192 IgG-saporin causes a major loss of synaptic content in rat olfactory bulb

An immunotoxin composed of a monoclonal antibody that recognizes the p75 nerve growth factor (NGF) receptor disulfide-linked to the ribosome-inactivating protein saporin selectively eliminates p75-expressing cholinergic neurons in the basal forebrain, while sparing other neurons in the forebrain, both cholinergic and noncholinergic. We now report the effect that intraventricular administration of this immunotoxin has on the synaptic content of the olfactory bulb, one of the major terminal fields of the cholinergic basal forebrain system. Control substances or immunotoxin were given to rats followed by a 2-week survival. Unilateral transection of the olfactory tract and peduncle was also studied. Both qualitative and quantitative evaluation of olfactory bulbs processed for synaptophysin immunohistochemistry indicated dramatic loss of synapses in the four regions of neuropil evaluated (glomeruli, outer and inner halves of the external plexiform layer, and internal plexiform layer) compared with the administration of control substances. Surgical transection of the bulb produced a visually similar decrement, but quantitative studies showed synaptic loss to be consistently greater following tract transection. The effects of these two insults on the glial response were remarkably different. Transection produced an obvious hyperplasia and hypertrophy of both astrocyte and microglial elements, while immunotoxin produced small, almost undetectable reactions by these two cell types. The results in the glomeruli strongly suggest an effect of the immunotoxin on either periglomerular cells or olfactory nerve terminals, whether directly by NGF receptor (+) structures or by trans-synaptic mechanisms. We conclude that the immunotoxin produces a specific and large loss of synapses that does not produce much glial response.