Reviewer #1 (Public Review):
Mating influences many behaviours such as enhanced oviposition, suppressed mating, and a change in dietary preference. In this study, Boehm et al explore the circuit basis of the mated female's enhanced preference for polyamines.
A previous study from this group had identified a mechanism by which mating reduced sensitivity of the olfactory sensory neurons resulting in a preference for higher concentrations of polyamines after mating. However, the preference for polyamines outlasts this mechanism by many days. So, in this study, the authors explore central brain circuits that might encode this persistent behavioural switch. Briefly, they identify neurons within the mushroom body - intrinsic neurons, output neurons and dopaminergic neurons (DAN) - that are involved in this behaviour. They also identify output neurons of the lateral horn that are involved in it.
The behaviour itself consists of two phases: 1) the mating experience, and 2) the subsequent expression of the polyamine preference. The authors use behavioural assays and neurogenetics to demonstrate that:
1. The ability to detect odours via the OR67d neurons at the time of mating is necessary to bring about the behavioural switch.
2. Activity of the intrinsic neurons of the mushroom body is necessary at both times - during the mating and the expression - to bring about the behavioural switch.
3. They identify one set of dopaminergic neurons - B1 DANs - that are sufficient but not necessary ***at the time of mating*** to induce the switch in virgin females.
4. They identify a second set of dopaminergic neurons - B2 DANs - that are necessary to ***at the time of expression*** to demonstrate the increased polyamine preference ******in mated females.
5. They identify a set of mushroom body output neurons (MBONs) downstream of the B1-DANs and show that output from the B1 region is necessary and sufficient at the time of mating for the expression of polyamine preference.
6. They identify MBONs downstream of the B2 DANs and find that they play no role at the time of mating, but that they are necessary and sufficient at the time of expression to suppress the polyamine preference.
7. They identify a set of output neurons of the lateral horn and find that they are necessary at the time of expression of polyamine preference.
The authors also use functional imaging to show that there are no brain-wide changes upon mating in the encoding of one of the polyamines. They explore how cVA (an odour they believe is relevant at the time of mating) is represented in the neurons they have identified. They find that the B1 DANs show enhanced representation of cVA post mating, however, their MBONs do not alter their response to cVA post mating. The B2-MBONs respond to both putrescine and cVA and show no alteration in their response post mating.
In summary, the authors have identified a mechanism similar to associative learning that operates across the mushroom body and lateral horn, to 'learn' the experience of mating and express it as an enhanced preference to a nutritionally rich food source.