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Octopamine has been identified and measured in individual neurons from Aplysia californica. Neither dopamine nor norepinephrine was detected in these cells. Thus, in Aplysia there may be separate populations of catecholaminergic and monophenolaminergic cells. Octopamine may have functions of its own in the central nervous system of mollusks.

Extract from the abdominal ganglia of Aplysia was fractionated by high performance liquid chromatography. Several large uv absorbing peaks were found. One of these peaks from the ganglionic extract was analyzed by displacement assay with the use of Mytilus edulis membranes. The results revealed the substance in this peak was able to displace 3H-D-Ala2, met5-enkephalinamide. In electrophysiological studies of abdominal and cerebral neurons, depolarizing and hyperpolarizing responses were obtained from some neurons, including neurons in the identified cerebral B cell cluster. A smaller population of cells exhibited biphasic responses. Some of the responses could be depressed by prior naloxone treatment. In conclusion, an endogenous opioid system, using substance related to but distinct from the enkephalins, may exist in Aplysia.

Acute isolation of hippocampal CA3 pyramidal cells using trypsin produces neurons which respond to kainate and quisqualate but not N-methyl-D-aspartate (NMDA). Incubation of 6- to 12-day-old cultured hippocampal neurons or slices of pyriform cortex with trypsin irreversibly removes the NMDA responses normally present without significant effect on responses to kainate or quisqualate. These data indicate that the NMDA receptor has a trypsin-sensitive component which is necessary for agonist recognition or ion channel activation.

1. Fast Na+-, Cl-, and K+-Conductance increase responses to gamma-aminobutyric acid (GABA) show times to peak similar to the comparable ionic responses to acetylcholine (ACh). 2. On some identified neurons, both putative transmitters elicit responses due to the same conductance change. For example, in cell R2 both substances cause an increase in Cl- conductance. Receptors for GABA and ACh on R2 do not cross desensitize and therefore are distinct. The ACh but not the GABA response is blocked by alpha-bungarotoxin and strychnine. 3. In R2 both responses reverse at -58 mV, and the Cl- ionophore (for both responses) appears to be partially permeant to propionate and isethionate, but impermeant to acetate, sulfate, and methylsulfate. 4. The Cl- responses but not the Na+ responses to both ACh and GABA are blocked by both picrotoxin and bicuculline, the classical GABA antagonists. 5. These results are compatible with the hypothesis that the ionophores associated with receptors to different neurotransmitters but mediating the same ionic conductance change have many common properties and may, in fact, be identical. Bicuculline and picrotoxin may be specific blockers of the Cl- ionophore, not the GABA receptor.

The effects of ionophoretically applied N-methyl-DL-aspartate (NMDA) and aspartate on identified pyramidal neurons in rat piriform cortex were examined in isolated, submerged, and perfused brain slices. NMDA was more potent than aspartate in eliciting neuronal discharge. Perfusion of the acidic amino acid antagonists, DL-2-amino-5-phosphonovalerate (APV), 10(-6) or 10(-5) M, DL-2-amino-7-phosphonoheptanoate (APH), 10(-5) M, and gamma-D-glutamylglycine (gamma DGG), 10(-5) M, selectively blocked the response to NMDA without effect on the response to aspartate. At higher concentrations which blocked responses to both NMDA and aspartate, gamma DGG blocked kainate responses and depressed glutamate and quisqualate responses. These results suggest that in piriform neurons NMDA and aspartate act at distinct receptor sites, not a common receptor site, and that both of these sites are distinct from those that mediate responses to glutamate, quisqualate, and kainate.

Using isolated, internally perfused bullfrog dorsal root ganglion cells we have studied the dose-response curves for gamma-aminobutyric acid (GABA) in the presence of internally or externally applied GABA antagonists. With external application of antagonists the inhibition of the GABA current by bicuculline was competitive and that by picrotoxin was noncompetitive. Picrotoxin but not bicuculline blocked when internally perfused.

The influence of monoamine transmitter candidates, acetylcholine and related substances on rhythmic depolarization shifts (field bursts) in the CA 1 area of hippocampal slices from rats in low calcium (0.2 mmol X 1(-1) ) high magnesium (4 mmol X 1(-1) ) was investigated. Acetylcholine (ACh), histamine (HA) and H2-agonists, noradrenaline (NA) and beta-agonists at nano- to micromolar concentrations as well as dopamine (DA) and 8-bromo-cyclic AMP at 100 mumol X 1(-1) accelerated the field bursts. H2-antagonists blocked HA actions, beta-antagonists blocked NA actions selectively; muscarinic antagonists blocked ACh, HA and NA actions. H1-agonists, serotonin, dopamine and adenosine slowed the field bursts at micromolar concentrations. These effects parallel the action of the tested substances on afterhyperpolarizations in CA 1 pyramidal cells. High sensitivity and specificity make this response of the field bursts an excellent model to study postsynaptic transmitter actions in the central nervous system.

We have studied the effects of curare on responses resulting from iontophoretic application of several putative neurotransmitters onto Aplysia neurons. These neurons have specific receptors for acetylcholine (ACh), dopamine, octopamine, phenylethanolamine, histamine, gamma-aminobutyric acid (GABA), aspartic acid, and glutamic acid. Each of these substances may on different specific neurons elicit at least three types of response, caused by a fast depolarizing Na+, a fast hyperpolarizing Cl-, or a slow hyperpolarizing K+ conductance increase. All responses resulting from either Na+ or Cl- conductance increases, irrespective of which putative transmitter activated the response, were sensitive to curare. Most were totally blocked by less than or equal to 10-4 M curare. GABA responses were less sensitive and were often only depressed by 10-3 M curare. K+ conductance responses, irrespective of the transmitter, were not curare sensitive. These results are consistent with a model of receptor organization in which one neurotransmitter receptor may be associated with any of at least three ionophores, mediating conductance increase responses to Na+, Cl-, and K+, respectively. In Aplysia nervous tissue, curare appears not to be a specific antagonist for the nicotinic ACh receptor, but rather to be a specific blocking agent for a class of receptor-activated Na+ and Cl- responses.

Autoradiographic analysis of [1-14C]2-deoxy-D-glucose-6-phosphate ([14C]2-DG-P) accumulation in the rattlesnake brain stem and optic tectum was used in an effort to map infrared and visual neuronal pathways. Visual stimulation with a standard stimulus (a heat lamp) resulted in dense labeling of the superficial layers of the optic tectum. Infrared stimulation with the same standard stimulus resulted in labeling at the first synaptic relay, the lateral descending nucleus of the trigeminal tract (LDN-V), but not at higher levels, including the optic tectum. Systematic comparison of electrophysiological properties of tectal neurons was performed using the standard stimulus. Responses of infrared units in one hemitectum and visual units in the other, elicited by the same stimulus used in the [14C]2-DG-P experiments, were analyzed. There were no clear differences in the number, maximal density, spread, or rates of accommodation of visual units and infrared units, although the locus of maximal density was more superficial for visual units. In general, infrared units generated a greater number of action potentials than did visual units. All infrared units responded only to onset of the stimulus but they varied greatly in their ability to maintain discharge for the full duration of the stimulus. Most visual units exhibited on-, off-, or on-off responses. Four units showed only inhibition of spontaneous activity during the visual stimulation. There were significant differences in the evoked responses elicited by visual and infrared stimulation in response to the standard stimulus. Infrared stimuli generated single, large, triphasic on-responses, whereas visual stimulation generated complex multiphasic and long-lasting on- and off-responses. The major infrared on peak reached maximal amplitude at greater depths and was larger than the major visual on peak. Amplitude of the infrared peak fell off more rapidly with distance from the locus of its maximum than did amplitude of the visual peak. These observations are consistent with the view that infrared stimulation is effective in discharging neurons but is not associated with intense synaptic excitation. In contrast, visual stimulation apparently does produce intense synaptic activity, as suggested by the duration, complexity, and spread of the visual evoked response. Failure of this synaptic activity to produce more spikes in visual units probably reflects either depolarizing spike inactivation or the admixture of excitatory and inhibitory actions. Our observations suggest that 2-deoxy-D-glucose uptake is not necessarily correlated with the degree of action potential activation of specific neuronal pathways. The amount of [14C]2-DG-P labeling may reflect the metabolic requirements for support of synaptic depolarization as well as that supporting action potentials.

Rhythmic activity in cardiac Purkinje fibers can be analyzed by using the voltage clamp technique to study pacemaker currents. In normally polarized preparations, pacemaker activity can be generated by two distinct ionic mechanisms. The standard pacemaker potential (phase 4 depolarization) involves a slow potassium current, IK2. Following action potential repolarization, the IK2 channels slowly deactivate and thus unmask a steady background inward current. The resulting net inward current causes the slow pacemaker depolarization. Epinephrine accelerates the diastolic depolarization by promoting more complete and more rapid deactivation of IK2 over the pacemaker range of potentials. The catecholamine acts rather selectively on the voltage dependence of the gating mechanism, without altering the basic character of the pacemaker process. The nature of the pacemaker depolarization is altered by intoxication with high concentrations of cardiac glycosides or aglycones. These compounds promote spontaneous impulses in Purkinje fibers by a mechanism that supersedes the ordinary IK2 pacemaker. The digitalis-induced depolarization is generated by a transient inward current that is either absent or very small in untreated preparations. The transient inward current is largely carried by sodium ions. Its unusual time course probably reflects an underlying subcellular event, the oscillatory release of calcium ions from intracellular stores.

1. Cl- conductance gated by gamma-aminobutyric acid (GABA) and L-glutamate in the medial pleural neurons of Aplysia was studied using conventional two-electrode voltage-clamp techniques and a continuous microperfusion system that allowed rapid and uniform agonist application. 2. Both GABA and glutamate elicited current responses that rapidly activated and then decayed (desensitized) during maintained presence of agonist. 3. Recovery from desensitization was rapid for both agonists. For intermediate concentrations of each agonist, recovery was approximately 90% complete within 1 min. 4. Dose-response experiments at a holding potential of -45 mV showed that 1) maximal peak responses to glutamate were on the average approximately 40% of those to GABA, 2) the dose-peak response curve for glutamate was steeper than that for GABA, and 3) time-to-peak (risetime) and desensitization half-time decreased monotonically with dose for GABA but were relatively insensitive to dose and more variable for glutamate. 5. Peak conductances elicited by GABA were voltage dependent, increasing with depolarization, whereas the Cl- conductance elicited by glutamate was voltage independent. Risetime and desensitization kinetics were insensitive to voltage for both agonists. 6. Lowering the temperature of the perfusion medium by 10 degrees C slowed activation and desensitization for both 0.1 mM GABA and 2 mM glutamate. The responses to the two agonists differed, however, in that the peak response to glutamate increased, whereas that to GABA decreased in amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)

The responses of 122 neurons in the area postrema of anesthetized dogs to 17 common transmitters and peptides were determined. Recordings were made from one barrel of a seven-barrel ionophoretic electrode. All neurons were silent at rest, but most could be detected and excited by the application of glutamate. The glutamate response was a brief, high-frequency response of less than 1-sec duration. Excitatory responses were also found to histamine, norepinephrine, serotonin, dopamine, apomorphine, angiotensin II, neurotensin, leucine enkephalin, vasoactive intestinal polypeptide, thyrotropin releasing hormone, gastrin, vasopressin, and substance P. While most neurons tested were excited by dopamine and apomorphine, approximately half of those studied were also excited by each of the other substances. Inhibitory responses were found to norepinephrine (6 of 15 cells) and histamine (3 of 45 cells). No responses were found to acetylcholine, somatostatin, or cholecystokinin. The responses to all 13 excitatory substances other than glutamate were similar. Typically these responses had a latency of 2-20 sec and lasted for 30 sec to 5 min on their first application. The frequency of discharge was usually low (approximately 0.5 Hz). Multiple applications of these agents often induced a maintained spontaneous discharge of low frequency. Each application also induced a transient incremental discharge at a frequency that rarely exceeded 2 Hz. The area postrema has been proposed to be the "chemoreceptor trigger zone" for emesis (Borison and Wang, 1953). All of the agents which excite area postrema neurons, with the exception of serotonin and norepinephrine, are emetic, while none of the three agents without excitatory effects is known to be emetic. Thus these results provide strong support for the central role of the area postrema in emesis. The similarity of response to so many substances on small neurons suggests a common ionic and/or metabolic mechanism underlying the response. The prolonged nature of the response to brief administration of these agents would seem to be appropriate for neurons which subserve a sensation and behavior such as nausea and vomiting.

Responses of neurons of the canine area postrema were recorded to ionophoretic application of insulin, apomorphine, leucine-enkephalin and glutamate. Each excited the neurons directly in a dose-dependent fashion. Like apomorphine and leucine enkephalin, which are known to induce emesis by activation of area postrema neurons, insulin given systemically induced emesis in intact dogs but not in animals with area postrema ablations. These results provide further support for a critical role of the area postrema in triggering the emetic reflex, and are the first definitive demonstration of a direct excitatory action of insulin on mammalian neurons.

We studied the possible role of the choroidal circulation in stabilizing the temperature environment of the retina and the retinal pigment epithelium. Temperature measurements were taken from the retina and choroid of the monkey eye while the ambient illumination and choroidal blood flow were varied. The results showed that the choroidal circulation plays a significant role in dissipating heat generated by the focusing of light at the macula and suggest that this function may be a primary reason for the relatively high values of choroidal blood flow.

The specific resistivity of the axoplasm of giant axons of squid and Myxicola was measured utilizing a single metal microelectrode subjected to alternating current in a circuit in which the voltage output varies with the conductivity of the thin layer of fluid at the exposed electrode tip. The average specific resistivity of stellar axons of Loligo pealei was 31 omegacm (1.55 times seawater [X SW]) while for Loligo opalescens it was 32 omegacm (1.30 X SW). Smaller giant axons had a higher average resistivity. Myxicola giant axons had a resistivity of 68 omegacm (2.7 X SW) in normal seawater, and 53 omegacm (2.1 X SW) in a hypertonic high-Mg++ seawater. The temperature dependence of squid axon resistivity does not differ from that of an equally conductive dilution of seawater.

Dogs exposed to 8 Gy 60Co gamma mid-abdominal irradiation exhibited emesis with an average latency of 102 min and an average of 7.4 episodes over 96 min. There were no significant changes in dogs subjected to a chronic bilateral subdiaphragmatic vagotomy, but emesis was prevented by ablation of the area postrema. Indomethacin pretreatment also prevented radiation-induced emesis in two of seven dogs and in the remainder reduced the average number of episodes. Domperidone pretreatment prevented radiation-induced emesis in all of four dogs tested. In electrophysiological studies recording from the area postrema the chemosensitive neurons were found to be normally silent in anesthetized preparations but excitable by a variety of emetic agents. After irradiation of the abdomen spontaneously active neurons were found with a discharge pattern that mirrored the behavioral pattern of postirradiation emesis. These studies are consistent with radiation-induced emesis being humorally mediated in the dog and implicate dopamine and/or prostaglandins as possible mediators.

The discharge patterns of single afferent fibers from rat pudendal nerve were studied as a function in temperature of the scrotal skin, an area known to function in temperature regulation. In a number of respects the population of temperature-sensitive afferents here differ from most previously described; 75% of temperature-sensitive afferents were also sensitive to mechanical stimulation. Of the 25% nonmechanosensitive units, half showed dynamic and static responses to cooling, while most of the remaining fibers gave only static discharges to warming. The most frequent thermal reaction of the mechanosensitive units was a dynamic-static cold response or a pure static warm response. However, fibers were also present with only dynamic or only static cold responses. Of the bimodal units, 20% had a dynamic cold response, but showed a minimal static discharge at intermediate temperatures (about 35 degrees C) and an increased discharge on both warming and cooling from that temperature. One unit had static and dynamic warm responses. Whereas a bursting discharge in the cold has previously been considered to be a distinguishing characteristic of specific cold receptors, in the pudendal afferents a bursting discharge on cooling or at low temperatures is common both in mechanosensitive and specific cold fibers. This observation and the identical discharge patterns and mechanisms underlying the thermosensitivity argue for the view that the mechanosensitive afferents participate in thermal sensation and/or regulation along with the specific temperature receptors. Hellon and Misra (7) have concluded that there is processing of thermal information from the scrotal skin at the level of the first synapse in the cord. However, in this study, we have found peripheral afferents which have most of the discharge properties that led Hellon and Misra to conclude that processing had occurred. The unusual characteristics of thermosensitive afferents of the pudendal nerve suggest that there is probably organ specificity of neuronal discharge properties.

The effects of the carbamate anticholinesterases neostigmine and pyridostigmine on the kinetics of desensitization of responses of isolated, voltage-clamped Aplysia neurons to microperfused acetylcholine (ACh) was examined. The peak ACh-induced current was potentiated at low carbamate doses and antagonized at higher doses (greater than 10(-5) M); neostigmine was more potent than pyridostigmine in producing both effects. These effects suggest two mechanisms of action of these compounds: (a) inhibition of acetylcholinesterase at low doses, which increases the effective ACh dose, and (b) direct antagonism of the response at higher concentrations, which is associated with a slowing of both the activation and desensitization of the ACh response. These compounds may therefore have direct actions on the excitatory ACh receptor in Aplysia neurons which are similar to the effects of these drugs at the vertebrate endplate.

Vipoxin, a 13,000-dalton polypeptide component of Russell's viper venom, has a dual pattern of effects on the responses of voltage-clamped Aplysia neurons to acetylcholine and biogenic amines. Application of low doses of vipoxin by pressure ejection reversibly antagonized all three types of ionic response to acetylcholine and carbachol. The blockade by vipoxin of acetylcholine responses was not prevented by eserine. The order of susceptibility of acetylcholine responses to blockade by vipoxin was Na+ greater than K+ greater than Cl-. Low doses of vipoxin also produced a reversible potentiation of excitatory responses to dopamine with a slower time course of onset and recovery. Inhibitory responses to dopamine (Cl-, K+) and both excitatory and inhibitory responses to histamine and 5-hydroxytryptamine were little affected by vipoxin. Higher doses of vipoxin directly evoked current responses which were always of the same ionic type as that evoked by acetylcholine or carbachol. Responses to cholinergic agonists and vipoxin were both blocked by cholinergic antagonists but not by antagonists of biogenic amine receptors, which reversibly antagonized the responses to amines on the same cell. These results suggest that vipoxin, which has no demonstrated actions on vertebrate acetylcholine receptors, acts as a partial agonist at all three types of acetylcholine receptor in Aplysia neurons. Our observations thus provide evidence for some degree of phylogenetic difference between vertebrate and molluscan acetylcholine receptors.

The effects of furosemide on action potentials and responses to several neurotransmitters have been studied in the neurons of Aplysia. Furosemide (10(-7) and 10(-3) M) does not visibly affect the normal action potential in R15 neurons. However, when TTX (30 microM) is used to block the sodium component in R15, the remaining spike (presumably the calcium component) is increased in amplitude in the presence of furosemide. Furosemide also alters transmitter-induced conductances. Furosemide greatly reduces the amplitude and shifts, in a depolarizing direction, the reversal potential of chloride-dependent responses to gamma-aminobutyric acid (GABA) and acetylcholine (ACh). This suggests that furosemide both blocks the chloride channel and inhibits a chloride pump. ACh-induced sodium responses were also reduced by furosemide but to a lesser extent than chloride responses. The potassium response to ACh and a voltage-dependent calcium response to serotonin were not altered. These results indicate that furosemide could alter synaptic responses both presynaptically by enhancement of calcium flux during the action potential and postsynaptically by blockade of chloride and sodium conductances.

Receptors to putative transmitters on A and B cells of Aplysia californica were identified and characterized. Neurons within each cluster were similar in responses to transmitters, but the neurons in A and B clusters differed. Both exhibited receptors to acetylcholine, dopamine, gamma-aminobutyric acid, glutamate, histamine and serotonin but not to octopamine or phenylethanolamine. Bi- or multiphasic responses to a single transmitter were common on both cell types. Inhibitory responses were more common on A than B cells where glutamate, histamine and serotonin were all excitatory. Each of these clusters appear homogeneous both in terms of presence of receptors and the ionic channels activated by the receptors.

The action of edrophonium on Aplysia neurons was studied using a concentration clamp technique which combines internal perfusion and a rapid drug application. Edrophonium elicited a dose-dependent inward current in the concentration range 10(-6) to 10(-4) M. At higher concentrations (10(-3) and 10(-2) M), the amplitude of the current often decreased and there was a rapid decay of the current. At these high concentrations, the current increased immediately after washing the neuron with normal solution. These results suggest that edrophonium blocks the ion channel which it opens. Removal of Na+ from the external solution greatly reduced the current amplitude by more than 90%. Removal of Ca2+ also reduced the amplitude of the response; however an increase of Ca2+ did not augment the response. These results suggest that Ca2+ does not carry the current, but is necessary for generation of an Na+-dependent inward current. Edrophonium, 10(-2) M, which completely blocked the current it induced within 20 s, did not significantly affect the voltage-dependent Na+ current. Tetrodotoxin, 1 x 10(-6) M, did not affect the edrophonium response. Hexamethonium, 1 x 10(-4) M, did not change the response elicited by edrophonium, while it significantly reduced the ACh response mediated by Na+. In some neurons edrophonium elicited an inward current, but ACh induced an outward current. Therefore the Na+ channels opened by edrophonium appear to be distinct from both the voltage-gated and ACh receptor-operated Na+ channels.

The responses to application of TRH were examined on 38 identified neurons in sensory motor cortex of cat. Two pyramidal tract (PT) and 3 nonpyramidal tract (NPT) neurons were directly excited by TRH. Two other NPT neurons were inhibited by TRH. TRH potentiated the excitatory action of ACh on 4 of 12 PT neurons and 1 of 2 unidentified cells. None of these neurons showed a direct effect of TRH. TRH did not potentiate the response to Glu on 12 cells tested, even when the same cell showed TRH modulation of the ACh response. None of the NPT cells examined showed TRH modulation of the excitatory ACh response. These results demonstrate that TRH has multiple actions in mammalian cerebral cortex, but that these actions, such as the modulation of the ACh responses, appear to exist on discrete populations of neurons.

Isolated pinnules of the gill of Aplysia contract when dopamine (DA) is perfused through the bath. The contraction is not blocked by high-Mg2+ seawater, and reflects excitatory receptors for DA on the smooth muscle cells of the gill. The pinnule often shows irregular, spontaneous contractions which are blocked by high-Mg2+ seawater and 30 mM CoCl2. These contractions reflect spontaneous activity of a peripheral nerve plexus. No other transmitter was found to be directly excitatory on the muscle fibers, although there are inhibitory receptors for serotonin (5-HT). Tactile stimulation of the pinnule evoked a two-component contractile reflex contraction due to activation of the peripheral nerve plexus. Acetylcholine, octopamine, and 5-HT but not several other transmitters depressed these responses, presumably due to inhibitory receptors on the neurons of the peripheral plexus.

Cerebral A neurons make excitatory contact with cerebral B cells. While glutamate, histamine and serotonin all have excitatory receptors on B cells, the A-to-B cells synaptic potential was not found sensitive to curare, strychnine, antihistaminergic or antiserotoninergic drugs, nor did bath-perfused glutamate or serotonin cause desensitization of the synaptic response. We conclude that none of these candidates is the transmitter at this synapse.

The kinetic properties of desensitization onset of excitatory cholinergic responses were studied in isolated, voltage-clamped Aplysia neurons. Desensitization of the acetylcholine (ACh)-induced current in response to microperfused acetylcholine occurred in two phases, and was best modelled as the sum of two exponential components plus a constant. Both exponential components were accelerated by increasing ACh dose. At the higher ACh doses the current decline was dominated by the fast exponential component, and the ratio of the plateau-peak current was reduced. Over the range of membrane potentials -50 to -110 mV, no change in the kinetics of desensitization onset was observed. The mean time constants of both exponential components were doubled by cooling from 20 degrees C to 5 degrees C. These results demonstrate that, as at the vertebrate neuromuscular junction, the onset of desensitization of this ACh response involves at least two processes which are dose- and temperature-sensitive. The lack of voltage dependence contrasts with results from vertebrate preparations, and indicates a fundamental difference between the properties of the excitatory ACh response in Aplysia neurons and the vertebrate neuromuscular junction.

Gamma-aminobutyric acid (GABA) is one of the most important neurotransmitters in the brain. In an effort to understand the operation of the GABA receptor-ionophore complex, the antagonism of GABA responses by four substances was studied in bullfrog dorsal root ganglion cells by concentration-clamp and internal-perfusion techniques. Two antagonists (bicuculline and Zn2+) were competitive; two (picrotoxin and penicillin) were noncompetitive. However, significant changes in the kinetics of activation and inactivation were produced by the antagonists, including those that were competitive. The causes of these changes may be important clues to the structure and operation of the GABA receptor-ionophore complex.

The apparent cytoplasmic resistivity of two different giant cells has been measured using an extension of a previously developed single microelectrode technique. Each cell is penetrated by a metal microelectrode whose complex impedance is measured as a function of frequency between 500 kHz and 5.7 MHz. By plotting the measured impedance data on the complex Z plane and extrapolating the data to infinite frequency, the substantial effects of electrode polarization can be overcome. For Aplysia giant neurons and muscle fibers of the giant barnacle, the extrapolated cytoplasmic specific resistivities are 40 and 74 omega-cm, respectively, at infinite frequency. The barnacle data are in excellent agreement with sarcoplasmic resistivity values derived from the measured cable properties of other marine organisms, and from high frequency conductivity cell measurements in intact barnacle muscle tissue. In the Aplysia neurons, the frequency-dependent part of the electrode impedance is larger when the electrode is in a cell than when it is in an electrolyte solution with the same specific resistivity as the aqueous cytoplasm; however, the phase angle of the frequency-dependent component of the electrode impedance is the same in both cases. This suggests that the high apparent values of cytoplasmic resistivity found using the single microelectrode technique at lower frequencies probably reflect an artifact caused by reduction of the effective surface area of the electrode by intracellular membranes, with a corresponding increase in its polarization impedance.

The resting membrane potential (RMP) of Aplysia neurons is very temperature-dependent, and in some cells increases with increasing temperature by as much as 2 mv/ degrees C. RMP at room temperature may significantly exceed the potassium equilibrium potential, which can be determined by measurement of the equilibrium point of the spike after potential. The hyperpolarization on warming is completely abolished by ouabain, replacement of external Na(+) by Li(+), removal of external K(+), and by prolonged exposure to high Ca(++), while it is independent of external chloride but is increased by cocaine (3 x 10(-3)M). In an identified cell that shows a marked temperature dependence of RMP, both the potassium equilibrium potential and the membrane resistance were found to be relatively independent of temperature. The hyperpolarization on warming, which may increase RMP by as much as 50%, can most reasonably be ascribed to the activity of an electrogenic Na(+) pump.

The internal conductivity of Aplysia neuron somata was measured by passing constant current pulses across a calibrated four-electrode array. The intracellular medium is less than one-tenth as conductive as seawater. The low conductivity probably results from structured cell water since ions are present in quantity and do not appear to be bound.

1. The organization of primary afferent depolarization (PAD) produced by excitation of peripheral sensory and motor nerves was studied in the frog cord isolated with hind limb nerves.2. Dorsal root potentials from sensory fibres (DR-DRPs) were evoked on stimulation of most sensory nerves, but were largest from cutaneous, joint and flexor muscle afferents. With single shock stimulation the largest cutaneous and joint afferent fibres gave DR-DRPs, but potentials from muscle nerves resulted from activation of sensory fibres with thresholds to electrical stimulation higher than 1.2-1.5 times the threshold of the most excitable fibres in the nerve. This suggests that PAD from muscle afferents is probably due to excitation of extrafusal receptors.3. Dorsal root potentials produced by antidromic activation of motor fibres (VR-DRPs) were larger from extensor muscles and smaller or absent from flexor muscles. The VR-DRPs were produced by activation of the lowest threshold motor fibres.4. Three types of interactions were found between test and conditioning DRPs from the same or different nerves. With maximal responses occlusion was usually pronounced. At submaximal levels linear summation occurred. Near threshold the conditioning stimulus frequently resulted in a large facilitation of the test DRP. All three types of interactions were found with two DR-DRPs, two VR-DRPs or one DR-DRP and one VR-DRP.5. The excitability of sensory nerve terminals from most peripheral nerves was increased during the DR-DRP. The magnitude of the excitability increase varied roughly with the magnitude of the DR-DRP evoked by the conditioning stimulus.6. There was a marked excitability increase of cutaneous and extensor muscle afferent terminals during the VR-DRP. Flexor muscle afferent terminals often showed no excitability changes to ventral root stimulation. In those experiments where afferent terminals from flexor muscles did show an excitability increase, the effects were smaller than those of cutaneous and extensor terminals.7. The VR-DRPs appear to reflect activity of a negative feed-back loop from extensor motoneurones on to sensory fibres from cutaneous and extensor muscles. This system may have a role in modulating the ballistic movement of the frog. DR-DRPs, on the contrary, are widespread in origin and distribution. PAD from sensory fibres may function to sharpen contrast between incoming afferent information.

An electrogenic sodium pump contributes to the membrane potential in squid nerve cell bodies, imparting a temperature dependence to the resting potential that is abolished by strophanthidin. The existence of a potential produced by the pump in the soma but not the axon is correlated with a higher membrane resistance in the soma. Thus, membranes from different parts of a neuron may have functionally significant differences in resistance.

Temperature increases cause a regular and reproducible increase in the frequency of generation of pacemaker potentials in most Aplysia neurons specialized for this type of activity which can only be explained as a direct stimulating effect of temperature upon the ionic mechanisms responsible for pacemaker potentials. At the same time all cells in the visceral ganglion undergo a membrane potential hyperpolarization of approximately 1-2 mv/ degrees C warmed. In spite of the marked variation in resting membrane potential the critical firing threshold remains at a constant membrane potential level at all temperatures in the absence of accommodative changes. The temperature-frequency curves of all types of cells are interpreted as a result of the interaction between the effects of temperature on the pacemaker-generating mechanism and resting membrane potential. Previous observations on the effects of temperature on excitability of mammalian neurons suggest that other types of neurons may undergo similar marked shifts in resting membrane potential with temperature variation.

Extracellular action potentials were recorded from 80 neurons in the sensorimotor cortex of the cat as brain temperature was varied by 4 degrees to 8 degrees C. The discharge rate of 37 percent of the neurons studied increased with increasing brain temperature. The discharge rate varied inversely with temperature in 11 percent of the neurons.