Neural Pathways/physiology; Neurons/metabolism; Receptors, Opioid/genetics; Sleep/physiology; Analgesics, Opioid/administration & dosage; Analgesics, Opioid/metabolism; Analgesics, Opioid/pharmacology; Animals; Dynorphins/administration & dosage; Dynorphins/metabolism; Dynorphins/pharmacology; Electroencephalography; Hypothalamus/cytology; Hypothalamus/drug effects; Hypothalamus/metabolism; Immunohistochemistry; In Situ Hybridization; Male; Narcotic Antagonists; Neural Pathways/drug effects; Neural Pathways/metabolism; Neurons/cytology; Neurons/drug effects; Neurotransmitter Agents/administration & dosage; Neurotransmitter Agents/metabolism; Neurotransmitter Agents/pharmacology; Oligopeptides/administration & dosage; Oligopeptides/metabolism; Oligopeptides/pharmacology; Preoptic Area/cytology; Preoptic Area/drug effects; Preoptic Area/metabolism; Proto-Oncogene Proteins c-fos/metabolism; RNA, Messenger/genetics; RNA, Messenger/metabolism; Rats; Rats, Sprague-Dawley; Receptors, Opioid/agonists; Receptors, Opioid, kappa/agonists; Receptors, Opioid, kappa/antagonists & inhibitors; Receptors, Opioid, kappa/genetics; Receptors, Opioid, mu/agonists; Receptors, Opioid, mu/antagonists & inhibitors; Receptors, Opioid, mu/genetics; Synaptic Transmission/drug effects; Synaptic Transmission/physiology; Wakefulness/physiology