Neuronal activity and subsequent calcium influx activates a signaling cascade that causes transcription factors in the nucleus to rapidly induce an early-response program of gene expression. This early-response program is composed of transcriptional regulators that in turn induce transcription of late-response genes, which are enriched for regulators of synaptic development and plasticity that act locally at the synapse.

GABAergic inhibitory neurons are known to play a critical regulatory role in memory formation and learning. During motor learning, pyramidal neurons (PNs) of the primary motor cortex (M1) undergo spine reorganization and firing pattern refinement. Cortical PNs are directly inhibited and regulated by two inhibitory neuronal subtypes: somatostatin-expressing interneurons (SOM-INs) and parvalbumin-expressing interneurons (PV-INs). Interestingly, SOM-mediated inhibition has been shown to regulate the observed dynamics of PNs during motor learning. Despite our expanded understanding, the molecular mechanisms that underlie these processes remain unclear. Here, I identified that the immediate-early gene transcription factor, NPAS4, is selectively expressed in a subset of SOM-INs, but not in PV-INs or PNs, during the head-fixed pellet reaching motor learning task. Furthermore, I characterized its expression pattern within the SOM-INs of M1 and found that there was no change at early phases; but as training progressed, there was a gradual increase and plateau in the number of NPAS4-expressing SOM-INs. In collaboration with other lab members, we showed that Npas4 region- and cell-type specific deletion within SOM-INs of M1, impaired motor skill acquisition and disrupted the motor learning-induced spine reorganization. In addition, I validated and employed the novel NRAM system to examine if NPAS4 is continually expressed within the same subset of SOM-INs and found that an ensemble of SOM-INs repetitively express NPAS4 at various phases of learning. Lastly, chronic in vivo two-photon Ca²⁺ imaging during training showed that the ensemble of NPAS4-expressing SOM-INs had reduced activity during task-related movements compared to other SOM-INs. Together, our results reveal an important instructive role of NPAS4 within the microcircuits of M1, in which it modulates the inhibition of a distinct subset of SOM-INs during motor learning to promote spine stabilization of downstream task-related PNs that are important for motor skill acquisition.

In seasonal mammals the duration of nocturnal melatonin secretion accurately reflects the environmental photoperiod. The endocrine rhythm is decoded by a specialised portion of the pituitary gland (the pars tuberalis, PT) which then relays this information to the pars distalis and hypothalamus, precipitating huge annual changes in physiology and behaviour. However how the PT decodes the melatonin signal is currently unknown. Melatonin influences gene transcription in the ovine PT at its onset and offset, and the phase relationship of these two groups is believed to form the underlying mechanism by which the PT integrates seasonal time. The transcripts induced at melatonin offset are understood to be under the control of a seasonally gated cAMP mechanism. Conversely processes involved in melatonin-mediated gene induction are currently not understood.The work in this thesis ultimately aims to reveal how the seasonal melatonin signal is decoded by the PT. To this end melatonin-mediated gene induction has been characterised through RNAseq, the highly displaced cohorts submitted to bioinformatic promoter analysis and the paradigm tested though in vitro modelling techniques.In this study a 1.5 h infusion with melatonin acutely regulated 219 transcripts in the ovine PT (115 induced, 104 repressed, >1.5 fold change), confirming previous association of several genes (including Cry1, MT1, Gadd45g, Nampt and Npas4) to rapid melatonin control. Gross promoter analysis of these groups indicated that the induced gene cohort was significantly enriched for GC content and CpG islands suggesting the involvement of epigenetic mechanisms of transcriptional control. Further bioinformatic analysis specifically implicated the importance of transcription factors ZFP161 and PAX5 in melatonin-mediated gene induction in the PT. Several immortalised cell lines were screened for the presence of a functional melatonin receptor. Two strains (MCF7 oMT1 and NES2Y) exhibited significant attenuation of forskolin-mediated cAMP accumulation when co-treated with melatonin, a hallmark of melatonin Gαi-coupled protein receptor signalling. These lines were subsequently evaluated as models of melatonin-mediated gene induction of the sheep PT through ovine promoter reporter assays of Cry1, Nampt, NeuroD1 and Npas4. However, treatment with melatonin failed to evoke a reporter response suggesting that the cell line models were inadequately equipped to reflect PT biology. Subsequently a protocol was established to culture ovine PT explants culture which faithfully recapitulated melatonin mediated transcriptional dynamics in vitro, providing a possible tool for the future investigation of the PT. Lastly, previous work has shown the transcriptional profile of Npas4 to peak highly and transiently, pre-empting the expression of other melatonin-induced genes. Using a COS7 cell line model, heterologously-expressed NPAS4 was shown to form functional heterodimeric partnerships with ARNT and ARNTL and transactivate both Cry1 and Nampt promoter reporters through novel binding sites. Collectively these data indicated NPAS4 to act as an immediate activator of melatonin regulated circuits