Presynaptic and interactive peptidergic modulation of reticulospinal synaptic inputs in the lamprey

The modulatory effects of neuropeptides on descending inputs to the spinal cord have been examined by making paired recordings from reticulospinal axons and spinal neurons in the lamprey. Four peptides were examined; peptide YY (PYY) and cholecystokinin (CCK), which are contained in brain stem reticulospinal neurons, and calcitonin-gene-related peptide (CGRP) and neuropeptide Y (NPY), which are contained in primary afferents and sensory interneurons, respectively. Each of the peptides reduced the amplitude of monosynaptic reticulospinal-evoked excitatory postsynaptic potentials (EPSPs). The modulation appeared to be presynaptic, because postsynaptic input resistance and membrane potential, the amplitude of the electrical component of the EPSP, postsynaptic responses to glutamate, and spontaneous miniature EPSP amplitudes were unaffected. In addition, none of the peptides affected the pattern of N-methyl-D-aspartate (NMDA)-evoked locomotor activity in the isolated spinal cord. Potential interactions between the peptides were also examined. The "brain stem peptides" CCK and PYY had additive inhibitory effects on reticulospinal inputs, as did the "sensory peptides" CGRP and NPY. Brain stem peptides also had additive inhibitory effects when applied with sensory peptides. However, sensory peptides increased or failed to affect the amplitude of reticulospinal inputs in the presence of the brain stem peptides. These interactive effects also appear to be mediated presynaptically. The functional consequence of the peptidergic modulation was investigated by examining spinal ventral root responses elicited by brain stem stimulation. CCK and CGRP both reduced ventral root responses, although in interaction both increased the response. These results thus suggest that neuropeptides presynaptically influence the descending activation of spinal locomotor networks, and that they can have additive or novel interactive effects depending on the peptides examined and the order of their application.     

Sustained depolarizing potentials in reticulospinal axons during evoked seizure activity in lamprey spinal cord

1. Intracellular recordings were made from lamprey reticulospinal axons (Müller axons) during seizures evoked by electrical stimulation of the isolated spinal cord in saline containing either 0 Cl or 1 mM picrotoxin. The seizures had tonic and clonic-phases similar to ictal seizures in mammalian brain. 2. During seizures Müller axons were depolarized by 10-15 mV. These seizure-depolarizations were not due to any direct effect of the evoking stimulus on the Müller axons themselves nor were they initiated by an accumulation or extracellular potassium. 3. A decrease in axonal input resistance occurred during a seizure-depolarization. Also, the amplitude of a seizure-depolarization was decreased by depolarizing the axon 5-15 mV with injected current. Further, hyperpolarizing the axon increased the amplitude of the seizure-depolarization, but the growth flattened out beyond 30-40 mV of hyperpolarization. The decrease in input resistance during the seizure-depolarization and the dependence of the response amplitude on axonal membrane potential suggested that the seizure-depolarization was an excitatory synaptic potential. However, the failure of the seizure-depolarization amplitude to continue to grow at membrane potentials greater than 30 mV negative to the resting potential was not consistent with this interpretation. 4. A synaptic conductance change as the cause of the seizure-depolarization was ruled out by setting the axonal membrane potential at different levels with injected current and monitoring the input resistance of the axon before and during seizure-depolarizations. It was found that no change in input resistance occurred during the seizure-depolarization when the axon was hyperpolarized more than approximately 30 mV, the same potential at which the growth in the response amplitude ceased. From analysis of these data and the passive current-voltage properties of Müller axons it is concluded that the seizure-depolarization is not a chemical synaptic potential, but rather the result of the passive injection of depolarizing current into the axons. 5. The source of the depolarizing current which flows into Müller axons during seizures is probably paroxysmal action-potential activity in spinal motoneurons and interneurons, many of which are electrically coupled to Müller axons.     

Post-tetanic potentiation, habituation and facilitation of synaptic potentials in reticulospinal neurones of lamprey

1. Synaptic potentials evoked by electrical stimulation of cranial nerves were recorded in giant reticulospinal neurones (Müller cells) of lamprey. A variety of patterns of stimulation was employed to explore further the functional properties of the pathways intervening between the cranial nerve fibres and Müller cells.

2. Simultaneous low intensity stimulation of two different cranial nerves produced excitatory short-latency synaptic potentials whose amplitudes summed linearly.

3. Tetanic (10/sec) stimulation of a cranial nerve depressed the evoked short-latency synaptic response, but following the tetanus the synaptic response was potentiated above control amplitude for several minutes. Tetanic stimulation of one cranial nerve had no effect upon the synaptic responses evoked by stimulation of other cranial nerves.

4. Low-frequency stimulation (1/sec to 1/20 sec) of a cranial nerve produced a progressive decrease in the amplitude of the evoked short-latency synaptic response. This phenomenon was termed synaptic habituation because its characteristics were functionally similar to behavioural habituation in animals.

5. Habituation of the synaptic response to stimulation of one cranial nerve had no effect on the synaptic responses produced by stimulation of other cranial nerves.

6. Synaptic afterdischarges lasting from several seconds to several minutes were recorded in Müller cells. They occurred both spontaneously and in response to strong electrical stimulation of cranial nerves. For several minutes following an afterdischarge the amplitudes of short-latency synaptic potentials produced by stimulation of any one of the cranial nerves were increased as much as twofold. This facilitation occurred equally well whether the short-latency synaptic responses had been habituated or not.

7. A theoretical cell-wiring diagram is proposed to account for the properties of short-latency evoked synaptic responses and synaptic afterdischarges and for the facilitation of short-latency responses by afterdischarges.

     

Role of apamin-sensitive k(ca) channels for reticulospinal synaptic transmission to motoneuron and for the afterhyperpolarization

Single motoneurons and pairs of a presynaptic reticulospinal axon and a postsynaptic motoneuron were recorded in the isolated lamprey spinal cord, to investigate the role of calcium-dependent K(+) channels (K(Ca)) during the afterhyperpolarization following the action potential (AHP), and glutamatergic synaptic transmission on the dendritic level. The AHP consists of a fast phase due to transient K(+) channels (fAHP) and a slower phase lasting 100-200 ms (sAHP), being the main determinant of spike frequency regulation. We now present evidence that the sAHP has two components. The larger part, around 80%, is abolished by superfusion of Cd(2+) (blocker of voltage-dependent Ca(2+) channels), by intracellular injection of 1,2-bis-(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA; fast Ca(2+) chelator), and by apamin (selective toxin for K(Ca) channels of the SK subtype). While 80% of the sAHP is thus due to K(Ca) channels, the remaining 20% is not mediated by Ca(2+), either entering through voltage-dependent Ca(2+) channels or released from intracellular Ca(2+) stores. This Ca(2+)-independent sAHP component has a similar time course as the K(Ca) portion and is not due to a Cl(-) conductance. It may be caused by Na(+)-activated K(+) channels. Glutamatergic excitatory postsynaptic potentials (EPSPs) evoked by single reticulospinal axons give rise to a local Ca(2+) increase in the postsynaptic dendrite, mediated in part by N-methyl-D-aspartate (NMDA) receptors. The Ca(2+) levels remain elevated for several hundred milliseconds and could be expected to activate K(Ca) channels. If so, this activation should cause a local conductance increase in the dendrite that would shunt EPSPs following the first EPSP in a spike train. We have tested this in reticulospinal/motoneuronal pairs, by stimulating the presynaptic axon with spike trains at different frequencies. We compared the first EPSP and the following EPSPs in the control and after blockade with apamin. No difference was observed in EPSP amplitude or shape before and after apamin, either in normal Ringer or in Mg(2+)-free Ringer removing the voltage-dependent block of NMDA receptors. In conclusion, the local Ca(2+) entry during reticulospinal EPSPs does not cause an activation of K(Ca) channels sufficient to affect the efficacy of synaptic transmission. Thus the integration of synaptic signals at the dendritic level in motoneurons appears simpler than would otherwise have been the case.     

Morphological correlates of synaptic transmission in lamprey spinal cord.

The dye Procion brown was used to identify in the light and electron microscope, synaptic contacts made between monosynaptically coupled neurons in the lamprey spinal cord whose synaptic interaction had been recorded. Synaptic contacts were made on different dendrites of the postsynaptic cell at different distances from the soma. Some of the contacts were made on dentritic spines and some on the smooth shaft of the dentrites. Serial sections through synaptic contacts made on dendritic processess of the postsynaptic cells were used for three-dimensional reconstruction of the synapses using computer graphics techniques. The computer reconstructions and detailed examination of the serial EM micrographs revealed the large proliferation of membrane involved in making these en passant synapses as well as the morphological changes due to stimulation of the presynaptic axon. These changes include depletion of synaptic vesicles and formation of complex vesicles and synaptic cisternae. Besides chemical synaptic contacts, four electrotonic contacts were located, confirming the mixed electrochemical synaptic response recorded from the postsynaptic cell. The mean quantum content was estimated and compared with the estimate of the available transmitter pool, assuming the quantal release hypothesis applies at these synapses. The total transmitter pool was estimated by counting all synaptic vesicles in all synaptic contacts. It was estimated that about 6% of the total transmitter pool is available for release at these synapses. This compares with less than 1% at the neuromuscular junction and about 20% at sympathetic synapses. These results support the hypothesis that synaptic vesicles may be recycled as described by Heuser and Reese (22) at the neuromuscular junction. Ongoing studies are investigating the effect on a variety of synaptic junctions to stimulation for different periods of time of presynaptic axons. The methods described in this study can also be used to test the models of synaptic interaction on dendritic trees described by Rall (39) and Jack and Redman (24).     

The relationship between some measures of synaptic ultrastructure as a function of distance from the soma on lamprey reticulospinal neurons

Summary Synaptic junctions located on the dendrites of lamprey (Petromyzon marinus) reticulospinal neurons labelled with intracellularly-injected horseradish peroxidase were studied. The normal ultrastructure of the synaptic junctions was defined and several quantitative measures made from each junction in order to test the hypothesis that distally-located synapses are ultrastructurally different from those located at proximal dendritic sites. A total of 820 contacts from one neuron and 279 from a second neuron ranging from 20 to 340 m from the soma were quantified. The vast majority of the presynaptic endings contained round, clear-cored vesicles and formed an asymmetrical membrane differentiation with the postsynaptic dendrite. A small fraction of the population contained flattened or pleomorphic vesicles and these synapses were equally distributed with respect to distance from the soma. Many of the terminals contained a few large dark- and clear-cored vesicles. Four quantitative measures of each synaptic contact were made. These included vesicle number, length of differentiated membrane, vesicle area and terminal area. Four ratios relating the different quantitative measures were also calculated. Each ratio or measurement from the synaptic junctions was plotted as a function of distance from the soma to determine if differences existed at any distance. It was found that synaptic junctions are uniformly similar and that distal junctions did not differ significantly (P > 0.05) from those at proximal dendritic sites. It is concluded that if distal synapses do compensate for their remote location they do this in some other way, possibly by increasing the number of synaptic contacts made by each presynaptic axon.     

5-Hydroxytryptamine modulates spike frequency regulation in reticulospinal neurons involved in the control of locomotion in lamprey.

To investigate the effects of 5-hydroxytryptamine (5-HT) on reticulospinal neurons involved in the initiation and control of locomotion in lamprey, 5-HT (10 mM) was locally pressure ejected on the dorsal surface of the brainstem during intracellular recordings from identified reticulospinal neurons in the in vitro brainstem-spinal cord preparation. 5-HT induced a reduction of the late afterhyperpolarization (AHP) following the spike due to a reduction of a Ca(2+)-activated K+ current. In addition, 5-HT caused a resting membrane hyperpolarization in a proportion of these cells. Due to the 5-HT induced reduction of the AHP, reticulospinal cells, including those that became hyperpolarized by an application of 5-HT, discharged at a higher rate after 5-HT as a response to the same excitatory drive.     

Glycine-gated chloride channels depress synaptic transmission in rat hippocampus

An inhibitory role for strychnine-sensitive glycine-gated chloride channels (GlyRs) in mature hippocampus is beginning to be appreciated. We have reported previously that CA1 pyramidal cells and GABAergic interneurons recorded in 3- to 4-wk-old rat hippocampal slices express functional GlyRs, dispelling previous misconceptions that GlyR expression ceases in early development. However, the effect of GlyR activation on cell excitability and synaptic circuits in hippocampus has not been fully explored. Using whole cell current-clamp recordings, we show that activation of strychnine-sensitive GlyRs through exogenous glycine application causes a significant decrease in input resistance and prevents somatically generated action potentials in both CA1 pyramidal cells and interneurons. Furthermore, GlyR activation depresses the synaptic network by reducing suprathreshold excitatory postsynaptic potentials (EPSPs) to subthreshold events in both cell types. Blockade of postsynaptic GlyRs with the chloride channel blocker 4, 4'-diisothiocyanatostilbene-2-2'-disulfonic acid (DIDS) or altering the chloride ion driving force in recorded cells attenuates the synaptic depression, strongly indicating that a postsynaptic mechanism is responsible. Increasing the local glycine concentration by blocking reuptake causes a strychnine-sensitive synaptic depression in interneuron recordings, suggesting that alterations in extracellular glycine will impact excitability in hippocampal circuits. Finally, using immunohistochemical methods, we show that glycine and the glycine transporter GlyT2 are co-localized selectively in GABAergic interneurons, indicating that interneurons contain both inhibitory neurotransmitters. Thus we report a novel mechanism whereby activation of postsynaptic GlyRs can function to depress activity in the synaptic network in hippocampus. Moreover, the co-localization of glycine and GABA in hippocampal interneurons, similar to spinal cord, brain stem, and cerebellum, suggests that this property is likely to be a general characteristic of inhibitory interneurons throughout the CNS.     

Responses of reticulospinal neurons in intact lamprey to pitch tilt

In the swimming lamprey, a postural control system maintains a definite orientation of the animal's longitudinal axis in relation to the horizon (pitch angle). Operation of this system is based on vestibular reflexes. Important elements of the postural network are the reticulospinal (RS) neurons, which are driven by vestibular input and transmit commands for postural corrections from the brain stem to the spinal cord. Here we describe responses to vestibular stimulation (rotation of the animal in the pitch plane) in RS neurons of intact lampreys. The activity of neurons was recorded from their axons in the spinal cord by chronically implanted arrays of macroelectrodes. From the multielectrode recordings of mass activity, discharges in individual axons were extracted by means of a spike-sorting program, and the axon position in the spinal cord and its conduction velocity were determined. Vestibular stimulation was performed by rotating the animal in steps of 45 degrees throughout 360 degrees or by periodical "trapezoid" tilts between the nose-up and -down positions. Typically, the RS neurons exhibited both dynamic responses (activity during movement) and static responses (activity in a new sustained position). The neurons were classified into two groups according to their pattern of response. Group UP neurons responded preferentially to nose-up rotation with maximal activity at 0-135 degrees up. Group DOWN neurons responded preferentially to nose-down rotation with maximal activity at 0-135 degrees down. Neurons of the two groups also differed in the position of their axons in the spinal cord and axonal conduction velocity. An increase in water temperature, which presumably causes a downward turn in swimming lampreys, affected the activity in the UP and DOWN groups differently, so that the ratio UP responses to DOWN responses increased. We suggest that the UP and DOWN groups mediate the opposing vestibular reflexes and cause the downward and upward turns of the animal, respectively. The lamprey will stabilize the orientation in the pitch plane at which the effects of UP and DOWN groups are equal to each other. In addition to the main test (rotation in the pitch plane), the animals were also tested by rotation in the transverse (roll) plane. It was found that 22% of RS neurons responding to pitch tilts also responded to roll tilts. The overlap between the pitch and roll populations suggests that the RS pathways are partly shared by the pitch and roll control systems.     

Responses of reticulospinal neurons in the lamprey to lateral turns

When swimming, the lamprey maintains a definite orientation of its body in the vertical planes, in relation to the gravity vector, as the result of postural vestibular reflexes. Do the vestibular-driven mechanisms also play a role in the control of the direction of swimming in the horizontal (yaw) plane, in which the gravity cannot be used as a reference direction? In the present study, we addressed this question by recording responses to lateral turns in reticulospinal (RS) neurons mediating vestibulospinal reflexes. In intact lampreys, the activity of axons of RS neurons was recorded in the spinal cord by implanted electrodes. Vestibular stimulation was performed by periodical turns of the animal in the yaw plane (60 degrees peak to peak). It was found that the majority of responding RS neurons were activated by the contralateral turn. By removing one labyrinth, we found that yaw responses in RS neurons were driven mainly by input from the contralateral labyrinth. We suggest that these neurons, when activated by the contralateral turn, will elicit the ipsilateral turn and thus will compensate for perturbations of the rectilinear swimming caused by external factors. It is also known that unilateral eye illumination elicits a contralateral turn in the yaw plane (negative phototaxis). We found that a portion of RS neurons were activated by the contralateral eye illumination. By eliciting an ipsilateral turn, these neurons could mediate the negative phototaxis.     

Activity of reticulospinal neurons during locomotion in the freely behaving lamprey

The reticulospinal (RS) system is the main descending system transmitting commands from the brain to the spinal cord in the lamprey. It is responsible for initiation of locomotion, steering, and equilibrium control. In the present study, we characterize the commands that are sent by the brain to the spinal cord in intact animals via the reticulospinal pathways during locomotion. We have developed a method for recording the activity of larger RS axons in the spinal cord in freely behaving lampreys by means of chronically implanted macroelectrodes. In this paper, the mass activity in the right and left RS pathways is described and the correlations of this activity with different aspects of locomotion are discussed. In quiescent animals, the RS neurons had a low level of activity. A mild activation of RS neurons occurred in response to different sensory stimuli. Unilateral eye illumination evoked activation of the ipsilateral RS neurons. Unilateral illumination of the tail dermal photoreceptors evoked bilateral activation of RS neurons. Water vibration also evoked bilateral activation of RS neurons. Roll tilt evoked activation of the contralateral RS neurons. With longer or more intense sensory stimulation of any modality and laterality, a sharp, massive bilateral activation of the RS system occurred, and the animal started to swim. This high activity of RS neurons and swimming could last for many seconds after termination of the stimulus. There was a positive correlation between the level of activity of RS system and the intensity of locomotion. An asymmetry in the mass activity on the left and right sides occurred during lateral turns with a 30% prevalence (on average) for the ipsilateral side. Rhythmic modulation of the activity in RS pathways, related to the locomotor cycle, often was observed, with its peak coinciding with the electromyographic (EMG) burst in the ipsilateral rostral myotomes. The pattern of vestibular response of RS neurons observed in the quiescent state, that is, activation with contralateral roll tilt, was preserved during locomotion. In addition, an inhibition of their activity with ipsilateral tilt was clearly seen. In the cases when the activity of individual neurons could be traced during swimming, it was found that rhythmic modulation of their firing rate was superimposed on their tonic firing or on their vestibular responses. In conclusion, different aspects of locomotor activity-initiation and termination, vigor of locomotion, steering and equilibrium control-are well reflected in the mass activity of the larger RS neurons.     

Spinal locomotor inputs to individually identified reticulospinal neurons in the lamprey

Locomotor feedback signals from the spinal cord to descending brain stem neurons were examined in the lamprey using the uniquely identifiable reticulospinal neurons, the Müller and Mauthner cells. The same identified reticulospinal neurons were recorded in several preparations, under reduced conditions, to address whether an identified reticulospinal neuron shows similar locomotor-related oscillation timing from animal to animal and whether these timing signals can differ significantly from other identified reticulospinal neurons. Intracellular recordings of membrane potential in identified neurons were made in an isolated brain stem-spinal cord preparation with a high-divalent cation solution on the brain stem to suppress indirect neural pathways and with D-glutamate perfusion to the spinal cord to induce fictive swimming. Under these conditions, the identified reticulospinal neurons show significant clustering of the timings of the peaks and troughs of their locomotor-related oscillations. Whereas most identified neurons oscillated in phase with locomotor bursting in ipsilateral ventral roots of the rostral spinal cord, the B1 Müller cell, which has an ipsilateral descending axon, and the Mauthner cell, which has a contralateral descending axon, both had oscillation peaks that were out of phase with the ipsilateral ventral roots. The differences in oscillation timing appear to be due to differences in synaptic input sources as shown by cross-correlations of fast synaptic activity in pairs of Müller cells. Since the main source of the locomotor input under these experimental conditions is ascending neurons in the spinal cord, these experiments suggest that individual reticulospinal neurons can receive locomotor signals from different subsets of these ascending neurons. This result may indicate that the locomotor feedback signals from the spinal locomotor networks are matched in some way to the motor output functions of the individual reticulospinal neurons, which include command signals for turning and for compensatory movements.     

Synapsin I injected presynaptically into goldfish mauthner axons reduces quantal synaptic transmission

1. Synapsin I was injected into a vertebrate presynaptic axon to analyze its action on quantal synaptic transmission. Two microelectrodes were used for simultaneous intracellular recording from pairs of identified neurons in the goldfish brain. The postsynaptic electrode was placed in a cranial relay neuron (CRN) within 100 microns of its synapse with the Mauthner neuron. The presynaptic electrode impaled the Mauthner axon (M-axon) 50-200 microns from the first electrode. 2. Spontaneous miniature excitatory postsynaptic potentials (mEPSPs) and evoked postsynaptic potentials (EPSPs) were recorded at steady states before and after synapsin I was microinjected into the presynaptic M-axon. Responses were digitized and subsequently analyzed by computer for quantal parameters. 3. In 12 experiments, injection of synapsin I resulted in a reduction in transmission. The decrease in EPSP amplitude began approximately 30 s after the injection, reached a plateau within 10 min, and appeared to be reversible and dose dependent. 4. Injection of synapsin I decreased quantal content (m), with no effect on postsynaptic receptor sensitivity or on amount of transmitter per quantum. Further analysis based on the simplest binomial model for quantal release revealed that synapsin I consistently reduced the number of quantal units available for release (n) although the probability of release (p) was either unchanged or slightly increased. Injected synapsin I may thus bind to presynaptic vesicles and prevent transmitter quanta from entering a pool subject to evoked release.     

Identification of interneurons with contralateral, caudal axons in the lamprey spinal cord: synaptic interactions and morphology

1. As part of a continuing investigation of the organization of the spinal cord of the lamprey, propriospinal interneurons with axons projecting contralaterally and caudally (CC interneurons) were surveyed with intracellular recordings. 2. CC interneurons were identified by recording their axon spikes extracellularly in the spinal cord during intracellular stimulation of the cell body. The axon projections of Cc interneurons were confirmed after intracellular injection and development of horseradish peroxidase. 3. Intracellular stimulation of CC interneurons produced synaptic potentials in myotomal motoneurons, lateral interneurons and other CC interneurons that lay caudally on the opposite side of the spinal cord. Most CC interneurons were inhibitory, but some were excitatory. 4. CC interneurons were divided into three classes on the basis of reticulospinal Müller cell inputs. CC1 interneurons were excited by the ipsilateral Müller cell B1 and the contralateral Mauthner cell. CC1 interneurons were inhibitory. They were excited polysynaptically by ipsilateral sensory dorsal cells and were inhibited by contralateral dorsal cells. They were distinguished morphologically by having no rostral axon branch and no contralateral dendrites. CC1 interneurons were phasically active during fictive swimming with their peak depolarizations preceding those of myotomal motoneurons by about 0.15 cycle. 5. CC2 interneurons were also inhibitory, but they were distinguished from CC1 interneurons by their excitation from the ipsilateral Müller cells B2-4 nd by their thin rostral and thicker caudal axonal branches on the contralateral side of the spinal cord. 6. CC3 interneurons were excitatory, and they were inhibited by the ipsilateral Müller cell I1. CC3 interneurons could have contralateral dendrites and bifurcating axons, and they had lower average axonal conduction velocities than CC1 and CC2 interneurons. 7. Inhibitory CC interneurons may be important for motor coordination in the lamprey. Movements of the lamprey body during reflexes and swimming consist of contraction and relaxation of myotomal muscles on opposite sides of the body. By being coactive with ipsilateral myotomal motoneurons, inhibitory CC interneurons could contribute to the inhibition of contralateral motoneurons during these movements.     

Responses of reticulospinal neurons in intact lamprey to vestibular and visual inputs

A lamprey maintains the dorsal-side-up orientation due to the activity of postural control system driven by vestibular input. Visual input can affect the body orientation: illumination of one eye evokes ipsilateral roll tilt. An important element of the postural network is the reticulospinal (RS) neurons transmitting commands from the brain stem to the spinal cord. Here we describe responses to vestibular and visual stimuli in RS neurons of the intact lamprey. We recorded activity from the axons of larger RS neurons with six extracellular electrodes chronically implanted on the surface of the spinal cord. From these multielectrode recordings of mass activity, discharges in individual axons were extracted by means of a spike-sorting program, and the axon position in the spinal cord and its conduction velocity were determined. Vestibular stimulation was performed by rotating the animal around its longitudinal axis in steps of 45 degrees through 360 degrees. Nonpatterned visual stimulation was performed by unilateral eye illumination. All RS neurons were classified into two groups depending on their pattern of response to vestibular and visual stimuli; the groups also differed in the axon position in the spinal cord and its conduction velocity. Each group consisted of two symmetrical, left and right, subgroups. In group 1 neurons, rotation of the animal evoked both dynamic and static responses; these responses were much larger when rotation was directed toward the contralateral labyrinth, and the dynamic responses to stepwise rotation occurred at any initial orientation of the animal, but they were more pronounced within the angular zone of 0-135 degrees. The zone of static responses approximately coincided with the zone of pronounced dynamic responses. The group 1 neurons received excitatory input from the ipsilateral eye and inhibitory input from the contralateral eye. When vestibular stimulation was combined with illumination of the ipsilateral eye, both dynamic and static vestibular responses were augmented. Contralateral eye illumination caused a decrease of both types of responses. Group 2 neurons responded dynamically to rotation in both directions throughout 360 degrees. They received excitatory inputs from both eyes. Axons of the group 2 neurons had higher conduction velocity and were located more medially in the spinal cord as compared with the group 1 neurons. We suggest that the reticulospinal neurons of group 1 constitute an essential part of the postural network in the lamprey. They transmit orientation-dependent command signals to the spinal cord causing postural corrections. The role of these neurons is discussed in relation to the model of the roll control system formulated in our previous studies.     

SK channels regulate excitatory synaptic transmission and plasticity in the lateral amygdala

At glutamatergic synapses, calcium influx through NMDA receptors (NMDARs) is required for long-term potentiation (LTP); this is a proposed cellular mechanism underlying memory and learning. Here we show that in lateral amygdala pyramidal neurons, SK channels are also activated by calcium influx through synaptically activated NMDARs, resulting in depression of the synaptic potential. Thus, blockade of SK channels by apamin potentiates fast glutamatergic synaptic potentials. This potentiation is blocked by the NMDAR antagonist AP5 (D(-)-2-amino-5-phosphono-valeric acid) or by buffering cytosolic calcium with BAPTA. Blockade of SK channels greatly enhances LTP of cortical inputs to lateral amygdala pyramidal neurons. These results show that NMDARs and SK channels are colocalized at glutamatergic synapses in the lateral amygdala. Calcium influx through NMDARs activates SK channels and shunts the resultant excitatory postsynaptic potential. These results demonstrate a new role for SK channels as postsynaptic regulators of synaptic efficacy.     

Carbenoxolone inhibition of voltage-gated Ca channels and synaptic transmission in the retina

We show that carbenoxolone, a drug used to block hemichannels in the retina to test the ephaptic model of horizontal cell inhibitory feedback, has strong inhibitory effects on voltage-gated Ca channels. Carbenoxolone (100 microM) reduced photoreceptor-to-horizontal cell synaptic transmission by 92%. Applied to patch-clamped, isolated cone photoreceptors, carbenoxolone inhibited Ca channels with an EC(50) of 48 microM. At 100 microM, it reduced cone Ca channel current by 37%, reduced depolarization-evoked [Ca(2+)] signals in fluo-4 loaded retinal slices by 57% and inhibited Ca channels in Müller cells by 52%. A synaptic transfer model suggests that the degree of block of Ca channels accounts for the reduction in synaptic transmission. These results suggest broad inhibitory actions for carbenoxolone in the retina that must be considered when interpreting its effects on inhibitory feedback.