Modulation of cellular and synaptic variability in the lamprey spinal cord

Variability is increasingly recognized as a characteristic feature of cellular, synaptic, and network properties. While studies have traditionally focused on mean values, significant effects can result from changes in variance. This study has examined cellular and synaptic variability in the lamprey spinal cord and its modulation by the neuropeptide substance P. Cellular and synaptic variability differed in different types of cell and synapse. Substance P reduced the variability of subthreshold locomotor-related depolarizations and spiking in motor neurons during network activity. These effects were associated with a reduction in the variability of spiking in glutamatergic excitatory network interneurons and with a reduction in the variance of excitatory interneuron-evoked excitatory postsynaptic potentials (EPSPs). Substance P also reduced the variance of postsynpatic potentials (PSPs) from crossing inhibitory and excitatory interneurons, but it increased the variance of inhibitory postsynpatic potentials (IPSPs) from ipsilateral inhibitory interneurons. The effects on the variance of different PSPs could occur with or without changes in the PSP amplitude. The reduction in the variance of excitatory interneuron-evoked EPSPs was protein kinase A, calcium, and N-methyl-d-aspartate (NMDA) dependent. The NMDA dependence suggested that substance P was acting postsynaptically. This was supported by the reduced variability of postsynaptic responses to glutamate by substance P. However, ultrastructural analyses suggested that there may also be a presynaptic component to the modulation, because substance P reduced the variability of synaptic vesicle diameters in putative glutamatergic terminals. These results suggest that cellular and synaptic variability can be targeted for modulation, making it an additional source of spinal cord plasticity.     

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).     

Developmental changes in short-term synaptic depression in the neonatal mouse spinal cord

We examined age-dependent changes in short-term synaptic depression of monosynaptic excitatory postsynaptic potentials (EPSPs) recorded in lumbar motoneurons in hemisected spinal cords of neonatal Swiss-Webster mice between postnatal day 2 (P2) and 12 (P12). We used four paradigms that sample the input-output dependence on stimulation history in different but complementary ways: 1) paired-pulse depression; 2) steady-state depression during constant frequency trains; 3) modulation during irregular stimulation sequences; and 4) recovery after high-frequency conditioning trains. Paired-pulse synaptic depression declined more than steady-state depression during 10-pulse trains at frequencies from 0.125 to 8 Hz in this age range. Depression during sequences of irregular stimulations that more closely mimic physiological activation also declined with postnatal age. On the other hand, the overall rate of synaptic recovery after a 4-Hz conditioning train exhibited surprisingly little change between P2 and P12. Control experiments indicated that these observations depend primarily, if not exclusively, on changes in presynaptic transmitter release. The data were examined using quantitative models that incorporate factors that have been suggested to exist at more specialized central synapses. The model that best predicted the observations included two presynaptic compartments that are depleted during activation, plus two superimposed processes that enhance transmitter release by different mechanisms. One of the latter produced rapidly-decaying enhancement of transmitter release fraction. The other mechanism indirectly enhanced the rate of renewal of one of the depleted presynaptic compartments. This model successfully predicted the constant frequency and irregular sequence data from all age groups, as well as the recovery curves following short, high-frequency tetani. The results suggest that a reduction in release fraction accounts for much of the decline in synaptic depression during early postnatal development, although changes in both enhancement processes also contribute. The time constants of resource renewal showed surprisingly little change through the first 12 days of postnatal life.     

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.     

Slow dorsal-ventral rhythm generator in the lamprey spinal cord

In the isolated lamprey spinal cord, a very slow rhythm (0.03-0.11 Hz), superimposed on fast N-methyl-D-aspartate (NMDA)-induced locomotor activity (0.26-2.98 Hz), could be induced by a blockade of GABA(A) or glycine receptors or by administration of (1 s, 3 s)-l-aminocyclopentane-1,3-dicarboxylic acid a metabotropic glutamate receptor agonist. Ventral root branches supplying dorsal and ventral myotomes were exposed bilaterally to study the motor pattern in detail. The slow rhythm was expressed in two main forms: 1) a dorsal-ventral reciprocal pattern was the most common (18 of 24 preparations), in which bilateral dorsal branches were synchronous and alternated with the ventral branches, in two additional cases a diagonal dorsal-ventral reciprocal pattern with alternation between the left (or right) dorsal and the right (or left) ventral branches was observed; 2) synchronous bursting in all branches was encountered in four cases. In contrast, the fast locomotor rhythm occurred always in a left-right reciprocal pattern. Thus when the slow rhythm appeared in a dorsal-ventral reciprocal pattern, fast rhythms would simultaneously display left-right alternation. A longitudinal midline section of the spinal cord during ongoing slow bursting abolished the reciprocal pattern between ipsilateral dorsal and ventral branches but a synchronous burst activity could still remain. The fast swimming rhythm did not recover after the midline section. These results suggest that in addition to the network generating the swimming rhythm in the lamprey spinal cord, there is also a network providing slow reciprocal alternation between dorsal and ventral parts of the myotome. During steering, a selective activation of dorsal and ventral myotomes is required and the neural network generating the slow rhythm may represent activity in the spinal machinery used for steering.     

Sensory cells in the spinal cord of the sea lamprey

1. Intracellular recordings were made from dorsal cells in the spinal cord of the sea lamprey.2. Dorsal cells were excited by mechanical stimulation of the ipsilateral skin and were established as first-order sensory cells using a variety of physiological criteria. Receptive fields were mapped.3. Dorsal cells were subdivided into three functional types on the basis of their responses to mechanical stimulation of the skin. Touch (T) cells gave rapidly adapting responses to indentation of the skin. Pressure (P) cells gave slowly adapting responses to indentation of the skin. Nociceptive (N) cells gave slowly adapting responses to severe (often destructive) indentation of the skin. The three types also differed in their spontaneous activity and in their response to repeated stimulation.4. Pressure and nociceptive cells were excited by heating the skin over the receptive field. The amount of heat necessary to excite nociceptive cells was greater than that necessary for pressure cells and often left a permanent burn mark on the skin.5. The three types of dorsal cell also showed differences in resting potential, membrane time constant, spike threshold, and response to sustained depolarization.     

Pulsatile spinal cord surrogate for intradural neuromodulation studies

We have designed, built and tested a novel spinal cord surrogate that mimics the low-amplitude cardiac-driven pulsations of the human spinal cord, for use in developing intradural implants to be used in a novel form of neuromodulation for the treatment of intractable pain and motor system dysfunction. The silicone surrogate has an oval cross section, 10 mm major axis × 6 mm minor axis, and incorporates a 3 mm diameter × 3 cm long angioplasty balloon that serves as the pulsation actuator. When pneumatically driven at 1 Hz and 1.5 atmospheres (≈ 1140 mm Hg), the surrogate's diametric pulsation is ≈ 100 μm, which corresponds well to in vivo observations. The applications for this surrogate are presented and discussed.     

Distribution of serotonergic neurons and processes in the lamprey spinal cord

The distribution of serotonin in the spinal cord in two species of lamprey, Ichthyomyzon unicuspis and Petromyzon marinus, was studied by indirect immunofluorescence techniques. Multipolar cell bodies containing serotonin-like immunoreactivity were found along the length of the spinal cord, along the midline and slightly ventral to the central canal. These cell bodies send a diffuse projection of processes throughout the spinal cord, including: (1) a dense projection to the ventral surface; (2) a strong projection to the ventromedial longitudinal fiber tracts; (3) a less intense projection to the dorsal longitudinal fiber tracts; and (4) a weak projection to the lateral fiber tracts. Lesion experiments showed that processes descending from the brain or rostral spinal cord provide a major projection to the lateral fiber tracts and smaller contributions to the dorsal and ventromedial fiber tracts. Fluorescent processes were also observed in the dorsal roots and serotonergic peripheral cell bodies were seen adjacent to the dorsal roots.Our results suggest that the serotonergic innervation of the lamprey spinal cord arises from three sources: spinal interneurons, descending tracts and peripheral (possibly sensory) input. This provides an anatomical substrate for our recent finding^12,13 that serotonin modulates the central pattern generator for locomotion in the lamprey spinal cord.     

Transmitter phenotypes of commissural interneurons in the lamprey spinal cord

The fundamental network for locomotion in all vertebrates contains a central pattern generator or CPG that produces the required motor output in the spinal cord. In the lamprey spinal cord different classes of interneuron's forming the core CPG circuitry have been characterized based on their morphological and electrophysiological features. The commissural interneuron's (C-INs) represent one essential component of CPG that have been implicated in controlling left–right alternation of the motor activity during swimming. However, it is still unclear if the C-INs displays a homogenous neurotransmitter phenotype and how they are distributed. In this paper we investigated the segmental distribution of glycine, glutamate and GABA-immunoreactive (ir) C-INs by combining retrograde Neurobiotin tracing with specific antibodies for these transmitters. The C-INs were more abundant in caudal and rostral segments adjacent to the injection site and their number gradually decreased in more distal segments, suggesting that these interneurons project over a short distance. The glycine-ir neurons represented around 50% of the total C-INs, while glutamate-ir neurons represented only 29%. Both types of C-INs were homogenously distributed over different segments along the spinal cord. Finally, no Neurobiotin labeled C-INs displayed GABA-ir, although many interneurons were ir to GABA, suggesting that GABAergic interneurons are not directly responsible for controlling left–right alternation of activity during locomotion in lamprey. Overall, these results show that the C-INs display a gradual rostrocaudal distribution and consist of both glycine- and glutamate-ir neurons. The difference in the proportion of inhibitory and excitatory C-INs represents an anatomical substrate that can ensure the predominance of alternating activity during locomotion.     

Neurotrophins and synaptic plasticity in the mammalian spinal cord

The pathway mediating the monosynaptic stretch reflex has served as an important model system for studies of plasticity in the spinal cord. Its usefulness is extended by evidence that neurotrophins, particularly neurotrophin-3 (NT-3), which has been shown to promote spinal axon elongation, can modulate the efficacy of the muscle spindle-motoneurone connection both after peripheral nerve injury and during development. The findings summarized here emphasize the potential for neurotrophins to modify function of both damaged and undamaged neurones. It is important to recognize that these effects may be functionally detrimental as well as beneficial.     

GABA- and glycine-immunoreactive terminals contacting motoneurons in lamprey spinal cord

Double postembedding GABA- and glycine-immunostaining was performed on the lamprey (Lampetra fluviatilis) spinal cord after previous HRP labeling of motoneurons. Immunopositive boutons contacting motoneurons were counted and distinguished as GABA (39%), glycine (30%) and both GABA+glycine-immunopositive (31%). Densely-packed, flattened synaptic vesicles were only observed in glycine-immunopositive boutons while GABA-immunoreactive and GABA+glycine-immunoreactive boutons contained rounded or oval synaptic vesicles. Dense-core vesicles of different diameters were associated with conventional synaptic vesicles in 74% of GABA-only-immunopositive boutons, 50% of double GABA+glycine-immunopositive boutons, but were only observed in 9% of glycine-only-immunopositive boutons. The presence of terminals immunoreactive to either GABA or glycine contacting the motoneurons suggests that there is a morphological substrate for both GABAergic and glycinergic postsynaptic inhibition of motoneurons in the lamprey spinal cord.     

Cholinergic modulation of the locomotor network in the lamprey spinal cord

Acetylcholine (ACh) was found here to be a strong modulator of swimming activity in the isolated spinal cord preparation of the adult lamprey (Ichthyomyzon unicuspis). During fictive swimming induced with either D-glutamate or N-methyl-D-aspartate, addition of ACh (200 microM) significantly reduced the cycle period of ventral root bursts to 54%, intersegmental phase lag to 32%, and ventral root burst proportion to 80% of control levels. Effects of ACh were apparent at concentrations as low as 1 microM. Both nicotinic and muscarinic receptors are involved, in that application of either nicotinic or muscarinic agonists alone significantly reduced cycle period. There is sufficient endogenous ACh in the spinal cord to modulate ongoing fictive swimming, as shown by application of the cholinesterase inhibitor eserine (physostigmine). Eserine (20 microM) significantly reduced the cycle period to 78% and phase lag to 58% of control levels, and these effects were reversed with the addition of cholinergic blockers. Addition of only a nicotinic or muscarinic antagonist, mecamylamine (10 microM) or scopolamine (20 microM), respectively, to the spinal cord during fictive swimming produced significant increases in cycle period and phase lag, suggesting that both types of cholinergic receptors participate in endogenous cholinergic modulation. It is concluded that ACh is an endogenous modulator of the locomotor network in the lamprey spinal cord and that ACh may take part in the regulation of cycle period, intersegmental coupling, and ventral root burst duration.     

Spinobulbar neurons in lamprey: cellular properties and synaptic interactions

An in vitro preparation of the nervous system of the lamprey, a lower vertebrate, was used to characterize the properties of spinal neurons with axons projecting to the brain stem [i.e., spinobulbar (SB) neurons)]. To identify SB neurons, extracellular electrodes on each side of the spinal cord near the obex recorded the axonal spikes of neurons impaled with sharp intracellular microelectrodes in the rostral spinal cord. The ascending spinal neurons (n = 144) included those with ipsilateral (iSB) (63/144), contralateral (cSB) (77/144), or bilateral (bSB) (4/144) axonal projections to the brain stem. Intracellular injection of biocytin revealed that the SB neurons had small- to medium-size somata and most had dendrites confined to the ipsilateral side of the cord, although about half of the cSB neurons also had contralateral dendrites. Most SB neurons had multiple axonal branches including descending axons. Electrophysiologically, the SB neurons were similar to other lamprey spinal neurons, firing spikes throughout long depolarizing pulses with some spike-frequency adaptation. Paired intracellular recordings between SB and reticulospinal (RS) neurons revealed that SB neurons made either excitatory or inhibitory synapses on RS neurons and the SB neurons received excitatory input from RS neurons. Mutual excitation and feedback inhibition between pairs of RS and SB neurons were observed. The SB neurons also received excitatory inputs from primary mechanosensory neurons (dorsal cells), and these same SB neurons were rhythmically active during fictive swimming, indicating that SB neurons convey both sensory and locomotor network information to the brain stem.     

Synaptic regeneration and glial reactions in the transected spinal cord of the lamprey

Summary We have examined axonal growth and synaptic regeneration in identified giant neurons of the transected lamprey spinal cord using intracellular injection of horseradish peroxidase. Wholemounts together with serial section light and electron microscopy, show that axons from identified Müller and Mauthner reticulospinal neurons grow across the lesion and regenerate new synaptic contacts. Relatively normal swimming returns in these animals by 3–4 weeks after spinal transection. This occurs despite the formation of regenerated synapses in regions of the cord that are not usually occupied by these neurons.The regenerating axons branch profusely in contrast to their unbranched state in the normal animal. In addition to showing the two synaptic configurations found normally, synapses may be formed by slender sprouts from the growing giant axon. These sprout type synaptic contacts appear unique to the regenerating neuron. Only regenerated chemical synapses were seen; the morphologically mixed chemical and electrical (gap junction) synaptic complex common in the normal animal was not observed at regenerated synapses.The site of spinal transection in the functionally recovered animal shows an increase in the number of ependymal and glial cells. Ependymal-like cells appear in regions away from the central canal. The expanded ependymal and glial processes covering the peripheral surface of the injured cord become convoluted, in contrast to their normal smooth configuration. There is no collagen within the cord at the site of transection but a considerable deposition is seen external to the cord surface.Axonal growth across a spinal lesion and subsequent synaptic regeneration can be examined in single identifiable giant interneurons in the spinal cord of the larval lamprey. This preparation may be used as an assay to investigate factors that could contribute to functional recovery following central nervous system injury in the higher vertebrates.     

Commissural interneurons in rhythm generation and intersegmental coupling in the lamprey spinal cord.

Commissural interneurons in rhythm generation and intersegmental coupling in the lamprey spinal cord. To test the necessity of spinal commissural interneurons in the generation of the swim rhythm in lamprey, longitudinal midline cuts of the isolated spinal cord preparation were made. Fictive swimming was then induced by bath perfusion with an excitatory amino acid while recording ventral root activity. When the spinal cord preparation was cut completely along the midline into two lateral hemicords, the rhythmic activity of fictive swimming was lost, usually replaced with continuous ventral root spiking. The loss of the fictive swim rhythm was not due to nonspecific damage produced by the cut because rhythmic activity was present in split regions of spinal cord when the split region was still attached to intact cord. The quality of this persistent rhythmic activity, quantified with an autocorrelation method, declined with the distance of the split spinal segment from the remaining intact spinal cord. The deterioration of the rhythm was characterized by a lengthening of burst durations and a shortening of the interburst silent phases. This pattern of deterioration suggests a loss of rhythmic inhibitory inputs. The same pattern of rhythm deterioration was seen in preparations with the rostral end of the spinal cord cut compared with those with the caudal end cut. The results of this study indicate that commissural interneurons are necessary for the generation of the swimming rhythm in the lamprey spinal cord, and the characteristic loss of the silent interburst phases of the swimming rhythm is consistent with a loss of inhibitory commissural interneurons. The results also suggest that both descending and ascending commissural interneurons are important in the generation of the swimming rhythm. The swim rhythm that persists in the split cord while still attached to an intact portion of spinal cord is thus imposed by interneurons projecting from the intact region of cord into the split region. These projections are functionally short because rhythmic activity was lost within approximately five spinal segments from the intact region of spinal cord.     

Neuropeptide-mediated facilitation and inhibition of sensory inputs and spinal cord reflexes in the lamprey

The effects of neuromodulators present in the dorsal horn [tachykinins, neuropeptide Y (NPY), bombesin, and GABAB agonists] were studied on reflex responses evoked by cutaneous stimulation in the lamprey. Reflex responses were elicited in an isolated spinal cord preparation by electrical stimulation of the attached tail fin. To be able to separate modulator-induced effects at the sensory level from that at the motor or premotor level, the spinal cord was separated into three pools with Vaseline barriers. The caudal pool contained the tail fin. Neuromodulators were added to this pool to modulate sensory inputs evoked by tail fin stimulation. The middle pool contained high divalent cation or low calcium Ringer to block polysynaptic transmission and thus limit the input to the rostral pool to that from ascending axons that project through the middle pool. Ascending inputs and reflex responses were monitored by making intracellular recordings from motor neurons and extracellular recordings from ventral roots in the rostral pool. The tachykinin neuropeptide substance P, which has previously been shown to potentiate sensory input at the cellular and synaptic levels, facilitated tail fin-evoked synaptic inputs to neurons in the rostral pool and concentration dependently facilitated rostral ventral root activity. Substance P also facilitated the modulatory effects of tail fin stimulation on ongoing locomotor activity in the rostral pool. In contrast, NPY and the GABAB receptor agonist baclofen, both of which have presynaptic inhibitory effects on sensory afferents, reduced the strength of ascending inputs and rostral ventral root responses. We also examined the effects of the neuropeptide bombesin, which is present in sensory axons, at the cellular, synaptic, and reflex levels. As with substance P, bombesin increased tail fin stimulation-evoked inputs and ventral root responses in the rostral pool. These effects were associated with the increased excitability of slowly adapting mechanosensory neurons and the potentiation of glutamatergic synaptic inputs to spinobulbar neurons. These results show the possible behavioral relevance of neuropeptide-mediated modulation of sensory inputs at the cellular and synaptic levels. Given that the types and locations of neuropeptides in the dorsal spinal cord of the lamprey show strong homologies to that of higher vertebrates, these results are presumably relevant to other vertebrate systems.     

Afferent input modulates neurotrophins and synaptic plasticity in the spinal cord

The effects of eliminating or decreasing neuromuscular activity on the expression of neurotrophins and associated molecules in the spinal cord and subsequent effects on spinal cord plasticity were determined. Spinal cord isolation (SI), which eliminates any supraspinal and peripheral monosynaptic input to the lumbar region but maintains the motoneuron-muscle connectivity, decreased the levels of brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT-3) mRNA and protein in the isolated segments. Synapsin I, an important mediator for the effects of BDNF on synaptic plasticity, also was lower in the lumbar region of SI rats. In contrast, the levels of BDNF, synapsin, and growth-associated protein (GAP-43) were increased in the cervical spinal cord enlargement rostral to the isolated region, most likely reflecting an increased use of the forelimbs in the SI rats. GAP-43 levels were also increased in the lumbar spinal cord region, probably associated with compensatory mechanisms related to the deafferentation. In a separate set of experiments, the soleus muscle was paralyzed unilaterally via intramuscular botulinum toxin type A (BTX-A) injection to determine the effects of reducing the propioceptive input, of this normally highly active muscle on neurotrophin expression in the spinal cord. BDNF and synapsin I mRNAs were lower and NT-3 levels were higher in the lumbar hemicord ipsilateral to the BTX-A injection. Combined, these results indicate that the level of supraspinal and muscle afferent input plays an important role in modulating the levels of BDNF and NT-3 in the spinal cord.     

Primary receptor for inhibitory transmitters in lamprey spinal cord neurons.

The action of glycine and GABA on isolated lamprey spinal cord neurons was investigated by means of intracellular perfusion and concentration clamp techniques. These amino acids activated desensitizing chloride ionic conductances. The concentrations of agonists evoking half-maximum effects (ED50) were equal to 16 microM and 1.5 mM for glycine- and GABA-activated currents, respectively. Increase in the transmitter concentration led to a decrease in the time constant of desensitization. Current-voltage relationships of glycine- and GABA-activated currents were strongly dose-dependent. At low agonist concentrations the time constant of activation decreased with membrane hyperpolarization. Glycine- and GABA-activated currents exhibited complete cross-desensitization. The specific glycine antagonist, strychnine, suppressed both glycine- and GABA-activated currents to the same degree. Selective antagonists of GABA receptors, bicuculline and picrotoxin, produced equal blocking effects on glycine- as well as GABA-evoked responses. In the cells studied, taurine activated desensitizing ionic conductance. Responses evoked by taurine and glycine applications demonstrated complete cross-desensitization. Taurine-activated currents were sensitive to strychnine, bicuculline and picrotoxin. These results suggest the existence of one receptor-channel complex for the main inhibitory transmitters in lamprey spinal cord neurons. 3,4-Dioxy-L-beta-phenylalanine evoked desensitizing strychnine-sensitive ionic responses which exhibited cross-desensitization with glycine-activated currents.     

Efficiency of electrical transmission in reticulomotoneuronal synapses of lamprey spinal cord

Summary Synaptic responses in motoneurons in the isolated spinal cord of the lamprey during stimulation of the reticulospinal axons were examined. Chemical transmission in the synapses was partially or completely blocked by temperature reduction of the perfusing solution, pentobarbitone application or substitution of Mn²⁺ ions for Ca²⁺ in the perfusate. Excitatory postsynaptic potentials (EPSPs) which were indifferent to the influences mentioned above, had an amplitude of 6–12 mV and were capable of evoking action potentials (APs) in motoneurons due to their high amplitude, the absence of a shunting effect at the postsynaptic membrane and the fast rise-time of the wave front. The suggestion is made that the electrical transmission is involved in functioning of the lamprey nervous system. Its stability and efficiency are likely to ensure functional connection between the brain and spinal cord under such unfavourable conditions when the chemical transmission does not operate and when the ability for locomotion would be prerequisite for the individual to survive.