Serotonin-containing neurons in lobsters: origins and characterization of inhibitory postsynaptic potentials.

1. The serotonin-containing neurons in the A1 ganglion of the lobster have been shown to act as "gain setters" in neuronal circuits that control the adoption of behaviorally relevant postures. These neurons are subject to tonic inhibition, which has been proposed as an important regulator of their activity. This study explores the pharmacological nature and anatomic location of the neurons responsible for inhibition of these A1 cells; the role played by inhibitory inputs in controlling the firing rates of these neurons is also examined. 2. Three classes of inhibitory postsynaptic potentials (IPSPs) are distinguished in the somata of A1 serotonin-containing neurons. The most common (type I) has amplitudes ranging from 0.4 to 1.5 mV; types II (2-5 mV) and III (< 0.4 mV) are less often seen. 3. Type I IPSPs are reversibly blocked by picrotoxin, a gamma-aminobutyric acid antagonist, but not by serotonin or octopamine antagonists known to act at other lobster synapses. 4. Elimination of type I IPSPs by reversible or irreversible blockage of conduction from the A3 ganglion results in a firing rate increase of approximately 50% in A1 serotonin-containing neurons; IPSP recovery results in a firing rate decrease of corresponding magnitude. Connective transections that do not affect IPSPs do not cause a firing rate increase. 5. Lesions studies suggest that type I IPSPs originate in neurons whose somata are located near the midline of the A3 ganglion; a cell impaled in this region showed action potentials that correlated with IPSPs in an A1 serotonin-containing neuron.