RNA splicing regulates axonal expression of GlyR α3. Hippocampal GlyRs are implicated in the regulation of glutamatergic synaptic transmission (27, 29). Here, we focused on the long (L) RNA splice variant of GlyR α3 because of its preponderance in brain (27). Unlike the short GlyR α3K variant, GlyR α3L contains exon 8A (30, 31), which codes for the TEAFALEKFYRFSDT peptide located in the large cytoplasmic loop between transmembrane domains 3 and 4 (TM3-4). Exon 8A was shown to confer particular subcellular trafficking and clustering properties on GlyR α3 (27, 28). To further investigate the relevance of this exon, we searched for interaction partners of GlyR α3L. To this end, the glutathione S-transferase–tagged (GST-tagged) α3L TM3-4 loop was used for cosedimentation assays with adult mouse brain lysate and mass spectrometric fingerprint analysis (Figure 1A and Supplemental Table 1; supplemental material available online with this article; doi: 10.1172/JCI71472DS1). Our approach identified the vesicular trafficking factor SEC8 as a putative determinant of subcellular GlyR α3L localization (Supplemental Table 1, Exoc4). In fact, SEC8 belongs to the exocyst complex protein family of vesicular trafficking factors and was shown to be involved in targeting membrane material to presynaptic sites (32), as with glycine transporter GlyT1 targeting to presynaptic glutamatergic terminals (33, 34). To corroborate that the α3L TM3-4 loop interacts with SEC8, we probed the adult mouse brain proteins that cosedimented with GST::α3L or GST::α3K for SEC8 immunoreactivity (Figure 1B and full uncut gels are shown in the Supplemental Material). Indeed, our Western blot analysis with a SEC8 antibody confirmed cosedimentation of SEC8 with the TM3-4 loop of the GlyR α3L RNA splice variant (Figure 1B), while the α3K TM3-4 loop without exon 8A was far less effective. Thus, the peptide TEAFALEKFYRFSDT encoded by exon 8A of the Glra3 gene facilitates interaction of GlyR α3 with SEC8.

Figure 1 The vesicular trafficking factor SEC8 is a new interaction partner of GlyR α3L and allows axonal receptor expression. (A) Proteins interacting with GST::α3L in the presence of adult mouse brain lysate were excised and analyzed with mass spectrometry. Supplemental Table 1 provides a list of significant hits. (B) Western blot with an SEC8 antibody confirms the cosedimentation of SEC8 with GlyR α3L and identifies the spliced exon 8A coding for TEAFALEKFYRFSDT of GlyR α3L as the SEC8 interaction domain. Equal GST bead loading with α3K- or α3L-loops was verified with Coomassie staining (B, left panel). (C) SEC8 bound with the GlyR α3L TM3-4 loop in HEK293 cells. The GlyR α3 TM3-4 loop harbored an intrinsic NLS, which led to the translocation of SEC8::EGFP to the nucleus of HEK293 cells upon coexpression of the α3L-loop. (D–G) To investigate the role of SEC8 in subcellular GlyR α3L trafficking, primary hippocampal neurons were cotransfected with HA-tagged full-length GlyR α3L and SEC8::EGFP or EGFP. Arrows indicate the presumptive axonal compartment devoid of MAP2, and arrowheads point to the somatodendritic MAP2-positive compartment. Note that SEC8::EGFP coclustered with GlyR α3L in the MAP2-negative compartment, an activity that was not observed upon coexpression of EGFP. Scale bars: 10 μm (C), and 5 μm (D–G).

To verify the biochemical data, we tested for the interaction of α3K/L TM3-4 loops with SEC8 in a cellular context. For this purpose, we cotransfected human embryonic kidney 293 (HEK293) cells with DsRed-Express–tagged α3K or α3L TM3-4 loops and EGFP-tagged SEC8 (Figure 1C and Supplemental Figures 1–3). We performed quantification of nuclear versus cytoplasmic fluorescence and analysis of colocalization of α3 loops and SEC8::EGFP in the nucleus using fluorescence intensity measurements within circular regions of interest and line scans (Supplemental Figure 2 and Table 1). The fact that α3 loops harbor an effective nuclear localization sequence (NLS) (ref. 35, Supplemental Figure 2A, and Table 1) was particularly useful, because coexpression of the DsRed-E–tagged GlyR α3L, not α3K, loop increased the ratio between nuclear and cytoplasmic SEC8::EGFP fluorescence (Supplemental Figure 2A and Table 1) and revealed α3L loop–dependent translocation of SEC8::EGFP to the nucleus. In agreement with the α3L loop–dependent change in the nuclear gray level ratio between integrated DsRed-E and EGFP fluorescence intensities (Table 1), line scans of nuclear fluorescence profiles and pixel-wise correlation analysis further revealed an almost perfect overlap of SEC8::EGFP and α3L-NLS::DsRed-E, but not α3K-NLS::DsRed-E (Supplemental Figure 2B, C). Taken together with the observation that SEC8::EGFP expressed alone was not able to access the nucleus of HEK293 cells even when it was equipped with different NLSs (Supplemental Figures 1–3 and Table 1), these results demonstrate that the RNA splicing–dependent L insert TEAFALEKFYRFSDT was responsible for nuclear SEC8 translocation.

Table 1 Summary of the quantification of integrated fluorescence intensities measured within circular (5 μm in diameter) regions of interest that were nucleus centered or positioned in the perinuclear cytoplasmic compartment of transfected HEK293 cells

Furthermore, we wanted to know whether SEC8 also influences subcellular trafficking of full-length GlyR α3L in neurons. Therefore, we coexpressed SEC8::EGFP and HA-tagged full-length GlyR α3L (27) in primary hippocampal neurons. Figure 1, D and E, shows that GlyR HA-α3L accessed the presumptive axonal compartment (devoid of microtubule-associated protein 2 [MAP2]) when SEC8::EGFP was coexpressed, whereas GlyR HA-α3L clusters were retained in the MAP2-positive somatodendritic compartment when EGFP was coexpressed (Figure 1, F and G; arrowheads). GlyR HA-α3L clusters colocalized with SEC8::EGFP in MAP2-negative neuronal processes (Figure 1, D and E; arrows). To verify axonal trafficking of HA-α3L upon coexpression of SEC8::EGFP, we used neurofilament M to stain the axonal compartment (Supplemental Figure 4, A and B). Colocalized clusters of HA-α3L and SEC8::EGFP decorated the axonal arbor in 85.2% (23 of 27) of SEC8::EGFP-positive neurons, whereas a minor fraction (7.7%, 2 of 26) of EGFP-coexpressing neurons showed axonal trafficking of HA-α3L. Quantification of GlyR HA-α3L immunofluorescence using line scans in cytoplasmic, dendritic, and neurofilament M–positive axonal compartments revealed a significant SEC8::EGFP-dependent increase in axonal GlyR α3L expression (Table 2, axon/soma and axon/dendrite ratios). Pixel-wise correlation analysis of HA-α3L and SEC8::EGFP signal intensities further revealed a strong positive correlation between HA-α3L immunofluorescence and SEC8::EGFP fluorescence in axons (Supplemental Figure 4, B and C), which confirmed colocalization of both proteins. We found that a large number of colocalized, large (>1 μm in diameter) axonal GlyR HA-α3L and SEC8::EGFP clusters were associated with the vesicular glutamate transporter VGluT (185 of 215 clusters in 23 of 27 SEC8::EGFP-coexpressing neurons, Supplemental Figure 4B, arrows), a vesicular axonal and presynaptic marker of glutamatergic neurons, whereas only some smaller axonal GlyR HA-α3L clusters in the EGFP-coexpressing neurons (2 of 26) colocalized with VGluT (5 of 24). Collectively, these results demonstrate that SEC8::EGFP facilitates axonal trafficking of HA-tagged GlyR α3L, and they suggest that SEC8 tethers GlyR α3L and VGluT in axonal cargo vesicles on their way to presynaptic terminals.

Table 2 Summary of the quantification of integrated fluorescence intensities measured with line scans (10 μm in length) in axons, dendrites, and somata of transfected primary hippocampal neurons

RNA editing leads to spontaneous channel opening of GlyR α3L. RNA editing of GlyR α3–coding mRNA leads to gain of function (26). In fact, substitution of the proline residue by the aliphatic hydrophobic amino acid leucine at position 185 in the mature GlyR α3 polypeptide impacts the conformation of the ligand binding domain and thereby confers increased accessibility of the neurotransmitters glycine and GABA to the agonist binding site (24). Due to the close proximity of the RNA-edited position to the plasma membrane, thermodynamic rules also imply a spontaneous channel opening of RNA-edited GlyR α3L185L in the absence of an agonist. We investigated this possibility by electrophysiological comparison of non–RNA-edited α3L185P and RNA-edited α3L185L in transfected HEK293 cells (Figure 2). The currents recorded in the absence of added glycine reflected agonist-independent channel opening of RNA-edited GlyR α3L185L (Figure 2A, red trace), while corresponding currents of the nonedited GlyR α3L185P had a much smaller amplitude (Figure 2A, black trace, and Figure 2C). Moreover, a commonly used concentration (1 μM) of the well-known competitive GlyR antagonist strychnine was not able to fully block agonist-independent channel opening of RNA-edited GlyR α3L185L (Figure 2B, top red trace, and Figure 2C). A high concentration of strychnine (10 μM) was required to fully block this type of GlyR α3L activity in the nominal absence of glycine (Figure 2B, bottom red trace, and Figure 2C). This strychnine concentration also blocks GABA type A receptors (GABAARs) (36) and, hence, is not suitable for the discrimination of glycine and GABA effects on neuronal function in slice preparations.

Figure 2 GlyR α3L185L activation in the nominal absence of glycine. (A) Representative recording traces of α3L185P- and α3L185L-dependent chloride currents in the nominal absence of glycine (driving force: 50 mV). (B) Representative traces show effects of increasing strychnine concentrations on agonist-independent GlyR α3L activity. (C) Quantification of the effects of 1 μM and 10 μM strychnine on basal currents in HEK293 cells expressing either GlyR α3L185P (black bars) or GlyR α3L185L (red bars). Values represent the difference between the basal currents in the presence and absence of strychnine (1 μM or 10 μM). Traces shown in A and B belong to continuous recordings. Numbers of investigated cells are provided in parentheses. Note that a high dose of strychnine was required to fully block spontaneous GlyR α3L185L activity. Strychnine effects were fully reversible upon washout (not shown). Data represent the means ± SEM. **P < 0.01; ***P < 0.001.

Knockin mouse model for characterization of GlyR α3L185L function in the brain. In order to investigate the functional role of GlyR α3L in vivo, we generated a knockin mouse line for Cre-dependent neuron type–specific expression of the gain-of-function α3L receptor variant. For this purpose, we used a targeting vector containing the cDNA copy of the HA-tagged GlyR α3L185L RNA variant (ref. 24 and Supplemental Figure 5) for recombination with the X chromosomal Hprt gene locus. Cre recombinase–dependent excision of the floxed STOP cassette upstream of the DNA coding for HA-tagged GlyR α3L185L enables minigene protein expression in different neuron types.

Endogenous GlyR α3 is expressed in principal cells in stratum granulosum (27) and stratum pyramidale (37) as well as in fast-spiking interneurons (Supplemental Figure 6). Therefore, we studied the functional impact of GlyR α3L185L on the two cognitively relevant neuron types (principal cells and fast-spiking interneurons) by mating homozygous Hprtα3L185L+/+ females with heterozygous Camk2aCre+/– males (38) or homozygous PvalbCre+/+ males (39), which excises the STOP cassette and induces neuron type–specific GlyR α3L185L protein expression in principal cells or parvalbumin-positive neurons, respectively. Western blot analyses with an HA epitope–directed antibody indeed confirmed full-length (48 kDa) HA-α3L185L GlyR expression in the hippocampus and cortex of male offspring that were hemizygous for the HA-α3L185L allele (Hprtα3L185L+/0) and heterozygous for the Camk2aCre or PvalbCre alleles (Supplemental Figure 7). Male Hprtα3L185L+/0 mice were used as control animals. We also verified at a functional level that Cre recombinase induced gain-of-function GlyR α3L185L protein expression. For this purpose, Cre recombinase was expressed in primary neuron cultures of Hprtα3L185L+/0;Hprtα3L185L+/+ mice, and whole cell patch clamp recording was performed. Cre-positive neurons indeed responded with transmembrane chloride currents to the application of a low (10 μM) glycine concentration (Supplemental Figure 8), which confirmed Cre-dependent excision of the floxed STOP cassette and, hence, induction of gain-of-function GlyR α3L185L protein expression.

Anatomical evidence for presynaptic GlyR α3L185L expression in vivo. We determined the characteristics of HA-α3L185L GlyR expression in principal glutamatergic neurons (Hprtα3L185L+/0;Camk2aCre+/–) using pre-embedding double immunochemistry and ultrastructural analysis with electron microscopy. In Hprtα3L185L+/0;Camk2aCre+/– animals, HA-α3L185L immunoreactivity was found predominantly at VGluT1-positive glutamatergic terminals (Figure 3A), establishing asymmetrical synapses with dendritic shafts and predominantly with dendritic spines of putative principal cells in the inner molecular layer of the dentate gyrus (n = 341 terminals in 2 animals), as well as in the stratum radiatum of cornu ammonis subfield 1 (CA1) (n = 391) and in the stratum lucidum of CA3 (n = 242). Approximately 40% of the VGluT1-positive presynaptic boutons (immunogold particles) showed immunoreactivity against HA-α3L185L (peroxidase reaction end product) in these areas, whereas 30%–45% of the terminals were immunopositive for the glutamate transporter, but not for the receptor protein (Figure 3B). In addition, we found a small subpopulation of axon terminals (3.6%) making asymmetrical synapses with postsynaptic dendritic spines to be immunopositive for HA-3L185L, but not for VGluT1 in the dentate gyrus (Figure 3B). Thus, GlyR HA-α3L185L specifically targets a subset of glutamatergic presynaptic terminals in mice with Camk2aCre-dependent GlyR HA-α3L185L expression. In another set of experiments, we identified VGluT1-positive terminals by the peroxidase reaction end product, and we detected immunoreactivity against HA-α3L185L with gold particles that allowed us to precisely visualize the location of the receptor protein at presynaptic terminals (Figure 3, C and D). Immunogold particle labeling of HA-α3L185L revealed that the receptor protein was preferentially located over presynaptic vesicles (Figure 3, C and D). We found that approximately one-third of the immunoparticles in the investigated boutons were associated with synaptic or extrasynaptic terminal membranes (Figure 3, C and D, arrows; percentage of immunogold particles in the dentate gyrus: 34.9 ± 7.3, CA3: 22.4 ± 1.5, CA1: 33.1 ± 8.6; means ± SD). Thus, the ultrastructural analysis is consistent with the proposed membrane topology of GlyRs, and it corroborates presynaptic GlyR α3L expression at glutamatergic synapses in vivo. We detected no immunoreactivity against HA-α3L185L in tissues obtained from control mice (Hprtα3L185L+/0).

Figure 3 Ultrastructural evidence for presynaptic GlyR α3L185L expression at hippocampal glutamatergic synapses. (A) Electron micrograph shows the distribution of HA-tagged GlyR α3L185L (peroxidase reaction end-product) in VGluT1-positive boutons (b, immunogold particles) that established asymmetrical glutamatergic synapses with dendritic spines (s) and occasionally with dendritic shafts (den) in the stratum radiatum of the CA1 area. (B) Quantification of the percentage fractions of colocalized immunoreactivities. Using peroxidase staining of HA-GlyR α3L185L, the mean percentages (± SEM) of double-labeled (VGluT1/HA), α3L185L-positive (HA), VGluT1-positive, and nonlabeled terminals were determined in the inner molecular layer of the dentate gyrus (DG), the stratum radiatum of CA1 (CA1), and the stratum lucidum of CA3 (CA3). (C and D) To reveal the membrane topology of GlyR α3L185L, immunoreactivity for the receptor subunit was examined using immunogold labeling. Particles were mainly located on the luminal side of the glutamatergic vesicles (C), and occasionally inside the synaptic cleft (C and D, arrows). Scale bars: 200 nm.

To characterize subcellular HA-α3L185L trafficking in parvalbumin-positive interneurons, we evaluated HA immunoreactivity in slice preparations from Hprtα3L185L+/0;PvalbCre+/– mice using confocal laser scanning microscopy, image deconvolution, and 3D reconstruction of multichannel images acquired in the stratum pyramidale. Quantification of colocalization between HA-α3L185L and parvalbumin using well-established Pearson’s correlation and Manders’ overlap coefficients (40) clearly revealed that GlyR HA-α3L185L overlapped with parvalbumin (Supplemental Figure 9; Pearson’s coefficient: 0.83 ± 0.01, Manders’ coefficient: 0.964 ± 0.019; means ± SEM). More importantly, GlyR HA-α3L185L signals were also congruent with the vesicular inhibitory amino acid transporter (VIAAT) (Supplemental Figure 9; Pearson’s correlation coefficient and Manders’ overlap coefficient of colocalization between HA-α3L185L and VIAAT: 0.73 ± 0.03 and 0.953 ± 0.056, respectively; means ± SEM). VIAAT is a well-characterized presynaptic marker of GABAergic synapses, hence, our high-resolution imaging approach demonstrates presynaptic expression of HA-α3L185L-GlyRs at hippocampal parvalbumin-positive synapses. Collectively, targeted expression of HA-α3L185L in our knockin mouse model led to presynaptic receptor localization in vivo.

Presynaptic GlyR α3L185L expression in vivo facilitates neurotransmitter release. To corroborate presynaptic GlyR α3L185L expression at a functional level, we performed a commonly used assay to determine the neurotransmitter release property of synapses. The paired-pulse recording approach examines the response properties of synapses during repetitive stimulation. By ruling out postsynaptic mechanisms during two stimulations, this technique can provide information about the capacity of the presynaptic neurotransmitter release machinery to satisfy repetitive demands. A ratio of less than one between the second and first response indicates fatigue of this machinery (paired-pulse depression) and, hence, reflects a high probability of neurotransmitter release upon the first stimulus. Likewise, a ratio greater than one (paired-pulse facilitation) indicates a low probability of transmitter release upon the first stimulus (41). To rule out concomitant recruitment of GABAergic synapses and their impact on glutamatergic synaptic transmission, we performed paired-pulse recordings in the whole-cell configuration in the presence of gabazine (1 μM) and saclofen (100 μM), which block GABAARs and GABA type B receptors, respectively (Figure 4A). Camk2aCre-dependent GlyR α3L185L protein expression began around postnatal day 15 (Supplemental Figure 10), and whole-cell recordings performed at this age revealed that the paired-pulse ratio was significantly decreased in slices from Hprtα3L185L+/0;Camk2aCre+/– mice compared with those from control Hprtα3L185L+/0 animals (Figure 4B, 150.2 ± 10.8% vs. 253.2 ± 24.3%). Because spontaneous channel openings of GlyR α3L185L in the nominal absence of glycine necessitate a high dose of strychnine for full receptor antagonism (above, Figure 2), we used 10 μM strychnine here. We found that strychnine leveled the difference between genotypes in paired-pulse recordings (Figure 4B, 226.7 ± 24.1% vs. 225.7 ± 30.8%), confirming that the observed genotype-specific difference was due to targeted GlyR α3L185L expression. Thus, presynaptic GlyR α3L185L expression in Hprtα3L185L+/0;Camk2aCre+/– animals increased synaptic glutamate release.

Figure 4 Presynaptic GlyR α3L185L expression facilitates neurotransmitter release. (A) Sample traces of evoked glutamatergic postsynaptic currents recorded in response to repetitive stimulation of Schaffer collaterals with two pulses separated by a 50-ms interstimulus interval in Hprtα3L185L+/0 (black) and Hprtα3L185L+/0;Camk2aCre+/– (red) mice. Traces show responses normalized to the first pulse. (B) Quantification of paired-pulse ratios measured between responses of the second pulse to the first pulse. Note that the paired-pulse ratio was significantly decreased in Hprtα3L185L+/0;Camk2aCre+/– mice compared with that in control animals, indicating increased synaptic glutamate release in animals with presynaptic GlyR α3L185L expression in principal cells. Also note that acute strychnine application was able to level the differences between genotypes. **P = 0.0033; ***P = 0.0001. Data represent the means ± SEM.

Facilitation of presynaptic GABA release due to targeted GlyR α3L185L expression in parvalbumin-positive interneurons should increase perisomatic inhibition and hence affect glutamatergic synaptic transmission. To address this possibility, we recorded evoked field potentials in area CA1 in response to Schaffer collateral stimulation in the absence of GABAAR antagonists (Figure 5, A and B). While volleys generated by glutamatergic fibers were not influenced by the genotype (Figure 5C), the amplitudes of field potentials were indeed significantly reduced in slices from Hprtα3L185L+/0;PvalbCre+/– animals (Figure 5D) . The chloride transporter antagonist bumetanide (50 εM) minimized differences between genotypes (not shown). These results demonstrate that presynaptic GlyR α3L185L expression in parvalbumin-positive interneurons increases perisomatic inhibition.

Figure 5 Perisomatic inhibition is increased in mice with presynaptic GlyR α3L185L at GABAergic synapses of parvalbumin-positive interneurons. (A and B) Sample traces of evoked glutamatergic field potentials (stimulation intensity: 0.03 mA) recorded in area CA1 of control animals (Hprtα3L185L+/0) and mice with GlyR α3L185L expression in parvalbumin-positive interneurons (Hprtα3L185L+/0;PvalbCre+/–). (C and D) Quantification of the relation between stimulation intensity and fiber volley (FV) (C) or amplitude of field potentials recorded upon Schaffer collateral stimulation (D). Note that amplitudes recorded in slices from Hprtα3L185L+/0;PvalbCre+/– mice were significantly decreased upon stimulation at different intensities. *P < 0.05. Data represent the means ± SEM.

Neuron type–specific GlyR α3L185L effects on network properties and excitability in vivo. To address the impact of enhanced neuronal function on a neural network substrate of cognitive function, we investigated high-frequency (gamma) oscillatory network activity in Hprtα3L185L+/0;Camk2aCre+/–, Hprtα3L185L+/0;PvalbCre+/–, and Hprtα3L185L+/0 control mice using local field potential recordings in hippocampal subfields CA1 and CA3. Gamma rhythm in the hippocampus could be induced in all three genotypes following bath application of 400 nM kainate, as described previously (42). The power spectra of the oscillations in slices of the different genotypes showed a clear peak in the 40-Hz frequency band (Figure 6A and Table 3). However, the power of gamma oscillations in the CA1 region was significantly reduced in slices from Hprtα3L185L+/0;Camk2aCre+/– and Hprtα3L185L+/0;PvalbCre+/– mice (Table 3). To assess hippocampal network excitability, the latency to recurrent epileptiform discharge upon blockage of GABA-ergic inhibition with a low dose (2.5 μM) of the competitive GABAAR antagonist bicuculline methiodide (BIC) was measured first in Hprtα3L185L+/0;Camk2aCre+/– animals (Figure 6B). Network disinhibition with 2.5 μM BIC indeed induced epileptiform network activity (Figure 6B), and the latency to pathological activity was significantly shorter in slices from Hprtα3L185L+/0;Camk2aCre+/– mice compared with those from Hprtα3L185L+/0 mice (7.0 ± 1.0 minutes vs. 10.5 ± 1.1 minutes) (Figure 6C), which is in agreement with the facilitation of presynaptic glutamate release due to targeted GlyR α3L185L expression (see Figure 4). Increased network excitability in Hprtα3L185L+/0;Camk2aCre+/– mice should also increase behavioral seizure activity. Therefore, we administered an i.p. injection of kainate and measured behavioral seizure activity according to the Racine classification (Figure 6D). Behavioral seizures were indeed exacerbated in Hprtα3L185L+/0;Camk2aCre+/– mice, and pronounced differences between the two genotypes were observed 50–60 minutes after the kainate injection (Figure 6D).

Figure 6 Increased network excitability in Hprtα3L185L+/0;Camk2aCre+/– mice. (A) Sample traces of oscillatory network activity recorded in CA3. Power spectra of kainate-induced gamma oscillation in slices from control Hprtα3L185L+/0 and Hprtα3L185L+/0;Camk2aCre+/– mice exhibited a clear peak at 40 Hz. However, the power of gamma oscillation was significantly reduced in slices from all animals with targeted GlyR α3L185L protein expression (see Table 3 for details). (B and C) Recurrent epileptiform discharge (RED) occurred earlier following application of the GABAAR antagonist BIC (2.5 μM) in slices from Hprtα3L185L+/0;Camk2aCre+/– animals. (D) Hprtα3L185L+/0;Camk2aCre+/– animals also showed more severe seizures upon i.p. kainate injection than did Hprtα3L185L+/0 control mice. Racine score: stage 0, normal behavior; stage 1, chewing and facial movements; stage 2, head nodding; stage 3, forelimb clonus; stage 4, rearing; stage 5, rearing, falling, and loss of posture. Data represent the means ± SEM. *P < 0.05; **P < 0.01.

Table 3 Properties of gamma oscillations in hippocampal subfields CA1 and CA3

In contrast to Hprtα3L185L+/0;Camk2aCre+/– animals, Hprtα3L185L+/0;PvalbCre+/– mice required a higher dose (10 μM) of the competitive GAB AAR antagonist BIC to generate epileptiform discharge in slice preparations (Figure 7). Qualitatively, we obtained the same results with different concentrations of another competitive GABAAR antagonist, i.e., 0.3 μM and 3 μM gabazine (Supplemental Figure 11). Thus, consistent with the data from the field potential recordings (see Figure 5), presynaptic GlyR α3L185L expression at parvalbumin-positive GABAergic synapses decreased network excitability. Collectively, these results demonstrate that GlyR α3L185L persistently increased neuronal impact in the neural network through a presynaptic mode of action in glutamatergic principal cells and parvalbumin-positive interneurons.

Figure 7 Decreased network excitability in Hprtα3L185L+/0;PvalbCre+/– mice. (A and B) Comparison of the effects of different BIC concentrations (A, 2.5 μM; B, 10 μM) on the incidence of RED. (C) Quantification of the percentage of slices with epileptiform activity. Note that a high dose (10 μM) of the competitive GABAAR antagonist BIC was required to elicit epileptiform activity in Hprtα3L185L+/0;PvalbCre+/– mice.

Consequences of neuronal enhancement for synaptic plasticity and behavior in vivo. Bidirectional synaptic plasticity, in the form of long-term potentiation and depression (LTP and LTD), is deemed to be the cellular substrate for associative spatial memory (43–45). However, recent evidence also demonstrated that LTP at the Schaffer collateral synapse influences discrimination of competing or overlapping memories and corresponding behavioral responses. To determine whether neuronal enhancement influences bidirectional plasticity of glutamatergic synaptic transmission and its relation to performance in learning and memory tasks, we used the reward-based 8-arm radial maze test, which addresses discriminative associative memory formation, and compared short- (working) and long-term (reference) memory of mice with enhanced function of glutamatergic principal cells or parvalbumin-positive interneurons (Figure 8, A and B). The learning curve in Figure 8A shows that working memory was impaired in Hprtα3L185L+/0;Camk2aCre+/– mice, since they required more trials than control Hprtα3L185L+/0 or Hprtα3L185L+/0;PvalbCre+/– animals. Furthermore, reference memory was selectively impaired in Hprtα3L185L+/0;Camk2aCre+/– mice (Figure 8B). For control purposes, we verified that the genotype did not influence motor coordination or hedonic behavior (Supplemental Figure 12). These results demonstrate that Hprtα3L185L+/0;Camk2aCre+/– mice were impaired in working memory formation and deficient in reference memory in a behavioral test that addressed discriminative associative memory (46).

Figure 8 Cell type–specific impairment of memory in GlyR α3L185L–expressing mice. (A) Quantification of errors made by animals of different genotypes in the 8-arm radial maze test. Working memory performance was determined according to the number of revisiting events during a fully baited 8-arm radial maze test. A high number of errors (working memory errors) indicated a poor working memory. (B) Quantification of errors made during analysis of reference memory. To this end, only 50% of the arms were baited, and the ability of animals to remember the arms that had food pellets was determined according to the number of visits to the nonbaited arms (reference memory errors). Data represent the means ± SEM. *P < 0.05; **P < 0.01; ***P < 0.001.

As LTP deficiency at the Schaffer collateral synapse was shown to impair discriminative associative learning (46), we expected Hprtα3L185L+/0;Camk2aCre+/– animals to be deficient in LTP. However, LTP could successfully be evoked by an appropriate stimulation protocol in all animal groups (Figure 9A and Table 4), while LTD could not be elicited specifically in Hprtα3L185L+/0;PvalbCre+/– mice (Figure 9B and Table 4). Quantitative analysis of bidirectional synaptic plasticity (LTP/LTD ratio) furthermore revealed that bidirectional synaptic plasticity was compromised in Hprtα3L185L+/0;PvalbCre+/– mice compared with Hprtα3L185L+/0;Camk2aCre+/– animals (Table 4). Thus, much to our surprise, Hprtα3L185L+/0;PvalbCre+/– mice performed normally in the discriminative associative learning and memory tests despite a reduced magnitude of bidirectional synaptic plasticity, whereas Hprtα3L185L+/0;Camk2aCre+/– mice did not — although their LTP/LTD ratio did not differ from that of control animals. For control purposes, we confirmed that strychnine leveled the genotypic differences (Figure 9, C and D, and Table 4).

Figure 9 Cell type–specific effects of GlyR α3L185L protein expression on bidirectional synaptic plasticity of glutamatergic transmission. (A and B) GlyR α3L185L–dependent effects on LTP and LTD. Normalized responses recorded in slices from the different genotypes are represented by color-coded symbols (black: Hprtα3L185L+/0, red: Hprtα3L185L+/0;Camk2aCre+/–, blue: Hprtα3L185L+/0;PvalbCre+/–). Note that GlyR α3L185L expression in PvalbCre-positive mice impaired expression of LTD. Also note that GlyR α3L185L expression did not affect the ratio between the magnitudes of LTP and LTD when expressed in Hprtα3L185L+/0;Camk2aCre+/– or Hprtα3L185L+/0 animals, whereas it reduced this ratio if expressed in Hprtα3L185L+/0;PvalbCre+/– animals (see Table 4 for values). (C and D) Quantification of strychnine effects on LTP and LTD. Strychnine leveled the differences between genotypes in both LTP and LTD (see Table 4 for values). Data represent the means ± SEM.

Table 4 Summary of the changes in EPSP slopes during recording of bidirectional synaptic plasticity

An alternative explanation for the impaired discriminative associative learning of Hprtα3L185L+/0;Camk2aCre+/– mice is a failure of sensory context–dependent formation of small neuronal assemblies (1). Therefore, we revisited our data on hippocampal network oscillation in the gamma band and evaluated the characteristics of this type of cognitively relevant network activity on a longer time scale (Figure 10). We found that gamma oscillation was stable over a long period in Hprtα3L185L+/0 and Hprtα3L185L+/0;PvalbCre+/– animals (Figure 10, A and B). However, with increased network excitability, it was consistently disrupted by EPSPs in slices from Hprtα3L185L+/0;Camk2aCre+/– animals (Figure 10C). In fact, this type of pathological network activity disrupted gamma oscillations in the vast majority (11 of 13, 85%) of Hprtα3L185L+/0;Camk2aCre+/– mice. Consistently, we observed that cognitive performance was affected, as these animals were substantially impaired in their ability to discriminate familiar and novel objects during the novel object recognition task (Supplemental Figure 12E). Thus, impaired cognitive function could underlie and explain poor performance in discriminative associative working and reference memory. On the other hand, Hprtα3L185L+/0;PvalbCre+/– mice performed as well as Hprtα3L185L+/0 control animals at the novel object recognition task, but they showed anxiety-related behavior in corresponding tests (Figure 11; light/dark preference, open field, and elevated plus maze tests). Hprtα3L185L+/0;PvalbCre+/– mice were indistinguishable from control animals with respect to motor coordination and locomotion (Supplemental Figure 12, A–C), ruling out motor skills impairment as a reason for their anxiety-related behavior. Collectively, these data identify a critical role for parvalbumin-positive interneurons in anxiety, and they demonstrate that factors other than bidirectional synaptic plasticity can cause impairment of discriminative associative memory.

Figure 10 Recurrent epileptiform discharge disrupts gamma frequency network oscillation. (A and B) Sample traces illustrating that the control Hprtα3L185L+/0 and Hprtα3L185L+/0;PvalbCre+/– animals displayed stable and regular gamma network oscillation. (C) Representative trace showing recurrent epileptiform discharge in Hprtα3L185L+/0;Camk2aCre +/– animals. Arrows indicate the characteristic depression of network activity following pathological network activity. Also note that high-frequency ripple oscillatory activity preceded hypersynchronous neuronal discharge (band-pass filtered, 120-300 Hz, see Supplemental Figure 14). (D) Each peak during gamma network oscillation represents the activity of sensory context–dependent neuronal assemblies (black triangles). (E) Schematic illustrating the conflict of interest of neurons (red triangles) due to their participation in hypersynchronous network activity.