Neurotransmission plays contrasting roles in the maturation of inhibitory synapses on axons and dendrites of retinal bipolar cells

Neuronal output is modulated by inhibition onto both dendrites and axons. It is unknown whether inhibitory synapses at these two cellular compartments of an individual neuron are regulated coordinately or separately during in vivo development. Because neurotransmission influences synapse maturation and circuit development, we determined how loss of inhibition affects the expression of diverse types of inhibitory receptors on the axon and dendrites of mouse retinal bipolar cells. We found that axonal GABA but not glycine receptor expression depends on neurotransmission. Importantly, axonal and dendritic GABA_A receptors comprise distinct subunit compositions that are regulated differentially by GABA release: Axonal GABA_A receptors are down-regulated but dendritic receptors are up-regulated in the absence of inhibition. The homeostatic increase in GABA_A receptors on bipolar cell dendrites is pathway-specific: Cone but not rod bipolar cell dendrites maintain an up-regulation of receptors in the transmission deficient mutants. Furthermore, the bipolar cell GABA_A receptor alterations are a consequence of impaired vesicular GABA release from amacrine but not horizontal interneurons. Thus, inhibitory neurotransmission regulates in vivo postsynaptic maturation of inhibitory synapses with contrasting modes of action specific to synapse type and location.Interneurons of the CNS control neuronal excitability through release of γ-aminobutyric acid (GABA) and glycine. How inhibition modifies neuronal output depends largely on the types of presynaptic interneurons making synapses onto a postsynaptic cell, and the location and densities of these synapses (1–3). Moreover, inhibitory receptor types with distinct transmitter affinities and kinetics present on the axon or dendrites of an individual neuron can critically shape its output (3–6). Although much is known about how different inhibitory synapses shape the spatiotemporal activity patterns of mature neurons, it is less clear what factors regulate the expression of inhibitory receptors at these synapses during development in vivo. Is the expression of distinct inhibitory receptor types within a cellular compartment (axon or dendrite) regulated coordinately or independently? Conversely, is the expression of the same receptor type at different cellular compartments of an individual neuron regulated by common or separate factors?To answer these questions, we assessed expression of inhibitory receptors on the axon and dendrites of individual glutamatergic retinal neurons in mice with genetically suppressed inhibition. We generated retina-specific knockouts of the vesicular inhibitory amino acid transporter (VIAAT), which mediates uptake of GABA or glycine into synaptic vesicles (7, 8). We perturbed inhibition because it has been found previously to influence pre- and postsynaptic maturation of GABAergic synapses (9–12). However, whether inhibitory receptor expression at the “input” and “output” compartments of an individual neuron is coordinately regulated by activity remains unknown. We focused on retinal bipolar cells (BCs) because of the rich variety of inhibitory synapses found on these neurons. Moreover, the many types of BCs enabled us to determine whether inhibitory transmission plays a uniform or diverse role in regulating inhibitory synapses across cell types that signal in parallel. We compared the postsynaptic maturation of GABA- and glycinergic synapses on cone BCs (CBCs) versus rod BCs (RBCs) that operate at different light levels. Among CBCs, we analyzed both ON and OFF BC types, which depolarize or hyperpolarize to light increments, respectively (13).     

Electrical coupling between bipolar cells in carp retina

Intracellular recordings were made simultaneously from pairs of neighboring bipolar cells by advancing two independent microelectrodes into retinas of carp (Cyprinus carpio). Bipolar cells were identified by their response properties and in several samples were verified by intracellular injection of Lucifer yellow. Current of either polarity injected into one member of the bipolar cell pair elicited a signconserving, sustained potential change in the other bipolar cell without any significant delay. This electrical coupling was reciprocal, and it was observed between cell types similar in function and in morphology. Our results strongly suggest that there is a spatial summation of signals at the level of bipolar cells, which makes central receptive field areas much larger than their dendritic fields.     

Retarded developmental expression and patterning of retinal cone opsins in hypothyroid mice

Color vision is mediated by cone photoreceptors that express opsin photopigments with sensitivities to different light wavelengths. Most mammals, including mice, differentially express M and S opsins for response to medium-long and short wavelengths, respectively. Previous studies demonstrated that a thyroid hormone receptor (TRbeta2) is critical for opsin patterning: in TRbeta2-deficient mice, M opsin is lost and all cones instead express S opsin. Here, to investigate the requirement for thyroid hormone in cone development, we studied Tshr(-/-)mice as a model of congenital hypothyroidism. The onset of M opsin expression in Tshr(-/-)mice was severely delayed until after postnatal d 17 (P17), and M opsin expression failed to attain normal levels at older adult ages. S opsin showed a subtler change with an extended distribution pattern over the superior-inferior axis of the retina. Similar opsin abnormalities were detected in wild-type C57BL/6J mice made hypothyroid by methimazole treatment. In Tshr(-/-) mice, T(3) treatment from P8 recovered significant M opsin expression at P17. Tshr(-/-) mice produced normal numbers of cones, indicating that the major requirement for thyroid hormone is in opsin patterning rather than in cone generation. The phenotype is similar to, although milder than, that caused by loss of TRbeta2 and indicates the necessity for thyroid hormone for cone maturation.     

Feedforward lateral inhibition in retinal bipolar cells: input-output relation of the horizontal cell-depolarizing bipolar cell synapse.

Lateral inhibition is the ubiquitous strategy used by visual neurons for spatial resolution throughout the animal kingdom. It has been a puzzle whether lateral inputs in retinal bipolar cells are mediated by the horizontal cell (HC)-cone feedback synapse, by the HC-bipolar cell feedforward synapse, or by both. By blocking the central inputs of the depolarizing bipolar cells (DBCs) with L-2-amino-4-phosphonobutyrate, we were able to eliminate the contribution of the feedback synapse and to demonstrate the postsynaptic light response in DBCs mediated by the HC-DBC feedforward synapse. The HC-DBC feedforward synapse contributes roughly one-third of the surround response whereas the HC-cone-DBC feedback synapse probably contributes the rest.     

A circadian clock regulates rod and cone input to fish retinal cone horizontal cells.

In the vertebrate retina, the light responses of post-receptor neurons depend on the ambient or background illumination. Using intracellular recording, we have found that a circadian clock regulates the light responses of dark-adapted fish cone horizontal cells. Goldfish were maintained on a 12-hr light/12-hr dark cycle. At different times of the day or night, retinas were superfused in darkness for 90 min ("prolonged darkness"), following which horizontal cells were impaled without the aid of any light flashes. In some of the experiments, fish were kept in constant darkness for 3-48 hr prior to surgery. After prolonged darkness during the night, but not during the day, the light responses of L-type cone horizontal cells resembled those of rod horizontal cells with respect to threshold, waveform, intensity-response functions, and spectral sensitivity. Following light sensitization during the night and day, the light responses of rod and cone horizontal cells were clearly different with respect to threshold, waveform, intensity-response functions, and spectral sensitivity. Under conditions of constant darkness for two full light/dark cycles, average responses of cone horizontal cells to a bright light stimulus during the subjective day were greater than during the subjective night. Prior reversal of the light/dark cycle reversed the 24-hr rhythm of cone horizontal cell responses to bright lights. In addition, following one full cycle of constant darkness, average cone horizontal cell spectral sensitivity during the subjective night closely matched that of rod horizontal cells, whereas average cone horizontal cell spectral sensitivity during the subjective day was similar to that of red (625 nm) cones. These results indicate that the effects of dark adaptation depend on the time of day and are regulated by a circadian clock so that cone input to cone horizontal cells predominates in the day and rod input predominates in the night.     

Heterogeneous expression of connexins in rabbit sinoatrial node cells: correlation between connexin isotype and cell size.

OBJECTIVE: Intercellular coupling through gap junctions allows the morphologically and functionally heterogeneous sinoatrial node to synchronize and drive the atrial muscle. The purpose of this study was to identify the connexin isotypes expressed by sinoatrial node cells and to analyse the density of connexins in relation to cell size. METHODS: Labeling for the different connexins using isotype-specific antibodies was assessed in cells isolated from the rabbit sinoatrial node by immunoconfocal microscopy. RESULTS: Sinoatrial node cells with a cell projection area smaller than 800 microm(2) were devoid of immunolabeling for connexin43. Such small cells showed high levels of connexin45 labeling (compared to that in large cells) and low levels of connexin40 labeling. Sinoatrial node cells with a projection area between 800 and 1200 microm(2) had a lower amount of connexin45 label and again a small amount of connexin40 but an increased amount of connexin43 label. In the larger sinoatrial node cells, some colocalization of connexin45 and connexin43 immunolabeled spots was observed. CONCLUSIONS: Rabbit sinoatrial node cells are heterogeneous in terms of connexin expression, and there is a clear cell size-dependence in pattern of connexin expression. Small (putative central) cells express connexin45 but not connexin43, whereas larger (putative peripheral) cells express both connexin45 and connexin43. The co-localization of connexin43 and connexin45 in larger cells raises the possibility that heterotypic or heteromeric connexin43/connexin45 channels could be present in gap junctions at the periphery of the sinoatrial node.     

Calcium influx selects the fast mode of endocytosis in the synaptic terminal of retinal bipolar cells

To investigate the regulation of endocytosis by Ca(2+), we have made capacitance measurements in the synaptic terminal of depolarizing bipolar cells from the retina of goldfish. After a brief depolarization, all of the excess membrane was retrieved rapidly (tau approximately 1 s). But when the rise in free [Ca(2+)] was reduced by the introduction of Ca(2+) buffers [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) or EGTA], a large fraction of the membrane was retrieved by a second, slower mechanism (tau > or = 10 s). The block of fast endocytosis by EGTA could be overcome by increasing the amplitude of the Ca(2+) current, demonstrating that Ca(2+) influx was the trigger for fast endocytosis. These manipulations of the Ca(2+) signal altered the relative proportions of fast and slow endocytosis but did not modulate the rate constants of these processes. A brief stimulus that triggered fast endocytosis did not generate a significant rise in the spatially averaged [Ca(2+)], indicating that Ca(2+) regulated endocytosis through an action close to the active zone. The slow mode of retrieval occurred at the resting [Ca(2+)]. These results demonstrate that Ca(2+) influx couples fast endocytosis and exocytosis at this synapse.     

Glutamate and 2-amino-4-phosphonobutyrate evoke an increase in potassium conductance in retinal bipolar cells.

Although there is general agreement that L-glutamate can produce a depolarizing inward current to account for the hyperpolarizing (OFF) bipolar cell response, the conductance mechanism underlying the depolarizing (ON) response has been difficult to establish satisfactorily. To investigate the ionic bases of the center responses, we studied the whole-cell currents controlled by L-glutamate and its analogues in solitary bipolar cells from salamander retina. We report here two groups of isolated bipolar cells: one group responded to L-glutamate with the previously described inward current [Attwell, D., Mobbs, P., Tessier-Lavigne, M. & Wilson, M. (1987) J. Physiol. (London) 387, 125-161] and a second group showed an outward current that reversed at about -70 mV. Both were associated with an increase in membrane conductance. In addition, DL-2-amino-4-phosphonobutyrate, a compound diagnostic for ON-bipolar cell activity [Slaughter, M. M. & Miller, R. F. (1981) Science 211, 182-185], elicited outward currents that closely resembled those seen in response to L-glutamate and, furthermore, that were shown to arise from an increase in conductance to potassium ions. Thus the presence of two distinct conductances controlled by L-glutamate in solitary cells would provide one mechanism for generating the ON and OFF light responses at the bipolar cell level in the intact retina.     

TRPM1 is a component of the retinal ON bipolar cell transduction channel in the mGluR6 cascade

An essential step in intricate visual processing is the segregation of visual signals into ON and OFF pathways by retinal bipolar cells (BCs). Glutamate released from photoreceptors modulates the photoresponse of ON BCs via metabotropic glutamate receptor 6 (mGluR6) and G protein (Go) that regulates a cation channel. However, the cation channel has not yet been unequivocally identified. Here, we report a mouse TRPM1 long form (TRPM1-L) as the cation channel. We found that TRPM1-L localization is developmentally restricted to the dendritic tips of ON BCs in colocalization with mGluR6. TRPM1 null mutant mice completely lose the photoresponse of ON BCs but not that of OFF BCs. In the TRPM1-L-expressing cells, TRPM1-L functions as a constitutively active nonselective cation channel and its activity is negatively regulated by Go in the mGluR6 cascade. These results demonstrate that TRPM1-L is a component of the ON BC transduction channel downstream of mGluR6 in ON BCs.     

Spatially and temporally distinct expression of fibroblast connexins after sheep ventricular infarction

OBJECTIVES: Myocardial infarction leads to extensive changes in the organization of cardiac myocytes and fibroblasts, and changes in gap junction protein expression. In the immediate period following ischemia, reperfusion causes hypercontraction, spreading the necrotic lesion. Further progressive infarction continues over several weeks. In reperfusion injury, the nonspecific gap junction channel uncoupler heptanol limits necrosis. We hypothesize that gap junction coupling and fibroblast invasion provide a substrate for progressive infarction via a gap junction mediated bystander effect. METHODS: A sheep coronary occlusion infarct model was used with samples collected at 12, 24 and 48 h, and 6, 12 and 30 d (days) post-infarction. Immunohistochemical labelling of gap junction connexins Cx40, Cx43, and Cx45 was combined with cell-specific markers for fibroblasts (anti-vimentin) and myocytes (anti-myomesin). Double and triple immunolabelling and confocal microscopy were used to follow changes in cardiac myocyte morphology, fibroblast content and gap junction expression after myocardial infarction. Gap junction protein levels and fibroblast numbers were quantified. RESULTS: Within 12 h of ischemia, myocyte viability is impaired within small islands in the ischemic region. These islands spread and fuse into larger infarct zones until 12 d post-infarction. Thereafter, surviving myocytes within the infarct and in the border-zone appear to become stabilized. Distant from the infarct, continuing myocyte disruption is regularly observed, even after 30 d. Cx43 becomes redistributed from intercalated discs to the lateral surface of structurally compromised myocytes within 12 d. Cx45 expressing fibroblasts infiltrate the damaged region within 24 h, becoming most numerous at 6-12 d post-infarction, with peak Cx45 levels at 6 d. Later, Cx43 expressing fibroblasts are observed, and the related Cx43 label increases over the 30 d observation period, even though fibroblast numbers decline after 12 d. Cx40 was only seen in vascular endothelium. CONCLUSIONS: Progressive infarction, identified by myocyte sarcomere disruption and subsequent cell loss, occurs in parallel with fibroblast invasion and gap junction remodeling. Two fibroblast phenotypes occur within infarcts, expressing either Cx43 or Cx45. Coupled fibroblasts may play a number of roles in tissue remodeling following myocardial infarction, including provision of a possible substrate for progressive infarction via a gap junction mediated bystander effect.     

An alternative pathway for rod signals in the rodent retina: rod photoreceptors, cone bipolar cells, and the localization of glutamate receptors

In the mammalian retina, extensive processing of spatiotemporal and chromatic information occurs. One key principle in signal transfer through the retina is parallel processing. Two of these parallel pathways are the ON- and OFF-channels transmitting light and dark signals. This dual system is created in the outer plexiform layer, the first relay station in retinal signal transfer. Photoreceptors release glutamate onto ON- and OFF-type bipolar cells, which are functionally distinguished by their postsynaptic expression of different types of glutamate receptors, namely ionotropic and metabotropic glutamate receptors. In the current concept, rod photoreceptors connect only to rod bipolar cells (ON-type) and cone photoreceptors connect only to cone bipolar cells (ON- and OFF-type). We have studied the distribution of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor subunits at the synapses in the outer plexiform layer of the rodent retina by immunoelectron microscopy and serial section reconstruction. We report a non-classical synaptic contact and an alternative pathway for rod signals in the retina. Rod photoreceptors made synaptic contact with putative OFF-cone bipolar cells that expressed the AMPA glutamate receptor subunits GluR1 and GluR2 on their dendrites. Thus, in the retina of mouse and rat, an alternative pathway for rod signals exists, where rod photoreceptors bypass the rod bipolar cell and directly excite OFF-cone bipolar cells through an ionotropic sign-conserving AMPA glutamate receptor.     

Suppressing thyroid hormone signaling preserves cone photoreceptors in mouse models of retinal degeneration

Cone phototransduction and survival of cones in the human macula is essential for color vision and for visual acuity. Progressive cone degeneration in age-related macular degeneration, Stargardt disease, and recessive cone dystrophies is a major cause of blindness. Thyroid hormone (TH) signaling, which regulates cell proliferation, differentiation, and apoptosis, plays a central role in cone opsin expression and patterning in the retina. Here, we investigated whether TH signaling affects cone viability in inherited retinal degeneration mouse models. Retinol isomerase RPE65-deficient mice [a model of Leber congenital amaurosis (LCA) with rapid cone loss] and cone photoreceptor function loss type 1 mice (severe recessive achromatopsia) were used to determine whether suppressing TH signaling with antithyroid treatment reduces cone death. Further, cone cyclic nucleotide-gated channel B subunit-deficient mice (moderate achromatopsia) and guanylate cyclase 2e-deficient mice (LCA with slower cone loss) were used to determine whether triiodothyronine (T3) treatment (stimulating TH signaling) causes deterioration of cones. We found that cone density in retinol isomerase RPE65-deficient and cone photoreceptor function loss type 1 mice increased about sixfold following antithyroid treatment. Cone density in cone cyclic nucleotide-gated channel B subunit-deficient and guanylate cyclase 2e-deficient mice decreased about 40% following T3 treatment. The effect of TH signaling on cone viability appears to be independent of its regulation on cone opsin expression. This work demonstrates that suppressing TH signaling in retina dystrophy mouse models is protective of cones, providing insights into cone preservation and therapeutic interventions.Rod and cone photoreceptors degenerate under a variety of pathological conditions, including a wide array of hereditary retinal diseases, such as retinitis pigmentosa, macular degeneration, and cone–rod dystrophies. Defects in a large number of genes are linked to inherited retinal degenerative disorders (www.sph.uth.tmc.edu/RetNet/disease.htm), including those encoding enzymes involved in the recycling of 11-cis retinal in the retinal pigment epithelium (RPE), retinoid isomerase (RPE65), and lecithin retinol acyltransferase (LRAT), and the phototransduction-associated proteins (opsins, subunits of transducin, cGMP phosphodiesterase PDE6, guanylate cyclase, and cyclic nucleotide-gated channel). There are currently no treatments for human retinal dystrophies. Despite a high genetic heterogeneity, the degenerating photoreceptors show common cellular disorder features, including oxidative damage (1, 2), endoplasmic reticulum stress (3, 4), and apoptosis (5, 6).Thyroid hormone (TH) signaling regulates cell proliferation, differentiation, and apoptosis. The role of TH signaling in retina regarding its regulation of cone opsin expression and patterning has been well documented (7, 8). Most mammals possess dichromatic color vision that is mediated by two opsins with peak sensitivities to medium-long (M, green) and short (S, blue) wavelengths of light (9, 10). In mouse, M- and S-opsins are expressed in opposing gradients such that varying amounts of both opsins are coexpressed in cones in midretinal regions, whereas M-opsin predominates in dorsal (superior) regions and S-opsin predominates in ventral (inferior) regions (10, 11) (Fig. S1). During development and in the adult postmitotic retina, TH signaling via its receptor type β2 (TRβ2) suppresses expression of S-opsin, induces expression of M-opsin, and promotes the dorsal–ventral opsin patterning (7, 8). Importantly, TH signaling has been associated with cone viability. Triiodothyronine (T3) treatment was shown to cause cone death in mice and this effect was reversed by deletion of TRβ2 gene (12). Excessive TH signaling was also shown to induce auditory defects and cochlear degeneration in mice (13). TH signaling has been associated with apoptosis of a variety of human cell lines, including lymphocytes (14), breast cancer cells (15), HeLa cells (16), and pituitary tumor cells (17), and TH signaling has been well documented in apoptotic tissue remodeling during anuran metamorphosis (18, 19). To determine whether TH signaling affects cone viability in inherited retinal degeneration, we investigated cone death/survival in retinal degeneration mouse models following TH signaling suppression and stimulation. Retinol isomerase RPE65-deficient (Rpe65^−/−) (a model of Leber congenital amaurosis, LCA) (20, 21) and cone photoreceptor function loss type 1 (cpfl1) mice (PDE6C mutation, a model of achromatopsia) (22), displaying fast and severe cone degeneration, were used to determine whether suppressing TH signaling with antithyroid treatment reduces cone degeneration. Cone cyclic nucleotide-gated channel B subunit-deficient (Cngb3^−/−) (a model of achromatopsia) (23) and guanylate cyclase 2e-deficient (Gucy2e^−/−) (another model of LCA) mice (24), displaying relatively slow progressive and moderate cone degeneration, were used to determine whether stimulating TH signaling (with T3 treatment) deteriorates cones. We report here that cone survival was greatly improved in Rpe65^−/− and cpfl1 mice following TH signaling suppression, whereas cone degeneration was significantly increased in Cngb3^−/− and Gucy2e^−/− mice following TH signaling stimulation, demonstrating a protective role of suppressing TH signaling in cones.     

Angiotensin II regulates migration in mouse cultured mesangial cells: evidence for the presence of receptor subtype-specific regulation

The biological effects of angiotensin II (AngII) are mediated by two major subtypes of AngII receptors, type 1 (AT1R) and type 2 (AT2R). In this study, we attempted to elucidate the role of AngII subtype receptor-specific regulation in migration and proliferation of mouse cultured mesangial (MSG) cells. We found that 100 nM AngII stimulated weak migration of MSG cells. Cell motility increased more in the presence of AT2R than in the presence of AT1R, and it was suppressed by guanylate cyclase inhibitors. On the other hand, the activation of AT1R resulted in increased cell numbers, while AT2R activation inhibited cell proliferation. Moreover, high concentrations of glucose (25 mM) stimulated the expression of AT2R but not AT1R. These results indicate that there are receptor subtype-specific roles in MSG cells, and it is therefore possible that the activation of AT2R stimulates repair of glomerular tissue defect, by regulation of migration and proliferation of MSG cells. Taken together, these results suggest that the relative concentrations of AT1R and AT2R are important factors in the regulation of AngII function in glomerular tissue, and alterations in the concentrations of these receptors may contribute to progression of or protection from diabetic nephropathy.     

Electrical synapses connect a network of gonadotropin releasing hormone neurons in a cichlid fish

Initiating and regulating vertebrate reproduction requires pulsatile release of gonadotropin-releasing hormone (GnRH1) from the hypothalamus. Coordinated GnRH1 release, not simply elevated absolute levels, effects the release of pituitary gonadotropins that drive steroid production in the gonads. However, the mechanisms underlying synchronization of GnRH1 neurons are unknown. Control of synchronicity by gap junctions between GnRH1 neurons has been proposed but not previously found. We recorded simultaneously from pairs of transgenically labeled GnRH1 neurons in adult male Astatotilapia burtoni cichlid fish. We report that GnRH1 neurons are strongly and uniformly interconnected by electrical synapses that can drive spiking in connected cells and can be reversibly blocked by meclofenamic acid. Our results suggest that electrical synapses could promote coordinated spike firing in a cellular assemblage of GnRH1 neurons to produce the pulsatile output necessary for activation of the pituitary and reproduction.Development and function of the reproductive system in vertebrates depends on the timing and levels of signaling by gonadal sex steroids (1, 2). Production of these steroids is controlled by neurons expressing gonadotropin-releasing hormone (GnRH1), which comprise the final output of the brain to the hypothalamic-pituitary-gonadal axis. During vertebrate development, GnRH1 neurons originate outside the central nervous system in the olfactory placode and migrate into the basal forebrain (3–6). These neurons signal to the pituitary via the decapeptide GnRH1 to effect the release of the gonadotropins, follicle stimulating hormone and luteinizing hormone, which in turn stimulate steroid production by the gonads. It has long been known that this release depends on coordinated, pulsatile GnRH1 release, not simply elevated levels (7, 8), requiring some level of synchronization in the output of these neurons. Episodic activation of the pituitary gonadotropes has been observed in multiple vertebrate taxa, including mammals and fish (9–12), however, mechanisms that underlie this required coordinated activity of GnRH1 neurons are unknown. Synchrony could in principle derive from coincident input from a “pacemaker” neural population, from direct coupling of GnRH1 neurons, or from a combination of mechanisms. Gap junction-mediated coupling has been suspected to play a role, as synchronous firing can be observed in neurons mechanically isolated from brain slices and in cultures of embryonic mouse and primate neurons, and immortalized mouse GnRH1 neurons express the connexin proteins that constitute gap junctions (13–15). However, no evidence for gap junctions among adult GnRH1 cells in vivo has been found (16, 17).To search for the origin of synchrony among these neurons, we used a unique model system for analysis of GnRH1 neurons, Astatotilapia burtoni, a cichlid fish. GnRH1 neurons in males of this species exhibit dynamic morphological plasticity caused by changes in their social status (18–21). Here we use transgenic dominant male A. burtoni to perform paired recordings from GnRH1 neurons, and report that they are reciprocally connected by electrical synapses. These findings suggest that gap junctions contribute to the coordinated firing of these neurons necessary for reproductive function.     

Electrical and chemical synapses among parvalbumin fast-spiking GABAergic interneurons in adult mouse neocortex

Networks of gamma-aminobutyric acid (GABA)ergic interneurons connected via electrical and chemical synapses are thought to play an important role in detecting and promoting synchronous activity in the cerebral cortex. Although the properties of electrical and chemical synaptic interactions among inhibitory interneurons are critical for their function as a network, they have only been studied systematically in juvenile animals. Here, we have used transgenic mice expressing the enhanced green fluorescent protein in cells containing parvalbumin (PV) to study the synaptic connectivity among fast-spiking (FS) cells in slices from adult animals (2-7 months old). We have recorded from pairs of PV-FS cells and found that the majority of them were electrically coupled (61%, 14 of 23 pairs). In addition, 78% of the pairs were connected via GABAergic chemical synapses, often reciprocally. The average coupling coefficient for step injections was 1.5% (n = 14), a smaller value than that reported in juvenile animals. GABA-mediated inhibitory postsynaptic currents and potentials decayed with exponential time constants of 2.6 and 5.9 ms, respectively, and exhibited paired-pulse depression (50-ms interval). The inhibitory synaptic responses in the adult were faster than those observed in young animals. Our results indicate that PV-FS cells are highly interconnected in the adult cerebral cortex by both electrical and chemical synapses, establishing networks that can have important implications for coordinating activity in cortical circuits.