Background illumination effects upon in vitro conditioning in Hermissenda

Elementary properties of axonal calcium currents in type B photoreceptors in Hermissenda crassicornis

Axons of the type B photoreceptors form synapses with hair cells and interneurons that are involved in classical conditioning in Hermissenda. We examined the differences in the Ca2+ channels expressed in the soma and axons of the B photoreceptors by direct functional recordings of single-channel currents. Although the soma of the B cells express two Ca2+ current subtypes, a transient BayK 8644-insensitive (approximately 17 pS) current and a sustained BayK 8644-sensitive (approximately 10 pS) current, the axons expressed only the latter. The axonal Ca2+ current activated at potentials positive to -20 mV. Moreover, the Ca2+ channels are distributed heterogeneously along the length of the axon, with the higher channel density (approximately 10-15 channel microm(-2)) occurring at the distal one-third of the isolated axons, with respect to the soma. The regions of Ca2+ channel clusters may represent the presynaptic site of the photoreceptor-interneuron synapses. Furthermore, the high-density clusters of Ca2+ channels may augment postsynaptic responses. The results of the present study represent the first direct recordings of Ca2+ currents at presumed synaptic sites. Expression of different Ca2+ channel subtypes at distinct compartments of the type B photoreceptors may generate diverse Ca2+ domains that may be required for neuronal plasticity in Hermissenda.

In vitro conditioning induces morphological changes in Hermissenda type B photoreceptor

Short- and long-term synaptic plasticity are considered to be cellular substrates of learning and memory. The mechanisms underlying synaptic plasticity especially with respect to morphology, however, are not known. In vitro conditioning in molluscan preparations is a well established form of short-term synaptic plasticity. Five paired presentations of light and vestibular stimulation to the isolated nervous system of Hermissenda results in an increase in excitability of the identified neuron, the type B photoreceptor, indicated by 2 measures, an increase in the input resistance and a cumulative depolarization after the cessation of light stimulus recorded from the cell soma. The terminal branches of type B photoreceptors iontophoretically injected with fluorescent dye were analyzed using computer-aided 3-dimensional reconstruction of images obtained using a confocal microscope under 'blind' conditions. The terminal branches contracted along the centro-lateral axis within an hour after conditioning, paralleling the increase in neuronal excitability. These data suggest that in vitro conditioning in Hermissenda is a form of short-term synaptic plasticity that involves changes in macromolecular synthesis.