Experience-dependent plasticity of gustatory insular cortex circuits and taste preferences.

Early experience with food influences taste preference in adulthood. How gustatory experience influences development of taste preferences and refinement of cortical circuits has not been investigated. Here, we exposed weanling mice to an array of taste solutions and determined the effects on the preference for sweet in adulthood. We demonstrate an experience-dependent shift in sucrose preference persisting several weeks following the termination of exposure. A shift in sucrose palatability, altered neural responsiveness to sucrose, and inhibitory synaptic plasticity in the gustatory portion of the insular cortex (GC) were also induced. The modulation of sweet preference occurred within a restricted developmental window, but restoration of the capacity for inhibitory plasticity in adult GC reactivated the sensitivity of sucrose preference to taste experience. Our results establish a fundamental link between gustatory experience, sweet preference, inhibitory plasticity, and cortical circuit function and highlight the importance of early life nutrition in setting taste preferences.

An Insula-Central Amygdala Circuit for Guiding Tastant-Reinforced Choice Behavior.

For animals to survive, they must reliably predict during foraging which substances are suitable for consumption. Despite extensive study, the neural circuit mechanisms underlying such adaptive behavior remain poorly understood. Here, using a tastant (sucrose/quinine)-reinforced "go/no-go" task in male and female mice, we examined the anatomical and functional connectivity of the circuit linking the insular cortex (IC) to the central amygdala (CeA) and the role of this circuit in the establishment of appropriate behavioral responses. Using anatomic tracing approaches combined with optogenetics-assisted circuit mapping, we found that the gustatory region of the IC sends direct excitatory projections to the lateral division of the CeA (CeL), making monosynaptic excitatory connections with distinct populations of CeL neurons. Specific inhibition of neurotransmitter release from the CeL-projecting IC neurons prevented mice from acquiring the "no-go" response, and impaired the "go" responses in the go/no-go task. Furthermore, selective activation of the IC-CeL pathway with optogenetics drove unconditioned lick suppression in thirsty animals, induced aversive responses, and was sufficient to instruct conditioned action suppression in response to a cue predicting the optogenetic activation. These results indicate that activities in the IC-CeL circuit are critical for establishing taste-reinforced behavioral responses, including avoidance responses to an aversive tastant, and are sufficient to drive learning of anticipatory avoidance. Our findings suggest that the IC-CeL circuit plays an important role in guiding appropriate choices during foraging.SIGNIFICANCE STATEMENT An animal's ability to predict which substances are suitable for consumption and then produce an appropriate action to those substances is critical for survival. Here we found that activity in the circuit that links the insular cortex (IC) to the central amygdala (CeA) is necessary for establishing appropriate behavioral responses to taste-predicting cues. This neural circuit seems to be particularly tuned to avoid an unpleasant tastant, and is also sufficient to drive learning of such avoidance responses. These results suggest that the IC-CeA circuit is critical for generating appropriate behavioral responses during foraging when facing different choices.

Characterization of the amplificatory effect of norepinephrine in the acquisition of Pavlovian threat associations.

The creation of auditory threat Pavlovian memory requires an initial learning stage in which a neutral conditioned stimulus (CS), such as a tone, is paired with an aversive one (US), such as a shock. In this phase, the CS acquires the capacity of predicting the occurrence of the US and therefore elicits conditioned defense responses. Norepinephrine (NE), through ß-adrenergic receptors in the amygdala, enhances threat memory by facilitating the acquisition of the CS-US association, but the nature of this effect has not been described. Here we show that NE release, induced by the footshock of the first conditioning trial, promotes the subsequent enhancement of learning. Consequently, blocking NE transmission disrupts multitrial but not one-trial conditioning. We further found that increasing the time between the conditioning trials eliminates the amplificatory effect of NE. Similarly, an unsignaled footshock delivered in a separate context immediately before conditioning can enhance learning. These results help define the conditions under which NE should and should not be expected to alter threat processing and fill an important gap in the understanding of the neural processes relevant to the pathophysiology of stress and anxiety disorders.

Primary auditory cortex regulates threat memory specificity.

Distinguishing threatening from nonthreatening stimuli is essential for survival and stimulus generalization is a hallmark of anxiety disorders. While auditory threat learning produces long-lasting plasticity in primary auditory cortex (Au1), it is not clear whether such Au1 plasticity regulates memory specificity or generalization. We used muscimol infusions in rats to show that discriminatory threat learning requires Au1 activity specifically during memory acquisition and retrieval, but not during consolidation. Memory specificity was similarly disrupted by infusion of PKMζ inhibitor peptide (ZIP) during memory storage. Our findings show that Au1 is required at critical memory phases and suggest that Au1 plasticity enables stimulus discrimination.

ß-Adrenergic Receptors Regulate the Acquisition and Consolidation Phases of Aversive Memory Formation Through Distinct, Temporally Regulated Signaling Pathways.

Memory formation requires the temporal coordination of molecular events and cellular processes following a learned event. During Pavlovian threat (fear) conditioning (PTC), sensory and neuromodulatory inputs converge on post-synaptic neurons within the lateral nucleus of the amygdala (LA). By activating an intracellular cascade of signaling molecules, these G-protein-coupled neuromodulatory receptors are capable of recruiting a diverse profile of plasticity-related proteins. Here we report that norepinephrine, through its actions on ß-adrenergic receptors (ßARs), modulates aversive memory formation following PTC through two molecularly and temporally distinct signaling mechanisms. Specifically, using behavioral pharmacology and biochemistry in adult rats, we determined that ßAR activity during, but not after PTC training initiates the activation of two plasticity-related targets: AMPA receptors (AMPARs) for memory acquisition and short-term memory and extracellular regulated kinase (ERK) for consolidating the learned association into a long-term memory. These findings reveal that ßAR activity during, but not following PTC sets in motion cascading molecular events for the acquisition (AMPARs) and subsequent consolidation (ERK) of learned associations.

Molecular mechanisms of threat learning in the lateral nucleus of the amygdala.

Pavlovian threat conditioning is a behavioral paradigm that has been successfully utilized to define the mechanisms underlying threat (fear) memory formation. The amygdala is a temporal lobe structure required for the acquisition, consolidation, and expression of threat (fear) memories. In particular, the lateral nucleus of the amygdala (LA) is the major input structure of the amygdala and is required for all aspects of threat learning and memory. The LA expresses many neurotransmitter and neuromodulator receptors. This chapter covers the molecular mechanisms that occur downstream of these receptors and how they influence LA-dependent Pavlovian threat learning.

Rhesus monkey alpha7 nicotinic acetylcholine receptors: comparisons to human alpha7 receptors expressed in Xenopus oocytes.

An alpha7 nicotinic acetylcholine receptor sequence was cloned from Rhesus monkey (Macaca mulatta). This clone differs from the mature human alpha7 nicotinic acetylcholine receptor in only four amino acids, two of which are in the extracellular domain. The monkey alpha7 nicotinic receptor was characterized in regard to its functional responses to acetylcholine, choline, cytisine, and the experimental alpha7-selective agonists 4OH-GTS-21, TC-1698, and AR-R17779. For all of these agonists, the EC(50) for activation of monkey receptors was uniformly higher than for human receptors. In contrast, the potencies of mecamylamine and MLA for inhibiting monkey and human alpha7 were comparable. Acetylcholine and 4OH-GTS-21 were used to probe the significance of the single point differences in the extracellular domain. Mutants with the two different amino acids in the extracellular domain of the monkey receptor changed to the corresponding sequence of the human receptor had responses to these agonists that were not significantly different in EC(50) from wild-type human alpha7 nicotinic receptors. Monkey alpha7 nicotinic receptors have a serine at residue 171, while the human receptors have an asparagine at this site. Monkey S171N mutants were more like human alpha7 nicotinic receptors, while mutations at the other site (K186R) had relatively little effect. These experiments point toward the basic utility of the monkey receptor as a model for the human alpha7 nicotinic receptor, albeit with the caveat that these receptors will vary in their agonist concentration dependency. They also point to the potential importance of a newly identified sequence element for modeling the specific amino acids involved with receptor activation.

Molecular dissection of tropisetron, an alpha7 nicotinic acetylcholine receptor-selective partial agonist.

The alpha7 nicotinic acetylcholine receptor (nAChR)-selective partial agonist tropisetron is a conjugate of an indole and a tropane group. We tested compounds structurally related to either the indole or tropane domains of tropisetron on oocytes expressing human alpha7. alpha4beta2, or alpha3beta4 nAChR or rat 5HT(3A) receptors. The simple compounds tropane and tropinone had alpha7-selective agonist activity comparable to that of tropisetron. Tropinone was more efficacious than tropisetron but 100-fold less potent. Some tropane compounds had antagonist activity on alpha3beta4 nAChR but no effect on alpha4beta2 nAChR. Some tropanes also affected the responses of 5HT3 receptors to serotonin. Tropisetron was more potent at inhibiting alpha3beta4 receptors (IC(50)=1.8+/-0.6) than was tropane or tropinone, suggesting that the presence of the indole group has a large impact on the potency of tropisetron, both as an alpha7 agonist and as an alpha3beta4 antagonist. The further reduced structures of dimethyl piperidinium and 1-methylpyrrolidine also had agonist activity on alpha7 receptors, suggesting that the minimal activating pharmacophore of these compounds, as with tetramethylammonium, may simply be the charged nitrogen, while additional structure elements impact subtype selectivity, potency, and efficacy. It has previously been reported that 5-hydroxyindole (5HI) can potentiate alpha7 receptor responses to acetylcholine (ACh). However, the site where 5HI binds to the receptor is not known. We tested the hypothesis that the tropisetron binding site might overlap the 5HI site and thereby produce a block of 5HI potentiation. Our results indicate that the indole portion of tropisetron is not likely to be binding to the same site where 5HI binds to potentiate alpha7 receptor responses since 5HI can greatly potentiate responses of tropisetron, tropinone, and other partial agonists such as 4OH-GTS-21.

Tolerance for inequity may increase with social closeness in chimpanzees.

Economic decision-making depends on our social environment. Humans tend to respond differently to inequity in close relationships, yet we know little about the potential for such variation in other species. We examine responses to inequity in several groups of chimpanzees (Pan troglodytes) in a paradigm similar to that used previously in capuchin monkeys (Cebus apella). We demonstrate that, like capuchin monkeys, chimpanzees show a response to inequity of rewards that is based upon the partner receiving the reward rather than the presence of the reward alone. However, we also found a great amount of variation between groups tested, indicating that chimpanzees, like people, respond to inequity in a variable manner, which we speculate could be caused by such variables as group size, the social closeness of the group (as reflected in length of time that the group has been together) and group-specific traditions.