Reinforcement From Pharmacological Effects of Ethanol in Newborn Rats Evgeniy S. Petrov, Elena I. Varlinskaya, and Norman E. Spear Background: The preweanling period in the rat is characterized by acceptance of substantial amounts of ethanol and susceptibility to its reinforcing effects. It has been unclear, however, whether the neurobio- logical basis of ethanol reinforcement properties at this age is in ethanol’s olfactory, gustatory, or pharma- cological effects. Methods: The effectiveness of intraperitoneal (ip) ethanol as a reinforcer for newborn (3-hr-old) rats was tested toward separation of the orosensory and pharmacological sources of ethanol reinforcement. Responsiveness to a test nipple by pups given such pairings was compared with that of pups given unpaired presentations of the nipple and ethanol. Results: Reinforcement was assessed in terms of response to a surrogate nipple 1 hr after a single pairing of a similar nipple providing water (conditioned stimulus) and ip injection of ethanol (0.125, 0.25, 0.50, or 0.75 g/kg; unconditioned stimulus). Significant effects of ethanol reinforcement occurred with the lower doses (0.125 and 0.25 g/kg); higher doses of ethanol (0.50 and 0.75 g/kg) had no significant reinforcement effect. A second experiment determined that for conditioning with ip ethanol as the unconditioned stim- ulus, a conditioned stimulus consisting of only ingesting water or only suckling on an empty nipple also yielded significant reinforcing effects of ethanol, although with less strength than their combination. Both reinforcing doses of ethanol, 0.125 and 0.25 g/kg, yielded detectable concentrations of ethanol in the blood 5 min after injection, which were sustained at a significantly lower level 60 min after administration. Conclusions: These data indicate that aside from possible, and likely weak, hematogenic sources of gustatory and olfactory attributes of ethanol, the basis of ethanol’s reinforcement effect in neonatal rats is primarily pharmacological. For the pharmacological effects of ethanol to be reinforcing for the neonatal rat, concurrent appetitive activity on a nipple providing a fluid may be necessary for a substantial effect with this paradigm. Key Words: Ethanol, Reinforcing Effects, Neonatal Rat, Suckling, Conditioning. R ECENT EXPERIMENTAL FINDINGS indicate that infancy (the preweaning period, which usually ends on postnatal day 21) is a unique period characterized by in- creased affinity to ethanol (Lee et al., 1998; McKinzie et al., 1999; Varlinskaya et al., 1999) and high susceptibility to its reinforcing effects (Cheslock et al., 2001; Petrov et al., 2001). Given that relatively frequent infantile exposure to ethanol, either in the context of suckling or aside from it (Blaumslag, 1986; Croce, 1977; Fildes, 1986; Flores-Heurta et al., 1992; Mennella, 1999), can change responsiveness to ethanol later in life (Abate et al., 2002; Molina et al., 1999; Pepino et al., 2002; Spear and Molina, 2001), increased sensitivity to the reinforcing properties of ethanol during infancy is of great concern. Having a minimal history of reinforcement of any kind, neonates provide distinct advantages for determining the neurobiological basis of the reinforcing properties of etha- nol related to its gustatory and pharmacological effects. However, studies of alcohol reinforcement early in ontog- eny have been limited due to the difficulty in adapting the operant and Pavlovian testing procedures commonly used for adult animals (Cunningham et al., 1993; Grant, 1995; Samson, 1987) to infants. An important feature of any animal learning paradigm is its ecological and ontogenetic relevance, which includes an accommodation of natural behaviors peculiar to a given species at a given age. Suckling is an essential age-relevant form of well organized behavior common to all mammals, around which almost all experiences occur early in life. A recently developed technique using an artificial (surrogate) nipple to which neonatal rats readily respond allows precise study of the reinforcing properties of fluids, such as etha- nol, in the context of suckling behavior (Cheslock et al., 2000, 2001; Nizhnikov et al., 2002; Petrov et al., 1997, 1999, 2000, 2001). From the Center for Developmental Psychobiology, Department of Psy- chology, Binghamton University–State University of New York, Binghamton, New York. Received for publication April 10, 2003; accepted July 15, 2003. Supported by grants from the NIAAA [1RO1AA11960, 1R21AA12762, and 1RO1AA13098 (NES) and 1RO1 AA12453 (EIV)]. Reprint requests: Elena I. Varlinskaya, PhD, Department of Psychology, Binghamton University–SUNY, Binghamton, NY 13902-6000; Fax: 607-777- 2677; E-mail: varlinsk@binghamton.edu. Copyright © 2003 by the Research Society on Alcoholism. DOI: 10.1097/01.ALC.0000089960.62640.58 0145-6008/03/2710-1583$03.00/0 ALCOHOLISM:CLINICAL AND EXPERIMENTAL RESEARCH Vol. 27, No. 10 October 2003 Alcohol Clin Exp Res, Vol 27, No 10, 2003: pp 1583–1591 1583