D.M. Dwyer

1. Flavour preference learning and palatability: Impairments by basolateral, but not central, amygdala lesions. D.M. Dwyer

2. Flavour preference learning • A preference for a neutral or disliked flavour can be created in a several ways. For example: • Pairing with nutrients • Pairing with a particularly palatable flavour. • Conditioning produces a preference for the CS+ over the CS- and to some extent an increase in CS+ consumption in extinction. • But beyond the consumption differences little is known about the effects of preference conditioning or the mechanisms underpinning this consumption change.

3. Change in CS palatability? • The examination of orofacial responses in the taste-reactivity test is one example of analyses that go beyond mere consumption to examine “palatability”. • Some, but not all, conditioned preferences are accompanied by an increase in CS palatability. • Compare Myers & Sclafani (2001) with Myers & Sclafani (2003). • But taste reactivity method is less than ideal for many reasons. • E.g. it is very labour intensive, creates a context change, only examines short exposures to fluids.

4. Microstructural analysis of licking • Rats consuming fluids produce sustained runs of rapidly occurring rhythmic licks (clusters) separated by pauses of varying lengths. • Mean cluster size has a positive monotonic relationship with concentration of sucrose (& other palatable flavours) and a negative relationship with concentration of unpalatable quinine. • This supports the idea that cluster size is an index of stimulus palatability (and there is further support from pharmacological studies). • Other factors (e.g. inter-lick interval within clusters) relatively unaffected by most manipulations.

5. Current aims • Lick analysis has a number of practical and theoretical advantages over taste reactivity. • E.g. it can be automated & applied to entire drinking sessions without any impact on the animal observed. • Current study aimed at examining lesion effects in flavour preference conditioning (basolateral v central amygdala) with fructose as the reinforcer. • Almost no cross-talk between microstructure & taste reactivity literatures. • So current study should also re-emphasise common ground between these methods of assessing palatability.

6. Experimental background • Lesions of the whole amygdala have been shown to completely prevent flavour preference learning with nutrient and palatability reinforcers • IG maltodextrin: Touzani & Sclafani, 2005; Oral sucrose: Gilbert et al., 2003; Oral saccharin: Sakai & Yamamoto, 2001. • But large lesions mean rather unspecific neural basis for these effects. • Consider the debate about whether damage to fibres of passage underpins the inconsistent effects of amygdala lesions on CTA. • Thus current study examined effects of BLA and CN lesions on palatability based preferences.

7. Methods • Naïve hooded Lister: 12 sham lesions, 15 quinolinic acid lesions of the BLA, 14 NMDA lesions of the CN. • Note: originally 20 per lesion group but N reduced after histology. • All 85% of free feeding weight during training and test. • CSs = 0.05% grape & cherry Kool Aid plus 0.1% saccharin. US = 8% Fructose. • 4 CS+ and 4 CS- sessions given. All sessions 30min. • 2-bottle test (CS+ v CS-) then 2 × 1-bottle tests (one each with CS+ & CS-) in extinction. Pattern of tests repeated and data averaged over test sessions.

8. 2-Bottle Consumption 12 10 8 CS+ Consumption (g) 6 CS- 4 2 0 Sham BLA CN Lesion • Preferential consumption of CS+ over CS- in all groups. • But size of preference reduced in BLA group compared to both Sham and CN (which did not differ).

9. 1-Bottle Consumption 14 12 10 CS+ 8 Consumption (g) 6 CS- 4 2 0 Sham BLA CN Lesion • Clear interaction between lesion condition & CS. • Sham and CN lesions show higher consumption of CS+ than CS- but BLA does not.

10. 1-Bottle Cluster Size 40 35 30 25 Cluster Size 20 15 10 5 0 Sham BLA CN Lesion • Clear interaction between lesion condition & CS. • Sham and CN lesions show higher cluster sizes with CS+ than CS- but BLA does not.

11. Results Summary • Analysis of cluster size suggests that preference conditioning increaces palatability of the CS+. • BLA lesions reduced, but did not totally prevent, palatability based preference learning. • Sham lesioned animals displayed enhanced palatability reaction to CS+. This not seen in BLA lesioned animals but is seen in CN lesions.

12. Discussion • Conditioned change in cluster size consistent with examination of orofacial reactions. • Supports the idea that both techniques are sensitive to the same sort of manipulations. • BLA lesion prevents conditioned changes in palatability and additional consumption of the CS+ in 1-bottle tests. • Thus palatability change appears to contribute to additional CS+ consumption in 1-bottle tests. • Palatability change cannot underpin all of the conditioned change in CS preference produced by pairing the CS+ with a palatable taste as BLA animals still showed some preference conditioning without palatability change.

13. General Summary • Many other applications of the lick-analysis technique are possible. For example I have used this to examine: • The effects of NMDA antagonists on palatability. • How taste aversions based on different mechanisms (e.g. nausea, activity or drugs of abuse) can actually have different effects of the palatability of the avoided food. • How contrasts between foods can lead to an inflation of the palatability differences between them. • Whether chronic social stress might reduce the hedonic reaction to sucrose. • Basically, in any rat (possibly also mouse) model where the question of “how nice” a substance might be is raised, lick analysis might provide an answer.