Rationale: Intermittent delivery of an important commodity (e.g., food pellets) generates excessive behaviors as an adjunct to the schedule of reinforcement (adjunctive behaviors) that are hypothesized to be due to conflict between engaging and escaping a situation where reinforcement is delivered, but at suboptimal rates.
Objectives: This study characterized the endocrine correlates during schedule-induced polydipsia of water and ethanol using a longitudinal approach in non-human primates.
Methods: Plasma adrenocorticotropic hormone (ACTH) and cortisol were measured in samples from awake cynomolgus monkeys (Macaca fascicularis, 11 adult males) obtained at the onset, mid-day, and offset of their 12-h light cycle. The monkeys were induced to drink water and ethanol (4 % w/v, in water) using a fixed time (FT) 300-s interval schedule of pellet delivery. The induction fluid changed every 30 sessions in the following order: water, 0.5 g/kg ethanol, 1.0 g/kg ethanol, and 1.5 g/kg ethanol. Following induction, ethanol and water were concurrently available for 22 h/day.
Results: The FT 300-s schedule gradually increased ACTH, but not cortisol, during water induction to a plateau sustained throughout ethanol induction in every monkey. Upon termination of the schedule, ACTH decreased to baseline and cortisol below baseline. Diurnal ACTH and cortisol were unrelated to the dose of ethanol, but ACTH rhythm flattened at 0.5 g/kg/day and remained flattened.
Conclusions: The coincidence of elevated ACTH with the initial experience of drinking to intoxication may have altered the mechanisms involved in the transition to heavy drinking.