La propuesta de Constructivismo Social en Lev Vygotsky.

The properties of voltage gated sodium channels that determine both the threshold and frequency o f the generated action potential can be modulated by phosphorylation. In the intracellular regions of these channels there are several consensus phosphorylation sites for protein kinase A and C. In cultured neurones and intact synaptosomes substantial cAMP dependent phosphorylation of N av l .2 was seen to occur (Costa & Catterall, 1984). Smith & Goldin (1996) have also shown that brain N a v l.2 sodium channel can be phosphorylated by protein kinase A on the four serine residues on the loop between domain I and domain II. Dascal & Lotan (1991) have also shown that phosphorylation by protein kinase C on the occurs on serine residue in domain III of N av l.2 sodium channel.

1.10.2. Inflam m atory mediators:

England et al, 1996 and Gold et al, 1996 in their studies have independently shown that several inflammatory mediators released in response to injury are capable of directly sensitising a subpopulations of primary afferent neurones. PGE2, serotonin and adenosine were seen to decrease the activation threshold and increase the rate of activation and inactivation and increase the magnitude of TTX-r Na^ currents.

Studies performed in vivo suggested that directly acting inflammatory mediators over a period of few minutes including PGE2, serotonin and adenosine produce hyperalgesia via a cAMP/protein kinase A (PKA) second messenger cascade. The effect of these compounds were mimicked by compounds that increase intracellular

concentration of cAMP, prolonged by agents that blocked the breakdown of cAMP and blocked by agents that inhibit adenylate cyclase or PKA (Aley & Levine, 1999).

It is well established that inflammatory mediators can modulate TTX-r currents in DRG. Recently it has been shown that sodium channel gene expression is altered in inflammatory models of pain. Tanaka et al, (1998) have shown that there is an increase in Nayl.S mRNA expression following carageenan injection into the hind paw with an increase in the amplitude of the TTX-r sodium current consistent with the idea of abnormal sodium channel expression to the pathophysiology o f inflammatory pain, Tate et al (1998) have reported an increase in NavL9 mRNA following injection of complete Freunds adjuvant and Gould et al (1998) reported an increase in sodium channel immunoreactivity following injection of Freund's adjuvant.

Fitzgerald et al, 1999 showed with site-directed mutagenesis o f the TTX-r sodium channel N ayl.S that this channel is phosphorylated after activation of PKA. PKA induced phosphorylation results in changes in gating properties similar to those induced by inflammatory mediators suggesting that inflammatory mediator induced modulation of TTX-r currents reflects a direct phosphorylation of the channel.

1.10.3. NGF:

Damage sensing small diameter neurones fall into two categories with distinct trophic factor requirements. About one-half of the neurones express the nerve growth factor (NGF) specific tyrosine kinase receptor trkA and p75 and are IB4', whereas a separate population which can be surface labelled with isolectin B4 (IB4^) are trkA and p75 negative and have a trophic requirement for glia-derived neurotrophic factors (GDNF)

and express another tyrosine kinase receptor c-ret. Navl.8 is expressed in these two distinct types of sensory neurones, one bearing trkA and p75 receptors and another expressing GDNF receptor c-ret.

As well as inflammatory mediators, there is evidence that NGF is a key regulator of nociceptive thresholds. NGF application has been shown to cause dramatic decreases in thermal thresholds of pain perception in animal models and NGF levels rise in damaged tissues. NGF is known to act through the high affinity receptor trkA to increase the TTX-resistant current in PC 12 cells (Omri & Meiri, 1990) in culture. Omri & Meiri have also demonstrated the induction of Na^ currents by NGF in DRG neurones in serum-free medium. Recent evidence has shown that NGF up-regulates the expression of N av l .8 transcript in vivo in axotomised neurones in rat (Waxman et al, 1999). They found that in vitro NGF delivered to DRG cell bodies acts to down- regulate N av l.3 mRNA and maintains high levels of N a v l.8 mRNA. Dib-Hajj et aï,

(1998a) extended the study in vivo, and showed that following axotomy, the administration of exogenous NGF to the proximal nerve stump results in partial rescue of N a v l.8 mRNA levels and of TTX-resistant currents in small DRG neurones. These observations suggest that some of the changes observed following axotomy in the sciatic nerve are due to loss of access to peripheral pools of neurotrophic factors. Although N a v l.8 does not appear to be completely dependent on NGF for expression, there is up-regulation of both the transcript and the protein together with the appearance of an unusual and non-functional splice variant in addition of NGF to sensory neurones in culture (Okuse et al, 1997).

In contrast to the up-regulation of TTX-r currents with inflammatory mediators, a variety of manipulations that leads to neuropathic conditions leads to down-regulation of Navl.8. This suggests that Navl.8 may play a more significant role in inflammatory than neuropathic pain states. Altered sodium channel activity in peripheral neurones is associated with the development of inflammatory and neuropathic pain where altered patterns of sodium channel transcripts, as well as changes in post-translational modifications, have been observed (Waxman et al, 1999; Black et al, 1999; Dib-Hajj

et al, 1999). Neuropathic pain that results from direct damage to peripheral nerves seems to result from ectopic action potential propagation initiated at the site of injury, this activity is tetrodotoxin sensitive (Lyu et al, 2000). Trophic factors such as NGF are known to have important effects in regulating sodium channel expression. Overexpression of NGF results in large increases in N ayl.2 and p2 transcripts in hyper-NGF mice and mRNA levels for Nayi l, Na%,Nayl.6, Nayl.S, Nayl.9 and p i were also greater (Fjell et al, 1999). When peripheral nerve are damaged, there is loss of expression of Nayl.S and Nayl.9 transcripts, but an increase in Nayl.3 type sodium channel. Ectopic application of glial derived neurotrophic factor can reverse these changes, and is also able to reverse neuropathic pain behaviour in animal models (Boucher et al, 2000).