MOLECULAR MEMORY SYSTEMS IN NEURAL REPAIR

The neurorehabilitation of brain injury relies on treatment principles that have often existed unchanged through the many decades since their discovery. As many other major diseases benefit from advances in molecular medicine, rehabilitation of brain injury still utilizes traditional physical medicine principles within the domains of physical and occupational therapy. While these modalities have a clear role in promoting neurological recovery, there are no drugs that have been shown effective in stimulating neural repair after stroke and other brain injuries. There are several hints in the normal patterns of human recovery after CNS injury for a way forward to molecular treatment for stroke recovery. In cognitive terms, recovery after brain injury appears to rely on the “re-learning” of movement patters that were normally routine prior to the injury. This notion has led to the idea that formal learning and memory rules may be applied to brain injury rehabilitation. Indeed, this idea that some aspect of learning is the neuronal basis for recovery has led to ad hoc attempts to treat brain injured patients with any available drug that might also stimulate learning, memory or attention: including serotonin reuptake inhibitors, dopamine agonists, Ritalin, modafinil and amphetamines. Because these drugs were developed to treat conditions other than neurorehabilitation from brain injury, and because they act with less specificity on many neurotransmitter systems in the brain, these agents have not held up in clinical trials for stroke recovery. Research in this area within in the Carmichael lab determines how defined molecular memory systems might play a role in repair and recovery in stroke.

Our work has initially focused on two cellular signaling system that plays a role in memory formation: tonic or extrasynaptic GABA signaling and AMPA receptor signalng. Tonic GABA signaling is predominantly an extrasynaptic process that determines the shunt current on neurons. Greater tonic GABA signaling leads to a relative hypo-excitable state and a diminished propensity to fire action potentials to a given input; decreased tonic GA signaling leads to an increased tendency to fire action potentials to any given input. In contrast, phasic GABA signaling is the synaptic GABA transmission associated with direct inhibitory cell or interneuron input. Tonic GABA currents can be modulated independently from phasic GABA currents because two GABA receptor subunits are preferentially incorporated into tonic GABA receptors: alpha 5 and delta.

We have found that stroke induces a hypoexcitable state in pyramidal neurons in adjacent motor cortex, for several weeks after the stroke (Clarkson et al, Nature, 2010). This hypoexcitable state occurs because of a decrease in levels and activity of astrocytic uptake of GABA, via decreased levels of the GAT3/4 transporter. The increase in extracellular GABA has the initial effect of limiting infarct size in the first day after the stroke, but the persistence of this diminished GABA uptake leads to increased inhibitory tone and decreased recovery over time. We have found that blocking tonic GABA currents with alpha5 or delta blocking approaches, in pharmacological or genetic studies, reduces tonic GABA currents after stroke to control levels and improves behavioral recovery (Clarkson et al. Nature in press). These data identify a novel pharmacological target for stroke recovery, and suggest an important avenue in further research studies in assessing how molecular memory systems might be involved in neural recovery and/or repair after stroke.

Figure. Summary of Tonic Inhibition in Stroke and Stroke Recovery.

Clarkson et al Nature 2010 Summary

Upper right. Small strokes in the mouse alter the way brain cells (neurons) perceive inhibitory signals.  1. Neurons normally send inhibitory signals through a chemical called GABA. GABA is cleared from the brain by uptake pumps (pinwheels). 2. After stroke one of these uptake pumps is decreased. This lets GABA build up in the brain. 3. The increased GABA has a greater inhibitory effect through a subset of GABA receptors, called extrasynaptic GABA receptors. This means that the brain has an increased inhibition after stroke. 4. These extrasynaptic GABA receptors have a different chemical composition than then synaptic GABA receptors. Drugs target these extrasynaptic, or tonic, GABA receptors and block them. Blocking tonic GABA currents restores the normal brain excitability level and improves recovery of limb function in mice. 5. There is an important time component to blocking tonic GABA inhibition after stroke. Early after stroke, the increase in tonic GABA inhibition is protective for the brain. Blocking tonic inhibition on the day of the stroke causes an increase in infarct size. However, blocking tonic GABA inhibition later, such as 3 days after stroke, does not increase the amount of damage and improves recovery.  These results show that the brain uses tonic GABA inhibition to limit stroke damage. The problem is that the increased tonic GABA inhibition then persists for weeks and limits recovery. Blocking tonic GABA inhibition during the later phases promotes recovery.

In addition to tonic GABA signaling in motor recovery after stroke, a second area of study has focused on excitatory glutamate signalng. Glutmate signaling through the AMPA receptor is important in learning and memory and in homeostatic synaptic scaling. AMPA receptor signaling also leads to BDNF induction, which mediates LTP, dendritic and axonal plasticity. We used gain and loss of function studies in AMPA receptor signaling to identify a role for this system in motor recovery after stroke (Clarkson et al J Neurosci, in press). These studies found that AMPA receptor activation induces motor recovery after stroke.: inducing AMPA receptor signaling  promotes motor recovery and blocking AMPA receptor signaling inhibits motor recovery. Importantly this AMPA receptor effect occurs through its activity-dependent induction of BDNF. Activation of the AMPA receptor, such as through positive allosteric modulators (termed AMPAkines), that does not induce BDNF does not promote recovery.

Three other important points emerged from these studies on excitatory signaling and motor recovery after stroke. First, the locus of the AMPA receptor-induced BDNF effect is in the peri-infarct cortex, and not diffusely in the post-stroke brain. Second, the timing of inducing AMPA receptor signaling for motor recovery is a crucial issue: inducing AMPA receptor signaling early in stroke (day one) increases infarct size; inducing AMPA receptor signaling later (day 5 after stroke) has no effect on infarct size but promotes recovery. Third, postiive allosteric modulators of the AMPA receptor, such as AMPAkines, are promising  therapy for human stroke because they can be given even 5 days after the stroke and promote recovery.