High Levels Of Spontaneous Activity Research Articles

Parallel analyses of nociceptive neurones in rat superior colliculus by using c-fos immunohistochemistry and electrophysiology under different conditions of anaesthesia.

Multiple sensory inputs to the superior colliculus (SC) play an important role in guiding head and eye movements toward or away from biologically significant stimuli. Much is now known about the visual, auditory, and somatosensory response properties of SC neurones that mediate these behavioural reactions. Rather less is known about the responses of SC neurones to noxious stimuli, and thus far, most of this information has been obtained in anaesthetised animals. Therefore, the purpose of the present study was to use the c-fos immunohistochemical technique and standard extracellular electrophysiology as parallel measures of nociceptive activity in the SC under different conditions of anaesthesia. In unanaesthetised animals, experimental and control treatments induced a qualitatively similar pattern of Fos-like immunoreactivity (FLI) in the SC, which was quantitatively related to the severity or biologic salience of the treatment; thus, baseline control < control injections of saline < a nonpainful stressor (immobilisation) < noxious injections of formalin. Compared with baseline levels, urethane and avertin anaesthesia induced FLI expression in the SC intermediate layers, although the FLI response to both noxious stimulation and control conditions was differentially suppressed in different layers of the SC by anaesthesia. Parallel electrophysiologic recordings found that anaesthesia was associated with high levels of spontaneous activity in the SC intermediate layers, often in neurones which were also nociceptive. High rates of background spike activity were also induced in the SC intermediate layers by noxious stimulation in chronically recorded awake animals. Although these results point to some differences between the nociceptive responses of SC neurones in anaesthetised and unanaesthetised animals, both data sets support the view that there are different populations of nociceptive neurones in the rodent SC that may be related to different adaptive functions of pain.

Chemosensitivity of nociceptive, mechanosensitive afferent nerve fibres in the guinea-pig ureter.

The mechanosensitivity and chemosensitivity of afferent fibres were investigated in an in vitro preparation of the guinea-pig ureter. Electrophysiological recordings were obtained from 5 U-1 (low mechanical threshold, contraction-sensitive) and 74 U-2 units (high threshold). U-2 units had significant higher levels of spontaneous activity, lower conduction velocities, higher mechanical thresholds (U-1: 7 mmHg; U-2: 39 mmHg), less pronounced phasic responses and longer latencies in the response to distensions than the U-1 units. For chemical stimulation, guinea-pig urine (> 800 mosmol/L), bradykinin and capsaicin were applied intraluminally. The responses of U-1 units mainly corresponded to the contractions induced by the chemical stimulation. The vast majority of the U-2 units were excited by urine, bradykinin (threshold: 0.1-1 microM) and capsaicin (threshold: 0.03-0.3 microM). The responses to urine could be mimicked by high concentrations of potassium ions (> 200 mM), but not by an equiosmolar solution of NaCl, urea and mannitol. Chemical stimulation could also result in a transient sensitization of the U-2 units to mechanical stimuli. In the anaesthetized guinea-pig, pseudo-affective responses could be evoked by ureteric distension (threshold: 30-60 mmHg) and serosal application of capsaicin. Intraluminal application of urine in vivo did not evoke any reactions, suggesting that the responses of the U-2 units to urine might be due to an impaired barrier function of the urothelium in vitro. The data are in agreement with the hypothesis that U-2 units are visceral polymodal nociceptors. Since the U-1 units were also able to encode at least noxious mechanical stimuli, their involvement in visceral nociception cannot be excluded.

Sodium urate arthritis: effects on the sensory properties of articular afferents in the chicken

The physiological properties of joint capsule mechanoreceptors in the ankle joint of the chicken were studied in the 3-h period immediately after intra-articular injection of microcrystal sodium urate. The electrical activity was recorded from single C- and A-delta sensory fibres dissected from the parafibular nerve. C-fibres showed high levels of spontaneous activity and receptive fields that varied from single spots 1 mm in diameter up to 4×4 mm. Thresholds to mechanical stimulation ranged from 0.1 to 8 g and 80% of the units responded to movement of the joint. A-delta fibres showed little spontaneous activity and receptive fields that varied from 1 mm to 9×1 mm. Thresholds to mechanical stimulation ranged from 0.1 to 16 g and 17% responded to joint movement. A comparison of the physiological properties of the C- and A-delta fibres in sodium urate arthritis with similar fibres in normal and monoarthritic animals indicated an increased sensitivity in the C-fibres but not in the A-delta fibres. Sensitisation was observed in the significantly increased receptive field size, decreased response thresholds, increased response to joint movement and the high level of spontaneous activity. These changes in the sensitivity of the joint capsule C-fibre receptors provides peripheral neural evidence for the pain experienced in acute gouty arthritis.

Neural assemblies and laminar interactions in the cerebral cortex.

Neural assemblies are assumed to become organized and to operate within the cerebral cortex, and so must be constrained by the cytological and physiological properties of this laminated structure. A hypothesis of such assemblies is presented, based on important details of neuronal architecture and physiology in different cortical laminae. Laminae II, III and VI, which are the origin and termination of most cortico-cortical projections, are regarded as the site of storage of most of the information encoded by assemblies - a neuronal 'library'. Laminae II and III are the most sensitive coincidence detectors, and therefore probably initiate the process of assembly formation. However, these three laminae have very low levels of spontaneous activity in the waking state, and so active cell assemblies cannot base their functioning on these laminae alone. Lamina V pyramidal cells have a much higher level of spontaneous activity. Thus, indirect pathways between 'library' cells, via lamina V pyramidal cells, are likely to be more secure than direct ones. It is proposed that direct links between 'library' cells become stabilized by Hebbian strengthening, once the recipient 'library' cell has been 'primed' by neural activity transmitted indirectly via lamina V neurones. Thus lamina V neurones could catalyse the process of assembly formation. Given this proposal, lamina V cells, in their interaction with 'library' cells, would code information in terms of precisely timed individual impulses, but would employ a code based on slower frequency changes in their descending influences upon neural centres in the brainstem and spinal cord. Predictions for single unit and electrographic experiments are discussed.

Why do cats need a dorsal cochlear nucleus?

The dorsal cochlear nucleus (DCN), one of the three major divisions of the cochlear nucleus (CN), has a complex internal structure, multiple inputs (some of them non-auditory), and multiple output pathways. Response properties of DCN units are accordingly complex. The principal cells of the DCN have type IV response characteristics, characterized by relatively high levels of spontaneous activity and inhibition by high level best frequency (BF) tones. We showed previously that type IV units are inhibited by two separate inhibitory mechanisms, one of them sensitive to narrow band stimuli and the other to wide band stimuli. One result of the wide band inhibition of type IV units is their sensitivity to spectral notches in the region of their BF - stimuli with such notches inhibit type IV units. The source of the narrow band inhibition is an interneuron in the DCN which has type II response characteristics - it does not have spontaneous activity and is strongly activated by BF tones. The neurons giving rise to type II responses are presumably vertical cells, which also project to other divisions of the CN. From anatomical studies, it is known that type IV units are also inhibited by a third system, which carries non-auditory information; movements of the pinna inhibit type IV units through this system. We hypothesize that type IV units signal important events to the auditory system by being inhibited. Such events are either auditory, e.g. spectral maxima and minima, or non-auditory, such as the somatosensory inputs from the pinnae. We hypothesize that the projection of type II units to the ventral cochlear nucleus (VCN) plays a role in reducing the effects of spectral notches introduced by the pinnae in the core auditory pathway. We conclude that although the DCN lies close to the auditory periphery, it already performs sophisticated tasks of auditory processing.

Presynaptic or postsynaptic location of receptors for angiotensin II and substance P in the medial solitary tract nucleus.

1. Microinjection of angiotensin (Ang) II or substance P (SP) into the medial nucleus tractus solitarii (nTS) produces similar decreases in arterial pressure and heart rate. We previously reported that some medial nTS neurons responsive to SP were also excited by Ang II, and that Ang II increased the release of SP from medulla slices. Both electrophysiological and anatomic data suggest that the cardiovascular effects of these peptides may be mediated by a common neuronal pathway consisting of SP-containing vagal afferent fibers with presynaptic Ang II receptors that innervate medial nTS neurons with SP receptors. To evaluate the validity of this model, we established the presynaptic or postsynaptic location of the receptors for Ang II and SP that mediate excitation of medial nTS neurons by determining the capacity of each peptide to activate the cell before and after blocking synaptic transmission in rat dorsal medulla slices. 2. Extracellular recordings were obtained from 55 medial nTS neurons responsive to Ang II or SP in 400-microns horizontal slices of the dorsal medulla. Neuronal excitation by Ang II and SP was tested before, during, and after reversal of synaptic blockade with low-Ca2+ (0.2 mM), high Mg2+ (5 mM) artificial cerebrospinal fluid (aCSF). Elimination of synaptically evoked short latency responses of the neuron to current pulses applied to afferent fibers in the solitary tract (TS) documented blockade of synaptic transmission by low-Ca2+ aCSF. In most cases, the basal firing rate of the cell increased slowly during perfusion with low-Ca2+ aCSF and stabilized after approximately 30 min at a higher level of spontaneous activity. Responses to the peptides and TS stimulation were also documented after synaptic blockade had been reversed by adding aCSF containing 2-mM Ca2+. 3. Of the 55 medial nTS neurons, 41 were responsive to Ang II; whereas, 50 of the 55 cells were responsive to SP. The neurons were divided into three subgroups on the basis of their responsiveness to Ang II and SP. Although most neurons were responsive to both Ang II and SP (n = 36), five other cells were excited only by Ang II, and 14 neurons were activated only by SP. Of the 55 neurons, 26 were also responsive to L-glutamate: 14 of 17 cells responsive to both Ang II and SP, all 5 neurons excited by Ang II but not by SP, and 7 of 10 neurons responsive only to SP were also excited by L-glutamate. The latency of the action potentials evoked by TS stimulation was much shorter in those neurons responsive only to Ang II (3.6 ms) than in cells excited by both Ang II and SP (6.8 ms) or responsive only to SP (7.4 ms). 4. In 21 of the 36 medial nTS neurons responsive to both Ang II and SP, Ang II continued to excite the cell when synaptic responses to TS stimulation were prevented by low-Ca2+ aCSF, but had no effect on the firing rate of the other 15 neurons during synaptic blockade. Excitation induced by Ang II was also prevented in two of the five medial nTS neurons responsive only to Ang II when synaptic transmission in the slice was blocked. Low-Ca2+ aCSF failed to prevent excitation by SP or L-glutamate in all medial nTS cells responsive to these agonists (n = 50 and n = 26, respectively). In contrast to these observations in medial nTS neurons, Ang II-induced excitation was not altered during synaptic blockade in any of the six dmnX cells studied. No responses to SP or L-glutamate were blocked in dmnX neurons, as also seen in the medial nTS. 5. When all medial nTS neurons responsive to Ang II were examined, the latencies of the response to TS stimulation were significantly shorter in those neurons with presynaptic Ang II receptors than in the group of cells with postsynaptic receptors. In addition, neurons with presynaptic Ang II receptors were distributed differently within the medial nTS than cells with postsynaptic Ang II receptors.(ABSTRACT TRUNCATED)

Hindbrain signal processing in the lateral line system of the dwarf scorpionfish Scopeana papillosus

Recordings were made from primary afferent fibres and secondary projection neurones (crest cells) in the mechanosensory lateral line system of the dwarf scorpionfish. Crest cells were identified by antidromic stimulation from the contralateral midbrain. Differences between primary afferent fibre and crest cell response characteristics are indicative of signal processing by the neuronal circuitry of the medial octavolateralis nucleus. There are a number of differences between primary afferent fibres and crest cells. Primary afferents have relatively high levels of spontaneous activity (mean close to 40 impulses s-1) and many of them are strongly modulated by ventilation. By contrast, crest cells have a much lower rate of spontaneous activity that is not obviously modulated by ventilation. Primary afferents show a simple tonic response to a maintained stimulus, whereas crest cells show a variety of temporal response properties, but in general show a phasic/tonic response to the same prolonged stimulus. Afferents are most sensitive to frequencies of stimulation around 100 Hz; in contrast, crest cells show a strong suppression of activity at this frequency. Crest cells are most responsive around 50 Hz. These afferent/secondary comparisons show similarities with those reported for allied electrosensory and auditory pathways.

Song‐selective auditory input to a forebrain vocal control nucleus in the zebra finch

Neurons in nuclei on the motor pathway for vocalizations in songbirds are known to respond to sound stimuli. The auditory responses in one such nucleus, robustus archistriatalis (RA), were characterized by making multi-unit recordings in awake and anesthetized adult male zebra finches and in birds that had received lesions of the input to RA from the lateral part of the magnocellular nucleus of the anterior neostriatum (LMAN) or the Higher Vocal Center (HVC). In awake birds, RA neurons have a high level of spontaneous activity and vigorous auditory responses to song stimuli. Significantly greater responses are seen to the bird's own song (BOS) than to BOS played in reverse (REV) or to the songs of conspecifics (CON). Under ketamine-xylazine anesthesia, spontaneous activity is reduced, response latency increases and responses to BOS, REV and CON are indistinguishable. Responses obtained under urethane anesthesia are similar to those seen in awake birds. Thus, the pattern and selectivity of auditory responses in RA depend on the animal's state. Auditory responses in RA are qualitatively unchanged following lesion of the input to RA from LMAN, indicating that this pathway is not required for the sensory processing that underlies the preference for BOS on the vocal production pathway. Our results show that an input other than that from LMAN must be primarily responsible for auditory responses in RA. The direct projection from HVC is the most likely pathway by which song selective auditory information arrives in RA, since lesioning HVC abolished auditory responses in RA.

Convergence of visceral and somatic inputs onto subnucleus reticularis dorsalis neurones in the rat medulla.

1. In anaesthetized rats, recordings were made within the medullary subnucleus reticularis dorsalis (SRD) from neurones that exhibited convergence of nociceptive inputs from the entire surface of the body. Neurones with total nociceptive convergence responded to supramaximal percutaneous electrical stimuli (2 ms duration) with early and late peaks due to the activation of A delta and C fibres, respectively, no matter which part of the body was stimulated. Neurones with partial nociceptive convergence responded to identical stimuli with an A delta peak of activation regardless of which part of the body was stimulated and with a C peak of activation from some, mainly contralateral, parts of the body. The characteristics of the responses of these neurones to graded colo-rectal distension (< or = 100 mmHg) were analysed. 2. The majority of neurones with total nociceptive convergence (n = 13 out of 16) responded to colo-rectal distension by increasing their firing rates. Although these neurones were virtually unresponsive to the lowest pressure employed (12.5 mmHg), they increased their discharges monotonically for distensions in the 26-100 mmHg range and these responses were sometimes followed by after-discharges. One of these neurones, which exhibited a high level of spontaneous activity, was inhibited during colo-rectal distension. None of the neurones with partial nociceptive convergence recorded (n = 10) ever changed its firing rate during increases of intracolonic pressure up to 100 mmHg. 3. It is concluded that neurones with total nociceptive convergence give monotonic stimulus-response relationships for colo-rectal distensions. Thus, neurones with total nociceptive convergence can encode the strength of visceral stimuli, probably within the noxious range, just as they have previously been shown to do for thermal and mechanical cutaneous stimuli. Together with previous electrophysiological and neuroanatomical findings, this study provides further evidence for the convergence of noxious inputs onto single SRD neurones. 4. It is suggested that neurones with total nociceptive convergence could be a link in spino-bulbospinal loops involved in autonomic reactions to strong visceral stimulation. In addition, SRD neurones could be an important supraspinal relay in the mechanisms of visceral pain.

Oxygen consumption in relation to sustained exercise and social stress in Arctic charr (Salvelinus alpinus L.)

AbstractRates of oxygen consumption were measured repeatedly in feeding groups of Arctic charr (Salvelinus alpinus L.) subjected to moderate sustained exercise or allowed to perform routine swimming activity (control). Oxygen consumption did not differ significantly between treatments. Agonistic activity, assessed in terms of bite mark frequencies, were significantly reduced in exercising fish. Consequently, high levels of spontaneous activity displayed by the control fish in performing agonistic interactions may have led to substantial increases in energetic costs resulting in energy expenditures equivalent to those shown by fish performing forced swimming at moderate speeds.Plasma cortisol levels measured at the end of the experiment suggested that both control and exercising fish were subjected to chronic stress with cortisol levels of exercising fish being significantly higher than those of controls. The validity of using plasma cortisol levels as an indicator of stress per se is discussed.

Afferent influences on brainstem auditory nuclei of the chick: Nucleus magnocellularis neuronal activity following cochlea removal

Elimination of presynaptic elements often results in marked changes, such as astrophy and death, in postsynaptic neurons in the central nervous system. These transneuronal changes are particularly rapid and profound in young animals. In order to understand the cellular events underlying transneuronal regulation it is necessary to explore changes in the local environment of neurons following manipulations of their afferents. In previous investigations we have documented a variety of rapid and marked cellular changes in neurons of the cochlear nucleus of neonatal chicks (n. magnocellularis) following cochlea removal. In adult chickens, however, these transneuronal changes are either absent or minor. The goals of the studies presented here were to examine changes in the electrical activity of nucleus magnocellularis cells and their afferents following removal of the cochlea and to determine if these changes were similar in adult and neonatal animals. Two measures of electrical activity were used; multiunit recording with microelectrodes and incorporation of radiolabeled 2-deoxyglucose (2-DG). Microelectrode recordings revealed high levels of spontaneous activity in n. magnocellularis and n. laminaris, the binaural target of n. magnocellularis neurons. Neither puncturing of the tympanic membrane nor removal of the columella causes significant changes in spontaneous activity, although the latter results in a profound hearing loss (40–50 dB). Removal of the cochlea, on the other hand, results in immediate cessation of all extracellular electrical activity in the ipsilateral n. magnocellularis. Recordings from the same location for up to 6 h failed to reveal any return of spontaneous activity. When the electrode tip was placed in n. laminaris, unilateral cochlea removal had no discernible effect on extracellularly recorded spontaneous activity, probably due to the high levels of excitatory input from the intact ear. Bilateral cochlea removal, however, completely eliminated activity in n. laminaris. 2-DG studies conducted 1 h to 8 days following unilateral cochlea removal revealed marked decreases in 2-DG incorporation in the ipsilateral n. magnocellularis and bilaterally in the n. laminaris target of the ablated cochlea. No compensatory return of 2-DG incorporation was observed for up to 8 days. Comparisons of adult and neonatal chicks failed to reveal significant differences in the effects of cochlea removal on multiunit activity or 2-DG incorporation, suggesting that age differences in transneuronal regulation are due to intrinsic biochemical differences in young and adult neurons rather than differences in the proportion of synaptic input that has been abolished.

Suppression and excitation in auditory nerve fibers of the goldfish, Carassius auratus

The suppression of background spike activity in the absence of deliberate acoustic stimulation occurs in fibers of the goldfish saccular nerve tuned in the region of 250 Hz. Suppression is most robust in the frequency range between 450 and 1050 Hz, the range of CF for the mid- and high-frequency saccular fibers. Suppression of background activity tends to occur following the suppressor tone offset (‘off-suppression’), even though the spike response during the suppressor is below the background rate. This suggests that the suppressor tone is excitatory at the level of the hair cells and their synapses onto saccular afferents. Tones at the low- frequency edge of the suppression region may show net excitation at low intensity levels, and net suppression at higher levels. This suggests that the spike response observed is the result of the relative strengths of excitatory and suppressive effects which operate simultaneously. The magnitude and frequency of best suppression tends to increase with stimulus intensity. A suppressing tone produces transient excitation at onset. In fibers with high levels of spontaneous activity, a spike response ‘rebound’ often occurs 20 to 50 ms following the suppressing tone offset. These ‘on’ and ‘off’ effects are not due to energy ‘splatter’ in the stimulus domain. Suppression by tones can also be observed in non-spontaneous fibers when the background spike activity is evoked by noise. In these cases, however, off-suppression following a suppressed response and the ‘rebound’ seldom occurs. Possible sites of suppression are the hair cells and their synapses, the spike-initiation zones of the saccular afferents, and efferent inhibition. The most likely site seems to be the spike-initiation zones of saccular afferents. An important consequence of suppression for hearing is the sharpening of frequency response areas for low frequency fibers, and the partial preservation of frequency analysis in saccular fibers stimulated well above threshold.

The characteristics and regional distribution of afferent fibres in the chorda tympani of the cat.

1. Experiments were carried out on the cat's tongue to investigate regional variations in sensitivity to gustatory and thermal stimuli and to determine whether taste buds responding to particular stimuli are unique to specific areas. 2. In ten cats, integrated whole-nerve activity was recorded from the left chorda tympani during stimulation of the anterior, middle or posterior segments of the tongue. The level of activity produced by gustatory or thermal stimuli increased towards the posterior segment of the tongue but the relative activity evoked by each stimulus was similar for each segment. 3. Recordings were made from 108 single units dissected from the chorda tympani; fifty-five responded principally to gustatory stimuli, twenty were principally thermosensitive and thirty-three were purely mechanosensitive. 4. The gustatory units could be subdivided according to the stimuli which evoked the most vigorous discharge; there were sixteen salt units, eighteen acid units, nineteen salt-acid units and two quinine units. The salt units had a higher level of spontaneous activity than the other groups and the acid units supplied more fungiform papillae with a greater separation between them. 5. The two quinine units were both located near to the mid-line posteriorly but all other groups of gustatory, thermosensitive and mechanosensitive units were distributed over all regions of the dorsal surface of the tongue and did not have higher discharge rates in specific areas. 6. Individual stimulation of the fungiform papillae supplied by a single unit revealed that similar responses were evoked from each papilla but stimulation of a single papilla supplied by two or more units could evoke a different response in each unit. 7. This study revealed that the relative sensitivity to gustatory and thermal stimuli was similar in each region of the tongue and that taste buds responding to particular stimuli are not confined to specific areas.

Electrophysiology of morphologically identified mossy cells of the dentate hilus recorded in guinea pig hippocampal slices

A specific population of cells located in the hilus of the hippocampal fascia dentata was studied in guinea pig hippocampal slices using standard intracellular recording techniques. Twenty-one such cells were characterized using electrophysiological techniques and were identified morphologically as mossy cells following intracellular injection of the fluorescent dye Lucifer yellow. These cells had a resting membrane potential (mean, -64.6 mV), action potential amplitude (mean, 78.6 mV), action potential duration (mean, 2.2 msec), and time constant (mean, 24.2 msec) similar to those of hippocampal pyramidal cells of area CA3. Rectification seen in their I-V curves, and their ability to fire action potentials in accommodating trains or bursts in response to injected current pulses, were also similar to those of area CA3 pyramidal cells. However, these cells could be distinguished from area CA3 pyramidal cells by their higher input resistance (mean, 97.4 M omega) and higher level of spontaneous activity. The synaptic responses of mossy cells were also different from those of CA3 pyramidal cells. First, mossy cells responded to low levels of stimulation in all areas of the hippocampal slice that were tested, even areas as remote as area CA1. Second, the responses of mossy cells to stimulation consisted primarily of EPSPs. Hyperpolarizing IPSP-like events followed EPSPs in some cells, but the hyperpolarizations were small and monophasic, even after the cell was depolarized with current injection. This response contrasts with the smaller EPSP and the prominent, biphasic IPSP elicited by afferent stimulation of area CA3 pyramidal cells. The physiological and morphological characteristics of these cells suggest that they could play an important role in the integration of electrical activity in the hippocampus.

Effects of low-level lead exposure on 24 h activity patterns in the mouse

Spontaneous activity of male mice chronically exposed to lead acetate or sodium acetate was tested for periods of 24 h in their home cages and normal housing groups. Animals receiving 0.25% lead acetate showed significantly higher levels of spontaneous activity than did distilled water controls during the early part of the dark phase in two experiments, and significantly lower activity levels during the latter part. Mice receiving 0.025% lead acetate were also significantly more active than controls early in the dark phase, and either significantly higher or no different from controls latterly. Animals receiving 0.025% sodium acetate were significantly less active than controls early in the dark phase, but later were equally active. Weight loss in lead-treated animals was variable and not correlated with changes in activity levels.

Abnormal impulse discharge in primary afferent axons injured in the peripheral versus the central nervous system.

Chronic injury to sensory axons in the rat peripheral nerve induces pathophysiologic changes in the axolemma at the cut nerve end, which are reflected in spontaneous ectopic impulse discharge and hyperexcitability to a range of depolarizing stimuli. We asked whether sensory axons injured in the central nervous system (CNS) also respond in this way. Primary afferent axons were severed in the sciatic nerve and, alternatively, in the midcervical or upper lumbar dorsal column (DC). Measurements of abnormal discharge from myelinated afferents showed high levels of spontaneous activity generated at the nerve injury site, especially during the period 3-16 days postoperatively, but comparatively little activity generated at the DC lesion site at any postoperative time. There was a corresponding difference in ectopic hyperexcitability to mechanical and adrenergic stimulation, and to depolarization with topical K+. DC lesion sites were not made more excitable by concurrent transection of the sciatic nerve, or by placing an autologous graft of excised sciatic nerve tissue into the DC defect at the time of initial surgery. Transection sites on dorsal roots L4 and L5 yielded abnormal discharge similar to that of sciatic nerve neuromas, indicating that the relative silence of DC transection sites was related to the CNS environment and not to position with respect to the sensory cell body.

Effects of pineal factors on the action potentials of sympathetic neurons.

Neurons from rat superior cervical ganglia were grown in coculture with pineal cells. Action potentials of neurons in cocultures were 25% longer and were 25% greater in amplitude than those recorded from neurons grown in the presence of ganglionic nonneuronal cells alone. Neurons showed an increase in action potential duration with increasing time in culture. This may have been related to the recovery of nonneuronal cell populations after an initial exposure to the antimitotic agent Ara-C. In cultures not initially exposed to Ara-C, a subsequent exposure after 7 days in culture resulted in a shortening of the action-potential duration. Neuronal cultures were exposed to gel slabs containing the pineal indolamines, serotonin, N-acetylserotonin, and melatonin. Serotonin and N-acetylserotonin showed no effect on the neuronal action potentials at the concentrations tested. Melatonin caused an increase in action-potential duration that was associated not with an increase in action-potential amplitude, but with a decrease in action-potential rise rates. The effects of long-term exposure in melatonin appeared to be reversible in some cells but not in all. Short-term effects of melatonin were observed in older cultures and in younger cultures after the cells were stimulated repeatedly. Older cultures also had higher levels of spontaneous activity. The dependence of the short-term effects of melatonin on electrical activity may suggest a role for melatonin as a neuromodulator.

Prior activity influences the velocity of impulses in frog and cat optic nerve fibers

Under normal visual stimulation, simultaneous recording from ganglion cells in the retina and from the axons of these cells in the brain revealed activity-dependent differences in the velocity of impulse propagation. In frog ganglion cells, spikes initiated 5–500 ms after a previous impulse showed supernormal increases in conduction velocity of up to 17%; spikes initiated 500–2000 ms after a previous one traveled more slowly than a spike initiated after a long period of rest. Cat ganglion cell impulses showed much smaller supernormality (maximum 3%), but exhibited pronounced slowing due to refractoriness and long-term fatigue associated with their high levels of spontaneous activity.

Postnatal development of corticotectal neurons in the kitten striate cortex: An electrophysiological study

In cats ranging in age from the second week to adult, the postnatal functional development of striate cortical neurons projecting to the superior colliculus (SC) was studied by observing their visual responses, axonal conduction velocities and spontaneous activities. One-hundred-and-fifty-seven cells were identified as projecting to the SC (C-T cells) on the basis of antidromic activation from SC. At the second week of age, 57% of the C-T cells had receptive fields of the complex type and the others were visually unresponsive. C-T cells with ‘special complex’ properties, which are characteristic of the adult C-T cells, first appeared at the third week and made up 19% of the total. The proportion of such C-T cells increased very rapidly and attained the adult-like value at the fourth week. The spontaneous activity of the C-T cells was very sparse or absent at the second week and increased thereafter with age. The proportion of cells with the high level of spontaneous activity found in the adult increased with the same time-course as did the special complex cells. The axonal conduction velocity of C-T cells was slower than 2 m/s until the third week. C-T cells with the faster velocities, suggestive of axonal myelination, appeared at the fourth week and thereafter the average value increased very rapidly until the weeks 8–9. These results indicate that the most remarkable functional development of C-T cells may occur around the fourth week of age. By comparison with previous data on postnatal development of neurons in the cortex and SC, we suggest that C-T cells may be one of the early-maturing groups of cells in the striate cortex and their maturation may exert a crucial influence on the development of visual response properties of SC neurons.

Effects of neurotensin, substance P and methionine-enkephalin on colonic motility.

The influence of neurotensin, substance P and methionine-enkephalin (met-enkephaline) on proximal and distal colonic motility was studied in anesthetized cats. When administered i.v, at a dose of 14 pmol x kg-1 x min-1, neurotensin increased basal smooth muscle tone and antiperistaltic activity in the proximal colon. After higher doses of neurotensin the basal muscle tone also increased in the distal colon. Substance P at a dose of 17 pmol x kg-1 x min-1 selectively and powerfully stimulated the distal colon, thus causing a mass contraction pattern with a concomitant decrease in peristaltic amplitude. In the proximal part of the colon 17 pmol x kg -1 x min-1 of substance P exerted an inhibitory action on the peristaltic amplitude in animals with a high level of spontaneous activity. In addition, the basal muscle tone was slightly decreased. In animals with sluggish spontaneous activity, however, no effects were detected in the proximal colon. With increasing doses of substance P forceful contractions of the proximal colon were also registered. Met-enkephalin at a dose of 40 pmol x kg-1 x min-1 increased the smooth muscle basal tone with no effect on the peristaltic activity in the proximal or distal segments. The effect on basal tone was blocked by naloxone. Thus, neurotensin, substance P and met-enkephalin have distinct motor actions on the colon. At low doses neurotensin may stimulate the churning and mixing functions of the proximal colon. Substance P exerts its major effects on the distal part with a mass contraction response and met-enkepahlin increases the basal muscle tone equally in the proximal and distal parts of the colon.