Sex differences in μ-opioid regulation of coerulear-cortical transmission

Abstract

Stress-induced activation of locus coeruleus (LC)-norepinephrine (NE) projections to the prefrontal cortex are thought to promote cognitive responses to stressors. LC activation by stressors is modulated by endogenous opioids that restrain LC activation and facilitate a return to baseline activity upon stress termination. Sex differences in this opioid influence could be a basis for sex differences in stress vulnerability. Consistent with this, we recently demonstrated that μ-opioid receptor (MOR) expression is decreased in the female rat LC compared to the male LC, and this was associated with sexually distinct consequences of activating MOR in the LC on cognitive flexibility. Given that the LC-NE system affects cognitive flexibility through its projections to the medial prefrontal cortex (mPFC), the present study quantified and compared the effects of LC-MOR activation on mPFC neural activity in male and female rats. Local field potential (LFPs) were recorded from the mPFC of freely behaving male and female rats before and following local LC microinjection of the MOR agonist, DAMGO, or vehicle. Intra-LC DAMGO altered the LFP power spectrum selectively in male but not female rats, resulting in a time-dependent increase in the power in delta and alpha frequency bands. LC microinfusion of ACSF had no effect on either sex. Together, the results are consistent with previous evidence for decreased MOR function in the female rat LC and demonstrate that this translates to a diminished effect on cortical activity that can account for sex differences in cognitive consequences. Decreased LC-MOR function in females could contribute to greater stress-induced activation of the LC and increased vulnerability of females to hyperarousal symptoms of stress-related neuropsychiatric pathologies.

Keywords: Cognition; Cortex; Locus coeruleus; Opioids; Sex differences.

Figure 1.. Histological verification of intra-LC injection and mPFC electrode placement.

Brightfield photomicrograph of a Neutral red counterstained section through the LC and mPFC showing histological verification of the single injection site and recording electrode placement, respectively. (A) Arrow points to the LC, and the arrowhead points to the pontamine sky blue dye, which is localized to the LC (Cb, cerebellum; V ventricle). (B) Neutral red counterstained section through the prelimbic mPFC (PrL) showing histological verification of the recording electrode placement. Arrow points to the electrode track (IL, infralimbic mPFC).

Figure 2.. Intra-LC DAMGO resulted in a time-dependent oscillatory activity of mPFC in male but not female rats.

Representative spectrograms (frequency through time) of mPFC LFP activity from a single male (A) and female rat (B). Frequency is on the y axis, and time on the x-axis. The color column shows the relative power spectrum values (hotter colors represent greater power). DAMGO (10 pg in 200 nl) was microinfused into the LC at white horizontal line and arrowhead. (C and D) The bar graphs represent the mean raw power spectral density values at different frequency bands (delta 2–4 Hz, theta 4–8 Hz, alpha 8–12 Hz, and beta 12–20 Hz) in male (C) and female rats (D), determined 10 min before (pre-injection), 0–10 min, 10–15 min, and 45–60 min after intra-LC DAMGO on mPFC network activity. Error bars represent ± SEM. *p<0.05.

Figure 3.. Lack of effect of intra-LC ACSF on mPFC network activity.

Bar graphs represent the mean raw power in different frequency bands at different times before and after intra-LC ACSF (200 nl). Males (A) and Females (B) at different time blocks (0–10, 10–15, 45–60 minutes after Intra-LC DAMGO injection). Error bars represent ± SEM.

Figure 4.. Qualitative effects of intra-LC clonidine on mPFC network activity.

Representative spectrograms (frequency through time) of mPFC LFP activity from a single male (A) and female (B) rat before and after clonidine (50 ng in 200 nl). Frequency is on the y axis, and time on the x-axis. The color column shows the relative power spectrum values (hotter colors represent greater power). The horizontal white line and the arrowhead indicate the time of microinfusion. Single male (C) and female (D) rat’s power spectral density plots corresponding to their respective time-frequency spectrograms shown above.