Plastic Change in the Auditory Minimum Threshold Induced by Intercollicular Effects in Mice.

In the auditory pathway, the commissure of the inferior colliculus (IC) interconnects the two ICs on both sides of the dorsal midbrain. This interconnection could mediate an interaction between the two ICs during sound signal processing. The intercollicular effects evoked by focal electric stimulation for 30 min could inhibit or facilitate auditory responses and induce plastic changes in the response minimum threshold (MT) of IC neurons. Changes in MT are dependent on the best frequency (BF) and MT difference. The MT shift is larger in IC neurons with BF differences ≤2 kHz than in those with BF differences >2 kHz. Moreover, MTs that shift toward electrically stimulated IC neurons increase with the increasing MT difference between the two ICs. The shift in MT lasts for a certain period of time and then returns to previous levels within ~150 min. The collicular interactions are either reciprocal or unilateral under alternate stimulating and recording conditions in both ICs. Our results suggest that intercollicular effects may be involved in the acoustic experience-dependent plasticity of the MT of IC neurons.

The complete mitochondrial genome of Southern pig-tailed macaque, Macaca nemestrina, and comparative mitochondrial genomics of Macaca species.

In this study, we report the complete mitochondrial genome sequence of Southern pig-tailed, Macaca nemestrina for the first time. The genome is found to be 16,560 bp in length and has a base composition of A (32.25%), G (12.31%), C (30.51%), and T (24.93%), indicating that the percentage of A + T (57.18%) was higher than G + C (42.82%). Similar to other monkeys, it contains a typically conserved structure including 13 protein-coding genes, 22 transfer RNA genes, 2 ribosomal RNA genes, and 1 control region (D-loop). Most of the genes were located on the H-strand except for the ND6 gene and 8 tRNA genes. To obtain a more complete understanding of the evolutionary history of Macaca genus, 11 mitochondrial genomes were used for phylogenetic analysis. This mitochondrial sequence reported here would be useful to uncover the monkey's evolution and add a new genetic resource for the genus Macaca.

Local neuronal circuits that may shape the discharge patterns of inferior collicular neurons.

The discharge patterns of neurons in auditory centers encode information about sounds. However, few studies have focused on the synaptic mechanisms underlying the shaping of discharge patterns using intracellular recording techniques. Here, we investigated the discharge patterns of inferior collicular (IC) neurons using intracellular recordings to further elucidate the mechanisms underlying the shaping of discharge patterns. Under in vivo intracellular recording conditions, recordings were obtained from 66 IC neurons in 18 healthy adult mice (Mus musculus, Km) under free field-stimulation. Fifty-eight of these neurons fi red bursts of action potentials (APs) to auditory stimuli and the remaining eight just generated local responses such as excitatory (n = 4) or inhibitory (n = 4) postsynaptic potentials. Based on the APs and subthreshold responses, the discharge patterns were classified into seven types: phasic (24/58, 41.4%), phasic burst (8/58,13.8%), pauser (4/58, 6.9%), phasic-pauser (1/58, 1.7%), chopper (2/58, 3.4%), primary-like tonic (14/58, 24.1%) and sound-induced inhibitory (5/58,8.6%). We concluded that (1) IC neurons exhibit at least seven distinct discharge patterns; (2) inhibition participates in shaping the discharge pattern of most IC neurons and plays a role in sculpting the pattern, except for the primary-like tonic pattern which was not shaped by inhibition; and (3) local neural circuits are the likely structural basis that shapes the discharge patterns of IC neurons and can be formed either in the IC or in lower-level auditory structures.

Neural interactions in unilateral colliculus and between bilateral colliculi modulate auditory signal processing.

In the auditory pathway, the inferior colliculus (IC) is a major center for temporal and spectral integration of auditory information. There are widespread neural interactions in unilateral (one) IC and between bilateral (two) ICs that could modulate auditory signal processing such as the amplitude and frequency selectivity of IC neurons. These neural interactions are either inhibitory or excitatory, and are mostly mediated by γ-aminobutyric acid (GABA) and glutamate, respectively. However, the majority of interactions are inhibitory while excitatory interactions are in the minority. Such unbalanced properties between excitatory and inhibitory projections have an important role in the formation of unilateral auditory dominance and sound location, and the neural interaction in one IC and between two ICs provide an adjustable and plastic modulation pattern for auditory signal processing.

Bilateral collicular interaction: modulation of auditory signal processing in amplitude domain.

In the ascending auditory pathway, the inferior colliculus (IC) receives and integrates excitatory and inhibitory inputs from many lower auditory nuclei, intrinsic projections within the IC, contralateral IC through the commissure of the IC and from the auditory cortex. All these connections make the IC a major center for subcortical temporal and spectral integration of auditory information. In this study, we examine bilateral collicular interaction in modulating amplitude-domain signal processing using electrophysiological recording, acoustic and focal electrical stimulation. Focal electrical stimulation of one (ipsilateral) IC produces widespread inhibition (61.6%) and focused facilitation (9.1%) of responses of neurons in the other (contralateral) IC, while 29.3% of the neurons were not affected. Bilateral collicular interaction produces a decrease in the response magnitude and an increase in the response latency of inhibited IC neurons but produces opposite effects on the response of facilitated IC neurons. These two groups of neurons are not separately located and are tonotopically organized within the IC. The modulation effect is most effective at low sound level and is dependent upon the interval between the acoustic and electric stimuli. The focal electrical stimulation of the ipsilateral IC compresses or expands the rate-level functions of contralateral IC neurons. The focal electrical stimulation also produces a shift in the minimum threshold and dynamic range of contralateral IC neurons for as long as 150 minutes. The degree of bilateral collicular interaction is dependent upon the difference in the best frequency between the electrically stimulated IC neurons and modulated IC neurons. These data suggest that bilateral collicular interaction mainly changes the ratio between excitation and inhibition during signal processing so as to sharpen the amplitude sensitivity of IC neurons. Bilateral interaction may be also involved in acoustic-experience-dependent plasticity in the IC. Three possible neural pathways underlying the bilateral collicular interaction are discussed.

Modulation of amplitude sensitivity by bilateral collicular interaction among different frequency laminae.

In the ascending auditory pathway, the commissure of the inferior colliculus (IC) interconnects the two ICs and may therefore mediate bilateral collicular interaction during sound processing. In this study, we show that electrically stimulates one IC produces facilitation or suppression of acoustically evoked response of neurons in the other IC. The facilitated IC neurons (14%) are located in bilateral corresponding frequency laminae while the suppressed IC neurons (86%) are widespreadly located in bilateral different frequency laminae. Whereas induced facilitation increases the dynamic range but decreases the slope of the rate-amplitude function of modulated IC neurons, induced suppression produces the opposite effect. As a result, bilateral collicular facilitation increases the sensitivity of modulated IC neurons to a wider range of sound amplitude while bilateral collicular suppression improves the sensitivity of modulated IC neurons to minor change in sound amplitude over a narrower range of sound amplitude. The degree of suppression is significantly greater for suppressed IC neurons located in bilateral corresponding frequency laminae than in non-corresponding frequency laminae. We suggest that bilateral collicular interaction through the commissure of the IC may play a role in modulation of amplitude sensitivity and in shaping the binaural property of IC neurons.