title: Involvement of GABAergic modulation of antinociception induced by morphine microinjected into the ventrolateral orbital cortex.
abstract: Previous studies have shown that microinjection of morphine into the prefrontal ventrolateral orbital cortex (VLO) produces antinociception. The current study examined whether gamma-aminobutyric acid (GABA) containing neurons in the VLO were involved in this antinociception. Under light anesthesia, the GABA(A) receptor antagonist bicuculline and picrotoxin or agonist muscimol and THIP was microinjected into the VLO in non-morphine-treated (control) and morphine-treated (microinjection into the VLO) rats. Noxious heat-evoked tail flick (TF) latencies (TFLs) were measured in all of these groups of rats every 5 min. Bicuculline or picrotoxin (100, 200, 500 ng in 0.5 microl) depressed the TF reflex in a dose-related fashion. A smaller dose (100 ng) of bicuculline or picrotoxin microinjected into VLO significantly enhanced the VLO morphine-evoked inhibition of the TF reflex. In contrast, administration of muscimol (250 ng) or THIP (1.0 microg) significantly attenuated the morphine-induced antinociception in the VLO morphine-treated rats. These results suggest that the GABA(A) receptor is involved in the modulation of VLO morphine-induced antinociception, and provide a behavioral support for the hypothesis that morphine may directly inhibit the GABAergic inhibitory interneurons leading to indirect activation of the descending antinociceptive pathway through a disinhibitory effect on the VLO output neurons and depression of the nociceptive inputs at the spinal cord level.
authors.affiliation: Key Laboratory of Environment and Genes Related to Diseases, Ministry of Education, Department of Physiology and Pathophysiology, Xi'an Jiaotong University School of Medicine, Xi'an, Shaanxi 710061, The People's Republic of China.
authors.forename: Chao-Ling
authors.initials: CL
authors.lastname: Qu
authors.forename: Jing-Shi
authors.initials: JS
authors.lastname: Tang
authors.forename: Hong
authors.initials: H
authors.lastname: Jia
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D001522:Behavior, Animal; D001640:Bicuculline / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005260:Female; D018755:GABA Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D007555:Isoxazoles / Q000494:pharmacology; D008297:Male; D008845:Microinjections / Q000379:methods; D009020:Morphine / Q000494:pharmacology*; D009118:Muscimol / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology*; D009474:Neurons / Q000187:drug effects* / Q000502:physiology; D009619:Nociceptors / Q000502:physiology*; D010147:Pain Measurement / Q000379:methods; D010852:Picrotoxin / Q000494:pharmacology; D017397:Prefrontal Cortex / Q000166:cytology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011930:Reaction Time / Q000187:drug effects; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D001640:Bicuculline; D018755:GABA Agonists; D018756:GABA Antagonists; D007555:Isoxazoles; D009020:Morphine; D009118:Muscimol; D009294:Narcotics; D010852:Picrotoxin; D005680:gamma-Aminobutyric Acid; C015542:gaboxadol