Infantile sensitivity to ethanol's motivational effects: Ethanol reinforcement during the third postnatal week.

BACKGROUND: Although tests specific to newborn rats have frequently verified their susceptibility to the reinforcing properties of ethanol, demonstration of comparable reinforcing effects in older infants has been elusive. Using a second-order conditioning procedure, the present study assessed in preweanling rats whether pairing with early postabsorptive effects of ethanol would render intraorally delivered gustatory stimuli capable of positive reinforcement for association with a salient texture. Direct reinforcing effects of ethanol were also evaluated through intake tests of gustatory stimuli previously paired with the drug. Blood ethanol levels (BELs) were determined for each of the ethanol doses used.
METHODS: Pups (14 days old) were stimulated with intraoral infusion of sucrose (10% v/v), water, or quinine (0.0045% w/v) 5 minutes after being intragastrically (i.g.) administered 0.00, 0.25, 0.50, or 2.00 g/kg ethanol (Experiments 1 and 2). These stimuli were then briefly presented while pups experienced a rough texture (sandpaper). Rats were subsequently evaluated in a 2-way texture location test (sandpaper vs smooth surface). In Experiment 3, sucrose, water, or quinine was paired with early postabsorptive effects of ethanol (0.00, 0.50, or 2.0 g/kg). Consumption of these stimuli was later assessed. Motor activity patterns during the intake test were also evaluated. In Experiment 4, BELs corresponding to 0.25, 0.50, or 2.0 g/kg ethanol were determined 5 and 20 minutes after i.g. administration (time periods were in accord with the onset and offset of intraoral stimulation used in the previous experiments).
RESULTS: Intraoral infusion of sucrose, water, or quinine, while under a state of sobriety and paired with sandpaper, resulted in roughly 50% preference for this texture. Sandpaper preferences were significantly elevated in pups that had experienced sucrose or water in a nonsober state-while under the effects of ethanol (Experiments 1 and 2). This indicated reinforcing effects of the ethanol intoxication. Pairing ethanol intoxication directly with consumption of sucrose, water, or quinine did not affect their later consumption. Yet, there were clear indications that this pairing resulted in conditioned behavioral activity patterns. Blood ethanol levels corresponding to the ethanol doses used here ranged between 10 and 150 mg%.
CONCLUSIONS: Infants appear sensitive to pharmacological reinforcing properties of low and relatively high ethanol doses. This sensitivity was revealed indirectly, by pairing gustatory stimuli with ethanol intoxication and then allowing these stimuli to act as second-order reinforcement for a quite different (tactile) stimulus. Behavioral activation elicited by the gustatory stimuli previously paired with a state of intoxication seems to compete with the expression of ethanol's motivational properties as assessed through intake tests.