Neurotensin, TIDA neurons, and the pregnancy-induced plasticity of a neuroendocrine circuit

Our goal is to understand how the brain prepares the mammalian body for the enormous challenges of pregnancy, nursing, and parental behaviour. As many of these adaptations are driven by the reproductive hormone prolactin (Prl), this proposal focuses upon the pregnancy induced plasticity of tuberoinfundibular dopaminergic (TIDA) neurons - the highly specialised brain cells that control the release of Prl from the pituitary.

If, via their regulation of Prl release, TIDA neurons are to effectively tune physiology and behaviour to ensure we meet both immediate and future reproductive demands, then they must be able to achieve two things. First, in order to actively promote conditions conducive to reproduction, they must be able to REACT to relevant information, in real time. Second, given the effort required to marshal the enormous resources demanded by pregnancy and parental behaviours, it is essential that they be able to 'plan ahead' and PREDICT what is needed in the future. This project is designed to investigate the role of the brain peptide neurotensin (NT) in driving the remarkable cellular and circuit plasticity that enables the TIDA network to fulfil these computational tasks.

If left to their own devices, the pituitary cells that produce Prl, will engage in continuous hormone release. Accordingly, TIDA neurons control Prl levels by via inhibition, continuously delivering the inhibitory 'NO' signal, dopamine (DA). As such, when an increase in Prl is required, changes in Prl regulating factors - such as NT - cause TIDA neurons to reduce their DA output. Under basal, non-pregnant conditions, such increases in Prl are powerfully restricted in size and duration by a process of negative feedback - an inhibitory loop where Prl supresses its own release by exciting TIDA neurons and increasing the 'NO' signal. During pregnancy, however, the expectant mother requires extreme and enduring hyperprolactinaemia. Correspondingly, communication between the TIDA circuit and the pituitary must change from negative to positive feedback. Yet, how this conversation switches from "NO" to "GO" remains obscure. Current data suggests that during pregnancy the TIDA system remains responsive to Prl, but ceases to release DA, replacing this NO signal with a GO factor. A prominent candidate for this GO signal is NT, the TIDA production of which increases enormously during pregnancy.

To investigate the dynamic role of NT in both reactive and predictive mechanisms of TIDA control, I will test the following three step hypothesis:
1) TIDA neurons can REACT directly to NT.
2) NT neurons - cells that control ingestive behaviour, metabolism and stress - talk to TIDA neurons, relaying vital reproduction relevant information.
3) During pregnancy the TIDA system undergoes a 'signal switch', replacing the 'NO' factor DA with the 'GO' signal NT - a PREDICTIVE step that enables the lactotrophic axis to drive the enduring prolactin release needed to prepare the body for the rigors of gestation, parturition and nursing.

To test these predictions, I will use validated mouse models that will enable me to selectively visualise, monitor, and manipulate, both TIDA and NT neurons. When coupled with anatomical and physiological approaches, I will be able to determine not only which NT neurons 'talk' to the TIDA circuit, but how NT affects the cellular and circuit properties of the TIDA network. Finally, I will use cutting-edge CRISPR-based techniques to prevent the TIDA circuit from producing NT and then observe how the removal of this state-dependent 'GO' factor impacts fertility, nursing and parental behaviour.

This research will reveal novel multi-level understanding regarding the adaptive dynamism of the neuroendocrine networks governing prolactin release and reproductive behaviour, and will uncover new mechanisms of signal plasticity by which neuronal networks adapt circuit performance to both prevailing circumstance and future requirement.

Technical Summary
The pituitary hormone prolactin (Prl) co-ordinates physiology to give mammals, male or female, the best chance of successful reproduction and rearing of offspring. Reproduction and parental behaviours are expensive, state-dependent processes - demanding enormous resources and complex adaptations. If Prl is to effectively organise animal physiology and marshal the means necessary to meet these dynamic reproductive requirements, then tuberoinfundibular dopamine (TIDA) neurons -the specialised brain cells that control Prl release - must be able to both REACT to what is happening now and PREDICT what is needed in the future. This project is designed to investigate the role of the brain Peptide neurotensin (NT) in driving the remarkable cellular and circuit plasticity that enables the TIDA network to fulfil these computational tasks. To this end, I have formulated a tripartite hypothesis.

1) TIDA neurons react to NT
2) NT-ergic neurons - prominent regulators of energy and fluid homeostasis, thermoregulation and stress - relay reproduction relevant information to the TIDA system
3) During pregnancy, the TIDA system undergoes a 'signal switch', replacing the inhibitory signal dopamine with the Prl releasing factor NT - a predictive step that enables the lactotrophic axis to escape negative feedback and promote the enduring hyperprolactinaemia that prepares the mother for the rigors of gestation, parturition and nursing.

To test my hypothesis, I will use targeted electrical and optical recordings to assess the in vivo and ex vivo impact of NT on the cellular and circuit performance of the TIDA system. I will adopt a retrograde tracing and optogenetic circuit mapping approach to identify and interrogate the NT-ergic components that interact with TIDA neurons. Before finally, utilising CRISPR based gene editing to conditionally delete NT from TIDA neurons - a loss of function experiment that will reveal NT's role in the predictive plasticity of the TIDA system.