Institute of Neurophysiology and Cellular Biophysics

Model animal: Xenopus laevis

The African clawed frog (Xenopus laevis) is an amphibian of the order Anura. It originates from the southeastern portion of Sub-Saharan Africa, but as an invasive species it now occupies freshwater areas all over the world. It lives in warm, stagnant grassland ponds as well as in streams in arid and semi-arid regions. The adult frog is a scavenger and eats living, dead, or dying arthropods and other pieces of organic waste, whereas the tadpoles feed on small organic particles, such as algae. To locate food it uses predominantly its acute sense of smell. It spends most of its time underwater and comes to surface to breathe. Respiration is predominantly through its well developed lungs, there is little cutaneous respiration.

Xenopus laevis, especially at its larval stages, is an excellent model system to study the olfactory system. The husbandry and breeding of this species is relatively easy and, as it is a poikilothermal animal, all experiments can be carried out at room temperature. Larval Xenopus laevis do not have a lamina cribrosa and therefore the olfactory mucosa is not separated from the olfactory bulb by a bony structure. This is a great advantage for the preparation of acute slices of the olfactory epithelium and the olfactory bulb (OB) and of so called nose-brain preparations.

Olfactory system of Xenopus laevis

The nose of the adult Xenopus laevis is made up of three interconnected chambers that form three different epithelia for the detection of different classes of odorants. The largest chamber forms the principal cavity ("air nose"), which is permanently filled with air and can be closed underwater by a membrane. The other two chambers, the lateral olfactory cavity ("water nose") and the vomeronasal organ are permanently filled with water. The olfactory epithelium (OE) of the principal cavity serves for the detection of air-borne, the olfactory epithelium of the lateral cavity detects water-borne stimuli. The vomeronasal organ (VNO) is implicated with the detection of pheromones. Premetamorphotic animals posses two separate chambers, the principal cavity, which is the larval ("water nose") and the vomeronasal organ.

All of the three epithelia, like in other vertebrates, consist of three main cell types: olfactory receptor neurons (ORNs) which transmit the olfactory information from the nose to the olfactory bulb in the brain via the olfactory nerve (ON), sustentacular supporting cells which share common properties with glial and epithelial cells, and basal cells including olfactory stem cells which maintain the regenerative capacity of the olfactory epithelium. The ORNs in the water nose and in the air nose are endowed with olfactory receptors (ORs) of different classes named class I ORs and class II ORs, respectively. The class I ORs are related to ORs of fish, whereas class II ORs are related to ORs of mammals.

Olfactory receptor neurons of the vomeronasal organ express neither class I nor class II ORs. As other vertebrates, Xenopus laevis possesses at least two types of ORNs, one type having cilia and the other type having microvilli. Axon terminals of ORNs synapse directly onto second-order neurons in the OB forming spheroidal structures called glomeruli. Thereby, ORNs from the principal- and the lateral cavity project to glomeruli in the main olfactory bulb (MOB), whereas receptor cells from the vomeronasal system project to glomeruli in the accessory olfactory bulb (AOB).

The second-order neurons in the OB are called mitral cells. These cells output the olfactory information to higher olfactory centers. In addition to mitral cells, numerous interneuron types exist in the OB, including periglomerular cells which synapse within and between glomeruli, and granule cells which synapse with mitral cells. The function of these interneurons is to modulate the olfactory information before it leaves the OB.