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The Journal of Neurophysiology Vol. 84 No. 1 July 2000, pp. 367-375
Copyright ©2000 by the American Physiological Society
Department of Neurobiology and Anatomy, University of Texas Medical School, Houston, Texas 77225
Crow, Terry and
Lian-Ming Tian.
Monosynaptic Connections Between Identified A and B
Photoreceptors and Interneurons in
Hermissenda: Evidence for Labeled-Lines. J. Neurophysiol. 84: 367-375, 2000. The cellular and synaptic organization of
the eye of the nudibranch mollusk Hermissenda is
well-documented. The five photoreceptors within each eye are mutally
inhibitory and can be classified into two types: A and B based on
electrophysiological and anatomical criteria. Two of the three type B
and two type A photoreceptors can be further identified according to
their medial or lateral positions within each eye. In addition to
reciprocal synaptic connections between photoreceptors, photoreceptors
also project to second-order neurons in the cerebropleural ganglion.
The second-order neurons receive convergent synaptic input from two
additional sensory pathways; however, it has not been previously
established if lateral A, lateral B, or medial B photoreceptors
converge onto the same second-order neurons. To determine the specific
synaptic organization of these components of the visual system, we have examined monosynaptic connections between identified lateral and medial
type A and B photoreceptors and second-order cerebropleural (CP)
interneurons. We found that monosynaptic connections between identified
lateral A and lateral and medial B photoreceptors and CP interneurons
follow a labeled-line principle. Illumination of the eyes or extrinsic
depolarizing current applied to identified photoreceptors evoked
excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs,
respectively) in different CP interneurons. The PSPs in CP interneurons
followed one-for-one spikes in the photoreceptors and could be elicited
in artificial seawater solutions containing high divalent cations.
Identified photoreceptors projected to more than one CP interneuron and
expressed both excitatory and inhibitory connections with the different
CP interneurons. In examples where a monosynaptic connection between a
lateral B photoreceptor and a CP interneuron was identified, lateral A, medial A, or medial B photoreceptors did not project to the same CP
interneuron. Moreover, when connections between medial B and CP
interneurons were identified, lateral A, medial A, and lateral B
connections were not found to project to the same CP interneuron. Similar results were obtained for a lateral A and CP interneuron connection. These results indicate that divergent labeled-lines exist
between specific photoreceptors and second-order CP interneurons and
potential convergence of synaptic input from primary and secondary elements of the visual system must occur at sites that are postsynaptic to the CP interneurons.
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