Ionizing radiation-induced alterations in the electrophysiological properties of Aplysia sensory neurons.

It is clear that ionizing radiation can alter neuronal function. Recently it has been suggested that radiation can directly influence neurons and/or the neuronal microenvironment. We have developed a simple in vitro model system utilizing the marine mollusc Aplysia californica to test this hypothesis. We show that ionizing radiation at doses of 5, 10 or 15 Gy produces complex effects on the electrophysiological properties of a population of Aplysia nociceptive sensory neurons at 24 and 48 h post irradiation. These results add support to the notion that ionizing radiation can directly influence neurons and/or the neuronal microenvironment. Furthermore, they demonstrate that Aplysia may be used as a useful model system to study radiation-induced neuronal plasticity.

Immune-mediated alterations in nociceptive sensory function in Aplysia californica.

Nerve injury in Aplysia californica is accompanied by a profound long-lasting enhancement of the excitability of nociceptive sensory neurons that have axons in injured nerves. It is likely that a variety of signals are involved in triggering this injury-induced sensory plasticity. The objective of the present study was to determine whether cells of the cellular defense system (hemocytes) play a role in the modulation of sensory excitability following injury. In support of such an idea, we have shown previously that the induction of a cellular defense reaction close to sensory axons is accompanied by an increase in the excitability of sensory neurons with axons close to responding hemocytes. Furthermore, in the present study, we verified that, following axonal crush, numerous hemocytes accumulate at the injured site on the nerve. Using a hemocyte/nervous system co-culture preparation, we found that there were no significant differences in the expression of injury-induced sensory plasticity between sensory neurons incubated in the presence or absence of hemocytes. To overcome some potential limitations of our co-culture preparation, we used the endotoxin lipopolysaccharide (LPS) as a tool to activate the hemocytes. Sensory cells incubated in the presence of LPS and hemocytes were significantly more excitable than sensory cells incubated in the presence of LPS alone. We speculate that the addition of LPS to the incubation medium containing hemocytes enhanced the release of hemocyte-derived cytokine-like factors such as interleukin-1 and tumor necrosis factor. These cytokine-like factors may act as signals to modulate the expression of injury-induced sensory hyperexcitability.

Neural-immune interactions--an evolutionary perspective.

Efforts to understand how the immune system can influence nervous system function are hampered by the complexity of mammalian nervous and immune systems. The marine mollusc Aplysia californica has recently emerged as a useful model system to investigate cellular mechanisms underlying neural-immune interactions. Aplysia has a relatively simple, well-characterized nervous system that is accessible for intracellular recording. Moreover, it shares with mammals basic cellular defensive responses to non-self or wounded-self, i.e. the accumulation of numerous defense cells (hemocytes) around foreign objects or at injured sites. We have shown that the excitability of a population of nociceptive sensory neurons in Aplysia can be influenced by the presence of hemocytes close to their axons. These sensory neurons also show profound, long-lasting increases in their excitability following axonal injury. Hemocytes are attracted to injured sites on peripheral nerves, and we have developed an in vitro nervous system-hemocyte coculture system to demonstrate that hemocytes can also influence the expression of this injury-induced sensory hyperexcitability. Immunoreactive interleukin-1 (IL-1) and tumor necrosis factor have been identified in Aplysia. Preliminary in vitro studies showing that IL-1 can modulate the expression of injury-induced sensory hyperexcitability raise the interesting possibility that hemocyte-derived cytokine-like factors can modulate sensory neuron functioning. The relevance of this work to more phylogenetically advanced organisms is also discussed.

A simple systems approach to neural-immune communication.

The marine mollusc Aplysia californica is emerging as a useful model system to study neuralimmune communication at the mechanistic level because it has a well characterized nervous system that is easily accessible and it shares with mammals similar basic cellular defensive responses to wounded or non-self, i.e. the accumulation of defense cells (haemocytes) at the target site. Loose ligation of peripheral nerves in Aplysia induces a cellular defense response as evidenced by the accumulation of numerous haemocytes around the ligature. The excitability of nociceptive sensory neurons having axons close to the responding haemocytes is significantly increased. Haemocytes also accumulate at regions of axonal injury. The finding that human recombinant IL-1 beta can enhance the expression of injury-induced sensory hyperexcitability coupled with the detection of (ir)IL-1 in Aplysia haemolymph raises the interesting possibility that cytokines released from activated haemocytes attracted to an injured nerve or to a foreign body close to peripheral nerves may modulate nociceptive sensory function. The feasibility of using results from simple system such as Aplysia to formulate testable hypotheses in more complex systems is also discussed.

Role of peri-axonal inflammation in the development of thermal hyperalgesia and guarding behavior in a rat model of neuropathic pain.

Loose ligation of a rat's sciatic nerve produces hyperalgesia to thermal stimuli and elicits guarding behavior directed at the afflicted paw. The present experiments test whether localized inflammation induced by the suture material used to ligate the nerve is critical to the development of hyperalgesia. Daily injections of dexamethasone reduced the inflammatory response induced by the sutures and blocked the development of guarding behavior and thermal hyperalgesia. In a second experiment inflammation associated with cotton sutures was enhanced by soaking the sutures in Freund's adjuvant prior to ligation. This caused an augmentation of thermal hyperalgesia and guarding behavior. These results suggest that inflammation around the nerve is critical for the development of guarding behavior and thermal hyperalgesia in this model of neuropathic pain.

Induction of a cellular defense reaction is accompanied by an increase in sensory neuron excitability in Aplysia.

The complexity of vertebrate immune and nervous systems makes detailed cellular analysis of neuroimmune interactions a challenging prospect. The immune systems of invertebrates, although much less complex than their vertebrate counterparts, share basic cellular defense responses to wounded self or nonself. We have developed a simple model system to study neuroimmune interactions using an invertebrate preparation well suited to detailed cellular analysis. Loose ligation of peripheral nerves in Aplysia induced a cellular defense reaction evidenced by the accumulation of large numbers of amebocytes at the ligation site. From 5 to 30 d after ligation, the excitability of the soma of sensory neurons having axons in ligated nerves was increased compared to contralateral sensory neurons with axons in nonligated nerves. Spike threshold and afterhyperpolarization were reduced, and spike amplitude and duration were increased. Spike accommodation was also decreased such that sensory neurons on the ligated side fired more spikes to a 1 sec intracellular depolarizing pulse than control sensory neurons. These effects are unlikely to be accounted for by ligation-induced injury of sensory axons since both morphological and electrophysiological evidence indicated that the axons in ligated nerves were healthy and able to conduct action potentials. Amebocytes activated by the presence of nonself may release factors that lead to a central sensitization of sensory neurons with axons in close proximity to the amebocytes.

Comparative analysis of hyperexcitability and synaptic facilitation induced by nerve injury in two populations of mechanosensory neurones of Aplysia californica.

Long-term effects of nerve injury on electrophysiological properties were compared in two populations of mechanosensory neurones in Aplysia californica: the J and K clusters in the cerebral ganglia and the VC clusters in the pleural ganglia. Following crush of cerebral nerves containing their axons, the cerebral J/K sensory neurones showed long-term changes that were quite similar to alterations previously described in the VC sensory neurones after either axonal injury or aversive learning. These changes include synaptic facilitation, an increase in soma excitability and spike duration, and a decrease in spike threshold and afterhyperpolarization. In addition, simultaneous crush of both the cerebral and pedal nerves in the same animals produced alterations in the cerebral J/K sensory neurones and pleural VC sensory neurones that were virtually identical. The incidence of hyperexcitability was the same in cerebral J/K and pleural VC sensory neurones when all their axons were crushed, even though the former population includes many neurones that probably have appetitive functions while the latter population appears to be made up exclusively of neurones with defensive functions. Long-term plasticity in both sensory populations failed to occur when nerves lacking axons of the tested neurones were crushed, even when the crush site was very close to the somata of the sensory neurones. This axonal specificity argues against a role for delayed activation of facilitatory interneurones in triggering the plasticity. Several observations are consistent with a triggering role for either (1) intracellular signals released directly by axonal injury or (2) extracellular signals released locally by other axons or injured support cells, or by immunocytes attracted to the injured site.

Rapid amplification and facilitation of mechanosensory discharge in Aplysia by noxious stimulation.

1. Little is known about modulation of action potential discharge in Aplysia mechanosensory neurons during defensive responses. The present studies examined rapid effects of noxious stimulation (occurring within 0.5-10 s) on the number of action potentials evoked by test stimuli delivered to the tail. Responses were monitored in the somata of mechanonociceptors in the pleural ganglion. A major hypothesis to be tested was that an important function of previously described alterations of membrane conductances in the sensory neuron soma is to generate an after-discharge that amplifies sensory signals during severe noxious stimulation of the cell's receptive field. 2. Discharge of spikes evoked by a moderate tap to one part of a sensory neuron's receptive field on the tail was enhanced by strong shock delivered 10 s earlier to another part of the field. Part of this enhancement appears to be due to a decrease in conduction block in central regions of the sensory neuron. 3. Repeated delivery of innocuous, moderately intense tail shock at 5-s intervals caused a progressive increase ("wind-up") of discharge, whereas repeated delivery of weak tail shock had no significant effect on discharge. In some cases the increase in action potential number involved a buildup of afterdischarge. 4. A single strong tail pinch sometimes induced an afterdischarge lasting < or = 2 s. Afterdischarge could also be induced in the isolated nervous system by intense electrical stimulation of the nerve containing the sensory neuron's main axon. 5. Several observations suggest that afterdischarge requires cooperative effects of a relatively large number of coactivated fibers in the test pathway. In contrast to pinching stimuli (which stimulated a larger part of the tail), intense, punctate stimulation with von Frey hairs failed to produce afterdischarge. Weaker tail or nerve stimulation failed to produce afterdischarge, even when short-latency, high-frequency discharge was evoked in the sensory neuron. 6. Cooperative effects on afterdischarge may differ from those involved in activity-dependent enhancement of presynaptic facilitation because simultaneous pairing of high-frequency activation of a single test sensory neuron with strong stimulation of a peripheral nerve lacking an axon of the tested sensory neuron was not sufficient to produce afterdischarge. The cooperative effects on afterdischarge may function to encode information about both the severity and spatial extensiveness of an injury. 7. Artificial hyperpolarization of the soma often reversibly reduced or abolished afterdischarge evoked by stimulating the nerve or tail. Thus the afterdischarge is often generated in or near the sensory neuron soma.(ABSTRACT TRUNCATED AT 400 WORDS)

Activity-dependent depression of mechanosensory discharge in Aplysia.

1. Inhibition of action potential discharge in Aplysia mechanosensory neurons after noxious stimulation has not been described previously. The present studies investigated depressive effects of prolonged noxious stimulation and repetitive intracellular activation on the number and latency of action potentials evoked by test stimuli applied to the tail or the nerve innervating the tail. Action potential discharge was monitored in the somata of mechanonociceptors in the pleural ganglia. 2. Repeated brief pinches delivered at 5-s intervals to a sensory neuron's receptive field on the tail initially caused intense activation (10-25 spikes recorded in the soma) followed by a progressive decrease or "wind-down" of spike number during subsequent pinches. 3. Repeated application to the tail of noxious shock that caused intense activation of sensory neurons (10-22 spikes during the initial shock) produced progressive wind-down of discharge similar to that produced by repeated tail pinch. However, sensory neurons that showed lower activation (1-9 spikes) to the same shock displayed wind-up of discharge during the 10 shocks. These results suggested that prolonged, intense activation depresses subsequent action potential discharge. 4. Changes in the time required for spikes evoked in the tail to reach the central soma were used as an indicator of changes in the excitability and/or conduction velocity of peripheral branches. Repeated pinch within a sensory neuron's receptive field caused an increase in the latency of discharge elicited by test shocks within the receptive field that lasted > or = 10 min. Repetitive intracellular stimulation of the sensory neuron soma caused a similar increase in latency. 5. Repetitive soma activation decreased the number of spikes evoked 10 s later by a test shock in the sensory neuron's receptive field, indicating that spike activity depresses the initiation and/or conduction of spikes in peripheral branches. Surprisingly, repeated pinch to the receptive field caused no significant change in the number of spikes evoked by the same test shock. This difference suggests that tail pinch produces concomitant facilitatory effects that oppose the depressive effects of intense spike activity. 6. Depressive effects of repeated pinch and repetitive soma activation were expressed in the axon between the receptive field and the CNS. Spikes evoked by brief test shocks delivered to the nerve containing the axon of the recorded sensory neuron showed a transient increase in latency (perhaps due to a decrease in conduction velocity) after either procedure. Repeated pinch, but not repetitive soma activation, also caused an increase in spike threshold in the nerve.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine receptor-mediated spinal antinociception in the normal and haloperidol pretreated rat: effects of sulpiride and SCH 23390.

Nociceptive tail flick latencies (TFL) were recorded in response to noxious thermal stimuli applied to lightly anaesthetized rats. The effects of intrathecally administered dopamine receptor agonists alone and combined with dopamine receptor antagonists were examined upon the TFL. Experiments were repeated on animals made supersensitive to dopamine following withdrawal from 28 day administration of haloperidol. In untreated animals the D2-receptor agonist LY 171555 and apomorphine produced an increase in TFL. In contrast, the Di-receptor agonist SKF 38393 had no significant effect on TFL. TFL. Following haloperidol-induced dopamine-supersensitivity, SKF 38393 produced an increase in TFL. In contrast, LY171555 and apomorphine had minimal effects on TFL in this preparation. In animals not treated with haloperidol, the dopamine receptor antagonists SCH 23390 and (+/-)-sulpiride both blocked the increase in TFL produced by the D2-agonists. SCH23390 and (+/-)-sulpiride also blocked the increase in TFL produced by SKF 38393 in haloperidol-supersensitized animals. The antinociceptive action of intrathecally administered dopamine agonists appears to be mediated via D2-receptors. Whether the antinociception produced by SKF 38393 is exclusively contingent upon the activation of D1-receptors in the dopamine-supersensitive animal is as yet unresolved.