Håkan Aldskogius

Central neuron–glial and glial–glial interactions following axon injury

Axon injury rapidly activates microglial and astroglial cells close to the axotomized neurons. Following motor axon injury, astrocytes upregulate within hour(s) the gap junction protein connexin-43, and within one day glial fibrillary acidic protein (GFAP). Concomitantly, microglial cells proliferate and migrate towards the axotomized neuron perikarya. Analogous responses occur in central termination territories of peripherally injured sensory ganglion cells. The activated microglia express a number of inflammatory and immune mediators. When neuron degeneration occurs, microglia act as phagocytes. This is uncommon after peripheral nerve injury in the adult mammal, however, and the functional implications of the glial cell responses in this situation are unclear. When central axons are injured, the glial cell responses around the affected neuron perikarya appears to be minimal or absent, unless neuron degeneration occurs. Microglia proliferate, and astrocytes upregulate GFAP along central axons undergoing anterograde, Wallerian, degeneration. Although microglia develop into phagocytes, they eliminate the disintegrating myelin very slowly, presumably because they fail to release molecules which facilitate phagocytosis. During later stages of Wallerian degeneration, oligodendrocytes express clusterin, a glycoprotein implicated in several conditions of cell degeneration. A hypothetical scheme for glial cell activation following axon injury is discussed, implying the injured neurons initially interact with adjacent astrocytes. Subsequently, neighbouring resting microglia are activated. These glial reactions are amplified by paracrine and autocrine mechanisms, in which cytokines appear to be important mediators. The specific functional properties of the activated glial cells will determine their influence on neuronal survival, axon regeneration, and synaptic plasticity. The control of the induction and progression of these responses are therefore likely to be critical for the outcome of, for example, neurotrauma, brain ischemia and chronic neurodegenerative diseases.

Chronic pain-related syndrome in rats after ischemic spinal cord lesion: a possible animal model for pain in patients with spinal cord injury

&NA; We examined a pain‐related syndrome, which includes mechanical allodynia and autotomy, in rats after ischemic spinal cord injury photochemically induced by laser irradiation for 5–20 min. This procedure results in an acute allodynia‐like phenomenon which lasts for several days and is possibly related to dysfunction of the GABAB system in the spinal cord. In some animals this is followed by a chronic allodynia‐like symptom with an onset varying between 1 week and 1.5 months after injury, expressed as a clearly painful reaction to light pressure applied to a skin area at or near the dermatome of the injured spinal segments. In the majority of rats the allodynia persists over several months, in some cases accompanied by autotomy of the hind paws. Pharmacological studies indicated that the allodynia in the majority of rats could be relieved by systemic tocainide (75 mg/kg). Morphine was only effective at a sedative dose (5 mg/kg). The allodynia was not relieved by baclofen, muscimol, clonidine or carbamazepine. Low‐dose systemic pentobarbital (5 mg/kg) had a slight beneficial effect. Guanethidine (20 mg/kg s.c.) did not abolish the allodynia in most of the rats. Histological examination revealed massive damage in the spinal cord. The dorsal roots of the irradiated segments were also injured. No morphological abnormalities were seen in the dorsal root ganglia. The mechanism that may account for this chronic pain‐related syndrome in spinally injured rats probably involves abnormalities in the central nervous system. The allodynia seen in chronic spinally injured rats was similar to some painful symptoms in patients after spinal cord injury or stroke. It is suggested that the chronic allodynia‐like phenomenon may represent an animal model for studying the mechanisms of chronic central pain.

Nerve cell degeneration and death in the trigeminal ganglion of the adult rat following peripheral nerve transection.

SummaryTrigeminal ganglia of normal rats and of adult rats subjected to unilateral transection of the infraorbital nerve were studied by light and electron microscopy. Counts of ganglion cells in ganglia on operated and unoperated sides were made following long postoperative survival times. The ultrastructural changes in ganglia of the operated side were studied from 3 to 70 days postoperatively. The quantitative observations show that a considerable loss of ganglion cells takes place on the operated side. The ultrastructural observations demonstrate the occurrence of ganglion cell degeneration, nerve fibre degeneration and phagocytosis by satellite and Schwann cells. The results are compatible with the view that degeneration of trigeminal afferents in the brain stem following lesions of peripheral trigeminal nerve branches is related to retrograde degeneration of trigeminal ganglion cells.

Allodynia-like effects in rat after ischaemic spinal cord injury photochemically induced by laser irradiation

&NA; We report behaviours suggesting the presence of allodynia elicited by non‐noxious brushing and mechanical pressure following photochemically induced ischaemic spinal cord injury in the rat. Female rats were intravenously injected with Erythrosin B and the T10 vertebra was irradiated with a laser beam for 1, 5 or 10 min. These procedures initiated an intravascular photochemical reaction, resulting in ischaemic spinal cord injury. After irradiation a clear allodynia was observed in most rats, The animals vocalized intensely to light touch during gentle handling and were clearly agitated to light brushing of the flanks. The vocalization threshold in response to the mechanical pressure measured with von Frey hairs was markedly decreased during this period. In some animals the existence of spontaneous pain was suggested by spontaneous vocalization. The duration of the allodynia varied among animals from several hours to several days. The severity and duration of allodynia seemed not to be related to the duration of irradiation. In sham‐operated rats a slight, transient allodynia was also noted around the wound within a few hours after surgery, which was effectively relieved by systemic morphine (2 mg/kg, i.p.). Morphine (2 mg/kg, i.p.) also partially relieved the allodynia in spinally injured rats 4 h after irradiation. However, morphine, even at a higher dose (5 mg/kg, i.p.), failed to alleviate the allodynia in spinally injured rats 24–48 h after the injury. Systemic injection of the GABAB agonist baclofen (0.01‐0.1 mg/kg, i.p.), but not the GABAA agonist muscimol (1 mg/kg, i.p.), effectively relieved allodynia during this period. Pretreatment with guanethidine 24 h and just prior to the irradiation (20 mg/kg, s.c.) did not prevent the occurrence of allodynia in spinal cord injured rats, The present observation is the first to show that ischaemic spinal cord injury could result in cutaneous mechanical allodynia. This phenomenon is resistant to morphine and may not involve the sympathetic system. Histological examination of allodynic animals 3 days after spinal cord injury revealed considerable morphological damage in the dorsal spinal cord of a rat irradiated for 5 min. The related dorsal roots were also slightly affected in this animal, while the dorsal root ganglia were normal. However, in rats irradiated for 1 min, despite the existence of strong allodynia, no damage could be found at this time in the spinal cord, dorsal roots or dorsal root ganglia. It is suggested that functional deficits in the GABAB system in the spinal cord may be related to this allodynia‐like phenomenon. Allodynia following laser‐induced spinal cord injury may be a useful pain model for testing the efficacy of analgesic drugs against central pain of ischaemic origin.

The reaction of primary sensory neurons to peripheral nerve injury with particular emphasis on transganglionic changes

This paper reviews light- and electron microscopic, histochemical and physiological evidence which demonstrate that peripheral nerve injury in mammals is followed by profound structural and functional changes in the central terminals of the affected primary sensory neurons. Available evidence indicates that at least some of these so-called transganglionic changes are the result of ganglion cell degeneration and death, although other mechanisms are probably in effect as well. Existing data suggest that this ganglion cell death does not effect all types of ganglion cells equally, but do not permit a clearcut answer to the question of which kinds of ganglion cells are affected more than others. Results from studies with microtubule inhibitors and antibodies to nerve growth factor are compatible with the notion that depletion of retrogradely transported trophic factors is involved in the production of certain transganglionic changes. This issue needs further examination, however. Physiological studies indicate marked alterations in certain primary afferent synaptic connections after peripheral nerve lesions. So far, these changes have not been satisfactorily correlated with the structural changes induced by similar lesions. Further studies on the structural and functional response of primary sensory neurons to peripheral nerve injury are likely to contribute to the understanding of the frequent failure to regain normal sensory functions after peripheral nerve lesions in man, as well as of the basic aspects of lesion-induced changes in general in the peripheral and central nervous system.

Expression of mRNAs for neurotrophin receptors in the dorsal root ganglion and spinal cord during development and following peripheral or central axotomy

Expression of mRNAs for the protein tyrosine kinases trk, trkB and trkC, encoding essential components of high-affinity neurotrophin receptors, was studied in the spinal cord and dorsal root ganglion during normal development and in the adult rat following peripheral and central axon injury. Northern blots revealed multiple trkB transcripts in the embryonic, early postnatal and adult spinal cord with different patterns of expression during development. The levels of 9.0 kb and 4.8 kb trkB transcripts, encoding a full-length trkB receptor, increased progressively during embryonic development with maximal levels around birth, followed by a decline at adulthood. In contrast, the level of 7.5/7.0 kb trkB transcripts, encoding a truncated trkB receptor, reached maximal levels shortly after birth and similar levels remained in the adult animal. In the spinal cord a 4.7kb trkC transcript was detected with maximal levels shortly after birth. In situ hybridization revealed a uniform labeling throughout the spinal cord for both trkB and trkC mRNAs with maximal intensities of labeling shortly after birth. The level of the 2.4 kb trkB transcript in the spinal cord increased 5-fold 8 days after a crush lesion of the sciatic nerve or the dorsal root, while no change was seen in the levels of the other trkB transcripts. No change in the 4.7 kb trkC mRNA was seen following these two injuries, although increased levels of several smaller size trkC transcripts were observed. For both trkB and trkC, similar size transcripts as seen in the spinal cord were also detected in adult rat dorsal root ganglia. Consistent with previous observations of decreased levels of cytoskeletal proteins after peripheral and central axotomy, the level of neurofilment light chain mRNA decreased markedly in the dorsal root ganglia following a crush lesion of the sciatic nerve or of the dorsal root. A small decrease was also seen in the level of preprotachykinin-A mRNA encoding the protein precursor of substance P. In the same animals, the levels of all five trkB transcripts increased 3-fold in the dorsal root ganglia in response to these two injuries. A small increase was also seen in the level of trkC mRNA. The level of brain-derived neurotrophic factor (BDNF) mRNA increased two-fold in the dorsal root ganglia following either of the two lesions, while no change was detected in trk mRNA following these two injuries.(ABSTRACT TRUNCATED AT 400 WORDS)

Comprehensive immunofluorescence and lectin binding analysis of vibrissal follicle sinus complex innervation in the mystacial pad of the rat.

The innervation of the vibrissal follicle sinus complexes (FSCs) in the mystacial pad of the rat was examined by lectin binding histofluorescence with the B subunit of Griffonia simplicifolia (GSA) and by immunofluorescence with a wide variety of antibodies for neuronal related structural proteins, enzymes, and peptides. Only anti‐protein gene product 9.5 labeled all sets of innervation. Several types of mechanoreceptors were distributed to specific different targets by medium to large caliber myelinated axons. All were positive for 200 kDa neurofilament subunit, peripherin, and carbonic anhydrase. Their endings expressed synaptophysin. Labeling for the 160 kDa neurofilament subunit, calbindin, and parvalbumin varied. Anti‐Schwann cell protein S100 was completely co‐extensive with the axons, terminal arbors, and endings of the mechanoreceptor afferents including Merkel innervation. At least 15 different sets of unmyelinated innervation were evident based upon distribution and labeling characteristics. They consisted of four basic types: 1) peptidergic; 2) GSA binding; 3) peptidergic and GSA binding; and 4) nonpeptidergic and GSA negative (peptide‐/GSA‐). Previous studies had not revealed that several major sets of unmyelinated innervation were peptide‐/GSA‐. The unmyelinated innervation had detectable peripherin but not 160 kDa or 200 kDa neurofilament subunits. GSA‐positive axons uniquely lacked anti‐S100 immunoreactivity. The dense circumferentially oriented unmyelinated innervation of the inner conical body contained major sets of peptide‐/GSA‐ and GSA innervation as well as a smaller peptidergic GSA component. A small contingent of sympathetic and possibly parasympathetic innervation was affiliated with microvasculature in the FSCs. This study confirms and refutes some previous hypotheses about biochemical and morphological relationships between peripheral innervation and sensory ganglion cells. J. Comp. Neurol. 385:149–184, 1997.

A quantitative analysis of the microglial cell reaction in central primary sensory projection territories following peripheral nerve injury in the adult rat.

The time course of the microglial cell reaction in central nervous system primary sensory projection territories has been examined following peripheral nerve injury in the adult rat using qualitative and quantitative analysis of immunoreactivity with the monoclonal antibody OX-42, which recognises the complement receptor CR3. The regions examined included the gracile nucleus, the column of Clarke and the spinal cord dorsal horn (superficial and deep laminae separately) after unilateral sciatic nerve transection, and the spinal trigeminal nucleus following unilateral infraorbital nerve transection. In all territories examined a qualitative increase in OX-42 immunoreactivity was observed 24 h postlesion. Further, quantitative analysis revealed an exponential development of the OX-42 immunoreactivity, with a peak at one week postlesion, thereafter showing a slow exponential decline. Our results show that the signal (or signals) that induces the microglial cell response in primary sensory projection territories is rapid in comparison to previously described central degenerative changes following peripheral nerve lesions (transganglionic degeneration). These findings are compatible with the hypothesis that activated microglia play a pathogenetic role in the development of transganglionic degeneration.

Glial Responses to Synaptic Damage and Plasticity

We review three principally different forms of injury‐induced synaptic alterations. (1) Displacement of presynaptic terminals from perikarya and dendrites of axotomized neurons, (2) central changes in primary afferent terminals of peripherally axotomized sensory ganglion cells, and (3) anterograde Wallerian‐type degeneration following interruption of central axonal pathways. All these instances rapidly acativate astrocytes and microglia in the vicinity of the affected synaptic terminals. The evidence suggests that activated astrocytes play important and direct roles in synapse elimination and in the processes mediating collateral reinnervation. The roles of microglia are enigmatic. They undergo activation close to axotomized motoneuron perikarya, where synapse displacement occurs, but not adjacent to axotomized intrinsic central nervous system neurons, where synapse displacement also occurs. Microglia are also rapidly activated around central primary sensory terminals of peripherally axotomized sensory ganglion cells. Occasional phagocytosis of degenerating axon terminals by microglia occur in the latter situation. However, the role of microglia may be more oriented toward the general tissue conditions rather than specifically toward synaptic terminals. J. Neurosci. Res. 58:33–41, 1999.

Decreased GABA immunoreactivity in spinal cord dorsal horn neurons after transient spinal cord ischemia in the rat

The number of GABA-like immunoreactive (LI) cells in lamina I-III of the rat spinal cord was significantly decreased bilaterally 48-72 h after photochemical induction of transient spinal cord ischemia compared to sham-operated controls. No significant changes in the number of GABA-LI cells were observed at cervical level. The number of GABA-LI cells was restored 2 weeks after ischemia. These data, together with recent behavioral and electrophysiological findings, suggest that decreased intraneuronal GABA levels after spinal cord ischemia may underlie the development of the temporary pain-like response to innocuous mechanical stimuli (allodynia) in rats after transient spinal cord ischemia.