Elsevier

Neuroscience

Volume 90, Issue 1, 5 February 1999, Pages 119-130
Neuroscience

Enhanced dopamine efflux in the amygdala by a predictive, but not a non-predictive, stimulus: facilitation by prior repeated d-amphetamine

https://doi.org/10.1016/S0306-4522(98)00464-3Get rights and content

Abstract

Extracellular levels of dopamine within the amygdala were monitored using in vivo microdialysis during performance of an appetitive Pavlovian conditioning task in sensitized rats and unsensitized controls. Animals received exposure either to d-amphetamine or to vehicle for seven consecutive days (2 mg/kg/day, i.p.) in the home cage. Training began following a further seven injection-free days. Animals were exposed to two session types: during conditioning sessions, a stimulus (tone or light) immediately preceded sucrose pellet delivery. During control sessions, the alternative stimulus was also presented, but not in temporal proximity to an otherwise identical schedule of pellet delivery. There was a total of three alternating presentations of each session type during training. Sensitization enhanced Pavlovian conditioned approach behaviour to the stimulus predictive of imminent pellet delivery, and was without effect upon approach behaviours either to the food pellets themselves or to the control stimulus. Extracellular levels of dopamine within the amygdala were assessed during the fourth conditioning and control sessions. Mesoamygdaloid dopamine efflux increased significantly during the conditioning test session, but not during the control session, and this dopaminergic response was more marked in rats with prior repeated d-amphetamine experience.

Hence, these results add to evidence suggesting a role for amygdaloid dopamine in appetitive Pavlovian conditioning, and in the facilitation of associative learning following prior experience of d-amphetamine.

Section snippets

Subjects

A total of 14 male Lister hooded rats was used (Charles River, Margate, Kent, U.K.), weighing 300–350 g immediately prior to surgery. Animals were housed in pairs under a 12-h : 12-h light–dark cycle (lights on 08.00). Housing conditions were maintained at 22±2°C and 55±10% humidity. Following recovery from surgery, animals were reduced to 85% of their free-feeding weight by restriction of access to food. Access to water remained ad libitum. All experimental procedures used were subject to U.K.

Histology

Histological examination verified that all dialysis sites were located in the vicinity of the central nucleus of the amygdala, and were within ±0.6 mm of the intended coordinates in the rostral–caudal dimension (see Fig. 2a). Glial reaction was of very limited extent, and restricted to the location of the probe (Fig. 2b).

Initial tray approach

During these initial training sessions, sensitized and control animals showed similar approach responses to the sucrose pellet US (Fig. 3; main effect of Group: F1,12=0.9, NS;

Discussion

The present findings suggest that dopamine overflow in the amygdala increases in response to the presentation of a stimulus of acquired motivational or affective significance. Extracellular levels of dopamine within the amygdala were found to increase during a session containing presentations of a stimulus immediately preceding those of a sucrose US (CS+US test session), but not during a session in which the stimulus was explicitly unpaired with the same US (CS−/US control session). In both

Conclusions

The present results suggest that mesoamygdaloid dopamine is stimulated during exposure to an initially neutral stimulus of acquired motivational significance. Prior repeated d-amphetamine was additionally found to enhance this mesoamygdaloid dopamine response and the acquisition of affective significance by a stimulus associated with an event of intrinsic biological significance. These data may therefore highlight a role for the mesoamygdaloid dopamine projection in appetitive Pavlovian

Acknowledgements

We thank the Wellcome Trust for their financial support of this work. We gratefully acknowledge the extensive support and assistance of Presearch Ltd (Letchworth Garden City, U.K.).

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