Auditory processing assessment suggests that Wistar audiogenic rat neural networks are prone to entrainment.

Epilepsy is a neurological disease related to the occurrence of pathological oscillatory activity, but the basic physiological mechanisms of seizure remain to be understood. Our working hypothesis is that specific sensory processing circuits may present abnormally enhanced predisposition for coordinated firing in the dysfunctional brain. Such facilitated entrainment could share a similar mechanistic process as those expediting the propagation of epileptiform activity throughout the brain. To test this hypothesis, we employed the Wistar audiogenic rat (WAR) reflex animal model, which is characterized by having seizures triggered reliably by sound. Sound stimulation was modulated in amplitude to produce an auditory steady-state-evoked response (ASSR; -53.71Hz) that covers bottom-up and top-down processing in a time scale compatible with the dynamics of the epileptic condition. Data from inferior colliculus (IC) c-Fos immunohistochemistry and electrographic recordings were gathered for both the control Wistar group and WARs. Under 85-dB SLP auditory stimulation, compared to controls, the WARs presented higher number of Fos-positive cells (at IC and auditory temporal lobe) and a significant increase in ASSR-normalized energy. Similarly, the 110-dB SLP sound stimulation also statistically increased ASSR-normalized energy during ictal and post-ictal periods. However, at the transition from the physiological to pathological state (pre-ictal period), the WAR ASSR analysis demonstrated a decline in normalized energy and a significant increase in circular variance values compared to that of controls. These results indicate an enhanced coordinated firing state for WARs, except immediately before seizure onset (suggesting pre-ictal neuronal desynchronization with external sensory drive). These results suggest a competing myriad of interferences among different networks that after seizure onset converge to a massive oscillatory circuit.

Micro-endoscopic system for functional assessment of neural circuits in deep brain regions: Simultaneous optical and electrical recordings of auditory responses in mouse's inferior colliculus.

In vivo Ca2+ imaging is a powerful method for the functional assessment of neural circuits. Although multi-photon excitation fluorescence microscopy has been widely used, observation of circuits in deep brain regions remains challenging. Recently, observing these deep regions has become possible via an endoscope consisting of an optical fiber bundle or gradient-index lens. We have designed a micro-endoscope system that enables simultaneous optical recording of fluorescence and electrical recording of neural activity. Using this system, we recorded auditory responses by simultaneously detecting changes in the fluorescence intensity of a Ca2+ indicator dye, multi-unit activities (MUA), and local field potentials (LFP) in the mouse's inferior colliculus (IC). Such simultaneous optical and electrical recordings enabled detailed comparison of electrically recorded phenomena (MUA and LFP) and optically recorded Ca2+ response. By systematically changing sound frequency and intensity, we determined the frequency tuning of the recording site. The best frequency shifted higher as the probe advanced more deeply, demonstrating that the system is capable of optically measuring the dorso-ventral organization of IC (i.e., tonotopicity). Thus, our new micro-endoscope system will be useful in the neurophysiological studies of a wide range of brain circuits, including those within the auditory system.

Quantitative assessment of developing afferent patterns in the cat inferior colliculus revealed with calbindin immunohistochemistry and tract tracing methods.

The central nucleus of the inferior colliculus (CNIC) is comprised of an orderly series of fibrodendritic layers. These layers include integrative circuitry for as many as 13 different ascending auditory pathways, each tonotopically ordered. Calcium-binding proteins, such as calbindin-D28k (CB), may be useful neurochemical markers for specific subsets of afferent input in these layers and their spatial organization that are developmentally regulated. In this study, CB-immunohistochemistry was used to examine 1-42 postnatal-day-old kitten and adult cat CNIC and anterograde tracers were used to label afferent projections from the lateral superior olivary nucleus (LSO) to the CNIC at similar ages. A distinct axonal plexus that is CB-immunopositive is described. This CB-afferent compartment is present at birth and persists throughout the ages examined. Already at birth, the CB-immunostained plexus in kitten CNIC is organized into discrete bands that are approximately 75 microm thick and 500 microm long. In adult CNIC, the periodic banded pattern of CB-immunostained fibers is similar to that in kittens albeit bands are thicker (145 microm) and longer (700 microm). Growth in band thickness in adult cat appears proportional to growth of the IC, whereas length of the dense CB-immunostained bands is somewhat more focused in the central region of fibrodendritic layers. The banded pattern of the CB-immunostained plexus is well correlated with the location and dimension of afferent projections from the LSO in newborn kitten labeled with carbocyanine dye, 1,1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate and in adult cat labeled with wheat germ agglutinin conjugated with horseradish peroxidase. The results reveal a neurochemical marker for one type of synaptic compartment in CNIC layers, banding, that is organized before hearing onset in kittens, but that may undergo some postnatal pruning.