“…Extensive studies have shown that the enkephalinergic system is altered both with nigrostriatal damage and subsequent treatments to alleviate the functional consequences of dopaminergic denervation, such as L-dopa administration. For instance, nigrostriatal degeneration increases preproenkephalin-A (PPE-A) mRNA levels in the indirect pathway, that is, striatal neurons projecting to the external globus pallidus; this contrasts with effects on PPE-B mRNA levels that are decreased in striatal outputs to the internal globus pallidus, or the direct pathway (Morissette et al, 1997,Cenci et al, 1998,Duty et al, 1998,Henry et al, 1999,Quik et al, 2002,Tel et al, 2002,Henry et al, 2003,Aubert et al, 2006. LIDs, on the other hand, are linked to increases in PPE-B transcript levels, while changes in PPE-A do not correlate well with the development of dyskinesias (Cenci et al, 1998,Duty et al, 1998,Henry et al, 1999,Quik et al, 2002,Tel et al, 2002,Henry et al, 2003,Aubert et al, 2006.…”