Effects of para-chlorophenylalanine and tryptophan on learning of a brightness discrimination in rats.

Both the learning of brightness discrimination, discrimination reversal and a simple oddity problem, and the pain-sensitivity to electric shock were studied in rats treated with 100 mg/kg of D, L-parachlorophenylalanine (PCPA), 100 mg/kg of D, L-tryptophan (TP) or 2.0 ml/kg of control solution (CS). At the end of the last behavioral session all animals were killed and the levels and rate of 5-hydroxytryptamine (5-HT) synthesis were measured. PCPA treatments produce rats which are more effective in coping with a problem; these animals were similar in that both the levels and turnover rate of brain 5-HT and the pain-sensitivity were lower. Results are tentatively explained suggesting that these differences in learning ability could be a result of the drug's action in pain-sensitivity. The possibility that 5-HT might be responsible for these differences is discussed.