Precise and Pervasive Phasic Bursting in Locus Coeruleus during Maternal Behavior in Mice.

The noradrenergic locus coeruleus (LC) mediates key aspects of arousal, memory, and cognition in structured tasks, but its contribution to naturalistic behavior remains unclear. LC activity is thought to multiplex distinct signals by superimposing sustained ("tonic") firing patterns reflecting global brain states, such as arousal and anxiety, and rapidly fluctuating ("phasic") bursts signaling discrete behaviorally significant events. Manipulations of the LC noradrenergic system broadly impair social behavior, but the temporal structure of LC firing and its relationship to social interaction is unknown. One possibility is that tonic firing may increase in the presence of social partners; it is also possible that phasic bursts may accompany specific social events. We used chronic in vivo electrophysiology and fiber photometry to measure single-unit and population neural activity in LC of freely behaving mice during their interactions with pups. We find that pup retrieval elicits remarkably precise phasic activity in LC that cannot be attributed merely to sensory stimuli, motor activity, or reward. Correlation of LC activity with retrieval events shows that phasic events are most closely related to specific subsequent behaviors. The reliability and magnitude of phasic responses strongly suggest that these events are coordinated across LC and broadcast noradrenaline (NA) release throughout the brain. We also observed slow changes in tonic firing when females performed distinct maternal behaviors such as nest building and pup grooming. We therefore propose that LC signals state changes during sustained interactions and contributes to goal-directed action selection during social behavior with globally broadcast NA release.SIGNIFICANCE STATEMENT Locus coeruleus (LC) releases noradrenaline (NA) brain wide, influencing many cognitive, emotional, and physiological processes. Multifunctionality of LC is maintained by multiplexing NA signaling via brief "phasic" patterns of bursting and slowly changing "tonic" firing. Manipulations of NA impair social behavior, yet the structure of LC activity with respect to specific social events is unknown. We measured LC activity in mice freely interacting with pups. We find that pup retrieval elicits precisely timed and pervasive phasic activation of LC that anticipates specific behaviors. We also found that LC neurons exhibited slow fluctuations in firing during sustained behaviors. We propose that LC simultaneously contributes to goal-directed social action selection with globally broadcast NA release and signals social state changes with increased tonic firing.

Relative Contributions of Specific Activity Histories and Spontaneous Processes to Size Remodeling of Glutamatergic Synapses.

The idea that synaptic properties are defined by specific pre- and postsynaptic activity histories is one of the oldest and most influential tenets of contemporary neuroscience. Recent studies also indicate, however, that synaptic properties often change spontaneously, even in the absence of specific activity patterns or any activity whatsoever. What, then, are the relative contributions of activity history-dependent and activity history-independent processes to changes synapses undergo? To compare the relative contributions of these processes, we imaged, in spontaneously active networks of cortical neurons, glutamatergic synapses formed between the same axons and neurons or dendrites under the assumption that their similar activity histories should result in similar size changes over timescales of days. The size covariance of such commonly innervated (CI) synapses was then compared to that of synapses formed by different axons (non-CI synapses) that differed in their activity histories. We found that the size covariance of CI synapses was greater than that of non-CI synapses; yet overall size covariance of CI synapses was rather modest. Moreover, momentary and time-averaged sizes of CI synapses correlated rather poorly, in perfect agreement with published electron microscopy-based measurements of mouse cortex synapses. A conservative estimate suggested that ~40% of the observed size remodeling was attributable to specific activity histories, whereas ~10% and ~50% were attributable to cell-wide and spontaneous, synapse-autonomous processes, respectively. These findings demonstrate that histories of naturally occurring activity patterns can direct glutamatergic synapse remodeling but also suggest that the contributions of spontaneous, possibly stochastic, processes are at least as great.

Hybrid multiphoton volumetric functional imaging of large-scale bioengineered neuronal networks.

Planar neural networks and interfaces serve as versatile in vitro models of central nervous system physiology, but adaptations of related methods to three dimensions (3D) have met with limited success. Here, we demonstrate for the first time volumetric functional imaging in a bioengineered neural tissue growing in a transparent hydrogel with cortical cellular and synaptic densities, by introducing complementary new developments in nonlinear microscopy and neural tissue engineering. Our system uses a novel hybrid multiphoton microscope design combining a 3D scanning-line temporal-focusing subsystem and a conventional laser-scanning multiphoton microscope to provide functional and structural volumetric imaging capabilities: dense microscopic 3D sampling at tens of volumes per second of structures with mm-scale dimensions containing a network of over 1,000 developing cells with complex spontaneous activity patterns. These developments open new opportunities for large-scale neuronal interfacing and for applications of 3D engineered networks ranging from basic neuroscience to the screening of neuroactive substances.