Joyce Besheer and Clyde Hodge in their Behavioral Pharmacology and Pharmacogenomics Laboratory at the Bowles Center for Alcohol Studies located on the campus of The University of North Carolina at Chapel Hill.

Using Drug Discrimination Procedures to Examine the Stimulus Properties of Ethanol

Joyce Besheer and Clyde Hodge
Bowles Center for Alcohol Studies
University of North Carolina – Chapel Hill

Drugs of abuse produce distinctive subjective effects in humans (e.g., the feeling of “drunkenness” that can accompany alcohol drinking).  These subjective effects play a major role in the onset and maintenance of drug taking behaviors.  We are studying the discriminative stimulus properties (i.e., subjective effects) of ethanol in rats, using a two-lever drug discrimination procedure.  Briefly, rats are trained to discriminate the stimulus properties of ethanol (1 g/kg) vs. water as described below and illustrated in Figure 1.  We use standard modular test chambers (ENV-008) placed within sound-attenuating cubicles (ENV-022MD).  Each cubicle is equipped with an exhaust fan that provides ventilation and also serves to mask external sounds.  The chambers are set up such that two response levers (ENV-112CM) are located on the right wall of each chamber.  A stimulus light is located above each response lever and a liquid dispenser (ENV-202M) fitted with a 0.1 ml dipper cup is centered between the levers.  The chambers are illuminated by an 8-W light located on the middle of the left wall 28 cm from the floor of the chamber.  The chambers are interfaced to a computer that uses MED-PC IV software (SOF-735) to control the sessions and record data.

Figure 1: Schematic depiction of the training and testing procedures used for examining discriminative stimulus properties of drugs.  Adapted from Hodge et al., 2006, Alcohol Clin Exp Res.

On a typical ethanol training session, rats receive ethanol (1g/kg IG) and are immediately placed in the chamber.  For the first 10 min of the session, the chambers are dark, the levers are retracted, and the dipper is in the down position so that the animals do not have access to the fluid.  After 10 min, the house light is turned on, and both levers are introduced into the chamber signaling the start of the session.  The schedule of reinforcement for the sessions is FR10.  That is, after ten responses on the ethanol-appropriate lever (e.g., left lever) the dipper arm is raised and animals are presented with access to a 10% (w/v) sucrose solution for 4 sec and then the dipper arm is returned to its lowered position.  Responses on the other lever (e.g., right lever) are recorded but do not produce any consequence.  The session continues as described and after 15 min the levers are retracted and the house light is turned off signaling the end of the session.  On a water training session, the same procedure is used except that water is injected before placement in the chamber and the other lever (e.g., right lever) is now the “active” lever.  Water (W) and ethanol (E) training days are varied on a double alternation schedule (W, W, E, E …).  After approximately 20 training sessions of each (ethanol and water), animals discriminate ethanol from water accurately and we are able to begin testing.  We use test sessions to assess whether various compounds produce ethanol-like effects or interfere with ethanol’s discriminative stimulus properties (see Figure 1).  Test sessions are similar to the training sessions except that 10 responses on either lever produce access to the sucrose reinforcement and the sessions are 2 min in duration.

	Figure 2: GABAA receptors in the nucleus accumbens and the amygdala mediate the discriminative stimulus properties of ethanol.  Adapted from Besheer et al. 2003; Hodge and Cox 1998.

To date we have used this procedure to investigate involvement of specific brain regions and interaction between brain regions in ethanol’s stimulus properties.  Using microinjections we have found that GABA_A receptors in the nucleus accumbens and amygdala mediate the discriminative stimulus properties of ethanol and play a more influential role in ethanol’s discriminative stimulus properties than GABA_A receptors in other brain regions (Figure 2). We have also shown that animals trained to discriminate between ethanol and water are sensitive to the discriminative stimulus properties of consumed (i.e., self-administered) ethanol.  Further, we have identified the involvement of a novel receptor system in ethanol’s stimulus properties (i.e., metabotropic glutamate receptor-subtype 5).  In all, we continue to use the ethanol discrimination procedure to investigate the neurobiology of ethanol’s stimulus properties.  Ultimately, understanding these mechanisms has numerous implications for development of therapeutic interventions in alcoholism, especially given that the discriminative stimulus properties of drugs can be important determinants of abuse liability.

Hodge CW, Grant KA, Becker HC, Besheer J, Crissman AM, Platt DM, Shannon EE, Shelton KL (2006). Understanding how the brain perceives alcohol: neurobiological basis of ethanol discrimination. Alcohol Clin Exp Res., 30: 203-213.

Besheer J, Cox AA, Hodge CW (2003). Coregulation of ethanol discrimination by the nucleus accumbens and amygdala. Alcohol Clin Exp Res., 27:450-456.

Hodge CW, Cox AA (1998). The discriminative stimulus effects of ethanol are mediated by NMDA and GABA(A) receptors in specific limbic brain regions. Psychopharmacology (Berl). 139:95-107.