Research reportRegulation of GABA release via NMDA and 5-HT1A receptors in guinea pig dentate gyrus
Introduction
The hippocampal computations can be understood in terms of a simple trisynaptic circuit, which is a purely feedforward network and is considered to be excitatory. Both extrinsic inputs and local interneurons modulate activity in the above circuit. The local circuit within each hippocampal region consists minimally of a principal neuron and an associated inhibitory neuron, but can also include a recurrent excitatory synapse [4]. Cholinergic and aminergic projections from other brain regions to the hippocampal formation can play important roles, and acetylcholine, norepinephrine and serotonin are widely distributed in the hippocampus, where they exert a number of neuromodulatory effects [4].
In the dentate gyrus, there are basket cells as GABAergic inhibitory interneurons which exhibit multiple reciprocal synaptic interactions [4]. The dentate gyrus receives cholinergic, noradrenergic and serotonergic innervation 2, 5, 20. The presence of muscarinic, α- and β-adrenoceptors and 5-HT receptors have also been shown in the dentate gyrus 8, 27, 28. However, little is known about their interactions with GABAergic interneurons in the dentate gyrus. Recently, the effects of serotonin on glutamatergic and GABAergic neurons in the dentate gyrus were reported 12, 21, 27. Furthermore, the dentate gyrus contains a very high concentration of 5-HT1A sites [26]and certain dentate GABAergic interneurons are innervated by serotonergic fibers 11, 13, 16. Therefore, interest needs to be focused on the effects of 5-HT on GABAergic neurons via 5-HT1A receptors in the dentate gyrus.
The inhibitory role of 5-HT in long-term potentiation (LTP) has been recently reported in the CA1 region 7, 32, the CA3 region [37]and the dentate gyrus [29]. GABA may also participate in LTP of hippocampus 9, 23, 34, 40. Serotonergic modulation of the GABAergic system in LTP remains unclear. Thus, it is of considerable interest to investigate the physiological interactions among GABAergic neurons, glutamatergic neurons and serotonergic neurons in the dentate gyrus.
In this study we aimed to establish whether the mechanism by which the serotonergic pathway and the glutamatergic neuron control the function of the dentate gyrus also involves the modulation of GABAergic neurons. Here we report the presence of the NMDA and 5-HT1A receptors on GABAergic neurons and the involvement of NMDA and 5-HT1A receptors in the GABA release from guinea pig dentate gyrus, using the superfusion and high performance liquid chromatography (HPLC) technique.
Section snippets
Superfusion procedure
Adult guinea pigs of either sex, weighing 250–350 g, were decapitated. The brains were rapidly removed and put in ice-cold Krebs–Ringer solution of the following composition (mM): NaCl 118, KCl 4.8, CaCl2 2.5, MgSO4 1.19, NaHCO3 25.0, KH2PO4 1.18 and glucose 11. The dentate gyrus was dissected out at 4°C and slices (10 mg wet wt.) were cut at a thickness of 0.6 mm with a razor blade and a sliding guide. Slices were placed in a superfusion chamber and then superfused at a constant rate of 0.5
NMDA- or NAN-190-evoked release of endogenous GABA from dentate gyrus
The spontaneous release of GABA from slices of dentate gyrus became stable 60 min after the start of superfusion. The spontaneous release of GABA after 60 min from dentate gyrus was 78.7±3.2 pmol/min/mg protein (n=35). The release of GABA evoked by NMDA or NAN-190 was reproducible, since removal of NMDA or NAN-190 resulted in a return to spontaneous levels of GABA efflux. Thus, there were no significant differences in the level of the GABA release evoked by the first, second or third
Discussion
Each hippocampal region sends excitatory projections not only to the next region in the sequence but also to one or two after it. The only exception is the dentate gyrus, which is projected by only the perforant path and does not send projections beyond the CA3 region [4]. Although there are several types of neurons in the dentate gyrus, excitatory glutamatergic and inhibitory GABAergic neurons appear to play important roles in local interactions. Little is known about aminergic anatomic
Acknowledgements
We thank Dr. Routtenberg for critical comments. This work was supported by grants from the Ministry of Education, Science and Culture, Japan and Uehara Memorial Foundation.
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