The effects of the loss of target cells upon photoreceptor inputs in the fly's optic lobe.

The sensitivity of sensory neurons to target cell denervation varies in the CNS. We have examined the effects of surgically interrupting the output axons of the first optic neuropil, or lamina, in the optic lobe of the fly (Musca domestica), upon the receptor terminal inputs to the lamina. Two of the output interneurons are the monopolar cells L1 and L2, which are found as a pair in each of the unit modules or cartridges of the lamina neuropil. The lamina axons of L1 and L2 degenerate rapidly (within 0.5 h) in a retrograde direction from their lesion site, but there is no sign of retrograde transneuronal degeneration to the receptor terminals, across the input synapse. At each of these synaptic sites, L1 and L2 are invariable contributors to two of the four elements of a postsynaptic tetrad. Not only do the receptor terminals persist, but the presynaptic ribbons at the tetrad sites do also, opposite the degenerated spines of L1 and L2, indicating their lack of target dependence at least over the longest period of post-lesion recovery (48 h) examined. The areal density of presynaptic sites was conserved in the face of the degenerative loss of L1 and L2, as were the numbers of capitate projections (glial invaginations into receptor terminals). The stability of both synaptic density and capitate projection number indicates that they are predominantly influenced by the receptor terminals, which are still intact. A reduction in the number of mitochondrial profiles was one of the few observed changes in the receptor terminals. The results reflect the autonomy which the terminals have, during development, from their interneurons; they especially reflect the role of the terminals in the adult, in maintaining the presynaptic site of their afferent synapses, the tetrads.

A new, possibly serotonergic, neuron in the lamina of the blowfly optic lobe: an immunocytochemical and Golgi-EM study.

With a modified Golgi impregnation technique a new neuron type, with extensive tangential processes, was discovered in the optic lamina of the blowfly, Calliphora erythrocephala. This cell is unusual since the majority of its varicose processes reside in a layer distal to the synaptic layer of the lamina. Using antibodies to 5-HT it was found that this new cell type was 5-HT immunoreactive, and varicosities of Golgi impregnated neurons analyzed electron-microscopically contain dense core vesicles.

Differentiation of fly visual interneurons after laser ablation of their central targets early in development.

To test the differentiation of visual interneurons that had their targets removed before axogenesis, we ablated neuronal precursors in brains of first instar fly larvae by using a laser microsurgery unit. We describe ablations that resulted in the elimination of the third neuropil region (the lobula complex) of the optic lobes. Neural differentiation in the more peripheral second and first neuropil regions (the medulla and the lamina) was thus studied in the absence of the lobula complex. It was found that the medulla neuropil differentiated with normal columnar and layered organization. The neuropil, however, folded along its central surface. The only connection between the medulla and more central neuropil (the midbrain) was via a bundle of axons (the Cuccati bundle) present also in the normal optic lobes. Some types of neurons that normally connect the medulla and the lobula complex could be identified. These appeared to end in a disorganized neuropil mass in the center of the folded medulla. The differentiation of the lamina neuropil also appeared normal in flies with the lobula complex eliminated and the medulla folded. Also, in optic lobes where the medulla was severely disorganized and/or reduced due to laser ablations, the lamina neuropil appeared more or less normal. The results suggest that lamina and medulla nerve cells can differentiate and develop normal neuropil patterns in absence of their appropriate targets.

Cytochrome c as a high resolution marker of neurons for light and electron microscopy.

Cytochrome c is taken up by lesioned neurons in the central and peripheral nervous system of the house fly. The enzyme reaction product is evenly distributed in cell bodies, axons, dendrites and axon terminals after 3-6 h uptake. No uptake into undamaged neurons, nor any transneuronal uptake could be demonstrated. Pre- and postsynaptic relations of cytochrome c-labeled neurons can be resolved.