Responses Of Interneurons Research Articles

Auditory behavior of the cricket

1. Many one-eared female crickets (Gryllus campestris andGryllus bimaculatus), whether the loss is due to surgery or to developmental accident, can maintain characteristic, stable courses with respect to the direction of male calling song, when tested on the Kramer spherical tread-mill. The error angles are usually less than 90°, so that Regen's (1924) remark about the success of such females in finding singing males is supported. 2. Some females with developmental unilateral hearing deficit, confirmed both histologically and by sparing of tracking after removal of the suspect leg, track nearly as accurately as normal animals with two ears. 3. Experiments in which tympana were immobilized with wax, whether tracking behavior or interneuron responses were monitored, show that the anterior tympanum — considered for some time to be irrelevant to hearing — mediates appreciable sound input to the auditory receptors, at least when the posterior tympana are blocked. 4. The above results resolve two current paradoxes regarding comparison of tympanum immobilization in behavior with the mechanics of receptor excitation: First, because even unambiguously one-eared animals can maintain stable sound-oriented courses, such tracking performance with waxed tympana does not argue that total tympanal immobilization spares some auditory function of that ear (cf. Schmitz et al. 1983). The argument assumes that tracking would not be expected if one ear were silenced; the assumption is evidently false. Second, it has been unclear why waxing of both posterior tympana raises tracking threshold only ca. 20 dB (Schmitz 1983) whereas the tympanalmechanics studies of Kleindienst et al. (1983) have suggested a much greater deficit. Our finding that one must wax the anterior tympana as well, in order to produce deficit greater than ca. 20 dB, clarifies the situation. That is, in the biophysical experiments both tympana of the leg were immobilized by the procedures used (pressure matching and water immersion), whereas in most current behavioral tympanum-waxing experiments (but not in those of Bailey and Thomson 1977) the anterior tympana have been ignored. 5. A novel behavioral effect was observed with all four tympana blocked. Thresholds for orientation were ca. 90–100 dB, but in every case the animals tended to walk away from the loudspeaker. 6. The search for an explanation of one-eared tracking leads us to analogies with the recovery from imbalance following such surgery as hemilabyrinthectomy in vertebrates. A ‘phantom tympanum’, on the side with deficit, could in principle participate in corrective tracking, by the usual notions of bilateral comparison.

Intracellular recording from hippocampal CA 1 interneurons before and after development of long-term potentiation

Synaptic efficacy is modified following a brief train of high frequency stimulation (HFS) to a cell's afferent fibers (long-term potentiation, LTP). Modulation of synaptic efficacy could be achieved through changes in excitability of interneurons. To test this hypothesis, we used the in vitro hippocampal slice to record intracellularly from CA 1 basket cell interneurons before and following a period of HFS to fibers in stratum radiatum. Stimulation of stratum radiatum consistently produced action potential discharge in the basket cell interneuron prior to the discharge of a population spike in stratum pyramidale. This result is consistent with findings that basket cells may receive direct excitatory afferent input from neurons extrinsic to the CA 1 region, and mediate a powerful feed-forward inhibition. Interneuron responses were recorded under 3 expeimental conditions, in which stimulus intensity was varied. The level of basket cell excitability, as assessed by responses to orthodromic input, was not decreased following a train of HFS. Rather, the evoked EPSP in basket cells was often enhanced following high frequency stimulation to the cell's afferent fibers. These findings support the notion that decrement in basket cell activity is not responsible for LTP in CA 1 hippocampus.

Interneurons in the tritocerebrum of the crayfish

In isolated head preparations of the freshwater crayfish Orconectes limosus 268 local and projecting interneurons with branches in the tritocerebrum have been penetrated with glass microelectrodes and characterized for their sensory inputs. Using 3 criteria (sensory modality, site of receptors, response type of interneurons), the interneurons found were divided into 16 classes. The interneurons were either unimodal mechanoreceptive (89%) or bimodal (9% responding to mechanical and chemical stimuli, 2% responding to mechanical and visual stimuli). No trimodal interneurons were found. Within each modality the neurons received mostly bilateral input (70% of all interneurons responding to antennal stimulation, 84% of all chemosensitive interneurons). If the input was lateralized it was more often ipsilateral. The types of interneuronal responses evoked by sensory stimulation were: neurons that were exclusively excited (84%), those that were exclusively inhibited (10%), those that were excited or inhibited depending on the modality or laterality of the stimulus (6%), those showing long lasting excitatory aftereffects (3%), and those showing excitation or inhibition upon identical stimulation depending on the state of the neurons while being stimulated (1%). Interneurons that responded to mechanical antennal stimulation responded best either to low (10 Hz) or to high (100 Hz) stimulus frequencies. Six neurons responded best to a certain phase relationship between the movements of both antennal flagellae.

Response of spinal interneurons elicited by peripheral nerve stimulation and their after-discharge

Response of spinal interneurons elicited by peripheral nerve stimulation and their after-discharge

Responses of interneurons in the cat cervical cord to vestibular tilt stimulation.

The responses of interneurons in the cervical spinal cord of the decerebrate cat to whole-body tilt were studied with a goal of identifying spinal elements in the production of forelimb vestibular postural reflexes. Interneurons both in the cervical enlargement and at higher levels, from which propriospinal neurons have been identified, were examined, both in animals with intact labyrinths and in animals with nonfunctional semicircular canals (canal plugged). Most cervical interneurons responding to tilt respond best to rotations in vertical planes aligned within 30 degrees of the roll plane. Two to three times as many neurons are excited by side-up roll tilt as are excited by side-down roll. In cats with intact labyrinths, most responses have dynamics proportional either to (and in phase with) the position of the animal or to a sum of position and tilt velocity. This is consistent with input from both otolith organs and semicircular canals. In animals without functioning canals, the "velocity" response is absent. In a few cells (8 out of 76), a more complex response, characterized by an increasing gain and progressive phase lag, was observed. These response dynamics characterize the forelimb reflex in canal-plugged cats and have been previously observed in vestibular neurons in such preparations.

Cholinergic suppression: a postsynaptic mechanism of long-term associative learning.

Food avoidance learning in the mollusc Pleurobranchaea entails reduction in the responsiveness of key brain interneurons in the feeding neural circuitry, the paracerebral feeding command interneurons (PCNs), to the neurotransmitter acetylcholine (AcCho). Food stimuli applied to the oral veil of an untrained animal depolarize the PCNs and induce the feeding motor program (FMP). Atropine (a muscarinic cholinergic antagonist) reversibly blocks the food-induced depolarization of the PCNs, implicating AcCho as the neurotransmitter mediating food detection. AcCho applied directly to PCN somata depolarizes them, indicating that the PCN soma membrane contains AcCho receptors and induces the FMP in the isolated central nervous system preparation. The AcCho response of the PCNs is mediated by muscarinic-like receptors, since comparable depolarization is induced by muscarinic agonists (acetyl-beta-methylcholine, oxotremorine, pilocarpine), but not nicotine, and blocked by muscarinic antagonists (atropine, trifluoperazine). The nicotinic antagonist hexamethonium, however, blocked the AcCho response in four of six cases. When specimens are trained to suppress feeding behavior using a conventional food-avoidance learning paradigm (conditionally paired food and shock), AcCho applied to PCNs in the same concentration as in untrained animals causes little or no depolarization and does not initiate the FMP. Increasing the concentration of AcCho 10-100 times, however, induces weak PCN depolarization in trained specimens, indicating that learning diminishes but does not fully abolish AcCho responsiveness of the PCNs. This study proposes a cellular mechanism of long-term associative learning--namely, postsynaptic modulation of neurotransmitter responsiveness in central neurons that could apply also to mammalian species.

Sensitization of the crayfish lateral giant escape reaction

Most behavioral reactions that habituate can also be dishabituated by strong stimuli. In the best studied cases, dishabituation seems to be the result of an independent "sensitization" of the behavioral reaction that compensates for habituation without necessarily abolishing it. Crayfish lateral giant (LG) neuron-mediated escape reactions are one of the most fully analyzed behavioral reactions that are prone to habituation; however, sensitization/dishabituation of LG escape has not previously been reported. Here, the effect of strong AC shocks to head or abdomen on the ability of 0.1 msec "test" shocks to sensory roots innervating the tailfan to elicit an LG escape response was examined. Following single AC shocks, test shock threshold for eliciting LG escape reliably fell 5-80% and recovered over 15 min to 1 hr. When AC shocks and test shocks alternated at 90 sec intervals, test shock threshold rapidly dropped to an asymptote that was maintained as long as AC shocks were given (up to 2 hr); following such repeated AC shocks, recovery often required a number of hours but was complete within 24. Comparable sensitization is seen in the response of interneuron A, the largest of a set of sensory interneurons that links afferents to LGs. AC shocks (to either head or tail) no longer sensitize abdominal LG reflex circuitry if the nerve cord is severed between thorax and abdomen. Thus, sensitization appears to depend on a neurally conducted influence that arises in the rostral half of the animal. Pharmacological evidence suggests that octopamine may mediate the sensitization.

Ultrastructural study of cholecystokinin-immunoreactive cells and processes in area CA1 of the rat hippocampus.

We used light and electron microscopic immunocytochemical methods to examine the structure of neuronal perikarya and processes containing cholecystokinin-like immunoreactivity (CCK-IR) in area CA1 of the rat hippocampus. The morphology of stained perikarya, their positions within all laminae, and the orientation of their dendrites indicate that CCK-IR is located in interneurons. These cells were seen in the electron microscope to have deeply folded nuclei and to receive both symmetric and asymmetric synaptic junctions on their cell somata and dendritic shafts. Their dendrites are essentially spine-free, but form bulges at the site of some asymmetric synaptic junctions. Axonal varicosities containing CCK-IR make symmetric synaptic junctions with cell somata and dendritic shafts of both pyramidal and non-pyramidal neurons. In addition, CCK-IR varicosities form symmetric junctions with unstained non-pyramidal neurons and with CCK-IR cells, suggesting either recurrent innervation of one cell on itself or interaction between interneurons. The presence of CCK-IR varicosities and synaptic junctions on pyramidal cells is in agreement with physiological data which indicate that CCK has a direct postsynaptic action. The observation of CCK-IR varicosities forming synaptic junctions on non-pyramidal cells suggests that CCK might also modify the response of interneurons.

Neuronal organization of coeruleo-spinal connections in cats

Electrical stimulation of locus coeruleus in anesthetized cats was accompanied by a decrease in the inhibitory action of flexor reflex afferents (FRA) on extensor motoneurons. No changes in excitatory reactions of spinal motoneurons were observed. Microinjections of aspartic acid or chlorpromazine depressed FRA inhibitory action; injection of procaine and noradrenaline potentiated this action. The responses of differente spinal interneurons during activation of locus coeruleus were also studied. A conclusion is made that the inhibitory action of FRA is depressed by coeruleo-spinal influences on the level of the last-order inhibitory interneuron of the FRA system.

Abdominal positioning interneurons in crayfish: projections to and synaptic activation by higher CNS centers.

Intracellular recording, stimulation, and Lucifer dye injections were used to characterize abdominal positioning interneurons from the neuropile of the second through sixth abdominal ganglia of the crayfish, Procambarus clarkii. Motor outputs of these cells were recorded with extracellular electrodes placed on various flexion and extension roots along the nerve cord. In an effort to assess the functional relationships between the postural interneurons in the abdomen and those known to exist in the circumesophageal connectives ( CECs ), a stimulus pulse train was delivered to each of the CECs while monitoring the intracellular responses of the impaled interneurons. Abdominal positioning interneurons were grouped into four general categories based on their responses to CEC stimulation: 1) those that projected their axons directly through the CECs ; 2) those that were remotely activated to spiking; 3) those locally activated to produce EPSPs or IPSPs; and 4) those that were not affected by CEC stimulation. The majority of abdominal positioning interneurons encountered in this study evoked flexion (N = 82), with relatively fewer evoking extension (N = 29). A major difference appeared between the two classes. Whereas 39% of the flexion interneurons had axons coursing to the brain, only 7% of the axons of extension interneurons coursed rostrally beyond the thoracic level. Finally, the large majority of those flexion and extension interneurons that lacked processes in the CEC received synaptic inputs at various levels along the lower CNS from other CEC neurons. Thus, control of abdominal positioning involves neurons at all levels in the CNS - some sequentially organized, others forming "through" pathways, but all with multiple input sites.

Tonic neck reflex of the decerebrate cat: response of spinal interneurons to natural stimulation of neck and vestibular receptors.

In order to investigate the neural basis of the tonic neck reflex, we studied the response of neurons in the cervical spinal cord of decerebrate, paralyzed cats to neck rotation about the longitudinal axis (roll), to vestibular stimulation produced by roll tilt, and to a combination of these stimuli. Most neurons were outside the motoneuron nuclei and were arbitrarily classified as interneurons. Three types of preparation were used--one with intact labyrinths, one acutely labyrinthectomized, and one with acute spinal transection. The activity of 115 neurons recorded extracellularly was modulated by sinusoidal neck rotation in the range 0.02-4 Hz; their behavior was sufficiently linear for sinusoidal analysis. The phase and gain of the responses of neurons in all three preparations were similar except that the absolute gain in cats with intact labyrinths was higher than that of the others. The location of neurons in segments C4-C8 was mainly in laminae 7-8. Some neurons were excited by rotation of the chin to the ipsilateral side (type I) and others by contralateral chin rotation (type II). The dynamic behavior of type I and type II neurons was the same; phase was flat over most of the frequency range and close to the phase of peak neck rotation, while gain enhancement occurred at higher frequencies. This behavior was similar to that of the neckforelimb reflex evoked in unparalyzed intact-labyrinth and labyrinthectomized cats. In cats with intact labyrinths, vestibular input to neurons whose activity was modulated by the neck stimulus was studied using whole-body roll tilt. Many neurons received otolith input; some received canal input. Neck and vestibular inputs to spinal neurons always had opposite polarities (complementary inputs). Thus, type I neurons were always excited by tilt to the ipsilateral side (ipsilateral ear down) while type II neurons were excited by tilt to the contralateral side. Combined neck and vestibular stimulation indicated that the dynamic behavior of neurons was determined by a linear summation of the responses to these stimuli. Interaction of neck and vestibular input at the neuron level was similar to that observed previously at the reflex level in forelimb extensor muscles.(ABSTRACT TRUNCATED AT 400 WORDS)

Actions of opiates, substance P, and serotonin on the excitability of primary afferent terminals and observations on interneuronal activity in the neonatal rat's dorsal horn in vitro

Approximately 5 segments of lumbo-thoracic spinal cord together with connected dorsal root ganglia were removed from 1–11-day-old rats and maintained in vitro. Dorsal root afferents, recorded from the ganglion and stimulated at the root entry zone, had conduction velocities typical of unmyelinated fibers (< 2 m/s). The spinal terminals of individual afferents showed increased excitability with bath application of substance P and serotonin and decreased excitability with morphine sulfate, [ d-ala 2]methionine-enkephalinamide, manganese ions and magnesium ions. Naloxone by itself elicited no change in excitability, although it appeared to reduce the ongoing effect of opiates. Neurons recorded extracellularly in the dorsal horn responded to afferent volleys with one or more of 3 distinct phases; an excitation roughly coincident with the volley's arrival, a 50–300 ms period of inhibition, and a late excitation of 150–300 ms latency. The excitability results are accounted for by a model in which substance P, γ-aminobutyric acid and possibly other depolarizing agents are contained in interneurons which synapse on afferent terminals. These interneurons could receive inhibitory enkephalinergic input, and, in the neonate but not the adult, excitatory serotoninergic input. An alternate scheme would have enkephalin and serotonin acting directly on afferent terminals, although perhaps by non-synaptic diffusion since the appropriate synapses have not been seen in histochemical studies. Such an action for enkephalin might explain the existence of opiate receptor on afferent terminals. The interneuronal responses to afferent volleys are parallel in most aspects to those found in the dorsal horns of adult mammals in vivo.

Olfactory excitation of interneurons in the brain of the spiny lobster

Multimodal interneurons descending the circumesophageal connectives from the brain to the lower nervous system in decapod crustaceans respond to stimulation of cephalic sensory structures. Using intracellular recording and staining and backfilling techniques, we have examined the complexity of olfactory stimulants necessary to excite descending interneurons in the spiny lobster, and have characterized the dendritic branching pattern and responsiveness to multimodal stimulation of some of these neurons. The excitation of descending interneurons by different chemical stimulants or by different concentrations of any one stimulant appears to vary only in intensity, not in temporal pattern (Figs. 1, 2). Responses to chemical stimulation of chemoexcited interneurons at this level in the olfactory pathway resemble responses of primary receptors. Variation in the responses of chemoexcited interneurons is attributable to differences between chemical stimulants (Table 2). Two mixtures of amino acids plus betaine at concentrations proportional to their occurrence in potential foods (‘crab’ and ‘urchin’; Table 1) are more excitatory than a mixture of inverted composition or a single component, glutamic acid. Another single component, taurine, is as excitatory as equimolar ‘crab’ and ‘urchin’ mixtures. Individual chemoexcited interneurons exhibit different response spectra (Fig. 1; Tables 3, 4). Most are excited by taurine, for which there are specific antennular receptors, as well as by glutamic acid or betaine, indicating that convergence of odor qualities occurs within the olfactory pathway. Many interneurons exhibit similar concentration-response functions for taurine (Fig. 3) regardless of their responsiveness to other chemical stimulants (Table 3) or modalities (Table 4). Many chemoexcited interneurons receive bilateral chemical, tactile and visual inputs. Spatial convergence of olfactory inputs is indicated by responses of many cells to stimulation of both the ipsilateral and contralateral antennules. A few cells respond only to ipsilateral chemical stimulation. The morphological diversity of descending interneurons excited by stimulation of cephalic structures is evidenced by the variety of their dendritic branching patterns (Figs. 4–6). One striking characteristic of many chemoexcitated interneurons is the presence of extensive branching in the ipsilateral antennary neuropil (Fig. 6). Chemoexcited interneurons do not appear to contact the olfactory lobes, where the majority of the primary afferents terminate, and thus they presumably transmit at least third-order chemosensory information. Because it is unlikely that chemosensory information is further processed per se in the subesophageal nervous system, our results suggest that the chemosensory output of the spiny lobster brain is encoded by a heterogeneous population of interneurons.

Corticofugal influences on segmental responses of lumbar spinal ventral horn interneurons in cats

Experiments on cats using extra- and intracellular recording methods showed that stimulation of the motor cortex of both hemispheres leads to considerable modulation of responses to stimulation of cutaneous and muscular lower limb afferents in spinal ventral horn interneurons in segments L6, 7. Three types of conditioning corticofugal effect were observed: facilitation, inhibition, and facilitation followed by inhibition (biphasic effect), and inhibitory effects predominated. The duration of facilitation of responses did not exceed 30–40 msec. The characteristics of the time course of inhibition varied: in some cases it began with relatively short intervals (8–15 msec), in other cases with an interval of 30–40 msec; its duration was 125–500 msec, or sometimes more. The effect of cortical stimulation on responses to stimulation of various afferent inputs of the same interneuron was shown to differ. The character of the conditioning corticofugal effect correlated with the latent period of segmental responses: facilitation was observed only in responses with a relatively short latent period (under 5 msec); responses with a longer latent period were mainly inhibited. The type of cortical effect also depended on the function performed by the activated afferent input. It is suggested that differential descending control of segmental polysynaptic responses recorded in ventral horn interneurons with wide convergence of afferent influences takes place in the initial stages of the reflex are. The mechanism of this control is discussed.

Electrical stimulation of the tympanal nerve as a tool for analysing the responses of auditory interneurons in the locust

1. Electrical stimulation of the tympanal nerve ofLocusta migratoria L. elicits 1∶1 firing in individual auditory receptor fibres (Figs. 1, 2). With a stimulus amplitude of 500 mV (pulse duration 0.5 ms) all afferent fibres within the nerve appear to be activated and to fire synchronously. 2. Electrical stimulation of the tympanal nerve also activates auditory interneurons in the metathoracic ganglion; in general, activation occurs with higher temporal accuracy than when conventional sound stimulation is used (Fig. 2). EPSP and IPSP responses with different synaptic delays can be found (Figs. 2, 3), and two magnitudes of delay are observed. Monosynaptic input from auditory receptor fibres is indicated for two units (Fig. 3 a, b) where the synaptic delays are 0.8 and 1.0 ms. For the other interneurons studied, the delay ranges from 2.6–6.3 ms, regardless of whether the postsynaptic responses are excitatory or inhibitory. The mono- or polysynaptic connections of the receptor input to these neurons are discussed. 3. The electrical stimulation technique bypasses the transduction process of the auditory receptors and the number of spikes and the response latency can therefore be manipulated independently. Using this method, we show that the response in an auditory interneuron is strongly affected by varying only the spike rate in the peripheral nerve (Fig. 4). From these and other observations it can be concluded that interaural differences in spike rate as well as in response latency both are important features of directional coding in the auditory system of insects.

Predictive coding: a fresh view of inhibition in the retina.

Interneurons exhibiting centre--surround antagonism within their receptive fields are commonly found in peripheral visual pathways. We propose that this organization enables the visual system to encode spatial detail in a manner that minimizes the deleterious effects of intrinsic noise, by exploiting the spatial correlation that exists within natural scenes. The antagonistic surround takes a weighted mean of the signals in neighbouring receptors to generate a statistical prediction of the signal at the centre. The predicted value is subtracted from the actual centre signal, thus minimizing the range of outputs transmitted by the centre. In this way the entire dynamic range of the interneuron can be devoted to encoding a small range of intensities, thus rendering fine detail detectable against intrinsic noise injected at later stages in processing. This predictive encoding scheme also reduces spatial redundancy, thereby enabling the array of interneurons to transmit a larger number of distinguishable images, taking into account the expected structure of the visual world. The profile of the required inhibitory field is derived from statistical estimation theory. This profile depends strongly upon the signal: noise ratio and weakly upon the extent of lateral spatial correlation. The receptive fields that are quantitatively predicted by the theory resemble those of X-type retinal ganglion cells and show that the inhibitory surround should become weaker and more diffuse at low intensities. The latter property is unequivocally demonstrated in the first-order interneurons of the fly's compound eye. The theory is extended to the time domain to account for the phasic responses of fly interneurons. These comparisons suggest that, in the early stages of processing, the visual system is concerned primarily with coding the visual image to protect against subsequent intrinsic noise, rather than with reconstructing the scene or extracting specific features from it. The treatment emphasizes that a neuron's dynamic range should be matched to both its receptive field and the statistical properties of the visual pattern expected within this field. Finally, the analysis is synthetic because it is an extension of the background suppression hypothesis (Barlow & Levick 1976), satisfies the redundancy reduction hypothesis (Barlow 1961 a, b) and is equivalent to deblurring under certain conditions (Ratliff 1965).

Force-sensitive interneurons in the spinal cord of the cat.

The input-output properties of interneurons mediating spinal reflexes were investigated by extracellularly recording the response of interneurons to excitation from muscle receptors in the ankle extensor muscles of decerebrated, spinal cats. A population ofinterneurons in the intermediate region ofthe spinal cord is potently excited by increases in muscle force. Unlike the discharge of Golgi tendon organs, which accurately encodes moment-to-moment variations in the force of a single muscle, the discharge of these interneurons depends in a dynamic and usually nonlinear way on the force in several muscles. Powerful input from unidentified mechanoreceptors in muscle, presumably free nerve endings, is at least partly responsible for these properties. These force-sensitive interneurons are more likely to mediate clasp knife-type inhibition than simple negative force feedback.

Synaptic mechanisms of monaural and binaural processing in the locust

(1) Responses of auditory interneurones were recorded intracellularly within the metathoracic ganglion of the locust when stimulating each tympanic membrane with a piezoelectric transducer. Thus, in contrast to conventional sound stimulation, each of the two ears could be activated independently from the other at variable intensities, duration and stimulus onsets. By means of this ‘earphone-like’ stimulation technique the binaural integration properties of auditory interneurons could be analysed. (2) A minority of units (3 out of 43) was affected by input from one side only. Their synaptic input was purely excitatory and the intensity characteristics reflected those of auditory receptor fibres. (3) Most interneurones received input from both ears, each being excitatory or one excitatory or one excitatory and one inhibitory. In some units the unilateral synaptic response already included both an EPSP and an IPSP. As a result of varying temporal interactions between the EPSP and the IPSP within the unilaterally evoked complex response the intensity characteristics differed widely from unit to unit. (4) With binaural simultaneous stimulation the complexity of the postsynaptic responses of most interneurones increased as the synaptic input from both ears coincided at the level of the recorded interneurone. Although both ears were stimulated symmetrically (at the same time and intensity), units were recorded where the latencies of ipsilateral and contralateral synaptic input were different. Contralateral inhibition could either follow or precede ipsilateral excitation and in some cases both EPSP and IPSP had the same latency. On the basis of these findings the binaural synaptic mechanisms of directional coding are discussed and compared with corresponding results under free field stimulus conditions.

Neonatal rat spinal cord slice preparation: postsynaptic effects of neuropeptides on dorsal horn neurons

In the neonatal rat spinal cord slice preparation responses of the dorsal horn interneurons to iontophoretic or bath application of methionine-enkephalin (ME), substance P (SP) and somatostatin (SS) were qualitatively similar to those obtained in intact spinal cord. Thus, SP powerfully excited almost all neurons tested ( 15 16 ), while ME and SS depressed neuronal discharges in 13 14 and 4 6 units respectively. In some dorsal horn neurons the iontophoretic application of ME caused a marked depression of the SP-induced excitation. Angiotensin II (AgII) had no effect on dorsal horn units (n = 8). In the slices perfused with a Ca 2+-free, Mg 2+-high Krebs solution the extracellularly recorded effects of ME, SP and SS were not significantly modified, suggesting that the peptides were acting directly on postsynaptic sites. The results also indicate that the in vitro rat spinal cord slice preparation can be successfully utilized for further studies on the cellular mechanisms of actions of neuropeptides, particularly in relation to synaptic transmission processes in the dorsal horn.

The inulin space of the lamprey spinal cord

The distribution of [ 14C]inulin was measured in isolated spinal cords of larval and feeding stage adult forms of sea lamprey ( Petromyzon marinus) and expressed in per cent of total cord wet weight. In larval cord the apparent inulin space reached a plateau value of 32–33% within 2.5 min. This correlates well with electrophysiological experiments in which 10 −7 M tetrodotoxin added to the perfusion fluid blocked the responses of giant interneurons to both intracellular and rostral cord stimulation in 1 to 2 min. Thus the plateau level of inulin space probably represents the extracellular space. [ 14C]Mannitol did not reach a steady distribution space even after 30 min of incubation. Therefore, mannitol is not an accurate extracellular space indicator in the isolated lamprey spinal cord. The inulin space increased with increasing temperature of incubation. Average inulin spaces for larval spinal cords incubated at 5, 10 and 22 °C were approximately 26%, 33% and 42% respectively. The inulin space of isolated adult lamprey spinal cords was about 18–19%. Since in larvae the inulin space did not vary consistently with the sizes (and therefore presumably the ages) of the animals, it is likely that the reduction in inulin space during maturation does not occur gradually during the larval phase, but probably occurs during transformation. The difference between the inulin spaces of isolated larval and adult spinal cords is reflected qualitatively in the electron microscopic appearance of the extracellular space. We conclude that the inulin space in the lamprey spinal cord behaves similarly to the picture of the mammalian brain extracellular space which has emerged in recent years. Because of the rapidity of inulin diffusion in the lamprey cord and the unambiguous time-dependent behavior of the inulin space of the isolated lamprey cord, the latter would seem to be a useful model for the extracellular space of the vertebrate central nervous system.