Environment; Receptors, Adrenergic, alpha-2/physiology; Receptors, Adrenergic, beta/physiology; Stress, Physiological/physiopathology; Submandibular Gland/physiology; Adrenal Glands/drug effects; Adrenal Glands/innervation; Adrenal Glands/physiology; Adrenergic alpha-Agonists/administration & dosage; Adrenergic alpha-Agonists/blood; Adrenergic alpha-Agonists/pharmacology; Adrenergic beta-Agonists/administration & dosage; Adrenergic beta-Agonists/pharmacology; Animals; Circadian Rhythm; Clonidine/administration & dosage; Clonidine/pharmacology; Darkness; Dose-Response Relationship, Drug; Efferent Pathways/drug effects; Efferent Pathways/physiology; Immobilization; Injections, Intravenous; Isoproterenol/administration & dosage; Isoproterenol/pharmacology; Lighting; Male; Norepinephrine/blood; Pineal Gland/drug effects; Pineal Gland/innervation; Pineal Gland/physiology; Rats; Rats, Wistar; Receptors, Adrenergic, alpha-2/drug effects; Receptors, Adrenergic, beta/drug effects; Saliva/metabolism; Submandibular Gland/drug effects; Submandibular Gland/innervation; Submandibular Gland/metabolism; Sympathetic Nervous System/drug effects; Sympathetic Nervous System/physiology