LONG-TERM CHANGES IN c-FOS EXPRESSION IN THE INFERIOR COLLICULUS AFTER UNILATERAL CORTICAL ABLATION

MIGUEL MERCHÁN CIFUENTES

Neuroscience Institute of Castilla y León (INCyL), University of Salamanca, Department of Cell Biology and Pathology, Medical School, University of Salamanca, 37007-Salamanca, Spain.

The inferior colliculus (IC), the main auditory midbrain center, receives convergent ascending projections from multiple auditory brainstem nuclei, commissural projections from the opposite IC, and descending projections from the cerebral auditory cortex (AC). To further analyze the role of the corticocollicular projections, we are investigating to what degree unilateral ablations of the AC modify sound-induced expression of c-Fos in the inferior colliculus (IC) of albino wistar rats.

The animals of control groups (n=3), were anaesthetized, and exposed to a 5 kHz pure tone (80 dB SPL), with duration of 300 ms each 800 ms, and a repetition frequency of 1.25 s for one hour. Another control group (n=3) were used, but not was stimulated with sound (silence control). In lesioned groups, the rats were anaesthetized and the AC is unilaterally ablated by surgical aspiration. 15 or 90 days after the ablation, the animals are reanesthetized and stimulated as described above. 100 minutes after acoustic stimulation, the rats, fully asleep, are perfused with 4% formaldehyde and their brains are processed immunocytochemically to visualize the c-Fos protein. Analysis of the results will include a qualitative assessment of the distribution of c-Fos-immunoreactive (Fos-IR), as well as a stereological, quantitative analysis of the number of Fos-IR in the IC, also a densitometry study.

Our results reveal that the number of c-Fos-IR neurons in the IC was lower 15 days after the cortical ablation than in the control animals. The decrease was significant in all three IC subdivisions (central nucleus [CNIC], dorsal cortex [DCIC] and external cortex [ECIC]), and less pronounced in the CNIC than in the DCIC and ECIC. In rats stimulated 90 days after the cortical ablation the number of Fos-IC neurons in the IC was higher than in the rats stimulated 15 days after the lesion, but lower than in the control animals. A densitometry study shows similar results to stereological numbers, but also show changes in the distribution patrons of c-Fos expression in the inferior colliculus after unilateral auditory cortical ablation.