Completeness and selectivity of ricin "suicide transport" lesions in rat dorsal root ganglia

A ‘Best Practices’ Approach to Neuropathologic Assessment in Developmental Neurotoxicity Testing—for Today

A key trait of developmental neurotoxicants is their ability to cause structural lesions in the immature nervous system. Thus, neuropathologic assessment is an essential element of developmental neurotoxicity (DNT) studies that are designed to evaluate chemically-induced risk to neural substrates in young humans. The guidelines for conventional DNT assays have been established by regulatory agencies to provide a flexible scaffold for conducting such studies; recent experience has launched new efforts to update these recommendations. The present document was produced by an ad hoc subcommittee of the Society of Toxicologic Pathology (STP) tasked with examining conventional methods used in DNT neuropathology in order to define the ‘best practices’ for dealing with the diverse requirements of both national (EPA) and international (OECD) regulatory bodies. Recommendations (including citations for relevant neurobiological and technical references) address all aspects of the DNT neuropathology examination: study design; tissue fixation, collection, processing, and staining; qualitative and quantitative evaluation; statistical analysis; proper control materials; study documentation; and personnel training. If followed, these proposals will allow pathologists to meet the need for a sound risk assessment (balanced to address both regulatory issues and scientific considerations) in this field today while providing direction for the research needed to further refine DNT neuropathology ‘best practices’ in the future.

A model of motor neuron loss: selective deficits after ricin injection

This study characterizes a model of motor neuron (MN) loss on the molecular, cellular, and behavioral levels. Injection of the toxic lectin Ricinus communis agglutinin I (RCA I or ricin) caused cellular deficit and loss of function by damaging the sciatic nerve. Since the sciatic nerve supplies movement to most of the lower limb, damaging this motor system models lower limb paralysis and the deficits that occur in diseases like amyotrophic lateral sclerosis (ALS) and infantile progressive spinal muscular atrophy (SMA). We used motor-, sensorimotor-, locomotor-, and reflex-based tests to demonstrate loss of function after ricin injection. Loss of function was also demonstrated by decreased retrograde transport, and supported by measurements of muscle wasting. Histochemical and molecular methods were used to characterize sciatic nerve damage in axons and cell bodies, including apoptotic cell death in MNs. This battery of tests documents the extent of the ricin-induced damage and provides a baseline that can be used to judge the efficacy of MN treatment strategies in preclinical studies.

Visceral and orofacial somatic afferent fiber terminals converge onto the same neuron in paratrigeminal nucleus: An electron microscopic study in rats

The paratrigeminal nucleus (Pa5) receives visceral sensory inputs through the vagus (X) and glossopharyngeal (IX) nerves and somatic sensory inputs through the trigeminal (V) nerve. In the present study, transganglionic transport of the WGA-HRP and Wallerian degeneration was used to identify whether two kinds of primary afferent fiber terminals converge onto a single neuron in the Pa5 at the utrastructural level. It was found that HRP-labeled and degenerated terminals originating from the IX and/or X nerves and infraorbital nerve formed asymmetrical synapses with unlabeled dendrites in the Pa5. Furthermore, approximately 7% (43/630) HRP-labeled and 31% (43/137) degenerated terminals formed synaptic connections with the same dendritic profiles simultaneously in the dorsal division of the Pa5. These results may provide a neuroanatomical substrate for integration of viscerosomatic sensory inputs associated with visceral and cardiovascular reflexes in the Pa5.

Selective inhibition of neuronal protein synthesis by retrogradely transported ricin

The ability of the lectin, ricinus communis agglutinin I (ricin120), to undergo retrograde axonal transport and cause degeneration of neuronal cell bodies has been frequently exploited to establish the origin of peripheral axons. Since this cytotoxic action of ricin results from its inactivation of ribosomes, the retrogradely transported lectin was employed in the present study to inhibit protein synthesis in dorsal root ganglion (DRG) neurons whose axons project into the lumbar nerve trunk of bullfrog tadpoles. The procedure was developed to examine, during tadpole metamorphosis, the ratio of fast-transported radiolabeled protein accumulating at the proximal side of a nerve trunk ligature to the total newly synthesized protein in the cell bodies of origin. The relatively small diameter and fragility of the developing lumbar nerve trunks necessitated introduction of ricin by bath application to the cut nerve end rather than by intraneural injection. Consistent uptake of ricin was achieved by pretreatment with the phospholipase A2 inhibitor, mepacrine, that blocks resealing of severed nerve fibers. Optimal time and dosage of ricin were established by determining the maximal achievable inhibition of [35S]methionine into DRG protein. In stage XVI tadpoles, maximal inhibition of approximately to 65% was observed after 16 h incubation in 2.5 mg/ml ricin. As evidence that neuronal protein synthesis was effectively suppressed, there was no detectable anterograde axonal transport of [35S]protein subsequent to ricin treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Collateral sprouting in the electrosensory lateral line lobe of weakly electric teleosts (gymnotiformes) following ricin ablation

Sprouted collateral axons were observed in the electrosensory lateral line lobe (ELL) of gymnotiform teleosts (Apteronotus leptorhynchus) following the ablation of the supraorbital branch of the anterior lateral line nerve. Ablation was accomplished by using microinjections of the toxic lectin ricin. Sprouted axons were followed for up to 26 weeks postablation. Ricin exposure severely reduced axonal numbers and the peripheral electroreceptors in the region innervated by these fibers. To visualize sprouted fibers, intact lateral line afferent nerve branches were anterogradely labelled with the neuronal tract tracers horseradish peroxidase or cobalt chloride, or the monoclonal antibody Q26A3. Within the four somatotopically organized ELL segments, sprouted collaterals were first observed two weeks after ricin injection in the medial and centromedial segments, and four weeks postinjection in the centrolateral and lateral segments. Sprouting involved intrasegmental, horizontally directed axons from adjacent nerve branch terminal fields, and mixed intra- and extrasegmental, dorsally directed axons from the ELL deep fiber layer. The sprouting response was robust but variable in its timing, peaking between 6 and 12 weeks. Subsequently, the intrasegmental, horizontally directed fibers were retained but the mixed dorsally directed fibers, including all extrasegmental axons, were retracted. Therefore, this sprouting response appears to consist of a collateral overproduction followed by a selective axonal retraction. In our view, the most likely explanation for this axonal retraction is that the descending inputs from the isthmus and the cerebellum, as well as commissural fibers from the contralateral ELL, maintain established somatotopic relationships by eliminating somatotopically mismatched sprouted collaterals.

The use of a neurotoxic lectin, volkensin, to induce loss of identified motoneuron pools

In this study we investigated degeneration of defined motor pools in the adult rat spinal cord and the associated changes in spinal cord in dorsal root ganglia and peripheral nerve. Degeneration of motoneurons was induced by the neurotoxic lectin, volkensin. This substance is taken up by the axons and retrogradely transported to the cell body, where it inhibits proteosynthesis and kills the neuron. Accordingly, in adult Wistar rats the peroneal or the sciatic nerve was injected with 5.0 ng volkensin, and the effect of this single injection was investigated at different intervals after the operation. Retrograde labelling by horseradish peroxidase was used to reveal the extent of cell death and glial repair was studied by immunostaining with different glial cell markers. Degenerating cells were observed in the ventral horn of the lumbar spinal cord and L4 and L5 dorsal root ganglia as early as four days after volkensin treatment and by two weeks no retrogradely labelled motoneurons could be found in the treated peroneal pool. These changes were accompanied by severe muscle weight loss. Examination of the ventral horn of the spinal cord on the treated side revealed many hypertrophic astrocytes and reactive microglial cells expressing an increased level of complement receptor type 3 immunoreactivity. In the volkensin-injected peripheral nerve, distinct signs of Wallerian-like degeneration could be observed. Schwann cells identified by immunostaining to S-100 protein appeared to be preserved. Interestingly, at later stages after volkensin injection (four to eight weeks), some retrogradely labelled motoneurons were seen in the peroneal pool; their number occasionally reached 18.4% of the control pool. The dorsal root ganglia showed extensive loss of neurons and numerous abnormal neurons were found throughout the period of the study. These findings suggest that some motoneurons are able to recover from exposure to volkensin and temporary arrest of proteosynthesis. Despite this, volkensin-induced selective motoneuron death in the adult rat can be a useful experimental model for degenerative motoneuron disease.

Neural lesioning with ribosome-inactivating proteins: suicide transport and immunolesioning

Toxic lectins, plant proteins that inactivate ribosomes, irreversibly inhibit protein synthesis with high efficiency. After intraneural (subepineurial) microinjection, these agents are taken up by axons and are retrogradely transported to the perikarya, where they result in cell death. These 'suicide transport' toxins can produce pathway-specific lesions that are useful in several types of experiment, including cellular localization of neurotransmitter receptors. The toxins can be coupled to monoclonal antibodies to produce immunotoxins: reagents that can make highly selective lesions of specific types of neurons. Central or peripheral neurons that express the low-affinity NGF receptor are selectively destroyed by the immunotoxin 192 IgG-saporin. Development of other anti-neuronal immunotoxins should provide a variety of powerful selective lesioning tools.

Quantitative study of neuronal degeneration induced by Ricinus toxin and crush of postganglionic nerves in the ciliary ganglion of quail

The effects of Ricinus toxin on the neurons of the ciliary ganglia were investigated in the quail. The neuronal death and the morphological alterations of the ganglionic cells were assessed following injection of the toxin in the anterior chamber of the eye or after application of the toxin on the postganglionic nerves at a crush site. A 45% loss of choroid neurons without loss of ciliary neurons was observed after postganglionic nerve crush alone. Injection of the toxin in the anterior chamber of the eye led to a selective loss of ciliary neurons (38%). Application of the toxin to the crushed postganglionic nerves led to a loss from both neuronal populations (40% of total neurons). This work indicates that different procedures result in selective lesion of the different neuronal populations in the ciliary ganglion.

Neurochemical mediators of the behavioural effects of receptor-selective substance P agonists administered intrathecally in the rat

Recently, two compounds have been developed, designated septide and senktide, which are highly selective agonists for the substance P receptor, types NK-1 and NK-3, respectively. Each of these, when injected intrathecally in awake rats, produced a distinct and non-overlapping constellation of sensory and behavioural effects which were subsets of the symptoms evoked by intrathecal administration of substance P. Prior systemic administration of 5-hydroxytryptamine (5-HT), alpha-adrenergic and opiate receptor antagonists, at doses sufficient to block the behavioural effects of the corresponding receptor agonists, did not alter responses to intrathecally injected septide or senktide. This was so, even for symptoms which suggested inhibitory mediation, hypoalgesia and (transient) motor flaccidity. Septide and senktide, administered by lumbar puncture and by indwelling catheter, produced identical results. Finally, in contrast to some other peptides, flaccid paralysis induced by senktide was not accompanied by spinal necrosis.

Ultrastructural observations of non-selective effects of ricin treatment (RCA-120) in the rat dorsal root ganglion

The effects of ricin (RCA-120) on non-injected dorsal root ganglion (DRG) cells, sharing the same DRG as the injected ones, were studied after ricin injections into the tibial nerve and B-HRP injections into the peroneal nerve. Numerous DRG cells containing B-HRP reaction product and exhibiting signs of advanced degeneration were observed. The findings suggest that ricin may be released from dying injected DRG neurons and taken up by adjacent non-injected DRG cells, which subsequently degenerate.

Neuronotoxic effects of monoclonal anti-Thy 1 antibody (OX7) coupled to the ribosome inactivating protein, saporin, as studied by suicide transport experiments in the rat

As a first attempt to develop suicide transport agents based upon antineuronal antibodies, we studied an immunotoxin directed against the Thy 1 antigen which is on rat neurons. The immunotoxin was composed of mouse monoclonal anti-Thy 1 antibody (OX7) and the ribosome-inactivating protein, saporin, and was prepared using the heterobifunctional cross linker, SPDP, which provides a disulfide linkage between the two protein components. This immunotoxin reliably and selectively destroyed ipsilateral vagal motor and sensory neurons after injection into the cervical vagus. Injection of the immunotoxin into the caudate nucleus produced destruction of the ipsilateral substantia nigra, pars compacta and intralaminar thalamic nuclei (parafascicular and central median). Anti-mouse IgG immunoperoxidase staining confirmed axonal transport of OX7 by vagal sensory and motor neurons and by caudate afferents and efferents. Systemic toxicity was not observed with OX7-saporin. The neuronotoxic effects of OX7-saporin were specific since injections of a similarly constructed immunotoxin of irrelevant specificity or a mixture of OX7 and saporin were without suicide transport activity. These results show the feasibility of using immunotoxins as suicide transport agents.

Dose- and time-dependent selective and non-selective effects of ricin (RCA 120) on rat primary sensory neurons

The distribution of degenerating fibers in the spinal cord was studied in Fink-Heimer-stained sections following treatment of the tibial nerve with ricinus communis agglutinin (RCA 120). The ricin was either injected into the nerve or applied in a capsule on the transected nerve. Short survival times and low doses of ricin resulted in degeneration in somatotopically appropriate parts of the medial dorsal horn. Longer survival times and higher doses resulted in degeneration which progressively expanded into inappropriate areas in the central and lateral parts of the dorsal horn and in deeper laminae regardless of the mode of application. Furthermore, the effect of a ricin injection into the tibial nerve on transganglionic transport of choleragenoid horseradish peroxidase (B-HRP) in the peroneal nerve was studied following a simultaneous or delayed B-HRP injection. A simultaneous ricin and a B-HRP injection resulted in primary afferent HRP labeling in the gray matter, regardless of the dose of ricin. Following a delayed B-HRP injection almost no primary afferent labeling was seen in the gray matter, unless a very low dose of ricin was injected. This study shows that treatment of a peripheral nerve with a high dose of ricin and a long survival time may result in a considerable non-selective degeneration of fibers in the spinal cord. A selective degeneration may, however, be obtained by using lower doses or shorter survival times.

Anti-ricin antibody protects against systemic toxicity without affecting suicide transport

Ricin is an effective suicide transport agent which reliably destroys sensory and motor neurons in anatomically selective fashion after peripheral nerve application. In the present study, we gave subcutaneous injections of commercially available antiricin antibody (ARA) at the time of ricin application to peripheral nerves. The ARA protected rats against systemic ricin poisoning without altering the suicide transport activity of ricin. This approach removes a significant barrier to the routine use of ricin and should prove useful when ricin is employed to ablate large peripheral nerves. However, antibody protection permits use of such large ricin doses that the toxin may diffuse within the CNS destroying neurons adjacent to those that project through the injected nerve. Consequently, antibody and ricin doses must be optimized for any particular experimental situation.

Cortical organization after treatment of a peripheral nerve with ricin: an evaluation of the relationship between sensory neuron death and cortical adjustments after nerve injury

The present study was designed to assess whether cortical changes after peripheral nerve damage are related to the degree of death of primary sensory neurons in the damaged nerve. The cytotoxin ricin was injected into the sciatic nerves of adult rats to kill primary sensory neurons with axons through the injection site. Following periods of 6-101 days, the S-I hindpaw map was evaluated with neurophysiological techniques and compared with the hindpaw maps of previously studied normal adult rats and adult rats that had undergone adult or neonatal sciatic section at a comparable level of the nerve. These comparisons allowed evaluation of cortical functional organization following different degrees of sensory neuron loss after sciatic nerve injury. There were three main results. 1) The comparison of ricin-treated and normal adult rats indicated that ricin treatment interrupted inputs from the sciatic skin territory on the hindpaw and caused a limited increase in the size of the cortical area that was activated by stimulation of hindpaw skin innervated by the remaining saphenous nerve. 2) The cortical maps of rats that had undergone adult ricin treatment (relatively large primary neuron loss) or section during adulthood (small to moderate primary neuron loss) were similar. In both groups, only the saphenous hindpaw skin was represented in cortex, and the cortical area that was activated by stimulation of the saphenous hindpaw skin had undergone a comparable limited enlargement. 3) The comparison of ricin-treated adult rats (relatively large primary neuron loss) and adult rats that had undergone neonatal section (relatively large primary neuron loss) indicated that cortical organization differed after these treatments. In particular, after ricin treatment the cortical area that was activated by stimulation of the saphenous hindpaw skin was larger than the comparable area in neonatal denervates, and the topographical progressions between the hindpaw and adjacent body representations were not as variable as after neonatal section. These findings indicate that cortical maps are altered after injection of ricin into a nerve. The similarity in cortical organization after ricin treatment (relatively large sensory neuron loss) and nerve section in adults (relatively small sensory neuron loss) and the differences in cortical organization after ricin treatment and nerve section in neonates (both relatively large sensory neuron loss) indicate cortical changes do not covary as a simple function of the degree of peripheral neuron death.

Selective neuronal destruction by Ricinus communis agglutinin I and its use for the quantitative determination of sciatic nerve dorsal root ganglion cell numbers

The selectivity of the neurotoxic lesion of Ricinus communis agglutinin I (RCAI) in rat dorsal root ganglia was examined. RCAI was injected in the sural nerve on one side. Two weeks later, the injected nerve, as well as the ipsilateral peroneal nerve, were examined in 1-micron-thick plastic embedded sections in the light microscope. The injected nerves showed a complete or almost complete Wallerian-like degeneration of myelinated fibers, but there were no signs of fiber damage in the uninjected nerves, which to a large extent originate in the same ganglia as the injected ones. We conclude that RCAI does not diffuse into and destroy ganglion cells adjacent to those that have transported the substance. We then used this selectivity in the effect of RCAI to determine indirectly the relative number of neurons in dorsal root ganglia L4-L6 which contribute to the sciatic nerve. Three weeks after unilateral injections of RCAI in the sciatic nerve, the L4-L6 dorsal root ganglion cells were counted bilaterally. On average, relative neuronal numbers between injected and uninjected sides were 0.36, 0.15 and 0.64 for L4, L5 and L6 respectively. From these data we conclude that the sciatic nerve receives on average of 64%, 85% and 36%, respectively of its sensory contribution from these ganglia.

Acute and chronic effects of the neurolytic agent ricin on dorsal root ganglia, spinal cord, and nerves

The short- and long-term effects of ricin injections into nerves have been evaluated with light microscopy in the dorsal root ganglia, spinal cord, and peripheral nerves in rats and cats. Dorsal root ganglion cells initially exhibited chromatolysis, followed by gliosis and cell death. These changes were associated with Fink-Heimer degeneration in the somatotopically appropriate region of the dorsal horn. There were no signs of chromatolysis in dorsal horn neurons in ricin-injected animals, but chromatolytic motoneurons were observed. Ricin produced acute necrosis of injected nerves and dissolution of axoplasm. At long survival times (greater than 4 weeks) some apparently regenerating axons were seen in the injection sites of rats. Cell counts indicated that a substantial percentage of dorsal root ganglion neurons associated with the injected nerves were killed, but the presence of regenerating axons suggested that some cells survived the ricin treatment. Although the lesion may not always be complete, even with maximum sublethal doses, this method appears to be useful for specifically destroying afferent fibers associated with a particular nerve without transynaptic destruction of dorsal horn neurons.