Proximo-distal Increase of Enzymic Activity in the Dorsal Spinal Tracts.

IT IS known that enzyme distribution is not the same among various fibre tracts, but except for LUBLINSKA’S (1 962) report of a proximo-distal decrease of cholinesterase along peripheral nerves there is little information about differences of enzyme activity along the course of a given fibre tract. This paper presents biochemical and histochemical data of the distribution of lactate dehydrogenase (LDH) along the tractus gracilis (Goll’s tract) and the tractus cuneatus (Burdach’s tract). The enzymes cytochrome oxidase (CYO), succinate dehydrogenase (SDH), malate dehydrogenase (MDH), glutamate dehydrogenase (GDH), and glucose 6-phosphate dehydrogenase (G-6-PDH) were also studied histochemically. The data show a gradual proximodistal increase of most of these enzymes in the axons of both fibre tracts; however, the increase is much more pronounced in the tractus gracilis than in the tractus cuneatus. The possible significance of this phenomenon is discussed. Additional studies evidenced an inverse distribution of LDH in glial cells and axons along the course of the tractus gracilis: in the caudal part of the tractus gracilis, the LDH activity was strong in many glial cells and weak in the axons; in the cranial-most part of the tract, there was no LDH activity in glial cells but it was very intense in the axons. MATERIAL A N D METHODS Assay methods. Biochemical determinations of the LDH activity at different levels of the tractus gracilis and tractus cuneatus were made in ten cats; two cats were used for preliminary establishment of a standard procedure for sampling. The results of studies on eight cats are included in the biochemical data. The cats were sacrificed with sodium pentobarbitone and the medulla oblongata and 6 cm of the cervical spinal cord were removed. The tissue was transferred to ice-cold saline and used within 10min. Transverse dorso-ventral incisions were made into the cord to divide the dorsal tracts into segments exactly 5 mm in length. By comparing the features of the cross section of the cord with the dorsal fissure, sulci and roots at its dorsal surface, the positions of the tractus gracilis and cuneatus were readily determined, and wedge-shaped samples were cut from each tract from the dorsal surface of the cord. In the beginning, after the assay sample was removed the remainder of the specimen was fixed and embedded in paraffin, and the stained sections were studied to determine if assay samples were free of contamination by adjacent tissue. After a little practice, uncontaminated samples of both tracts were obtained without difficulty. The wet weight of each sample, 23-7 mg, was determined immediately. These samples were transferred to chilled glass tissue-grinder tubes and homogenized with a motor driven pestle in ice-cold tris buffer, pH 7.4. Concentration of homogenates was 10 mg of tissue per ml of buffer. The incubation medium was slightly modified from that of ALLEN and SLATER (1961) for cytochemical determination of LDH: 1 ml sodium lactate, 0.5 M ; 1 ml NAD, 0.01 M ; 1 ml potassium cyanide,