Chronic morphine increased CB2 receptors on spinal neurons in rats | Research

The takeaway

Chronic morphine administration produced a larger increase in neuronal CB2 receptor expression in the spinal dorsal horn than peripheral nerve injury alone, suggesting CB2-opioid receptor interactions may contribute to combined cannabinoid-opioid treatment effects.

Pain researchers, opioid pharmacologists, cannabinoid-opioid combination therapy investigators

Morphine produced larger CB2 upregulation than peripheral nerve injury

What the researchers found

CB2 receptors were predominantly expressed on neurons (NeuN-labeled cells) rather than microglia in the spinal dorsal horn. Chronic morphine administration (10 days) significantly increased CB2 receptor labeling across all surgery groups in both deep and superficial dorsal horn regions. Morphine produced a larger CB2 upregulation than surgery alone.

Why it matters

The synergistic pain-relieving effects of opioids and cannabinoids are well-documented but poorly understood. This finding that morphine upregulates neuronal CB2 receptors suggests a mechanism through which opioids may prime the system for cannabinoid-mediated pain relief.

The numbers in context

24 rats; 3 surgery groups x 2 drug conditions; CB2 predominantly on NeuN+ neurons; morphine significantly increased CB2 in deep and superficial dorsal horn; morphine effect larger than CCI surgery effect

How the study worked

24 male Sprague Dawley rats assigned to chronic constriction injury (CCI), sham surgery, or pain-naive groups, with half receiving daily morphine (5 mg/kg) for 10 days. Fluorescent immunohistochemistry on spinal cord sections assessed CB2 colocalization with neuronal (NeuN) and microglial (CD11b) markers.

What this study cannot tell us

Male rats only. Immunohistochemistry provides expression data but not functional receptor activity. Single morphine dose and duration tested. Does not prove the CB2 upregulation translates to enhanced cannabinoid analgesia.

How to read the evidence

Novel mechanistic finding with rigorous histological methods, but limited by male-only design and lack of functional behavioral validation.

When this study was published

Published in 2021.

The bigger picture

If morphine increases CB2 receptor availability, then adding cannabinoid treatment during or after opioid therapy could provide enhanced pain relief. This has implications for opioid-sparing strategies using combination cannabinoid-opioid approaches.

Questions still open

Does the morphine-induced CB2 upregulation enhance responses to cannabinoid pain medications? Could this mechanism be leveraged to reduce opioid doses through cannabinoid co-administration?

Common questions

What does this mean for combining opioids and cannabinoids?

If morphine increases CB2 receptor expression in the spinal cord, it may make the pain system more responsive to cannabinoid treatment. This could explain why opioid-cannabinoid combinations often show synergistic pain relief and suggests a biological basis for opioid-sparing strategies.

Were CB2 receptors on neurons or immune cells?

Predominantly on neurons. CB2 receptors were originally thought to be mainly on immune cells, but this study confirmed their presence on spinal neurons and showed that both surgery and morphine increased their neuronal expression.

Read the original research

Morphine Induces Upregulation of Neuronally Expressed CB2 Receptors in the Spinal Dorsal Horn of Rats.

Cannabis and cannabinoid research, 6(2), 137-147

View on PubMed ↗Publisher’s paper ↗

Citation

Grenier, Patrick; Sunavsky, Adam; Olmstead, Mary C. (2021). Morphine Induces Upregulation of Neuronally Expressed CB2 Receptors in the Spinal Dorsal Horn of Rats.. Cannabis and cannabinoid research, 6(2), 137-147. https://doi.org/10.1089/can.2020.0004