Formation of Specific Connections in Culture by Identified Leech Neurones Containing Serotonin, Acetylcholine and Peptide Transmitters

Abstract

Connections made in culture by identified leech neurones have been studied using pairs of cells that contain various transmitters. These cells were: motoneurones innervating the heart; the anterior pagoda neurones (which contain both acetylcholine and an FMRFamide-like peptide); the Retzius cells (which contain serotonin); and the pressure sensory neurones, which contain an unidentified transmitter. Heart motoneurones (HE) and anterior pagoda (AP) cells in culture reacted with antiserum against the peptide FMRFamide. The immunoreactive peptide was found in dense-core vesicles at presumptive sites of transmitter release. In culture the HE, AP and Retzius cells formed non-rectifying electrical synapses with one another within 2 days. Some pairs of cells made connections resembling those seen in normal leech ganglia; others formed novel connections seen only in culture. Electrical connections made by sensory P cells with HE and AP cells showed rectification. Confirming earlier results, P cells never established electrical connections with Retzius cells. In certain pairs of neurones, chemically mediated synaptic interactions developed. Thus, stimulation of AP cells evoked hyperpolarizing synaptic potentials that arose after a delay in Retzius cells and in P cells. Similarly, stimulation of HE cells gave rise to delayed, slow synaptic potentials in AP cells and in isolated heart muscle fibres in culture. P cells, which in culture are never presynaptic to Retzius cells, made chemical connections with AP cells. These results support the conclusion that identified leech neurones in culture make synaptic connections with certain specific target cells while ignoring others.