Axonal regeneration following retrochiasmatic deafferentation in young and adult rats.

Regeneration of nerve fibres after hypothalamic knife cuts was studied by the anterograde transport of horseradish peroxidase [HRP] in female rats of various ages. Retrochiasmatic frontal cuts were made in 2, 5, 7 and 11-day-old and in adult rats. Four, 6, 7 and 12 months later HRP was injected rostral to the cut-line in the suprachiasmatic area. HRP-stained nerve fibres ran rostro-caudally from the injection site through the cut-line in animals operated upon at 2, 5 and 7 days of age. In contrast to the former group, animals operated on the 11th day of life as well as in adult rats no HRP-stained nerve fibres could be seen passing through the cut-line which was marked by scar formation. In one animal operated in adulthood a bundle of nerve fibres noticed 7 months after the surgery turned medially at the caudal end of the cut-line and spread over the frontally deafferented area. The character of these newly formed fibres was different: part of them showed a varicose appearance, the others exhibited even contours. The present findings indicate that the deafferented [or isolated] hypothalamus remains neuronally isolated from the environment if the operation is carried out later than the end of the first week of life. Operations made during the first postnatal week do not leave permanent traits in the brain.