Q:

How does Sodium Valproate cause neural plasticity

I have been reading a fascinating paper: Valproate reopens critical-period learning of absolute pitch
18 individuals were given Sodium Valproate (VPA) for a fortnight during which they trained on a pitch-training game. Results suggest that VPA reopens the plasticity window that normally closes by adolescence.
However, the paper seems to suggest that the exact mechanism of action is unknown.

Valproic acid is believed to have multiple pharmacological actions,
  including acute blockade of GABA transaminase to enhance inhibitory
  function in epileptic seizures and enduring effects on gene
  transcription as an histone deacetlyase (HDAC) inhibitor (Monti et
  al., 2009). Of relevance here is the epigenetic actions of this drug,
  as enhancing inhibition does not reactivate brain plasticity in
  adulthood (Fagiolini and Hensch, 2000), but reopening chromatin
  structure does (Putignano et al., 2007). While systemic drug
  application is a rather coarse treatment, the effects may differ
  dramatically by individual cell type (TK Hensch and P Carninci,
  unpublished observations). VPA treatment mimics Nogo receptor deletion
  to reopen plasticity for acoustic preference in mice (Yang et al.,
  2012), suggesting a common pathway through the regulation of
  myelin-related signaling which normally closes critical period
  plasticity (McGee et al., 2005). Future work will address the cellular
  actions of VPA treatment in the process of reactivating critical
  periods. Future MRI studies will also be needed to establish whether
  HDAC inhibition by VPA induces hyperconnectivity of myelinated,
  long-range connections concurrent with renewed AP ability (Loui et
  al., 2011).

So it is saying that the standard use of VPA is to increase GABA levels (which keeps firing rate down -- it is used as an antiepileptic), however it also acts as an HDAC inhibitor, which means it causes unwrapping of chromatin and consequently increased mRNA transcription, maybe even transcription of genes that would normally be entirely deactivated in an adult.
So my guess is that some protein is getting produced that messages neurons to generate new axon/dendrite growth and/or new synaptic connections.
Can anyone clarify how VPA might accomplish plasticity?
EDIT (one month later): I have more detail, but I still can't quite make the connection. Here goes:
Neurites get wrapped by myelin/oligodendrocyte, which produces and exudes some of the chemical messagers {Nogo, OMgp, MAG}. The membrane surface of the neurite contains nogo 66 receptors (NgR-s) that get triggered by these messagers and inform the neuron to inhibit axon-growth. Somehow the 'HDAC inhibition' property of VPA is unwinding DNA enough to alter transcription rates of certain proteins, and one of these must be disabling the NgR.
But how is this happening?

A:

As far as I can see this paper is being a little misleading, by saying "VPA mimics Nogo-66 receptor deletion".
The action of VPA doesn't seem to be related to this receptor.
It seems that blocking this receptor and applying VPA both increase plasticity, but it is like taking a car or taking a train -- entirely different modes of transport that achieve the same effect.
VPA seems to facilitate LTP through increased availability of relevant proteins.
That is unconnected with growing new neural structure.
The principal problem with growing new structure is, as hinted at the end of the question, that the adolescent/adult brain secretes a chemical that gets picked up by Nogo 66 receptor, which signals to collapse the axon growth cone.
It's why adult humans can't recover from spinal injuries. The axons won't reconnect.
It so happens that a small molecule Nogo Antagonist has recently been developed by Professor Strittmatter.  He was kind to reply to my query, and I learned that this molecule is currently in the early stages of FDA approval. It's a very exciting discovery!
I would caution anyone against taking VPA -- I have been encountering chest pains since experimenting with it (even though I dropped the experiment after a week due to pain). It looks as though this pain is inflammation of the stomach lining and a common reaction to VPA (VPA is an acid!).
If someone is determined to take VPA, they should at least investigate taking it together with Omeprazole, which discourages the stomach from producing excess acid.
I am disappointed that I now need to take omeprazole quazi-regularly, it appears that my brief VPA experiment has caused some long-term upset in my chemical balance.
Also of note is that (again discovered by Strittmatter) Ibuprofen has been found to also interrupt the signalling pathway that leads to axon growth collapse. But it requires a high dosage, and ibuprofen also causes a similar imbalance.

