Mechanisms of Olfaction - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 33

Unlike OSNs, VNO neurons expressing the same receptors project to more than
one glomerulus, and each glomerulus in the AOB may receive input from multiple
types of VNO neurons [ 137 ]. In addition, afferent AOB mitral cells also contact
more than one glomerulus with their dendrites, increasing the complexity [ 138 ].
The AOB mitral cells send the information to different brain areas from the areas
that receive information from the OB. The hypothalamus plays an important role in
exerting effects starting from VNO activation.
The VNO is essential in mediating innate behaviors and physiological changes
triggered by external chemical cues like the synchronization of the estrous cycle
amongst females housed together [ 139 ], pregnancy block caused by the exposure
of mated female mice to a strange male [ 140 ], enhancement of sexual receptivity in
females [ 141 ], early onset of puberty in females in response to exposure to males
[ 142 ], nursing behavior of lactating dams [ 8 ], male aggressive behaviors [ 17 ] and
attraction to the odors of the opposite sex [ 7 ], all of which are either completely lost
or partially affected in the mice lacking TRPC2 [ 5 , 136 , 143 ]. More recently, a third
family of receptors has been discovered in the VNO called formyl peptide receptor-
like proteins (FPRs) [ 144 ], which are expressed in a set of neurons that do not ex-
press other known VNO receptors. Since some FPR members are found in immune
cells, the VNO may provide sensitivity to molecules associated with disease and
inflammation [ 145 ]. In humans, though a VNO can be clearly detected in the em-
bryonic stages [ 146 ], the organ in adults does not contain any cells with properties
of sensory neurons [ 147 ]. TRPC2 is pseudogenized in humans, supporting the idea
that the human VNO is vestigial. It is interesting to note that a few V1Rs seem to be
intact in humans, though the vast majority of the VNO receptors are pseudogenized
[ 16 , 129 , 148 - 151 ].

2.9 Olfactory System in Aquatic Vertebrates

Fish carry out chemosensation via olfaction, gustation and general chemosensation
mediated by solitary chemosensory cells. Unlike terrestrial animals, the fish ol-
factory system detects water-soluble chemicals. They detect four main classes of
odorants: amino acids, gonadal steroids, bile acids and prostaglandins [ 152 ]. Fish
have a single olfactory organ called an olfactory rosette, which contains 3 types of
OSNs: ciliated, microvillous and crypt cells, and projects a tightly fasciculated ol-
factory nerve to the OB (Fig. 2.6 ) [ 153 ]. These 3 types of neurons differ from each
other based on morphology (number of cilia, length of dendrite), position (depth in
the OE) and their receptor expression profile. The type of receptors expressed by
crypt cells remains unknown, but the ciliated cells express ORs and the microvil-
lous cells express V2R-like receptors [ 154 - 156 ]. mRNA for V1R-like receptors
has been found in the OE [ 157 ]. Each class of these receptors is believed to use the
same or similar signaling pathways as their mammalian orthologs. The fish receptor
repertoire is smaller than that of mammals, but their receptors are more diverse
in sequence [ 158 , 159 ] and the repertoire size itself varies significantly amongst