Effects of opioid receptor blockade on defensive behavior elicited by electrical stimulation of the aversive substrates of the inferior colliculus in Rattus norvegicus (Rodentia, Muridae).
BACKGROUND
Electrical or chemical stimulation of some structures of the midbrain tectum, such as the dorsal periaqueductal gray matter, deep layers of the superior colliculus and inferior colliculus induce fear and flight behavior. These structures constitute the main neural substrates commanding defensive behavior in brainstem. Many neurotransmitters are implicated in the modulation of aversion at the mesencephalic level.


OBJECTIVE
The aim of this work is to investigate the involvement of opioid mechanisms in modulation of defensive behavior in dorsal mesencephalon.


METHODS
Male Wistar rats were fixed in a stereotaxic frame and a chemitrode was implanted into the midbrain, targeted to the central nucleus of the inferior colliculus. In the present study, the effects of peripheral and central administration of naloxone, naltrexone or naloxonazine on aversive thresholds (freezing and escape reactions) elicited by electrical stimulation of the midbrain tectum were determined.


RESULTS
Peripherally and centrally administered naloxone caused a significant increase in the freezing and flight thresholds elicited by electrical stimulation of the aversive substrates of the inferior colliculus. These effects were confirmed by peripheral and central administration of naltrexone and by microinjections of naloxonazine in inferior colliculus.


CONCLUSIONS
These findings suggest that endogenous opioids are involved in the modulation of the aversive behavior elicited by midbrain tectum stimulation. Since microinjections of naloxonazine in the central nucleus of the inferior colliculus caused a significant increase in the aversive thresholds elicited by electrical stimulation of this structure, it is possible that micro1 opioid receptor located in this nucleus may be critically implicated in this neural circuitry.
Coimbra
N C
NC
Departamento de Farmacoligia, Faculdade de Medicina de Ribeirão Preto da Universidade de São Paulo, Brasil. nccoimbr@fmrp.usp.br
Osaki
M Y
MY
Eichenberger
G C
GC
Ciscato
J G
JG
Jucá
C E
CE
Biojone
C R
CR
D000818:Animals; D004558:Electric Stimulation; D004924:Escape Reaction / Q000187:drug effects; D007245:Inferior Colliculi / Q000187:drug effects* / Q000502:physiology; D008297:Male; D008636:Mesencephalon / Q000187:drug effects* / Q000502:physiology; D009270:Naloxone / Q000494:pharmacology*; D009292:Narcotic Antagonists / Q000494:pharmacology*; D009294:Narcotics / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar
D009292:Narcotic Antagonists; D009294:Narcotics; D009270:Naloxone