S. Ching et al., SELECTION OF TRANSMITTER RESPONSES AT SITES OF NEURITE CONTACT DURINGSYNAPSE FORMATION BETWEEN IDENTIFIED LEECH NEURONS, Journal of physiology, 468, 1993, pp. 425-439
1. Pressure sensitive (P) neurons of the leech Hirudo medicinalis show
both an inhibitory, Cl--dependent response and a depolarizing, cation
ic response to pipette application of serotonin (5-HT). Serotonergic R
etzius (R) neurons in culture reform inhibitory, Cl--dependent synapse
s with P neurons but fail to elicit the extrasynaptic, depolarizing re
sponse to 5-HT. We have examined the localization of the selection of
5-HT responses by testing the sensitivity of P cell growth cones and n
eurites to 5-HT application. 2. As measured by intracellular recording
at the P cell soma, synaptic release of 5-HT from R cell processes ac
tivated only the Cl--dependent response in P cell neurites. Focal appl
ication of 5-HT from a micropipette depolarized uncontacted P cell gro
wth cones and neurites. In contrast, processes from the same P cells t
hat were contacted by R cells were rarely depolarized by 5-HT applicat
ion unless the application pipette was moved along the neurites away f
rom the sites of contact. 3. The channels underlying the depolarizing
response to 5-HT were identified in patch clamp recordings from P cell
growth cones. These cation channels showed rare, brief openings in th
e absence of 5-HT. Application of 5-HT in the bath (outside the patch
pipette) increased channel activity in uncontacted P cell growth cones
but not in growth cones of the same P cells contacted by R cells.4. W
e conclude that the selection of transmitter responses during synapse
formation was localized to discrete sites of contact between the synap
tic partners.