RESUMEN
Numerous observations suggest diverse and modulatory roles for serotonin (5-HT) in cortex. Because of the diversity of cell types and multiple receptor subtypes and actions of 5-HT, it has proven difficult to determine the overall role of 5-HT in cortical function. To provide a broader perspective of cellular actions, we studied the effects of 5-HT on morphologically and physiologically identified pyramidal and nonpyramidal neurons from layers I-III of primary somatosensory and motor cortex. We found cell type-specific differences in response to 5-HT. Four cell types were observed in layer I: Cajal Retzius, pia surface, vertical axon, and horizontal axon cells. The physiology of these cells ranged from fast spiking (FS) to regular spiking (RS). In layers II-III, we observed interneurons with FS, RS, and late spiking physiology. Morphologically, these cells varied from bipolar to multipolar and included basket-like and chandelier cells. 5-HT depolarized or hyperpolarized pyramidal neurons and reduced the slow afterhyperpolarization and spike frequency. Consistent with a role in facilitating tonic inhibition, 5-HT2 receptor activation increased the frequency of spontaneous IPSCs in pyramidal neurons. In layers II-III, 70% of interneurons were depolarized by 5-HT. In layer I, 57% of cells with axonal projections to layers II-III (vertical axon) were depolarized by 5-HT, whereas 63% of cells whose axons remain in layer I (horizontal axon) were hyperpolarized by 5-HT. We propose a functional segregation of 5-HT effects on cortical information processing, based on the pattern of axonal arborization.
Asunto(s)
Lisina/análogos & derivados , Neuronas/metabolismo , Serotonina/metabolismo , Corteza Somatosensorial/metabolismo , Potenciales de Acción/efectos de los fármacos , Potenciales de Acción/fisiología , Animales , Antagonistas de Aminoácidos Excitadores/farmacología , Antagonistas del GABA/farmacología , Técnicas In Vitro , Interneuronas/efectos de los fármacos , Interneuronas/metabolismo , Potenciales de la Membrana/efectos de los fármacos , Potenciales de la Membrana/fisiología , Inhibición Neural/efectos de los fármacos , Inhibición Neural/fisiología , Neuronas/efectos de los fármacos , Técnicas de Placa-Clamp , Potasio/metabolismo , Células Piramidales/efectos de los fármacos , Células Piramidales/metabolismo , Ratas , Receptores de Serotonina/efectos de los fármacos , Receptores de Serotonina 5-HT3 , Serotonina/farmacología , Agonistas de Receptores de Serotonina/farmacología , Corteza Somatosensorial/citología , Corteza Somatosensorial/efectos de los fármacosRESUMEN
Transporters are thought to assist in the termination of synaptic transmission at some synapses by removing neurotransmitter from the synapse. To investigate the role of glutamate transport in shaping the time course of excitatory transmission at the mossy fiber-granule cell synapse, the effects of transport impairment were studied using whole-cell voltage- and current-clamp recordings in slices of rat cerebellum. Impairment of transport by L-trans-pyrrolidine-2,4-dicarboxylate (PDC) produced a prolongation of the decay of the AMPA receptor-mediated current after a repetitive stimulus, as well as prolongation of single stimulus-evoked EPSCs when AMPA receptor desensitization was blocked. PDC also produced a prolongation of both single and repetitive-evoked NMDA receptor-mediated EPSCs. Enzymatic degradation of extracellular glutamate did not reverse the PDC-induced prolongation of AMPA receptor-mediated current after a repetitive stimulus, suggesting that transporter binding sites participate in limiting glutamate spillover. In current-clamp recordings, PDC dramatically increased the total area of the EPSP and the burst duration evoked by single and repetitive stimuli. These data indicate that glutamate transporters play a significant role in sculpting the time course of synaptic transmission at granule cell synapses, most likely by limiting the extent of glutamate spillover. The contribution of transporters is particularly striking during repetitive stimulus trains at physiologically relevant frequencies. Hence, the structural arrangement of the glomerulus may enhance the contribution of transporters to information processing by limiting the extent of glutamate spillover between adjacent synapses.
Asunto(s)
Cerebelo/fisiología , Potenciales Evocados/fisiología , Neuronas/fisiología , Transmisión Sináptica/fisiología , Envejecimiento , Animales , Ácido Aspártico/farmacología , Benzotiadiazinas/farmacología , Ácidos Dicarboxílicos/farmacología , Estimulación Eléctrica , Potenciales Evocados/efectos de los fármacos , Femenino , Técnicas In Vitro , Masculino , Inhibidores de la Captación de Neurotransmisores/farmacología , Técnicas de Placa-Clamp , Pirrolidinas/farmacología , Ratas , Ratas Sprague-Dawley , Receptores AMPA/fisiología , Transmisión Sináptica/efectos de los fármacos , Factores de TiempoRESUMEN
It is thought that galanin, a 29 amino acid neuropeptide, is involved in various neuronal functions, including the regulation of food intake and hormone release. Consistent with this idea, galanin receptors have been demonstrated throughout the brain, with high levels being observed in the hypothalamus. However, little is known about the mechanisms by which galanin elicits its actions in the brain. Therefore, we studied the effects of galanin and its analogs on synaptic transmission using an in vitro slice preparation of rat hypothalamus. In arcuate nucleus neurons, application of galanin resulted in an inhibition of evoked glutamatergic EPSCs and a decrease in paired-pulse depression, indicating a presynaptic action. The fragments galanin 1-16 and 1-15 produced a robust depression of synaptic transmission, whereas the fragment 3-29 produced a lesser degree of depression. The chimeric peptides C7, M15, M32, and M40, which have been reported to antagonize some actions of galanin, all produced varying degrees of depression of evoked EPSCs. In a minority of cases, C7, M15, and M40 antagonized the actions of galanin. Analysis of mEPSCs in the presence of TTX and Cd2+, or after application of alpha-latrotoxin, indicated a site of action for galanin downstream of Ca2+ entry. Thus, our data suggest that galanin acts via several subtypes of presynaptic receptors to depress synaptic transmission in the rat arcuate nucleus.
Asunto(s)
Núcleo Arqueado del Hipotálamo/metabolismo , Ácido Glutámico/metabolismo , Inhibición Neural/fisiología , Receptores de la Hormona Gastrointestinal/metabolismo , Animales , Núcleo Arqueado del Hipotálamo/química , Complemento C7/farmacología , Agonistas de Aminoácidos Excitadores/farmacología , Femenino , Proteínas de Unión al GTP/metabolismo , Galanina/análogos & derivados , Galanina/farmacología , Masculino , Neuropéptidos/farmacología , Técnicas de Placa-Clamp , Fragmentos de Péptidos/farmacología , Ratas , Ratas Wistar , Receptores AMPA/fisiología , Receptores de Galanina , Receptores de la Hormona Gastrointestinal/agonistas , Venenos de Araña/farmacología , Sustancia P/análogos & derivados , Sustancia P/farmacología , Transmisión Sináptica/efectos de los fármacos , Transmisión Sináptica/fisiología , Ácido alfa-Amino-3-hidroxi-5-metil-4-isoxazol Propiónico/farmacologíaRESUMEN
The cellular mechanism underlying the genesis of the long-lasting alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptor-mediated excitatory postsynaptic currents (EPSCs) at the mossy fiber (MF)-unipolar brush cell (UBC) synapse in rat vestibular cerebellum was examined with the use of whole cell and excised patch-clamp recording methods in thin cerebellar slices. Activation of MFs evokes an all-or-none biphasic AMPA-receptor-mediated synaptic current with a late component that peaks at 100-800 ms, which has been proposed to originate from an entrapment of glutamate in the MF-UBC synaptic cleft and is generated by the steady-state activation of AMPA receptors. Bath application of cyclothiazide, which blocks desensitization of AMPA receptors, produced a dose-dependent enhancement of the amplitude of the synaptic current (median effective dose 30 microM) and slowing of the rise time of the fast EPSC. N-methyl-D-aspartate-receptor-mediated EPSCs in UBCs were not potentiated in amplitude or time course by cyclothiazide (100 microM). The dose-response relations for the steady-state current evoked by glutamate acting at AMPA receptors in excised outside-out patches from UBC and granule somatic membranes was biphasic, peaking at 50 microM and declining to 50-70% of this value at 1 mM glutamate. When glutamate was slowly washed from patches to simulate the gradual decline of glutamate in the synapse, a late hump in the transmembrane current was observed in patches from both cell types. The delivery of a second MF stimulus at the peak of the slow EPSC evoked a fast EPSC of reduced amplitude followed by an undershoot of the subsequent slow current, consistent with the hypothesis that the peak of the slow EPSC reflects the peak of the biphasic steady-state dose-response curve. Estimates of receptor occupancy and glutamate concentration derived from the ratio of fast EPSC amplitudes, and the amplitude and polarity of the initial steady-state current in paired-pulse experiments, predict a slow decline of glutamate with a time constant of 800 ms, declining to ineffective concentrations at 5.4 s. Manipulation of cleft glutamate concentration by lowered extracellular calcium or delivery of brief stimulus trains abolished the slow EPSC and restored the undershoot to paired stimuli, respectively, in a manner consistent with a prolonged lifetime of glutamate in the cleft. The slow component of the EPSC was prolonged in duration by the glutamate reuptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate, suggesting that glutamate transport contributes to the time course of the synaptic current in UBCs. The data support the notion that the MF-UBC synapse represents an ultrastructural specialization to effectively entrap glutamate for unusually prolonged periods of time following release from MF terminals. The properties of the postsynaptic receptors and constraints on diffusional escape of glutamate imposed by synaptic ultrastructure and glutamate transporters act in concert to sculpt the time course of the resulting slow EPSC. This in turn drives a long-lasting train of action potentials in response to single presynaptic stimuli.
Asunto(s)
Polaridad Celular/fisiología , Cerebelo/fisiología , Ácido Glutámico/fisiología , Neuronas/fisiología , Sinapsis/fisiología , Animales , Benzotiadiazinas/farmacología , Cerebelo/citología , Diuréticos , Estimulación Eléctrica , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Femenino , Ácido Glutámico/metabolismo , Técnicas In Vitro , Masculino , Potenciales de la Membrana/fisiología , Técnicas de Placa-Clamp , Ratas , Ratas Sprague-Dawley , Receptores AMPA/antagonistas & inhibidores , Receptores AMPA/fisiología , Inhibidores de los Simportadores del Cloruro de Sodio/farmacología , Sinapsis/metabolismo , Núcleos Vestibulares/citología , Núcleos Vestibulares/fisiologíaRESUMEN
We examined the effects of peptides of the neuropeptide Y (NPY)/pancreatic polypeptide (PP) family on synaptic transmission in the arcuate nucleus in rat hypothalamic slices. Application of NPY produced two effects. In some cells NPY produced an outward current that had the properties of a K+ current. NPY also inhibited the evoked glutamatergic EPSC recorded in these arcuate neurons by a presynaptic mechanism. Although the effects of NPY on the K+ current reversed within a few minutes of washout of the peptide, its effects on the EPSC frequently were longer lasting (>30 min). Similar effects were observed using peptide YY or the NPY analog [Leu31, Pro34]NPY. Although K+ current activation by [Leu31,Pro34]NPY was blocked by the selective Y1 antagonist BIBP 3226, inhibition of the EPSC was blocked only partially. Other NPY-related peptides such as NPY(13-36), PP, and [D-Trp32]NPY also inhibited the EPSC. However, none of these peptides produced activation of the K+ current. Thus, activation of more than one NPY receptor produces synaptic inhibition in the arcuate nucleus. A Y1 receptor activates a K+ current postsynaptically, and several receptor types appear to inhibit the EPSC by a presynaptic mechanism.
Asunto(s)
Núcleo Arqueado del Hipotálamo/efectos de los fármacos , Neuropéptido Y/farmacología , Neurotransmisores/farmacología , Receptores de Neuropéptido Y/efectos de los fármacos , Transmisión Sináptica/efectos de los fármacos , Animales , Relación Dosis-Respuesta a Droga , Femenino , Masculino , Ratas , Ratas Sprague-DawleyAsunto(s)
Plasticidad Neuronal , Reflejo Vestibuloocular/fisiología , Potenciales de Acción , Animales , Tronco Encefálico/fisiología , Cerebelo/fisiología , Simulación por Computador , Movimientos Oculares/fisiología , Técnicas In Vitro , N-Metilaspartato/fisiología , Técnicas de Placa-Clamp , Postura , Ratas , Ratas Sprague-Dawley , Rotación , Canales Semicirculares/fisiología , Nervio Vestibular/fisiologíaRESUMEN
1. We examined the synaptic activation of N-methyl-D-aspartate (NMDA) receptors by stimulation of primary vestibular afferent projections to second-order neurons in the medial vestibular nucleus (MVN) using whole cell patch-clamp recording methods in rat brain stem slices maintained in vitro. 2. Stimulation of the vestibular nerve (nVIII) evoked monosynaptic excitatory postsynaptic potentials (EPSPs) in second-order MVN neurons. Bath application of the gamma-aminobutyric acid receptor antagonist bicuculline (10 microM) revealed a late, slow EPSP that was blocked by the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (D-AP5; 50 microM) and displayed a voltage-dependent reduction at hyperpolarized potentials in the presence of external magnesium (1 mM). The early component of the nVIII-evoked EPSP in the presence of bicuculline was blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX; 10 microM) and displayed linear current-voltage relations in the presence of external magnesium. 3. In some cells both components of the EPSP were blocked by DNQX, whereas only the late component was sensitive to D-AP5, indicating that NMDA receptors also mediate excitation via intrinsic pathways within MVN. 4. The NMDA receptor-mediated excitatory postsynaptic current (EPSC) evoked by nVIII stimulation was recorded in voltage-clamped MVN neurons in a magnesium-free saline containing bicuculline (10 microM) and DNQX (10 microM). At -80 mV the NMDA receptor-mediated EPSC (latency = 2.7 ms) displayed a slow rise time (10-90%, 5.8 ms) and exhibited a biexponential decay [time constant of fast component of decay (tau s) = 27.6 ms, time constant of slow component of decay (tau s) = 147.4 ms].(ABSTRACT TRUNCATED AT 250 WORDS)
Asunto(s)
Receptores de N-Metil-D-Aspartato/fisiología , Transmisión Sináptica/fisiología , Núcleos Vestibulares/fisiología , Vías Aferentes/fisiología , Animales , Mapeo Encefálico , Femenino , Potenciación a Largo Plazo/fisiología , Masculino , Potenciales de la Membrana/fisiología , Red Nerviosa/fisiología , Plasticidad Neuronal/fisiología , Neuronas/fisiología , Técnicas de Placa-Clamp , Ratas , Ratas Sprague-Dawley , Reflejo Vestibuloocular/fisiologíaRESUMEN
1. The effects of metabotropic glutamate receptor (mGluR) agonists on excitatory postsynaptic potentials (EPSPs) evoked by stimulation of mossy fibers (MF) and parallel fibers (PF) were examined in turtle cerebellar Purkinje cells. 2. The mGluR agonist 1S,3R-ACPD (1-25 microM) reversibly potentiated the amplitude of the MF-evoked EPSPs, but was without effect on PF-evoked EPSPs. The potentiation of MF-evoked EPSPs was dose-dependent, with a median effective dose (ED50) of approximately 4.4 microM. At higher doses (15-25 microM) 1S,3R-ACPD produced a direct depolarization of Purkinje cells in 58% of cells examined. 3. The enhancement of MF EPSPs by 1S,3R-ACPD was mimicked by 1S,3S-ACPD (50 microM) and blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovalerate (D-AP5), but not by the mGluR antagonist L-2-amino-3-phosphonopionic acid (L-AP3; 1 mM), or the 1R,3S isomer of ACPD (25-500 microM). 4. Quisqualate (1 microM) produced a biphasic effect on MF EPSPs, producing an initial blockade of the EPSP followed by a D-AP5-sensitive potentiation. 5. The potentiation of MF EPSPs by 1S,3R-ACPD was not blocked by prior exposure to the protein kinase C activator phorbol 12-myristate 13-acetate (10 microM), the protein kinase C inhibitor calphostin C (1 microM), the adenylate cyclase activator forskolin (25 microM), or the nitric oxide donator sodium nitroprusside (1 mM). Preincubation of the tissue for 24-48 h in pertussis toxin also failed to prevent the ability of 1S,3R-ACPD to potentiate the NMDA receptor-mediated component of the MF EPSP. PF EPSPs were also not significantly affected by these agents. 6. The results demonstrate that the mGluR agonists 1S,3R-ACPD, 1S,3S-ACPD, and quisqualate produce a potent, stereospecific potentiation of NMDA receptor-mediated transmission at the MF-granule cell synapse. Agents that modulate the intracellular messengers protein kinase C, adenylate cyclase, nitric oxide, or pertussis toxin-sensitive G proteins failed to mimic or block this effect. This would suggest that the potentiation of NMDA receptor-mediated transmission at this synapse is not mediated via these systems, and reflects a different site of action of mGluR agonists on the NMDA receptor. The observed interaction between mGluR and NMDA receptors in granule cells provides a means for activity-dependent modulation of synaptic transmission, which may play a role in synaptic integration at the MF-granule cell synapse.
Asunto(s)
Cerebelo/fisiología , Naftalenos , Receptores de Glutamato/fisiología , Receptores de N-Metil-D-Aspartato/fisiología , Transmisión Sináptica/fisiología , Tortugas/fisiología , 2-Amino-5-fosfonovalerato/farmacología , Toxina de Adenilato Ciclasa , Animales , Cerebelo/citología , Colforsina/farmacología , Cicloleucina/análogos & derivados , Cicloleucina/farmacología , Potenciales Evocados/fisiología , Técnicas In Vitro , Isomerismo , Óxido Nítrico/farmacología , Toxina del Pertussis , Compuestos Policíclicos/farmacología , Células de Purkinje/efectos de los fármacos , Ácido Quiscuálico/farmacología , Receptores de N-Metil-D-Aspartato/efectos de los fármacos , Sinapsis/efectos de los fármacos , Transmisión Sináptica/efectos de los fármacos , Acetato de Tetradecanoilforbol/farmacología , Factores de Virulencia de Bordetella/farmacologíaRESUMEN
1. The effects of the metabotropic glutamate receptor (mGluR) agonist 1S,3R-ACPD on excitatory postsynaptic potentials (EPSPs) evoked by stimulation of mossy fibers (MF) and parallel fibers (PF) were examined in turtle cerebellar Purkinje cells. 2. 1S,3R-ACPD (1-25 microM) reversibly potentiated the amplitude of the MF-evoked EPSPs and revealed a late, slow EPSP component, but was without effect on PF-evoked EPSPs. The potentiation of both components of MF-evoked EPSPs was dose dependent, with an ED50 of approximately 3 microM. At higher doses (15-25 microM) 1S,3R-ACPD produced a direct depolarization of Purkinje cells in 57% of cells examined. 3. The enhancement of MF EPSPs by 1S,3R-ACPD was blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovalerate (AP-5), but not by the mGluR antagonist L-2-amino-3-phosphonopionic acid (L-AP3; 1 mM), or the 1R,3S isomer of ACPD (25-500 microM). 4. The results demonstrate that mGluR activation by 1S,3R-ACPD produces a potent, stereospecific facilitation of NMDA receptor-mediated transmission at the MF-granule cell synapse.