Glutamate as a transmitter in the sensory pathway from prostomial lip to serotonergic Retzius neurons in the medicinal leech Hirudo.

The involvement of glutamate in putative ingestive sensory pathways affecting the excitability of serotonergic Retzius neurons (RZ) in the leech CNS was investigated with a pharmacological approach. Exposure of the prostomial lip to 150 mM NaCl and 1 mM arginine produced excitatory as well as inhibitory responses in RZ found in the reproductive segments, while only excitatory responses were elicted in standard midbody RZ. Antagonists of glutamatergic receptors of the kainate/quisqualate type effectively inhibited chemosensory dependent excitation of RZ. Antagonists of glutamatergic receptors of the N-methyl D-aspartate type were ineffective in this regard. Cephalic nerve stimulation, like chemical stimulation of the lip, produced segment-specific responses in midbody RZ. Both the polysynaptic and monosynaptic components of the excitatory response of standard midbody RZ following cephalic nerve stimulation were inhibited in the presence of the kainate/quisqualate antagonist DNQX. These data suggest a role for glutamate as a transmitter in the neural circuitry from receptors of the leech prostomial lip to serotonergic RZ.