To determine how the neurons in the nucleus are activated by 16OH and 16Ac, c-Fos immunoreactivity was also examined on the anterior subdivision of this nucleus Figure 9. The results showed that the c-Fos immunoreactivity pattern mirrored, to some extent, that found in the BNST, e. Figure 9. Figure To determine whether 16OH and 16Ac also activate the neurons in this nucleus, we performed c-Fos immunostaining and analyzed the data Figure The results showed that both male and female mice generated significantly more c-Fos immunoreactivity in the nuclei following the exposure to LOH, HOH, or HAc than the control Figure 11 and Table 1.
Comparison between sexes indicated that the female MPA was more sensitive to HOH than its male counterpart while the opposite was true in response to HAc. Comparison between the two pheromones indicated that only HOH was more effective than HAc on the female MPA and no any other significant differences were found Figure K—M Comparative analysis of the c-Fos data between different concentrations of the pheromones, between sexes, and between the two pheromones, respectively.
But the concentration-dependent increases in c-Fos signals were found for 16OH on both male and female VmH only. While the former is mainly responsible for detecting volatile odorants, the latter is largely for detecting less or non-volatile pheromones, although some cross-detections between these two have been reported as well Holy, ; Mohrhardt et al.
Unlike a relatively large number of odorants and their corresponding olfactory receptors, very few pheromones or their receptors have been structurally identified or deorphanized. Hexadecanol 16OH and hexadecyl acetate 16Ac are two semi-volatile and rare chemically pure pheromones that exert differential effects on male and female mice Zhang et al.
In-depth studies on these two pheromones can facilitate our understanding of the AOS mechanisms. Calcium imaging of the VNO slices indicated that a small percentage 0. Thus, given the similar percentages of the activated VSNs, we conclude that it is likely only one vomeronasal receptor each for detecting 16OH, 16Ac, or both. On the other hand, however, the basal VSNs also responded, to a lesser extent, to these chemicals.
Thus, it is also possible that there is another V2r responsive to each of 16OH and 16Ac or both, respectively. This discrepancy may be explained by the fact that VSN firing rates vary from 2. Further studies, however, are needed to validate this notion.
The overall profiles of c-Fos immunoreactivity in these nuclei were similar to that found in the BNST, and can be summarized as following: 1 dose-dependency: stronger c-Fos responses were induced by higher concentrations of 16OH and 16Ac than lower ones, respectively; 2 sexual dimorphism: the female nuclei responded more strongly to 16OH whereas male nuclei did so to 16Ac; and 3 the relative effectiveness between the two pheromones: significant differences in evoking c-Fos responses in the nuclei were found between the two pheromones only at their high concentrations but not at the low concentrations.
There are, however, some subtle variations. Neurons in the nuclei examined so far in this study are involved in various pheromone-induced activities. The MeA, which receives excitatory signals from the AOB and the PMCo, sends axons to different amygdala and hypothalamic areas, directing sex-specific behavioral responses as well as endocrine and autonomic responses Brennan and Zufall, ; Ishii et al. The MPA is known to be critical to control inter-male and pup-directed aggression, sexual behaviors, and parental care Wu et al.
Finally, some neurons in the VmH are involved in initiating aggression, parental caring, and mating as well Lin et al. However, the only documented behavioral responses induced by 16OH and 16Ac include attraction and aversion, with no evident aggression, mating, or parental caring observed yet Zhang et al.
Our c-Fos data indicate that 16OH- and 16Ac-activated neurons in the brain regions can be parts of the AOS circuit hubs, contributing to the modulation or fine-tuning of the behavioral outputs determined by these ensembles of neurons Li et al. Further investigations are needed to unravel the exact roles played by the 16OH- and 16Ac-activated neurons in the population coding of the pheromonal signals. Like many other pheromones, 16OH and 16Ac also evoke sexually dimorphic responses.
In the AOB and most other brain regions, it seems largely consistent that males responded more strongly to 16Ac, females more strongly to 16OH, especially at the high concentration, except in the male MPA and VmH Figures 6 — It is known that AOS brain regions between sexes differ in neuron numbers and sex hormone receptor expression Li and Dulac, For example, the enzyme aromatase can convert testosterone into estrogen, which acts on the neurons expressing the estrogen receptors, modulating their activity, while mating experience makes a subset of male MeA neurons more responsive Dey et al.
Additional studies are needed to reveal the behavioral and physiological consequences resulted from or modulated by 16OH and 16Ac sexual dimorphic effects. In conclusion, our study demonstrates the activation of VSNs by 16OH and 16Ac, and the activation of neurons in the circuit hubs of the AOS in a sexually dimorphic manner.
And activation of the neurons in the brain regions strongly suggests that 16OH and 16Ac can modulate the AOS outputs such as aggression, mating, and parental caring. Further studies on these two abundantly produced and chemically identified natural pheromones can facilitate our understanding of this ancient and vital system in many mammals. The funds received from the National Natural Science Foundation of China cover the open access publication fees.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Akiyoshi, S. Subpopulations of vomeronasal sensory neurons with coordinated coexpression of type 2 vomeronasal receptor genes are differentially dependent on Vmn2r1.
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Future research work should examine information perceived by the recipient, for example looking for evidence of behavioral or physiological responses mediated by scent-marks via bioassay tests Wyatt In addition, we focused on the volatile components of mandrill odor, but we are currently investigating the nonvolatile profile of such odor secretions.
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The sources of the pheromones are exocrine glands located all over the termite body. The principal exocrine structures considered pheromone-producing glands in Isoptera are the frontal, mandibular, salivary or labial, sternal, and tergal glands.
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