Daytime spikes in dopaminergic activity drive rapid mood-cycling in mice.

Disruptions in circadian rhythms and dopaminergic activity are involved in the pathophysiology of bipolar disorder, though their interaction remains unclear. Moreover, a lack of animal models that display spontaneous cycling between mood states has hindered our mechanistic understanding of mood switching. Here, we find that mice with a mutation in the circadian Clock gene (ClockΔ19) exhibit rapid mood-cycling, with a profound manic-like phenotype emerging during the day following a period of euthymia at night. Mood-cycling coincides with abnormal daytime spikes in ventral tegmental area (VTA) dopaminergic activity, tyrosine hydroxylase (TH) levels and dopamine synthesis. To determine the significance of daytime increases in VTA dopamine activity to manic behaviors, we developed a novel optogenetic stimulation paradigm that produces a sustained increase in dopamine neuronal activity and find that this induces a manic-like behavioral state. Time-dependent dampening of TH activity during the day reverses manic-related behaviors in ClockΔ19 mice. Finally, we show that CLOCK acts as a negative regulator of TH transcription, revealing a novel molecular mechanism underlying cyclic changes in mood-related behavior. Taken together, these studies have identified a mechanistic connection between circadian gene disruption and the precipitation of manic episodes in bipolar disorder.

Developmental alterations in CNS stress-related gene expression following postnatal immune activation.

Early-life adversity is associated with dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis and increased susceptibility to later-life psychopathology. Specifically, there is mounting evidence suggesting that the immune system plays an important role in central nervous system (CNS) development and in the programing of behavior. The current study investigated how early-life immune challenge affects the development of CNS stress neurocircuitry by examining the gene expression profile of corticotropin-releasing hormone (CRH), CRH receptors, and the major corticosteroid receptors within the limbic-hypothalamic circuit of the developing rodent brain. Mice were administered a 0.05 mg/kg lipopolysaccharide (LPS) injection on postnatal day (P) 3 and 5 and gene expression was assessed using in situ hybridization from P14 to P28. Target genes investigated were CRH, CRH receptor-1 (CRHR1), CRH receptor-2, the mineralocorticoid receptor, and the glucocorticoid receptor (GR). Early LPS challenge resulted in a transient decrease in CRHR1 mRNA expression in the cornuammonis 1 (CA1) and CA3 regions of the hippocampus that were accompanied by increased hippocampal GR mRNA expression in the CA1 region between P14 and P21. This was followed by increased hypothalamic CRH expression in LPS-mice on P28. Our current findings suggest that early-life LPS challenge impacts the developmental trajectory of CNS stress neurocircuitry. These results lend insight into the molecular basis for the later development of stress-related behaviors as previously described in early immune challenge rodents.

A developmental characterization of mesolimbocortical serotonergic gene expression changes following early immune challenge.

An immunogenic challenge during early postnatal development leads to long-term changes in behavioural and physiological measures reflecting enhanced emotionality and anxiety. Altered CNS serotonin (5-HT) signalling during the third postnatal week is thought to modify the developing neurocircuitry governing anxiety-like behaviour. Changes in 5-HT signalling during this time window may underlie increased emotionality reported in early immune challenge rodents. Here we examine both the spatial and temporal profile of 5-HT related gene expression, including 5HT1A, 2A, 2C receptors, the 5-HT transporter (5HTT), and tryptophan hydroxylase 2 (TPH2) during early development (postnatal day [P]14, P17, P21, P28) in mice challenged with lipopolysaccharide (LPS) during the first postnatal week. Expression levels were measured using in situ hybridization in regions associated with mediating emotive behaviours: the dorsal raphe (DR), hippocampus, amygdala, and prefrontal cortex (PFC). Increased TPH2 and 5HTT expression in the ventrolateral region of the DR of LPS-mice accompanied decreased expression of ventral DR 5HT1A and dorsal DR 5HTT. In the forebrain, 5HT1A and 2A receptors were increased, whereas 5HT2C receptors were decreased in the hippocampus. Decreased mRNA expression of 5HT2C was detected in the amygdala and PFC of LPS-treated pups; 5HT1A was increased in the PFC. The majority of these changes were restricted to P14-21. These transient changes in 5-HT expression coincide with the critical time window in which 5-HT disturbance leads to permanent modification of anxiety-related behaviours. This suggests that alterations in CNS 5-HT during development may underlie the enhanced emotionality associated with an early immune challenge.