Evidence for GABA involvement in midbrain control of medullary neurons that modulate nociceptive transmission.
The effects of GABA-related compounds microinjected into the midbrain periaqueductal gray (PAG) on the tail-flick reflex (TF) and on the activity of tail-flick related neurons in the rostral ventromedial medulla (RVM), were studied in barbiturate anesthesized rats. Neurons whose activity either decreased (off-cells) or increased (on-cells) immediately prior to TF were examined. Bicuculline and picrotoxin microinjected into the ventrolateral aspect of the caudal PAG inhibited the TF, increased the spontaneous activity of the off-cells and decreased that of the on-cells. Concomitant with the increase in TF latency, the TF-related deceleration of the off-cells and acceleration of the on-cells were reduced. These effects were reversed by a microinjection of muscimol (MUS) into the PAG. The analgesic effect of morphine microinjected into the PAG was also reversed by a MUS microinjection at the same site. These results support the hypothesis that a GABAergic synapse inhibits cells in the PAG which modulate nociceptive transmission at the spinal level through actions on neurons in the RVM.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects / Q000502:physiology*; D009020:Morphine / Q000037:antagonists & inhibitors; D009118:Muscimol / Q000494:pharmacology; D009434:Neural Pathways / Q000502:physiology; D010146:Pain / Q000503:physiopathology*; D010147:Pain Measurement; D010487:Periaqueductal Gray / Q000187:drug effects / Q000502:physiology*; D010852:Picrotoxin / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D005680:gamma-Aminobutyric Acid / Q000502:physiology*
D001640:Bicuculline; D009020:Morphine; D009118:Muscimol; D010852:Picrotoxin; D005680:gamma-Aminobutyric Acid