Morphine Dependence: Strong dependence, both physiological and emotional, upon morphine.Morphine: The principal alkaloid in opium and the prototype opiate analgesic and narcotic. Morphine has widespread effects in the central nervous system and on smooth muscle.Naloxone: A specific opiate antagonist that has no agonist activity. It is a competitive antagonist at mu, delta, and kappa opioid receptors.Substance Withdrawal Syndrome: Physiological and psychological symptoms associated with withdrawal from the use of a drug after prolonged administration or habituation. The concept includes withdrawal from smoking or drinking, as well as withdrawal from an administered drug.Narcotic Antagonists: Agents inhibiting the effect of narcotics on the central nervous system.Narcotics: Agents that induce NARCOSIS. Narcotics include agents that cause somnolence or induced sleep (STUPOR); natural or synthetic derivatives of OPIUM or MORPHINE or any substance that has such effects. They are potent inducers of ANALGESIA and OPIOID-RELATED DISORDERS.Matricaria: A plant genus of the family ASTERACEAE. M. chamomilla appears similar to Anthemis but this flower disk is conical and hollow and lacks chaffy bract scales and the odor is weaker. The common name of 'manzanilla' is confused with other meanings of the word. 'Matricaria chamomilla sensu' is classified by some as Tripleurospermum perforata. Other plants with similar common names include CHAMAEMELUM; TRIPLEUROSPERMUM and ANTHEMIS.Drug Tolerance: Progressive diminution of the susceptibility of a human or animal to the effects of a drug, resulting from its continued administration. It should be differentiated from DRUG RESISTANCE wherein an organism, disease, or tissue fails to respond to the intended effectiveness of a chemical or drug. It should also be differentiated from MAXIMUM TOLERATED DOSE and NO-OBSERVED-ADVERSE-EFFECT LEVEL.Receptors, Opioid, mu: A class of opioid receptors recognized by its pharmacological profile. Mu opioid receptors bind, in decreasing order of affinity, endorphins, dynorphins, met-enkephalin, and leu-enkephalin. They have also been shown to be molecular receptors for morphine.Morphine Derivatives: Analogs or derivatives of morphine.Behavior, Animal: The observable response an animal makes to any situation.Rats, Sprague-Dawley: A strain of albino rat used widely for experimental purposes because of its calmness and ease of handling. It was developed by the Sprague-Dawley Animal Company.Dose-Response Relationship, Drug: The relationship between the dose of an administered drug and the response of the organism to the drug.Analgesics, Opioid: Compounds with activity like OPIATE ALKALOIDS, acting at OPIOID RECEPTORS. Properties include induction of ANALGESIA or NARCOSIS.Brain: The part of CENTRAL NERVOUS SYSTEM that is contained within the skull (CRANIUM). Arising from the NEURAL TUBE, the embryonic brain is comprised of three major parts including PROSENCEPHALON (the forebrain); MESENCEPHALON (the midbrain); and RHOMBENCEPHALON (the hindbrain). The developed brain consists of CEREBRUM; CEREBELLUM; and other structures in the BRAIN STEM.