Inhibition of a cAMP-dependent Ca-activated K conductance by forskolin prolongs Ca action potential duration in lamprey sensory neurons.

Intracellular recordings from primary mechanosensory neurons (dorsal cells) of the lamprey spinal cord were made to test the membrane effects of forskolin, an activator of adenylate cyclase in these cells. At a concentration of 50 microM, forskolin was found to have a pronounced broadening effect on calcium action potentials (Ca APs) produced in the presence of voltage-activated K channel blockers (TEA, 3,4-DAP). Forskolin had no effect on passive membrane properties of the cells, such as resting potential or input resistance. Nor did it affect delayed rectification or Na APs and thus appeared not to block voltage-activated K channels. Forskolin's effect was evident only when a significant Ca component to the AP was present. It appeared not to increase the conductance of the Ca channel since its action was accompanied by a decrease in membrane conductance during the Ca AP, indicating instead an inhibition of a repolarizing Ca-activated conductance, other than a Ca-activated Cl conductance. The prolongation of Ca APs by forskolin, barium or the neurotransmitter GABA were all correlated in voltage-clamp with a decrease in outward current. Under the conductions used here, the major outward conductance in dorsal cells is a Ca-activated K conductance (gK(Ca]28, and it is concluded that the most probable mode of action for forskolin is via a cyclic AMP-mediated inhibition of this conductance. GABA has also been shown to prolong Ca APs in lamprey dorsal cells by inhibiting a repolarizing gK(Ca)28. Thus, the action of this transmitter may be mediated by an increase in intracellular cyclic AMP.

Activation of adenylate cyclase by forskolin prolongs calcium action potential duration in lamprey sensory neurons.

Calcium-dependent action potentials of primary sensory neurons in the isolated spinal cord of the lamprey were greatly prolonged in duration by forskolin, an activator of adenylate cyclase in other systems. This effect was dose-dependent over the tested range of 25-400 microM with an EC50 of 55 microM. Experiments were performed to establish a role for adenylate cyclase and adenosine 3',5'-cyclic monophosphate (cAMP) as mediators of the forskolin effect. The prolonging action of forskolin on the Ca action potential was significantly reduced in the presence of the adenylate cyclase inhibitor 2',5'-dideoxyadenosine. The inactive forskolin analogue 1,9-dideoxyforskolin did not prolong the duration of the Ca action potential, while forskolin treatment of the same cells produced a large and rapid increase in action potential duration. In addition, the prolonging action of forskolin was potentiated by the phosphodiesterase inhibitor, theophylline. It is concluded that forskolin acts in lamprey sensory neurons to activate adenylate cyclase and raise intracellular cAMP levels which in turn mediate the increase in Ca action potential duration.

Prolongation of calcium action potentials by gamma-aminobutyric acid in primary sensory neurones of lamprey.

Intracellular recordings from primary mechanosensory neurones (dorsal cells) in the lamprey spinal cord were used to test the membrane effects of a variety of putative neuromodulatory agents. gamma-Aminobutyric acid (GABA) produced a dose-dependent increase in the duration of mixed Na-Ca or pure Ca action potentials in these cells. L-Glutamate and glycine produced minimal broadening of Ca action potentials. Acetylcholine, noradrenaline, serotonin, met-enkephalin, D-glutamate and dopamine had no effect. The pharmacology of GABA's action appeared to be complex. While the GABAA receptor antagonists, bicuculline, picrotoxin and curare, did not block GABA's effect, both the GABAA receptor agonist, muscimol, and the GABAB-receptor agonist, baclofen, occasionally broadened Ca action potentials in these cells. GABA had no effect on the resting potential, passive current-voltage (I-V) characteristics and pure Na action potential of dorsal cells, ruling out an action on passive membrane channels, transmitter-activated channels, or on those voltage-dependent channels activated during the Na action potential. Thus, GABA affected dorsal cells only when a significant Ca current was evident. GABA appeared not to increase the conductance of the Ca channels since its action was accompanied by an increase in input resistance, suggesting an inhibition of Ca-dependent conductance that normally acts to repolarize the membrane during a Ca action potential. An inhibitory effect of GABA on a Ca-dependent Cl conductance was ruled out in experiments where the Cl gradient was altered by removal of extracellular Cl without affecting GABA-induced Ca action potential prolongation. Dorsal cells have a prominent Ca-dependent K conductance (gK(Ca], and it is this conductance that GABA may inhibit. Consistent with this was the observation that the hyperpolarizing after-potential that follows Ca action potentials in dorsal cells, which reflects gK(Ca) in these cells and whose duration is normally increased when the Ca action potential duration increases, was not prolonged when the Ca action potential was broadened by GABA. Further, the failure of GABA to prolong Ba action potentials was consistent with this proposed mechanism of action, since Ba apparently does not activate gK(Ca) in these cells. Forskolin, a specific adenylate cyclase activator, caused broadening of Ca action potentials in lamprey dorsal cells comparable in magnitude to that of GABA. Thus, an increase in intracellular cyclic AMP is a candidate for the intracellular mediator of GABA's effect on these cells.

Ultrastructural correlates of transmitter release in presynaptic areas of lamprey reticulospinal axons.

The ultrastructure of presynaptic areas of lamprey reticulospinal axons was studied before, during, and after periods of elevated transmitter release produced either by repetitive action potential activity or depolarization by elevated extracellular potassium. Controls for possible effects of these procedures per se were done by replacing extracellular Ca with Mg to block transmitter release. In some experiments the time course of ultrastructural changes during K depolarization and subsequent recovery were studied by fixing tissue samples at various times. Transmitter release produced by action potential activity (20/sec for 15 min) in the presence of extracellular Ca significantly and reversibly decreased the number of synaptic vesicles, the area occupied by the vesicles, and the density of synaptic vesicles. An unexpected finding was a reversible decrease in the length of the differentiated membrane during periods of increased transmitter release. Transmitter release significantly and reversibly increased the number of coated vesicles, expanded the presynaptic membrane, and increased the number of pleomorphic vesicles. K depolarization (50 mM K for 15 min) produced identical, reversible effects, except that the expansion of the presynaptic membrane, although significant, was relatively small and there was no change in the number of pleomorphic vesicles. Raising the temperature of the saline from 2 degrees C (K depolarization experiments) or 7 degrees C (action potential experiments) to 20 degrees C did not change the results qualitatively but did produce somewhat larger effects during stimulation and appeared to increase the speed of recovery. Action potential activity or K depolarization in control experiments with the Ca in the saline replaced by Mg had little or no effect on synaptic ultrastructure. Synaptic vesicles in lamprey reticulospinal axons never contacted the axonal membrane anywhere other than at the differentiated membrane. During periods of elevated transmitter release, although the absolute number of vesicles in contact with the differentiated membrane decreased, the percentage of total vesicles in contact with the differentiated membrane increased dramatically. This suggests that the differentiated membrane is the site of vesicle release and there is an active process of vesicle movement to this membrane. In the course of this work it was observed that presynaptic areas closer than approximately 2 mm to the site of axonal transection, regardless of the composition of the saline or the experimental conditions, showed ultrastructural changes typical of increased transmitter release.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium spike and calcium-dependent potassium conductance in mechanosensory neurons of the lamprey.

Action potentials (APs) of long duration (up to 1 s) followed by prolonged (0.5-5 s) hyperpolarizing afterpotentials (HAP) were recorded in lamprey primary mechanosensory neurons (dorsal cells) in isolated spinal cords exposed to either or both of the potassium channel blockers, tetrathylammonium (TEA) and 3,4-diaminopyridine (DAP). The membrane events underlying the prolonged AP and HAP were investigated in current clamp studies and were shown to be a Ca spike- and a Ca-dependent K conductance, respectively. The prolonged AP was accompanied by an increased membrane conductance and, unlike the normal Na AP in these cells, was not blocked by tetrodotoxin (TTX) or by replacement of external Na with choline or TEA. Reduction of [Ca]o from 10 to 0 mM reduced the amplitude and duration of the prolonged TTX-resistant AP but did not eliminate it within the 15-min washout period, probably because of Ca buffering in the spinal cord. The overshoot of the prolonged AP varied in amplitude as a linear function of the log of the external Ca concentration (2.5-10 mM) with a slope of 31.5 mV for a 10-fold change in Ca concentration, a value close to the 28 mV expected from the Nernst relation. Co (2 mM) and Cd (1 mM) blocked the prolonged APs. Ba and Sr substituted for Ca. The APs in Ba were extremely long lasting (up to 40 s). The HAPs following Ca spikes were 0.5-5 s in duration (peak to half amplitude) and were accompanied by an increased membrane conductance. The HAP varied in amplitude with the extracellular K concentration, reversed in sign at the presumed K equilibrium potential (-90 mV), and was insensitive to injected Cl. We conclude that HAP is a result of increased K conductance. The increase in K conductance during the HAP appeared to be dependent on Ca influx, because the amplitude and duration of the HAP varied with the extracellular Ca concentration and increased in duration during repetitive Ca spike activation, presumably as a result of accumulation of Ca intracellularly. Further, the HAP was absent following even very long lasting spikes in Ba, an ion that in other cells does not activate the Ca-dependent K conductance. Small regenerative depolarizations sometimes followed Ca spikes in dorsal cell somata. These are believed to reflect Ca spikes in discrete axonal regions at various electrotonic distances from the soma.

Glycine, GABA and synaptic inhibition of reticulospinal neurones of lamprey.

1. Intracellular recordings were made from the cell bodies and axons of giant reticulospinal neurones (Müller cells) of the lamprey and the effects of a variety of putative neurotransmitters tested. Bath-applied acetylcholine, carbamylcholine, norepinephrine, dopamine, histamine and serotonin were without effect. Glycine and gamma-aminobutyric acid (GABA) hyperpolarized and reduced the input resistance of cell bodies but had no effect on the membrane conductance of axons. 2. The threshold dose of bath-applied GABA or glycine for a conductance change in somata was about 0.5 mM and the maximum effect was reached at about 10 mM. The maximum conductance change produced by glycine was always greater than that produced by GABA. 3. Replacement of the sodium in the bathing saline with lithium or choline prolonged the conductance change produced by ionophoretically applied glycine or GABA, suggesting the presence of sodium-dependent uptake systems for glycine and GABA. 4. The reversal potentials for responses to ionophoretically applied glycine and GABA average about --83 mV, the same as that for the inhibitory post-synaptic potential (i.p.s.p.) produced in Müller cells by stimulation of the ipsilateral vestibular nerve. 5. The i.p.s.p. and drug responses appeared to involve an increase in chloride conductance, since their reversal potentials were shifted appropriately by changes in either internal or external chloride. 6. Changes in extracellular potassium concentration also changed i.p.s.p. and drug reversal potentials. However, these effects could be attributed to secondary changes in internal chloride. 7. The receptors for GABA and glycine appeared to be different because of the absence of cross-desensitization and because, at doses below 20 microM, picrotoxin and bicuculline selectively blocked GABA responses while strychnine selectively blocked glycine responses. 8. At concentrations of 20 microM, strychnine eliminated the i.p.s.p. while picrotoxin and bicuculline had no effect. Further, the i.p.s.p. and glycine response of Müller cells located in the isthmic region of the midbrain had the same threshold sensitivity to strychnine. However, the glycine response of other Müller cells was more sensitive to strychnine than was the i.p.s.p. 9. We conclude that glycine is a better candidate for the inhibitory transmitter onto Müller cells than is GABA.

Glutamate and synaptic excitation of reticulospinal neurones of lamprey.

1. Intracellular recordings were made from the cell bodies and axons of giant reticulospinal neurones (Müller cells) of the lamprey, and responses to bath- and ionophoretically applied glutamate and aspartate were studied. 2. Bath-applied glutamate and aspartate depolarized both cell bodies and axons, but there appeared to be an associated conductance increase only in the cell bodies. The depolarization of Müller axons by the bath-applied drugs probably resulted from the passive flow of current into them from spinal cells to which the axons are coupled electrically. 3. The reversal potentials for responses to ionophoretically applied glutamate and for excitatory post-synaptic potentials (e.p.s.p.s) evoked by stimulation of the contralateral vestibular nerve were directly determined in Müller cell bodies which had been damaged by penetration with low-resistance electrodes. The glutamate and e.p.s.p. reversal potentials were identical, the average difference in eight cells being 0.31 mV. The absolute value of the e.p.s.p.--glutamate reversal potential varied from --16 to --35 mV in different cells, with the more negative values occurring in less damaged cells with higher resting potentials. 4. Injection of Cl into Müller cell bodies had no effect on the e.p.s.p.--glutamate reversal potential. Reduction of the extracellular Na concentration to 1 over 10 normal produced a negative shift in the glutamate reversal potential. 5. It is proposed that the natural excitatory transmitter and glutamate produce identical conductance changes in Müller cells, involving an increase in Na and K conductance.

Evoked depolarizing and hyperpolarizing potentials in reticulospinal axons of lamprey.

1. Intracellular recordings were made from reticulospinal axons (Müller axons) in the lamprey spinal cord. Electrical stimuli applied to the spinal cord surface elicited depolarizing and hyperpolarizing 'synaptic-like' potentials in Müller axons. The physiological basis of these evoked potentials was investigated. 2. The depolarizing response was not the result of increased extracellular K, as demonstrated by the constancy of the undershoot of the axonal action potential during the depolarization, by the failure of the response to summate during repetitive stimulation and by the failure of the response amplitude to vary as predicted when the [K] of the saline was varied. 3. When the membrane potential of the axon was varied by passing current through a micro-electrode, the amplitude of the depolarizing evoked potential decreased at membrane potentials positive to the resting potential and increased up to a maximum when the axon was hyperpolarized by about 10 mV. The extrapolated 'reversal potential' for the depolarizing response was about 15 mV positive to the normal -80 mV resting potential of the axon. However, the amplitude of the response did not continue to grow with hyperpolarizations greater than 10 mV, and, thus, the response did not behave as would a normal depolarizing synaptic potential. 4. Müller axons make numerous electrical synapses with spinal motoneurones and interneurones, and this suggested that the depolarizing response might be a coupling potential. In agreement with this idea, quantitative correspondence was found between changes in the input resistance of the axon produced by the depolarizing response and the variation in the depolarizing response amplitude. Thus, although the depolarizing response mimicked in some ways the behaviour of an excitatory synaptic potential, we conclude that it is a coupling potential. 5. The hyperpolarizing response also appeared to be a coupling potential. Its amplitude was not changed by hyperpolarizing the axon up to 30 mV and was decreased by depolarizing the axon sufficiently to decrease the axon's input resistance. 6. It is proposed that both depolarizing and hyperpolarizing evoked potentials in lamprey Müller axons are a result of passive flow of current from cells activated by the spinal cord stimulus and electrically coupled to Müller axons.

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.

Physiological and anatomical characteristics of reticulospinalneurones in lamprey.

1. Intracellular records were obtained from giant reticulospinal cells (Müller cells) in the brain of adult lamprey. The cells had maximum resting potentials of -80 mV and action potentials with overshoots of 30 mV. Input resistances varied from 2 to 8 MOmega.2. Individual spontaneous excitatory and inhibitory synaptic potentials (e.p.s.p.s and i.p.s.p.s) were observed, as well as occasional high frequency bursts of excitatory potentials. Much of the spontaneous synaptic activity could be eliminated by elevating the Ca(2+) concentration in the bathing solution to 10-15 mM, suggesting that the synaptic potentials were due to spike activity in elements presynaptic to Müller cells.3. Electrical stimulation of cranial nerves produced synaptic responses in Müller cells. Ipsilateral vestibular nerve stimulation produced i.p.s.p.s; contralateral stimulation, e.p.s.p.s. Stimulation of either optic nerve produced mixed synaptic responses with e.p.s.p.s dominating in cells with large resting potentials. Trigeminal nerve stimulation produced mixed responses. Olfactory nerve stimulation produced excitation. Spinal cord stimulation produced e.p.s.p.s and i.p.s.p.s, the dominant effect being inhibition.4. In favourable preparations strong electrical stimulation of cranial nerves produced afterdisharges in Müller cells, lasting from a few seconds after stimulation of the olfactory and vestibular nerves to as long as several minutes after optic, trigeminal or spinal cord stimulation.5. Natural stimulation of tactile, visual and vestibular receptors resulted in synaptic responses similar to those produced by electrical stimulation of the cranial nerves. Fish odour applied to the olfactory mucosa produced no response.6. Iontophoretic application of L-glutamate to Müller cells produced depolarization accompanied by a decrease in input resistance. In addition, glutamate produced bursts of inhibitory and excitatory synaptic potentials, presumably by depolarizing excitatory or inhibitory nerve terminals or nearby cell bodies.7. Iontophoretic application of gamma-aminobutyric acid (GABA) resulted in a slight hyperpolarization, accompanied by a large reduction in input resistance. The reversal point both of the hyperpolarizations and of the spontaneous inhibitory post-synaptic potentials was about 6 mV greater than the resting potential.8. There were two types of synaptic ending on Müller cell bodies, one type containing round vesicles and the other containing ellipsoidal vesicles. These terminals were intermixed over the surface of the cell bodies and dendrites with no readily apparent segregation.9. Intracellular records from the spinal axons of Müller cells during electrical stimulation of cranial nerves and spinal cord showed, in addition to the normal propagating action potential activity which normally originates in the cell bodies, depolarizing, hyperpolarizing and biphasic evoked potentials. These membrane responses were grossly similar in appearance to synaptic potentials except that the large depolarizing potentials had unusually long decay times. The physiological basis of these potentials remains unclear.10. Electron microscopic examination showed very few synaptic endings afferent to Müller axons, a finding in contrast to the abundance of synaptic-like potentials recorded. However, the occasional synapses afferent to Müller axons were invariably located near an efferent synaptic region of the axon itself. This raises the possibility that a very limited number of synaptic regions of Müller axons may be subject to presynaptic modulation of transmitter release.11. The observations reported here support the idea that Müller cells in lamprey are an important motor outflow from the brain and serve to coordinate the lamprey's trunk responses to external sensory stimulation.

Effects of guanidine on transmitter release and neuronal excitability.

1. Guanidine hydrochloride (CH5N3-HCl) was applied to frog neuromuscular junctions blocked by reduced external Ca2+, or increased external Mg2+ concentration, or by both. Guanidine produced a dose-dependent increase in the average number of quanta released by presynaptic action potentials, the threshold dose being 0-1-0-2 mM. No post-synaptic effects were observed. 2. Guanidine also increased the excitability of the motor nerve fibres, as evidenced by multiple firing to single electrical stimuli and finally by spontaneous action potentials. These effects were studied in greater detail in giant axons (Müller axons) in the spinal cord of lamprey. Exposure to guanidine produced in these axons a progressive increase in excitability, manifested by repetitive firing to a single electrical stimulus, spontaneous membrane potential oscillations and spontaneous bursts of action potentials. Guanidine had no effect on the resting potential. 3. The effect of guanidine on the excitability of Müller axons was mimicked in every detail simply by reducing the divalent cation concentration of the bathing solution. 4. Guanidine also produced dose-dependent increases in the duration of action potentials in Müller axons. This effect always preceded in time the appearance of the excitability effects and was not mimicked by reducing the divalent cation concentration. It is suggested that the broadening of the action potential is separate from the excitability effects and may reflect a decrease of delayed rectification. 5. Guanidine (0-3 mM) increased the frequency of miniature end-plate potentials (min. e.p.p.) in solutions containing 2-11 mM-K+ in such a way as to shift the relationship between min. e.p.p. frequency and extracellular K+ toward lower values of K+. This effect was interpreted to mean that guanidine produced a depolarization of the nerve terminal which summed with the depolarization produced by a given concentration of K+. The calculated depolarization produced by 0-3 mM guanidine was 5-7 mV. 6. The effects of guanidine on evoked transmitter release, excitability, and min. e.p.p. frequency are consistent with a hypothesis which states that guanidine binds at or near fixed negative changes on the outside of nerve membrane and reduces the screening effect of divalent cations.

On the effect of calcium on the frequency of miniature end-plate potentials at the frog neuromuscular junction.

1. The effect of the extracellular Ca concentration on the frequency of miniature end-plate potentials (min. e.p.p.s) at the frog neuromuscular junction was studied. 2. In saline containing elevated K (5 or 11 mM), the frequency of min. e.p.p.s increased as Ca concentration was increased from 0-1 to 1-3 mM. However, with further increases of Ca concentration up to 10 mM, min. E.P.P. frequency declined. 3. In saline containing the normal concentration of K (2 mM), increasing Ca concentration from 0-1 to 10 mM produced a slight, monotonic increase in min. e.p.p. frequency. 4. The non-monotonic effect of Ca on min. e.p.p. frequency in preparations depolarized by elevated K is consistent with the existence of two opposing effects of Ca on transmitter release. Firstly, raising the external concentration of Ca increases the electrochemical potential for Ca entry, which tends to increase Ca influx and transmitter release. Secondly, increasing external Ca concentration increases electrostatic screening of fixed negative charges on the outer surface of the nerve terminal membrane. Such an increase in screening of charges near voltage-sensitive Ca gates would produce a hyperpolarization across the gates and they would tend to close, an effect which would tend to decrease Ca influx. The monotonic increase in min. e.p.p. frequency with increasing Ca concentration in 2 mM-K is consistent with the voltage insensitivity of the Ca gates at potentials close to the normal resting potential.

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.

Effects of iontophoretically applied drugs on spinal interneurons of the lamprey.

1. Intracellular records were obtained from giant interneurones in the isolated spinal cord of the sea lamprey. The cells had a mean resting potential of about 75 mV and action potentials with overshoots of about 35 mV. Their input resistances, measured by passing polarizing currents through the recording pipette, were in the range 3-7 MOmega.2. Iontophoretic ejection of gamma-aminobutyric acid (GABA) from a micropipette placed near the surface of a cell resulted in a slight hyperpolarization, accompanied by a marked reduction in input resistance. The reversal point for the potential change was about 5 mV greater than the resting membrane potential.3. Iontophoretic application of L-glutamate to the cells produced a depolarization with a decrease in input resistance much smaller than that accompanying a GABA potential of similar amplitude. The action potential amplitude was reduced by L-glutamate application. The reversal potential could not be determined accurately but appeared to be near zero membrane potential.4. Glutamate application produced, in addition, a burst of inhibitory synaptic potentials in the cell, presumably by depolarizing either inhibitory presynaptic nerve terminals or nearby inhibitory cell bodies.5. Acetylcholine (ACh) produced no detectable change in membrane resistance or potential.6. Application of the three drugs to first-order sensory cells in the spinal cord had no effect on their membrane properties.