The female urine sniffing test: a novel approach for assessing reward-seeking behavior in rodents

BACKGROUND

Abnormal hedonic behavior is a key feature of many psychiatric disorders. Several paradigms measure reward-seeking behavior in rodents, but each has limitations. We describe a novel approach for monitoring reward-seeking behavior in rodents: sniffing of estrus female urine by male mice, along with number of ultrasonic vocalizations (USVs) emitted during the test.

METHODS

The female urine sniffing test (FUST) was designed to monitor reward-seeking activity in rodents together with tests of helplessness and sweet solution preference. USVs and dopamine release from the nucleus accumbens (NAc) were recorded. Sniffing activity was measured in 1) manipulation-naive C57BL/6J and 129S1/SVImJ mice and Wistar-Kyoto rats; 2) stressed mice; 3) two groups of mice that underwent the learned helplessness paradigm-one untreated, and one treated with the SSRI citalopram; and 4) GluR6 knockout mice, known to display lithium-responsive, mania-related behaviors.

RESULTS

Males from all three strains spent significantly longer sniffing female urine than sniffing water. Males emitted USVs and showed significantly elevated NAc dopamine levels while sniffing urine. Foot-shock stress significantly reduced female urine sniffing time. Compared with mice that did not undergo the LH paradigm, LH males spent less time sniffing female urine, and citalopram treatment alleviated this reduction. Compared with their wildtype littermates, GluR6KO males sniffed female urine longer and showed enhanced saccharin preference.

CONCLUSIONS

In rodents, sniffing female urine is a preferred activity accompanied by biological changes previously linked to reward-seeking activities. The FUST is sensitive to behavioral and genetic manipulation and to relevant drug treatment.

Evidence for the involvement of the kainate receptor subunit GluR6 (GRIK2) in mediating behavioral displays related to behavioral symptoms of mania

The glutamate receptor 6 (GluR6 or GRIK2, one of the kainate receptors) gene resides in a genetic linkage region (6q21) associated with bipolar disorder (BPD), but its function in affective regulation is unknown. Compared with wild-type (WT) and GluR5 knockout (KO) mice, GluR6 KO mice were more active in multiple tests and super responsive to amphetamine. In a battery of specific tests, GluR6 KO mice also exhibited less anxious or more risk-taking type behavior and less despair-type manifestations, and they also had more aggressive displays. Chronic treatment with lithium, a classic antimanic mood stabilizer, reduced hyperactivity, aggressive displays and some risk-taking type behavior in GluR6 KO mice. Hippocampal and prefrontal cortical membrane levels of GluR5 and KA-2 receptors were decreased in GluR6 KO mice, and chronic lithium treatment did not affect these decreases. The membrane levels of other glutamatergic receptors were not significantly altered by GluR6 ablation or chronic lithium treatment. Together, these biochemical and behavioral results suggest a unique role for GluR6 in controlling abnormalities related to the behavioral symptoms of mania, such as hyperactivity or psychomotor agitation, aggressiveness, driven or increased goal-directed pursuits, risk taking and supersensitivity to psychostimulants. Whether GluR6 perturbation is involved in the mood elevation or thought disturbance of mania and the cyclicity of BPD are unknown. The molecular mechanism underlying the behavioral effects of lithium in GluR6 KO mice remains to be elucidated.

Developmental changes in agonist-induced retrograde signaling at parallel fiber-Purkinje cell synapses: role of calcium-induced calcium release

In cerebellar Purkinje cells (PCs), activation of postsynaptic mGluR1 receptors inhibits parallel fiber (PF) to PC synaptic transmission by retrograde signaling. However, results were conflicting with respect to whether endocannabinoids or glutamate (Glu) is the retrograde messenger involved. Experiments in cerebellar slices from 10- to 12-day-old rats and mice confirmed that suppression of PF-excitatory postsynaptic currents (EPSCs) by mGluR1 agonists was entirely blocked by cannabinoid receptor antagonists at this early developmental stage. In contrast, suppression of PF-EPSCs by mGluR1 agonists was only partly blocked by cannabinoid receptor antagonists in 18- to 22-day-old rats, and the remaining suppression was accompanied by an increase in paired-pulse facilitation. This endocannnabinoidindependent suppression of PF-EPSCs was potentiated by the Glu uptake inhibitor D-threo-beta-benzyloxyaspartate (D-TBOA) and blocked by the desensitizing kainate (KA) receptors agonist SYM 2081, by nonsaturating concentrations of 6-cyano-7-nitroquinoxaline-2-3-dione (CNQX) [but not by GYKI 52466 hydrochloride (GYKI)] and by dialyzing PCs with guanosine 5'-[beta-thio]diphosphate (GDP-betaS). An endocannnabinoid-independent suppression of PF-EPSCs was also present in nearly mature wild-type mice but was absent in GluR6(-/-) mice. The endocannnabinoid-independent suppression of PF-EPSCs induced by mGluR1 agonists and the KA-dependent component of depolarization-induced suppression of excitation (DSE) were blocked by ryanodine acting at a presynaptic level. We conclude that retrograde release of Glu by PCs participates in mGluR1 agonist-induced suppression of PF-EPSCs at nearly mature PF-PC synapses and that Glu operates through activation of presynaptic KA receptors located on PFs and prolonged release of calcium from presynaptic internal calcium stores.

GluR7 is an essential subunit of presynaptic kainate autoreceptors at hippocampal mossy fiber synapses

Presynaptic ionotropic glutamate receptors are emerging as key players in the regulation of synaptic transmission. Here we identify GluR7, a kainate receptor (KAR) subunit with no known function in the brain, as an essential subunit of presynaptic autoreceptors that facilitate hippocampal mossy fiber synaptic transmission. GluR7(-/-) mice display markedly reduced short- and long-term synaptic potentiation. Our data suggest that presynaptic KARs are GluR6/GluR7 heteromers that coassemble and are localized within synapses. We show that recombinant GluR6/GluR7 KARs exhibit low sensitivity to glutamate, and we provide evidence that presynaptic KARs at mossy fiber synapses are likely activated by high concentrations of glutamate. Overall, from our data, we propose a model whereby presynaptic KARs are localized in the presynaptic active zone close to release sites, display low affinity for glutamate, are likely Ca(2+)-permeable, are activated by single release events, and operate within a short time window to facilitate the subsequent release of glutamate.

Genetic and pharmacological studies of GluR5 modulation of inhibitory synaptic transmission in the anterior cingulate cortex of adult mice

In the anterior cingulate cortex (ACC), GluR5-containing kainate receptor mediated the small portion of excitatory postsynaptic current. However, little is known about its role in modulation of neurotransmitter release in this brain region. In the present study, we address this question by using selective GluR5 agonist and antagonist, as well as GluR5(-/-) mice. Our results showed that activation of GluR5 induced action potential-dependent GABA release, which is also required for the activation of voltage-dependent calcium channel and Ca(2+) influx. The effect of GluR5 activation is selective to the GABAergic, but not glutamatergic synaptic transmission. Endogenous activation of GluR5 also enhanced GABA release to ACC pyramidal neurons and the corresponding postsynaptic tonic GABA current. Our results suggest the somatodendritic, but not presynaptic GluR5, in modulation of GABA release. The endogenous GluR5 activation and the subsequent tonic GABA current may play an inhibitory role in ACC-related brain functions.

Localization of glutamate receptors to distal dendrites depends on subunit composition and the kinesin motor protein KIF17

Correct glutamate receptor localization in neurons is crucial for neurotransmission in the brain. Here we investigated the mechanisms underlying localization of kainate GluR5 receptors to dendrites in cultured hippocampal neurons. We find that the GluR5 distribution depends on association with GluR6 and KA2 subunits. The GluR5 subunit was expressed in distal dendrites only when GluR6 and KA2 subunits were present, whereas it was restricted to proximal dendrites in the absence of these subunits. The overlap between GluR5 distribution and the organization of microtubules in dendrites led us to examine whether KIF17, a microtubule motor protein expressed in distal dendrites, is involved in GluR5 localization to distal dendrites. We show here, for the first time that the microtubule motor protein KIF17 interacts with GluR6 and KA2 subunits and is required for GluR5 localization to distal dendrites, defining a novel mechanism that controls receptor localization in neurons.

Identification of an endoplasmic reticulum-retention motif in an intracellular loop of the kainate receptor subunit KA2

Neuronal kainate receptors are typically heteromeric complexes composed of GluR5-7 and KA1-2 subunits. Although GluR5-7 can exist as functional homomeric channels, the KA subunits cannot. KA2 is widely expressed in the CNS, and KA2/GluR6 heteromers are the most prevalent subunit composition in brain. Previous work has identified endoplasmic reticulum (ER)-retention motifs in the C terminus of KA2, which prevent surface expression of KA2 homomers. However, we find that, when these motifs are mutated, only a small fraction of KA2 is surface expressed. We now identify an additional ER retention motif in the intracellular loop region of KA2, which, when mutated together with the C-terminal motifs, significantly increases the level of KA2 surface expression. However, electrophysiological analysis of surface-expressed KA2 homomers indicates that they do not form functional ion channels. In heterologous cells, a large fraction of KA2 remains intracellular even when the trafficking motifs are mutated or when GluR6 is coexpressed. Therefore, we analyzed the trafficking of endogenous KA2 in vivo. We find that native KA2 surface expression is dramatically reduced in GluR6 knock-out mice compared with wild-type mice. In contrast, KA2 trafficking was unaffected in the GluR5 knock-out. Thus, our study demonstrates that trafficking motifs in both the intracellular loop and C terminus regulate KA2 surface expression; however, in neurons, GluR6 oligomerization is required for egress of KA2 from the ER and transport to the cell surface. The combination of these mechanisms likely prevents surface expression of nonfunctional KA2 homomers and ensures a high level of GluR6/KA2 heteromeric kainate receptors.

Intracellular Trafficking of KA2 Kainate Receptors Mediated by Interactions with Coatomer Protein Complex I (COPI) and 14-3-3 Chaperone Systems

Assembly and trafficking of neurotransmitter receptors are processes contingent upon interactions between intracellular chaperone systems and discrete determinants in the receptor proteins. Kainate receptor subunits, which form ionotropic glutamate receptors with diverse roles in the central nervous system, contain a variety of trafficking determinants that promote either membrane expression or intracellular sequestration. In this report, we identify the coatomer protein complex I (COPI) vesicle coat as a critical mechanism for retention of the kainate receptor subunit KA2 in the endoplasmic reticulum. COPI subunits immunoprecipitated with KA2 subunits from both cerebellum and COS-7 cells, and beta-COP protein interacted directly with immobilized KA2 peptides containing the arginine-rich retention/retrieval determinant. Association between COPI proteins and KA2 subunits was significantly reduced upon alanine substitution of this signal in the cytoplasmic tail of KA2. Temperature-sensitive degradation of COPI complex proteins was correlated with an increase in plasma membrane localization of the homologous KA2 receptor. Assembly of heteromeric GluR6a/KA2 receptors markedly reduced association of KA2 and COPI. Finally, the reduction in COPI binding was correlated with an increased association with 14-3-3 proteins, which mediate forward trafficking of other integral signaling proteins. These interactions therefore represent a critical early checkpoint for biosynthesis of functional KARs.

Endogenous activation of kainate receptors regulates glutamate release and network activity in the developing hippocampus

Kainate receptors (KARs) are highly expressed throughout the neonatal brain, but their function during development is unclear. Here, we show that the maturation of the hippocampus is associated with a switch in the functional role of presynaptic KARs. In a developmental period restricted to the first postnatal week, endogenous L-glutamate tonically activates KARs at CA3 glutamatergic synapses to regulate release in an action potential-independent manner. At synapses onto pyramidal cells, KARs inhibit glutamate release via a G-protein and PKC-dependent mechanism. In contrast, at glutamatergic terminals onto CA3 interneurons, presynaptic KARs can facilitate release in a G-protein-independent mechanism. In both cell types, however, KAR activation strongly upregulates inhibitory transmission. We show that, through the interplay of these novel diverse mechanisms, KARs strongly regulate the characteristic synchronous network activity observed in the neonatal hippocampus. By virtue of this, KARs are likely to play a central role in the development of hippocampal synaptic circuits.

Altered long-term synaptic plasticity and kainate-induced Ca2+ transients in the substantia gelatinosa neurons in GLUK6-deficient mice

Functional kainate receptors are expressed in the spinal cord substantia gelatinosa region, and their activation contributes to bi-directional regulation of excitatory synaptic transmission at primary afferent synapses with spinal cord substantia gelatinosa neurons. However, no study has reported a role(s) for kainate receptor subtypes in long-term synaptic plasticity phenomena in this region. Using gene-targeted mice lacking glutamate receptor 5 (GLU(K5)) or GLU(K6) subunit, we here show that GLU(K6) subunit, but not GLU(K5) subunit, is involved in the induction of long-term potentiation of excitatory postsynaptic potentials, evoked by two different protocols: (1) high-frequency primary afferent stimulation (100 Hz, 3 s) and (2) low-frequency spike-timing stimulation (1 Hz, 200 pulses). In addition, GLU(K6) subunit plays an important role in the expression of kainate-induced Ca2+ transients in the substantia gelatinosa. On the other hand, genetic deletion of GLU(K5) or GLU(K6) subunit does not prevent the induction of long-term depression. These results indicate that unique expression of kainate receptors subunits is important in regulating spinal synaptic plasticity and thereby processing of sensory information, including pain.

Kainate receptor-mediated synaptic transmission in the adult anterior cingulate cortex

Kainate (KA) receptors are expressed widely in the CNS. However, little is known about their functional characterization, molecular identity, and role in synaptic transmission in the forebrain of adult mice. Patch-clamp recordings in genetically modified mice show that postsynaptic KA receptors contribute to fast synaptic transmission in pyramidal neurons in the anterior cingulate cortex (ACC), a forebrain region critical for higher-order cognitive brain functions such as memory and mental disorders. Single-shock stimulation could induce small KA receptor-mediated excitatory postsynaptic currents (KA EPSCs) in the presence of picrotoxin, D-2-amino-5-phosphono-pentanoic acid, and a selective AMPA receptor antagonist, GYKI 53655. KA EPSCs had a significantly slower rise time course and decay time constant compared with AMPA receptor-mediated EPSCs. High-frequency repetitive stimulation significantly facilitated the KA EPSCs. Genetic deletion of the GluR6 or GluR5 subunit significantly reduced, and GluR5 and 6 double knockout completely abolished, KA EPSCs and KA-activated currents in ACC pyramidal neurons. Our results show that KA receptors contribute to synaptic transmission in adult ACC pyramidal neurons and provide a synaptic basis for the physiology and pathology of KA receptors in ACC-related functions.

Assessing the role of GLUK5 and GLUK6 at hippocampal mossy fiber synapses

It has been suggested recently that presynaptic kainate receptors (KARs) are involved in short-term and long-term synaptic plasticity at hippocampal mossy fiber synapses. Using genetic deletion and pharmacology, we here assess the role of GLU(K5) and GLU(K6) in synaptic plasticity at hippocampal mossy fiber synapses. We found that the kainate-induced facilitation was completely abolished in the GLU(K6)-/- mice, whereas it was unaffected in the GLU(K5)-/-. Consistent with this finding, synaptic facilitation was reduced in the GLU(K6)(-/-) and was normal in the GLU(K5)-/-. In agreement with these results and ruling out any compensatory effects in the genetic deletion models, application of the GLU(K5)-specific antagonist LY382884 [(3S,4aR,6S,8aR)-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid] did not affect short-term and long-term synaptic plasticity at the hippocampal mossy fiber synapses. We therefore conclude that the facilitatory effects of kainate on mossy fiber synaptic transmission are mediated by GLU(K6)-containing KARs.

Lack of the metabotropic glutamate receptor subtype 7 selectively impairs short-term working memory but not long-term memory

Metabotropic glutamate receptors (mGluRs), and in particular the mGluR group III receptors (subtypes 4, 6, 7, 8) are known to play a role in synaptic plasticity and learning. Here, we report the effect of mGluR7 gene ablation in different learning paradigms. In the acoustic startle response (ASR), no differences were seen between knockout (KO) mice and wildtype (WT) littermates in parameters including prepulse inhibition and habituation. In an open field test, no differences were seen between genotypes in motor activity, exploratory behaviour, and fearful behaviour. In a T-maze reinforced alternation working memory (WM) task, again no difference was seen between groups. However, when increasing the demands on working-memory in a 4-arm and 8-arm maze task, KO mice committed more WM errors than WT littermates thereby uncovering a highly significant difference between the two groups that persisted every day for the whole 9 days of the experiment. In a 4-arm maze with 2 arms baited, KO and wildtype mice committed the same number of LTM errors, whereas KOs committed more WM errors. Altogether, these findings suggest that a lack of mGluR7 mainly impairs short-term working but not long-term memory performance while having no effect on sensorimotor processing, non-associative learning, motor activity and spatial orientation. The effects on WM are task-dependent and become apparent in more complex but not simple learning tasks. We discuss how mGluR7 could influence WM.

Modulation of excitatory synaptic transmission in the spinal substantia gelatinosa of mice deficient in the kainate receptor GluR5 and/or GluR6 subunit

Functional kainate (KA) receptors (KARs) are expressed in the spinal cord substantia gelatinosa (SG) region, and their activation has a capacity to modulate excitatory synaptic transmission at primary afferent synapses with SG neurones. In the present study, we have used gene-targeted mice lacking KAR GluR5 and/or GluR6 subunits to determine the identity of the receptor subunits involved in the KA-induced modulation of excitatory transmission. Our findings reveal that KARs comprising GluR5 or GluR6 subunits can either suppress or facilitate glutamatergic excitatory transmission in the SG of acutely prepared adult mouse spinal cord slices. In the absence of synaptic inhibition mediated by GABA(A) and glycine receptors, a biphasic effect of kainate is characteristic with facilitation apparent at a low concentration (30 nM) and depression at a higher concentration (3 microM). In addition, GluR6-KARs, localizing pre- and postsynaptically, are critically involved in inhibiting transmission at both A delta and C fibre monosynaptic pathways, whereas presynaptic GluR5-KARs play a limited role in inhibiting the C fibre-activated pathway. The results obtained support the hypothesis that KARs are involved in bi-directional regulation of excitatory synaptic transmission in the spinal cord SG region, and that these actions may be of critical importance for nociception and the clinical treatment of pain.

Presynaptic kainate receptors impart an associative property to hippocampal mossy fiber long-term potentiation

Hippocampal mossy fiber synapses show an unusual form of long-term potentiation (LTP) that is independent of NMDA receptor activation and is expressed presynaptically. Using receptor antagonists, as well as receptor knockout mice, we found that presynaptic kainate receptors facilitate the induction of mossy fiber long-term potentiation (LTP), although they are not required for this form of LTP. Most importantly, these receptors impart an associativity to mossy fiber LTP such that activity in neighboring mossy fiber synapses, or even associational/commissural synapses, influences the threshold for inducing mossy fiber LTP. Such a mechanism greatly increases the computational power of this form of plasticity.

Loss of kainate receptor-mediated heterosynaptic facilitation of mossy-fiber synapses in KA2-/- mice

Multimeric assemblies of kainate (KA) receptor subunits form glutamate-gated ion channels that mediate EPSCs and function as presynaptic modulators of neurotransmitter release at some central synapses. The KA2 subunit is a likely constituent of many neuronal kainate receptors, because it is widely expressed in most neurons in the CNS. We have studied the effect of genetic ablation of this receptor subunit on synaptic transmission at the mossy-fiber-CA3 pyramidal cell synapse in hippocampal slices, where kainate receptors are localized to both presynaptic and postsynaptic sites. We found that both postsynaptic and presynaptic mossy-fiber kainate receptor function is altered in neurons from KA2-/- mice. The presynaptic facilitatory autoreceptor, which modulates glutamate release from mossy-fiber terminals, had a reduced affinity for exogenous agonists and synaptic glutamate. Although presynaptic facilitation attributable to homosynaptic glutamate release was normal at mossy-fiber synapses in KA2-/- neurons, heterosynaptic kainate receptor-mediated facilitation resulting from the spillover of glutamate from CA3 collateral synapses was absent. Consistent with a decrease in glutamate affinity of the receptor, the half-decay of the postsynaptic kainate-mediated EPSC was shorter in the knock-out mice. These results identify the KA2 subunit as a determinant of kainate receptor function at presynaptic and postsynaptic mossy-fiber kainate receptors.

Kainate receptor subunits underlying presynaptic regulation of transmitter release in the dorsal horn

Presynaptic kainate (KA) receptors regulate synaptic transmission at both excitatory and inhibitory synapses in the spinal cord dorsal horn. Previous work has demonstrated pharmacological differences between the KA receptors expressed by rat dorsal horn neurons and those expressed by the primary afferent sensory neurons that innervate the dorsal horn. Here, neurons isolated from KA receptor subunit-deficient mice were used to evaluate the contribution of glutamate receptor subunit 5 (GluR5) and GluR6 to the presynaptic control of transmitter release and to KA receptor-mediated whole-cell currents in these two cell populations [corrected]. Deletion of GluR6 produced a significant reduction in KA receptor-mediated current density in dorsal horn neurons, whereas GluR5 deletion caused no change in current density but removed sensitivity to GluR5-selective antagonists. Presynaptic modulation of inhibitory transmission between dorsal horn neurons was preserved in cells from either GluR5- or GluR6-deficient mice. In DRG neurons, in contrast, GluR5 deletion abolished KA receptor function, whereas deletion of GluR6 had little effect on peak current density but increased the rate and extent of desensitization. These results highlight fundamental differences in KA receptor physiology between the two cell types and suggest possible strategies for the pharmacological modulation of nociception.

Functional characterization of kainate receptors in the mouse nucleus accumbens

Kainate receptors are abundantly expressed in the nucleus accumbens but their functional characterization and their role in synaptic transmission has not yet been investigated. Using patch-clamp recordings in mouse nucleus accumbens slices, we show the presence of functional kainate receptors activated by low concentrations of kainate (100-300 nM) in medium size neurons. These somatodendritic receptors are comprised of the GluR6 subunit, since they are absent in GluR6-deficient mice. Kainate receptors do not directly participate in glutamatergic synaptic transmission evoked by electrical stimulation of cortical afferent fibers in nucleus accumbens neurons. However, application of low concentrations of kainate inhibits cortico-accumbens synaptic transmission, by increasing synaptic failure rate and increasing variation coefficient, thus indicating a presynaptic site of action. Presynaptic kainate receptors are observed both in GluR6 and in GluR5-deficient mice, but are absent in mice devoid of both subunits. Hence, at variance with somatodendritic kainate receptors, presynaptic kainate receptors on cortical afferents are composed of both GluR5 and GluR6 kainate receptor subunits. These results indicate that different subtypes of kainate receptors, representing distinct pharmacological targets, should play important roles in the synaptic integration properties of nucleus accumbens neurons.

Kainate receptors: knocking out plasticity

There is increasing evidence that kainate receptors contribute to both postsynaptic and presynaptic signaling. Studies of knockout mice have played a pivotal role in defining the functions of kainate receptors, including a recent study that implicates kainate receptors in frequency-dependent facilitation and long-term potentiation of hippocampal mossy fiber synapses.

Kainate receptors are involved in short- and long-term plasticity at mossy fiber synapses in the hippocampus

Kainate receptors alter the excitability of mossy fiber axons and have been reported to play a role in the induction of long-term potentiation (LTP) at mossy fiber synapses in the hippocampus. These previous studies have relied primarily on the use of compounds whose selectivity is unclear. In this report, we investigate short- and long-term facilitation of mossy fiber synaptic transmission in kainate receptor knockout mice. We find that LTP is reduced in mice lacking the GluR6, but not the GluR5, kainate receptor subunit. Additionally, short-term synaptic facilitation is impaired in GluR6 knockout mice, suggesting that kainate receptors act as presynaptic autoreceptors on mossy fiber terminals to facilitate synaptic transmission. These data demonstrate that kainate receptors containing the GluR6 subunit are important modulators of mossy fiber synaptic strength.

Identification of the kainate receptor subunits underlying modulation of excitatory synaptic transmission in the CA3 region of the hippocampus

To understand the physiological role of kainate receptors and their participation in seizure induction in animal models of epilepsy, it will be necessary to develop a comprehensive description of their action in the CA3 region of the hippocampus. Activation of presynaptic kainate receptors depresses excitatory synaptic transmission at mossy fiber and associational-commissural inputs to CA3 pyramidal neurons (Vignes et al., 1998; Bortolotto et al., 1999; Kamiya and Ozawa, 2000). In this study, we use gene-targeted mice lacking glutamate receptor 5 (GluR5) or GluR6 kainate receptor subunits to identify the receptor subunits that comprise the kainate receptors responsible for presynaptic modulation of CA3 transmission. We found that bath application of kainate (3 microm) profoundly reduced EPSCs at mossy fiber and collateral synapses in neurons from wild-type and GluR5(-/-) mice but had no effect on EPSCs in neurons from GluR6(-/-) mice. These results therefore contrast with previous studies that supported a role for GluR5-containing receptors at mossy fiber and associational-commissural synapses (Vignes et al., 1998; Bortolotto et al., 1999). Surprisingly, at perforant path synapses kainate receptor activation enhanced transmission; this potentiation was abolished in both GluR5 and GluR6 knock-out mice. Kainate receptors thus play multiple and complex roles to modulate excitatory synaptic transmission in the CA3 region of the hippocampus.

Subunit composition of kainate receptors in hippocampal interneurons

Kainate receptor activation affects GABAergic inhibition in the hippocampus by mechanisms that are thought to involve the GluR5 subunit. We report that disruption of the GluR5 subunit gene does not cause the loss of functional KARs in CA1 interneurons, nor does it prevent kainate-induced inhibition of evoked GABAergic synaptic transmission onto CA1 pyramidal cells. However, KAR function is abolished in mice lacking both GluR5 and GluR6 subunits, indicating that KARs in CA1 stratum radiatum interneurons are heteromeric receptors composed of both subunits. In addition, we show the presence of presynaptic KARs comprising the GluR6 but not the GluR5 subunit that modulate synaptic transmission between inhibitory interneurons. The existence of two separate populations of KARs in hippocampal interneurons adds to the complexity of KAR localization and function.

Kainate receptor-mediated responses in the CA1 field of wild-type and GluR6-deficient mice

Kainate receptors are abundantly expressed in the hippocampus. Mice with disruption of kainate receptor subunits allow the genetic dissection of the role of each kainate receptor subunits in the synaptic physiology of the hippocampus, as well as in excitotoxic processes. We have compared the action of domoate and kainate on CA1 pyramidal neurons in slices from wild-type and GluR6-/- mice. The difference in the amplitude of inward currents evoked by domoate and kainate between wild-type and GluR6-/- mice demonstrates the presence of functional kainate receptors in CA1 pyramidal neurons. Block of domoate-activated inward currents by the AMPA receptor antagonists 2,3-dihydroxy-6-nitro-7-sulfonyl-benzo(F)quinoxaline (1 microM) and 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine) (GYKI 53655) (50 microM) is complete in GluR6-/- mice but only partial in wild-type mice. In the presence of GYKI 53655, kainate receptor activation dramatically increases the frequency of spontaneous IPSCs in CA1 pyramidal cells from wild-type, as well as GluR6-/-, mice. This results from the kainate receptor-mediated activation of a sustained inward current and an increased action potential firing in afferent GABAergic interneurons of the CA1 field. These effects are observed in wild-type, as well as GluR6-/-, mice. Kainate receptors also decrease the amplitude of evoked IPSCs in CA1 pyramidal cells by increasing synaptic failures in wild-type and GluR6-/- mice. These results indicate that in CA1 pyramidal cells, distinct subtypes of kainate receptors mediate several functionally antagonistic effects.

Altered synaptic physiology and reduced susceptibility to kainate-induced seizures in GluR6-deficient mice

L-glutamate, the neurotransmitter of the majority of excitatory synapses in the brain, acts on three classes of ionotropic receptors: NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainate receptors. Little is known about the physiological role of kainate receptors because in many experimental situations it is not possible to distinguish them from AMPA receptors. Mice with disrupted kainate receptor genes enable the study of the specific role of kainate receptors in synaptic transmission as well as in the neurotoxic effects of kainate. We have now generated mutant mice lacking the kainate-receptor subunit GluR6. The hippocampal neurons in the CA3 region of these mutant mice are much less sensitive to kainate. In addition, a postsynaptic kainate current evoked in CA3 neurons by a train of stimulation of the mossy fibre system is absent in the mutant. We find that GluR6-deficient mice are less susceptible to systemic administration of kainate, as judged by onset of seizures and by the activation of immediate early genes in the hippocampus. Our results indicate that kainate receptors containing the GluR6 subunit are important in synaptic transmission as well as in the epileptogenic effects of kainate.