensive blood flow reorganization and more importantly that mild sensory stimulation can provide complete protection from impending stroke given early intervention. Such non-invasive, non-pharmacological intervention has clear translational potential.