SEROTONINERGIC, PEPTIDERGIC AND GABAERGIC INNERVATION OF THE VENTROLATERAL AND DORSOLATERAL MOTOR NUCLEI IN THE CAT S1 S2 SEGMENTS - AN IMMUNOFLUORESCENCE STUDY/

Indirect single- and double-staining immunofluorescence techniques wer e used to study the serotoninergic, peptidergic and GABAergic innervat ion of the ventrolateral (Onuf's nucleus) and dorsolateral (innervatin g intrinsic foot sole muscles) nuclei, located in the S1/S2 segments o f the cat spinal cord. The relative density of 5-hydroxytryptamine-, t hyrotropin-releasing hormone-, substance P- and gamma-aminobutyric aci d-immunoreactive axonal varicosities was similar in both nuclei. The h ighest relative density was recorded for varicosities immunoreactive t o gamma-aminobutyric acid, while those immunoreactive to 5-hydroxytryp tamine or thyrotropin-releasing hormone yielded the lowest values. The density of enkephalin-immunoreactive varicosities was higher in the v entrolateral than in the dorsolateral nucleus. Calcitonin gene-related peptide-like immunoreactivity could be seen in neurons of the ventrol ateral and dorsolateral nuclei. Occasionally, calcitonin gene-related peptide-immunoreactive axonal fibers were also encountered in these nu clei. Virtually all thyrotropin-releasing hormone-immunoreactive varic osities in the ventrolateral and dorsolateral nuclei also contained 5- hydroxytryptamine-like immunoreactivity, while a somewhat smaller numb er of them were co-localized with substance P. About 5-10% of the 5-hy droxytryptamine-immunoreactive varicosities were devoid of peptide-lik e immunoreactivity, and the number of 5-hydroxytryptamine-immunoreacti ve varicosities lacking thyrotropin-releasing hormonelike immunoreacti vity was higher in the dorsolateral than in the ventrolateral nucleus. Finally, the free fraction of substance P-immunoreactive varicosities , i.e., those lacking both 5-hydroxytryptamine and thyrotropin-releasi ng hormone, was about 39% in the ventrolateral and 26% in the dorsolat eral nucleus. Spinal cord transection at the lower thoracic level indu ced a depletion of 5-hydroxytryptamine and thyrotropin-releasing hormo ne-immunoreactive fibers from the ventrolateral and dorsolateral nucle i, indicating an exclusive supraspinal origin for these fibers. A redu ction in substance P-like immunoreactivity following spinal cord trans ection alone or spinal cord transection combined with unilateral dorsa l rhizotomy was also detected in both nuclei, suggesting a dual origin for substance P-immunoreactive fibers, i.e., both supra- and intraspi nal. The decrease in number of substance P-immunoreactive fibers was h owever smaller than expected from the analysis of the fraction of subs tance P-immunoreactive fibers co-localized with 5-hydroxytryptamine, i ndicating thus that the experimental lesions may have triggered a spro uting of substance P-immunoreactive axons originating from spinal cord sources. The distribution of gamma-aminobutyric acid in the ventrolat eral and dorsolateral nuclei was not affected by the different lesion paradigms. It is therefore assumed that these inputs are intrinsic to the spinal cord. Finally, both in the ventrolateral and the dorsolater al nucleus a small but statistically significant increase of axonal fi bers immunoreactive to enkephalin was seen in response to the experime ntal lesions.