Targeted invalidation of CCK2 receptor gene induces anxiolytic-like action in light-dark exploration, but not in fear conditioning test.
BACKGROUND
Evidence suggests that gamma-aminobutyric acid (GABA) and cholecystokinin (CCK) have opposite roles in the regulation of anxiety.


OBJECTIVE
The aim of our work was to study the behaviour of CCK(2) receptor deficient mice in light-dark exploration and fear conditioning tests. Moreover, the action of diazepam and methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), having the opposite effect on GABA(A) receptors, was evaluated on the exploratory behaviour in these mice. Expression levels of GABA(A) receptor subunit genes were also measured.


METHODS
Light-dark exploration and fear conditioning tests were used to determine changes in anxiety of mice. The action of diazepam (0.5-2 mg/kg i.p.) and DMCM (0.25-1 mg/kg i.p.) was studied in the light-dark box. The effect of DMCM was also evaluated in the motor activity test to demonstrate that its anti-exploratory action was not related to motor suppression. Expression levels of GABA(A) receptor subunit genes were determined by means of real-time polymerase chain reaction (qRT-PCR).


RESULTS
Female mice lacking CCK(2) receptors displayed increased exploratory activity in the light-dark box compared to their wild-type (+/+) littermates. Locomotor activity in the motility boxes and the intensity of freezing did not differ in wild-type (+/+) and homozygous (-/-) mice. Treatment with diazepam (0.5 mg/kg) increased the number of transitions in wild-type (+/+) animals, whereas in homozygous (-/-) mice diazepam (0.5-2 mg/kg) reduced exploratory activity. Administration of DMCM (0.25-1 mg/kg) induced an anxiogenic-like effect in homozygous (-/-) mice, but did not change their locomotor activity. Gene expression analysis established a 1.6-fold increase in the expression of the alpha2 subunit of GABA(A) receptors in the frontal cortex of homozygous (-/-) mice.


CONCLUSIONS
Genetic invalidation of CCK(2) receptors induced an anxiolytic-like action in exploratory, but not in conditioned models of anxiety. The observed reduction in anxiety in homozygous (-/-) mice is probably related to an increased function of GABAergic system in the brain.
Raud
Sirli
S
Department of Physiology, Biomedicum, University of Tartu, Estonia.
Innos
Jürgen
J
Abramov
Urho
U
Reimets
Ain
A
Kõks
Sulev
S
Soosaar
Andres
A
Matsui
Toshimitsu
T
Vasar
Eero
E
D000818:Animals; D001007:Anxiety / Q000209:etiology / Q000503:physiopathology*; D002243:Carbolines / Q000494:pharmacology; D003216:Conditioning, Operant; D003624:Darkness; D003975:Diazepam / Q000494:pharmacology; D005106:Exploratory Behavior / Q000502:physiology*; D005239:Fear / Q000502:physiology*; D005260:Female; D005625:Frontal Lobe / Q000378:metabolism; D015870:Gene Expression; D008027:Light; D051379:Mice; D008810:Mice, Inbred C57BL; D008815:Mice, Inbred Strains; D018345:Mice, Knockout; D009043:Motor Activity / Q000187:drug effects; D043887:Receptor, Cholecystokinin B / Q000172:deficiency / Q000235:genetics*; D011963:Receptors, GABA-A / Q000235:genetics
D002243:Carbolines; D043887:Receptor, Cholecystokinin B; D011963:Receptors, GABA-A; C034818:methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate; D003975:Diazepam