Glycine-Gated Chloride Channels Depress Synaptic Transmission in Rat Hippocampus

Abstract

An inhibitory role for strychnine-sensitive glycine-gated chloride channels (GlyRs) in mature hippocampus is beginning to be appreciated. We have reported previously that CA1 pyramidal cells and GABAergic interneurons recorded in 3- to 4-wk-old rat hippocampal slices express functional GlyRs, dispelling previous misconceptions that GlyR expression ceases in early development. However, the effect of GlyR activation on cell excitability and synaptic circuits in hippocampus has not been fully explored. Using whole cell current-clamp recordings, we show that activation of strychnine-sensitive GlyRs through exogenous glycine application causes a significant decrease in input resistance and prevents somatically generated action potentials in both CA1 pyramidal cells and interneurons. Furthermore, GlyR activation depresses the synaptic network by reducing suprathreshold excitatory postsynaptic potentials (EPSPs) to subthreshold events in both cell types. Blockade of postsynaptic GlyRs with the chloride channel blocker 4, 4'-diisothiocyanatostilbene-2-2'-disulfonic acid (DIDS) or altering the chloride ion driving force in recorded cells attenuates the synaptic depression, strongly indicating that a postsynaptic mechanism is responsible. Increasing the local glycine concentration by blocking reuptake causes a strychnine-sensitive synaptic depression in interneuron recordings, suggesting that alterations in extracellular glycine will impact excitability in hippocampal circuits. Finally, using immunohistochemical methods, we show that glycine and the glycine transporter GlyT2 are co-localized selectively in GABAergic interneurons, indicating that interneurons contain both inhibitory neurotransmitters. Thus we report a novel mechanism whereby activation of postsynaptic GlyRs can function to depress activity in the synaptic network in hippocampus. Moreover, the co-localization of glycine and GABA in hippocampal interneurons, similar to spinal cord, brain stem, and cerebellum, suggests that this property is likely to be a general characteristic of inhibitory interneurons throughout the CNS.

Role of Glycine Receptors in Glycine-Induced LTD in Hippocampal CA1 Pyramidal Neurons

“…Reduction of GlyRs suppression function facilitates LTD induced by repetitive paring of postsynaptic spike and presynaptic stimulations, whereas activation of GlyRs with high-level glycine (1.0 mM) counteracts this form of LTD. In this study, it is possible that activation of GlyRs by 1.0 mM glycine (in 4.0 mM Mg 2 + -containing perfusing medium) suppresses excitatory neurotransmission, as demonstrated by a recent study (Song et al, 2006). This effect might occur more rapidly than LTD induction and thus excludes further LTD elicited by 10 Hz induction protocol.…”

“…However, these results were obtained in the presence of the GlyR antagonist, strychnine, in all of the solutions to deliberately exclude the potential activation of GlyRs (Lu et al, 2001). A recent study reported the acute suppressive action of both EPSCs in CA1 pyramidal neurons and IPSCs in interneurons by glycine (Song et al, 2006). This inhibitory effect only lasted for around 15 min, and the EPSCs recovered after the washout of glycine.…”

“…However, it is still unknown whether GlyRs have a role in this inhibitory effect. Another recent study further demonstrated that GlyR activation could mediate the acute inhibitory effects of glycine on excitatory postsynaptic potentials (EPSPs) (Song et al, 2006). It is of great importance to know whether and how GlyR activation by high levels of glycine accumulation, which opens GlyR-gated chloride channels, could induce persistent changes in EPSCs.…”

Glycine receptors control the generation of projection neurons in the developing cerebral cortex

“…GlyRs containing a1 and a3 subunits are expressed in spinal cord and brainstem circuits implicated in startle disease, inflammatory pain and rhythmic breathing. 51 Although GlyRs are expressed in the hippocampus and cortex, [21][22][23][24][25][26][27] their role in development has been largely overlooked. Over the past few years, we and others have discovered that GlyRs are highly expressed by specific neurons and progenitors of the cerebral cortex where they control critical developmental features.…”

“…Postsynaptic GlyRs control synaptic transmission of sensory stimuli and are involved in the generation of motor outputs in the spinal cord. 20 These receptors are also expressed in the hippocampus [21][22][23][24][25] and in the cortex. 26,27 Importantly, GlyR heteromers cluster at the presynaptic membrane to control neurotransmitter release in the medial nucleus of trapezoid body in the brainstem, 28 the spinal cord 29,30 and the ventral tegmental area.…”