Behavioral processes mediating phencyclidine-induced decreases in voluntary sucrose consumption

Abstract

Prior exposure to phencyclidine (PCP) has been shown to decrease voluntary sucrose consumption in rats, which may indicate reduced reward function. To further characterize the effects of PCP on sucrose consumption, we examined the dose-response relationship between PCP and sucrose consumption, the longevity of the effect, the effects of repeated injections of PCP, variation of the PCP effect across sucrose concentrations, and the effects of PCP on gustatory hedonic responses. A single injection of PCP (2.5-20 mg/kg) dose-dependently suppressed sucrose consumption 20 h post-injection, with significant decreases after 15 and 20 mg/kg PCP. These decreases were sustained three days following withdrawal from PCP. Repeated injections of PCP (7.5 mg/kg bid for 7 days) decreased sucrose consumption 20 h after withdrawal, which returned to baseline on the second day. A single injection of PCP (15 mg/kg) suppressed 0.15 M sucrose more than 1 M sucrose consumption, with no effect on 0.3 M sucrose, suggesting that PCP suppressed intake of moderately rewarding taste stimuli. Finally, a single injection of PCP (15 mg/kg) suppressed brief access (20 s) licking for the majority of concentrations of sucrose solutions offered (0.031 M, 0.062 M, 0.125 M, 0.25 M, 0.5 M, and 1.0 M), while it had no effect on licking for 0.016 M sucrose, water, or for bitter quinine hydrochloride solutions (range: 0.94 mM-30 mM), suggesting that the PCP effect is specific to palatable taste stimuli without disruption of sensitivity to taste quality or intensity. We conclude that PCP produces moderate anhedonia as reflected through a specific decrease in the sustained consumption of moderately palatable sucrose solutions.

Figure 1

(a) Twenty hours following injection (Day 3), PCP produced a dose-dependent decrease in voluntary sucrose consumption (expressed as a %-Day 2 consumption) with no effect on water consumption (inset). *p < 0.05 SNK post-hoc test (b) Decreases in voluntary sucrose consumption seen following a single injection of 15 or 20 mg/kg PCP lasted for 3 days.

Figure 2

(a) Subchronic PCP decreased voluntary sucrose consumption 20 hours after the cessation of treatment; however, the effect was only significant on the first day following withdrawal. (b) Sucrose-induced water intake suppression was reversed for 2 days following cessation of subchronic PCP treatment.

Figure 3

(a) PCP produced an overall decrease in voluntary sucrose consumption across all sucrose concentrations twenty-four hours after injection. (b) PCP did not alter water consumption on Day 2 of the experiment (during which time PCP was administered). (c) Water consumption on Day 3 (immediately following the sucrose test) was increased by PCP.

Figure 4

(a) Concentration–response curve for sucrose (mean ± standard error) 4h before (filled circles) and 20h after (open circles) 15 mg/kg PCP treatment (n = 4). (b) Same data standardized to the water response for each rat.

Figure 5

Interlick interval (ILI) frequency distribution (for ILIs < 500 ms) for sucrose data depicted in Figure 4. While PCP reduced the number of licks (hence ILIs) expressed for sucrose, there was no significant shift in the mean of the ILI distribution, indicating no disruption of oromotor coordination after PCP treatment.

Figure 6

(a) Concentration–response curve for quinine hydrochloride (QHCl; mean ± standard error) 4h before (filled circles) and 20h after (open circles) 15 mg/kg PCP treatment (n = 7). (b) Same data standardized to the water response for each rat.