Physiology& Behavior, Vol. 47, pp. 601-603. ©Pergamon Press plc, 1990. Printed in the U.S.A. 0031-9384/90 $3.00 + .00 BRIEF COMMUNICATION Effects of Preferential Delta and Kappa Opioid Receptor Agonists on the Intake of Hypotonic Saline BLAKE A. GOSNELL, MARK J. MAJCHRZAK AND DEAN D. KRAHN University of Michigan, Department of Psychiatry, University Hospital, 8D8806, Box 0116 Ann Arbor, MI 48109-0116 Received 16 November 1989 GOSNELL, B. A., M. J. MAJCHRZAK AND D. D. KRAHN. Effects of preferential delta and kappa opioid receptor agonists on the intake ofhypotonic saline. PHYSIOL BEHAV 47(3) 601-603, 1990. - - A previous study has implicated central mu opioid receptors in the preference for salt solutions. Because mu, kappa and delta receptors are all thought to play a role in food intake and/or the mediation of palatability, we performed a series of experiments to determine whether preferential agonists at kappa and delta receptors might also stimulate the intake of salt solutions. When injected centrally into nondeprived rats, two selective agonists at delta receptors caused increases in the intake of 0.6% saline; the intake of concurrently available water was either unchanged or slightly increased. The selective kappa agonist U-50,488H had no effect on water or saline intake, whereas the preferential kappa agonist DAFPHEDYN caused a delayed increase in saline intake. These results indicate a role for central delta receptors in the preference for salt solutions, and are consistent with the suggestion that opioids play a role in the mediation of palatability. Opioids Palatability Taste preference Reward Drinking Saline intake SEVERAL reports have suggested that endogenous opioids play a role in mediating palatibility [see (5,12) for reviews]. Generally, opioid antagonists have been found to decrease the preference for (or intake of) palatable fluids (3, 5, 8), while opioid agonists increase preferences (2,8). Although most studies have dealt with the consumption or preferences for sweet-tasting fluids, there are indications that these opioid effects can also be obtained with nonsweet palatable fluids. Hypo- and isotonic salt solutions are preferred over water even in salt-replete rats (10), which suggests that these solutions are palatable to rats. Naloxone caused a selective reduction in the intake of hypotonic saline solutions (4), and reduced 0.9% saline intake at lower doses than those required to reduce water intake (11). In water-deprived rats, systemic injections of morphine increased rats' preference for isotonic saline (1). There are at least three types of opioid receptors, the best characterized being the mu, kappa and delta (6). Recently, we found that a selective mu agonist stimulates the intake of hypo- tonic (0.6%) saline in nondeprived rats (9). Because mu, kappa and delta receptors are all thought to play a role in food intake and/or the mediation of palatability (5, 7-9), we performed a series of experiments to determine whether preferential agonists at kappa and delta receptors might also stimulate the intake of salt solutions. The delta agonists used were [D-Thr 2, LeuS]enkephalin - Thr 6 (DTLET) and [D-Pen 2, D-PenS]enkephalin (DPDPE); the kappa agonists were U-50,488H and a recently re- ported dynorphin analog, [D-Ala2, (Fs)Phea]-dynorphinl_13-NH2 (DAFPHEDYN) (13). METHOD Male Sprague-Dawley rats (Charles River Laboratories, Inc., Wilmington, MA) were used in all experiments. At the time of surgery, body weights ranged from 239-345 g. They were individually housed in stainless steel cages, and food and water were available ad lib, except as described below. Lights were on for 12 hours daily, and all experiments were carried out in the light period. A 20-gauge cannula was implanted into the fight lateral ventricle of each rat with techniques similar to those previously reported (8,9). Cannula placement was verified by the observation of a drinking response to an ICV injection of angiotensin II (AII, 100 ng). All rats were tested at least once with AII, and all data from animals drinking less than 5 ml in 30 min on all tests were excluded. Adaptation to testing procedures began no sooner than 3 days after surgery. After an overnight period of water deprivation, rats were moved to individual test cages for adaptation to the testing procedure. One hour later, a 0.6% sodium chloride (saline) solution was presented to each rat in a 25-ml graduated buret fitted with a stainless steel sipper tube. After two hr of access to this solution, rats were returned to their home cages. After this first 601