Ce. Ribak et Cl. Morin, THE ROLE OF THE INFERIOR COLLICULUS IN A GENETIC MODEL OF AUDIOGENIC-SEIZURES, Anatomy and embryology, 191(4), 1995, pp. 279-295
Previous studies have shown the functional importance of the inferior
colliculus (IC) for the propagation and initiation of audiogenic seizu
res in several models of epilepsy in rats. A review of the cell types
and cytoarchitecture of the IC, including its three major subdivisions
, is presented. Significant increases in GABA levels and the number of
GABAergic neurons are found in the central nucleus of the IC (ICCN) o
f genetically epilepsy-prone rats (GEPR-9s) as compared to Sprague-Daw
ley rats that do not display audiogenic seizures. Two independent anat
omical methods were used to determine the number of GABAergic neurons,
immunocytochemistry and in situ hybridization. In both types of prepa
ration, the labeled cells in the ICCN appeared to be of different size
s but the number of small cells with diameters less than 15 mu m showe
d the greatest increase. Nissl-stained sections showed that the total
number of neurons in the ICCN was increased in GEPR-9s and indicated t
hat the increase in GABAergic neurons was not due to a change in the p
henotype of collicular neurons from non-GABAergic to GABAergic. The nu
mber of small neurons in Nissl-stained sections of the ICCN was shown
to correlate with seizure severity in the offspring of crosses made be
tween Sprague-Dawley rats and GEPR-9s. Furthermore, the GEPR-3s that d
isplay moderate seizures showed a significant increase in the number o
f small neurons in the ICCN, and the magnitude of this increase was pr
edicted from this correlation. Finally, the use of knife cuts through
the midbrain indicated that the ICCN sends an important projection to
the external nucleus and that this projection plays a vital role in th
e propagation of seizure activity from the site of seizure initiation
in the ICCN. It remains to be resolved how the increase in small GABAe
rgic neurons in the ICCN is responsible for the known pharmacological
defects observed at GABAergic synapses.