The properties and connections of nerve cells in leech ganglia maintained in culture

Abstract

Segmental ganglia of the central nervous system of the leech were maintained in culture medium outside the animal for several weeks at 16°C, and electrical recordings made from identified sensory and motor nerve cells. Ganglia were isolated and cultured singly, in chains and connected to the skin and muscles they normally innervate. Such preparations are suitable for a study of relatively long-term changes that occur as a result of denervation. The changes resemble those seen in isolated ganglia that had been kept in situ in the animal. (1) Resting and action potentials in sensory and motor neurones of isolated ganglia appear normal for up to ten weeks. The same cells can be tested at intervals of a few days. (2) Sensory cells, classified as touch (T), pressure (P) or nociceptive (N) according to their morphology and electrical properties, continue to respond selectively to stimuli of the appropriate modality applied to their receptive fields in the skin; action potentials in motor cells cause contractions in the appropriate muscles. (3) Chemical synaptic transmission between sensory and motor nerve cells changes markedly over the first three weeks; excitatory and inhibitory synaptic potentials can double in size and duration over a period of about two weeks in culture. (4) The balance between excitatory and inhibitory synaptic influences changes sequentially after isolation in culture. For example, P or N cells innervating a cell which erects skin annuli normally cause small depolarizing excitatory potentials. After 5 days in culture they initiate large inhibitory synaptic potentials, while by 15 days the balance between excitation and inhibition changes again, so that the predominant synaptic action is an abnormally large prolonged excitatory potential. (5) After more than three weeks synaptic potentials disappear and ganglia lose transparency. (6) The morphological appearance of T, P and N sensory cells has been compared in cultured and normal ganglia after injection of horseradish peroxidase. In cultured ganglia the major branching pattern appears normal but presumed sites of transmitter release become more conspicuous.