Background

In rats, pairing a novel flavored solution with nausea produced by the administration of emetic drugs such as lithium chloride (LiCl) results in the subsequent reduction in consumption of that flavor, a learning phenomenon termed conditioned taste aversion (CTA) (see Reilly and Schachtman, 2009 for a review). Critically, not only does CTA result in decreased consumption of that flavor, but there is also evidence of a change in its palatability or hedonic qualities, as suggested by John Garcia, the pioneer in studying this learning phenomenon (Garcia et al., 1955). However, CTA is not exclusively produced by emetic treatments. Pairing flavors with a wide variety of other aversive events, including pain produced by footshock or injection of hypertonic saline, as well as the administration of many drugs of abuse, reliably produces reductions in voluntary consumption (e.g., Pelchat et al., 1983; Parker, 1995; Arthurs et al., 2012). Thus, one central issue in this protocol is whether emetic and non-emetic treatments produce the same sorts of behavioral affective reactions to fluids. The taste reactivity (TR) protocol we describe here can be generalized for many uses, including determining whether a novel substance or intervention is aversive because it produces pain or nausea (and similarly whether a treatment ameliorates pain/nausea). Additionally, this procedure is amenable to be adapted for studying the neural mechanisms of hedonic evaluation (Inui and Simura, 2017), or for examining the role of anhedonia in psychiatric and neurological disorders such as depression and schizophrenia (Ward et al., 2012).

The optimal method affording a direct assessment of the hedonic impact of flavors is the TR test (Grill and Norgren, 1978). This involves examining orofacial reactions—stereotyped oromotor and somatic consummatory responses—elicited during the intraoral infusion of the fluid in the rat’s oral cavity. These orofacial responses can be classified as aversive (e.g., gaping, chin rubbing, and paw treading), elicited when infused with unpleasant sour or bitter tastes, or appetitive (e.g., tongue protrusions, mouth movements, and paw licks), elicited by pleasant, sweet tastes. When infused with a palatable taste (e.g., saccharin) previously paired with LiCl-induced nausea, rats display aversive responses, reflecting a shift in the hedonic value of the flavor from positive to negative. Importantly, this technique is selective to disgust, in contrast to the reduction in consumption that may simply reflect taste avoidance without a change in affective responses. As mentioned above, pairing a flavor with peripheral pain or with rewarding drugs, such as cocaine or amphetamine, results in a reduction in subsequent voluntary consumption of that flavor, but not in the production of orofacial aversive responses (Parker, 1995). This dissociation in the impact of emetic drugs and other aversive events supports the suggestion that a reduction in fluid intake may be motivated by two different processes: a taste associated with nausea causes a reduction in its palatability (CTA), whereas a taste associated with a drug of abuse or peripheral pain is avoided because it signals a potential danger or a disturbance in homeostasis regulation [taste avoidance learning (TAL)] (see Parker 2003, 2014).

The hedonic value of flavors can also be examined by analyzing the microstructure of licking behavior during voluntary consumption (Davis, 1989; Dwyer, 2012). The ingestive behavior of rodents consuming fluids consists of sustained runs of licks separated by pauses of varying length (clusters); the mean number of licks in a cluster (lick cluster size) is directly related to the nature and concentration of the solution ingested. Lick cluster size shows a positive monotonic relationship to the concentration of palatable sweet solutions, decreasing monotonically with an increased concentration of unpalatable quinine solutions. In the context of taste aversion learning, pairing an otherwise palatable taste with nausea results in a reduction of lick cluster size, similar to that produced by exposure to quinine. However, reductions in cluster size cannot be unambiguously attributed to conditioned disgust (for example, these do not distinguish between the effects of hypertonic saline and isotonic lithium chloride as described here) in the absence of careful observation of orofacial behavioral reactions directly indicative of a particular emotional response. With this issue in mind, here we use the orofacial reactivity protocol to selectively characterize hedonic responses to flavors paired with either nausea or internal pain caused by injection of hypertonic saline.