Delay Discounting and Impulsive Choice in the Rat

Adam C. Mar1, Trevor W. Robbins1

1 Department of Experimental Psychology and the Behavioural and Clinical Neuroscience Institute University of Cambridge, Cambridge
Publication Name:  Current Protocols in Neuroscience
Unit Number:  Unit 8.22
DOI:  10.1002/0471142301.ns0822s39
Online Posting Date:  April, 2007
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Abstract

Delay discounting refers to the degree to which immediate outcomes exhibit more influence over behavior than outcomes which are delayed. Impulsive choice, in the context of delay discounting, is generally considered as an increased preference for immediate over delayed outcomes, even where the delayed outcomes are more advantageous. In the past decade, there has been increasing use of delay‚Äźdiscounting paradigms to elucidate the physiological, pharmacological, and behavioral aspects of the putative neural circuitry underlying impulsive choice. This unit describes the assessment of impulsive choice in the rat using a delay‚Äźdiscounting procedure involving an operant response choice between a small reinforcer delivered immediately and a larger reinforcer delivered after a delay, which is progressively increased within a session. Variations of some of the main task parameters are also discussed, as well as their significance and interpretation.

Keywords: delay discounting; delayed reward; impulsive choice; impulsivity; temporal discounting; delayed reinforcement; decision making

     
 
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Table of Contents

  • Basic Protocol 1: A Progressive Delay‐Discounting Procedure for Rats
  • Alternate Protocol 1: Omission of Delays to Evaluate Sensitivity to Delay Duration and Preference for Reward Size
  • Alternate Protocol 2: Inclusion of Cued Delays to Hasten Acquisition of Delay Sensitivity And/Or To Alter Preference for the Delayed Reward
  • Alternate Protocol 3: Use of Descending Order of Delays to Address Preference Bias Due to use of Progressively Increasing Delays
  • Alternate Protocol 4: Assessment of Lesions, in Vivo Pharmacology, and Neurochemistry on Delay Discounting
  • Alternate Protocol 5: Modification of Other Task Parameters in Delay‐Discounting Procedures
  • Commentary
  • Literature Cited
  • Figures
     
 
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Materials

Basic Protocol 1: A Progressive Delay‐Discounting Procedure for Rats

  Materials
  • 2.5‐ to 4‐month‐old male rats, 350 to 400 g (e.g., Lister Hooded from Charles River Laboratories); typically, a minimum of eight rats per between‐subjects level
  • Standard laboratory rodent diet food pellets (e.g., Purina Laboratory Chow)
  • Food reward pellets (e.g., TestDiet 45 mg precision‐weight, purified‐ingredient rodent tablets)
  • Nontoxic, permanent marker (e.g., Impega solvent‐free permanent marker)
  • Operant Chambers with 30.5 cm long × 24.1 cm wide × 21.0 cm high inner working area (e.g., Med Associates, Campden Instruments, Lafayette Instruments) or constructed according to the following specifications (see Fig. ):
    • Two transparent polycarbonate side‐wall panels, one hinged to serve as a door entry for the animal (a and b in Fig. A)
    • Polycarbonate chamber roof: can include an optional hole (covered when not in use or small enough to ensure that the animal cannot escape from the chamber) to facilitate optional techniques such as infusions, microdialysis and/or electrophysiological recordings; c in Fig. A,B
    • Grid floor (d in Fig. A,B) consisting of parallel, 4.5‐mm stainless steel rods spaced 1.6 cm apart (center to center) and mounted 4 cm over a stainless steel waste pan (29.5 cm long × 24.1 cm wide × 1.9 cm high)
    • Aluminum back and front panels each with three modular columns to house input and/or output devices (e in Fig. A)
    • House light (28 V DC, 100 mA, 2.5 W) mounted 1 cm from the top of the central column of the back panel (f in Fig. A)
    • Two retractable response levers with adjustable tension mounted 8 to 10 cm above the grid floor in the left and right columns of the front panel (g in Fig. B)
    • Trough‐type food pellet receptacle (5.1 cm × 5.1 cm) mounted 2 cm above the grid floor in the central column of the front panel (h in Fig. B)
    • Continuous infrared photobeam (i in Fig. B) mounted across the entrance of the pellet receptacle to detect nose pokes (head entries) into the receptacle (i.e., when collecting food)
    • One stimulus light (e.g., 28 V, 100 mA, 1 W, 2.5 cm diameter) mounted above or within the food pellet receptacle (j in Fig. B)
    • Magazine‐type food pellet dispenser mounted outside the operant chamber to deliver 45‐mg pellets into the receptacle (k in Fig. A,B)
  • Sound‐attenuating, fan‐ventilated cubicle (inner dimensions 55.9 cm long × 40.6 cm wide × 38.1 cm high; l in Fig. A) to enclose each operant chamber, pellet dispenser and interfacing hardware
  • Personal computer meeting minimum speed and memory requirements of the operant control software and having free expansion slots or ports as necessary (see Hardware below)
  • Hardware to interface the output (response data) and input (e.g., control of light and pellet delivery) lines of the operant chambers to a personal computer: available from operant chamber suppliers or designed in‐house with passive connection panels to interface the operant chambers' lines through the computer's ISA or PCI slots using digital input/output boards (e.g., Amplicon 272 series or Avantech 1753 cards)
  • Software to control the operant chamber: available from operant chamber suppliers programmed (see steps 12, 17, and 22) using the control software's user interface language or generic input/output control software with a delayed discounting end‐user program (e.g., Whisker; see ) or programmed with most available programming languages
  • 70% ethanol
  • Standard statistical software package (e.g., SYSTAT, STATISTICA, or SPSS)

Alternate Protocol 1: Omission of Delays to Evaluate Sensitivity to Delay Duration and Preference for Reward Size

  • Two stimulus lights per operant chamber (see protocol 1 and j in Fig. B)

Alternate Protocol 2: Inclusion of Cued Delays to Hasten Acquisition of Delay Sensitivity And/Or To Alter Preference for the Delayed Reward

  • As required by the technique employed (e.g., see unit 7.2 for information on microdialysis in rodents)
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Figures

Videos

Literature Cited

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Internet Resources
   http://www.whiskercontrol.com.
  Source for the Whisker version 2.2 computer program written by R.N. Cardinal and M.F.R, Aitken in 2001.
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