Mitochondrial changes in preoptic neurons after capsaicin desensitization of the hypothalamic thermodetectors in rats

Effects of capsaicin on central monoaminergic mechanisms in the rat

The acute and chronic effects of capsaicin (s.c.) on the monoamines in the preoptic region + hypothalamus (RPO-H), spinal cord, substantia nigra and striatum were studied. Levels of DOPA, DA, DOPAC, HVA, 3-MT, NA, Trp, 5-HTP, 5-HT and 5-HIAA were determined by means of liquid chromatography (HPLC-EC). In response to acute capsaicin treatment, the levels of DA, DOPAC and DA synthesis rate (DOPA formation) were increased in a dose-dependent manner in the RPO-H and spinal cord. The disappearance rate of NA was accelerated in both regions. In substantia nigra, increased DOPAC levels were found whereas the levels of 3-MT were decreased in striatum after acute capsaicin treatment. Only minor changes on the levels of 5-HT and 5-HIAA in the regions studied were noted. Neonatal or adult capsaicin treatment failed to affect the levels of NA, DA and 5-HT (measured two months or five weeks after injection, respectively) in the regions studied. A capsaicin injection to rats pretreated with the drug as adults did not affect either the monoamines in the RPO-H and spinal cord or the body temperature. In contrast, in rats pretreated with capsaicin as neonates, a second injection of the drug to adult animals elicited hypothermia and changes in monoamines similar to those observed in naive animals.

Capsaicin fails to produce disturbances of autonomic heat and cold defence in an avian species (Anas platyrhynchos)

Capsaicin was intravenously administered to adult domestic ducks of 1.8-2.6 kg body weight, with a cumulative dose of 1.0 g/kg body weight given in 4-6 single infusions at intervals of 2-3 days. There were no acute, nociceptive or hypothermic effects, as typically seen in mammals. Before and after capsaicin treatment respiratory evaporative heat loss (REHL, w X kg-1), breathing frequency (BF, min-1) and metabolic heat production (M, w X kg-1) were determined in a warm environment (35-38 degrees C) as a function of core temperature, measured in the esophagus (Tes), which was altered by graded heat extraction with a colonic thermode. Capsaicin treatment reduced the rate at which REHL increased with increasing BF, however, this was compensated by a steeper increase of BF with rising Tes so that the relationship between Tes and REHL remained unchanged. The Tes threshold for activation of M was increased by 0.3 degrees C and the slope reduced by 27% after capsaicin, but identical maximum M values were attained before and after capsaicin at identical degrees of hypothermia. Skin temperature measurements revealed no influence of capsaicin on the threshold Tes values for skin vasoconstriction. It is concluded that capsaicin fails to exert effects in birds on those afferents and central neurons which are involved in thermo- and nociception, in contrast to mammals in which these perceptive functions become severely impaired.

Hypothalamic thermosensitivity in capsaicin-desensitized rats

In rats, we tested the hypothesis that capsaicin desensitization reduces hypothalamic warm thermosensitivity. We locally heated and cooled the hypothalamus using water-perfused thermodes while observing thermoregulatory variables. In untreated rats, a small dose of capsaicin had profound effects on thermoregulation. However, desensitizing rats to capsaicin had no effect on hypothalamic thermosensitivity for metabolic rate or changes in body temperature due to displacements of hypothalamic temperature. Contrary to current opinion, we conclude that capsaicin desensitization does not alter hypothalamic thermosensitivity to warm or cold.

Evidence for a capsaicin-sensitive vasomotor mechanism in the ventral medullary chemosensitive area of the cat

The effects of capsaicin applied to the exposed ventral surface of the medulla were studied on the mean arterial blood pressure, heart rate, respiration and sympathetic efferent nerve activity in chloralose-urethane-anaesthetized cats. The application of capsaicin produced a marked increase in the mean arterial blood pressure and sympathetic nerve activity, but not in the heart rate. The "intermediate area" proved to be the most sensitive to capsaicin. Pressor responses could be elicited repeatedly; tachyphylaxis was not noted provided a time interval of 30 min elapsed between consecutive applications. Repeated applications of capsaicin at intervals of less than 30 min led to tachyphylaxis. However, pressor responses evoked by either topical application of glutamate or pentamethylene-tetrazole or bilateral carotid occlusion could invariably be demonstrated during this period of tachyphylaxis. Histological studies revealed the existence of a hitherto unrecognized termination of capsaicin-sensitive nerve endings within the ventral medullary chemosensitive area of the cat. The results provide both functional and morphological evidence for the presence of a capsaicin-sensitive vasomotor mechanism in the ventral medullary chemo-sensitive area of the cat. It is suggested that the pressor effects of capsaicin applied to the ventral medullary chemo-sensitive area may be mediated by an activation of capsaicin-sensitive primary sensory afferents terminating in this area. Accordingly, capsaicin-sensitive neuronal mechanisms located in the ventral medullary chemosensitive area may play an important role in the central nervous regulation of blood pressure.

Capsaicin impairs preoptic serotonin-sensitive structures mediating hypothermia in rats

In an attempt to elucidate the possible mechanisms involved in the development of the thermoregulatory disturbances induced by systemic capsaicin treatment, the effects of 5-HT injected into the preoptic region or into the cisterna magna on the body temperature and on tail skin vasodilation were studied in control and capsaicin-treated rats. Intracisternal 5-HT elicited a comparable decrease in body temperature in both groups of animals. In contrast, intrapreoptic injection of different doses of 5-HT-induced tail skin vasodilation and hypothermia in the controls, but not in the capsaicin-treated rats. It is suggested that changes in the sensitivity of preoptic warm-responsive structures to 5-HT may contribute essentially to the specific thermoregulatory impairment brought about by systemic capsaicin treatment.

Neurophysiological effects of capsaicin

Data obtained from neonatally treated rats are fairly consistent. However, there is disagreement as to whether mechanical and thermal nociceptive thresholds are elevated or unchanged in this group. There are at least two major areas of disagreement in adult animal capsaicin research. Behavioral data are extremely variable. The thermal nociceptive threshold after systemic capsaicin has been reported to be both raised and lowered. After intrathecal capsaicin injection, the thermal nociceptive threshold was reported raised, but onset and duration of responses varied and some animals exhibited no changes. Capsaicin application to peripheral nerve, however, drastically increased thermal threshold. Mechanical pain threshold has been reported both increased and unchanged after systemic capsaicin treatment and unchanged after intrathecal injection. Obviously, capsaicin's effects upon pain perception are not fully understood. Although lower on the phylogenetic scale than many mammals, rodents exhibit complex individualistic behavior. Lower vertebrates may eventually provide more simple behavioral models for pain tolerance. Investigators also disagree as to whether C fibres can conduct action potentials after local capsaicin application. C fibre conduction was reported unaffected by capsaicin in an acute preparation and for 13-21 days after treatment. On the other hand, C fibre compound action potentials have been reported diminished for up to 2 h after capsaicin application. Additional conduction impairment studies will be useful in comparing peripheral and intrathecal capsaicin application. There is general agreement that, allowing for variation in dosages and route of administration, capsaicin causes central and peripheral C fibre damage, though never as extensive in adults as in neonates. Neonatal capsaicin treatment (always s.c.) results in destruction of C and some A delta fibres and their central terminals. Capsaicin causes degeneration of C terminals in the adult CNS only when applied centrally. In both neonates and adults, s.c. capsaicin depletes the putative 'pain' peptide neurotransmitter, SP, from peripheral and sensory neurons and the tissues they innervate but not from the gut. Capsaicin-induced SP depletion in neonates is permanent. Systemic administration to adult depleted SP from much the same areas as observed in neonates, but all areas but the medulla exhibited a slow, regional recovery. Intraventricular injection of capsaicin depleted SP in the adult medulla only, while other SP-containing areas affected by systemic injection remained intact.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of long warm and cold ambient exposures on food intake water intake and body weight in the capsaicin desensitized rat

Seventeen Sprague Dawley rats received, subcutaneously, 250 mg . kg-1 of capsaicin divided into 10 increasing doses (10-50 mg . kg-1) and administered on 7 successive days. Nine controls were treated with an isotonic saline solution using the same protocol. The rats spent, in succession, 5 weeks at 20 degrees C, 6 weeks at 33.5 degrees C, 6 weeks at 8 degrees C, 4 weeks at 30 degrees C and, finally, 5 weeks at 20 degrees C ambient temperature. Their mean food intake (FI), water intake (WI) and body weights were recorded daily. In the 2 groups of rats, FI was inversely related to ambient temperature. However, during the first few days of the exposures, FI in treated rats was greater than controls in the warm environment and less in the cold environment. In controls, WI increased linearly with ambient temperature in the warm environment. This relation was not found in treated rats: they drank less water than controls and lost body weight. During the first days at 8 degrees C ambient temperature, rectal temperature decreased in treated rats and two animals died. The results are similar to those described for rats with hypothalamic lesions. They may also be related to a peripheral effect of the drug.

The capsaicin sensitivity of the preoptic region is preserved in adult rats pretreated as neonates, but lost in rats pretreated as adults

Two days old rats were pretreated with subcutaneous injection of 50 mg/kg, and adult animals with either 20, 50 or 300 mg/kg capsaicin. The responsiveness of these and naive animals to microinjection into the preoptic region of capsaicin (10 micrograms) and to subcutaneously injected capsaicin (2 mg/kg) was studied at the age of 3--4 months by recording the tail skin vasodilatation and colon temperature, respectively. On preoptic injection of capsaicin, the reaction of neonatally-pretreated adult rats was similar to that of naive animals, while in all groups pretreated as adults the tail skin vasodilatation was abolished. In response to 2 mg/kg capsaicin administered subcutaneously, the group pretreated neonatally and the adults pretreated with 20 mg/kg capsaicin produced significantly less hypothermia than the naive animals. Rats pretreated as adults with 50 and 300 mg/kg capsaicin failed to show a hypothermic reaction. It is concluded that the sensitivity of the preoptic region to capsaicin is preserved when 2 days old rats are treated with the drug, but lost when adults are injected with capsaicin. These features of capsaicin sensitivity indicate a functioning preoptic and an impaired extrapreoptic thermoregulation in rats pretreated with capsaicin as neonates.

Dose-response relationships in the thermoregulatory effects of capsaicin

The hypothermic effect of capsaicin, the reduced responsiveness towards the hypothermic effect of the drug as well as the impairment of thermoregulation in the warm environment subsequent to the administration of different doses of capsaicin have been studies in rats. The mortality after capsaicin treatment has also been established. 1. Capsaicin given subcutaneously in doses of 1-10 mg/kg induced a dose-dependent fall in body temperature lasting for 2-5 h. A single dose of 10 mg/kg caused the most pronounced hypothermic effect reaching its maximum (3,4 degrees C) after 107 min. After higher doses (20-50 mg/kg) the fall in body temperature was less, being similar to that observed after the administration of 1-2 mg/kg. 2. A decreased sensitivity towards the hypothermic effect of a test dose of 2 mg/kg capsaicin, as well as an impaired tolerance to high ambient temperature have been found in rats 2 weeks after the pretreatment either with a single dose of 20-50 mg/kg or fractionated administration of 50 mg/kg capsaicin. The fractionated administration of a dose of 50 mg/kg capsaicin proved to be beneficial in decreasing mortality without affecting the desensitizing effect of capsaicin pretreatment. 3. After single doses of capsaicin the hypothermic period was followed by a dose-dependent hyperthermia which lasted for at least 2 days. A close correlation between the prolonged hyperthermic action and the desensitizing effect of capsaicin administration has also been established. The possible relationship between the desensitizing and hyperthermia inducing effect of capsaicin is briefly discussed.

Analgesia induced by neonatal capsaicin treatment in rats

Twenty-seven rats were treated by 475 mg kg-1 of subcutaneous capsaicin from 2 to 7 days of their life. After treatment, only 11 rats had survived. Their cutaneous heat, pressure, pricking and visceral hyperosmolarity nociceptive thresholds were measured between 50 and 170 days after treatment. The results showed a considerable, or even total, analgesia to cutaneous as well as visceral stimuli. The conclusion is that same pathways carried all the nociceptive messages independently of nature or localization of noxious stimuli.

Effects of capsaicin applied locally to adult peripheral nerve. I. Physiology of peripheral nerve and spinal cord

(1) Systemic capsaicin treatment of neonatal and adult rats is known to affect unmyelinated afferents. However, the systemic route of administration presents several disadvantages and in order to overcome these a method was explored where a single nerve in adult rats was locally treated. (2) Sciatic nerves were exposed and a 10 mm length was soaked for 15 min in 1.5% capsaicin in vehicle or in the vehicle alone (10% Tween 80, 10% ethyl alcohol in saline). (3) Both the capsaicin solution and the vehicle caused acute block of the C compound action potential while in contact with the nerve. Removal of the solutions, however, resulted in substantial recovery of C fibre conduction. The A fibre volley was totally unaffected. (4) 13-21 days after treatment, the size of the myelinated and unmyelinated volleys evoked by maximal stimulation of the capsaicin treated nerve were unchanged but there was a 20% decrease of conduction velocity in the C fibres. (5) The ability of the maximal C volley from the treated nerve to excite cells in the spinal cord was substantially decreased (by 50%) 13-21 days after local capsaicin.

Immunohistochemical studies on the effect of capsaicin on spinal and medullary peptide and monoamine neurons using antisera to substance P, gastrin/CCK, somatostatin, VIP, enkephalin, neurotensin and 5-hydroxytryptamine

After neonatal treatment of rats with capsaicin, the spinal cord, the spinal trigeminal nucleus and spinal and trigeminal ganglia were analysed with immunohistochemistry using antisera to several peptides and 5-hydroxytryptamine. A marked decrease was observed in substance P-, cholecystokinin-, somatostatin- and VIP-like immunoreactivity present in the central branches of primary sensory neurons in the spinal cord and in substance P- and somatostatin-like immunoreactivity in sensory ganglion cells. No definite depleting effect of capsaicin could be established on 5-hydroxytryptamine and peptides, such as enkephalin and neurotensin, present in centrally originating fibres in the dorsal horn of the spinal cord. The results demonstrate that the effects of capsaicin are not confined to substance P immunoreactive primary sensory neurons. The possibility is discussed that capsaicin effects specifically functioning rather than chemically specific primary sensory neurons.

Thermoregulation in adult rats which have been treated with capsaicin as neonates

1. Subcutaneous or intrahypothalamic injections of capsaicin produce hypothermia in the neonate rat. Repeated injections with increasing doses of capsaicin result in unresponsiveness to this drug (capsaicin-desensitization). 2. Young rats aged 8-10 days which had received serial injections of capsaicin solution (a cumulative dose of 4.63 mg per animal) or of the solvent alone were subsequently tested as adults for their ability to thermoregulate. 3. On exposure to an ambient temperature of 41 degrees C, adult rats which had been capsaicin-desensitized as neonates were unable to thermoregulate against overheating by means of autonomic responses whereas control littermates could maintain normal rectal temperature. However, autonomic thermoregulation against cold was unimpaired in the desensitized rats. 4. Skin-cooling operant behavior in heat stress was impaired in adult rats which had been capsaicin-desensitized as neonates whereas their skin-heated behavior was not different from that of control littermates. 5. These results suggest that the central and peripheral warm-receptors responsible for thermoregulation in the neonate rat are functionally mature at least inasmuch as they form a part of thermoregulatory system involved in lowering body temperature and can be desensitized by capsaicin. Moreover, such receptors, once desensitized 8-10 days after birth, apparently do not regain their function and are not regenerated or replaced during subsequent maturation of the animal.

The effect of ambient temperature on rectal temperature, food intake and short term body weight in the capsaicin desensitized rat

1. Subcutaneous injections of capsaicin (mean cumulative dose: 80.1 +/- 3.6 mg , kg-1) permanently reduced the capacity of rats to withstand a hot environment when deprived of water. With water available, hyperthermia was discrete or absent in capsaicinized rats in hot environment. 2. Desensitization was followed by a significant decrease in both food intake and body weight. Treated rats recovered normal body weight after 3 weeks. 3. In a hot environment, compared to controls, food intake was significantly increased in capsaicin desensitized rats which maintained their body weight. In cold environment, food intake was decreased in capsaicin desensitized rats which lost body weight. At normal ambient temperature, food intake and body weight were similar in the 2 groups. 4. Caloric intake adjustment at high and at low temperatures was therefore disturbed in capsaicin desensitized rats. It is concluded that hypothalamic thermodetectors implicated in both thermoregulation and food intake behaviour could be partially damaged by capsaicin.

Effect of capsaicin on morphine analgesia--possible involvement of hypothalamic structures

1. Rats were treated with 50 mg/kg capsaicin on the second day of life or at the age of 2--3 months. The effect of morphine on the nociceptive threshold, as determined by the reaction time in tail withdrawal test, was measured 3--4 and 1--2 months after capsaicin pretreatment, respectively. 2. The analgesic effect of morphine was markedly attenuated in rats treated with capsaicin in the adult age, while neonatal capsaicin treatment did not affect morphine analgesia. 3. Pretreatment of adult rats with capsaicin results in the impairment of certain hypothalamic preoptic neurones, while neonatal capsaicin treatment induces selective degeneration of chemosensitive primary sensory neurones without affecting hypothalamic neurones. Therefore, it is suggested that in the analgesic effect of morphine the capsaicin-sensitive neurones of the preoptic area are involved, and the contribution of spinal mechanisms might be of minor importance. Thus, the preoptic region may be an important link in endogenous pain controlling systems.

Salivary cooling, escape reaction and heat pain in capsaicin-desensitized rats

Salivary thermolytic mechanisms (weight of salivary glands, effect of desalivation on water intake and body temperature, grooming activity) as well as escape behaviour and reaction to heat pain were studied in capsaicin-desensitized and control rats exposed to various warm ambient temperatures. Body temperature of the desensitized rats increased more than the controls' at all the ambient temperatures studied (32, 34 and 36 degrees C); however, significant differences in the mechanism of salivary cooling were obtained only at 34 and 36 degrees C. Central impairment of saliva spreading in desensitized rats seems evident. Complete surgical desalivation did not increase hyperthermia of control and desensitized animals in warm environments. Therefore other mechanisms, primarily vasodilatatory, must also be involved in the rat's thermolytic normal response. Although desensitized rats did not show a tendency to escape from the warm environment their response to heat pain was normal. In conclusion, it is suggested that heat perception in desensitized animals is impaired; however, the existence of some capsaicin-insensitive thermolytic mechanisms (prone extension of the body) cannot be excluded.

The effect of capsaicin on the adenylate cyclase activity of rat brain

The effect of capsaicin on the adenylate cyclase activity in different regions of the rat brain (preoptic area of the hypothalamus, cerebral cortex and cerebellum) was investigated. Capsaicin added in vitro (10(-7)-10(-5) M) increased the adenylate cyclase activity of different brain regions. Following systemic capsaicin desensitization adenylate cyclase activity was significantly increased in the preoptic area. The enhanced adenylate cyclase activity in the preoptic area was inhibited by the vitro addition of capsaicin or 5-HT, whereas desensitization did not affect the in vitro activating effect of capsaicin in other brain regions (cerebral cortex, cerebellum). It is assumed that the pharmacological effect of capsaicin in the preoptic area is mediated through the activation of adenylate cyclase. Since capsaicin induces irreversible impairment of the function of warmsensitive hypothalamic neurons it is assumed that adenylate cyclase is involved in maintaining normal thermoregulatory functions.

The effect of capsaïcin on temperature regulation of the rat

1. Subcutaneous injection of capsaïcin (6-11 mg, or 21-66 mg cumulative), permanently reduced the capacity of rats to withstand a hot environment, as described by Jancsó-Gábor et al. (1970). 2. The treated rats thermoregulatory behaviour was not different from that of control rats, both in hot and cold environments. 3. Saliva secretion was decreased in a hot environment, and the weight of the submaxillary glands was reduced in capsaïcinized rats. 4. It is concluded that hyperthermia present in treated rats when subjected to a warm environment is not due to a disruption of sensu stricto temperature regulation, but rather to a decreased salivary secretion. Whether this decrease has a central or a peripheral origin is not known.

Functional and fine structural characteristics of the sensory neuron blocking effect of capsaicin

In the eye of rats the long-lasting specific desensitization induced by local or systemic capsaicin treatment is characterized by three phases: 1. complete insensitivity, 2. decreased sensitivity and a tendency to rapid adaptation, 3. normal initial sensitivity with a tendency to rapid adaptation to chemical pain stimuli. A low density of nicrovesicles and swollen mitochondria were found after local capsaicin treatment in certain nerve endings of the cornea of rats, but no signs of axonal degeneration or alteration in fine structure of non-neural elements were seen. Systemic capsaicin desensitization induced selective mitochondrial swelling in B type of neurons of the trigeminal ganglion which was demonstrable even 60 days after the pretreatment. Actinomycin-D, 8-azaguanine, 6-azauracil, aminopterin, mannomustin or cycloheximide in high doses did not alter the desensitizing effect of systemic capsaicin treatment. However, pretreatment of rats with colchicine or vinblastine prolonged the desensitizing effect of local capsaicin application, probably by inhibiting the axonal flow. It is concluded that capsaicin is a specific sensory neuron blocking agent having a practically irreversible effect in rats and guinea-pigs.