Spectral responses across a dorsal-ventral array of dermal sensilla in the medicinal leech.

How do animals use visual systems to extract specific features of a visual scene and respond appropriately? The medicinal leech, Hirudo verbana, is a predatory, quasi-amphibious annelid with a rich sensorium that is an excellent system in which to study how sensory cues are encoded, and how key features of visual images are mapped into the CNS. The leech visual system is broadly distributed over its entire body, consisting of five pairs of cephalic eyecups and seven segmentally iterated pairs of dermal sensilla in each mid-body segment. Leeches have been shown to respond behaviorally to both green and near ultraviolet light (UV, 365-375 nm). Here, we used electrophysiological techniques to show that spectral responses by dermal sensilla are mapped across the dorsal-ventral axis, such that the ventral sensilla respond strongly to UV light, while dorsal sensilla respond strongly to visible light, broadly tuned around green. These results establish how key features of visual information are initially encoded by spatial mapping of photo-response profiles of primary photoreceptors and provide insight into how these streams of information are presented to the CNS to inform behavioral responses.

Targeting a neuropeptide to discrete regions of the motor arborizations of a single neuron.

The heart excitor (HE) motor neuron in the leech Hirudo releases acetylcholine (ACh) and a peptide, FMRFamide, to regulate the contractile activity of the heart tube and associated side vessels. Consistent with Dale's principle, it was assumed that both neurotransmitters were localized to all presynaptic varicosities. However, we found discrete peptide-positive and peptide-negative varicosities associated with particular sites of innervation on the heart tube. We produced dual-labeled HE neurons by pressure injecting Neurobiotin into single HE cell bodies and applied anti-FMRFamide antibodies on the same preparations. Consistent with initial expectations, peptide-labeled varicosities were numerous and widely distributed along the heart tube and at one of the three side vessels, the latero-abdominal vessel. Nevertheless, some Neurobiotin-labeled varicosities along the heart tube lacked peptide label entirely. Moreover, there were dense and distinct peptide-negative innervations at the valve junctions of the latero-dorsal and latero-lateral vessels at each segment. Nevertheless, the peptide label was found in HE axons and varicosities that projected distally along the side vessels. Therefore, the more proximal peptide-negative clusters cannot simply be the result of restricted transport or deficient staining of peptide. Rather, we infer that FMRFamide is transported to (or selectively excluded from) discrete locations and that ACh is present in varicosities that lacked peptide. Such targeting of neurotransmitters could be described using a discrete targeting model of synaptic transmission. Compared with Dale's principle, this model may provide a more complete perspective of chemical communication than previously understood.