Using intracellular recording techniques, we have begun to examine the effects of 5-hydroxytryptamine (5-HT) on antennal-lobe (AL) neurones in the brain of the adult moth Manduca sexta. 5-HT modulated the responses of local interneurones and projection neurones, which were recognized on the basis of well-established electrophysiological criteria, to primary synaptic input elicited by electrical stimulation of the ipsilateral antennal nerve. 5-HT applied at low concentration (10(-8) mol l-1) reduced the excitatory responses evoked by electrical stimulation of the antennal nerve, whereas at high concentration (10(-4) mol l-1), 5-HT enhanced the responses. At 10(-4) mol l1, 5-HT increased cell input resistance, led to broadening of action potentials and caused increased cell excitability in many AL neurones.

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