Table 1.

Brief description of main behavioral paradigms used for studying opioid addiction symptoms

Physiological aspects of opioid addiction
Tolerance to antinociceptive effects	Measurement of antinociceptive efficacy of a given agonist over a period of time using thermal or mechanical nociceptive tests [31]
Withdrawal syndrome	Naloxone-precipitated or spontaneous withdrawal symptoms in dependent animals measured over a period of time. Most common withdrawal symptoms include: jumping, body/paw tremor, teeth chattering, rearings, wet-dog shakes, increased urination and defecations [32]

Subjective aspects of opioid addiction
Reward-associated behavior	Conditioned place preference paradigm allowing to measure animal’s preference toward the drug-associated area in comparison to the neutral environment [33]
Reinforcement-related behavior	Operant self-administration (intravenous, oral) of a substance under stable or progressive ratio conditions allowing to measure motivation to obtain drug [34] Relapse-related and drug-seeking behaviors during the abstinence period [35] Drug discrimination test used for identifying the abuse potential of a given compound in comparison to known drugs of abuse [36] Intracranial self-stimulation procedure which through electrical stimulation of brain reward region allows to assess the effects of various pharmacological manipulations on sensitivity to reward [37]