Voltage-dependent actions of endogenous and exogenous serotonin on identified neurones

1. Impulse activity in an identified serotonin-containing neuron (GSN) produces a slow excitatory synaptic response in another identified neuron, the A neuron. An axon process can be traced close to the follower neuron perikaryon after Lucifer Yellow injection of the GSN perikaryon. 2. The synaptic response is markedly voltage-sensitive being increased at depolarized potentials and almost abolished and not inverted at potentials in excess of about -55mV. 3. Serotonin locally applied produces a similar response. 4. The response to serotonin does not involve a change in conductance to either sodium or chloride ions, but calcium ions do appear to be important either because of their influence on potassium ion permeability or in a direct transfer of charge across the membrane. 5. Another follower neuron exhibits a complex GSN-induced synaptic response comprising a slow potential similar to that seen in the A neuron and also a fast, probably sodium dependent, potential. 6. In addition to producing a weak direct excitation of the A neuron, GSN-activation can also partly reverse accommodation and also prolong the duration of the impulse in the A neuron. 7. Exogenously applied serotonin produces a similar voltage-dependent inward current response in the GSN as seen in the A neuron. It is suggested that the receptors mediating the response on the GSNs may normally be involved in feedback regulation. 8. Cyproheptadine (reversibly), methergoline, mianserin and propranolol (all irreversibly) antagonised the response in the GSN. These agents probably all have action on the ionic mechanism underlying the serotonin response.