EXPRESSION OF CHOLINERGIC PHENOTYPE BY EMBRYONIC VENTRAL HORN NEURONSTRANSPLANTED INTO THE SPINAL-CORD IN THE RAT

Solid grafts of E12 embryonic spinal ventral horn were transplanted in to motoneuron-depleted adult lumbar spinal cord in the rat. A muscle w as implanted parallel to the vertebral column with its nerve inserted into the lumbar cord at the site of transplantation so as to provide a target for innervation by the grafted neurons. Previous retrograde la belling studies have shown that modest numbers of grafted motoneuron-l ike cells participate in the muscle's reinnervation and these are ofte n found outside the graft within the host spinal cord. However, Nissl stained sections show that larger numbers of neurons survive within ti ssue recognisable as being of graft origin. In this study we have exam ined the expression of acetylcholinesterase (AChE) and choline acetylt ransferase (ChAT) by neurons within the graft. These enzymes are invol ved in cholinergic neurotransmission and are characteristic of motoneu rons. Thirty-four to seventy days following transplantation the grafts contained numerous neurons with acetylcholinesterase (AChE) activity. Different patterns of AChE staining were observed which probably refl ected the degree of differentiation and maturation within the graft. A ChE positive neurons were found in isolation or in groups resembling d eveloping motor pools. Most of the AChE-positive neurons appeared imma ture with scant cytoplasm. However, neurons could be found which appea red relatively mature with a regularly shaped nucleus, prominent nucle olus and Nissl bodies. The grafts contained few AChE-positive axons an d no dense plexuses of varicose fibres around the neurons such as are found around motoneurons in the mature ventral horn. Comparisons betwe en the size of AChE-positive neurons in the graft and the size of AChE -positive neurons in the developing ventral horn found that the size o f grafted neurons to be intermediate between the sizes of spinal moton eurons at E19 and PO. Far fewer grafted neurons were found to be immun oreactive for choline acetyltransferase (ChAT) than histochemically re active for AChE. This was consistent with our findings in the spinal c ord during normal development where we found that fixation and stainin g procedures which labelled adult motoneurons failed to reliably demon strate ChAT immunoreactivety in normal motoneurons prenatally, althoug h AChE histochemical reactivity could be demonstrated as early as E16. We conclude that the grafts contain numbers of immature motoneurons w hich fail to proceed beyond a certain stage of development, perhaps be cause of a failure to form appropriate efferent and afferent connectio ns.