Depression of excitatory motoneurones by a single neurone in the leech central nervous system.

Intracellular staining techniques have been used to characterize the morphology of a newly identified neurone, cell 151, in the segmental ganglia of the leech. This neurone ramifies extensively within the neuropile and sends multiple extensions into roots and connectives. Strong dye coupling and non-rectifying electrical coupling were observed between the contralateral homologues. No action potentials were recorded from the cell body, but postsynaptic potentials and slow potential changes (greater than 1 s, greater than 15 mV) were observed. Upon injection of hyperpolarizing currents, the efferent spike activity, recorded extracellularly, was depressed in both the ipsi- and the contralateral roots of the ganglion. The depression was gradual and non-adapting and occurred reliably only within the ganglion where cell 151 is situated. Depolarization of cell 151 was without consequence for the tonic firing of isolated ganglia. Many identified excitatory motoneurones follow the hyperpolarization of cell 151. Currents can be exchanged between cell 151 and motoneurones via rectifying electrical synapses. Spontaneous hyperpolarizations of cell 151 were correlated with depression of spike frequencies, recorded in whole nerves as well as in identified motoneurones. The membrane potential of cell 151 was drastically altered by bursts from mechanosensory cells. The ability of cell 151 to distribute inhibition onto a great number of motoneurones and to curtail excessive neuronal activity is discussed.