Spinal motoneurones and the bulbospinal serotonininergic system in aged rats with behavioral deficits

The expression of neuropeptides, growth-related proteins and neurotrophin receptors were studied in motoneurons of young adult rats and in aged rats with symptoms of neuromuscular incapacities. Also studied were aging-related changes in the descending serotoninergic (5HT) bulbospinal system which constitutes a major afferent pathway to spinal motoneurons. Finally, aged rats were subjected to behavioral sensorimotor tests after treatment with drugs that increase the availability of 5HT in the brain and the spinal cord. A large number of rat lumbar motoneurons express [alpha]-CGRP in the young adult rat. The expression of [alpha]-CGRP in motoneurons; is influenced by changes in the synaptic input to the motoneurons as well as the integrity of the motoneurons. Thus, damage to motor axons induced a transient upregulation of [alpha]-CGRP, while peripheral nerve lesion which also involves sensory pathways, in addition, caused an brief upregulation of the unlesioned contralateral motoneurons. In spinal cord transected rats (SCT) the expression of [alpha]- CGRP in motoneurons below the lesion decreased progressively over the period studied. However, if the SCT was combined with a peripheral nerve transection, [alpha]-CGRP became upregulated in the motoneurons. Furthermore, selective lesioning of the serotoninergic (5HT) neurons, including those giving rise to the descending bulbospinal pathway, with 5,7dihydroxytryptamine (5,7-DHT) induced an upregulation of [alpha]- CGRP in the motoneurons. Only a small number (~15%) of the motoneurons that innervate the hind limb muscles were lost in aged rats and the remaining motoneurons expressed choline acetyltransferase (ChAT) mRNA at normal levels. The vast majority of the remaining motoneurons showed an increased expression of [alpha]-CGRP and growth-associated protein (GAP-43). Studies of individual aged rats revealed that the increase in number of GAP-43 mRNA positive motoneurons occurred in cases with the most severe clinical symptoms, while the increase in [alpha]-CGRP also was evident in rats with mild symptoms. The changes observed in aged motoneurons are quite similar to those seen in young adult motoneurons suffering from axon damage. Mature spinal motoneurons express mRNA encoding neurotrophin receptors trkB and trkC. In aged rats, there was a downregulation of trkB and trkC mRNA and an upregulation of p75NTR. The decreased levels of trk receptors may indicate a diminishing access to trk ligands in senescence, while the parallel upregulation of p75NTR may represent a compensatory mechanism. In response to axotomy both age groups showed an upregulation of p75NTR. mRNA and in young adult, but not in aged, rats there was a parallel increase in trkB mRNA. In spinal cord of aged rats a decreased number of 5HT fibres with a normal morphology was observed. Many of the 5HT fibers had an aberrant morphology, suggesting axon dystrophy/axon degeneration. These regressive sips showed a rostrocaudal gradient where the lumbosacral segments were more affected than the cervical and thoracic segments. The most extensive signs of degeneration could be found in aged animals with the most severe hind limb incapacities. The aberrant fibers also contained thyrotropin-releasing hormone (TRH) and/or substance P (SP) and/or galanin. Chemical analysis disclosed decreased levels of TRH and SP in the spinal cord, with the most pronounced decrease in the lumbar segments. 5HT levels in the spinal cord was unaltered but an in increase of 5- hydroxyindoleacetic acid (5HIAA) indicated an increased 5HT tum-over in aged rat spinal cord. Medulla oblongata contained decreased levels of 5HT, TRH and SP and increased levels of 5HIAA in aged rats. In contrast, the expression of TRH and SP mRNA was increased while the expression of aromatic L-amino acid decarboxylase AADC mRNA was unaltered in nucleus raph obscurus (NRO) and pallidus. (NRP) of aged rats. There was also an increased expression of the serotonin transporter protein mRNA in the aged animals. 'Me most marked mRNA increase was evident in animals with the most pronounced symptoms of hind limb motor dysfunction. Treatment with a selective serotonin reuptake inhibitor (fluoxetine) did not have any effect on explorative or motor behavior in aged animals. The aged rats reacted to a single dose of the 5HT precursor L-5hydroxitryptophan with 5HT-dependent behavior such as head-twitches, however, the response was clearly less vigorous than in young adult animals