D. Schmitz et al., SEROTONIN REDUCES SYNAPTIC EXCITATION IN THE SUPERFICIAL MEDIAL ENTORHINAL CORTEX OF THE RAT VIA A PRESYNAPTIC MECHANISM, Journal of physiology, 508(1), 1998, pp. 119-129
The superficial layers II and III of the entorhinal cortex, which form
the main cortical input to the hippocampus, receive a large serotoner
gic projection from the raphe nuclei and express 5-HT receptors at hig
h density. Here, we studied the effects of serotonin on the intrinsic
properties and excitatory synaptic transmission of the superficial med
ial entorhind cortex. 2. Intracellular and patch clamp recordings reve
aled that serotonin hyperpolarized only one-third of the cells, approx
imately, through a potassium conductance via a GTP-dependent process.
3. Serotonin depressed mixed as well as isolated lpha-amino-3-hydroxy-
5-methyl-4-isoxazolepropionic acid receptor (AMPAR)- and N-methyl-D-as
partic acid receptor (NMDAR)mediated excitatory postsynaptic potential
s/currents (EPSPs/EPSCs; approximately 40% reduction with 1 mu m serot
onin). 4. The effect of serotonin on EPSPs/EPSCs was similar in whole-
cell versus intracellular recordings; it did not require intracellular
GTP and was not visible in glutamate applications to excised patches.
Miniature EPSCs recorded in the presence of tetrodotoxin and bicucull
ine were reduced in frequency but not altered in amplitude. 5. The eff
ects of serotonin on intrinsic properties and EPSPs were partially mim
icked by 5-HT1A receptor agonists (+)-8-hydroxy-2-(di-n-propylamino)te
tralin hydrobromide (8-OH-DPBT) and 5-carboxamido-tryptamine maleate (
5-CT), and reduced by 5-HT1A receptor antagonists S-(-)-5-fluoro-8-hyd
roxy-DPAT hydrochloride (S-UH-301), ethoxyphenyl)-4-[4-(2-phthalimido)
butyl]piperazine hydrobromide (NAN-190) and spiperone. 6. We conclude
that serotonin potently suppresses excitatory synaptic transmission vi
a 5-HT1A receptors in layers II and III of the medial entorhinal corte
x by a presynaptic mechanism.