GLUTAMATE-IMMUNOREACTIVE AND GABA-IMMUNOREACTIVE SYNAPSES ON SYMPATHETIC PREGANGLIONIC NEURONS CAUDAL TO A SPINAL-CORD TRANSECTION IN RATS

Spinal cord injury destroys bulbospinal amino acid-containing pathways to sympathetic preganglionic neurons and severely disrupts blood pres sure control, resulting in resting or postural hypotension and episodi c hypertension. Almost all immunoreactivity far the excitatory amino a cid L-glutamate has been reported to disappear from autonomic areas of the cord caudal to a transection, apparently depriving autonomic neur ons of their major excitatory input. However, the magnitude of the neu rogenic episodic hypertension after cord injury suggests that excitato ry inputs to sympathetic preganglionic neurons must still be present. Moreover, the hypotension associated with high spinal injuries may ref lect a enhanced role for inhibitory transmitters, such as GABA. This a pparent contradiction regarding the presence of glutamate and lack of information about GABA prompted the present investigation. In rats sev en days after spinal cord transection, we examined identified sympathe tic preganglionic neurons caudal to the injury for the presence of syn apses or direct contacts from Varicosities that were immunoreactive fo r the amino acids, L-glutamate and GABA. Adrenal sympathetic pregangli onic neurons were retrogradely labelled with cholera toxin B subunit a nd amino acid immunoreactivity was revealed with post-embedding immuno gold labelling. In single ultrathin sections, 46% (98/212) of the syna pses or direct contacts on adrenal sympathetic preganglionic neurons w ere immunoreactive for glutamate and 39% (83/214) were immunoreactive for GABA. Analysis of inputs with the physical disector yielded simila r results for the two amino acids. The proportions of glutamatergic or GABAergic synapses on cell bodies and dendrites were similar. When al ternate ultrathin sections were stained to reveal glutamate or GABA im munoreactivity, either one or the other amino acid occurred in 78.4% ( 116/148) of inputs; 4.1% (6/148) of inputs contained both amino acids and 17.5% (26/148) of inputs contained neither. These results demonstr ate that nerve fibres immunoreactive for the neurotransmitter amino ac ids, glutamate and GABA, provide most of the input to sympathetic preg anglionic neurons caudal to a spinal cord transection. Synapses contai ning glutamate and GABA could provide the anatomical substrate For the exaggerated sympathetic reflexes and the low sympathetic tone that re sult from spinal cord injury. (C) 1997 IBRO. Published by Elsevier Sci ence Ltd.