A Novel Procedure for Evaluating the Reinforcing Properties of Tastants in Laboratory Rats: Operant Intraoral Self-administration

Summary

The current study evaluates a novel procedure for assessing the reinforcing effects of palatable solutions in laboratory rats: intraoral self-administration. To this end, operant responding (i.e. lever pressing) for intraoral infusions of sweet solutions at different concentrations was measured on continuous and progressive ratios schedules of reinforcement.

Abstract

This paper describes a novel method for studying the bio-behavioral basis of addiction to food. This method combines the surgical component of taste reactivity with the behavioral aspects of operant self-administration of drugs. Under very brief general anaesthesia, rats are implanted with an intraoral (IO) cannula that allows delivery of test solutions directly in the oral cavity. Animals are then tested in operant self-administration chambers whereby they can press a lever to receive IO infusions of test solutions. IO self-administration has several advantages over experimental procedures that involve drinking a solution from a spout or operant responding for solid pellets or solutions delivered in a receptacle. Here, we show that IO self-administration can be employed to study self-administration of high fructose corn syrup (HFCS). Rats were first tested for self-administration on a progressive ratio (PR) schedule, which assesses the maximum amount of operant behavior that will be emitted for different concentrations of HFCS (i.e. 8%, 25%, and 50%). Following this test, rats self-administered these concentrations on a continuous schedule of reinforcement (i.e. one infusion for each lever press) for 10 consecutive days (1 session/day; each lasting 3 hr), and then they were retested on the PR schedule. On the continuous reinforcement schedule, rats took fewer infusions of higher concentrations, although the lowest concentration of HFCS (8%) maintained more variable self-administration. Furthermore, the PR tests revealed that 8% had lower reinforcing value than 25% and 50%. These results indicate that IO self-administration can be employed to study acquisition and maintenance of responding for sweet solutions. The sensitivity of the operant response to differences in concentration and schedule of reinforcement makes IO self-administration an ideal procedure to investigate the neurobiology of voluntary intake of sweets.

Introduction

The study of the neurobiological and behavioral basis of addiction to food relies on the observations that, similarly to drugs of abuse, excessive consumption of palatable foods promotes behavioral dependence^1-4 and induces alterations in brain reward circuits in both humans^5-6 and laboratory animals^7-8. But, while there are several protocols and procedures for studying the addictive properties of drugs of abuse in laboratory rats, adapting these methods to assess "addictive" behaviors induced by food presents unique challenges. The goal of the current study was to apply principles of operant intravenous (IV) drug self-administ....

Protocol

1. Animal Care and Housing

All experiments are approved by the Animal Care Committee of the University of Guelph, and are carried out in accordance with the recommendations of the Canadian Council on Animal Care. The current study includes 22 adult male Sprague-Dawley rats (Charles River, QC) weighing 200-225 g at the beginning of the experiment.

1. Single house rats and maintain them on a reverse light/dark cycle (7:00 am OFF - 7:00 pm ON) with ad libitum access to food and water except when otherwise indicated, or during behavioral testing which occurs during the active cycle.
2. Habituate rats to the colony room and human....

Results

Acquisition of IO self-administration

Figure 3 represents operant responding on the active and inactive levers during self-administration of 8%, 25%, or 50% HFCS for 10 consecutive sessions. Rats acquire IO self-administration of HFCS within the first three sessions and maintain stable active lever responding (as described above) for the remainder of acquisition. Concentration-dependent differences in active lever responses are also evident: compared to 25%, responding i.......

Discussion

The current study illustrates a novel approach for evaluating the reinforcing properties of sweet solutions by combining methods traditionally employed to study behavior motivated by drugs of abuse (intravenous operant self-administration) and to assess the palatability of tastants (taste reactivity through intraoral infusions). To this end, under very brief isoflurane-induced anesthesia, rats are implanted with an IO cannula that allows for the delivery of test solutions directly into their mouth. Following recovery fro.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Reagents
Meloxicam	Boehringer Ingelheim Canada Ltd.		From the Ontario Veterinary College pharmacy
Procaine Penicillin G	Pen Aqueous, Wyeth Animal Health		From the Ontario Veterinary College pharmacy
Lidocaine HCl 2%			From the Ontario Veterinary College pharmacy
Marcaine 0.5 %			From the Ontario Veterinary College pharmacy
Lubricating eye ointment			Product can be bought at any pharmacy
2% Lidocaine Viscous Oral (Topical Anesthetic)	Pharmascience Inc.	CDMV # 14705	100 ml bottle
Isoflurane USP	Pharmaceutical Partners of Canada	CDMV # 108737	250 ml bottle
Bacti-Stat	Merck Sante Animale	CDMV # 6449	3.785 L bottle
Isopropyl alcohol (70%)	Perdu Pharma	Fisher # MPX18404	4 L bottle
Betadine 10%	McKesson Canada	CDMV # 104826	500 ml bottle
Super Germiphene	Ceva Animal Health	CDMV # 103629	454 ml bottle
Chlorhexidene (Novadent)	Zoetis	CDMV # 8908	236 ml bottle
High Fructose Corn Syrup	Natures Flavours	HFCS-55	1 gal bottle
Materials
PE90 tubing	Becton Dickinson and Company	VWR # CA-63019-080A	100 ft/coil
PE160 tubing	Becton Dickinson and Company	VWR # CA-63018-747	100 ft/coil
Polypropylene Mesh	Small Parts Inc.	CMP-0297-D
Soldering iron			Product can be bought at any hardware store
#64 Elastic bands	Staples Office supplies	Item # 13556	Product can be bought at any office supply store
15 G Thin-walled 3.5 in needles		VWR # CABD1108	12 needles/pack
Electric razor (1/2 in wide blade)			Product can be bought at any pet supply store
Precision Glide Needles 20 G needles (1½ in)	Becton Dickinson and Company	Fisher # 14-826D
Precision Glide Needles 16 G needles (1½ in)	Becton Dickinson and Company	Fisher # 14-826-5D
Operant conditioning chambers	Med Associates Inc.	ENV-008-CTC
Sound attenuating chamber	Med Associates Inc.	ENV-018M
MED PC IV software	Med Associates Inc.	SOF-735
Syringe Pumps	Razel Scientific Instruments
Disposable plastic swivel assembly	Med Associates Inc.	PHM-115I
Tygone Microbore tubing	Saint Gobain Performance Plastics	Fisher # 1417015B	500 ft/coil