Age-related changes in the response to thermal noxious heat and reduction of C-fibers by neonatal treatment with capsaicin.

Developmental changes of the response to nociceptive heat were examined in mice treated with capsaicin (50 mg/kg) on postnatal days 2-15. Tests of hot-plate (55 degrees C) and irradiation by infrared (IR test) were carried out after 10 days of capsaicin administration up to 120 days at intervals of 10 or 20 days. The time until forepaw (hot-plate) and hindpaw (IR test) withdrawal was assessed as the response latencies to suprathreshold and thermal threshold, respectively. Moreover, the numbers of unmyelinated C-fibers and myelinated fibers in the L4 dorsal roots of the animals treated on postnatal days 2 and 5 were counted on electron micrograph montages. Despite the marked reduction of C-fibers (60% mean decrease) in the 4 dorsal roots of the animals treated on postnatal day 2, thresholds were normal compared with those of controls. However, the animals treated with capsaicin on postnatal day 5 showed an apparent delay of thermal threshold latency only in the IR test, although the mean reduction of C-fibers was very likely the same as that of the animals pretreated on day 2. The reduction of C-fibers in mice treated on postnatal days 10 and 15 was lower than the animals treated on days 2 or 5, but their threshold latencies were significantly increased (hypoalgesia). A possible implication of these findings is discussed on the basis of the development of inhibitory systems in the intraspinal and supraspinal dorsal horn and sprouting from the surviving primary afferent neurons in the superficial dorsal horn.

Early morphological changes of primary afferent neurons and their processes in newborn mice after treatment with capsaicin.

Degenerating figures of dorsal root ganglion (DRG) neurons and their central and peripheral processes (dorsal root and saphenous nerve) and terminals (central terminals in the superficial dorsal horn and cutaneous nerve of the hind paw dorsal skin) of neonatal mice were examined 30 min, 1, 2 and 5 h, and 2, 3, 5, and 10 days after subcutaneous injection of capsaicin on post-natal day 2. Many small DRG neurons showed degeneration 1 h after treatment. Scarcely any features of degeneration were seen in the DRG and dorsal root 10 days after treatment. The degenerating aspects of terminal axons in the marginal layer of the superficial dorsal horn were characterized by enlarged round axons with closely packed osmiophilic materials, lamellar bodies, and loss of axoplasmic organelles. Two types of central terminals (C-terminals) showed degeneration in the substantia gelatinosa from 30 min after treatment onward. One type consisted of small, round, sinuous or slender dark terminals (CI-terminals), and the other of large, pale, round or angular terminals (CII-terminals). Those that degenerated markedly had homogeneously electron-dense axoplasm with dilated synaptic vesicles and inclusion bodies. Extensive degeneration of terminal axons in the marginal layer occurred 5 h after treatment, whereas conspicuous degeneration of C-terminals occurred from 30 min to 10 days after treatment in the substantia gelatinosa. CI-terminals showed marked degeneration during the first 3 days, whereas marked degeneration of CII-terminals occurred between 5 and 10 days after treatment. This time difference between the peaks of degeneration of CI- and CII-terminals indicates an important difference in the origins of these two types of capsaicin-sensitive, nociceptive fibers in the superficial dorsal horn; CI-terminals are derived from small DRG cells, whereas CII-terminals are derived from larger DRG cells. Unmyelinated axons in the dorsal root, saphenous nerve, and dorsal skin of the hind paw showed similar degeneration patterns 2 h after treatment to those of terminal axons in the marginal layer. Thus, the degenerating profiles in the marginal layer suggest that these axons arose from collaterals of unmyelinated primary axons descending or ascending within the marginal layer. Numerous enlarged degenerating axons showing vacuolation were conspicuous in the dorsal skin 3 days after treatment. The synchronous degeneration of the smaller DRG neurons, their central and peripheral processes, and their CI-terminals in the substantia gelatinosa supports the idea that the smaller DRG neurons are directly influenced by capsaicin, and that their degeneration is followed by centrifugal degeneration.

Quantitative electron-microscopic analyses of pulpal nerve fibres in the mouse lower incisor after neonatal capsaicin treatment.

A single dose of capsaicin (50 mg/kg) was injected subcutaneously into four mice on day 2 of life; four untreated mice were used as controls. Six months later, a drop of 30 microM capsaicin was instilled on to the cornea of all the mice and the number of times the eyes were wiped was counted to assess the effect of capsaicin on trigeminal sensory neurones. Ultrathin cross-sections were made of the apical pulp of the incisors on both sides of control (n = 8) and capsaicin-treated animals (n = 8). Electron micrographs of pulp nerves were taken and enlarged to a final magnification of x34,000. The numbers of unmyelinated axons in the pulps of all 16 incisors and of unmyelinated axons per Schwann cell in the pulps of four incisors each from the control and capsaicin-treated groups were counted. The short diameters of unmyelinated axons were measured with a computer-operated image analyser. The number of eye wipings was eight-fold less in the capsaicin-treated than in the normal group. This finding clearly indicated that capsaicin irreversibly affected the chemogenic nociceptive trigeminal neurones. The mean number of unmyelinated axons was 345 in controls and 217 (37.1% reduction) in capsaicin-treated animals. The number of unmyelinated axons of less than 0.6 microns dia was 41.5% less in capsaicin-treated mice than in controls. Thus, fine unmyelinated axons in the mouse incisor pulp are capsaicin sensitive, and they are assumed to be nociceptive fibres conveying pain stimuli from the tooth. Capsaicin affected Schwann cells, even those with few unmyelinated axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in features of degenerating primary sensory neurons with time after capsaicin treatment.

Capsaicin (50 mg/kg) was injected into new born mice and 5 and 12 h, and 1, 2, 3, and 5 days later, their lumbar dorsal root ganglia (DRG) with the nerve roots were fixed by immersion. The morphological changes which ensued with time after treatment were examined by light and electron microscopy. The findings were as follows: (a) rapid degeneration of certain smaller B-type neurons, indicating their prompt death, was seen 5 h after treatment. Later, accumulated neurofilaments appeared in larger B-type neurons. Fissures of the cytoplasm and cell fragmentation were also observed as particular features of degeneration. Finally, these degenerating neurons, destined to die, appeared as small round figures with a disorganized nucleus. Severely degenerated neurons were seen throughout the survival time after treatment, but seemed to be most numerous after 2-3 days. (b) Three days after treatment the Nissl substances of large A-type neurons appeared dispersed, forming ring-like bundles in the periphery of cells. Cytoplasmic rupture and large membrane-bound spaces with fine granular or fibrillar materials, indicating peripheral cytolysis, were also conspicuous. Some of these cells showed severe degeneration clearly leading to cell death. The A-type neurons began to degenerate later than the B-type neurons. (c) Satellite cells showed an increased amount of electron-opaque cytoplasm that contained large vacuoles and neuronal cell debris. Mitotic figures were increased in satellite cells 3 days after treatment. (d) Unmyelinated axons in the dorsal root of mice treated with capsaicin became enlarged with accumulation of neurofilaments, synaptic vesicles or various kinds of vesicles, multivesicular bodies and mitochondria. Numerous dense lamellar bodies appeared in the unmyelinted axons within DRG 3 days after treatment, but were scarcely seen in the dorsal roots. Degeneration of the myelinated fibers increased with time. Interestingly, capsaicin seemed to have both a direct and indirect action on DRG neurons: its direct action induced rapid degeneration of the smaller neurons, whereas its indirect action induced relatively slow degeneration of the larger neurons, causing chromatolytic changes similar to those induced by periphal nerve axotomy. The injury to DRG neurons due to the indirect action seemed to be induced retrogradely.