Figure 1. Serotonin induces heterogeneous effects on motoneurons.

Aa, scheme of the set-up. A slice in a continuously perfused recording chamber (arrows). Drugs were applied to the extracellular medium. Intracellular recordings from motoneurons. b and c, responses to depolarizing current pulses in 2 motoneurons. b, extracellular addition of 5-HT promoted a plateau potential characterized by a bistable firing that was abolished by a hyperpolarizing current pulse. c, another motoneuron had plateau properties characterized by an afterdischarge in control condition. Addition of serotonin inhibited the firing of the motoneuron. Ba, scheme of the iontophoresis set-up. A glass pipette filled with serotonin was either positioned close to a distal dendrite or close to the soma of the recorded cell. b, release of 5-HT close to a dendrite promoted a plateau potential with a latency that decrease with the amount of serotonin released. Traces separated for the clarity of the figure. c and d, the excitability of the motoneuron was tested by injecting depolarizing current pulses. c, release of 5-HT close to the soma promoted a plateau potential. d, when the motoneuron was hyperpolarized by means of a negative bias current, release of 5-HT at same intensity and same position as in Bc had an inhibitory effect characterized by a gradual decrease in the number of action potentials generated by the current pulse. All the recordings in B are from the same motoneuron. Fast synaptic potentials were blocked by a mixture of CNQX, AP5, Bicuculline and strychnine. Figure adapted from Perrier & Hounsgaard (2003); used with permission of the American Physiological Society.