The effects of R(+)-8-hydroxy-dipropylaminotetralin (8-OHDPAT) administration into
the dorsal raphe nucleus (DRN) or bilaterally into the dorsal hippocampus (HIP) on
fear behavior in a modified version of the light-dark transitions test and regional
brain monoamines (NA, DA, 5-HT) and their metabolites (MHPG, DOPAC, 5-HIAA) in the
hypothalamus, midbrain central gray matter, amygdala, hippocampus and pons were examined.
The experiments were performed on 36 male, 3-month old Wistar rats. Administration
of 8-OHDPAT (200 ng) into the DRN reduced time out from the illuminated part of the
chamber and time of motionless behavior in the illuminated part, increased the number
of returns from the dark to illuminated part and number of head dipping from the dark
to illuminated part without effect on time of motionless behavior in the dark part
and on time of locomotor activity in the illuminated as well as in dark part of the
chamber. HPLC analysis showed reduction of 5-HT content in the midbrain and amygdala,
reduction of 5-HIAA content in pons, increased 5-HIAA/5-HT ratio in the hippocampus
and increased DOPAC/DA ratio in the hypothalamus, midbrain, hippocampus and pons without
affecting the MHPG/NA ratio and NA content. The administration of 8-OHDPAT (100 ng
per site) into the HIP reduced time out from the illuminated part of chamber, time
of locomotor activity in the illuminated part and head dipping from the dark to illuminated
part without effect on the number of returns from the dark to illuminated part, time
of locomotor activity in the dark part and time of motionless in the illuminated as
well as in the dark part of chamber. HPLC analysis showed reduction of NA content
in the hypothalamus, amygdala and pons, increased the MHPG content in all the investigated
structures, increased MHPG/NA ratio in all the investigated structures except the
hypothalamus. Dopamine content decreased in the hypothalamus and amygdala, and DOPAC/DA
ratio increased in the amygdala and hippocampus. Concentrations of 5-HT, 5-HIAA and
5-HIAA/5-HT ratio were unchanged. The results obtained indicate that 8-OHDPAT acting
on the pre-synaptic 5-HT1A receptors decreases fear behavior and acting on 5-HT1A
post-synaptic receptors increases fear behavior in the light-dark transitions test.
The neurochemical base of anxiolytic and anxiogenic effects evoked by 8-OHDPAT is
being discussed.