Effects of midazolam on the expression of conditioned taste aversion in rats

In conditioned taste aversion (CTA), the animals learn to avoid a taste substance (conditioned stimulus, CS) which was previously associated with visceral distress (unconditioned stimulus, US). The present study examined the effects of administration of midazolam (MDZ), a benzodiazepine agonist, after the acquisition of CTA on the expression of CTA. After ingestion of 0.5 M sucrose (CS) was paired with an intraperitoneal (i.p.) injection of 0.15 M LiCl (US), control rats showed strong CTA to the CS. However, a systemic injection of MDZ (1.5 mg/kg, i.p.) before the retention test prevented conditioned animals from rejecting the CS, but in the subsequent retention tests where the drug was not administrated, those animals again showed strong aversions to the CS. Aversive taste reactivity patterns to the intraorally infused sucrose and 0.3 M dl-alanine in the conditioned animals were also diminished by the similar injection of MDZ, but not by a serotonergic anxiolytic agent, buspirone (2.5 or 5.0 mg/kg, i.p.). General taste sensory deficit might not be induced by MDZ because the drug injection did not impair conditioned aversions to 0.2 M NaCl and 0.01 M HCl. Infusion of MDZ into the basolateral nucleus of the amygdala (BLA) also attenuated conditioned aversions to sucrose. These results suggest that systemic or intra-BLA administrations of MDZ impair the expression of CTA selectively to sweet-tasting substances, implying that a transient MDZ-induced CTA expression deficit is due to the enhancement of palatability of CSs with preferable tastes rather than general anxiolytic or amnesic effects of MDZ.