The ventral and dorsal lateral geniculate nucleus of the rat: intracellular recordings in vitro

Phasic stimulation of the locus coeruleus: effects on activity in the lateral geniculate nucleus

Neurons in the locus coeruleus (LC) encode information related to behavioral state in a tonic pattern of firing and information related to the occurrence of a sensory stimulus in a phasic pattern of firing. The effects of phasic stimulation of the LC (6 pulses at 30 Hz), designed to approximate its physiological activation by sensory stimuli, were studied in the lateral geniculate nucleus (LGN) of anesthetized rats. Phasic stimulation of the LC significantly increased neuronal firing in the LGN with a mean latency 320 ms from onset of stimulation. Receiver operating characteristic analyses on a trial-by-trial basis showed that phasic LC stimulation can result in a highly discriminable signal in the LGN. This increased neuronal firing rate in the LGN was specific for the site of stimulation and was reduced by the norepinephrine synthesis inhibitor alpha-methyl-p-tyrosine and by intravenous WB-4101 (alpha 1-receptor antagonist). Neurons in the LGN have a single-spike firing mode when sensory information is faithfully relayed from retina to cortex and a burst-firing mode when the transfer of this information is degraded. Phasic LC stimulation reduced burst firing (2-5 ms interspike intervals, ISIs) at low frequencies (< or = 4 Hz) in the LGN, and for some neurons there was an absolute decrease in burst-like ISIs after LC stimulation, despite an increase in mean firing rate.

Prevention of Ca(2+)-mediated action potentials in GABAergic local circuit neurones of rat thalamus by a transient K+ current

1. Neurones enzymatically dissociated from the rat dorsal lateral geniculate nucleus (LGN) were identified as GABAergic local circuit interneurones and geniculocortical relay cells, based upon quantitative analysis of soma profiles, immunohistochemical detection of GABA or glutamic acid decarboxylase, and basic electrogenic behaviour. 2. During whole-cell current-clamp recording, isolated LGN neurones generated firing patterns resembling those in intact tissue, with the most striking difference relating to the presence in relay cells of a Ca2+ action potential with a low threshold of activation, capable of triggering fast spikes, and the absence of a regenerative Ca2+ response with a low threshold of activation in local circuit cells. 3. Whole-cell voltage-clamp experiments demonstrated that both classes of LGN neurones possess at least two voltage-dependent membrane currents which operate in a range of membrane potentials negative to the threshold for generation of Na(+)-K(+)-mediated spikes: the T-type Ca2+ current (IT) and an A-type K+ current (IA). Taking into account the differences in membrane surface area, the average size of IT was similar in the two types of neurones, and interneurones possessed a slightly larger A-conductance. 4. In local circuit neurones, the ranges of steady-state inactivation and activation of IT and IA were largely overlapping (VH = 81.1 vs. -82.8 mV), both currents activated at around -70 mV, and they rapidly increased in amplitude with further depolarization. In relay cells, the inactivation curve of IT was negatively shifted along the voltage axis by about 20 mV compared with that of IA (Vh = -86.1 vs. -69.2 mV), and the activation threshold for IT (at -80 mV) was 20 mV more negative than that for IA. In interneurones, the activation range of IT was shifted to values more positive than that in relay cells (Vh = -54.9 vs. -64.5 mV), whereas the activation range of IA was more negative (Vh = -25.2 vs. -14.5 mV). 5. Under whole-cell voltage-clamp conditions that allowed the combined activation of Ca2+ and K+ currents, depolarizing voltage steps from -110 mV evoked inward currents resembling IT in relay cells and small outward currents indicative of IA in local circuit neurones. After blockade of IA with 4-aminopyridine (4-AP), the same pulse protocol produced IT in both types of neurones. Under current clamp, 4-AP unmasked a regenerative membrane depolarization with a low threshold of activation capable of triggering fast spikes in local circuit neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

An oriented framework of neuronal processes in the ventral lateral geniculate nucleus of the rat demonstrated by NADPH diaphorase histochemistry and GABA immunocytochemistry

This study investigated the morphology and quantitative distribution of neurons containing NADPH diaphorase activity in the ventral lateral geniculate nucleus of the rat. The pattern of diaphorase staining revealed a strongly reactive lateral subdivision and a weakly staining medial subdivision. A characteristic feature of the diaphorase staining in the lateral part was its "stripe-like" appearance. These "diaphorase stripes" resulted from regions of strong somatic and neuropil diaphorase activity lying between unstained fibre bundles coursing dorsoventrally through the nucleus. Two distinct populations of diaphorase reactive cell types were present--class A and class B neurons. The ratio of class A to class B diaphorase neurons was approximately 14:1 (A:B). Diaphorase reactive neurons made up 73% of the total neuron population in the lateral subdivision, and 31% in the medial subdivision. A third population of cells was found exclusively in the optic tract--class C neurons. Quantitative analyses in the coronal and sagittal planes indicated that the principal processes of both class A and class B neurons were oriented preferentially--either parallel with, or perpendicular to the outlying optic tract. Diaphorase enzyme histochemistry in combination with GABA immunocytochemistry demonstrated the co-localization of GABA immunoreactivity in the majority of class B neurons, whereas class A and class C neurons were GABA immunonegative. Furthermore a large population of GABA-immunoreactive neurons was present that were not stained for diaphorase activity. From this and previous studies, it can be concluded that a high proportion of the diaphorase reaction class A neurons are geniculotectal projection cells, while diaphorase reaction class B neurons represent a numerically small subpopulation of "local-circuit" inhibitory neurons. Since diaphorase activity co-localizes with nitric oxide synthase, the results indicate the likely involvement of nitric oxide in the neuronal operations of both subpopulations of geniculotectal projection neurons and "local-circuit" GABAergic neurons in the rat's ventral lateral geniculate nucleus.

Barium-induced bistability in rat ocular motoneurones in vitro

Intracellular recordings were obtained from adult rat oculomotor neurones in an in vitro brainstem slice preparation. In motoneurones hyperpolarized to levels below -90 mV, depolarizing current injections induced voltage-dependent low-threshold plateau potentials. These potentials were triggered at rest when charge transfer through Ca2+ channels was increased by Ba2+ substitution. In such conditions, the membrane displayed voltage-dependent bistable properties similar to those described in other preparations and shown in vivo to be under neurotransmitter control. These results show that increasing inward currents in adult oculomotor neurones of the rat induce functionally different responses to slight shifts of membrane potential.

The central nucleus of the rat amygdala: in vitro intracellular recordings

Membrane properties of neurons from the central nucleus of the rat amygdala (ACe) were analyzed using intracellular current-clamp recordings from in vitro coronal slices of adult rat amygdala. Two types of neurons were identified and classified according to their accommodation characteristics and the nature of their afterhyperpolarizations (AHP). Type A neurons represented 74% of the population and were identified by a lack of accommodation and a medium-AHP (m-AHP) in response to transient (100 ms) depolarizing current injection. The m-AHP was defined by a fast decay time constant with a mean tau AHP = 113.6 ms. In both Type A and Type B ACe cells the m-AHP can be reduced with cadmium and rubidium. Type B neurons represented 26% of the population and were identified by the presence of accommodation and a long duration slow-AHP (s-AHP) following the m-AHP. The s-AHP was defined by a slow decay time constant with a mean tau AHP = 1.7 s. The s-AHP was similar to the AHP mediated by IAHP, a long duration calcium-dependent, noradrenaline-sensitive current present in hippocampal neurons. In Type B cells, the s-AHP was reduced by cadmium and noradrenaline. There was no significant difference between Type A and B ACe neurons in passive electrical properties such as the membrane input resistance (RiA = 113 M omega, RiB = M omega), and the membrane time constant (tau A = 15 ms, tau B = 16 ms). However, there was a statistically significant difference in the resting membrane potentials of Type A and B ACe neurons (RMPA = -67 mV; RMPB = -63 mV). These data suggest that the characteristic active membrane properties displayed by Type A and Type B neurons will determine the ability of each type to integrate and encode neuronal information.

The inward rectifier K+ current underlies oscillatory membrane potential behaviour in bovine pigmented ciliary epithelial cells

1. Fresh bovine, pigmented ciliary epithelial cells possess an inward rectifier current activated by hyperpolarization. This current was investigated using whole-cell patch-clamp techniques. At the holding potential of -70 mV, and with EK (potassium equilibrium potential) set at -84 mV, a small outward current flowed through the inward rectifier that was sensitive to external K+, becoming more outward in 0.5 mM K+ and progressively more inward in 20 and 50 mM K+. 2. The inward rectifier showed V-EK dependence; increasing [K+]o increased the inward conductance from 1.28 nS in 5 mM K+ to 7.42 nS in 50 mM K+. The conductance at a given V-EK was proportional to the square root of [K+]o. 3. It was blocked by external Cs+ but replacing K+ in the pipette with Cs+ blocked only outward ion movement through the inward rectifier. Inward rectification was also blocked by Ba2+ (85% with Ki (concentration giving half-maximal inhibition) = 3.1 x 10(-5) M) and TEA+ (74% with Ki = 2.9 x 10(-4) M). 4. The activation time constant was voltage dependent, decreasing from 5 ms to 0.7 ms over the voltage range -90 to -170 mV. With increasing hyperpolarization the current exhibited time-dependent decay. The time constant for this process was voltage sensitive but the steady-state inactivation was independent of external [K+]. 5. The current disappeared in culture within 8 days. 6. Solution flow over the cell inactivated the inward rectifier, a property that may be related to [K+]o. 7. In current clamp the cells exhibited an unstable region at a potential of around -70 mV. Once in this region oscillations and regenerative hyperpolarizing potentials were observed. This behaviour was eliminated by treatments that blocked (Cs+, Ba2+) or removed (0.5 mM K+) active inward rectification. 8. It is suggested that these oscillations may represent a process of cation loading, the first step in the secretion of aqueous humour.

A role for low-frequency, rhythmic synaptic potentials in the synchronization of cat thalamocortical cells

1. Low-frequency, rhythmic synaptic potentials and their ability to evoke and modulate membrane potential oscillations in thalamocortical (TC) cells of the cat dorsal lateral geniculate nucleus (dLGN) were investigated using intracellular recordings in a brain slice preparation. Three types of rhythmic synaptic potentials were distinguished: EPSPs, IPSPs and 'complex synaptic potentials' consisting of an IPSP followed by an EPSP. 2. The frequency of all three types of synaptic potentials was insensitive to changes in the membrane potential. At potentials positive to -50 mV, the EPSPs and the complex potentials gave rise to action potentials, while between -65 and -80 mV all three types of synaptic potential evoked low-threshold Ca2+ potentials. TC cells which displayed rhythmic synaptic potentials were either cells that showed spontaneous pacemaker oscillation or cells that were brought to oscillate by the rhythmic EPSPs or depolarizing (i.e. reversed IPSPs. 3. The low-frequency (1.9 +/- 0.2 Hz), rhythmic EPSPs were observed in 23 (out of 192) cells, were abolished by tetrodotoxin (TTX; n = 4) and by the combined application of DL-2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione (n = 3), and were insensitive to bicuculline (n = 4). Paired intracellular recordings (n = 32) demonstrated the presence of simultaneously occurring EPSPs in a pair of cells situated 75 microns apart. 4. The low-frequency (2.2 +/- 0.3 Hz), rhythmic IPSPs were observed in 5 (out of 192) cells, were blocked by bicuculline (n = 3), and reversed in polarity at -65 mV. The low-frequency (1.3 +/- 0.3 Hz), rhythmic 'complex potentials' were observed in 5 (out of 192) cells and were abolished by TTX (n = 2). 5. Intracellular depolarizing current pulses delivered at different phases of the pacemaker oscillations revealed the existence of two different types of phase resetting. Furthermore, a current pulse of critical amplitude and duration applied at a specific phase of the cycle abolished the pacemaker oscillations. 6. These results indicate that the low-frequency, rhythmic synaptic potentials recorded in TC cells of the dLGN (i) originate from other TC cells that are in the pacemaker oscillating mode, (ii) are capable of driving other TC cells to oscillate rhythmically, or of modulating the frequency of pacemaker oscillations, and (iii) provide a means by which oscillatory activities of TC cells can be synchronized in the absence of sensory, cortical and reticular thalamic inputs.

Modulation of visually evoked responses in units of the ventral lateral geniculate nucleus of the rat by somatic stimuli

Single unit activity was recorded from the ventral part of the lateral geniculate nucleus (vLGN) in rats anaesthetized with urethane. Most of the cells located laterally in the nucleus were excited by light. The studied vLGN neurones did not respond to electrical stimulation of the tail, but about half of them changed their response to light significantly when the light flash was paired with the electrical stimulation. When the tail stimulus preceded the light, the changes consisted in a pronounced facilitation of flash-evoked activity. When the electrical stimulus was applied after the flash in a forward conditioning paradigm, facilitations were less pronounced and responses of some neurones were suppressed. These results are in contrast to those of similar experiments on the dorsal LGN, neurones of which were mainly facilitated by the conditioning paradigm. Thus, light-evoked activity of ventral geniculate cells can be enhanced by arousal-related processes.

Immunohistochemical changes of neuronal calcium-binding proteins parvalbumin and calbindin-D-28k following unilateral deafferentation in the rat visual system

The neuron-specific calcium-binding proteins, parvalbumin and calbindin-D-28k, were studied in the subcortical visual system of normal and unilaterally deafferented albino rats. Immunohistochemistry with monoclonal antibodies was used on vibratome sections through optic tract (OT), dorsal lateral geniculate nucleus (dLGN), olivary pretectal nucleus (OPN), and superior colliculus (SC). In controls, OT stained strongly for parvalbumin and weakly for calbindin-D-28k. The dLGN contained a plexus of parvalbumin-positive fibers. In dLGN, calbindin-D-28k-antibodies showed strong labeling of some neurons with long dendrites and weak staining of the cytoplasm in other neurons. In OPN, parvalbumin stained a ring of neurons and terminals in the shell region, whereas calbindin-D-28k was contained in medial cell populations. In SC, parvalbumin was contained in fibers, terminals, and neurons throughout the visual layer. Calbindin-D-28k showed a laminar distribution of neurons with a predominance in deep portions of superficial grey matter and in ventral portions of stratum opticum. Following unilateral deafferentation induced by optic nerve section, retinal axons showed immunohistochemical changes related to Wallerian degeneration and target neurons reacted by changes of calcium-binding proteins. Parvalbumin and calbindin-D-28k immunostaining decreased during Wallerian degeneration of OT. In the deafferented dLGN, immunohistochemical labeling for calbindin-D-28k declined in strongly stained neurons from 4 to 21 days after lesion. Measurement of dendritic length per number of cells or per area of dLGN showed a significant decline for the contralateral side at 4, 8, and 21 days (ANOVA, P less than 0.05). In deafferented OPN, terminal-like staining for parvalbumin decreased and neuronal labeling was enhanced. In deafferented SC, the neuronal and dendritic staining for parvalbumin increased beginning from Day 1 on and persisting at Day 21, whereas fibers and terminal-like elements decreased in staining. Measurement of parvalbumin-positive neurons per area of SC showed a significant increase of labeling in the contralateral side from Day 1 to Day 21 (ANOVA, P less than 0.05). These studies show that cellular responses to deafferentation of visual neurons involve a regulation of calcium-binding proteins. The decline in staining for calbindin-D-28k in dLGN may relate to reduced retinal afferent activity. The progressive cellular changes in parvalbumin staining may be related to unmasking of intrinsic neurons after removal of parvalbumin-containing, afferent fibers and terminals. Additionally, the changes of parvalbumin labeling in SC neurons may reflect a plastic reorganization of local circuits known to occur in rat SC in response to deafferentation.

Computer model of ethosuximide's effect on a thalamic neuron

Ethosuximide appears to have a specific effect on the low-threshold calcium current in thalamic cells. This may be related to its efficacy in the treatment of absence epilepsy. We used a computer model of an individual thalamocortical neuron to better understand the alteration in the low-threshold calcium current under voltage clamp and to predict response to current injection in the presence of ethosuximide. The full model included nine voltage-sensitive ionic channels and a realistic dendritic morphology. The model reproduced the two major responses seen in tissue slices: repetitive spiking with depolarization and the low-threshold calcium spike elicited on release from hyperpolarization. The alteration in low-threshold calcium current with ethosuximide can be explained by a 10-mV depolarizing shift in the steady-state activation curve for this channel with a 10% reduction in maximum channel permeability. Simulations of current injection showed that ethosuximide diminished the low-threshold calcium spike while leaving the tonic firing pattern unaffected. Our results support the hypothesis that ethosuximide's effects on low-threshold calcium current might selectively alter the dynamics of slow bursting in thalamic cells.

Detecting changes in neuronal activities induced by N-methyl-D-aspartate receptor blockade using non-linear dynamics techniques

The dynamics of N-methyl-D-aspartate receptor blockade-induced transitions between two types of intracellularly recorded spontaneous membrane potential oscillation from cat thalamic neurons have been studied using non-linear dynamics techniques. We report that, as previously predicted by theoretical studies, the number of degrees of freedom of these oscillations (the minimal number of independent variables governing the activity) is small, i.e. they are low dimensional. The N-methyl-D-aspartate receptor antagonists DL-2-amino-5-phosphono-valeric acid and ketamine, which transformed one type of oscillation into another, decreased the calculated dimension. DL-2-Amino-5-phosphono-valeric acid had no effect on the dimension when Mg2+ was present in the perfusion medium. The decrease in dimension was gradual and its time-course had a sigmoidal shape. It is suggested that the application of the machinery of dynamical systems theory might help to detect and monitor drug-induced membrane potential state transitions and to identify the factors underlying membrane potential oscillations.

Intracellular recordings from rat thalamic VL neurons: a study combined with intracellular staining

Intracellular recordings from thalamic neurons receiving cerebellar inputs were performed under urethane anesthesia in the rat. A total of 64 neurons were recorded intracellularly with micropipettes filled with 4% biocytin solution (dissolved in 0.5 M K-acetate), and cerebellar-induced EPSPs (CN-EPSPs), the membrane resistance and firing properties were analyzed with intracellular current injections. The mean latency of CN-EPSPs was 1.9 +/- 0.8 ms and the mean input resistance measured in 10 neurons was 17.6 +/- 5.0 M omega. Thirty-two out of 35 stained neurons were analysed morphologically; 28 of these neurons were located in the VL, and 26 received CN-EPSPs. Their somata were round or polygonal in shape and the mean size was 22.5 x 15.2 microns. They had radially extending spinous dendrites, and the mean radii of the dendritic fields were 214.7 microns in the frontal and 171.4 microns in the sagittal planes. These morphological features were similar to those observed in the sensory relay nucleus of the thalamus.

Low-frequency oscillatory activities intrinsic to rat and cat thalamocortical cells

1. Low-frequency membrane potential oscillations recorded intracellularly from thalamocortical (TC) cells of the rat and cat dorsal lateral geniculate nucleus (dLGN) and of the rat ventrobasal nucleus (VB) maintained in vitro were investigated. On the basis of their electrophysiological and pharmacological properties, four types of activity were distinguished and named: the pacemaker oscillations, the spindle-like oscillations, the 'very slow' oscillations and the 'N-methyl-D-aspartate' (NMDA) oscillations. 2. The pacemaker oscillations (95 out of 173 cells) consisted of rhythmic, large-amplitude (10-30 mV) depolarizations which occurred at a frequency of 1.8 +/- 0.3 Hz (range, 0.5-2.9 Hz) and could often give rise to single or a burst of action potentials. Pacemaker oscillations were observed when the membrane potential was moved negative to -55 and positive to -80 mV, but in a given cell the upper and lower limits of this voltage range were separated by only 13.1 +/- 0.5 mV. Above -45 mV tonic firing consisting of single action potentials was seen in the cells showing this or the other types of low-frequency oscillations. 3. The spindle-like oscillations were observed in thirty-nine (out of 173) TC cells and consisted of rhythmic (2.1 +/- 0.3 Hz), large-amplitude depolarizations (and often associated burst firing) similar to the pacemaker oscillations but occurring in discrete periods every 5-25 s and lasting for 1.5-28 s. The spindle-like oscillations were observed when the membrane potential was moved negative to -55 and positive to -80 mV and in two cells they were transformed into continuous pacemaker oscillations by depolarization of the membrane potential to -60 mV. 4. Pacemaker and spindle-like oscillations were unaffected by tetrodotoxin (TTX) or by selective blockade of NMDA, non-NMDA, GABAA, GABAB, nicotinic, muscarinic, alpha- and beta-noradrenergic receptors. 5. The 'very slow' oscillations consisted of a TTX-insensitive, slow hyperpolarization-depolarization sequence (5-15 mV in amplitude) which lasted up to 90 s and was observed in nine dLGN cells and in two VB cells. The pacemaker and the spindle-like oscillations were recorded in one cell each which also showed the 'very slow' oscillations. 6. The 'NMDA' oscillations were observed only in a 'Mg(2+)-free' medium (0 mM-Mg2+, 2-4 mM-Ca2+; 64 out of 72 cells) and consisted of large-amplitude (10-25 mV) depolarizations that did not occur at regular intervals and were intermixed with smaller depolarizations present on the baseline and on the falling phase of the larger ones.(ABSTRACT TRUNCATED AT 400 WORDS)

Two inward currents and the transformation of low-frequency oscillations of rat and cat thalamocortical cells

1. The contribution of a slow, mixed Na(+)-K+, inward rectifying current (Ih) and the T-type Ca2+ current (IT) (that underlies low-threshold Ca2+ potentials) to the low-frequency oscillations observed in rat and cat thalamocortical (TC) cells in vitro was studied using current clamp and single-electrode voltage clamp recordings. 2. From a holding potential of -50 mV, voltage steps negative to -60 mV showed the presence of a slow, non-inactivating inward current, Ih. This current was unaffected by Ba2+ (1-4 mM), tetrodotoxin (0.5-1 microM) and TEA (20 mM, n = 6), reversibly blocked by Cs+ (1-3 mM), and its reversal potential (-33.0 +/- 1.2 mV) followed changes in the extracellular Na+ and K+, but not Cl-, concentration. 3. Application of Cs+ (1-3 mM) abolished the pacemaker oscillations (n = 9), while in six cells that did not show any oscillatory activity Cs+ first evoked the spindle-like oscillations that, in the continuous presence of these ions, were then transformed into the pacemaker oscillations before all activities were finally blocked: all these effects were accompanied by a hyperpolarization and a progressive decrease and final blockade of Ih. Cs+ had no effect on the 'N-methyl-D-aspartate' (NMDA) oscillations (n = 5) and Ba2+ (2 mM, n = 8) did not block the pacemaker, the spindle-like and the 'NMDA' oscillations. 4. In ten cells that showed the pacemaker oscillations selective activation of beta-adrenoceptors by 10-50 microM-noradrenaline (in the presence of alpha-noradrenergic antagonists) or by 20 microM-isoprenaline first transformed the pacemaker oscillations into the spindle-like oscillations that, in the continuous activation of beta-receptors, were finally abolished: all these effects were accompanied by a depolarization and a progressive increase of Ih. 5. In TC cells that showed the pacemaker oscillations application of 1-octanol (50-100 microM), an antagonist of T-type Ca2+ currents, reversibly blocked this activity but concomitantly decreased (50%) the cell input resistance (n = 5). Application of Ni2+ (0.2-0.5 mM, n = 13), another antagonist of IT reversibly blocked the pacemaker, the spindle-like and the 'NMDA' oscillations. 7. In cells showing the pacemaker oscillations it was found that the current developing from the most hyperpolarized potential of an oscillation cycle was an inward relaxation whose time course differed from that of Ih evoked at the same potential.(ABSTRACT TRUNCATED AT 400 WORDS)

A role for GABAB receptors in excitation and inhibition of thalamocortical cells

Gamma-aminobutyric acid (GABA) in the thalamus has mainly been associated with the inhibitory modulation of the sensory and cortical flow of information via a 'classical', chloride-dependent, GABAA receptor-mediated action. However, the discovery of a late, long-lasting potassium-dependent inhibitory postsynaptic potential (IPSP) mediated by GABAB receptors present on thalamocortical cells, has allowed new insights into our understanding of the physiological role of this neurotransmitter. In particular, work on the dorsal lateral geniculate nucleus indicates that together with a relatively weak inhibition, GABAB receptor-mediated IPSPs 'prepare' thalamocortical cells for burst firing by activating low-threshold calcium potentials. Thus, GABA in the thalamus can no longer be viewed only as a 'classical' inhibitory transmitter but also as a neuromodulator with a 'priming' role for burst firing excitation. This dual role of GABAB receptors in inhibition and excitation of thalamocortical cells might allow different interpretations of earlier findings in animals and humans, both in healthy and pathological conditions. It will also help to identify new functions for postsynaptic GABAB receptors in other parts of the central nervous system.

Properties of a hyperpolarization-activated cation current and its role in rhythmic oscillation in thalamic relay neurones

1. The physiological and functional features of time-dependent anomalous rectification activated by hyperpolarization and the current which underlies it, Ih, were examined in guinea-pig and cat thalamocortical relay neurones using in vitro intracellular recording techniques in thalamic slices. 2. Hyperpolarization of the membrane from rest with a constant-current pulse resulted in time-dependent rectification, expressed as a depolarizing sag of the membrane potential back towards rest. Under voltage clamp conditions, hyperpolarizing steps to membrane potentials negative to approximately -60 mV were associated with the activation of a slow inward current, Ih, which showed no inactivation with time. 3. The activation curve of the conductance underlying Ih was obtained through analysis of tail currents and ranged from -60 to -90 mV, with half-activation occurring at -75 mV. The time course of activation of Ih was well fitted by a single-exponential function and was strongly voltage dependent, with time constants ranging from greater than 1-2 s at threshold to an average of 229 ms at -95 mV. The time course of de-activation was also described by a single-exponential function, was voltage dependent, and the time constant ranged from an average of 1000 ms at -80 mV to 347 ms at -55 mV. 4. Raising [K+]o from 2.5 to 7.5 mM enhanced, while decreasing [Na+]o from 153 to 26 mM reduced, the amplitude of Ih. In addition, reduction of [Na+]o slowed the rate of Ih activation. These results indicate that Ih is carried by both Na+ and K+ ions, which is consistent with the extrapolated reversal potential of -43 mV. Replacement of Cl- in the bathing medium with isethionate shifted the chloride equilibrium potential positive by approximately 30-70 mV, evoked an inward shift of the holding current at -50 mV, and resulted in a marked reduction of instantaneous currents as well as Ih, suggesting a non-specific blocking action of impermeable anions. 5. Local (2-10 mM in micropipette) or bath (1-2 mM) applications of Cs+ abolished Ih over the whole voltage range tested (-60 to -110 mV), with no consistent effects on instantaneous currents. Barium (1 mM, local; 0.3-0.5 mM, bath) evoked a steady inward current, reduced the amplitude of instantaneous currents, and had only weak suppressive effects on Ih. 6. Block of Ih with local application of Cs+ resulted in a hyperpolarization of the membrane from the resting level, a decrease in apparent membrane conductance, and a block of the slow after-hyperpolarization that appears upon termination of depolarizing membrane responses, indicating that Ih contributes substantially to the resting and active membrane properties of thalamocortical relay neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Pacemaker-like and other types of spontaneous membrane potential oscillations of thalamocortical cells

During EEG-synchronized sleep, thalamic activity is characterized by rhythmic oscillations that till recently have been suggested to require the contribution of intra- and extra-thalamic inputs. The present experiments show that thalamocortical (TC) cells, mechanically and pharmacologically isolated from their intra-thalamic, cortical and brainstem inputs, are capable of different types of spontaneous membrane potential oscillations some of which resemble those observed in TC cells of the living animal during EEG-synchronization.

Distribution and synaptic organization of serotoninergic and noradrenergic axons in the lateral geniculate nucleus of the rat

Antisera raised against the monoamines serotonin (5-HT) and noradrenaline (NA) were employed in a study designed to provide a detailed description of the distribution, morphology, and synaptic organization of the serotoninergic and noradrenergic afferents in the lateral geniculate nucleus (LGN) of the rat. The distribution patterns of the two types of immunoreactive fibers were distinct and largely complementary to each other. NA axons were particularly concentrated in the dorsal lateral geniculate nucleus (LGd), with the ventral lateral geniculate nucleus (LGv) and the intergeniculate leaflet (IGL) receiving substantially fewer fibers. In contrast, 5-HT axons, although present throughout the LGN, were preferentially concentrated in the LGv and IGL. 5-HT and NA axon terminals and axonal varicosities, examined in single and serial ultrathin sections, formed conventional synapses in the extraglomerular neuropil. The types of synapses and the nature of the postsynaptic targets were different for the two monoamines. 5-HT afferents formed asymmetrical synapses on dendritic spines and shafts of both presumptive relay cells and interneurons but established symmetrical synapses on cell bodies. However, NA afferents formed almost exclusively symmetrical synapses on dendritic spines and shafts and made no contacts with cell bodies. The present findings suggest that the 5-HT and NA afferents of the rat LGN, which are likely to influence certain stages of visual processing, exhibit distinct organizational principles and act at restricted sites as do other classical neurotransmitter systems.

Postnatal development of the T calcium current in cat thalamocortical cells

The burst firing of thalamic cells in the adult cat is mainly controlled by a voltage-dependent membrane current that has recently been characterized as being similar to the low voltage-activated (i.e. low threshold), T-type, Ca2+ current originally described in sensory neurons. In those neurons so far studied, as well as in skeletal muscle, the low threshold Ca2+ current has been shown to decrease in amplitude or even disappear during embryogenesis or the first few weeks of postnatal life. We have now investigated the in vivo postnatal development of the low threshold Ca2+ current present in thalamocortical cells of the dorsal lateral geniculate nucleus of cats aged four to 100 days. The results show that the amplitude of the low threshold Ca2+ current triples from 0.5 nA in the first few days after birth to over 1.5 nA in the adult. However, this increase in amplitude is not accompanied by any change in its inactivation and activation properties, its latency to peak and the time dependence of inactivation removal. Because of these similarities during development and in adulthood it is likely that a major role of the low threshold Ca2+ current during neuronal development is the generation of oscillatory activities similar to those observed in adult thalamocortical cells.

The assembly of ionic currents in a thalamic neuron. II. The stability and state diagrams

In the previous model of a thalamic neuron (R.M. Rose & J.L. Hindmarsh, Proc. R. Soc. Lond. B237, 267-288 (1989], which we referred to as the z-model, the burst response was terminated by the slow activation of a subthreshold outward current. In this paper we show that similar results can be obtained if the burst response is terminated by slow inactivation of the subthreshold inward current, Isa. We illustrate the use of this new model, which we refer to as the ha-model, by using it to explain the response of a thalamic neuron to a double ramp current. The main aim of the paper is to show how the stability and state diagrams introduced previously can be used to explain various types of firing pattern of thalamic and other neurons. We show that increasing the threshold for the fast action potentials leads to low threshold spikes of increased amplitude. Also, addition of a second subthreshold inward current adds a new stability region, which enables us to explain the origin of plateau potentials. In addition, various types of subthreshold oscillation are produced by relocating a previously stable equilibrium point in an unstable region. Finally, we predict a sequence of responses to current steps from different levels of background current that extends the burst, rest, tonic sequence of thalamic neurons. The stability and state diagrams therefore provide us with a useful way of explaining further properties of thalamic neurons and appear to have further applications to other mammalian neurons.

Calcium currents in rat thalamocortical relay neurones: kinetic properties of the transient, low-threshold current

1. Calcium currents were recorded with whole-cell voltage-clamp procedures in relay neurones of the rat thalamus which had been acutely isolated by an enzymatic dissociation procedure. 2. Low-threshold and high-threshold Ca2+ currents were elicited by depolarizing voltage steps from holding potentials more negative than -60 mV. A transient current, analogous to the T-current in sensory neurones, was activated at low threshold near -65 mV and was completely inactivating at command steps up to -35 mV. Voltage steps to more depolarized levels activated a high-threshold current that inactivated slowly and incompletely during a 200 ms step depolarization. 3. The high-threshold current contained both non-inactivating and slowly inactivating components which were insensitive and sensitive to holding potential, respectively. 4. A 'T-type' current was prominent in relay neurones, in both absolute terms (350 pA peak current average) and in relation to high-threshold currents. The average ratio of maximum transient to maximum sustained current was greater than 2. 5. T-current could be modelled in a manner analogous to that employed for the fast Na+ current underlying action potential generation, using the m3h format. The rate of activation of T-current was voltage dependent, with a time constant (tau m) varying between 8 and 2 ms at command potentials of -60 to -10 mV at 23 degrees C. The rate of inactivation was also voltage dependent, and the time constant tau h varied between 50 and 20 ms over the same voltage range. With command potentials more positive than -35 mV, the inactivation of Ca2+ current could no longer be fitted by a single exponential. 6. Steady-state inactivation of T-current could be well fitted by a Boltzman equation with slope factor of 6.3 and half-inactivated voltage of -83.5 mV. 7. Recovery from inactivation of T-current was not exponential. The major component of recovery (70-80% of total) was not very voltage sensitive at potentials more negative than -90 mV, with tau r of 251 ms at -92 mV and 23 degrees C, compared to 225 ms at -112 mV. A smaller, voltage-sensitive component accounted for the remainder of recovery. 8. All kinetic properties, including rates of activation, inactivation, and recovery from inactivation, as well as the amplitude of T-current, were temperature sensitive with Q10 (temperature coefficient) values of greater than 2.5.(ABSTRACT TRUNCATED AT 400 WORDS)

A T-type Ca2+ current underlies low-threshold Ca2+ potentials in cells of the cat and rat lateral geniculate nucleus

1. The characteristics of a transient inward Ca2+ current (IT) underlying low-threshold Ca2+ potentials were studied in projection cells of the cat and rat dorsal lateral geniculate nucleus (LGN) in vitro using the single-electrode voltage-clamp technique. 2. In cat LGN slices perfused at 25 degrees C with a solution which included 1 mM-Ca2+ and 3 mM-Mg2+, IT could be evoked by depolarizing voltage steps to -55 mV from a holding potential (Vh) of -95 mV and was abolished by reducing [Ca2+]o from 1 to 0.1 mM. IT was also blocked by 8 mM-Mg2+ and 500 microM-Ni2+, but 500 microM-Cd2+ was a significantly less effective antagonist. 3. The inactivation of IT, which occurred at Vh positive to -65 mV, was removed as Vh approached -100 mV. The process of inactivation removal was also time dependent, with 800-1000 ms needed for total removal. Activation curves for IT showed a threshold of -70 mV and illustrated that IT was extremely voltage sensitive over the voltage range from -65 to -55 mV. 4. The decay phase of IT followed a single-exponential time course with a time constant of decay which was voltage sensitive and ranged from 20 to 100 ms. The mean peak conductance increase associated with IT was 8.4 nS (+/-0.9, S.E.M.). 5. In more 'physiological' conditions (35 degrees C and 1.5 mM-Ca2+, 1 mM-Mg2+) the voltage dependence of activation and inactivation were unaffected. However, the development and decay of IT proceeded more rapidly and only 500-600 ms were needed for total removal of inactivation. Under these conditions, the use of voltage ramps showed that depolarization rates of greater than 30 mV/s were necessary for IT activation. 6. The use of multiple voltage-step protocols illustrated that the process of inactivation removal was rapidly reversed by brief returns to a Vh of -50 mV. Furthermore, any delay in IT activation, once the LGN cell membrane potential was in the IT activation range, resulted in a current of reduced amplitude. 7. Although IT in rat LGN cells was briefer and had a shorter latency to peak, it was otherwise similar to that seen in cat LGN cells. 8. The characteristics of IT are very similar to those of the T-type Ca2+ currents of other excitable membranes. The properties of IT are discussed with respect to its role in generating the low-threshold Ca2+ potentials which are central to the oscillatory behaviour of thalamic projection cells.

Burst discharges of thalamic reticular neurons: an intracellular analysis in anesthetized rats

In order to analyze the mechanism of burst discharges intracellular recordings were made from 27 somatosensory thalamic reticular (S-TR) neurons in urethane-anesthetized rats. Burst discharges, composed of 2-7 spikes, were always superposed on a slow depolarization (SD) lasting for 40-60 ms, which appeared only when the membrane was hyperpolarized. The number of spikes superposed on an SD varied depending upon the amplitude of the SD. A single shock stimulation of the lemniscus medialis elicited a series of SDs, each without being preceded by a phasic hyperpolarizing potential. The SDs were repeated with spindle rhythms. Evidence has been provided that EPSPs contribute to the mechanism for triggering SDs. In spontaneous rhythmic SDs occurring with the rhythm of EEG spindles, steps representing EPSPs were recordable on the rising phase of each SD. It is suggested that excitatory synaptic inputs to S-TR neurons with the spindle rhythm are responsible for the rhythmic generation of SDs. Ventrobasal relay neurons are presumed as the source of the inputs.

Optic tract stimulation evokes GABAA but not GABAB IPSPs in the rat ventral lateral geniculate nucleus

The inhibitory postsynaptic potentials (IPSPs) evoked in neurons of the rat ventral geniculate nucleus (vLGN) by electrical stimulation of the optic tract and the action of GABA and baclofen on the same cells were studied using intracellular recording technique in an in vitro slice preparation. A short latency short duration IPSP always followed the monosynaptic excitatory postsynaptic potential (EPSP). This IPSP reversed in polarity at about -65 mV and was reversibly blocked by bicuculline (50 microM) thus indicating that it represents a GABAA receptor-mediated IPSP. No long-lasting IPSP was evoked in vLGN cells by stimulation of the optic tract, while in the same slice, long-lasting GABAB IPSPs were routinely recorded in the dorsal lateral geniculate nucleus. GABA applied by ionophoresis evoked a hyperpolarization that had a reversal potential close to -70 mV and was antagonized by bicuculline. Baclofen hyperpolarized vLGN neurons and its action was reversibly blocked by the selective GABAB antagonist phaclofen (1 mM). In the presence of bicuculline GABA also produced a hyperpolarization that had properties similar to that evoked by baclofen. These results indicate that, although functional GABAA and GABAB receptors are present on vLGN neurons, stimulation of the optic tract evokes only GABAA but not GABAB mediated IPSPs. The lack of long-lasting GABAB IPSPs could explain the absence of long-lasting inhibition observed in vLGN neurons in vivo following stimulation of the optic tract.

On the properties and origin of the GABAB inhibitory postsynaptic potential recorded in morphologically identified projection cells of the cat dorsal lateral geniculate nucleus

Intracellular recordings were performed from projection cells of the cat dorsal lateral geniculate nucleus in vitro to investigate the properties and origin of optic tract evoked inhibitory postsynaptic potentials mediated by GABAB receptors and their relationship to the physiologically different cell classes present in this nucleus. In all three main laminae of the dorsal lateral geniculate nucleus, stimulation of the optic tract evoked an excitatory postsynaptic potential followed by two inhibitory postsynaptic potentials. The first is a GABAA receptor mediated inhibitory postsynaptic potential since it was blocked by bicuculline, reversed in polarity following intracellular Cl- injection and had a reversal potential similar to the bicuculline sensitive hyperpolarizing effect of GABA. The second is a GABAB receptor mediated inhibitory postsynaptic potential. Its amplitude was not linearly related to membrane potential (maximal amplitude at -60 mV), it decreased when using frequencies of stimulation higher than 0.05 Hz and it was reversibly increased by addition of bicuculline to the perfusion medium. The reversal potential of GABAB inhibitory postsynaptic potentials was dependent on the extracellular K+ concentration but did not change in the presence of bicuculline or when recording with Cl- filled microelectrodes. While GABAA inhibitory postsynaptic potentials always abolished repetitive firing of projection cells, GABAB inhibitory postsynaptic potentials were able to block weak firing but unable to decrease strong activation of projection cells evoked by direct current injection. Optic tract evoked GABAB (as well as GABAA) inhibitory postsynaptic potentials could be recorded in slices which did not include the perigeniculate nucleus, thus indicating that they are generated by the interneurons of the dorsal lateral geniculate nucleus. Using intracellular injection of horseradish peroxidase, we have found that the GABAB inhibitory postsynaptic potentials are present in projection cells showing many different types of neuronal morphologies. In conclusion, GABA released from interneurons in the dorsal lateral geniculate nucleus is capable of evoking an early, short-lasting GABAA and a late, long-lasting GABAB inhibitory postsynaptic potential in projection cells with diverse morphology, indicating that the late inhibition in the dorsal lateral geniculate nucleus can no longer be associated exclusively with the recurrent inhibitory pathway through the perigeniculate nucleus.

The GABAB antagonist phaclofen inhibits the late K+-dependent IPSP in cat and rat thalamic and hippocampal neurones

Phaclofen (0.5-1 mM) reversibly inhibited the late, bicuculline resistant, K+ dependent IPSP recorded in projection cells of the cat and rat dorsal lateral geniculate nucleus and in rat hippocampal CA1 pyramidal neurones. At the same concentrations, phaclofen reversibly blocked the K+ dependent, bicuculline insensitive hyperpolarization evoked by GABA and baclofen but had no effect on the GABAA IPSP. These results represent conclusive evidence that GABAB receptors mediate the late K+ dependent IPSP in cortical and subcortical neurones.

Cl- - and K+-dependent inhibitory postsynaptic potentials evoked by interneurones of the rat lateral geniculate nucleus

1. Hyperpolarizing potentials evoked by electrical stimulation of the optic tract were studied in projection cells of the rat dorsal lateral geniculate nucleus (LGN) in vitro. In the same cells the effects of gamma-amino butyric acid (GABA), baclofen and acetylcholine (ACh) were also investigated. 2. In the majority of cells a short- (SHP) (34 ms) and a long-lasting (LHP) (240 ms) hyperpolarizing potential could be recorded in the presence and in the absence of a preceding EPSP. They were blocked by tetrodotoxin (1 microM) and were more sensitive than the monosynaptic EPSP to a low-Ca2+-high-Mg2+ solution. 3. The SHP was associated with a marked decrease (75%) in input resistance, was blocked by bicuculline (1-100 microM) and its reversal potential (-67 mV) was dependent on the extracellular Cl- concentration. 4. The LHP was associated with a smaller decrease (45%) in input resistance and its reversal potential (-76 mV) was dependent on the extracellular K+ concentration. It was increased by bicuculline (100% at 50 microM) and nipecotic acid (30% at 10 microM), blocked by Ba2+ (1 mM), and unaffected by eserine (1-10 microM), neostigmine (1-10 microM) or by recording with EGTA-filled electrodes. In the presence of bicuculline, a single LHP was able to evoke, as a rebound response, a low-threshold Ca2+ spike that was, however, not followed by another LHP (or any other long-lasting hyperpolarization). 5. Ionophoretic applications of GABA evoked in the same cell a Cl- -dependent hyperpolarization (reversal potential: -65 mV) and/or depolarization, both of which were associated with a marked decrease (91%) in input resistance and abolished by bicuculline. GABA was also able to evoke a bicuculline-insensitive, K+-dependent hyperpolarization that had a reversal potential of -75 mV and was associated with a smaller decrease (43%) in input resistance. 6. Baclofen, applied by ionophoresis, pressure ejection or in the perfusion medium (1-100 microM), produced a hyperpolarization that had a reversal potential of -79 mV and was associated with a decrease (45%) in input resistance. 7. In the majority of cells (thirty-seven out of forty) ACh evoked a slow depolarization and only in three cells a hyperpolarization which had a reversal potential of -80 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

The action of the corticofugal pathway on sensory thalamic nuclei: a hypothesis

The N-methyl-D-aspartate receptor has recently attracted great interest due to its nonlinear current-voltage behavior. In order to evoke a large depolarizing postsynaptic current, the synaptic-induced conductance change must be paired with a postsynaptic depolarization. This temporally tuned AND gate could underlie a number of different operations throughout the nervous system. We propose that the synapses made by the optical nerve onto projection cells in the mammalian dorsal lateral geniculate nucleus are of the N-methyl-D-aspartate type. [In this Commentary, we have pooled data regarding sensory thalamic nuclei from a number of different mammalian species. Unless otherwise mentioned, we have referred to the dorsal division of the cat lateral geniculate nucleus.] About half of all synapses in these cells--located almost exclusively in the peripheral two-thirds of the dendritic tree--are associated with axons originating in layer VI of visual cortex. It then follows that the massive corticogeniculate pathway controls the gain of the retinogeniculate pathway via its action on the N-methyl-D-aspartate receptors. Thus, near-simultaneous activation of the retinal and the cortical input will transiently enhance the geniculate cell response. Generalizing to other thalamic sensory nuclei, afferent information will be routed through the thalamus and on to the cortex as long as cortical activity is congruent with sensory input to the thalamus. Experimental evidence argues for such a mechanism to control the gain of the somatosensory input to the ventrobasal thalamic nucleus.

Low-threshold calcium spikes in hypothalamic neurons recorded near the paraventricular nucleus in vitro

From guinea-pig hypothalamic slices, intracellular studies demonstrate the existence of neurons responding to depolarizing current pulses by bursts of fast spikes riding on slow depolarizing potentials, when activated at the resting potential or from hyperpolarized levels (44 cells). Slow depolarizing potentials have a mean amplitude of 17.6 mV and a mean duration of 65.2 msec. They are also produced at the termination of hyperpolarizing current pulses. The ionic basis for these slow potentials have been investigated. Fast spikes constituting the burst discharge are blocked by TTX but the slow component is unaffected, being blocked by Co++ and enhanced by Ba++. Taken together, these results show that the slow depolarizing potentials are generated by a low-threshold Ca++ conductance which is de-inactivated by membrane hyperpolarization. When the neurons are spontaneously active, they exhibit bursts arising from slow depolarizing potentials reminiscent of those evoked by direct stimulation. They also show longer episodes of repetitive firing. Twelve neurons were intracellularly stained and were found in the periphery of the paraventricular nucleus (PVN), in close proximity to the groups of neurophysin-positive neuroendocrine neurons present in the lateral part of this nucleus. Injected neurons have the morphology of reticular cells, judging by their few multipolar, rectilinear and sparsely branched dendrites. Some of their processes are directed towards PVN. Because of their intrinsic electrophysiological properties and their possible relationships with PVN, the population of cells described in the present study may play a role in functions relating to the PVN.

Membrane properties of morphologically identified X and Y cells in the lateral geniculate nucleus of the cat in vitro

1. The membrane properties and the electrotonic features of cells in lamina A of the cat dorsal lateral geniculate nucleus (l.g.n.) were studied using an in vitro slice preparation. 2. Following intrasomatic injection of horseradish peroxidase (HRP) each neurone was classified as an X (n = 20) or a Y (n = 27) cell on the basis of its morphology. For both classes, the frequency distribution of soma area was similar to that reported in vivo where the identification of X and Y cells in lamina A of the cat l.g.n. was based on physiological criteria. 3. No difference was observed in the mean resting membrane potential between the two classes of cells. However, the input resistance (RN) of X cells was greater (82 M omega) and their membrane time constant (tau 0) longer (22 ms) than of Y cells (RN, 32 M omega; tau 0, 15 ms). 4. Using a simple neuronal model, the calculated electrotonic length (L) and the dendritic to somatic conductance ratio (rho) were similar for the two classes of cells. The mean value of L (0.7) and rho (1.9) suggests that both X and Y cells are electrically compact. 5. The specific membrane resistance (Rm, 28,000 omega cm2) of X cells, calculated using two different approaches, was found to be higher than that of Y cells (17,000 omega cm2). 6. The implication of these results for the integration of synaptic signals in the two classes of l.g.n. cells and the feasibility of differentiating between X and Y cells on the basis of their membrane properties are discussed.

An in vitro slice preparation of the cat lateral geniculate nucleus

A slice preparation of the cat thalamus containing the lateral geniculate nucleus and the terminal portion of the optic tract is described. Ultrastructurally the slices remain relatively normal for only a short time after cutting. Indeed most cellular elements deteriorate quickly with time but patches of relatively intact tissue were still present even 10 h after cutting and maintenance in a storage bath. However, for 4-5 h after cutting long-lasting intracellular recordings of high quality and stability were obtained, and intrasomatic injection of horseradish peroxidase used for the morphological identification of recorded neurones as X or Y cells.

On the excitatory post-synaptic potential evoked by stimulation of the optic tract in the rat lateral geniculate nucleus

1. The electrophysiological and pharmacological properties of the excitatory post-synaptic potentials (e.p.s.p.) evoked by electrical stimulation of the optic tract were studied in projection neurones of the ventral and dorsal lateral geniculate nucleus (l.g.n.) of the rat in vitro. 2. No difference was found in the rise time of e.p.s.p.s. recorded in the dorsal and ventral l.g.n. and in their threshold for action potentials. At membrane potentials more negative than -60 mV, e.p.s.p.s. in the dorsal l.g.n. were always followed by a Ca2+-dependent potential. Its amplitude could easily reach threshold for generating an action potential and thus evoke firing from an e.p.s.p. that was subthreshold at resting potential. No Ca2+ potential was observed to follow e.p.s.p.s. recorded in the ventral l.g.n. 3. At resting potential the excitability of dorsal and ventral cells was unaffected following an initial shock to the optic tract. However, in dorsal neurones, at potentials more negative than -60 mV, the presence of Ca2+ potentials evoked by the e.p.s.p.s. resulted in a period of decreased excitability. 4. Using intrasomatic injection of Cs+ the reversal potential (E) of the e.p.s.p. and of the depolarization produced by glutamate could be measured in the same l.g.n. neurone. They were: Eepsp, -0.9 mV; and Eglut, -3.9 mV. 5. gamma-D-glutamylglycine (DGG), an excitatory amino acid antagonist, reversibly inhibited the e.p.s.p. and depolarization produced by quisqualate and glutamate by a competitive action. The concentration of DGG that produced 50% inhibition (IC50) was 2.7 mM. 6. D-2-amino-5-phosphonovalerate (APV), the potent and selective N-methyl-D-aspartate (NMDA) antagonist, had no effect on the e.p.s.p. both in the presence and absence of Mg2+. The isomers of 2-amino-4-phosphonobutyrate (APB) were inactive or had a non-specific action on the e.p.s.p. 7. No difference could be detected in either the reversal potential or the action of the antagonists between neurones of the dorsal and the ventral l.g.n. 8. These results suggest that Ca2+-dependent potentials play an important role in modulating synaptic efficacy in principal neurones of the dorsal l.g.n. The quisqualate/kainate nature of the optic nerve receptors and the similarity of Eepsp and Eglut constitute strong support in favour of a glutamate-like substance as the transmitter of the optic nerve.