Parametric Changes in Response Equilibrium During an Intra-cranial Self Stimulation (ICSS) Task: Can Reward Value be Assessed Independently of Absolute Threshold?
Section snippets
INTRODUCTION
AS REVIEWED by Liebman [13], there are two fundamental assumptions underlying traditional bar-pressing measures of lateral hypothalamic ICSS used by pharmacologists: (i) response rate is predictive of the reward value of the stimulation, therefore; (ii) drug effects on response rates represent changes in reward. However, it is generally acknowledged that these assumptions are true only for a lower or submaximal range of stimulation intensities. For example, only if the current intensity is kept
Subjects and Surgical Procedures
Subjects were six adult male Sprague-Dawley rats (Charles-River, Wilmington, MA). Prior to surgery, and during experiments, subjects were individually housed in standard polycarbonate cages with free access to food and water. The colony room was maintained under a 12:12 light-dark cycle with lights on at 06.00. Each weighed 250–350 g at the time of surgery.
Subjects were anesthetized with a mixture of pentobarbital and chloral hydrate (EquithesinTM; 3.3 ml/kg, IP) and placed in the stereotaxic
RESULTS
Fig. 1 depicts rate-per-step data for subjects (n=6) responding under both autotitration schedules in the third of five consecutive constant-parameter trials. These data indicate that, although they begin with identical stimulation parameters (first 15 responses, step 1) and comparable rates, subjects responding during 5% current stepdowns exhibit a significantly (F(6,30) = 3.96) sharper drop-off in rate at subsequent steps than they do when responding during 5% frequency stepdowns. The sharper
DISCUSSION
Each schedule generated mean titration point data resulting from approximately 15–20 determinations within a 15-min trial. During five consecutive constant-parameter trials, titration point, rate, and number-of-resets data remained stable as subjects responded over the course of a 2.5-h testing session. This occurred under both autotitration schedules evaluated. Therefore, both autotitration tasks may serve as stable baseline schedules against which drug or parametric manipulations may be
Acknowledgements
This investigation was supported by NIH Grants DA00541 and DA03413, and by Research Scientist Award K05/DA00008 to S.G.H. Special thanks to Dr Darryl B. Neill for helpful discussion of the data.
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