PARK, D.S.*; EISTHEN, H.L.; Michigan State University, East Lansing: Terminal Nerve Peptides Modulate Activity in the Vertebrate Olfactory Epithelium

The terminal nerve is an anterior cranial nerve that extends between the nasal cavity and preoptic area and is hypothesized to serve a neuromodulatory function. Previous studies have shown that gonadotropin releasing hormone (GnRH), a peptide found in the terminal nerve, modulates activity of ion channels in salamander olfactory receptor neurons. Here, we demonstrate that GnRH modulates odorant responses in axolotls (Ambystoma mexicanum; n = 45). We recorded electrical field potentials (electro-olfactograms, or EOGs) in response to stimulation with four different amino acid odorants, L-methionine, L-cysteine, L-lysine, and L-glutamic acid. EOG responses to these odorants were recorded from the olfactory epithelium before, during, and after application of 10 �M GnRH. In general, EOG responses were reduced to 79% of the initial magnitude during GnRH application. Interestingly, EOG responses recovered and in many cases were enhanced during the subsequent wash period, reaching 120 – 150 % of the baseline magnitude after 30 min of washing. With most subjects, this protocol was repeated a total of 3 times, with 60-80 min between trials. Although the 4-min inter-stimulus interval did not lead to adaptation of EOG responses during the first trial, we frequently observed evidence of odorant adaptation during the second and third trials. Overall, the effect of GnRH on EOG responses was similar with all odorants tested and over the three consecutive trials. In addition, we found that higher concentrations of GnRH induced a greater effect, and that longer exposure to GnRH may induce a longer-lasting effect. These results demonstrate that GnRH modulates odorant responses, and support the hypothesis that the terminal nerve serves a neuromodulatory function in peripheral olfactory systems.