Sex differences in noradrenergic regulation of the medial prefrontal cortex in mice.

BACKGROUND

Norepinephrine (noradrenaline; NE) is a stress signal released from the locus coeruleus (LC) into the prefrontal cortex (PFC) to govern arousal, attention, and cognition. The LC is sexually dimorphic, and PFC NE dysfunction contributes to alcohol use disorder and several stress-related neuropsychiatric disorders that manifest differently in men and women. However, most preclinical studies of the medial PFC (mPFC) NE system have only used male subjects. Additionally, even though each mPFC subregion and layer forms unique circuits that mediate different aspects of cognitive behavior, their specific neuromodulation by NE is not understood.

METHODS

We comprehensively probed potential sex differences in the mouse mPFC NE system, starting with fluorescent tracing of the LC→mPFC circuit. Basal mPFC NE tissue content and adrenergic receptor mRNA were measured using high performance liquid chromatography and real-time quantitative polymerase chain reaction. Ex vivo electrophysiology assessed NE modulation of glutamate synapses in layers 2/3 and 5 of the prelimbic and infralimbic subregions of the mPFC. Finally, we used an α2 adrenergic receptor antagonist to increase NE release and tested for mPFC-associated reversal learning and episodic memory.

RESULTS

Females had a greater percentage of LC NE neurons→mPFC than males, with no differences in basal mPFC NE concentration or adrenergic receptor mRNA. NE increased mPFC glutamate release broadly in males, but its effects in females were restricted to prelimbic layer 5 and infralimbic layer 2/3. Finally, while there were dose-dependent effects of the α2 receptor antagonist on cognitive behavior, they did not vary between sexes.

CONCLUSIONS

We uncovered complex sex differences in LC→mPFC structure and mPFC NE function, and future studies should examine NE activation in the context of greater cognitive load, such as during alcohol withdrawal or periods of stress. Clinically, women exhibit greater stress-induced activation of the NE system, are more likely to be diagnosed with affective disorders, and are more likely to drink alcohol to regulate negative affect and stress reactivity than men. Therefore, our study highlights the importance of considering specific subpopulations (e.g. women, or people with comorbid stress and alcohol use disorders) during the development of new NE-based treatments. Norepinephrine (also known as noradrenaline) is a stress signal that regulates activity in the brain region the medial prefrontal cortex (mPFC) to optimize decision making, emotional processing, inhibitory control, and learning and memory. Its dysfunction contributes to both alcohol use disorder and stress-related neuropsychiatric disorders, though its role may differ between men and women. It is well known that the brain region that makes norepinephrine (i.e. locus coeruleus; LC) is larger and more complex in women and female rodents than their male counterparts. However, most preclinical studies have only used male subjects so the impact of these sex differences remains unclear. In this study, we used male and female mice to probe the LC→mPFC brain circuit and understand how norepinephrine shapes mPFC neuronal communication. We also tested how increasing norepinephrine levels alters cognitive behaviors that are mediated by the mPFC. We identified complex sex differences; female mice had a larger LC→mPFC circuit but their mPFC neurons were less sensitive to norepinephrine compared to male mice. There were no sex differences in norepinephrine's effects on reversal learning and episodic memory under baseline conditions, but future studies should examine whether sex differences emerge during alcohol withdrawal or periods of stress. This work expands our understanding of mPFC norepinephrine signaling in both sexes and highlights the importance of considering specific subpopulations (e.g. women, or people with comorbid stress and alcohol use disorders) during the development of new norepinephrine-based treatments.

HIGHLIGHTS

Females have a larger locus coeruleus → medial prefrontal cortex circuit than males. Basal norepinephrine levels and adrenergic receptor gene expression levels are similar in the medial prefrontal cortex of male and female mice. Norepinephrine broadly increases glutamate release onto medial prefrontal cortex pyramidal neurons in male mice. In female mice, norepinephrine-induced glutamate release is restricted to prelimbic cortex layer 5 and infralimbic cortex layer 2/3 pyramidal neurons. Norepinephrine altered medial prefrontal cortex-dependent cognitive behaviors (reversal learning and episodic memory) in mice, but there were no sex differences in its effects.