Opioid use disorder (OUD) is a chronic, relapsing neurobiological disease driven by persistent neuroplastic changes in the brain's reward, stress, and executive function circuits. Under normal physiology, endogenous opioid peptides (endorphins, enkephalins, dynorphins) bind mu, delta, and kappa opioid receptors to modulate pain, mood, and reward. Exogenous opioids (heroin, fentanyl, oxycodone) produce supraphysiological activation of mu-opioid receptors in the ventral tegmental area (VTA), triggering massive dopamine release in the nucleus accumbens (NAc) -- the brain's reward center -- creating intense euphoria that far exceeds natural reward signaling. With repeated exposure, neuroadaptive changes develop: mu-receptor downregulation and desensitization (tolerance), upregulation of the cAMP/adenylyl cyclase pathway (cellular dependence), and recruitment of anti-reward stress systems including corticotropin-releasing factor (CRF) in the extended amygdala (dysphoria and negative reinforcement). The transition from recreational use to compulsive use reflects a shift from positive reinforcement (using for euphoria) to negative reinforcement (using to avoid withdrawal and dysphoria). Medication-assisted treatment (MAT) targets these neurobiological mechanisms directly. Buprenorphine is a partial mu-opioid agonist with extremely high receptor binding affinity (Ki = 1 nM) but limited intrinsic activity (~40% maximal...