GLUTAMATE MICROSTIMULATION OF LOCAL INHIBITORY CIRCUITS IN THE SUPRAOPTIC
NUCLEUS FROM RAT HYPOTHALAMUS SLICES.
Jean-Pierre Wuarin.
Department of Anatomy and Neurobiology, Colorado State University, Fort
Collins, CO 80523
APStracts 4:135N, 1997.
ABSTRACT
The hypothesis of a local inhibitory input to the hypothalamic supraoptic
nucleus was tested with combined glutamate microstimulation and whole-cell
patch-clamp recordings in slices from rat hypothalamus. Synaptic activity in
supraoptic magnocellular neuroendocrine cells (MNCs) was monitored and
glutamate microdrops were applied in the perinuclear region of the supraoptic
nucleus to evoke firing of action potentials in putative presynaptic
inhibitory cells. The effect of glutamate microdrops applied in the
perinuclear region was tested on 57 supraoptic MNCs. In control conditions,
spontaneous excitatory (EPSCs) and inhibitory (IPSCs) postsynaptic currents
were observed at resting membrane potential in all MNCs tested. Glutamate
microstimulation evoked an abrupt increase in the frequency and size of
spontaneous IPSCs in eight MNCs. Forty-nine MNCs did not show any change in
the inhibitory synaptic input. Microapplication of glutamate in the periphery
of the supraoptic nucleus did not modify the amplitude or the frequency of
spontaneous EPSCs in any of the 57 MNCs tested. In the group of eight MNCs
that responded to glutamate microstimulation by an increase in inhibitory
input, two types of responses were observed. Four MNCs showed an increase in
both size and frequency of spontaneous IPSCs through the entire range of
amplitude. In the other four MNCs, local glutamate stimulation produced a
dramatic increase in the size of IPSCs and a lesser increase in the frequency
of the smaller IPSCs. The potential effect of the glutamate-evoked increase in
inhibitory input on the firing activity of MNCs was tested in current-clamp
conditions. Intracellular current injection was applied to evoke firing of
action potentials in six MNCs that had responded to local glutamate
microstimulation by an increase in inhibitory input. Glutamate microdrop
applications inhibited the evoked action potential firing in all six cells.
These results suggest: (1) local inhibitory interneurons are present in the
periphery of the supraoptic nucleus, (2) they contain functional glutamate
receptors, (3) they form inhibitory synapses with supraoptic MNCs, and (4)
activation of these interneurons inhibits firing in MNCs. These results
support the hypothesis that local inhibitory interneurons play a important
role in the firing activity of supraoptic MNCs.
Received 21 February 1997; accepted in final form 8 July 1997.
APS Manuscript Number J155-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 27 August 1997