Neuromodulatory effects of gonadotropin releasing hormone on olfactory receptor neurons

The terminal nerve is an anterior cranial nerve that innervates the lamina propria of the chemosensory epithelia of the nasal cavity. The function of the terminal nerve is ambiguous, but it has been suggested to serve a neuro modulatory role. We tested this hypothesis by exposing olfactory receptor n eurons from mudpuppies (Necturus maculosus) to a peptide, gonadotropin rele asing hormone (GnRH), that is found in cells and fibers of the terminal ner ve. We used voltage-clamped whole-cell recordings to examine the effects of 0.5-50 mu M GnRH on voltage-activated currents in olfactory receptor neuro ns from epithelial slices. We found that GnRH increases the magnitude, but does not alter the kinetics, of a tetrodotoxin-sensitive inward current. Th is increase in magnitude generally begins 5-10 min after initial exposure t o GnRH, is sustained for at least 60 min during GnRH exposure, and recovers to baseline within 5 min after GnRH is washed off. This effect occurred in almost 60% of the total number of olfactory receptor neurons examined and appeared to be seasonal: similar to 67% of neurons responded to GnRH during the courtship and mating season, compared with similar to 33% during the s ummer, when the sexes separate. GnRH also appears to alter an outward curre nt in the same cells. Taken together, these data suggest that GnRH increase s the excitability of olfactory receptor neurons and that the terminal nerv e functions to modulate the odorant sensitivity of olfactory receptor neuro ns.