ClonidineAdrenergic alpha-AgonistsAdrenergic alpha-2 Receptor AgonistsYohimbinePreanesthetic MedicationSympatholyticsReceptors, Adrenergic, alpha-2Adrenergic alpha-AntagonistsImidazoline ReceptorsAnalgesicsReceptors, Adrenergic, alphaInjections, SpinalGuanfacineDouble-Blind MethodAdjuvants, AnesthesiaDose-Response Relationship, DrugIdazoxanAdrenergic alpha-2 Receptor AntagonistsOxymetazolineAnalgesia, EpiduralBlood PressureMorphineNorepinephrinePrazosinPremedicationHeart RatePain, PostoperativePhentolamineAnalgesics, OpioidReceptors, AdrenergicAnesthetics, LocalDioxanesHypnotics and SedativesDrug InteractionsBupivacaineHemodynamicsPain MeasurementNaloxoneNeostigmineMethyldopaInjections, IntravenousPainGuanabenzAdministration, OralDexmedetomidineMedetomidineInjections, EpiduralTime FactorsImidazolesAnalgesia, ObstetricalAntihypertensive AgentsEpinephrineTreatment OutcomeSympathetic Nervous SystemMethoxamineAlfentanilDrug-Related Side Effects and Adverse ReactionsRats, Sprague-DawleyDesipramineAnalgesics, Non-NarcoticShivering