DEVELOPMENTAL PLASTICITY IN THE RESPIRATORY PATHWAY OF THE ADULT-RAT

Hemisection of the cervical spinal cord rostral to the phrenic nucleus paralyzes the ipsilateral hemidiaphragm. When the contralateral phrenic nerve is transected immediately after the hemisection, function may be restored to the paralyzed hemidiaphragm. This response, which occurs only in some mammalian species (e.g., the cat, but not the guinea pig), was termed the crossed phrenic phenomenon. Although the crossed phrenic response cannot be induced in the guinea pig when the hemisection and phrenicotomy are performed without delay, it is induced when the phrenicotomy is performed several hours after hemisection. The synaptic connections essential for the response may normally exist in a functionally ineffective state in guinea pigs. These ineffective connections are converted within a few hours after spinal cord injury to synapses capable of eliciting the crossed phrenic response. This study determined if functionally ineffective synapses are also present in the respiratory pathway of rats. Young rats (5-9 wk old) required an interoperative delay of 24 h or more to elicit the crossed phrenic response, whereas in older rats (26 wk of age) the crossed phrenic response occurred without any interoperative delay. Functionally ineffective synapses exist in young rats and these synapses are converted to functionally effective ones with age. This change in synaptic connections occurs in adult rats as they develop from young maturity to older maturity. The mechanisms for such developmental plasticity in the adult animal are not known.