Dr. Graciela Piñeyro: Partial Agonism, Receptor Recycling, and the Limits of Bias

For over a decade, biased agonism at opioid receptors offered a clean route to analgesics without the respiratory depression and tolerance that have defined opioid therapy. If the right ligand could engage G-protein signaling while sparing β-arrestin, pain relief might finally be separable from side effects. In this conversation, Dr. Graciela Piñeyro — who leads an opioid receptor pharmacology group at the CHU Sainte-Justine Research Center and the Université de Montréal — walks through what happened when her lab tested that hypothesis rigorously. Across 25 ligands profiled at the μ-opioid receptor, the team found no bias, only partial agonism. Recent β-arrestin knockout work from Australia, Germany, and the UK has converged on a similar sobering conclusion.

The conversation also traces her earlier work showing that δ-opioid receptors recycle from the lysosome — the compartment meant to degrade them — and how recycling patterns, not internalization itself, predict analgesic tolerance. For Dr. Piñeyro, a physician-turned-pharmacologist whose early project in Michel Bouvier's lab felt like "torture" until the right quantitative model turned her artifacts into real pharmacological responses, the throughline has never been building a career. It has been a refusal to treat messy data as noise until the right math arrived to make sense of it.

About the Guest

Dr. Graciela Piñeyro is a principal investigator at the CHU Sainte-Justine Research Center and a professor in the Department of Pharmacology and Physiology at the Université de Montréal. Trained as a physician in Uruguay, she began doing research in the only way then available to her — through a medical pharmacology department studying how benzodiazepines and alcohol affected sleep architecture. She completed her PhD at McGill University with Claude de Montigny, working on serotonin and antidepressant mechanisms, and her postdoctoral training with Michel Bouvier at the Université de Montréal, where she shifted into opioid receptors. Her lab now studies δ- and μ-opioid receptor signaling, biased agonism, receptor trafficking and recycling, and more recently the cannabinoid entourage hypothesis — combining BRET-based biosensors, quantitative pharmacology, and clustering approaches to connect in vitro signaling signatures to clinical side-effect profiles.

Scientific Themes of the Conversation

• The limits of biased agonism at the μ-opioid receptor — and why partial agonism keeps explaining what bias was supposed to

• δ-opioid receptor recycling from the lysosome — and why the post-internalization fate of a receptor predicts tolerance better than internalization itself

• Protean agonism and multiple active conformations — how the same ligand can behave as agonist or inverse agonist depending on receptor tone

• Quantitative pharmacology as rescue operation — using operational models and BRET-based biosensors to pull pharmacological signal out of what first looks like experimental noise

• The defense of simple systems — why HEK-cell data can still predict FDA pharmacovigilance outcomes, and where iPSC-derived somatic systems actually pay their cost

• The cannabinoid entourage hypothesis as a pharmacological question — turning folkloric claims about complex mixtures into testable quantitative work

Key Insights from the Conversation

Twenty-five μ-opioid ligands produced partial agonism, not bias. When Dr. Piñeyro's lab systematically compared G-protein and β-arrestin recruitment across 25 compounds — standard opioids and new ligands from Pfizer — they could not identify a ligand-bias signature. What they could do was classify the compounds by relative efficacy, and that classification alone predicted clinical side effects.

β-arrestin is not the villain it was made to be. Newer knockout work from groups in Australia, Germany, and the UK has shown that respiratory depression from opioids persists in β-arrestin-null mice. The mechanistic story that guided a decade of biased-ligand drug discovery needs rebuilding from the underlying biology, not from the pathway labels.

The δ-opioid receptor recycles from the lysosome, and the recycling pattern determines tolerance. Long treated as a non-recycling receptor destined for degradation, the δ-opioid receptor can in fact return from the lysosomal compartment. In Dr. Piñeyro's framework, the ligands that permit recycling are the ones that produce less tolerance, whether acute or chronic — an inversion of the older view that internalization itself was the key variable.

A quantitative model can turn an artifact into pharmacology. Early in her postdoc, Dr. Piñeyro watched the same ligand behave as an agonist under one condition and an inverse agonist under another. The resolution came not from a new experiment but from a model — protean agonism, as Kenakin had just described it, combined with André deLéan's software for estimating active receptor conformations — which revealed that the apparent contradiction was a quantitative signature of multiple active conformations.

HEK cells still earn their keep. Even as the field pushes toward iPSC-derived somatic systems, HEK-cell signaling profiles of μ-opioid ligands correctly predicted outcomes logged in the FDA pharmacovigilance database. Dr. Piñeyro argues that the predictive power of the simpler system, and the real cost of the more elaborate one, should be weighed honestly before the field walks away from what works.

The cannabinoid entourage hypothesis deserves a rigorous test. Instead of treating cannabinoids as single molecules, her group is asking whether complex mixtures of cannabinoids and terpenes produce analgesic responses that pure compounds cannot — and whether the mechanism involves allosterism across GPCRs, TRPs, and enzymes of the endocannabinoid system. The open question about THC's long-term effects on adolescent cognition sits alongside the analgesic one, and both require the same quantitative discipline.

Follow the questions, not the career. Asked what advice she gives young scientists, Dr. Piñeyro's answer is blunt: she does not think of herself as having built a career. She followed interests that pulled her. In a funding environment that rewards the opposite, the advice is harder to take than it sounds — and, in her telling, the only thing that sustained her through the years when the data refused to make sense.

Episode Timeline

00:00 — Introduction

01:57 — Meet Dr. Graciela Piñeyro

02:45 — From Uruguay to McGill: the cold email that changed a career

04:40 — The closed door that opened another: landing in Bouvier's lab

05:30 — The side project that became the real work

08:22 — Protean agonism and the model that rescued the data

11:00 — δ-opioid recycling from the lysosome and the question of tolerance

14:43 — 25 μ-opioid ligands, no bias, and the β-arrestin reversal

21:11 — Receptor tuning and the quantitative discipline behind the analysis

24:25 — The case for HEK cells in an age of iPSC enthusiasm

30:40 — Cannabinoids, the entourage hypothesis, and THC in teenage brains

43:20 — Follow the question, not the career

Timestamps were generated using AI for readability.

Selected Quotes

About this episode

Dr. Graciela Pineyro’s love for GPCR pharmacology started in Uruguay where she first worked on the serotonin receptors. This interest in research and pharmacology took Graciela to Canada where she stayed ever since she arrived for her Ph.D. work. Graciela has done extensive work on the molecular pharmacology of opioid receptors, exploring their signaling, trafficking, and their ability to activate different signaling pathways and signaling bias. Today, Graciela and her team’s efforts are directed towards the characterization of the pharmacological properties of cannabinoids in conjunction with terpenes for pain relief.

Dr. Graciela Pineyro on the web

• Dr. Graciela Pineyro on LinkedIn

• Dr. Graciela Pineyro - University of Montreal

• Dr. Graciela Pineyro - CHU Ste-Justine Research Centre

• Pineyro Lab Publications on Google Scholar

• Pineyro Lab on Pubmed

• Dr. GPCR Ecosystem