Unilateral absence of the terminal nerve and distribution of gonadotropin-releasing hormone immunoreactive neurons in the brain of the common mole-rat (Cryptomys, Rodentia)

Perioral secretions enable complex social signaling in African mole-rats (genus Fukomys)

Subterranean common mole-rats of the genus Fukomys (family Bathyergidae) live in large, cooperatively-breeding families. Odor cues have been hypothesized to play an important role in mediating social behaviors in the underground ecotope, but only little is known about the role of olfactory signaling in burrowing mammals. Here we characterize the so far neglected perioral glands of Fukomys and other African mole-rats as an important source of olfactory social information. Histology demonstrates these structures to be derived sebaceous glands that are developed regardless of sex and reproductive status. However, gland activity is higher in Fukomys males, leading to sexually dimorphic patterns of stain and clotting of the facial pelage. Behavioral assays revealed that conspecifics prefer male but not female perioral swabs over scent samples from the back fur and that male sebum causes similar attraction as anogenital scent, a known source of social information in Fukomys. Finally, we assessed volatile compounds in the perioral sebum of the giant mole-rat (Fukomys mechowii) via GCxGC-MS-based metabolomic profiling. Volatiles display pronounced sex-specific signatures but also allow to differentiate between intrasexual reproductive status groups. These different lines of evidence suggest that mole-rat perioral glands provide complex odor signals which play a crucial role in social communication.

Terminal nerve in the mouse-eared bat (Myotis myotis): ontogenetic aspects

As in other mammals, ontogenesis of the terminal nerve (TN) in the mouse-eared bat (Myotis myotis) starts shortly after the formation of the olfactory placode, a derivative of the ectoderm. During development of the olfactory pit, proliferating neuroblasts thicken the placodal epithelium and one cell population migrates toward the rostroventral tip of the telencephalon. Here they accumulate in a primordial terminal ganglion, which successively divides into smaller units. Initial fibers of the TN can be distinguished from olfactory fibers in the mid-embryonic period. The main TN fiber bundle (mfb) originates from the anteriormost ganglion in the nasal roof, whereas one or more inconstant smaller fiber bundles (sfb) originate from one or more smaller ganglia in the basal part of the rostral nasal septum. The fibers of the mfb and sfbs join in the posterior quarter of the nasal roof before reaching the central ganglion (M) located in the meninges medial to the olfactory bulb. From the mid-fetal period onward, a thin TN fiber bundle with some intermingled perikarya connects M to the brain by penetrating its wall rostral to the olfactory tubercle. Additional smaller ganglia may occur in this region. The TN and its ganglia persist in postnatal and adult bats but the number of perikarya is reduced here. Moreover, the different potential functions of the TN are discussed briefly.

Netrin 1-mediated chemoattraction regulates the migratory pathway of LHRH neurons

Luteinizing hormone-releasing hormone (LHRH) neurons migrate from the vomeronasal organ (VNO) to the forebrain in all mammals studied. In mice, the direction of LHRH neuron migration is dependent upon axons that originate in the VNO, but bypass the olfactory bulb and project caudally into the basal forebrain. Thus, factors that guide this unique subset of vomeronasal axons that comprise the caudal vomeronasal nerve (cVNN) are candidates for regulating the migration of LHRH neurons. We previously showed that deleted in colorectal cancer (DCC) is expressed by neurons that migrate out of the VNO during development [Schwarting et al. (2001) J. Neurosci., 21, 911-919]. We examined LHRH neuron migration in Dcc-/- mice and found that trajectories of the cVNN and positions of LHRH neurons are abnormal. Here we extend these studies to show that cVNN trajectories and LHRH cell migration in netrin 1 (Ntn1) mutant mice are also abnormal. Substantially reduced numbers of LHRH neurons are found in the basal forebrain and many LHRH neurons migrate into the cerebral cortex of Ntn1 knockout mice. In contrast, migration of LHRH cells is normal in Unc5h3rcm mutant mice. These results are consistent with the idea that the chemoattraction of DCC+ vomeronasal axons by a gradient of netrin 1 protein in the ventral forebrain guides the cVNN, which, in turn, determines the direction of LHRH neuron migration in the forebrain. Loss of function through a genetic deletion in either Dcc or Ntn1 results in the migration of many LHRH neurons to inappropriate destinations.