Distinct progressions of neuronal activity changes underlie the formation and consolidation of a gustatory associative memory

Acquiring new memories is a multi-stage process. Ample of studies have convincingly demonstrated that initially acquired memories are labile, and only stabilized by later consolidation processes. These multiple phases of memory formation are known to involve modification of both cellular excitability and synaptic connectivity, which in turn change neuronal activity at both the single neuron and ensemble levels. However, the specific mapping between the known phases of memory and the observed changes in neuronal activity remains unknown. Here we address this unknown in the context of conditioned taste aversion learning by continuously tracking gustatory cortex (GC) neuronal taste responses from alert rats in the 24 hours following a taste-malaise pairing. We found that the progression of neuronal activity changes in the GC depend on the neuronal organizational level. The population response changed continuously; these changes, however, were only reflected in the population mean amplitude during the acquisition and consolidation phases, and in the known quickening of the ensemble state dynamics after the time of consolidation. Together our results demonstrate how complex dynamics in different representational level of cortical activity underlie the formation and stabilization of memory within the cortex.

LTD at amygdalocortical synapses as a novel mechanism for hedonic learning

A novel, pleasant taste stimulus becomes aversive if associated with gastric malaise, a form of learning known as conditioned taste aversion (CTA). CTA is common to vertebrates and invertebrates and is an important survival response: eating the wrong food may be deadly. CTA depends on the gustatory portion of the insular cortex (GC) and the basolateral nucleus of the amygdala (BLA) however, its synaptic underpinnings are unknown. Here we report that CTA was associated with decreased expression of immediate early genes in rat GC of both sexes, and with reduced amplitude of BLA-GC synaptic responses, pointing to long-term depression (LTD) as a mechanism for learning. Indeed, association of a novel tastant with induction of LTD at the BLA-GC input in vivo was sufficient to change the hedonic value of a taste stimulus. Our results demonstrate a direct role for amygdalocortical LTD in taste aversion learning.

Encounters between freshwater crocodiles and invasive cane toads in north-western Australia: does context determine impact?

ABSTRACT The potent defensive chemicals of cane toads (Rhinella marina) protect them against predators that lack coevolved physiological tolerance to those toxins. That relative invulnerability may explain why major injuries (such as limb loss) appear to be rare in cane toads from most of their global range; however, we noted frequent predator-induced injuries (>4% of adults) in samples from within the toad’s native range (in French Guiana) and from a site (Lake Argyle) in north-western Australia. Toads at Lake Argyle enter the edge of the lake at night to rehydrate, exposing them to foraging freshwater crocodiles (Crocodylus johnstoni). Crocodiles rarely consume toads, but the attacks often result in loss of a limb. Because limbs contain relatively little toxin, attacks to the limbs expose a crocodile to nauseating but non-lethal amounts of toxin; and hence, facilitate taste aversion learning by the predator. The context of the encounters, such as differences in geography, may help to explain why the invasion of cane toads has not significantly impacted on crocodile populations at this site, in contrast to heavy impacts reported from nearby riverine systems.

LTD at amygdalocortical synapses as a novel mechanism for hedonic learning

A novel, pleasant taste stimulus becomes aversive if associated with gastric malaise, a form of learning known as conditioned taste aversion (CTA). CTA is common to vertebrates and invertebrates and is an important survival response: eating the wrong food may be deadly. CTA depends on the gustatory portion of the insular cortex (GC) and the basolateral nucleus of the amygdala (BLA) however, its synaptic underpinnings are unknown. Here we report that CTA was associated with decreased expression of immediate early genes in rat GC of both sexes, and with reduced amplitude of BLA-GC synaptic responses, pointing to long term depression (LTD) as a mechanism for learning. Indeed, association of a novel tastant with induction of LTD at the BLA-GC input in vivo was sufficient to change the hedonic value of a taste stimulus. Our results demonstrate a direct role for amygdalocortical LTD in taste aversion learning.