Descending pathways increase sensory neural response heterogeneity to facilitate decoding and behavior

The functional role of heterogeneous spiking responses of otherwise similarly tuned neurons to stimulation, which has been observed ubiquitously, remains unclear to date. Here, we demonstrate that such response heterogeneity serves a beneficial function that is used by downstream brain areas to generate behavioral responses that follows the detailed timecourse of the stimulus. Multi-unit recordings from sensory pyramidal cells within the electrosensory system of Apteronotus leptorhynchus were performed and revealed highly heterogeneous responses that were similar for all cell types. By comparing the coding properties of a given neural population before and after inactivation of descending pathways, we found that heterogeneities were beneficial as decoding was then more robust to the addition of noise. Taken together, our results not only reveal that descending pathways actively promote response heterogeneity within a given cell type, but also uncover a beneficial function for such heterogeneity that is used by the brain to generate behavior.

Serotonin increases population coding of behaviorally relevant stimuli by enhancing responses of ON but not OFF-type sensory neurons

How neural populations encode sensory input to generate behavioral responses remains a central problem in systems neuroscience. Here we investigated how neuromodulation influences population coding of behaviorally relevant stimuli to give rise to behavior in the electrosensory system of the weakly electric fish Apteronotus leptorhynchus. We performed multi-unit recordings from ON and OFF sensory pyramidal cells in response to stimuli whose amplitude (i.e., envelope) varied in time, before and after electrical stimulation of the raphe nuclei. Overall, raphe stimulation increased population coding by ON- but not by OFF-type cells, despite both cell types showing similar sensitivities to the stimulus at the single neuron level. Surprisingly, only changes in population coding by ON-type cells were correlated with changes in behavioral responses. Taken together, our results show that neuromodulation differentially affects ON vs. OFF-type cells in order to enhance perception of behaviorally relevant sensory input.

Vocal and Electric Fish: Revisiting a Comparison of Two Teleost Models in the Neuroethology of Social Behavior

The communication behaviors of vocal fish and electric fish are among the vertebrate social behaviors best understood at the level of neural circuits. Both forms of signaling rely on midbrain inputs to hindbrain pattern generators that activate peripheral effectors (sonic muscles and electrocytes) to produce pulsatile signals that are modulated by frequency/repetition rate, amplitude and call duration. To generate signals that vary by sex, male phenotype, and social context, these circuits are responsive to a wide range of hormones and neuromodulators acting on different timescales at multiple loci. Bass and Zakon (2005) reviewed the behavioral neuroendocrinology of these two teleost groups, comparing how the regulation of their communication systems have both converged and diverged during their parallel evolution. Here, we revisit this comparison and review the complementary developments over the past 16 years. We (a) summarize recent work that expands our knowledge of the neural circuits underlying these two communication systems, (b) review parallel studies on the action of neuromodulators (e.g., serotonin, AVT, melatonin), brain steroidogenesis (via aromatase), and social stimuli on the output of these circuits, (c) highlight recent transcriptomic studies that illustrate how contemporary molecular methods have elucidated the genetic regulation of social behavior in these fish, and (d) describe recent studies of mochokid catfish, which use both vocal and electric communication, and that use both vocal and electric communication and consider how these two systems are spliced together in the same species. Finally, we offer avenues for future research to further probe how similarities and differences between these two communication systems emerge over ontogeny and evolution.

Everything in modulation: neuromodulators as keys to understanding communication dynamics

Across the animal kingdom, the ability to produce communication signals appropriate to social encounters is essential, but how these behaviors are selected and adjusted in a context-dependent manner is poorly understood. This question can be addressed on many levels, including sensory processing by peripheral organs and the CNS, sensorimotor integration in decision-making brain regions, and motor circuit activation and modulation. Because neuromodulator systems act at each of these levels, they are a useful lens through which to explore the mechanisms underlying complex patterns of communication. It has been clear for decades that understanding the logic of input-output decision making by the nervous system requires far more than simply identifying the connections linking sensory organs to motor circuits; this is due in part to the fact that neuromodulators can promote distinct and temporally dynamic responses to similar signals. We focus on the vocal circuit dynamics of Xenopus frogs, and describe complementary examples from diverse vertebrate communication systems. While much remains to be discovered about how neuromodulators direct flexibility in communication behaviors, these examples illustrate that several neuromodulators can act upon the same circuit at multiple levels of control, and that the functional consequence of neuromodulation can depend on species-specific factors as well as dynamic organismal characteristics like internal state.

Postweaning Isolation Alters the Responses of Auditory Neurons to Serotonergic Modulation

Juvenile social experience, such as social isolation, has profound effects on communicative behavior, including signal production and reception. In the current study, we explored responsiveness to the neuromodulator serotonin as a potential mechanistic link between early life social isolation and auditory processing. The serotonergic system is sensitive to social isolation in many brain regions including the inferior colliculus (IC), an auditory midbrain nucleus. We investigated the effects of social experience on serotonergic responsiveness by measuring cFos, an immediate early gene product, in the IC of female mice. Serotonin was manipulated pharmacologically by administering fenfluramine, pCPA, or saline to mice that had undergone an extreme dichotomy in social experience after weaning: being housed in social groups versus individually. These mice were exposed to a 60-min recording of vocalizations from an opposite-sex interaction and perfused. Using immunohistochemistry, we measured the density of cFos-positive (cFos+) nuclei in the major subdivisions of the IC. Housing condition, drug treatment, and IC subregion all had a significant effect on cFos+ density. The central IC showed the highest density of cFos+ cells and also the most pronounced effects of housing condition and drug treatment. In the central IC, cFos+ density was higher following fenfluramine treatment than saline, and lower following pCPA treatment than fenfluramine. Individually housed mice showed a higher cFos+ density than socially housed mice in both of the pharmacological treatment groups, but not in the saline group. Drug treatment but not housing condition had strong effects on the behaviors of grooming, digging, rearing, and movement. Once the effects of drug condition were controlled, there were no across-individual correlations between cFos+ densities and behaviors. These findings suggest that the responses of auditory neurons to neuromodulation by serotonin are influenced by early life experience.

Serotonergic Modulation of Sensory Neuron Activity and Behavior in Apteronotus albifrons

Organisms must constantly adapt to changes in their environment to survive. It is thought that neuromodulators such as serotonin enable sensory neurons to better process input encountered during different behavioral contexts. Here, we investigated how serotonergic innervation affects neural and behavioral responses to behaviorally relevant envelope stimuli in the weakly electric fish species Apteronotus albifrons. Under baseline conditions, we found that exogenous serotonin application within the electrosensory lateral line lobe increased sensory neuron excitability, thereby promoting burst firing. We found that serotonin enhanced the responses to envelope stimuli of pyramidal cells within the lateral segment of the electrosensory lateral line lobe (ELL) by increasing sensitivity, with the increase more pronounced for stimuli with higher temporal frequencies (i.e., >0.2 Hz). Such increases in neural sensitivity were due to increased burst firing. At the organismal level, bilateral serotonin application within the ELL lateral segment enhanced behavioral responses to sensory input through increases in sensitivity. Similar to what was observed for neural responses, increases in behavioral sensitivity were more pronounced for higher (i.e., >0.2 Hz) temporal frequencies. Surprisingly, a comparison between our results and previous ones obtained in the closely related species A. leptorhynchus revealed that, while serotonin application gave rise to similar effects on neural excitability and responses to sensory input, serotonin application also gave rise to marked differences in behavior. Specifically, behavioral responses in A. leptorhynchus were increased primarily for lower (i.e., ≤0.2 Hz) rather than for higher temporal frequencies. Thus, our results strongly suggest that there are marked differences in how sensory neural responses are processed downstream to give rise to behavior across both species. This is even though previous results have shown that the behavioral responses of both species to envelope stimuli were identical when serotonin is not applied.