| Abstract: | Depletion of telencephalic noradrenaline (Ne), caused by lesion of the dorsal tegmental bundle, has been reported to increase persistance of non-reinforced responding in various operant tasks. This has been referred to as the dorsal bundle extinction effect (DBEE). In an effort to reproduce this effect, rats receiving 6-hydroxydopamine lesions of the dorsal Ne bundle (DB-OHDA) were compared to controls during extinction of a continous food rewarded (CRF) lever-response. While the lesion group showed an increase in responding during initial extinction, no significant difference in resistance to extinction using a 2-min non-response criterion was obtained. Moreover, no differences in reinforced response rates were observed with CRF, fixed ratio (FR-15, FR-30, FR-60) or variable interval (VI-30, VI-60, VI-20 s) schedules of reinforcement. In order to test the hypothesis that the DBEE is dependent on time of behavioral testing after surgery, subsequent experiments were performed where rats began CRF operant training 5, 17, 31 or 110 days post-lesion. No differences in resistance to extinction were observed between lesion and control rats at any post-lesion interval. Neonatal treatment with 6-OHDA which permanently lesions forebrain Ne terminals also failed to prolong extinction. Finally, when both DB-OHDA and neonatal rats were given a choice between water and saccharine the lesioned animals exhibited a neophobic reaction whereby they drank significantly less saccharine. We conclude that while the DBEE is not a reliably reproducible phenomenon other effects of forebrain Ne lesions, such as neophobia, appear to be robust. |
