AN AXOSOMATIC AND AXODENDRITIC MULTIPOLAR NEURON IN THE LIZARD CEREBRAL-CORTEX

The morphology and synaptic organisation of a type of multipolar neuro n of the lizard cerebral cortex were studied by Golgi impregnation, in tracellular injection of horseradish peroxidase, electron microscopy, and immunocytochemistry. It is a GABA-immunoreactive interneuron and m ost likely parvalbumin-immunoreactive. Its conspicuous axonal arbor is characterised by an initial segment arising from the soma or from a j uxtasomatic dendritic segment. The initial axon segment ramifies and g ives rise to thick myelinated segments that terminate in short unmyeli nated branches studded with thick boutons 'en passant' that (1) make a xosomatic synapses on bipyramidal neuronal somata and (2) synapse on i nitial apical dendritic segments of bipyramidal neurons forming climbi ng-like cartridges. The dendrites extend throughout the thickness of t he cortex, receiving synaptic input from a variety of sources of which the most prominent is that of zinc-positive boutons coming from granu le cells of the medial cortex. According to its synaptology, this inte rneuron may play a role in regulating the activity of bipyramidal neur ons by both feed-forward and feed-back inhibition mechanisms. From a c omparative standpoint, it may be related to the sparsely spiny or nons piny multipolar neurons of the stratum oriens of the mammalian hippoca mpus.