Synaptosomal tryptophan uptake and efflux following lesion of central 5-hydroxytryptaminergic neurones

Abstract

1. This study attempted to determine whether the activation of the tryptophan carrier in rat forebrain synaptosomes caused by depolarization or by extracellular sodium depletion occurred exclusively in 5-hydroxytryptaminergic nerve endings. 2. Ascending 5-hydroxytryptaminergic neurones were lesioned either electrolytically or by intraventricular administration of 5,7-dihydroxytryptamine. The extent of the lesion was assessed by comparing the uptake of [3H]-5-hydroxytryptamine (5-HT) in lesioned animals and in sham-operated controls. [3H]-5-HT uptake was reduced by 85.9 +/- 1.63% (mean +/- s.e. mean) in animals receiving electrolytic lesions, and by 87.4 +/- 4.51% in those receiving 5,7-dihydroxytryptamine. 3. The uptake of [3H]-tryptophan by synaptosomes from lesioned animals incubated in standard Na(+)-rich media was slightly lower (278.8 +/- 27.3 pmol mg-1 protein min-1) than that observed in sham-operated controls (360.6 +/- 30.3 pmol mg-1 protein min-1). However, uptake in the absence of extracellular Na+ was increased to a similar extent in both the sham-operated (539 +/- 54.5 pmol mg-1 protein min-1) and lesioned animals (507.2 +/- 42.4 pmol mg-1 protein min-1). 4. The efflux of [3H]-tryptophan in response to extracellular Na+ depletion was similar in sham-operated and lesioned animals. Release expressed as a percentage of tissue [3H]-tryptophan released in response to the pulse of Na(+)-free medium was 6.691 +/- 0.585 (n = 4) in sham-operated controls and 8.195 +/- 0.906 in lesioned animals. 5. The efflux of [3H]-tryptophan in response to K+ depolarization was also unchanged in lesioned animals when compared with sham-operated controls. Release, expressed as described above was, in sham-operated controls 3.76 +/- 0.41 (n = 4) and 4.09 +/- 0.30 in lesioned animals. 6. The results of this study show that the tryptophan carrier which is activated by depolarization or by extracellular Na+ depletion is not located exclusively on 5-hydroxytryptaminergic nerve endings. Moreover the contribution made by 5-hydroxytryptaminergic neurones appears to be only minor.