Biogenic amines in cephalopod retina.

Biochemical and ultrastructural studies by previous workers have suggested dopamine as a candidate neurotransmitter for the centrifugal neurons present in the cephalopod retina. We have examined the retina in 3 species of cephalopod mollusc and find strong green bands of glyoxylic acid-induced fluorescence in the synaptic plexus region. These results, together with data obtained from our Golgi impregnations, help to confirm previous speculations that the centrifugal neurons are dopaminergic.

Biogenic amines in the brain of the honeybee, Apis mellifera.

Fluorescence histochemistry with glyoxylic acid has been used in close conjunction with detailed anatomical studies (Mobbs 1982) to investigate the distribution of fluorogenic amines in the brain of the honeybee, Apis mellifera. In addition, the concentration and distribution of biogenic amines in the brain of the bee have been determined using highly sensitive radioenzymatic techniques and high performance liquid chromatography. The cerebral ganglia of the bee contain similar amounts of dopamine and serotonin, more dopamine than octopamine, and very low levels of noradrenaline. Slow fading green fluorescence, typical of catecholamines, was located throughout the cerebral ganglia, and was particularly intense in the central body and mushroom body neuropils. Results indicate that the fluorescence in the mushroom body neuropils is largely extrinsic in origin. Both dopamine and serotonin were detected in the calyces and alpha-lobe of the mushroom bodies, and in the antennal lobe. In the optic lobe, however, serotonin was found, but only low levels of dopamine were detected. Slow fading green fluorescence was replaced in the optic lobes by fluorescence which faded rapidly in the excitation light. The non-fluorogenic amine octopamine was found in the mushroom bodies and in the neuropils of the optic lobes, with the largest amounts of octopamine in the optic lobes associated with the neuropil of the medulla. The possibility that intrinsic neurones of the mushroom body neuropil are octopaminergic is discussed.

The fine structure of the ocelli of Schistocerca gregaria. The neural organisation of the synaptic plexus.

A study of the organisation of the locust dorsal ocellus shows that the structure is designed to provide the maximum possible effective aperture. The condenser-like cuticular lens and the dispersal of the rhabdome over a large proportion of the circumferential area of the retinula cells increases the light gathering power of the eye. The synaptic plexus of the ocellus has two major features: (i) the retinula cells are repeatedly and reciprocally connected by synapses and junctions, and (ii) there is an extensive lateral and feedback network between the receptors and interneurons. A unified structure is described for a synapse that presents differing profiles dependent upon the angle of section. A distinct morphological class of junction is described between retinula cells. The synaptic arrangements of morphologically identical retinula cells vary from cell to cell and the synaptic plexus is not organised with a high degree of spatial precision. The overall synaptic configurations are discussed in terms of the varied response characteristics of units in the ocellar nerve.

Information processing along the course of a visual interneuron.

Locust ocellar retinal cells are innervated by giant second order cells, 2 mm long, which show discrete zones of integration along their course, including a major zone in the axonal length of the neuron. The complex synaptic arrangements which exist between higher-order afferent and efferent cells and these second order cells along their course suggests that transmission takes place by the electrotonic spread of slow potentials. The size and accessibility of these visual interneurons offers a unique preparation for examining mechanisms of graded synaptic transmission.

The projection of ocellar neurons within the brain of the locust, Schistocerca gregaria.

Cobalt iontophoresis of the median and lateral ocellar nerves of Schistocerca gregaria, combined with silver impregnated sections of the brain, has demonstrated the projection area of the large and medium-sized ocellar afferent neurons. These neurons terminate within the brain and their cell bodies lie within the protocerebrum. Ocellar neurons project to two discrete areas on each side of the brain, each area receiving input from a different set of fibres. Both postero-dorsal complexes receive an imput from two large ipsilateral and two large median fibres. Their dendritic fields maintain an ordered spatial array relative to one another. The two antero-lateral complexes receive an imput from one large ipsilateral fibre and medium-sized ipsilateral and medium small-field afferent fibres. Each lateral ocellus has two large fibres in common with the median ocellus. These lateromedial fibres receive photoreceptor input from both ocelli but form no major arborisations within the brain. The lateral ocellar tracts appear to form a third ocellar association area since higher-order neurons branch amongst the lateral and latero-medial fibres within the tract. The axons of the higher-order neurons descend to the ventral cord via the circumoesophageal commissures.