Excitatory synaptic currents in Purkinje cells

The N-methyl-D-aspartate (NMDA) and non-NMDA classes of glutamate receptor combine in many regions of the central nervous system to form a dual-component excitatory postsynaptic current. Non-NMDA receptors mediate synaptic transmission at the resting potential, whereas NMDA receptors contribute during periods of postsynaptic depolarization and play a role in the generation of long-term synaptic potentiation. To investigate the receptor types underlying excitatory synaptic transmission in the cerebellum, we have recorded excitatory postsynaptic currents (EPSCS), by using whole-cell techniques, from Purkinje cells in adult rat cerebellar slices. Stimulation in the white matter or granule-cell layer resulted in an all-or-none synaptic current as a result of climbing-fibre activation. Stimulation in the molecular layer caused a graded synaptic current, as expected for activation of parallel fibres. When the parallel fibres were stimulated twice at an interval of 40 ms, the second EPSC was facilitated; similar paired-pulse stimulation of the climbing fibre resulted in a depression of the second EPSC. Both parallel-fibre and climbing-fibre responses exhibited linear current-voltage relations. At a holding potential of -40 mV or in the nominal absence of Mg2+ these synaptic responses were unaffected by the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV), but were blocked by the non-NMDA receptor antagonist 6-cyano-2,3-dihydro-7-nitroquinoxalinedione (CNQX). NMDA applied to the bath failed to evoke an inward current, whereas aspartate or glutamate induced a substantial current; this current was, however, largely reduced by CNQX, indicating that non-NMDA receptors mediate this response. These results indicate that both types of excitatory input to adult Purkinje cells are mediated exclusively by glutamate receptors of the non-NMDA type, and that these cells entirely lack NMDA receptors.     

Calcineurin-mediated LTD of GABAergic inhibition underlies the increased excitability of CA1 neurons associated with LTP

Coincident pre- and postsynaptic activity generates long-term potentiation (LTP), a possible cellular model of learning and memory. LTP has two components: (1) an increase in the excitatory postsynaptic potential (EPSP), and (2) an increase in the ability of the EPSP to generate a spike (E-S coupling of LTP). We have used pharmacological and genetic approaches to address the molecular nature of E-S coupling in CA1 pyramidal neurons. Blockade of the Ca2+-sensitive phosphatase, calcineurin, prevents induction of E-S coupling without interfering with LTP of the EPSP. Calcineurin produces its effect on E-S coupling by inducing a long-lasting depression (LTD) of the GABA(A)-mediated inhibitory postsynaptic potentials (IPSPs). This LTD of the IPSP was prevented by blockade of NMDA receptors. Thus, the tetanus that elicits NMDA-dependent LTP mediates a coordinately regulated double function. It produces LTP of the EPSP and, concomitantly, LTD of the IPSP that leads to enhancement of E-S coupling.     

Local interneurons and information processing in the olfactory glomeruli of the moth Manduca sexta

Intracellular recordings were made from the major neurites of local interneurons in the moth antennal lobe. Antennal nerve stimulation evoked 3 patterns of postsynaptic activity: (i) a short-latency compound excitatory postsynaptic potential that, based on electrical stimulation of the antennal nerve and stimulation of the antenna with odors, represents a monosynaptic input from olfactory afferent axons (71 out of 86 neurons), (ii) a delayed activation of firing in response to both electrical- and odor-driven input (11 neurons), and (iii) a delayed membrane hyperpolarization in response to antennal nerve input (4 neurons).Simultaneous intracellular recordings from a local interneuron with short-latency responses and a projection (output) neuron revealed unidirectional synaptic interactions between these two cell types. In 20% of the 30 pairs studied, spontaneous and current-induced spiking activity in a local interneuron correlated with hyperpolarization and suppression of firing in a projection neuron. No evidence for recurrent or feedback inhibition of projection neurons was found. Furthermore, suppression of firing in an inhibitory local interneuron led to an increase in firing in the normally quiescent projection neuron, suggesting that a disinhibitory pathway may mediate excitation in projection neurons. This is the first direct evidence of an inhibitory role for local interneurons in olfactory information processing in insects. Through different types of multisynaptic interactions with projection neurons, local interneurons help to generate and shape the output from olfactory glomeruli in the antennal lobe.Abbreviations AL antennal lobe - EPSP excitatory postsynaptic potential - GABA -aminobutyric acid - IPSP inhibitory postsynaptic potential - LN local interneuron - MGC macroglomerular complex - OB olfactory bulb - PN projection neuron - TES N-tris[hydroxymethyl]methyl-2-aminoethane-sulfonic acid     

Synaptic mechanisms of monaural and binaural processing in the locust

(1) Responses of auditory interneurones were recorded intracellularly within the metathoracic ganglion of the locust when stimulating each tympanic membrane with a piezoelectric transducer. Thus, in contrast to conventional sound stimulation, each of the two ears could be activated independently from the other at variable intensities, duration and stimulus onsets. By means of this ‘earphone-like’ stimulation technique the binaural integration properties of auditory interneurons could be analysed. (2) A minority of units (3 out of 43) was affected by input from one side only. Their synaptic input was purely excitatory and the intensity characteristics reflected those of auditory receptor fibres. (3) Most interneurones received input from both ears, each being excitatory or one excitatory or one excitatory and one inhibitory. In some units the unilateral synaptic response already included both an EPSP and an IPSP. As a result of varying temporal interactions between the EPSP and the IPSP within the unilaterally evoked complex response the intensity characteristics differed widely from unit to unit. (4) With binaural simultaneous stimulation the complexity of the postsynaptic responses of most interneurones increased as the synaptic input from both ears coincided at the level of the recorded interneurone. Although both ears were stimulated symmetrically (at the same time and intensity), units were recorded where the latencies of ipsilateral and contralateral synaptic input were different. Contralateral inhibition could either follow or precede ipsilateral excitation and in some cases both EPSP and IPSP had the same latency. On the basis of these findings the binaural synaptic mechanisms of directional coding are discussed and compared with corresponding results under free field stimulus conditions.     

A persistent postsynaptic modification mediates long-term potentiation in the hippocampus

Long-term potentiation (LTP) is a long-lasting enhancement of synaptic transmission that can be induced by brief repetitive stimulation of excitatory pathways in the hippocampus. One of the most controversial points is whether the process underlying the enhanced synaptic transmission occurs pre- or postsynaptically. To examine this question, we have taken advantage of the novel physiological properties of excitatory synaptic transmission in the CA1 region of the hippocampus. Synaptically released glutamate activates both NMDA and non-NMDA receptors on pyramidal cells, resulting in an excitatory postsynaptic potential (EPSP) with two distinct components. A selective increase in the non-NMDA component of the EPSP was observed with LTP. This result suggests that the enhancement of synaptic transmission during LTP is caused by an increased sensitivity of the postsynaptic neuron to synaptically released glutamate.     

Analysis of function of the glutamatergic and GABAergic mediator systems in the olfactory cortex of spontaneously hipertensive rats in vitro

The effect of persistent hypertension on neuronal activity and synaptic transmission has been studied on olfactory cortex slices of SHR rats. The profilies of focal potentials in hypertensive rats demonstrated a short duration of the 2-amino-3-(5-methyl-3-hydroxyisoxazol-4-yl)-propanoic acid (AMPA) component of excitatory postsynaptic potential (EPSP), a small amplitude and long duration of the N-methyl D-aspartate (NMDA) component of EPSP, and a large amplitude of the GABA_B-dependent slow inhibitory postsynaptic potentials. The sensitivity of glutamate receptors responsible for the generation of AMPA- and NMDA-mediated EPSPs was low after the exposure to 1 mM L-glutamate. The amplitudes of the AMPA- and NMDA-mediated EPSPs decreased. Tetanization of slices from hypertensive rats induced a short-term potentiation followed by a depression. The data obtained indicate that persistent hypertension has depressive effects on the basic glutamatergic and GABAergic parameters of synaptic activity of neurons as well as on learning and memory. Apparently, these processes were evoked by glutamate excitotoxicity in the brain of hypertensive rats.     

GABA-mediated synaptic inhibition of projection neurons in the antennal lobes of the sphinx moth,Manduca sexta

Responses of neurons in the antennal lobe (AL) of the moth Manduca sexta to stimulation of the ipsilateral antenna by odors consist of excitatory and inhibitory synaptic potentials. Stimulation of primary afferent fibers by electrical shock of the antennal nerve causes a characteristic IPSP-EPSP synaptic response in AL projection neurons. The IPSP in projection neurons reverses below the resting potential, is sensitive to changes in external and internal chloride concentration, and thus is apparently mediated by an increase in chloride conductance. The IPSP is reversibly blocked by 100 microM picrotoxin or bicuculline. Many AL neurons respond to application of GABA with a strong hyperpolarization and an inhibition of spontaneous spiking activity. GABA responses are associated with an increase in neuronal input conductance and a reversal potential below the resting potential. Application of GABA blocks inhibitory synaptic inputs and reduces or blocks excitatory inputs. EPSPs can be protected from depression by application of GABA. Muscimol, a GABA analog that mimics GABA responses at GABAA receptors but not at GABAB receptors in the vertebrate CNS, inhibits many AL neurons in the moth.     

The suprachiasmatic nucleus exhibits diurnal variations in spontaneous excitatory postsynaptic activity

A most prominent feature of neurons in the suprachiasmatic nucleus (SCN) is the circadian rhythm in spontaneous firing frequency. To disclose synaptic mechanisms associated with the rhythmic activity, the spontaneous postsynaptic activity was studied using whole-cell, patch clamp recordings in the ventral region of the SCN in slice preparations from rats. The synaptic events were compared between two time intervals corresponding to the highest and lowest electrical activity within the SCN during subjective daytime and nighttime, respectively. The gamma-aminobutyric acid (GABA)-mediated spontaneous inhibitory activity showed no diurnal variations, but the excitatory activity was markedly higher in frequency, without differences in amplitude, during the subjective day compared to the subjective night. Spontaneous and evoked inhibitory synaptic events were blocked by the GABA(A) receptor antagonist bicuculline. The alpha-amino-hydroxy-5-methylisoxazole-4-propionic acid (AMPA/kainate) receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX) blocked most of the excitatory activity. In addition, CNQX reduced the spontaneous inhibitory activity. The N-methyl-D-aspartate antagonist D-2-amino-5-phosphonopentanoic acid reduced the inhibitory activity to a lesser degree, and there was no significant difference in amplitude or frequency of synaptic events in control and Mg2+-free solutions, indicating that the AMPA receptor plays an important role in regulating the inhibitory release of GABA within the SCN. Ipsi- and contralateral stimulation of the SCN consistently evoked excitatory synaptic responses. Inhibitory synaptic responses occurred in some neurons upon increasing stimulus strength. In conclusion, this study shows that there is a substantial influence from spontaneous glutamatergic synapses on the ventral part of the SCN and that these exhibit daily variations in activity. Diurnal fluctuations in spontaneous excitatory postsynaptic activity within this network may contribute to the mechanisms for synchronization of rhythms between individual SCN neurons and may underlie the daily variations in the spontaneous firing frequency of SCN neurons.     

Corticofugal postsynaptic influences on red nucleus neurons

The responses of red nucleus neurons to stimulation of the sensorimotor cortex was studied on nembutal-anesthetized cats. Most of the rubrospinal neurons were identified according to their antidromic activation. Stimulation of the sensorimotor cortex was shown to evoke in the red nucleus neurons monosynaptic excitatory potentials with a latency of 1.85 msec, polysynaptic excitatory potentials (EPSP), and inhibitory postsynaptic potentials (IPSP) with a latency of 9–24 msec. The EPSP often produced spikes. The probability of generation of spreading excitation is greater with motor cortex stimulation. The monosynaptic EPSP are assumed to arise under the influence of the impulses arriving over the corticorubral neurons as a result of excitation of axodendritic synapses. The radial type of branching of red nucleus neurons facilitates the transition from electrotonically spreading local depolarization to an action potential triggered by the initial axonal segment. Polysynaptic EPSP and IPSP seem to be a result of activation of fast pyramidal neurons whose axon collaterals are connected via interneurons with the soma of the red nucleus neurons.L. A. Orbeli Institute of Physiology of the Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 43–51, January–February, 1971.     

Pharmacological characterization of postsynaptic potentials evoked in the bimodal pacemaker neuron of Helix pomatia L

Stimulation of various peripheral nerve trunks evokes very similar compound postsynaptic potentials (PSP) composed of one or more excitatory postsynaptic potentials (EPSP) followed by fast and slow inhibitory postsynaptic potentials (IPSP) on the identified RPal neuron of Helix pomatia L. Evoked EPSPs were reduced or blocked by nicotine, atropine and d-tubocurarine. The two components of IPSP were different in their pharmacological sensitivity. Slow IPSP was partly or totally eliminated by ergometrine and chlorpromazine and was reduced by atropine, nicotine as well as by propranolol. Fast IPSP was reduced only in the presence of ergometrine and could not be blocked by either of the applied drugs. Participation of cholinergic transmission seems to be essential in the evoked EPSP but its partial involvement in the slow IPSP can also be supposed. A dopaminergic mechanism may take part in the generation of both components of IPSP but the receptors responsible for the slow IPSP were sensitive to other catecholamine antagonists as well, referring to a more complex origin, or to the involvement of an unknown transmitter. Comparison of PSPs evoked by stimulation of different nerves shows that presynaptic areas belonging to various peripheral sources are overlapped on the RPal neuron, and they probably act by similar transmitter substances.     

Electrical advantages of dendritic spines

Many neurons receive excitatory glutamatergic input almost exclusively onto dendritic spines. In the absence of spines, the amplitudes and kinetics of excitatory postsynaptic potentials (EPSPs) at the site of synaptic input are highly variable and depend on dendritic location. We hypothesized that dendritic spines standardize the local geometry at the site of synaptic input, thereby reducing location-dependent variability of local EPSP properties. We tested this hypothesis using computational models of simplified and morphologically realistic spiny neurons that allow direct comparison of EPSPs generated on spine heads with EPSPs generated on dendritic shafts at the same dendritic locations. In all morphologies tested, spines greatly reduced location-dependent variability of local EPSP amplitude and kinetics, while having minimal impact on EPSPs measured at the soma. Spine-dependent standardization of local EPSP properties persisted across a range of physiologically relevant spine neck resistances, and in models with variable neck resistances. By reducing the variability of local EPSPs, spines standardized synaptic activation of NMDA receptors and voltage-gated calcium channels. Furthermore, spines enhanced activation of NMDA receptors and facilitated the generation of NMDA spikes and axonal action potentials in response to synaptic input. Finally, we show that dynamic regulation of spine neck geometry can preserve local EPSP properties following plasticity-driven changes in synaptic strength, but is inefficient in modifying the amplitude of EPSPs in other cellular compartments. These observations suggest that one function of dendritic spines is to standardize local EPSP properties throughout the dendritic tree, thereby allowing neurons to use similar voltage-sensitive postsynaptic mechanisms at all dendritic locations.     

Changes in membrane potential and postsynaptic potentials evoked by strychnine in neocortical neurons

Intracellular responses of neurons of the suprasylvian fissure to intracortical stimulation before and during topical cortical strychnine application was studied in experiments on immobilized, unanesthetized cats (a local anesthetic was used). Untreated cortical neurons responded to intracortical stimulation with a monosynaptic excitatory postsynaptic potential (EPSP) followed by an inhibitory postsynaptic potential (IPSP). Application of strychnine evoked epileptiform population activity and paroxysmal depolarizations of neuronal membrane potentials (MPs), followed by hyperpolarization. Increased hyperpolarizations, and the prolonged duration of their summation were responsible for an increased MP and reduced or abolished tonic spike activity. Intracellular application (as a result of diffusion from the microelectrode) of ethyleneglycoltetraacetate (EGTA) that blocked the calcium-dependent potassium membrane conductance (gK(Ca)) abolished the hyperpolarization. The development of epileptiform activity was accompanied by reduction of the IPSP, and an increase in the monosynaptic EPSP. The role of gK(Ca) and postsynaptic inhibition in epileptogenesis is discussed.I. I. Mechnikov State University, Odessa. Translated from Neirofiziologiya, Vol. 24, No. 6, pp. 684–691, November–December, 1992.     

Dopaminergic acceleration and GABAergic inhibition in extrinsic neural control of the hermit crab heart

The acceleratory and inhibitory cardio-regulatory nerves of hermit crabs (Aniculus aniculus, Dardanus crassimanus) were studied using histochemical, immunocytochemical and pharmacological tests. Glyoxylic acid-induced fluorescence was observed in two of three axons of the dorsal cardiac nerve. One axon of the nerve showed gamma-aminobutyric acid-like immunoreactivity. Effects of stimulation of cardio-acceleratory axons were blocked by the dopaminergic antagonists, haloperidol and chlorpromazine, but not by cholinergic, adrenergic or serotonergic blockers, suggesting that dopamine is the primary potential candidate for the neurotransmitter of cardio-accelerator neurons. Picrotoxin antagonized inhibition of the cardiac ganglion induced by gammaam-inobutyric acid and by cardio-inhibitory axons. Both small and large ganglionic cells may receive dopaminergic and GABAergic extrinsic neural control.Abbreviations ACh acetylcholine - CA cardio-accelerator - CA1 and CA2 first and second cardio-accelerators - CI cardio-inhibitor - EJP excitatory junction potential - GABA gamma-aminobutyric acid - EPSP excitatory postsynaptic potential - IPSP inhibitory postsynaptic potential - LGC large ganglionic cell - SGC small ganglionic cell - 5-HT serotonin     

Slower spontaneous excitatory postsynaptic currents in spiny versus aspiny hilar neurons.

In the hilar region of the rat hippocampus, large spontaneous excitatory postsynaptic currents (sEPSCs) mediated by non-NMDA glutamate receptors are present in both excitatory spiny mossy cells and inhibitory aspiny hilar interneurons, making these neurons ideal candidates for a comparative study using the tight seal whole-cell recording technique. Although sEPSCs have similar amplitude distributions, the rise and decay times are significantly slower in spiny versus aspiny neurons. Similar kinetic differences are observed in synaptic currents evoked by mossy fiber stimulation. These results demonstrate a physiological difference between the excitatory drive to excitatory and inhibitory neurons in the hilus that certainly contributes to differences in synaptic strength and that may be applicable to other brain regions. Furthermore, since the development or modification of individual spines or groups of spines may affect synaptic strength, these results may be pivotal in establishing a role for spines in modulating synaptic activity.     

Influence of forelimb afferents on the activity of lumbar spinal-cord motoneurons

Experiments were conducted on anesthetized cats with microelectrode recording to study the synaptic responses that develop in the lumbar motoneurons on stimulation of the afferent fibers of groups II and III in the nerves of the ipsilateral and contralateral forelegs. Stimulation of these afferents evoked predominantly inhibitory postsynaptic potentials (IPSP) in the extensor motoneurons and excitatory postsynaptic potentials (EPSP) in the flexor motoneurons. A basically inhibitory change in the rhythmic background activity developed under the influence of descending impulsation. The duration of the total inhibition of "spontaneous" motoneuron activity corresponded to the duration of the inhibitory influences exerted by the forelimb flexor-reflex afferents (FRA) on the interneurons. The interaction of the descending and segmental PSP resulted in inhibition and facilitation of the segmental responses in the motoneurons. The ultimate result of this interaction was determined by the shifts in the membrane potential of the motoneuron and by the effects created in the interneurons.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 58–67, January–February, 1971.     

Synaptic effects elicited in the Retzius cells of the leech Hirudo medicinalis by stimulation of the segmental roots

Electrical stimulation of the segmental roots of each ganglion of Hirudo medicinalis, elicits in both Retzius' cells inhibitory and excitatory effects. The IPSP and EPSP are chemical in nature, being dependent on the membrane potential, and suppressed by high Mg++. Selective inactivation of one RC shows that the responses of the contralateral RC are not due to electrotonic coupling between the two cells, but to synaptic actions impinging upon the membrane of both RCs. The two synaptic potentials appear to be mediated by two set of fibres with a different threshold to electrical stimulation. Their actions on the RCs appear to be polysynaptic on the basis of central latency. Simultaneous stimulation of two roots shows evidence for occlusion for IPSP and summation for EPSP, confirming the polysynaptic nature of the effects. The possible functional significance of the inhibitory and excitatory pathways, is discussed.     

Bidirectional plasticity gated by hyperpolarization controls the gain of postsynaptic firing responses at central vestibular nerve synapses

Linking synaptic plasticity with behavioral learning requires understanding how synaptic efficacy influences postsynaptic firing in neurons whose role in behavior is understood. Here, we examine plasticity at a candidate site of motor learning: vestibular nerve synapses onto neurons that mediate reflexive movements. Pairing nerve activity with changes in postsynaptic voltage induced bidirectional synaptic plasticity in vestibular nucleus projection neurons: long-term potentiation relied on calcium-permeable AMPA receptors and postsynaptic hyperpolarization, whereas long-term depression relied on NMDA receptors and postsynaptic depolarization. Remarkably, both forms of plasticity uniformly scaled synaptic currents evoked by pulse trains, and these changes in synaptic efficacy were translated into linear increases or decreases in postsynaptic firing responses. Synapses onto local inhibitory neurons were also plastic but expressed only long-term depression. Bidirectional, linear gain control of vestibular nerve synapses onto projection neurons provides a plausible mechanism for motor learning underlying adaptation of vestibular reflexes.     

Cholinergic interneurons in the feeding system of the pond snail Lymnaea stagnalis. II. N1 interneurons make cholinergic synapses with feeding motoneurons.

The N1 neurons are a population of interneurons active during the protraction phase of the feeding rhythm. All the N1 neurons are coupled by electrical synapses which persist in a high Mg/low Ca saline which blocks chemical synapses. Individual N1 spikes produce discrete electrotonic postsynaptic potentials (PSPS) in other N1 cells, but the coupling is not strong enough to ensure 1:1 firing. Bursts of N1 spikes generate compound PSPS in the feeding motoneurons. The sign (excitation or inhibition) of the N1 input corresponds with the synaptic barrage recorded during the protraction phase. Discrete PSPS are only resolved in a Hi-Di saline. Their variation in latency and number can be explained by variation in electrotonic propagation within the electrically coupled network of N1 cells. The excitatory postsynaptic potentials (ESPS) in the 1 cell are reduced by 0.5 mM antagonists hexamethonium (HMT), atropine (ATR), curare (d-TC) and by methylxylocholine (MeXCh), all of which block the excitatory cholinergic receptor (Elliott et al. (Phil. Trans. R. Soc. Lond. 336, 157-166 (Preceding paper.) (1992)). The 1 cell EPSPS were transiently blocked by phenyltrimethylammonium (PTMA), which is both an agonist and antagonist at the 1 cell excitatory acetylcholine (ACh) receptor (Elliott et al. 1992). The inhibitory postsynaptic potential (IPSP) in the 3 cell is blocked by bath applications of MeXCh and PTMA, which both abolish the response of the 3 cell to ACh (Elliott et. al. 1992). The effects of the cholinergic antagonists on the response of 4 cluster and 5 cells to N1 stimulation matches their response to ACh (Elliott et al. 1992). It is concluded that the population of N1 cells are multiaction, premotor cholinergic interneurons.     

Effects of maturation on synaptic potentials in the locust flight system

We have measured parameters of identified excitatory postsynaptic potentials from flight interneurons in immature and mature adult locusts (Locusta migratoria) to determine whether parameters change during imaginal maturation. The presynaptic cell was the forewing stretch receptor. The postsynaptic cells were flight interneurons that were filled with Lucifer Yellow and identified by their morphology. Excitatory postsynaptic potentials from different postsynaptic cells had characteristic amplitudes. The amplitude, time to peak, duration at half amplitude and the area above the baseline of excitatory postsynaptic potentials did not change with maturation. The latency from action potentials in the forewing stretch receptor to onset of excitatory postsynaptic potentials decreased significantly with maturation. We suggest this was due to an increase in conduction velocity of the forewing stretch receptor. We also measured morphological parameters of the postsynaptic cells and found that they increased in size with maturation. Growth of the postsynaptic cell should cause excitatory postsynaptic potential amplitude to decrease as a result of a decrease in input resistance, however, this was not the case. Excitatory postsynaptic potentials in immature locusts depress more than in mature locusts at high frequencies of presynaptic action potentials. This difference in frequency sensitivity of the immature excitatory postsynaptic potentials may account in part for maturation of the locust flight rhythm generator.Abbreviations EPSP excitatory postsynaptic potential - fSR forewing stretch receptor - IPSP inhibitory postsynaptic potential - SR stretch receptor     

Intrinsic and synaptic properties of neurons in the vocal-control nucleus lMAN from in vitro slice preparations of juvenile and adult zebra finches

A common theme of diverse neural systems is that circuits that are important for initial acquisition of learning do not necessarily serve as a substrate for the long-term storage of that memory. The neural basis of vocal learning in songbirds provides an example of this phenomenon, since a circuit that is necessary for vocal production during initial stages of vocal development apparently plays no subsequent role in controlling learned vocalizations. This striking functional change suggests the possibility of marked physiological changes in synaptic transmission within this circuit. We therefore examined intrinsic and synaptic properties of neurons in the cortical nucleus lMAN (lateral magnocellular nucleus of the anterior neostriatum), which forms part of this developmentally regulated circuit, in an in vitro preparation of the zebra finch forebrain. Although both functional and morphological characteristics of these neurons change substantially during vocal development, we did not observe widespread, substantive changes in the electrophysiological characteristics of juvenile versus adult lMAN neurons examined in vitro. Overall, both the intrinsic properties and synaptic responses of lMAN neurons were similar in slices from juvenile birds (at ages when lesions of lMAN disrupt vocal production) and in slices from adult birds (when lMAN lesions have no effect on song production). However, one intrinsic property that did vary between juvenile and adult cells was spike duration, which was longer in juvenile cells, suggesting the potential for activation of second-messenger cascades and/or enhanced synaptic transmission onto target cells of lMAN neurons. The pattern of synaptic response observed in both juvenile and adult cells suggests that lMAN projection neurons receive direct excitatory afferent inputs, as well as disynaptic inhibitory inputs from interneurons within lMAN. Activation of inhibitory interneurons rapidly curtails the excitatory response seen in projection neurons. This inhibition was abolished by bicuculline, indicating that the inhibitory interneurons normally exert their postsynaptic response via GABA_A receptors on projection neurons. The inhibitory response could also be blocked by CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), suggesting that the activation of inhibitory interneurons within lMAN may be governed primarily by AMPA receptors. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 642–658, 1998