Single, but not multiple pairings of sucrose and corticosterone enhance memory for sucrose drinking and amplify remote reward relativity effects

This study tested whether pre-training pairings of ingestion of a 32% sucrose solution and injection(s) of corticosterone (B) would enhance later ingestion in the absence of B, and whether these effects would carry over into later contrast-like effects when animals were subsequently shifted to 4% sucrose. Frequency-dependence of these pairings was also examined. Three groups of male Sprague-Dawley rats were adrenalectomized (ADX). A fourth group was sham ADX. Each ADX group received three presentations of sucrose and B (666 microg/kg, s.c.). One received unpaired presentations (separated by days), one received two unpaired presentations and one paired (i.e., simultaneous) presentation, and one received three paired presentations. Shams received three sucrose presentations paired with saline. Single, but not multiple pairings of B with ingestion of a 32% sucrose solution enhanced later sucrose ingestion, a memorial-like effect that carried over into later, opposite contrast-like effects upon presentation of a less-preferred 4% sucrose solution. These effects could not be easily ascribed to differences in training, other than the pairing regimen itself, nor to motivational differences at the time of testing, and were presumed to be memorial. The pairing and frequency-dependence of these appetitive phenomena are analogous to what is frequently observed during acute or chronic exposure to aversive situations and/or neuromodulatory stress hormones, in terms of their bidirectional effects on memory. Through effects on memory, stress hormones may modulate reward and reward relativity.     

An Examination of Sex and Masculinity/Femininity as Related to the Taste Sensitivity of Japanese Students

The purpose of the present study was to examine the influences of gender differences and masculinity–femininity on taste thresholds and food preferences. The participants were 108 Japanese undergraduate students (44 men and 64 women); their mean age was 19.6 years (SD = 1.3). Their detection and recognition thresholds for caffeine and sucrose were measured. Multiple regression analyses showed that women tended to have a lower detection threshold for caffeine than men did. The recognition threshold for caffeine was positively associated with scores for masculinity. For men, masculinity may be associated with smoking and drinking behavior, thereby resulting in a lower sensitivity for bitterness. Masculinity and femininity related to food preferences are also discussed.     

Evidence for associative learning in newly emerged honey bees (Apis mellifera)

The honey bee is a model organism for studies on the neural substrates of learning and memory. Associative olfactory learning using sucrose rewards is fast and reliable in foragers and older hive bees. However, researchers have so far failed to show any significant learning in newly emerged bees. It is generally argued that in these bees only part of the brain structures important for learning are fully developed. Here we show for the first time that newly emerged honey bees are capable of associative learning, if they are sufficiently responsive to sucrose. Responsiveness to sucrose, which can be measured using the proboscis extension response (PER), increases with age. Newly emerged bees are on average very unresponsive to sucrose. We show that if newly emerged bees displaying a PER to 10% sucrose or lower sucrose concentrations are conditioned to an odour, they show significant associative learning and early long-term memory. Nevertheless, the level of acquisition is still lower than in foragers. The general assumption that newly emerged honey bees are incapable of associative learning must therefore be reconsidered. Further, our study suggests that an age-dependent increase in responsiveness to rewarding stimuli is directly related to the development of early learning abilities. The decisive influence of responsiveness to rewarding stimuli in associative learning of newly emerged bees has far reaching consequences for studies on the development of associative learning capabilities in insects and vertebrates.     

Learning in honey bees with brain lesions: how partial mushroom-body ablations affect sucrose responsiveness and tactile antennal learning

Honey bees are ideal organisms for studying associative learning, because they can rapidly learn different sensory cues, even under laboratory conditions. Classical olfactory learning experiments have shown that the mushroom bodies (MBs), a prominent neuropil of the central nervous system of the bee, are involved in olfactory learning and memory formation. We tested whether the MBs are also involved in tactile antennal learning. As in olfactory learning, bees use the antennae during tactile learning to sense tactile cues. We produced specific MB ablations by applying the mitotic blocker hydroxyurea (HU). In Drosophila, HU-induced brain lesions of the MBs strongly impaired olfactory memory formation. As treatment with HU might also interfere with the processing of the reward stimulus, sucrose, we measured the responsiveness to sucrose stimuli in these bees. Treatment with HU led to partial ablations of the median MB sub-units on one or both sides of the brain. We analysed side-specific effects in double-blind tests, testing sucrose responsiveness separately for each antenna, and conditioning first one antenna and then the other in a reversal learning assay. HU-treated bees without detectable ablations were less responsive to sucrose and had a poorer learning performance than untreated controls. Partial MB ablation did not additionally affect responsiveness to sucrose or tactile antennal learning. Interestingly, bees with bilateral MB ablations did not differ from untreated controls in their learning performance during the first learning phase. During reversal learning, acquisition in these bees was significantly lower than that in untreated controls. It is concluded that HU treatment affects sucrose responsiveness and tactile learning even without detectable ablation of neuropils. The effects of MB ablations on tactile learning are not side-specific and not correlated with the volume of the ablated neuropil. Accepted after revision: 15 January 2001 ❚ Electronic Publication     

Repeated cocaine experience facilitates sucrose-reinforced operant responding in enriched and isolated rats

The purpose of the present experiment was to determine whether repeated cocaine exposure differentially affects sucrose-reinforced operant responding in rats raised in an enriched condition (EC) or an isolated condition (IC). Specifically, the performance of EC and IC rats pressing a lever for sucrose under a high fixed-ratio schedule (FR 30) prior to and after 10 days of exposure to cocaine (15 mg/kg, i.p.) or saline was compared. Regardless of rearing condition, rats repeatedly exposed to cocaine had shorter reacquisition latencies to complete a sucrose-reinforced FR 30 task than saline controls. The results suggest that cocaine exposure may have cross-sensitized both EC and IC rats to the reinforcing effects of sucrose or sucrose-associated cues, thus facilitating reacquisition of operant responding.