Rodent auditory perception: Critical band limitations and plasticity

What do animals hear? While it remains challenging to adequately assess sensory perception in animal models, it is important to determine perceptual abilities in model systems to understand how physiological processes and plasticity relate to perception, learning, and cognition. Here we discuss hearing in rodents, reviewing previous and recent behavioral experiments querying acoustic perception in rats and mice, and examining the relation between behavioral data and electrophysiological recordings from the central auditory system. We focus on measurements of critical bands, which are psychoacoustic phenomena that seem to have a neural basis in the functional organization of the cochlea and the inferior colliculus. We then discuss how behavioral training, brain stimulation, and neuropathology impact auditory processing and perception.

Inhibitory synaptic plasticity: spike timing-dependence and putative network function

While the plasticity of excitatory synaptic connections in the brain has been widely studied, the plasticity of inhibitory connections is much less understood. Here, we present recent experimental and theoretical findings concerning the rules of spike timing-dependent inhibitory plasticity and their putative network function. This is a summary of a workshop at the COSYNE conference 2012.

Perinatal exposure to a noncoplanar polychlorinated biphenyl alters tonotopy, receptive fields, and plasticity in rat primary auditory cortex

Noncoplanar polychlorinated biphenyls (PCBs) are widely dispersed in human environment and tissues. Here, an exemplar noncoplanar PCB was fed to rat dams during gestation and throughout three subsequent nursing weeks. Although the hearing sensitivity and brainstem auditory responses of pups were normal, exposure resulted in the abnormal development of the primary auditory cortex (A1). A1 was irregularly shaped and marked by internal nonresponsive zones, its topographic organization was grossly abnormal or reversed in about half of the exposed pups, the balance of neuronal inhibition to excitation for A1 neurons was disturbed, and the critical period plasticity that underlies normal postnatal auditory system development was significantly altered. These findings demonstrate that developmental exposure to this class of environmental contaminant alters cortical development. It is proposed that exposure to noncoplanar PCBs may contribute to common developmental disorders, especially in populations with heritable imbalances in neurotransmitter systems that regulate the ratio of inhibition and excitation in the brain. We conclude that the health implications associated with exposure to noncoplanar PCBs in human populations merit a more careful examination.

Dynamics of spontaneous activity in neocortical slices

The flow of activity in the cortical microcircuitry is poorly understood. We use calcium imaging to reconstruct, with millisecond and single-cell resolution, the spontaneous activity of populations of neurons in unstimulated slices from mouse visual cortex. We find spontaneous activity correlated among networks of layer 5 pyramidal cells. Synchronous ensembles occupy overlapping territories, often share neurons, and are repeatedly activated. Sets of neurons are also sequentially activated numerous times. Network synchronization and sequential correlations are blocked by glutamatergic antagonists, even though spontaneous firing persists in many "autonomously active" neurons. This autonomous activity is periodic and depends on hyperpolarization-activated cationic (H) and persistent sodium (Na(p)) currents. We conclude that the isolated neocortical microcircuit generates spontaneous activity, mediated by a combination of intrinsic and circuit mechanisms, and that this activity can be temporally precise.

Phosphorylation and local presynaptic protein synthesis in calcium- and calcineurin-dependent induction of crayfish long-term facilitation

Long-term facilitation at the crayfish opener muscle is elicited by prolonged high frequency stimulation, and arises from an increase in functional active zones, resulting in increased transmitter release. LTF induction depends critically upon presynaptic calcium accumulation and calcineurin (PP2B) activity. The protein synthesis dependence of this synaptic strengthening was investigated. LTF occurred without transcription, but the translation inhibitors cycloheximide and anisomycin, or local presynaptic injection of mRNA cap analog m7GpppG, impaired LTF expression. Both MAP kinase and phosphatidylinositol 3-OH kinase (PI3K) activation are implicated in this rapamycin-sensitive synaptic potentiation. This study defines an important role for protein synthesis in the expression of activity-dependent plasticity, and provides mechanistic insight for the induction of this process at presynaptic sites.