Passive cable properties of locust ocellar L-neurons

Summary The passive electrical cable properties of ocellar L-neurons were determined by applying current steps and recording the voltage transients using a two-electrode intracellular current clamp system. Morphological data were obtained following intracellular staining with Lucifer yellow.Two groups of neurons were distinguished physiologically. In the first group both the membrane time constant _m and the first equalizing time constant ₁ could be determined. In the second group only _m was measurable. The ratio of the physiological groups was equal to the ratio of the morphological types ML:(M1 plus M2) in the median ocellar nerve. Thus the first group probably consists of ML-type L-neurons. The passive cable properties of this group were calculated by combining the physiological and morphological data. The following values were obtained: electrotonic lengthL=1.35; membrane time constant _m =7.6 ms; length constant =0.22 cm; membrane resistivityR _m=2.0 · 10³ · cm²; membrane capacitanceC _m=3.8 F · cm^–2; intracellular resistivityR _i=24 · cm. Evidence is presented that the membrane parameters of the other types of L-neurons have the same values. The results are discussed with special reference to transmission in the ocellar system.     

Regeneration of goldfish retina: rod precursors are a likely source of regenerated cells

This study describes regeneration of the neural retina in juvenile goldfish. The retina was destroyed with an intraocular injection of ouabain, a technique introduced by Wolburg and colleagues (Maier and Wolburg, 1979; Kurz-Isler and Wolburg, 1982). We confirmed their observation that the level of damage produced by the toxin was graded, in that neurons in the inner retinal layers were preferentially destroyed, and only in the more severely affected retinas were cells in the outer nuclear layer (i.e., photoreceptor cells) damaged. Evidence of retinal regeneration could be seen beginning about 2 weeks after the injection of ouabain. In contrast to previous studies (Maier and Wolburg, 1979), we found that regeneration took place only in those retinas in which photoreceptors had been destroyed. In cases in which the outer nuclear layer was spared, no regeneration of inner layers occurred, even after 6 months. Thymidine autoradiography was used to document the regeneration of new retinal neurons and to show that rod precursors, like other dividing cells, were not destroyed by the ouabain, but in contrast showed an increased mitotic activity. Regeneration did not proceed uniformly, but was initiated at neurogenic foci scattered across the retina. These foci consisted of clusters of dividing neuroepithelial-like cells. The evidence is consistent with the proposal that these cells were derived from rod precursors. These results imply that rod precursors are capable of a wider range of developmental fates than they normally express.     

GABAergic transmission and chloride equilibrium potential are not modulated by pyruvate in the developing optic tectum of Xenopus laevis tadpoles

In the developing mammalian brain, gamma-aminobutyric acid (GABA) is thought to play an excitatory rather than an inhibitory role due to high levels of intracellular Cl(-) in immature neurons. This idea, however, has been questioned by recent studies which suggest that glucose-based artificial cerebrospinal fluid (ACSF) may be inadequate for experiments on immature and developing brains. These studies suggest that immature neurons may require alternative energy sources, such as lactate or pyruvate. Lack of these other energy sources is thought to result in artificially high intracellular Cl(-) concentrations, and therefore a more depolarized GABA receptor (GABAR) reversal potential. Since glucose metabolism can vary widely among different species, it is important to test the effects of these alternative energy sources on different experimental preparations. We tested whether pyruvate affects GABAergic transmission in isolated brains of developing wild type Xenopus tadpoles in vitro by recording the responsiveness of tectal neurons to optic nerve stimulation, and by measuring currents evoked by local GABA application in a gramicidin perforated patch configuration. We found that, in contrast with previously reported results, the reversal potential for GABAR-mediated currents does not change significantly between developmental stages 45 and 49. Partial substitution of glucose by pyruvate had only minor effects on both the GABA reversal potential, and the responsiveness of tectal neurons at stages 45 and 49. Total depletion of energy sources from the ACSF did not affect neural responsiveness. We also report a strong spatial gradient in GABA reversal potential, with immature cells adjacent to the lateral and caudal proliferative zones having more positive reversal potentials. We conclude that in this experimental preparation standard glucose-based ACSF is an appropriate extracellular media for in vitro experiments.     

Inositol phospholipids are probably not the source of arachidonic acid for eicosanoid synthesis in astrocytes

In astrocyte-enriched cultures of the rat cerebral cortex the Ca2+ ionophore A23187 provoked the breakdown of inositol phospholipids, the liberation of arachidonic acid and the release of prostaglandins E2, F2 alpha, I2 and thromboxane A2. However, agonists for receptors also coupled to inositol phospholipid metabolism in these cells failed to produce an increase in the release of both arachidonic acid and eicosanoids. Results suggest that the A23187-stimulated release of arachidonic acid and eicosanoids is caused by a phospholipase A2-mediated attack on lipids other than the inositol phospholipids. Moreover, receptors linked to inositol lipid turnover are not involved in the control of eicosanoid release from astrocytes.     

Freeze-fracture characteristics of the ocellar retina of the honeybee,Apis mellifica carnica pollm. (Hymenoptera : Apidae)

Freeze-fracture electron microscopy has been utilized to investigate membrane specializations in the dark-adapted retina of the ocellus of worker honeybee, Apis mellifica (Hymenoptera : Apidae).The distal segment of the photoreceptor cell is unilaterally composed of short microvilli, termed rhabdomeres. All microvilli are oriented in the same direction. Transversally fractured rhabdomerss show the typical hexagonal arrangement of the microvillar profiles. The plasma membrane limiting each microvillus, is rich in particles (4000/μm² on the protoplasmic (P) face). Two kinds of P-face particles are seen: one type is about 4nm in diameter and the other has diameters of 8 – 10nm. Because of their size and density, these particles may mainly correspond to photopigments. Plaques of exoplasmic (E) face particles present in the glial cells sheathing the photoreceptor cells, are thought to represent gap junctions. Special differentiations of the smooth endoplasmic reticulum (ER) are found in the perirhabdomeric cytoplasm closely associated with microvilli. The membranes of the ER are wound up to form large concentric bodies. These bodies may play an important role in formation of new microvilli. Furthermore, conspisuous bodies with stomata are found in the perirhabdomeric cytoplasm.     

Photopic action of thyrotropin-releasing hormone in the cat retina

The effects of iontophoretically applied thyrotropin-releasing hormone (TRH) on cat retinal brisk-sustained(X) and brisk-transient(Y) ganglion cells were studied in the intact eye in vivo. Under photopic illumination we found a differential action of TRH on ON- and OFF-centre cells: the maintained activity and light response were suppressed in ON-centre cells and enhanced in OFF-centre cells. This was true for both brisk-sustained(X) and brisk-transient(Y) cells. In contrast, TRH did not influence the ganglion cell discharge under scotopic stimulus conditions. These results indicate that TRH acts on neurons presynaptic to ganglion cells and these neurons are only active under photopic conditions. We suggest that a possible functional role of this specific action of TRH is in light adaptation.     

Noradrenergic and GABAergic systems in the medial hypothalamus are activated during hypoglycemia

Noradrenergic and GABAergic systems in the medial hypothalamus influence plasma glucose and may be activated during glucoprivation. Microdialysis probes were placed into the ventromedial nucleus (VMH), lateral hypothalamus (LHA), and paraventricular nucleus (PVH) of male Sprague-Dawley rats to monitor extracellular concentrations of norepinephrine (NE) and GABA. During systemic hypoglycemia, induced by insulin (1.0 U/kg), NE concentrations increased in the VMH (P < 0.05) and PVH (P = 0.06) in a bimodal fashion during the first 10 min and 20-30 min after insulin administration. In the VMH, GABA concentrations increased (P < 0.05) in a similar manner as NE. Extracellular NE concentrations in the LHA were slightly lower (P = 0.13), and GABA levels remained at baseline. The increases in NE and GABA in the VMH were absent during euglycemic clamp; however, NE in the PVH still increased, reflecting a direct response to hyperinsulinemia. On the basis of these data, we propose that the activity of noradrenergic afferents to the medial hypothalamus is increased during hypoglycemia and influences the activity of local GABAergic systems to activate appropriate physiological compensatory mechanisms.     

Direction-selective dendritic action potentials in rabbit retina

Dendritic spikes that propagate toward the soma are well documented, but their physiological role remains uncertain. Our in vitro patch-clamp recordings and two-photon calcium imaging show that direction-selective retinal ganglion cells (DSGCs) utilize orthograde dendritic spikes during physiological activity. DSGCs signal the direction of image motion. Excitatory subthreshold postsynaptic potentials are observed in DSGCs for motion in all directions and provide a weakly tuned directional signal. However, spikes are generated over only a narrow range of motion angles, indicating that spike generation greatly enhances directional tuning. Our results indicate that spikes are initiated at multiple sites within the dendritic arbors of DSGCs and that each dendritic spike initiates a somatic spike. We propose that dendritic spike failure, produced by local inhibitory inputs, might be a critical factor that enhances directional tuning of somatic spikes.     

Role of the GABAergic system in the mechanism of the stress-regulating action of phenibut

Disturbances in GABA-ergic inhibitory system were noted upon the exposure of thalamus and hypothalamus of experimental animals to stress of 18 hours and longer duration but not of 3 hours duration. Phenibut (I mg/kg) eliminates the symptoms of GABA-system disturbances revealed upon exposure to stress, decreases the tension of stress reaction and hyperglucocorticoidemia which causes hyperglycemia. In the bodies of intact rats phenibut causes "switching on" of adaptive GABA-system function, which is more physiological than the one appearing on short-term stress.     

Dynamics of action potential initiation in the GABAergic thalamic reticular nucleus in vivo

Understanding the neural mechanisms of action potential generation is critical to establish the way neural circuits generate and coordinate activity. Accordingly, we investigated the dynamics of action potential initiation in the GABAergic thalamic reticular nucleus (TRN) using in vivo intracellular recordings in cats in order to preserve anatomically-intact axo-dendritic distributions and naturally-occurring spatiotemporal patterns of synaptic activity in this structure that regulates the thalamic relay to neocortex. We found a wide operational range of voltage thresholds for action potentials, mostly due to intrinsic voltage-gated conductances and not synaptic activity driven by network oscillations. Varying levels of synchronous synaptic inputs produced fast rates of membrane potential depolarization preceding the action potential onset that were associated with lower thresholds and increased excitability, consistent with TRN neurons performing as coincidence detectors. On the other hand the presence of action potentials preceding any given spike was associated with more depolarized thresholds. The phase-plane trajectory of the action potential showed somato-dendritic propagation, but no obvious axon initial segment component, prominent in other neuronal classes and allegedly responsible for the high onset speed. Overall, our results suggest that TRN neurons could flexibly integrate synaptic inputs to discharge action potentials over wide voltage ranges, and perform as coincidence detectors and temporal integrators, supported by a dynamic action potential threshold.     

Convergence of antennal and ocellar inputs in the insect brain

Summary Unitary responses were recorded from the brain of the fleshfly, Boettcherisca peregrina, during olfactory or mechanical stimulation of the antenna, and simultaneous photic stimulation of the ocelli. Convergence from the two inputs, the antenna and the ocelli, was observed. The response to antennal stimulation was facilitated by photic stimulation in most units. The responses to the antennal stimuli were facilitated greatly at the peak of the photic response. Some units responded both to ocellar illumination and antennal stimulation. Their response to antennal stimulation seemed independent of the light-condition during the light-adapted state, but was facilitated at the onset of the ocellar illumination, and occluded just after its cessation. In addition, there were some units which responded to antennal stimulation but not to the ocellar illumination; some of them also showed facilitation of the response to antennal stimulation during ocellar illumination.