Neural Substrates of Sexual Motivation and Performance as Revealed by Neural Immediate-Early Gene Expression

Lique Coolen

It is most likely that olfactory information to the MPN does indeed pass through the MEApd. However, the neurons in MEApd that are involved in the relay of olfactory cues may not express Fos: there is a possibility that not the netire chain of neurons involved in this process express Fos. In agreement: there is no induction of Fos in anterior medial amygdala, although this brain area receives most of the input from the olfactory bulbs. This brings up two interesting points: 1) what does the Fos-IR that is present in MEApd after paradigms that introduce olfactory cues reflect? and 2) are other neurons involved in the relay of input to MPN and how can we visualize these cells?
1) Dave Edwards already argued in one of the replies that at this point we do not know for sure that the odor-induced Fos is indeed related to processing of olfactory cues. Rather, it can be related to arousal or motivation. In agreement, there is indication for the existence of an arousal-circuit that includes MEApd (Newman et al, 1997, Coolen and Veening unpublished). Another possibility is that the Fos-IR neurons are cells involved in the integration of olfactory and hormonal cues. Wood and Coolen (1997) have demonstrated that MEApd in a site where integration of odor cues and hormonal information is critical for male sexual behavior in Syrian hamsters. In addition, Coolen and Wood (1998) demonstrated that there are bidirectional connections between anterior MEA (which receives major input from the bulbs) and posterior MEA (where the majority of steroid-responsive neurons are present). Thus, steroid-responsive (androgen-receptor-containing) neurons in MEApd are in a position to receive olfactory input via anterior MEA. In contrast, Jennifer Swann (1997) demonstrated that castrated male hamsters express Fos in MEApd following vaginal secretion.
2) If the neurons that relay olfactory information to the MPN do not express Fos, how can we visualize these cells? Combined tract tracing could provide some answers. I have only performed one experiment to address this problem, and the data are very preliminary. I combined anterograde tracer injections with retrograde tracer injections in MPN, to visualize neurons that project to MPN that receive direct connections from the main olfactory bulbs. The males were then placed in estrous bedding to induce Fos in mEApd. Preliminary results confirm previous results presented in the paper in this symposium: very few neurons in MEApd that project to MPN were Fos-IR. In addition, Fos-IR neurons in MEApd do not receive direct contacts from the olfactory bulbs. Thus indicating that it is likely that interneurons are involved in the relay of olfactory cues, that do not express Fos-IR. However, please note that the anterograde injections were placed in the main olfactory bulbs, and do not include the accessory olfactory bulbs. Further studies are needed to address these questions.