Role of cholinergic, opioidergic and GABAergic neurotransmission of the dorsal hippocampus in the modulation of nociception in guinea pigs.
OBJECTIVE
Several physiological, pharmacological and behavioral lines of evidence suggest that the hippocampal formation is involved in nociception. The hippocampus is also believed to play an important role in the affective and motivational components of pain perception. Thus, our aim was to investigate the participation of cholinergic, opioidergic and GABAergic systems of the dorsal hippocampus (DH) in the modulation of nociception in guinea pigs.


METHODS
The test used consisted of the application of a peripheral noxious stimulus (electric shock) that provokes the emission of a vocalization response by the animal.


RESULTS
Our results showed that, in guinea pigs, microinjection of carbachol, morphine and bicuculline into the DH promoted antinociception, while muscimol promoted pronociception. These results were verified by a decrease and an increase, respectively, in the vocalization index in the vocalization test. This antinociceptive effect of carbachol (2.7 nmol) was blocked by previous administration of atropine (0.7 nmol) or naloxone (1.3 nmol) into the same site. In addition, the decrease in the vocalization index induced by the microinjection of morphine (2.2 nmol) into the DH was prevented by pretreatment with naloxone (1.3 nmol) or muscimol (0.5 nmol). At doses of 1.0 nmol, muscimol microinjection caused pronociception, while bicuculline promoted antinociception.


CONCLUSIONS
These results indicate the involvement of the cholinergic, opioidergic and GABAergic systems of the DH in the modulation of antinociception in guinea pigs. In addition, the present study suggests that cholinergic transmission may activate the release of endorphins/enkephalin from interneurons of the DH, which would inhibit GABAergic neurons, resulting in antinociception.
Department of Physiology, School of Medicine of Ribeirão Preto, University of São Paulo, Av. Bandeirantes 3900, CEP 14049-900, Ribeirão Preto, SP, Brazil.
Lys Angela
LA
Favaroni Mendes
Leda
L
Menescal-de-Oliveira
D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D018756:GABA Antagonists / Q000494:pharmacology; D006168:Guinea Pigs; D006624:Hippocampus / Q000502:physiology*; D008297:Male; D009292:Narcotic Antagonists / Q000494:pharmacology; D010146:Pain / Q000503:physiopathology*; D010147:Pain Measurement / Q000187:drug effects; D010275:Parasympathetic Nervous System / Q000502:physiology*; D010276:Parasympatholytics / Q000494:pharmacology; D010277:Parasympathomimetics / Q000494:pharmacology; D011957:Receptors, Opioid / Q000502:physiology*; D013238:Stereotaxic Techniques; D009435:Synaptic Transmission / Q000502:physiology*; D014828:Vocalization, Animal / Q000187:drug effects; D005680:gamma-Aminobutyric Acid / Q000502:physiology*
D000701:Analgesics, Opioid; D018756:GABA Antagonists; D009292:Narcotic Antagonists; D010276:Parasympatholytics; D010277:Parasympathomimetics; D011957:Receptors, Opioid; D005680:gamma-Aminobutyric Acid