A Neural System that Represents the Association of Odors with Rewarded Outcomes and Promotes Behavioral Engagement.

Odors are well known to elicit strong emotional and behavioral responses that become strengthened throughout learning, yet the specific cellular systems involved in odor learning and the direct influence of these on behavior are unclear. Here, we investigate the representation of odor-reward associations within two areas recipient of dense olfactory input, the posterior piriform cortex (pPCX) and the olfactory tubercle (OT), using electrophysiological recordings from mice engaged in reward-based learning. Neurons in both regions represent conditioned odors and do so with similar information content, yet the proportion of neurons recruited by conditioned rewarded odors and the magnitudes and durations of their responses are greater in the OT. Using fiber photometry, we find that OT D1-type dopamine-receptor-expressing neurons flexibly represent odors based on reward associations, and using optogenetics, we show that these neurons influence behavioral engagement. These findings contribute to a model whereby OT D1 neurons support odor-guided motivated behaviors.

Selective Attention Controls Olfactory Decisions and the Neural Encoding of Odors.

Critical animal behaviors, especially among rodents, are guided by odors in remarkably well-coordinated manners, yet many extramodal sensory cues compete for cognitive resources in these ecological contexts. That rodents can engage in such odor-guided behaviors suggests that they can selectively attend to odors. Indeed, higher-order cognitive processes-such as learning, memory, decision making, and action selection-rely on the proper filtering of sensory cues based on their relative salience. We developed a behavioral paradigm to reveal that rats are capable of selectively attending to odors in the presence of competing extramodal stimuli. We found that this selective attention facilitates accurate odor-guided decisions, which become further strengthened with experience. Further, we uncovered that selective attention to odors adaptively sharpens their representation among neurons in the olfactory tubercle, an olfactory cortex region of the ventral striatum that is considered integral for evaluating sensory information in the context of motivated behaviors. Odor-directed selective attention exerts influences during moments of heightened odor anticipation and enhances odorant representation by increasing stimulus contrast in a signal-to-noise-type coding scheme. Together, these results reveal that rats engage selective attention to optimize olfactory outcomes. Further, our finding of attention-dependent coding in the olfactory tubercle challenges the notion that a thalamic relay is integral for the attentional control of sensory coding.

Preservation of Essential Odor-Guided Behaviors and Odor-Based Reversal Learning after Targeting Adult Brain Serotonin Synthesis.

The neurotransmitter serotonin (5-HT) is considered a powerful modulator of sensory system organization and function in a wide range of animals. The olfactory system is innervated by midbrain 5-HT neurons into both its primary and secondary odor-processing stages. Facilitated by this circuitry, 5-HT and its receptors modulate olfactory system function, including odor information input to the olfactory bulb. It is unknown, however, whether the olfactory system requires 5-HT for even its most basic behavioral functions. To address this question, we established a conditional genetic approach to specifically target adult brain tryptophan hydroxylase 2 (Tph2), encoding the rate-limiting enzyme in brain 5-HT synthesis, and nearly eliminate 5-HT from the mouse forebrain. Using this novel model, we investigated the behavior of 5-HT-depleted mice during performance in an olfactory go/no-go task. Surprisingly, the near elimination of 5-HT from the forebrain, including the olfactory bulbs, had no detectable effect on the ability of mice to perform the odor-based task. Tph2-targeted mice not only were able to learn the task, but also had levels of odor acuity similar to those of control mice when performing coarse odor discrimination. Both groups of mice spent similar amounts of time sampling odors during decision-making. Furthermore, odor reversal learning was identical between 5-HT-depleted and control mice. These results suggest that 5-HT neurotransmission is not necessary for the most essential aspects of olfaction, including odor learning, discrimination, and certain forms of cognitive flexibility.

The Neural Representation of Goal-Directed Actions and Outcomes in the Ventral Striatum's Olfactory Tubercle.

The ventral striatum is critical for evaluating reward information and the initiation of goal-directed behaviors. The many cellular, afferent, and efferent similarities between the ventral striatum's nucleus accumbens and olfactory tubercle (OT) suggests the distributed involvement of neurons within the ventral striatopallidal complex in motivated behaviors. Although the nucleus accumbens has an established role in representing goal-directed actions and their outcomes, it is not known whether this function is localized within the nucleus accumbens or distributed also within the OT. Answering such a fundamental question will expand our understanding of the neural mechanisms underlying motivated behaviors. Here we address whether the OT encodes natural reinforcers and serves as a substrate for motivational information processing. In recordings from mice engaged in a novel water-motivated instrumental task, we report that OT neurons modulate their firing rate during initiation and progression of the instrumental licking behavior, with some activity being internally generated and preceding the first lick. We further found that as motivational drive decreases throughout a session, the activity of OT neurons is enhanced earlier relative to the behavioral action. Additionally, OT neurons discriminate the types and magnitudes of fluid reinforcers. Together, these data suggest that the processing of reward information and the orchestration of goal-directed behaviors is a global principle of the ventral striatum and have important implications for understanding the neural systems subserving addiction and mood disorders. SIGNIFICANCE STATEMENT: Goal-directed behaviors are widespread among animals and underlie complex behaviors ranging from food intake, social behavior, and even pathological conditions, such as gambling and drug addiction. The ventral striatum is a neural system critical for evaluating reward information and the initiation of goal-directed behaviors. Here we show that neurons in the olfactory tubercle subregion of the ventral striatum robustly encode the onset and progression of motivated behaviors, and discriminate the type and magnitude of a reward. Our findings are novel in showing that olfactory tubercle neurons participate in such coding schemes and are in accordance with the principle that ventral striatum substructures may cooperate to guide motivated behaviors.

Two distinct representations of social vocalizations in the basolateral amygdala.

Acoustic communication signals carry information related to the types of social interactions by means of their "acoustic context," the sequencing and temporal emission pattern of vocalizations. Here we describe responses to natural vocal sequences in adult big brown bats (Eptesicus fuscus). We first assessed how vocal sequences modify the internal affective state of a listener (via heart rate). The heart rate of listening bats was differentially modulated by vocal sequences, showing significantly greater elevation in response to moderately aggressive sequences than appeasement or neutral sequences. Next, we characterized single-neuron responses in the basolateral amygdala (BLA) of awake, restrained bats to isolated syllables and vocal sequences. Two populations of neurons distinguished by background firing rates also differed in acoustic stimulus selectivity. Low-background neurons (<1 spike/s) were highly selective, responding on average to one tested stimulus. These may participate in a sparse code of vocal stimuli, in which each neuron responds to one or a few stimuli and the population responds to the range of vocalizations across behavioral contexts. Neurons with higher background rates (≥1 spike/s) responded broadly to tested stimuli and better represented the timing of syllables within sequences. We found that spike timing information improved the ability of these neurons to discriminate among vocal sequences and among the behavioral contexts associated with sequences compared with a rate code alone. These findings demonstrate that the BLA contains multiple robust representations of vocal stimuli that can provide the basis for emotional/physiological responses to these stimuli.

The olfactory tubercle encodes odor valence in behaving mice.

Sensory information acquires meaning to adaptively guide behaviors. Despite odors mediating a number of vital behaviors, the components of the olfactory system responsible for assigning meaning to odors remain unclear. The olfactory tubercle (OT), a ventral striatum structure that receives monosynaptic input from the olfactory bulb, is uniquely positioned to transform odor information into behaviorally relevant neural codes. No information is available, however, on the coding of odors among OT neurons in behaving animals. In recordings from mice engaged in an odor discrimination task, we report that the firing rate of OT neurons robustly and flexibly encodes the valence of conditioned odors over identity, with rewarded odors evoking greater firing rates. This coding of rewarded odors occurs before behavioral decisions and represents subsequent behavioral responses. We predict that the OT is an essential region whereby odor valence is encoded in the mammalian brain to guide goal-directed behaviors.

Social vocalizations of big brown bats vary with behavioral context.

Bats are among the most gregarious and vocal mammals, with some species demonstrating a diverse repertoire of syllables under a variety of behavioral contexts. Despite extensive characterization of big brown bat (Eptesicus fuscus) biosonar signals, there have been no detailed studies of adult social vocalizations. We recorded and analyzed social vocalizations and associated behaviors of captive big brown bats under four behavioral contexts: low aggression, medium aggression, high aggression, and appeasement. Even limited to these contexts, big brown bats possess a rich repertoire of social vocalizations, with 18 distinct syllable types automatically classified using a spectrogram cross-correlation procedure. For each behavioral context, we describe vocalizations in terms of syllable acoustics, temporal emission patterns, and typical syllable sequences. Emotion-related acoustic cues are evident within the call structure by context-specific syllable types or variations in the temporal emission pattern. We designed a paradigm that could evoke aggressive vocalizations while monitoring heart rate as an objective measure of internal physiological state. Changes in the magnitude and duration of elevated heart rate scaled to the level of evoked aggression, confirming the behavioral state classifications assessed by vocalizations and behavioral displays. These results reveal a complex acoustic communication system among big brown bats in which acoustic cues and call structure signal the emotional state of a caller.

Automated classification of mouse pup isolation syllables: from cluster analysis to an Excel-based "mouse pup syllable classification calculator".

Mouse pups vocalize at high rates when they are cold or isolated from the nest. The proportions of each syllable type produced carry information about disease state and are being used as behavioral markers for the internal state of animals. Manual classifications of these vocalizations identified 10 syllable types based on their spectro-temporal features. However, manual classification of mouse syllables is time consuming and vulnerable to experimenter bias. This study uses an automated cluster analysis to identify acoustically distinct syllable types produced by CBA/CaJ mouse pups, and then compares the results to prior manual classification methods. The cluster analysis identified two syllable types, based on their frequency bands, that have continuous frequency-time structure, and two syllable types featuring abrupt frequency transitions. Although cluster analysis computed fewer syllable types than manual classification, the clusters represented well the probability distributions of the acoustic features within syllables. These probability distributions indicate that some of the manually classified syllable types are not statistically distinct. The characteristics of the four classified clusters were used to generate a Microsoft Excel-based mouse syllable classifier that rapidly categorizes syllables, with over a 90% match, into the syllable types determined by cluster analysis.

A novel coding mechanism for social vocalizations in the lateral amygdala.

The amygdala plays a central role in evaluating the significance of acoustic signals and coordinating the appropriate behavioral responses. To understand how amygdalar responses modulate auditory processing and drive emotional expression, we assessed how neurons respond to and encode information that is carried within complex acoustic stimuli. We characterized responses of single neurons in the lateral nucleus of the amygdala to social vocalizations and synthetic acoustic stimuli in awake big brown bats. Neurons typically responded to most of the social vocalizations presented (mean = nine of 11 vocalizations) but differentially modulated both firing rate and response duration. Surprisingly, response duration provided substantially more information about vocalizations than did spike rate. In most neurons, variation in response duration depended, in part, on persistent excitatory discharge that extended beyond stimulus duration. Information in persistent firing duration was significantly greater than in spike rate, and the majority of neurons displayed more information in persistent firing, which was more likely to be observed in response to aggressive vocalizations (64%) than appeasement vocalizations (25%), suggesting that persistent firing may relate to the behavioral context of vocalizations. These findings suggest that the amygdala uses a novel coding strategy for discriminating among vocalizations and underscore the importance of persistent firing in the general functioning of the amygdala.