DAMAGED 5-HT (5-hydroxytryptamine) axons in the central nervous system have been observed to undergo vigorous sprouting1,2. Regeneration of these axons to their original termination sites has been seen several months after intraventricular administration of selective 5-HT neurotoxic drugs; and in the spinal cord this regrowth has been correlated with a return of normal 5-HT receptor function as measured by a hindlimb stretch reflex3. These findings raise two questions. First, is the pattern of re-innervation after injury dependent on some regenerative property of the damaged axons themselves, or is it primarily controlled by the denervated terminal field? And second, can re-innervation take place not only by regeneration of the original axons but also by collateral sprouting of chemically identical but anatomically separate undamaged fibres; and , if so, will function be restored? In the work reported here we have taken advantage of the fact that the hippocampus receives two separate inputs of 5-HT fibres from the median raphe nucleus, and we have shown that the unilateral removal of one input induces re-innervation of the unoccupied sites by the other. This collateral sprouting occuring in a single hippocampus is accompanied by disappearance of an asymmetrical behavioural response, suggesting that not only structural but functional recovery has occurred. Our results, therefore, suggest that the 5-HT terminal field plays a major part in the reorganisation of this system after injury to the brain.
ASJC Scopus subject areas