Behavior maintained under fixed-interval and second-order schedules by intravenous injections of endogenous...

The effectiveness of i.v. injections of the endogenously occurring amines beta-phenylethylamine (PEA), N-methyl phenylethylamine (NMPEA) and phenylethanolamine in maintaining schedule-controlled behavior was investigated in dogs. Behavior was maintained under either a fixed-interval (FI) 5-min schedule of i.v. drug injection or a second-order FI 5-min schedule where every fifth response (FR 5) resulted in a 2-sec visual stimulus and the first FR 5 completed after the interval elapsed resulted in both the visual stimulus and i.v. drug injection [FI 5-min (FR 5:S)]. Experimental sessions, with 10 intervals per session, were conducted 5 days/week. Each drug injection was followed by a 5-min timeout period to minimize the direct effects of the drugs on responding. As the dose per injection increased, rates of responding maintained under both schedules by PEA and NMPEA first increased and then decreased. When saline was substituted for drug, responding occurred at very low rates. PEA and NMPEA were approximately equieffective and equipotent in maintaining responding under the FI 5-min schedule. PEA maintained somewhat higher rates under the FI 5-min (FR 5:S) schedule; rates maintained by NMPEA under the second-order schedule were comparable to those maintained under the simple FI schedule. Phenylethanolamine failed to consistently maintain responding under either schedule. Injections of PEA and NMPEA controlled overall patterns of positively accelerated responding under both schedules, whereas the local pattern of responding under the second-order schedule was under the control of both the brief stimulus presentations as well as drug delivery.(ABSTRACT TRUNCATED AT 250 WORDS)