Steven R. Glaum

Acute Regulation of Synaptic Transmission by Metabotropic Glutamate Receptors

Two principal classes of glutamate receptors have been identified: (1) ligand-gated ion channels and (2) G-protein-coupled “metabotropic” receptors (Sugiyama et al., 1989). Activation of ionotropic α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA), kainate (KA), and N-methyl-d-aspartate (NMDA) receptors represents the principal route of fast excitatory transmission in the CNS. However, it is becoming increasingly clear that synaptic transmission also appears to be influenced by the actions of glutamate on metabotropic glutamate receptors (mGluRs) at both pre- and postsynaptic sites. At least seven mGluR subtypes (mGluR1–7) plus several splice varients have been identified by molecular biological methods (see Chapter 1). Expression of these receptors in a variety of cell types has shown that they are capable of interacting with most of the commonly recognized second-messenger systems. As detailed elsewhere in this volume, each expressed mGluR subtype also displays unique pharmacological specificity and shows a particular preference for one of the effector systems (Nakajima et al., 1993; Tanabe et al., 1993). However, which mGluRs mediate the various acute effects of mGluR activation on synaptic transmission and the underlying mechanisms are still poorly understood. In this chapter, we will review the current understanding regarding acute regulation of synaptic transmission by mGluRs and examine possible mechanisms of this regulation.

The actions of phenylglycine derived metabotropic glutamate receptor antagonists on multiple (1S,3R)-ACPD responses in the rat nucleus of the tractus solitarius

The effects of the metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] and a series of phenylglycine-derived putative mGluR antagonists were examined on electrophysiological responses mediated by glutamate and GABA receptors in the nucleus of the tractus solitarius (NTS) in transverse brainstem slices of the rat. Monosynaptic excitatory currents (EPSCs) evoked by electrical stimulation in the region of the tractus solitarius (TS) were reduced in the presence of (1S,3R)-ACPD in > 90% of neurons recorded in the dorsomedial subdivision of the NTS adjacent to the area postrema (AP). Monosynaptic evoked inhibitory currents (IPSCs) were similarly inhibited by (1S,3R)-ACPD. The inward current evoked by pressure application of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (IAMPA) was potentiated in the presence of (1S,3R)-ACPD, whereas the outward current evoked by the gamma-amino-butyric acid-A (GABA-A) receptor agonist muscimol (IMUSC) was inhibited. (1S,3R)-APCD also produced a postsynaptic inward current (IK(ACPD)) associated with a decrease in membrane conductance in approximately 50% of cells. The novel mGluR antagonists (S)-4-carboxy-3-hydroxy-phenylglycine (4C3H-PG), (R,S)-4-carboxy-phenylglycine (4C-PG) and (R,S)-alpha-methyl-4-carboxy-phenylglycine (alpha M4C-PG) reversibly antagonized the effects of (1S,3R)-ACPD on EPSCs IPSCs, IAMPA and IMUSC. The first two compounds also displayed weak agonist activity. However, none of the antagonists significantly inhibited IK(ACPD) at concentrations which blocked (1S,3R)-ACPD effects on synaptic transmission. These results suggest that pharmacologically distinct mGluRs may be present in the NTS.

5-Hydroxytryptamine-3 receptors modulate synaptic activity in the rat nucleus tractus solitarius in vitro.

Whole-cell patch clamp recordings were made from neurons in the rat nucleus tractus solitarius (NTS) in transverse brainstem slices. 5-Hydroxytryptamine (5-HT, 100 microM) and the selective 5-HT3 receptor agonist 2-methyl-5-HT (2-CH3-5-HT, 100 microM) depolarized 86% of NTS neurons at resting membrane potential (Vm). This response was resistant to tetrodotoxin (TTX) and Co2+ application. In addition, 2-CH3-5-HT (500 nM-100 microM) increased the amplitude and frequency of both excitatory and inhibitory spontaneous synaptic potentials. This effect was also TTX-resistant, but was abolished by Co2+. The effects of 2-CH3-5-HT on EPSPs and IPSPs evoked by electrical stimulation of the tractus solitarius (TS) were analyzed separately in the presence of bicuculline or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), respectively. Concentrations of 2-CH3-5-HT between 500 nM and 1 microM decreased the amplitude of evoked EPSPs and IPSPs with similar potency. The selective 5-HT3 receptor antagonists ICS 205-930 (10 nM) and MDL 72222 (10 microM) reversibly blocked the effects of 2-CH3-5-HT at all doses examined. It is concluded that 5-HT3 receptors can mediate both pre- and postsynaptic responses in the NTS.

Neuronal Ca2+ Channels and Their Regulation by Excitatory Amino Acids

It is generally accepted that glutamate is the most widely distributed excitatory neurotransmitter in the central nervous system. Under normal conditions glutamate released at central synapses causes fast neuronal excitation and also produces long-term changes in synaptic efficacy which may underlie phenomena such as learning and memory. However, under pathological conditions, such as those prevailing during periods of ischemia/ hypoxia, abnormally large quantities of glutamate or similar substances are released from neurons and probably from glial cells as ell.^,^ These excessive quantities of glutamate produce a greater-than-normal activation of glutamate receptors, and this has been found to produce neuronal toxicity (“excitotoxicity”) in most parts of the brain.2 It is now believed that glutamate exerts both its physiological and pathophysiological effects through actions at at least four distinct types of receptor^.^ A question of great current interest is how glutamate exerts these effects at a molecular level. It has recently become clear that Ca2+ probably plays a key role in mediating the ability of glutamate to increase synaptic strength and also its long-term toxic effects under abnormal circumstances. 2*5 Thus removal of external Ca2+ has been reported to block both glutamate-mediated ‘‘long-term potentiation,” a manifestation of synaptic pla~t ic i ty ,~ and also glutamatemediated neuroto~icity.~~’ The purpose of this paper is to review current knowledge of our understanding of the ways in which glutamate can influence Ca2+ metabolism and disposition in central neurons. Before embarking on this particular task, it is important to review what we know at this time about the pharmacology of glutamate receptors. As discussed above, it is currently thought that at least four types of glutamate receptors exist. These are generally defined through the use of archetypal glutamate agonists and in some cases glutamate antagonists as weL4 The first receptor type we shall consider is that defined through the action of the glutamate agonist kainic acid. The second receptor is defined through the actions of the glutamate agonist quisqualic acid and the third through the actions of the glutamate agonist N-methyl-D-aspartic acid (NMDA). Finally, we shall discuss a second type of quisqualic-acid-specific glutamate receptor which appears to work in quite a different manner from the other three just defined. A summary of the pharmacology of these four types of glutamate receptors is provided in TABLE 1.

GABAB receptors modulate a tetanus-induced sustained potentiation of monosynaptic inhibitory transmission in the rat nucleus tractus solitarii in vitro

Whole-cell recordings were made in the nucleus tractus solitarii (NTS) in transverse brainstem slices from rats. Monosynaptic GABAA-receptor-mediated inhibitory postsynaptic currents (IPSCs) or potentials (IPSPs) were evoked (0.1-0.2 Hz) by electrical stimulation within and medial to the tractus solitarius in the presence of the ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) or 6,7-dinitroquinoxaline-2,3-dione (DNQX; 10 microM) and D-amino-5-phosphonopentanoic acid (APV; 50 microM). A brief period of tetanic stimulation (20 Hz, 2 s) resulted in posttetanic (< 5 min, 69 of 73 recordings) and sustained potentiation (> 15 min, 31 of 73 recordings) of the IPSP/Cs. Sustained potentiation was not due to alterations in the reversal potential of IPSP/Cs. Both pre- and post-tetanus IPSP/Cs were completely blocked by the GABAA antagonist bicuculline (10 microM). Postsynaptic responses to pressure ejection of the GABAA-receptor agonist muscimol were unaltered in cells displaying sustained potentiation. Sustained potentiation of IPSP/Cs could be induced by tetanus in the presence of either metabotropic glutamate receptor antagonists or bicuculline. However, sustained potentiation could not be induced in the presence of the GABAB-receptor antagonists 2-OH-saclofen (400 microM) or CGP35348 (3-amino-propyl-(diethoxymethyl)phosphinic acid, 100 microM), although a subsequent tetanus following washout induced sustained potentiation. Posttetanic potentiation was unaffected by GABAB-receptor antagonists. These data suggest that neuronal or terminal excitability of GABAergic interneurons in the NTS is enhanced following brief periods of increased frequency of activation in vitro. This novel phenomenon within the rat medulla may be involved in the temporal modulation of autonomic reflex sensitivity observed during certain behavioral states, such as the defense reaction.