Serotonergic Modulation of Sensory Neuron Activity and Behavior in Apteronotus albifrons

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.

5-HT1A Receptors Alter Temporal Responses to Broadband Vocalizations in the Mouse Inferior Colliculus Through Response Suppression

Neuromodulatory systems may provide information on social context to auditory brain regions, but relatively few studies have assessed the effects of neuromodulation on auditory responses to acoustic social signals. To address this issue, we measured the influence of the serotonergic system on the responses of neurons in a mouse auditory midbrain nucleus, the inferior colliculus (IC), to vocal signals. Broadband vocalizations (BBVs) are human-audible signals produced by mice in distress as well as by female mice in opposite-sex interactions. The production of BBVs is context-dependent in that they are produced both at early stages of interactions as females physically reject males and at later stages as males mount females. Serotonin in the IC of males corresponds to these events, and is elevated more in males that experience less female rejection. We measured the responses of single IC neurons to five recorded examples of BBVs in anesthetized mice. We then locally activated the 5-HT1A receptor through iontophoretic application of 8-OH-DPAT. IC neurons showed little selectivity for different BBVs, but spike trains were characterized by local regions of high spike probability, which we called "response features." Response features varied across neurons and also across calls for individual neurons, ranging from 1 to 7 response features for responses of single neurons to single calls. 8-OH-DPAT suppressed spikes and also reduced the numbers of response features. The weakest response features were the most likely to disappear, suggestive of an "iceberg"-like effect in which activation of the 5-HT1A receptor suppressed weakly suprathreshold response features below the spiking threshold. Because serotonin in the IC is more likely to be elevated for mounting-associated BBVs than for rejection-associated BBVs, these effects of the 5-HT1A receptor could contribute to the differential auditory processing of BBVs in different behavioral subcontexts.

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.