# Repeated naloxone-induced morphine withdrawal alters blood brain barrier and blood spinal cord barrier integrity in mice

## TL;DR

Repeated opioid withdrawal in mice causes changes in the brain and spinal cord barriers, allowing larger molecules to pass through in specific regions.

## Contribution

This study reveals region-specific changes in barrier permeability due to opioid withdrawal in male and female mice.

## Key findings

- Evans Blue extravasation increased in specific brain regions in male and female mice after opioid withdrawal.
- NaFl extravasation remained stable in most regions, except for a reduction in female mice.
- Opioid withdrawal alters BBB and BSCB integrity in a sex- and region-dependent manner.

## Abstract

Passage of molecules across the central nervous system is tightly regulated by the blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB), which restrict entry of many substances, including opioid medications. Here, we examined the effects of opioid withdrawal on BBB and BSCB integrity by measuring extravascular levels of peripherally injected dyes – Evans Blue (high molecular weight) and sodium fluorescein (NaFl, low molecular weight) – in the brain and spinal cord. In morphine-dependent male and female mice, repeated naloxone challenge induced robust withdrawal behaviors concomitant with region specific dye extravasation. In a fixed dose morphine paradigm, Evans Blue extravasation was highest within the cortex, hippocampus, cerebellum, and brainstem (pons and medulla) in male mice, and in the hypothalamus in female mice. By contrast, NaFl extravasation remained unchanged in both sexes. In an escalating dose morphine paradigm, Evans Blue extravasation was most prominent in the brainstem (pons and medulla) of both sexes, as well as in the lumbar of male mice and cervical spinal cord of female mice. NaFl extravasation in these regions was unchanged in male but reduced in female mice. These findings suggest that repeated opioid withdrawal alters permeability of the BBB and BSCB in discrete regions of the brain and spinal cord.