Age-dependent failure of axon regeneration in organotypic culture of gerbil auditory midbrain

Inferior colliculus (IC) slice cultures from postnatal (P) day 6-8 gerbils exhibit axonal regeneration across a lesion site, and these regrowing proce sses can form synapses. To determine whether regenerative capacity is lost in older tissue, as occurs in vivo, slices from P12-21-day animals were gro wn under similar conditions. While these cultures displayed a near complete loss of neurons over 6 days in vitro, glutamate receptor antagonists (AP5 and/or CNQX) significantly enhanced survival, particularly at P12-15. In co ntrast, several growth factors or high potassium did not improve neuron sur vival. Therefore, axonal regeneration was assessed following complete trans ection of the commissure in AP5/CNQX-treated IC cultures from P12 animals. Neurofilament staining revealed that transected commissural axons survived for 6 days in vitro, but only a few processes crossed the lesion site and t hese axons did not extend into the contralateral lobe, In contrast, there w as robust axonal sprouting and growth within one lobe of the IC, remote fro m the lesion site. When P6 and P12 tissue was explanted onto a coated subst rate, the P6 axons grew onto the substrate, but the P12 axons were seemingl y prevented from reaching the substrate by a veil of nonneuronal cells. Coc ulture of the IC and one of its afferent populations, the lateral superior olive, provided a similar finding, indicating that failure to regenerate wa s a general property at the age examined. These data show that neuron survi val is not sufficient to permit axon regeneration at P12, and suggest that P12 lesion sites manufacture a prohibitive substrate since process outgrowt h is blocked specifically at the commissure transection. (C) 1999 John Wile y & Sons, Inc.