The intrastriatal injection of thrombin in rat induced a retrograde apoptotic degeneration of nigral dopaminergic neurons through synaptic elimination

We have performed intrastriatal injection of thrombin and searched for distant effects in the cell body region. In striatum, thrombin produced a slight loss of striatal neurons as demonstrated by neural nuclei immunostaining – a non-specific neuronal marker – and the expression of glutamic acid decarboxylase 67 mRNA, a specific marker for striatal GABAergic interneurons, the most abundant phenotype in this brain area. Interestingly, striatal neuropil contained many boutons immunostained for synaptic vesicle protein 2 and synaptophysin which colocalize with tyrosine hydroxylase (TH), suggesting a degenerative process with pre-synaptic accumulation of synaptic vesicles. When we studied the effects on substantia nigra, we found the disappearance of dopaminergic neurons, shown by loss of TH immunoreactivity, loss of expression of TH and dopamine transporter mRNAs, and disappearance of FluoroGold-labelled nigral neurons. The degeneration of substantia nigra dopaminergic neurons was produced through up-regulation of cFos mRNA, apoptosis and accumulation of α-synuclein shown by colocalization experiments. Thrombin effects could be mediated by protease-activated receptor 4 activation, as protease-activated receptor 4-activating peptide mimicked thrombin effects. Our results point out the possible relationship between synapse elimination and retrograde degeneration in the nigral dopaminergic system.