THE SEROTONINERGIC BULBOSPINAL SYSTEM AND BRAIN-STEM-SPINAL CORD CONTENT OF SEROTONIN-LIKE, TRH-LIKE, AND SUBSTANCE-P-LIKE IMMUNOREACTIVITYIN THE AGED RAT WITH SPECIAL REFERENCE TO THE SPINAL-CORD MOTOR NUCLEUS

The 5-hydroxytryptamine (5HT) containing bulbospinal pathway was studi ed with immunohistochemical (IF) and chemical techniques in 2-3 and 30 months old male Sprague-Dawley rats. The coexisting neuropeptides sub stance P (SP), thyrotropin-releasing hormone (TRH) and galanin were al so analysed. Furthermore, the expression of mRNA encoding aromatic L-a mino acid decarboxylase (AADC), prepro-TRH, and preprotachykinin (prep ro-SP) was analysed with in situ hybridization (ISH) in the midline ra phe nuclei in the lower brainstem. The results showed a decreased numb er of axonal 5HT fibers with a normal morphology in the ventral horn o f the aged rat lumbosacral spinal cord, and several 5HT immunoreactive (IR) fibers with an aberrant morphology, suggestive of axonal degener ation, were intermingled. This was evident in both the dorsal and vent ral horn of the spinal cord. The 5HT-IR fibers with an aberrant morpho logy usually also contained TRH-and/or SP- and/or galanin-like immunor eactivity (LI) in the ventral horn. These signs of degeneration were c learly less evident in the thoracic and cervical spinal cord segments. Moreover, these changes varied between aged litter-mates. This was in agreement with behavioural signs of motor disturbances, present in ab out 40% of the aged rats and which in all cases were confined to the h indlimbs. Chemical analyses disclosed significantly lower levels of TR H-LI and, in particular, SP-LI in both the ventral and dorsal quandran ts of the spinal cord in the aged rat compared to young adults. The di fferences were largest in the lumbar regions of the spinal cord. Corre sponding analysis of 5HT and 5-hydroxyindoleacetic acid (5HIAA) in the same tissue specimens revealed largely unaltered levels of 5HT and a slight increase in 5HIAA, indicating the possibility of an increased 5 HT turnover in the aged rat spinal cord. Neurons in nucleus raphe obsc urus and nucleus raphe pallidus were immunoreactive to 5HT, and after pretreatment with colchicine to TRH-, SP-, and galanin-LI as well. The re was no obvious difference in number of labeled cells, or labeling i ntensity, between colchicine-treated young adult and aged rats, althou gh, in the corresponding region of medulla oblongata, chemical analysi s disclosed significantly lower levels of 5HT, TRH, and, in particular , SP in untreated aged rats. In contrast, in situ hybridization analys is revealed increased mRNA levels encoding prepro-TRH and prepro-SP in old rats, while mRNA content encoding AADC mRNA was similar in young adult and aged rats. The results demonstrate an age related degenerati on of the 5HT axon arborizations in the spinal cord with a clear rostr ocaudal variance correlated to the motor deficiencies observed in the aged rat. Aberrant axons often also contained TRH, SP, and galanin. Th e marked decline in spinal TRH and SP combined with the increase level s of medullary SP and TRH mRNA suggest increased peptide turnover in t he aged rat. There was also indications for an increased 5HT turnover. Finally, chemical results showed that local and/or primary afferent p eptide systems in the dorsal horn may be affected in the aged rat. (C) 1993 Wiley-Liss, Inc.