Modulation and selection of neurotransmitter responses during synapse formation between identified leech neurons

Development of Ca2+ hotspots between Lymnaea neurons during synaptogenesis

Calcium (Ca2+) channel clustering at specific presynaptic sites is a hallmark of mature synapses. However, the spatial distribution patterns of Ca2+ channels at newly formed synapses have not yet been demonstrated. Similarly, it is unclear whether Ca2+ 'hotspots' often observed at the presynaptic sites are indeed target cell contact specific and represent a specialized mechanism by which Ca2+ channels are targeted to select synaptic sites. Utilizing both soma-soma paired (synapsed) and single neurons from the mollusk Lymnaea, we have tested the hypothesis that differential gradients of voltage-dependent Ca2+ signals develop in presynaptic neuron at its contact point with the postsynaptic neuron; and that these Ca2+ hotspots are target cell contact specific. Fura-2 imaging, or two-photon laser scanning microscopy of Calcium Green, was coupled with electrophysiological techniques to demonstrate that voltage-induced Ca2+ gradients (hotspots) develop in the presynaptic cell at its contact point with the postsynaptic neuron, but not in unpaired single cells. The incidence of Ca2+ hotspots coincided with the appearance of synaptic transmission between the paired cells, and these gradients were target cell contact specific. In contrast, the voltage-induced Ca2+ signal in unpaired neurons was uniformly distributed throughout the somata; a similar pattern of Ca2+ gradient was observed in the presynaptic neuron when it was soma-soma paired with a non-synaptic partner cell. Moreover, voltage clamp recording techniques, in conjunction with a fast, optical differential perfusion system, were used to demonstrate that the total whole-cell Ca2+ (or Ba2+) current density in single and paired cells was not significantly different. However, the amplitude of Ba2+ current was significantly higher in the presynaptic cell at its contact side with the postsynaptic neurons, compared with non-contacted regions. In summary, this study demonstrates that voltage-induced Ca2+ hotspots develop in the presynaptic cell, concomitant with the appearance of synaptic transmission between the soma-soma paired cells. The appearance of Ca2+ gradients in presynaptic neurons is target cell contact specific and is probably due to a spatial redistribution of existing channels during synaptogenesis.

Serotonin depletion does not prevent intrinsic sensitization in the leech

Intrinsic sensitization is a form of behavioral facilitation that is distinct from the extrinsic sensitization normally studied. To examine whether intrinsic and extrinsic sensitization are mediated by different physiological processes, the effects of 5,7-dihydroxytryptamine-induced serotonin (5-HT) depletion on intrinsic sensitization of the leech whole-body shortening response were observed. Previous experiments have shown that 5-HT depletion disrupts dishabituation and extrinsic sensitization of this behavior in the leech. Intrinsic sensitization was observed in preparations from both control and 5-HT-depleted animals, indicating that this form of behavioral facilitation was not affected by 5-HT depletion. The differences in the effects of 5-HT depletion on intrinsic versus extrinsic sensitization suggest that there are distinct neurophysiological processes mediating these two forms of behavioral facilitation. In addition, 5-HT depletion appeared to disrupt a putative extrinsic form of habituation of the shortening reflex. These data support the hypothesis that both intrinsic and extrinsic processes of neuromodulation mediate habituation and sensitization.

Laser-induced native fluorescence (LINF) imaging of serotonin depletion in depolarized neurons

Since certain neurotransmitters exhibit native fluorescence we can monitor this property to disclose intracellular changes that result from neurotransmitter release. Isolated Retzius neurons of the leech are known to release serotonin (5-HT) during depolarization. Using intensified CCD technology coupled with UV laser (305 nm) excitation we observed depolarization and calcium-dependent reductions in native fluorescence in the axon, as well as in the cortex of the cell body. When taken together with data obtained from single-cell capillary electrophoresis, we demonstrate that this laser-induced native fluorescence can be reliably used to study spatial and temporal changes in intracellular transmitter content that accompany calcium-dependent secretion.