AUGMENTATION OF NOCICEPTIVE REFLEXES AND CHRONIC DEAFFERENTATION PAINBY CHEMICAL LESIONS OF EITHER DOPAMINERGIC TERMINALS OR MIDBRAIN DOPAMINERGIC-NEURONS

Neurodegenerative diseases affecting the midbrain dopaminergic system have been reported to produce spontaneous pains like in Parkinson's di sease. Using various pain tests for acute (hot plate test, HPT, tail f lick, TFT, paw pressure test, PPT and paw immersion test, PIT) and chr onic deafferentation (autotomy, AT, following peripheral neurectomy) p ains in rats, we have investigated the effects on these tests of selec tive chemical lesions with 6-hydroxydopamine (6-OHDA) or/and kainic ac id (KA) either in the striatum or in the substantia nigra (SN) and ven tral tegmental area (VTA), 6-OHDA lesions of dopaminergic terminals in the striatum decreased significantly the latencies of all nociceptive reflexes (HPT from 11.7 +/- 1.45 s to 7 +/- 1.35 s, TFT from 4.5 +/- 0.15 s to 3.2 +/- 0.16 s and PPT on the contralateral leg from 2.07 +/ - 0.45 s to 1.05 +/- 0.085 s) and accelerated the time of onset (from 10.82 +/- 2.3 days to 3.1 +/- 0.52 days) and end (from 29.5 +/- 5.6 da ys to 5.2 +/- 1.1 days) of AT. These effects were not modified by simu ltaneous injection of KA and 6-OHDA in the striatum. 6-OHDA lesions in the SN-VTA produced comparable effects to those of similar injections in the striatum, while KA lesions in the SN-VTA did not produce signi ficant changes in the latencies of nociceptive reflexes or in the AT c riteria. These results suggest that the dopaminergic system plays a ma jor role in the processing of nociceptive information in the striatum and the limbic areas.