Journal of Neuroscience, Vol 8, 4718-4727, Copyright © 1988 by Society for Neuroscience
Selection of postsynaptic serotonin receptors during reinnervation of an identified leech neuron in culture
P Drapeau and S Sanchez-Armass
Neurosciences Unit, Montreal General Hospital Research Institute, Quebec, Canada.
Serotoninergic Retzius neurons reform an inhibitory synapse onto
pressure-sensitive mechanosensory (P) neurons when the cells are removed
from the nervous system of the leech and are juxtaposed in tissue culture.
The somas of P cells in situ and single (uninnervated) P cells in culture
have both a depolarizing and Cl-dependent hyperpolarizing response to
application of the transmitter serotonin (5- HT). Synaptic release of 5-HT
by a Retzius cell in situ and in culture evokes a Cl-dependent postsynaptic
response but does not appear to activate the depolarizing receptors. We
have characterized the ionic currents induced by synaptically released and
applied 5-HT in voltage- clamped P cells in culture in order to determine
the responses to transmitter and their modifications following innervation.
When 5-HT was applied to single P cells, 2 types of channels were activated
by 5- HT and could be distinguished by changing the ionic composition of
the superfusion solution. In an impermeant cation (TrisCl) solution, a
5-HT- dependent Cl current was activated. When single P cells were
superfused with a Cl-free solution (Cl replaced by impermeant SO4), 5-HT
activated a monovalent cation current. Following innervation of a P cell by
a cocultured Retzius cell, the reversal potential of the peak postsynaptic
current depended on the Cl gradient and the synaptic response was blocked
by the Cl channel blocker 9-anthracenecarboxylic acid. Thus, synaptic
release of 5-HT activated solely the Cl channels and not the cationic
channels. Pipette application of 5-HT onto innervated P cells activated a
Cl conductance comparable in magnitude to the synaptic response. In
contrast, the cationic conductance was reduced roughly 5-fold. It is
concluded that innervation of a P cell by a Retzius cell resulted in
clustering of the synaptic 5-HT receptors, which activate Cl channels and
reduction of the nonsynaptic, cationic response. The result is a selection
of receptors in the cultured P cell that mimics the pattern observed in
vivo.