Inhibition by tetanus toxin of sodium-dependent, high-affinity [3H]5-hydroxytryptamine uptake in rat synaptosomes

Tetanus toxin (TeTx) is a powerful clostridial neurotoxin that inhibits Ca²⁺-dependent neurotransmitter secretion as do the botulinum neurotoxins (BoNTs). We found that TeTx (but not BoNT/A) produced a specific time- and dose-dependent inhibition of Na⁺-dependent [³H]5-hydroxytryptamine (serotonin, 5-HT) uptake in rat CNS synaptosomes. This effect was found in all CNS tryptaminergic areas, being maximal in the hippocampus and occipital cortex. TeTx produced the maximum reduction in [³H]5-HT uptake after 30 min of preincubation, being significant also at lower doses (10^-12 M) or shorter incubation times (10 min). Serotonin transport inhibitors such as fenfluramine (IC₅₀, 11.0 ± 0.9 μM), paroxetine (IC₅₀, 33.5 ± 0.1 μM), and imipramine (IC₅₀, 89.9 ± 5.7 μM) were 3 or 4 orders of magnitude less potent than TeTx (IC₅₀, 8.7 ± 1.0 nM). Of the two fragments of TeTx, (the C-terminal portion of the neurotoxin heavy chain, which is responsible for the binding to the nerve tissue) was consistently more effective than the L- H(N) fragment (the light neurotoxin chain disulfide linked to the N-terminal portion of the heavy chain, which is responsible for the toxic metalloprotease action) as inhibitor of [³H]5-HT uptake in synaptosomal preparations (56 ± 5% and 95 ± 3% with respect to control, respectively). Antagonism of the toxin-induced [³H]5-HT uptake blockade could not be reversed by zinc chelators but did have the ability to antagonize the TeTx inhibition of basal and K⁺-evoked [³H]5-HT release in rat synaptosomes. The reduction in serotonin accumulation induced by TeTx could be responsible for some tetanic symptoms that have been related to the serotonergic system.