Physiological characteristics and reflex activation of DUM (octopaminergic) neurons of locust metathoracic ganglion

Intracellular recordings were made from single or pairs of somata of the dorsal unpaired median (DUM) neurons of the metathoracic ganglion of the locust Schistocerca gregaria and the grasshopper Romalea microptera, during reflex actions, direct electric excitation and orthodromic and antidromic neural stimulation. Some, possibly all, of these neurons are unique, identifiable individuals in regard to their targets, which are specific peripheral muscles. Their physiological properties and the ways they are activated synaptically are, however, similar. Large, overshooting action potentials, comprising three components, occur. The first component in time is small and represents an excitatory synaptic potential for orthodromic stimulation or an axon spike (AS) for antidromic stimulation, electrotonically conducted into the soma. The second component is larger, being an electrotonically conducted integrating segment spike (ISS). The final component is the soma spike (SS). Neither AS nor ISS have a late positive phase, but there is a large, prolonged one for SS. The latter, combined with rapid accommodation, determine a low maximum firing rate for the neurons. Most nerves entering the ganglion make excitatory inputs onto each DUM neuron, which is readily driven to spike by electric excitation of either connective. There is a great deal of spontaneous excitatory synaptic input to each DUM neuron and a high proportion of it is common. Although there is no detectable electrical coupling between the cells, there is about 30% synchronous firing, apparently due to the common inputs; independent excitation and inhibition also occur. All sensory modalities tested have inputs to the neurons, which tend to fire constantly at a low rate (1 per 3–4 sec). In reflex actions, DUM neurons tend to fire before motor output occurs. It is suggested that the cells will be found to have many functions serving a general role comparable to that achieved by the release of adrenaline in vertebrates.     

Conflicting effects of excitatory synaptic and electric coupling on the dynamics of square-wave bursters

Using two-cell and 50-cell networks of square-wave bursters, we studied how excitatory coupling of individual neurons affects the bursting output of the network. Our results show that the effects of synaptic excitation vs. electrical coupling are distinct. Increasing excitatory synaptic coupling generally increases burst duration. Electrical coupling also increases burst duration for low to moderate values, but at sufficiently strong values promotes a switch to highly synchronous bursts where further increases in electrical or synaptic coupling have a minimal effect on burst duration. These effects are largely mediated by spike synchrony, which is determined by the stability of the in-phase spiking solution during the burst. Even when both coupling mechanisms are strong, one form (in-phase or anti-phase) of spike synchrony will determine the burst dynamics, resulting in a sharp boundary in the space of the coupling parameters. This boundary exists in both two cell and network simulations. We use these results to interpret the effects of gap-junction blockers on the neuronal circuitry that underlies respiration.     

The Sites of Action of Pericardial Organ Extract and 5-Hydroxytryptamine in the Decapod Crustacean Heart

Responses of isolated, perfused hearts of Homarm americanusto brief, internal application of extracts of pericardial organs(PO's) ofCancer borealis, or 5-hydroxytryptamine (5HT) are verysimilar over a thousand-told range of concentration: an increasein rate and amplitude of beating. These reach their maxima afterwashing out has begun, and recover within ten minutes. Externalapplication is ineffective and the substances do not interactwith effects of stretch stimulation. Intracellular recordingfrom heart muscle fibers reveals facilitation of depolarizationto bursts at heart beat frequencies. There may be some effectof 5HT directly on neuromuscular facilitation. Responses recordedfrom isolated cardiac ganglia show increased burst rate, burstduration, or both. Thresholds and the range of concentrationsfor which coordinated responses are recorded correspond to thosefor perfused hearts. It is concluded that the major sites ofaction of PO extract and 5HT are in the cardiac ganglion. 5HTtachyphlaxis and LSD block effects of 5HT, but not of PO extractor accelerator nerve stimulation. Intracellular recordings fromthe large ganglion cells show no effects on resting, synaptic,or spike potentials. Changes in membrane potential to currentpulses revealed no changes in membrane resistance or in theresistance of the electrotonic pathway between cells. Resultsof selective application to large or small cells suggested thatPO extract may contain a rate-increasing substance and one prolongingthe duration of bursts. The former, and 5HT, may influence pacemakerpotentials; the latter may increase the number of spikes a unitcan produce before becoming refractory.     

A hemicord locomotor network of excitatory interneurons: a simulation study

Locomotor burst generation is simulated using a full-scale network model of the unilateral excitatory interneuronal population. Earlier small-scale models predicted that a population of excitatory neurons would be sufficient to produce burst activity, and this has recently been experimentally confirmed. Here we simulate the hemicord activity induced under various experimental conditions, including pharmacological activation by NMDA and AMPA as well as electrical stimulation. The model network comprises a realistic number of cells and synaptic connectivity patterns. Using similar distributions of cellular and synaptic parameters, as have been estimated experimentally, a large variation in dynamic characteristics like firing rates, burst, and cycle durations were seen in single cells. On the network level an overall rhythm was generated because the synaptic interactions cause partial synchronization within the population. This network rhythm not only emerged despite the distributed cellular parameters but relied on this variability, in particular, in reproducing variations of the activity during the cycle and showing recruitment in interneuronal populations. A slow rhythm (0.4–2 Hz) can be induced by tonic activation of NMDA-sensitive channels, which are voltage dependent and generate depolarizing plateaus. The rhythm emerges through a synchronization of bursts of the individual neurons. A fast rhythm (4–12 Hz), induced by AMPA, relies on spike synchronization within the population, and each burst is composed of single spikes produced by different neurons. The dynamic range of the fast rhythm is limited by the ability of the network to synchronize oscillations and depends on the strength of synaptic connections and the duration of the slow after hyperpolarization. The model network also produces prolonged bouts of rhythmic activity in response to brief electrical activations, as seen experimentally. The mutual excitation can sustain long-lasting activity for a realistic set of synaptic parameters. The bout duration depends on the strength of excitatory synaptic connections, the level of persistent depolarization, and the influx of Ca²⁺ ions and activation of Ca²⁺-dependent K⁺ current.     

Effects of phase on homeostatic spike rates

Recent experimental results by Talathi et al. (Neurosci Lett 455:145–149, 2009) showed a divergence in the spike rates of two types of population spike events, representing the putative activity of the excitatory and inhibitory neurons in the CA1 area of an animal model for temporal lobe epilepsy. The divergence in the spike rate was accompanied by a shift in the phase of oscillations between these spike rates leading to a spontaneous epileptic seizure. In this study, we propose a model of homeostatic synaptic plasticity which assumes that the target spike rate of populations of excitatory and inhibitory neurons in the brain is a function of the phase difference between the excitatory and inhibitory spike rates. With this model of homeostatic synaptic plasticity, we are able to simulate the spike rate dynamics seen experimentally by Talathi et al. in a large network of interacting excitatory and inhibitory neurons using two different spiking neuron models. A drift analysis of the spike rates resulting from the homeostatic synaptic plasticity update rule allowed us to determine the type of synapse that may be primarily involved in the spike rate imbalance in the experimental observation by Talathi et al. We find excitatory neurons, particularly those in which the excitatory neuron is presynaptic, have the most influence in producing the diverging spike rates and causing the spike rates to be anti-phase. Our analysis suggests that the excitatory neuronal population, more specifically the excitatory to excitatory synaptic connections, could be implicated in a methodology designed to control epileptic seizures.     

L9 modulation of gill withdrawal reflex habituation in Aplysia.

Repeated tactile stimulation of the siphon in Aphysia normally results in habituation of the gill withdrawal reflex and a concomitant decrease in the amplitude of the excitatory synaptic input ot gill motor neurons in the abdominal ganglion. It was found, however, that induced low-level tonic activity in motor neuron L9, which does not itself elicit a gill withdrawal movement, prevented habituation of the reflex from occurring. Further, in preparations already habituated, this tonic low-level activity brought about a reversal of habituation. Although tonic L9 activity prevented the occurrence of habituation or brought about its reversal, it did not interfere with the synaptic decremental process which normally accompanies gill reflex habituation. Motor neurons L7 and LDG1 were found not to possess this ability of L9 to modulate gill reflex habituation. Evidence suggests that L9's modulatory effect is mediated in the periphery, in the gill and not centrally in the abdominal ganglion.     

Dual effects of proctolin on the rhythmic burst activity of the cardiac ganglion

The neuropeptide proctolin has distinguishable excitatory effects upon premotor cells and motorneurons of Homarus cardiac ganglion. Proctolin's excitation of the small, premotor, posterior cells is rapid in onset (5–10 s) and readily reversible (< 3 min). Prolonged bursts in small cells often produce a “doublet” ganglionic burst mode via interactions with large motorneuron burst-generating driver potentials. In contrast to small cell response, proctolin's direct excitatory effects upon motorneuron are slow in onset (60–90 s to peak) and long-lasting (10–20 min). The latter include: (a) a concentration-dependent (10^?9–10^?7M) depolarization of the somatic membrane potential; (b) increases in burst frequency and (c) enhancement of the rate of depolarization of the interburst pacemaker potential. Experiments on isolated large cells indicate: (a) the slow depolarization is produced by a decrease in the resting G_K and (b) proctolin can produce or enhance motorneuron autorhythmicity. A two-tiered non-hierarchical network model is proposed. The differential pharmacodynamics exhibited by the two cell types accounts for the sequential modes of ganglionic burst activity produced by proctolin.     

Integrative Synaptic Mechanisms in the Caudal Ganglion of the Crayfish

A study of activity recorded with intracellular micropipettes was undertaken in the caudal abdominal ganglion of the crayfish in order to gain information about central fiber to fiber synaptic mechanisms. This synaptic system has well developed integrative properties. Excitatory post-synaptic potentials can be graded, and synaptic potentials from different inputs can sum to initiate spike discharge. In most impaled units, the spike discharge fails to destroy the synaptic potential, thereby allowing sustained depolarization and multiple spike discharge following single pulse stimulation to an afferent input. Some units had characteristics which suggest a graded threshold for spike generation along the post-synaptic fiber membrane. Other impaled units responded to afferent stimulation with spike discharges of two distinct amplitudes. The smaller or "abortive" spikes in such units may represent non-invading activity in branches of the post-synaptic axon. On a few occasions one afferent input was shown to inhibit the spike discharge initiated by another presynaptic input.     

How voltage-gated ion channels alter the functional properties of ganglion and amacrine cell dendrites

The present study compares the structure and function of retinal ganglion and amacrine cell dendrites. Although a superficial similarity exists between amacrine and ganglion cell dendrites, a comparison between the branching pattern of the two cell types reveals differences which can only be appreciated at the microscopic level. Whereas decremental branching is found in ganglion cells, a form of non-decremental or "trunk branching" is observed in amacrine cell dendrites. Physiological differences are also observed in amacrine vs ganglion cells in which many amacrine cells generate dendritic impulses which can be readily distinguished from those of the soma, while separate dendritic impulses in ganglion cell dendrites have not been reported. Despite these differences, both amacrine and ganglion cell dendrites appear to contain voltage-gated ion channels, including TTX-sensitive sodium channels. One way to account for separate dendritic impulses in amacrine cells is to have a higher density of sodium channels and we generally find in modeling studies that a dendritic sodium channel density that is more than about 50% of that in the soma is required for excitatory, synaptic currents to give rise to local dendritic spike activity. Under these conditions, impulses can be generated in the dendrites and propagate for some distance along the dendritic tree. When the soma generates impulse activity in amacrine cells, it can activate, antidromically, the entire dendritic tree. Although ganglion cell dendrites do not appear to generate independent impulses, the presence of voltage-gated ion channels in these structures appears to be important for their function. Modeling studies demonstrate that when dendrites lack voltage-gated ion channels, impulse activity evoked by current applied to the cell body is generated at rates that are much higher than those observed physiologically. However, by placing ion channels in the dendrites at a reduced density compared to those of amacrine cells, the firing rate of ganglion cells becomes more physiological and the relationship between frequency and current (F/I relationship) can be precisely matched with physiological data. Recent studies have demonstrated the presence of T-type calcium channels in ganglion cells and our analysis suggests that they are found in higher density in the dendrites compared to the soma. This is the first voltage-gated ion channel which appears more localized to the dendrites than other cell copartments and this difference alone cries for an interpretation. The presence of a significant T-type calcium channel density in the dendrites can influence their integrative properties in several important ways. First, excitatory synaptic currents can be augmented by the activation of T-type calcium channels, although this is more likely to occur for transient rather than sustained synaptic currents because T-type currents show strong inactivation properties. In addition, T-type calcium channels may serve to limit the electrical load which dendrites impose on the spike initiation process and thus enhance the speed with which impulses can be triggered by the impulse generation site. This role whill enhance the safety factor for impulses traveling in the orthograde direction.     

Cerebral neurons underlying prey capture movements in the pteropod mollusc,Clione limacina

The pteropod mollusc Clione limacina is a highly specialized carnivore which feeds on shelled pteropods and uses, for their capture, three pairs of oral appendages, called buccal cones. Contact with the prey induces rapid eversion of buccal cones, which then become tentacle-like and grasp the shell of the prey. In the previous paper, a large group of electrically coupled, normally silent cells (A motoneurons) has been described in the cerebral ganglia of Clione. Activation of A neurons induces opening of oral skin folds and extrusion of the buccal cones. The present study continues the analysis of the electrical properties of A motoneurons.Brief intracellular stimulation of an A neuron can produce prolonged firing (afterdischarge), lasting up to 40 s, in the entire population of A neurons. Afterdischarge activity is based on an afterdepolarization evoked by an initial strong burst of A neuron spikes. The data suggest that this afterdepolarization represents excitatory synaptic input from unidentified neurons which in turn receive excitatory inputs from A neurons, thus organizing positive feedback. The main functional role of this positive feedback is the spread and synchronization of spike activity among all A neurons in the population. In addition, it serves to transform a brief excitatory input to A neurons into their prolonged and stable firing, which is required during certain phases of feeding behavior in Clione.     

Negative relationship between odor-induced spike activity and spontaneous oscillations in the primary olfactory system of the terrestrial slug Limax marginatus

Although primary olfactory systems in various animals display spontaneous oscillatory activity, its functional significance in olfactory processing has not been elucidated. The tentacular ganglion, the primary olfactory system of the terrestrial slug Limax marginatus, also displays spontaneous oscillatory activity at 1-2 Hz. In the present study, we examined the relationship between odor-evoked spike activity and spontaneous field potential oscillations in the tentacular nerve, representing the pathway from the primary olfactory system to the olfactory center. Neural activity was recorded from the tentacular nerve before, during and after application of various odors (garlic, carrot, and rat chow) to the sensory epithelium and the changes in firing rate and spontaneous oscillations were analyzed. We detected the baseline amplitude of the oscillations and baseline spike activity before stimulation. Odor stimulations for 20 s or 60 s evoked a transient increase in the firing rate followed by a decrease in the amplitude of spontaneous oscillations. The decrease in the amplitude was larger in the first 8 s of stimulation and subsequently showed recovery during stimulation. The amplitude of the recovered oscillations often fluctuated. Odor-evoked spikes appeared when the amplitude of the recovered oscillations was transiently small. These results suggest that the large oscillations could inhibit spike activity whereas the first transient increase in spike activity was followed by the decrease in the oscillation amplitude. Our results indicate that there is a significant negative correlation between spontaneous oscillations and odor-evoked spike activity, suggesting that the spontaneous oscillations contribute to the olfactory processing in slugs.     

L9 modulation of gill withdrawal reflex habituation in Aplysia

Repeated tactile stimulation of the siphon in Aplysia normally results in habituation of the gill withdrawal reflex and a concomitant decrease in the amplitude of the excitatory synaptic input to gill motor neurons in the abdominal ganglion. It was found, however, that induced low-level tonic activity in motor neuron L₉, which does not itself elicit a gill withdrawal movement, prevented habituation of the reflex from occurring. Further, in preparations already habituated, this tonic low-level activity brought about a reversal of habituation. Although tonic L₉ activity prevented the occurrence of habituation or brought about its reversal, it did not interfere with the synaptic decremental process which normally accompanies gill reflex habituation. Motor neurons L₇ and LDG₁ were found not to possess this ability of L₉ to modulate gill reflex habituation. Evidence suggests that L₉'s modulatory effect is mediated in the periphery, in the gill and not centrally in the abdominal ganglion.     

Functional role of serotonergic neuromodulation in Aplysia.

The serotonergic metacerebral cell (MCC) of the mollusk Aplysia produces slow synaptic potentials in motor neurons of the buccal muscle, and increases the rate of ongoing rhythmic burst output of the buccal ganglion. In addition, the MCC acts peripherally to enhance the strength of buccal muscle contractions that are produced by firing of motor neurons. The potentiation of contraction is not associated with any detectable changes of resting membrane potential of muscle cells. Although MCC activity produces a small enhancement of excitatory junctional potentials, several experiments clearly indicate that the MCC has a direct potentiating effect on excitation-contraction coupling. The data suggest that potentiation of contraction might be mediated by cAMP. For example, activity of the MCC enchances the rate of accumulation of cAMP in buccal muscle, application of phosphodiesterase resistant analogs of cAMP potentiates muscle contraction, and a phosphodiesterase inhibitor enhances the effect of MCC stimulation. Recordings from free-moving animals indicate that the MCC becomes activated by exposure of the animal to food stimuli, and that the activation parallels the presence of a food-arousal state. Food-arousal is characterized by enhanced strength and increased frequency of biting responses. Both these effects can result from activity of the MCC. Thus, in this system, modulatory synaptic actions function to provide the substrate for a type behavioral modulation.     

Integrative Neurophysiology of the Lobster Cardiac Ganglion

The synchronized bursts of impulses produced by the nine neuronsof the isolated Homarus cardiac ganglion are usually initiatedby Cell 7. Activity in all other cells commences with very shortlatency thereafter. Impulses in most cells originate in triggerzones located 1–2 mm from the cell body, but the firstseveral impulses in Cells 8 and 9 frequently originate in distaltrigger zones some distance from the somata. Large cells fireat a high initial frequency, dropping rapidly to a low frequencyplateau. Small cells exhibit a more tonic behavior and fireat intermediate rates. More anterior small cells tend to firefaster than more posterior ones. The major synaptic interactionsare the impulse-mediated excitatory ones from small cells tolarge cells, and possibly to more anterior small cells. Thereare weak interactions from large cells back onto small cells,and very specific interactions from Cells 1 and 2 onto 3_A, 4_A,5_A, and 3_B 4_B 5_B respectively. The large discrete EPSPs generatedin large cells by small cell impulses appear to be the explanationfor "discrete positioning" in large-cell firing patterns. Inthis situation, large-cell impulses only fire at discrete timesduring the burst, regardless of the actual large-cell pattern. The overall view is of a two-layered neural system in whichthe small cells possess an endogenous oscillatory driver potential,synchronized by synaptic and electrotonic interactions, anddriving a train of impulses in each cell. This activates excitatorysynapses on the large cells, which combined with a triggereddriver potential in each large cell, produces synchronized trainsof motor impulses which activate the heart muscle, causing theheartbeat.     

Electrophysiology of Supramedullary Neurons in Spheroides maculatus : I. Orthodromic and antidromic responses

This series of three papers presents data on a system of neurons, the large supramedullary cells (SMC) of the puffer, Spheroides maculatus, in terms of the physiological properties of the individual cells, of their afferent and efferent connections, and of their interconnections. Some of these findings are verified by available anatomical data, but others suggest structures that must be sought for in the light of the demonstration that these cells are not sensory neurons. Analysis on so broad a scale was made possible by the accessibility of the cells in a compact cluster on the dorsal surface of the spinal cord. Simultaneous recordings were made intracellularly and extracellularly from individual cells or from several, frequently with registration of the afferent or efferent activity as well. The passive and active electrical properties of the SMC are essentially similar to those of other neurons, but various response characteristics have been observed which are related to different excitabilities of different parts of the neuron, and to specific anatomical features. The SMC produce spikes to direct stimuli by intracellular depolarization, or by indirect synaptic excitation from many afferent paths, including tactile stimulation of the skin. Responses that were evoked by intracellular stimulation of a single cell cause an efferent discharge bilaterally in many dorsal roots, but not in the ventral. Sometimes several distinct spikes occurred in the same root, and behaved independently. Thus, a number of axons are efferent from each neuron. They are large unmyelinated fibers which give rise to the elevation of slowest conduction in the compound action potential of the dorsal root. A similar component is absent in the ventral root action potential. Antidromic stimulation of the axons causes small potentials in the cell body, indicating that the antidromic spikes are blocked distantly to the soma, probably in the axon branches. The failure of antidromic invasion is correlated with differences in excitability of the axons and the neurite from which they arise. As recorded in the cell body, the postsynaptic potentials associated with stimulation of afferent fibers in the dorsal roots or cranial nerves are too small to discharge the soma spike. The indirect spike has two components, the first of which is due to the synaptically initiated activity of the neurite and which invades the cell body. The second component is then produced when the soma is fired. The neurite impulse arises at some distance from the cell body and propagates centrifugally as well as centripetally. An indirect stimulus frequently produces repetitive spikes which are observed to occur synchronously in all the cells examined at one time. Each discharge gives rise to a large efferent volley in each of the dorsal roots and cranial nerves examined. The synchronized responses of all the SMC to indirect stimulation occur with slightly different latencies. They are due to a combination of excitation by synaptic bombardment from the afferent pathways and by excitatory interconnections among the SMC. Direct stimulation of a cell may also excite all the others. This spread of activity is facilitated by repetitive direct excitation of the cell as well as by indirect stimulation.     

Spatial Distribution of Excitatory Synapses on the Dendrites of Ganglion Cells in the Mouse Retina

Excitatory glutamatergic inputs from bipolar cells affect the physiological properties of ganglion cells in the mammalian retina. The spatial distribution of these excitatory synapses on the dendrites of retinal ganglion cells thus may shape their distinct functions. To visualize the spatial pattern of excitatory glutamatergic input into the ganglion cells in the mouse retina, particle-mediated gene transfer of plasmids expressing postsynaptic density 95-green fluorescent fusion protein (PSD95-GFP) was used to label the excitatory synapses. Despite wide variation in the size and morphology of the retinal ganglion cells, the expression of PSD95 puncta was found to follow two general rules. Firstly, the PSD95 puncta are regularly spaced, at 1–2 µm intervals, along the dendrites, whereby the presence of an excitatory synapse creates an exclusion zone that rules out the presence of other glutamatergic synaptic inputs. Secondly, the spatial distribution of PSD95 puncta on the dendrites of diverse retinal ganglion cells are similar in that the number of excitatory synapses appears to be less on primary dendrites and to increase to a plateau on higher branch order dendrites. These observations suggest that synaptogenesis is spatially regulated along the dendritic segments and that the number of synaptic contacts is relatively constant beyond the primary dendrites. Interestingly, we also found that the linear puncta density is slightly higher in large cells than in small cells. This may suggest that retinal ganglion cells with a large dendritic field tend to show an increased connectivity of excitatory synapses that makes up for their reduced dendrite density. Mapping the spatial distribution pattern of the excitatory synapses on retinal ganglion cells thus provides explicit structural information that is essential for our understanding of how excitatory glutamatergic inputs shape neuronal responses.     

Electrical potentials from the eye and optic nerve of Strombus: effects of electrical stimulation of the optic nerve.

1. Photic stimulation of the mature eye of Strombus can evoke in the optic nerve 'on' activity in numerous small afferent fibres and repetitive 'off' bursts of afferent impulses in a smaller number of larger fibres. 2. Synchronous invasion of the eye by electrically evoked impulses in small optic nerve fibres (apparently the 'on' afferents, antidromically activated) can evoke a burst of impulses in the larger 'off' fibres which propagate away from the eye. Invasion of the eye via one branch of optic nerve can evoke an answering burst in another branch. 3. Such electrically evoked bursts are similar to light-evoked 'off' bursts with respect to their impulse composition, their ability to be inhibited by illumination of the eye, and their susceptibility to MgCl2 anaesthesia. 4. Invasion of the eye by a train of repetitive electrically evoked impulses in the absence of photic stimulation can give rise to repetitive 'off' bursts as well as concomitant oscillatory potentials in the eye which are similar to those normally evoked by cessation of a photic stimulus. 5. The electrically evoked 'off' bursts appear to be caused by an excitatory rebound following the cessation of inhibitory synaptic input from photoreceptors which can be antidromically activated by electrical stimulation of the optic nerve. 6. The experimental results suggest that the rhythmic discharge of the 'off' fibres evoked by the cessation of a photic stimulus is mediated by the abrupt decrease of inhibitory synaptic input from the receptors.     

Identification of two cardioacceleratory neurons in the isopod crustacean, Ligia exotica and their effects on cardiac ganglion cells

We identified two pairs of cardioacceleratory (CA1, CA2) neurons in the central nervous system of the isopod Ligiaexotica and examined their effects on the cardiac ganglion (CG). CA1 neurons had cell bodies in the 2nd thoracic ganglion and had arborizations in the subesophageal ganglion and the 1st and 2nd thoracic ganglia. CA2 neurons had cell bodies in the 3rd thoracic ganglion and had arborizations in the 2nd, 3rd and 4th thoracic ganglia. They sent axons to the heart through the ipsilateral 3rd roots of the ganglia where their cell bodies were located. Repetitive stimulation of the CA1 axon rapidly increased the burst frequency of the CG, and that of CA2 rather slowly. The increased burst rate caused by the CA1 stimulation was significantly higher than that caused by CA2. Overall depolarization of a quiescent CG cell produced by the CA1 stimulation was significantly larger in amplitude than that produced by CA2. Facilitation was obviously seen in the excitatory post-synaptic potentials evoked by the CA1 stimulation. These results show that the synaptic properties of CA1 and CA2 neurons are different, suggesting that they have different functional roles in heart regulation. Accepted: 19 July 1997     

Synaptic inputs to the ganglion cells in the tiger salamander retina

The postsynaptic potentials (PSPs) that form the ganglion cell light response were isolated by polarizing the cell membrane with extrinsic currents while stimulating at either the center or surround of the cell's receptive field. The time-course and receptive field properties of the PSPs were correlated with those of the bipolar and amacrine cells. The tiger salamander retina contains four main types of ganglion cell: "on" center, "off" center, "on-off", and a "hybrid" cell that responds transiently to center, but sustainedly, to surround illumination. The results lead to these inferences. The on-ganglion cell receives excitatory synpatic input from the on bipolars and that synapse is "silent" in the dark. The off-ganglion cell receives excitatory synaptic input from the off bipolars with this synapse tonically active in the dark. The on-off and hybrid ganglion cells receive a transient excitatory input with narrow receptive field, not simply correlated with the activity of any presynaptic cell. All cell types receive a broad field transient inhibitory input, which apparently originates in the transient amacrine cells. Thus, most, but not all, ganglion cell responses can be explained in terms of synaptic inputs from bipolar and amacrine cells, integrated at the ganglion cell membrane.     

Intracellular electrophysiology of mammalian peptidergic neurons in rat hypothalamic slices

The magnocellular neuropeptidergic cells (MNCs) of the paraventricular and supraoptic nuclei have been a model for biochemical and physiological studies of peptidergic neurons in the mammalian brain, but nearly all the electrophysiological studies of these vasopressinergic and oxytocinergic neuroendocrine cells are based on extracellular recordings. This paper reviews recent literature on electrophysiological properties of neurons in the magnocellular nuclei in which the rat in vitro slice preparation and intracellular recording were used. Spontaneously occurring action potentials and synaptic potentials (excitatory and inhibitory) have been observed in hypothalamic slices. The spike patterns have included slow and irregular firing, short rapid bursts of inactivating spikes, and slow phasic discharge with prolonged active and silent periods. Some studies have shown that increased osmolality causes neuronal firing, but this area is controversial. Intracellular injections of lucifer yellow have shown that some MNCs are dye-coupled and electron microscopic observations with the freeze-fracture technique have revealed occasional gap junctions, thus suggesting that some MNCs are electrotonically coupled. Both excitatory and inhibitory postsynaptic potentials have been evoked with extracellular stimulation. Therefore, action potentials, synaptic potentials, burst discharges, and probably electrotonic coupling have been found with intracellular recording in mammalian neuroendocrine cells. Future studies with intracellular recording and staining followed by immunohistochemical identification of cells should provide significant new information on the membrane physiology and synaptic pharmacology of vasopressinergic and oxytocinergic cells.