Agahari, Fransiscus Adrian
Description
As axons from the raphe nuclei densely innervate the
somatosensory cortex, the modulation of transmitter release by
serotonin (5-HT) was investigated in pyramidal cells in layer II
of rat barrel cortex. The idea was that 5-HT, via presynaptic
5-HT2 receptors (5-HT2R) coupled to Gq proteins, activates Ca2+
release from stores to increase spontaneous transmitter release.
Addition of 10 µM 5-HT, in the presence of TTX and gabazine,
caused a waxing and waning of...[Show more] the instantaneous mEPSC frequency.
Specifically, 5-HT increased the frequency by 28 ± 7% within 5
minutes (phase 1). Later, within 5 – 12 minutes, it dropped to
below control (-15 ± 3%; phase 2). Thereafter, it resurged back
to 27 ± 7% (phase 3). Concomitantly, the mEPSC amplitude
remained unaffected.
These changes in spontaneous release were mediated by 5-HT2CR and
5-HT2AR, with the former providing a larger contribution. The
downstream signalling was verified by blocking PLCβ, IP3R, and
Ca2+ release from stores. The findings were consistent with the
activation of a classical Gq cascade. Inhibiting PKC by Gö 6983
rendered the increase sustained, suggesting that a
phosphorylation caused the reduction in frequency after reaching
an initial peak.
These findings were restricted to a subset of cells (47%), which
were subsequently termed responders. No change occurred in
non-responders. The two groups differed by the size of the
reduction in input resistance (Rin) and the change in holding
current. For responders, the former was large, but the latter
small. In contrast, for non-responders, the former was small, but
the latter large.
In connected pairs of pyramidal cells in this layer, 5-HT
depressed the EPSC amplitude by 49 ± 3% without significantly
altering the paired-pulse ratio. This depression occurred
downstream of 5-HT2R activation. It was caused by Gβγ, because
it was blocked by Gβγ-binding peptides (mSIRK/ct-SNAP-25). As
Gβγ most likely inhibited voltage-dependent Ca2+ channels,
limited influx caused the EPSC depression.
Because 5-HT depressed EPSCs in most pairs, specificity in the
connectivity between responders and non-responders must exist. In
fact, responders were typically post-, whereas non-responders
presynaptic. Consistent with this idea, the mEPSC frequency only
increased in post-, but not presynaptic cells. Furthermore,
postsynaptic cells showed a large drop in Rin associated with a
small outward current. Conversely, presynaptic cells showed a
small drop in Rin together with a considerable
hyperpolarization.
My results revealed that, in contrast to the classical tenet of
Katz’ hypothesis of transmitter release, spontaneous
transmitter release increased, whereas evoked release depressed
downstream of 5-HT2R activation. The two mechanisms dissociated
at the Gq protein level. Spontaneous release was increased by
Ca2+ release from stores, whilst evoked release was depressed
most likely via inhibition of VDCC by Gβγ. Because of their
restriction to responders, these mechanisms would differentially
affect the neocortical microcircuitry.
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