002%3908( 94)0008 1-6 Nclrrophormorolog~ Vol. 33, No. IO. pp. Il?l-I 182. 1994 Copyright ICI 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0028-3908/94 $7.00 + 0.00 Comparison of Delta Opiate Receptor Agonist Induced Reward and Motor Effects Between the Ventral Pallidum and Dorsal Striatum P. I. JOHNSON* and J. R. STELLAR Department of Psychology, Norlheasrern Universiry, 125 Nighhgale Hall, Bosron, MA 021 IS, U.S.A. (Accepted 5 July 1994) Summary-The role of the ventral pallidurn and the dorsalstriatumin mediating the rewarding effects of the delta receptorspecific agonist[2-D-penicillamine, 5-D-penicillamine]enkephalin (DPDPE) were evaluated in the rat using the intracranial self-stimulation paradigm. Reward shifts were indicated by the changein frequency required to maintain half-maximalresponding while motor/performance changes were identified by increases or decreases in the maximum responding. Each hour-long test session consisted of three identical,consecutive 20 min rate-frequency curves. In an effort to ascertain possible heterogeneity of function along the rostrocaudal axis, DPDPE (0.0 nmol= saline dose, 0.3 nmol= low dose,I .O nmol= medium dose, 3.0 nmol= high dose)was microinjectedinto either the rostra1or caudal region of the two structures. Microinjections into the caudate werepositioned directly abovethe ventral pallidurnplacements resulting in centromedial or caudomedial caudate placements. DPDPE microinjections into the rostra] ventral pallidum resulted in a significant rewardincrease (28% increase or -0.14 log Hz shift) only at the high dose. In contrast, caudal ventral pallidal DPDPE microinjections showed a dose-response effect with rewardincreases of 19, 22 and 31% (-0.09, -0.11 and -0.16 log Hz) for the low, medium and high dose, respectively. DPDPE microinjections into the centromedial caudate resulted in a large rewardincrease (29% or -0.15 log Hz) at the high dose, while caudomedial caudate DPDPE microinjections had no effect on reward. Motor/ performance effects tended to follow the pattern of reward effects, with most regions showing motor increases rangingfrom 25 to 75% over baseline activity. The only exceptionwasfound in the caudomedial caudate, where microinjectionsof the high dose of DPDPE resulted in an approximate 20% suppression of motor/performance activity. These results demonstrate that the ventral pallidum andthe mediocentral caudate play a role in modulatingopiate rewards, and adds to the growing body of literature regarding the regional heterogeneity within the caudate and ventral pallidum. Keywords-DPDPE, intracranial self-stimulation, rostrocaudal heterogeneity, opioid agonist, caudate, delta opiate receptor. The ventral pallidum receives the principle efferent traditionally formed by the ventral tegmental area and projection of the nucleus accumbens (Heimer et al., nucleus accumbens (Johnson et al., 1993; Koob and 1991; Nauta et al., 1978; Zahm and Brog, 1992; Zahm Swerdlow, 1988). For example, behavioral activation and Heimer, 1990, 1993), which contains enkephalin studies have shown that stimulation of either the mu- or fibers (Zahm et al., 1985). In addition, the ventral delta-opiate receptor in the ventral pallidum causes an pallidurn has been shown to contain a light to moderate increase in motor activation (Austin and Kalivas, 1990; distribution of the mu, delta and kappa opioid receptors Hoffman et al., 1991; Napier, 1992, 1993), which is (Churchill et al., 1990;Herkenham et al., 1984; Mansour consistent with the results seen in accumbens motor et al., 1988; Mansour et al., 1994), and has been impli- studies (Cunningham and Kelley, 1992; Daugb et al., cated in opiate reward and motor functions (Austin and 1988; Havermann and Kuschinsky, 1985). Additional Kalivas, 1990; Hubner and Koob, 1990; Johnson et al., motor function similarities found between the accum- 1993;Napier, 1992). Basedon functional similarities and bens and ventral pallidum include biphasic motor effects anatomical connections, the ventral pallidum has been caused by high doses of the mu-opiate receptor agonist proposed to continue the limbic motor/reward pathway Tyr-D-Ala-Gly-NMe-Phe-Gly-ol-enkephalin (DAMGO) (DaugC et al., 1988, 1989; Johnson et al., 1993) and the finding that delta-opiate receptor activation produces a *To whom correspondence should be addressed. smaller motor increase when compared to relatively 1171