S. Okoyama et al., ANATOMICAL PLASTICITY IN THE MEDIAL SUPERIOR OLIVE FOLLOWING ABLATIONOF THE INFERIOR COLLICULUS IN NEONATAL AND ADULT-RATS, Hearing research, 88(1-2), 1995, pp. 71-78
We evaluated the consequences of unilateral ablation of the inferior c
olliculus (IC) upon the ascending projection from the medial superior
olive (MSG) to the IC. Ablation of the IC was performed in rats aged b
etween postnatal day 1 (P1) and maturity. All the rats were given inje
ctions of Fluoro-Gold (FG) into the ipsilateral IC at birth (PO) (befo
re the ipsilateral IC was ablated in any case) so that growth of early
-developing axons to the ipsilateral IC could be examined for any labe
led neurons in the ipsilateral MSG. Upon reaching adulthood, the rats
received injections of Fluoro-Ruby (FR) into the contralateral (intact
) IC so that aberrant crossed projections to the intact IC could be ex
amined for any labeled neurons in the ipsilateral MSG. These rats were
killed 2 days after FR injections. The number of surviving cells in t
he ipsilateral MSO were counted in Nissl-stained sections for quantita
tive analysis of retrograde degeneration. The results show that: (1) t
he total number of neurons was reduced to 64-34% in the ipsilateral MS
O as a result of IC ablation; (2) cell reduction by retrograde degener
ation followed a U-shaped curve with a maximal effect in rats operated
at P7 (reduced to 34%); (3) adult ablation of the IC led to retrograd
e degeneration that was less severe than that in late neonatal (P7) ab
lation; (4) an aberrant projection from the MSO to the contralateral I
C occurred in rats operated at P1 and P3 but not in rats operated at P
7 or maturity. Thus, our findings suggest that growth of late-developi
ng axons is a major factor in the plasticity of this system of project
ion.