Brain Research, 558 (1991) 141-144 © 1991 Elsevier Science Publishers B.V. All rights reserved. 0006-8993/91/$03.50 ADONIS 000689939124818H BRES 24818 141 Acute amphetamine-induced subsensitivity of Alo dopamine autoreceptors in vitro V. Seutin 1, P. Verbanck 2, L. Massotte 1 and A. Dresse 1 1Laboratory of Pharmacology, University of Lidge, Lidge (Belgium) and 2 Laboratory of Medical Psychology, Free University of Brustels, Brussels (Belgium) (Accepted 28 May 1991) Key words: Amphetamine; Ventral tegmental area; Dopaminergic neuron; Autoreceptor; Brain slice; Extracellular recording ExtraceUular recordings were obtained from spontaneously active, presumed dopamine (DA) neurons of the ventral tegmental area (VTA) of the rat in a slice preparation. Bath-applied (+)-amphetamine (AMPH) (1-30/~M) induced a concentration-dependent decrease in the firing rate of these neurons, which tended to saturate with the highest concentrations used (n = 11). This inhibitory effect was dependent on the activation of D 2 receptors since it was reversed by the D2 antagonist sulpiride (n = 8). However, the most striking effect of AMPH was the induction of a prominent subsensitivity of DA autoreceptors: whereas in 18 out of 20 control neurons, the D2 agonist BHT 920 (100 nM) produced a rapid and complete inhibition of the firing, this was observed in none out of 11 neurons 10 min after the end of the application of AMPH (1-30/~M) (P < 0.001). In these cells, the mean percent inhibition produced by BHT 920 was only 47 + 8%. This subsen$itivity remained unchanged after 20 min and declined after one hour. This effect was specific, since the sensitivity of GABAa receptors to baelofen (500 nM-1 /~M) was not modified by the application of AMPH (n = 12). These results suggest that AMPH-induced DA autoreceptor subsensitivity can be produced acutely and may be the first step in a cascade of events leading to behavioral sensitization to this compound. Administration of (+)-amphetamine (AMPH) to rats produces an increase in motor activity which is thought to be mediated chiefly by the dopamine (DA) mesolimbic system 9. At the neuronal level, the main effects of AMPH are an increase of the release of DA and an inhibition of its uptake 4. Long-term treatment with AMPH produces an increase in the motor response of the animal to the drug 13. This phenomenon, called behav- ioral sensitization, is thought to be one of the most valid animal models of AMPH-induced psychosis in humans 12. A lot of studies have tried to delineate the neurobio- logical substrate(s) of this sensitization 12 and there is substantial evidence for DA autoreceptor subsensitivity occurring in the ventral tegmental area (VTA) or A10 after chronic treatment with AMPH 8'19. In previous experiments in slices of the VTA, it has been observed that during a continuous application of D 2 agonists like BHT 9201'15 or apomorphine, their inhibi- tory effect on the firing of some neurons could markedly decrease over time (Seutin et al., unpublished). More- over, such an acute tolerance to D 2 agonists has also been described in the substantia nigra 11. The aim of the present study was therefore to examine whether prolonged application of AMPH in the VTA in vitro, which would presumably increase the concentra- tion of DA at the level of the autoreceptors 18, could also induce an acute subsensitivity of these receptors. Methods which were used have been described pre- viously 14A5. Briefly, male Wistar rats (180-280 g) were anaesthetized with chloral hydrate (400 mg/kg i.p.) and given 100% oxygen to breathe for 5 min. After decapi- tation, the brain was quickly removed and cooled in ice-cold artificial cerebrospinal fluid (ACSF) which had the following composition (in mM/1): NaC1 130, KC1 5, NaH2PO 4 1.25, NaHCO 3 24, D-glucose 10, CaCI2 2, MgSO4 1.25. A piece of mesencephalon was prepared and cut in transverse sections by means of a vibratome. The thickness of the slices was about 0.4 mm. The slice containing the VTA was drawn into a pipette and transferred to a perfusion chamber modified from the design of Haas et al. 6. The slice was positioned at the interface of a continuous flow of oxygenated ACSF (1 ml/min) and a humidified mixture of 0 2 95% and CO 2 5%. The temperature was set at 34.5 + 0.5 °C. The VTA was identified visually, using a zoom stereomicroscope (Wild M7A). Extracellular single-cell recordings were performed with glass microelectrodes filled with ACSF and having an impedance of a few MI2. Correspondence: A. Dresse, Laboratoire de Pharmacologie, Institut de Pathologie (B 23), Universit~ de Lirge, 4000 Sart-Tilman par Lirge 1, Belgium.