Distribution of serotonin and FMRF-amide in the brain of Lymnaea stagnalis with respect to the visual system.

Despite serotonin's and FMRF-amide's wide distribution in the nervous system of invertebrates and their importance as neurotransmitters, the exact roles they play in neuronal networks leaves many questions. We mapped the presence of serotonin and FMRF-amide-immunoreactivity in the central nervous system and eyes of the pond snail Lymnaea stagnalis and interpreted the results in connection with our earlier findings on the central projections of different peripheral nerves. Since the chemical nature of the intercellular connections in the retina of L. stagnalis is still largely unknown, we paid special attention to clarifying the role of serotonin and FMRF-amide in the visual system of this snail and compared our findings with those reported from other species. At least one serotonin- and one FMRF-amidergic fibre were labeled in each optic nerve, and since no cell bodies in the eye showed immunoreactivity to these neurotransmitters, we believe that efferent fibres with somata located in the central ganglia branch at the base of the eye and probably release 5HT and FMRF-amide as neuro-hormones. Double labelling revealed retrograde transport of neurobiotin through the optic nerve, allowing us to conclude that the central pathways and serotonin- and FMRF-amide-immunoreactive cells and fibres have different locations in the CNS in L. stagnalis. The chemical nature of the fibres, which connect the two eyes in L. stagnalis, is neither serotoninergic nor FMRF-amidergic.

Afferent and efferent pathways in the visual system of the freshwater snail Planorbarius corneus.

Afferent and efferent neural elements of the retina and central ganglia in the freshwater snail Planorbarius corneus were labelled using retrograde transport of neurobiotin through the optic nerve. Axons of at least some photoreceptor cells become direct contributors to the optic nerve as no synaptic junctions could be detected. The processes enter the cerebral ganglion and form a dense bundle of thin afferent fibres, the so-called optical neuropil. Efferent neurons were revealed in all ganglia, except the buccal ones. Some of the ascending axons branch in the cerebral ganglia, cross the cerebro-cerebral commissure, reach the contralateral eye and form arborizations in the eye cup. Some efferent neurons send axons to different peripheral nerves as well: n.n. intestinalis, pallialis dexter, pallialis sinister internus et externus. Serotonin- and FMRF-amide-ergic fibres were revealed in the optic nerve. These fibres belong to those central neurons which send their axons to the ipsilateral eye only. They form abundant varicoses in the eye cup and nuclear layer of the retina, and possibly help to regulate retinal sensitivity to light.