title: Defensive behaviors evoked from the ventrolateral periaqueductal gray of the rat: comparison of opioid and GABA disinhibition.
abstract: Microinjection of morphine into the ventrolateral periaqueductal gray (PAG) disinhibits output neurons resulting in immobility and antinociception. Disinhibition also can be produced by microinjection of the GABA antagonist bicuculline. If morphine and bicuculline disinhibit the same class of neurons, then the behavioral effects evoked should be the same. Microinjection of morphine (5 microg/0.4 microl) into the ventrolateral PAG produced antinociception in 46 of 85 rats (54%). Subsets of rats with and without morphine antinociception were subsequently injected with bicuculline (2.5, 5, 10, and 25 ng/0.4 microl) into the same PAG site. Microinjection of bicuculline produced an increase in hot plate latency that was independent of the effect of the prior morphine microinjection. Bicuculline administration also produced an increase in locomotor activity in most rats, not immobility as with morphine microinjections. These differences between morphine and bicuculline microinjections indicate three things: (a) disinhibition of PAG neurons whether by morphine or bicuculline is an effective means of producing antinociception; (b) the circuitry underlying the behavioral effects of morphine and bicuculline differ; (c) the ventrolateral PAG appears capable of supporting a range of defensive behaviors from immobility to flight.
authors: Department of Psychology, Washington State University Vancouver, 98686, USA. morgan@vancouver.wsu.edu
authors: Michael M
authors: MM
authors: Morgan
authors: Cecilea C
authors: CC
authors: Clayton
mesh_terms: D000701:Analgesics, Opioid / Q000008:administration & dosage; D000818:Animals; D001640:Bicuculline / Q000008:administration & dosage*; D004924:Escape Reaction / Q000187:drug effects* / Q000502:physiology; D018756:GABA Antagonists / Q000008:administration & dosage; D046353:Immobility Response, Tonic / Q000187:drug effects / Q000502:physiology; D008297:Male; D008845:Microinjections; D009020:Morphine / Q000008:administration & dosage*; D009043:Motor Activity / Q000187:drug effects / Q000502:physiology; D009433:Neural Inhibition / Q000187:drug effects* / Q000502:physiology; D009474:Neurons / Q000187:drug effects; D017288:Pain Threshold / Q000187:drug effects* / Q000502:physiology; D010487:Periaqueductal Gray / Q000166:cytology / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology
chemical_list: D000701:Analgesics, Opioid; D018756:GABA Antagonists; D009020:Morphine; D001640:Bicuculline