Olfactory system, but not terminal nerve, carries sex pheromone information.
The terminal nerve (TN) runs along or within the olfactory pathway of most
vertebrate species including man. Recent studies in teleosts have suggested
that the TN is the chemosensory pathway mediating responses to sex pheromones. The
following support this hypothesis: 1) the TN innervates olfactory lamellae and the brain
areas that control sexual behavior, 2) endocrinological and behavioral
responses are impaired by lesions of the medial olfactory tract (MOT) through
which the TN axons project centrally, and 3) sperm release is evoked by
electrical stimulation of the MOT as well as the optic nerve which contains
retinopetal TN axons. The evidence is
indirect and not conclusive, because the MOT and the optic nerve carry mitral
cell axons and retinal ganglion cell axons respectively, in addition to TN axons. Which system, TN or olfactory, mediates sex
Strong candidates for sex pheromones have recently been identified in goldfish. A steroid, 17α, 20 β-dihydroxy-4-pregnen-3-one (17,20P), is released from ovulating female goldfish into the water and causes an elevation in blood gonadotropin in the male, which in turn increases milt (sperm and seminal fluid) volume. 17,20 P does not affect behavior. By contrast, prostaglandin (PG) F2α and its metabolite, 15-keto-PGF2α, rapidly induce male courtship behavior. All these compounds are detected by goldfish at concentrations of as low as 10-13 - 10-10 M. Now we can answer the above question by examining neural responses of the TN and olfactory system to these sex pheromones.
Sex pheromones evoked clear responses in some olfactory bulb neurons. In contrast, no ganglion cells of the TN(TN cells) responded to any of these sex pheromones or any other odorants we tested. Odorants used include not only pure chemicals such as 17,20P, PGs, L-serine and taurocholic acid, but also "crude" solutions such as water in which ovulated female fish were kept, egg wash water, tank water containing male and female goldfish, food extract and human finger rinse. Thus, our evidence indicates that sex pheromone information is conveyed by the classical olfactory system, not by the TN.
Spontaneous firing of TN cells, however , was suppressed by pressure applied to the body surface or by a pinch to the tail. Although goldfish do not often come into contact with each other during most of their life, the male vigorously pushes and butts the female's body during courtship. This contact may cause suppression of the TN cell activity, which leads to changes in neural activity of target brain areas and/or changes in release of luteinizing hormone-releasing hormone from this nerve. The TN may play a role in some aspects of reproductive processes in this way.