Rationale: There are increasing reports of sex differences in the etiology
of drug abuse in humans. A nonhuman primate model is useful for examining s
ex as a variable in drug abuse. Objectives: To determine whether there are
sex differences in the acquisition of oral phencyclidine (PCP) self-adminis
tration and to compare the effect of altered feeding conditions on drug sel
f-administration in male and female monkeys. Methods: Acquisition of orally
delivered PCP was studied using 7 female and 11 male adult rhesus monkeys.
Initially, the monkeys were not food restricted, and they were given acces
s to water under concurrent fixed-ratio (FR) 1 schedules during daily 3-h s
essions. Each lip-contact response on a drinking spout resulted in a 0.3 ml
liquid delivery. After baseline levels of water intake were obtained for 5
days, water was replaced with PCP (0.125 mg/ml) at both drinking spouts. B
ody weights were then reduced to 85% of free-feeding weights, and the monke
ys were fed 30 min before the session began. The FR value was increased fro
m 1 to 2, 4. and 8, at both drinking spouts. As a final step in the procedu
re, water and PCP were concurrently available at the two spouts under FR 8
schedules. Acquisition of PCP-reinforced behavior was considered to have oc
curred if PCP intake was consistently greater than water intake. Results: L
ip-contact responses and liquid deliveries were not significantly different
between the females and males throughout the acquisition period, but there
was a significant increase in responding and decrease in liquid intake as
FR increased, and a significant increase in PCP consumption due to food res
triction that did not differ in males and females. On a milligram per kilog
ram basis, female monkeys consumed nearly twice as much PCP as the males; h
owever, this effect was not significant. The females showed significantly h
igher PCP than water intake while the males consumed approximately equal am
ounts of PCP and water. Of the seven females, 100% met the acquisition crit
erion of significantly greater PCP than water intake, while only 36.4% of t
he males met the criterion. Conclusion: These results concur with previous
rat studies and indicate that female monkeys are mon likely than males to a
cquire drug-reinforced behavior.