Introduction

1. A. B. Activity vs. Amphetamine Self-Administration Novelty Preference vs. Amphetamine Self-Administration Number of Self-Infusions Number of Self-Infusions • Figure 5.Strain Differences in Activity and Novelty Preference as Predictors of Amphetamine Self-Administration • Points for each individual strain based on their average locomotor activity and average number of amphetamine infusions on the FR5 schedule. No significant correlation was obtained. • B. Points for each individual strain based on their average novelty preference and average number of amphetamine infusions on the FR5 schedule. A significant correlation was found between novelty preference and amphetamine infusions on the FR5, r = 0.601, p < .05. Results Introduction There is now considerable evidence to support a relationship between individual differences in response to inescapable novelty and response to stimulant drugs of abuse. Rats categorized as high responders (HR) self-administer more amphetamine than rats categorized as low responders (LR; Piazza et al., 1989). While these individual differences may represent heritable traits, the majority of this evidence has been derived from experiments using outbred rats (Exner and Clark, 1993; Gingras and Cools, 1997; Higgins et al., 1994; Hooks et al., 1991; 1992; Piazza et al., 1989) or using only two inbred strains (Camp et al., 1994; Kosten et al., 1997). More recently, individual differences in novelty seeking have been found to also predict response to stimulant drugs. These latter studies have found that novelty seeking predicts amphetamine self-administration, with high novelty seekers showing greater amphetamine self-administration than low novelty seekers (Cain et al., 2004; 2005). However, these studies also only used rats from an outbred population. The purpose of the present experiment was to determine if the relationship between these predictor and outcome variables is, at least in part, genetically based. An initial experiment was conducted using outbred male Sprague-Dawley rats to establish that reliable acquisition of amphetamine self-administration could be obtained using an autoshaping procedure. Then, rats from 12 inbred rat strains were screened for their response to both escapable and inescapable novelty, and assessed for acquisition of amphetamine self-administration. The various available strains chosen offered a diversity of characterized phenotypes (traits) which were useful in analyzing the critical determinants of novelty-seeking behavior. By holding environmental factors constant in this experiment, interstrain differences in behavioral responses to amphetamine may be attributed to differences in the genotype between strains. Figure 1.Acquisition of Amphetamine Self-Administration using an Autoshaping Procedure in Sprague-Dawley Rats. An ANOVA revealed a significant main effect of autoshaping session, F(10,100) = 8.516, p<.001. Figure 2.Strain Differences in Acquisition of Amphetamine Self-Administration During the Autoshaping Phase. Among the inbred strains, an ANOVA revealed a significant interaction between strain and session, F(110,640) = 2.02, p<.001. Individual strain main effects of session were observed for the following strains: BUF, WKY, ACI, BN, LEW, DSS, FH, WF and WAG, p<.05. Differences Among Inbred Rat Strains in Novelty Seeking, Locomotor Activity and Amphetamine Self-Administration Meyer AC1,2, Dawahare ER1, Rahman, S1,2, and Bardo MT1,21Department of Psychology, University of Kentucky, Lexington, KY 405362Center for Drug Abuse Research Translation, Lexington, KY 40536 Number of Self-Infusions Methods Subjects. Male Sprague-Dawley (SD) outbred rats and male rats from the following inbred strains were used: ACI; BDIX (BD9); Brown Norway (BN); Buffalo (Buf); Dahl salt sensitive (DSS); Fawn Hooded (FH); Fischer (F344); Lewis (LEW); Spontaneous hypertensive rat (SHR); Wistar Albino Glaxo (WAG); Wistar Furth (WF); Wistar Kyoto (WKY). Rats were housed individually with ad libitum access to food (except as noted) and water in the home cage, which was maintained in a colony room on a 12-hr/12-hr light/dark cycle. Drugs. d-Amphetamine sulfate (Sigma; St. Louis, MO) was prepared in 0.9% NaCl (saline). Apparatus. Locomotor activity was recorded using an automated monitoring system with Versamax System software (AccuScan Instruments Inc., Columbus, OH). Activity was measured as photobeam interruptions, expressed as total distance traveled (cm). Novelty place preference was measured using an automated conditioned place preference chamber (ENV-013, Med Associates, St. Albans, VT) consisting of three distinct compartments. Each chamber was interfaced to a personal computer running MED-PC IV (Med Associates) software. Amphetamine self-administration was assessed in an operant conditioning chamber (ENV-001, Med Associates St. Albans, VT) that was enclosed in a sound attenuating compartment. Located on the front panel of the chamber was a 5 x 4.2 cm opening that allowed access to a recessed food tray. Two metal response levers on either side of the food tray were located 7.3 cm above a metal-grid floor. A white cue light was centered above each response lever. A white house light was centered 20.3 cm above the metal-grid floor on the wall opposite the response levers. Drug infusions were delivered via a syringe pump. A water-tight swivel allowed attachment of the catheter tubing from a 10-ml syringe to the head mount of the rat within the chamber. Individual Difference Tests. To assess activity in inescapable novelty, rats were placed individually in the locomotor apparatus for 30 min. Total distance traveled was recorded in 5-minute intervals and summed for total distance traveled over the entire 30 min. On the next day, rats were assessed for novelty place preference. Animals were habituated to either the black or white compartment (counterbalanced) for 30-min sessions on two consecutive days. On the following day, animals were placed in the center gray compartment with access to both the black and white compartments. Time spent in both the black and white compartments was monitored for 15 min. The percentage of time spent in the novel compartment was then calculated as the duration in the novel compartment, divided by the sum of the duration in both the novel and familiar compartments. Amphetamine Self-administration. Animals were surgically implanted with a chronic indwelling jugular catheter under anesthesia. Following recovery from surgery, phase 1 began. Phase 1: Autoshaping. Acquisition of amphetamine self-administration began using an autoshaping procedure while animals were food restricted (20 g/day). For 5 consecutive days, rats were given a 60-min autoshaping session, followed 30 min later by a 60-min contingent amphetamine self-administration session. During the autoshaping session, the house light was on and the inactive lever (no programmed consequence) was extended. For the first 15-min of each autoshaping session, the active lever was extended 10 times at random intervals and remained extended for 10 sec. If the lever was pressed, a 0.1 mg/kg infusion of amphetamine was delivered immediately; if the lever was not pressed during the 10-sec extension, a non-contingent infusion of amphetamine was delivered when the lever retracted. For the remaining 45 min, only the inactive lever was extended. For the contingent amphetamine self-administration session, both levers were extended and 0.1 mg/kg infusions of amphetamine were contingent using a FR1/20-sec signaled time out schedule of reinforcement; during the 20-sec time out interval, both cue lights above the levers were illuminated and responding was not reinforced. Following the 5 days of autoshaping, rats underwent 3 more days of 60-min contingent self-administration sessions under food restriction. Rats were then placed on free feed and phase 2 began. Phase 2: Incremented FR Schedule. Self-administration continued with the schedule of reinforcement being incremented from FR1 to FR5, with rats spending 3 days on each FR value. Phase 3: Dose Response. Following phase 2, rats then spent 3 consecutive days on the FR5 at each of the following doses, assessed in descending dose order: 0.056 mg/kg/infusion, 0.03 mg/kg/infusion, 0.01 mg/kg/infusion, and 0.001 mg/kg/infusion. Figure 3. Strain Differences Across Incrementing FR Values in Amphetamine Self-Administration. An ANOVA revealed a significant interaction between strain and FR value, F(44,212) = 1.52, p<.03. Number of Self-Infusions Figure 4. Strain Differences Across Varying Doses of Amphetamine. An ANOVA revealed a significant interaction between strain and dose, F(44,180) = 2.56, p<.001. Individual strain main effects of dose were observed for the following strains: BUF, SHR, and DSS, p<.05. Number of Self-Infusions Conclusions Reliable acquisition of amphetamine self-administration was obtained using an autoshaping procedure in outbred Sprague-Dawley rats. Significant differences in locomotor activity, novelty preference and amphetamine self-administration were obtained across different inbred rat strains, indicating a genetic influence for each of these behaviors. For most strains, the number of amphetamine infusions earned was similar across gradually increasing FR values, suggesting that intake was regulated for each strain. Strain-dependent differences in the dose effect function for amphetamine self-administration were obtained. Among inbred strains, there was no relation between locomotor activity in inescapable novelty and amphetamine self-administration on the terminal FR5 schedule. Among inbred strains, there was a relation between novelty preference and amphetamine self-administration on the terminal FR5 schedule, suggesting a genetic link between novelty seeking and drug abuse vulnerability. ACKNOWLEDGMENTS We acknowledge the expert consultation of Dr. John Crabbe. Research supported by USPHS grant DA05312.