Capsaicin impairs preoptic serotonin-sensitive structures mediating hypothermia in rats

Vagal neurocircuitry and its influence on gastric motility

A large body of research has been dedicated to the effects of gastrointestinal peptides on vagal afferent fibres, yet multiple lines of evidence indicate that gastrointestinal peptides also modulate brainstem vagal neurocircuitry, and that this modulation has a fundamental role in the physiology and pathophysiology of the upper gastrointestinal tract. In fact, brainstem vagovagal neurocircuits comprise highly plastic neurons and synapses connecting afferent vagal fibres, second order neurons of the nucleus tractus solitarius (NTS), and efferent fibres originating in the dorsal motor nucleus of the vagus (DMV). Neuronal communication between the NTS and DMV is regulated by the presence of a variety of inputs, both from within the brainstem itself as well as from higher centres, which utilize an array of neurotransmitters and neuromodulators. Because of the circumventricular nature of these brainstem areas, circulating hormones can also modulate the vagal output to the upper gastrointestinal tract. This Review summarizes the organization and function of vagovagal reflex control of the upper gastrointestinal tract, presents data on the plasticity within these neurocircuits after stress, and discusses the gastrointestinal dysfunctions observed in Parkinson disease as examples of physiological adjustment and maladaptation of these reflexes.

Transient receptor potential (TRP) channels: a clinical perspective

Transient receptor potential (TRP) channels are important mediators of sensory signals with marked effects on cellular functions and signalling pathways. Indeed, mutations in genes encoding TRP channels are the cause of several inherited diseases in humans (the so-called 'TRP channelopathies') that affect the cardiovascular, renal, skeletal and nervous systems. TRP channels are also promising targets for drug discovery. The initial focus of research was on TRP channels that are expressed on nociceptive neurons. Indeed, a number of potent, small-molecule TRPV1, TRPV3 and TRPA1 antagonists have already entered clinical trials as novel analgesic agents. There has been a recent upsurge in the amount of work that expands TRP channel drug discovery efforts into new disease areas such as asthma, cancer, anxiety, cardiac hypertrophy, as well as obesity and metabolic disorders. A better understanding of TRP channel functions in health and disease should lead to the discovery of first-in-class drugs for these intractable diseases. With this review, we hope to capture the current state of this rapidly expanding and changing field.

A critical re-evaluation of the specificity of action of perivagal capsaicin

Perivagal application of capsaicin (1% solution) is considered to cause a selective degeneration of vagal afferent C fibres and has been used extensively to examine the site of action of many gastrointestinal (GI) neuropeptides. The actions of both capsaicin and GI neuropeptides may not be restricted to vagal afferent fibres, however, as other non-sensory neurones have displayed sensitivity to capsaicin and brainstem microinjections of these neuropeptides induce GI effects similar to those obtained upon systemic application. The aim of the present study was to test the hypothesis that perivagal capsaicin induces degeneration of vagal efferents controlling GI functions. Experiments were conducted 7-14 days after 30 min unilateral perivagal application of 0.1-1% capsaicin. Immunohistochemical analyses demonstrated that, as following vagotomy, capsaicin induced dendritic degeneration, decreased choline acetyltransferase but increased nitric oxide synthase immunoreactivity in rat dorsal motor nucleus of the vagus (DMV) neurones. Electrophysiological recordings showed a decreased DMV input resistance and excitability due, in part, to the expression of a large conductance calcium-dependent potassium current and the opening of a transient outward potassium window current at resting potential. Furthermore, the number of DMV neurones excited by thyrotrophin-releasing hormone and the gastric motility response to DMV microinjections of TRH were decreased significantly. Our data indicate that perivagal application of capsaicin induced DMV neuronal degeneration and decreased vagal motor responses. Treatment with perivagal capsaicin cannot therefore be considered selective for vagal afferent C fibres and, consequently, care is needed when using perivagal capsaicin to assess the mechanism of action of GI neuropeptides.

Capsaicin and nitric oxide synthase inhibitor interact to evoke a hypothermic synergy

The present study investigated the effect of a drug combination of capsaicin and L-NAME on hypothermia in rats. Capsaicin administration (0.1, 0.25, 0.5, 1 and 2mg/kg, i.m.) caused a significant hypothermia. L-NAME (50mg/kg, i.p.), a nonspecific nitric oxide synthase (NOS) inhibitor, was ineffective. For combined administration, progressively increasing doses of capsaicin (0.1, 0.25, 0.5, 1 and 2mg/kg, i.p.) were given with a non-hypothermic dose of L-NAME (50mg/kg, i.p.). Experiments revealed that L-NAME (50mg/kg, i.p.) enhanced the hypothermic response to capsaicin (0.25, 0.5, 1, and 2mg/kg, i.m.). Comparison of the graded dose-effect curves for capsaicin alone and capsaicin plus L-NAME revealed a significant difference (P<0.05), thus indicating synergy for the drug interaction. To determine if L-NAME acted centrally, a fixed dose of L-NAME (1mg/rat, i.c.v.) was given with graded doses of capsaicin (0.25, 0.5, 1, and 2mg/kg, i.m.). L-NAME (1mg/rat, i.c.v.) only enhanced the hypothermia at a single dose of capsaicin (0.5mg/kg, i.m.). The super-additive hypothermia produced by the concurrent administration of capsaicin and L-NAME (50mg/kg, i.p.) is the first evidence of synergy for a drug combination of capsaicin and a NOS inhibitor. The synergy is apparent only when L-NAME is given systemically, thus indicating that the inhibition of peripheral NO production enhances the hypothermic response to capsaicin.

Transient receptor potential ion channels as participants in thermosensation and thermoregulation

Living organisms must evaluate changes in environmental and internal temperatures to mount appropriate physiological and behavioral responses conducive to survival. Classical physiology has provided a wealth of information regarding the specialization of thermosensory functions among subclasses of peripheral sensory neurons and intrinsically thermosensitive neurons within the hypothalamus. However, until recently, the molecular mechanisms by which these cells carry out thermometry have remained poorly understood. The demonstration that certain ion channels of the transient receptor potential (TRP) family can be activated by increases or decreases in ambient temperature, along with the recognition of their heterogeneous expression patterns and heterogeneous temperature sensitivities, has led investigators to evaluate these proteins as candidate endogenous thermosensors. Much of this work has involved one specific channel, TRP vanilloid 1 (TRPV1), which is both a receptor for capsaicin and related pungent vanilloid compounds and a "heat receptor," capable of directly depolarizing neurons in response to temperatures >42 degrees C. Evidence for a contribution of TRPV1 to peripheral thermosensation has come from pharmacological, physiological, and genetic approaches. In contrast, although capsaicin-sensitive mechanisms clearly influence core body temperature regulation, the specific contribution of TRPV1 to this process remains a matter of debate. Besides TRPV1, at least six additional thermally sensitive TRP channels have been identified in mammals, and many of these also appear to participate in thermosensation. Moreover, the identification of invertebrate TRP channels, whose genetic ablation alters thermally driven behaviors, makes it clear that thermosensation represents an evolutionarily conserved role of this ion channel family.

Preclinical neuropharmacology of naratriptan

The basic CNS neuropharmacology of naratriptan is reviewed here. Naratriptan is a second-generation triptan antimigraine drug, developed at a time when CNS activity was thought not to be relevant to its therapeutic effect in migraine. It was, however, developed to be a more lipid-soluble, more readily absorbed and less readily metabolized variant on preexisting triptans and these variations conferred on it a higher CNS profile. Naratriptan is a 5-HT(1B/1D) receptor agonist with a highly selective action on migraine pain and nausea, without significant effect on other pain or even other trigeminal pain. Probable sites of therapeutic action of naratriptan include any or all of: the cranial vasculature; the peripheral terminations of trigeminovascular sensory nerves; the first-order synapses of the trigeminovascular sensory system; the descending pain control system; and the nuclei of the thalamus. Naratriptan may prevent painful dilatation of intracranial vessels or reverse such painful dilatation. Naratriptan can prevent the release of sensory peptides and inhibit painful neurogenic vasodilatation of intracranial blood vessels. At the first order synapse of the trigeminal sensory system, naratriptan can selectively suppress neurotransmission from sensory fibers from dural and vascular tissue, while sparing transmission from other trigeminal fibers, probably through inhibition of neuropeptide transmitter release. In the periaqueductal gray matter and in the nucleus raphe magnus, naratriptan selectively activates inhibitory neurons which project to the trigeminal nucleus and spinal cord and which exert inhibitory influences on trigeminovascular sensory input. Naratriptan has also a therapeutic effect on the nausea of migraine, possibly exerting its action at the level of the nucleus tractus solitarius via the same mechanisms by which it inhibits trigeminovascular nociceptive input. The incidence of naratriptan-induced adverse effects in the CNS is low and it is not an analgesic for pain other than that of vascular headache. In patients receiving selective serotonin uptake inhibitors (SSRIs) naratriptan may cause serotonin syndrome-like behavioral side effects. The mechanism of action involved in the production of behavioral and other CNS side effects of naratriptan is unknown.

A neurochemical mechanism for hypoxia-induced anapyrexia

Hypoxia evokes a regulated decrease in body temperature, a response that has been termed anapyrexia, but the mechanisms involved are poorly understood. Therefore, the present study was undertaken to test the hypothesis that hypoxia-induced anapyrexia results from the activation of cAMP- and cGMP-dependent pathways in the preoptic region (PO). Adult male Wistar rats weighing 230-260 g were used. Body temperature was monitored by biotelemetry, and the levels of cAMP and cGMP were determined in the anteroventral third ventricular region (AV3V), where the PO is located. Using immunohistochemistry, we observed that the PO contains a high density of cAMP- and cGMP-containing cells. Interestingly, hypoxia exposure raised the levels of cAMP and cGMP in the AV3V. Intra-PO microinjection of Rp-cAMPS, an inhibitor of cAMP-dependent protein kinase, attenuated hypoxia-induced anapyrexia. Similarly, intra-PO microinjection of the mixed beta-adrenoceptor/serotonin (5-HT(1A)) receptor antagonist propranolol also impaired the drop in body temperature in response to hypoxia. The reduction in body temperature evoked by intra-PO serotonin, but not epinephrine, was blocked by Rp-cAMPS, indicating the involvement of a preoptic serotonin-cAMP pathway in the development of anapyrexia. Moreover, microinjection of N(G)-monomethyl-l-arginine, an inhibitor of nitric oxide (NO) synthesis, or Rp-cGMPS, an inhibitor of cGMP-dependent protein kinase, into the PO also attenuated hypoxia-induced anapyrexia. In conclusion, the present study supports that hypoxia-induced anapyrexia results from the activation of the serotonin-cAMP and NO-cGMP pathways in the PO.

Age-related changes in capsaicin-induced degeneration in rat brain

Previous results indicate that the pattern of capsaicin-induced degeneration in the rat central nervous system is age-related. Experiments utilizing capsaicin's selective neurodegenerative effects to study the function of central neural circuits will therefore require a detailed understanding of capsaicin's central neurotoxicity in rats of different ages. The goal of this experiment was to characterize the degeneration induced in the rat brain by systemic treatment with capsaicin at different ages (10, 15, 20, 25, 30 or 75 days, or 11 months), using a cupric silver stain to label degenerating neurons. Results revealed degenerating cell bodies in the ventromedial brainstem in capsaicin-treated rats of all age groups, though they were more numerous in adult rats than in pups. In addition, many areas contained capsaicin-induced nerve terminal degeneration both in rat pups and in adult rats. These areas were the substantia gelatinosa of the spinal cord dorsal horn; the solitary tract; the nucleus of the solitary tract, visceral portion; the area postrema; the trigeminal nerve and spinal trigeminal nucleus; the medial nucleus of the inferior olive; the rostral, dorsomedial and dorsolateral interpeduncular subnuclei and overlying interfascicular nucleus; the supramammillary area; the lateral septal nucleus; the bed nucleus of the stria terminalis, anterior medial portion; the optic nerve and tract; the suprachiasmatic nucleus, ventroposterolateral portion; the magnocellular subnucleus of the ventrolateral geniculate nucleus; the intergeniculate leaf; the medial pretectal nucleus and the olivary pretectal nucleus. In several, but not all of these areas, the apparent density of degenerating terminals was significantly less in adult rats than in pups. In other brain sites, capsaicin-induced degeneration was observed only in rats younger than 30 days of age. These areas were the lateral habenula, medial part; the sphenoid nucleus; and the stria medullaris. Still other brain sites lost their sensitivity to capsaicin sometime between 30 and 75 days of age. These areas were the bed nucleus of the stria terminalis, medial posteromedial part; the medial preoptic nucleus, central part; the septohypothalamic nucleus; the ventral reuniens area; and the ventromedial hypothalamic nucleus. Adult rats 75 days and 11 months of age did not differ detectably in their response to capsaicin. Thus, loss or attenuation of capsaicin sensitivity is not progressive throughout life. It does not occur in all capsaicin-sensitive sites. Where it does occur, loss of sensitivity occurs prior to adulthood and follows a distinct and reproducible time course that may differ for different sites.

Capsicum--production, technology, chemistry, and quality. Part V. Impact on physiology, pharmacology, nutrition, and metabolism; structure, pungency, pain, and desensitization sequences

The spice Capsicum is the fruit of the cultivated species of the genus Capsicum (family, Solanaceae), C. annuum principally, and C. frutescens L. to a lesser extent. A third variety of C. annuum var. annuum fruits, the large-sized, fleshy bell capsicum is used as a fresh vegetable and valued for its aroma, color, and crisp texture, but with no pungency. This variety is not considered in this series of reviews covering primary processing, production, international trade, chemistry, and biochemistry of functional components--the red keto carotenoids, the aromatic volatiles and the pungent capsaicinoids in Parts I to III. The valid qualitative aspects correlating the specific components of capsicum and their sensory responses are critically covered in Part IV. In this the concluding part of the series of reviews, the significant preference of the spice for initially evoking an aversive response, its potent physiological and pharmacological effects, and the aspects of structure-activity relationships of the pungent stimuli of the capsaicinoids are reviewed. The beneficial effects particularly associated with long usage by some ethnic groups and its safe consumption levels, with a critical review of the studies on the gastrointestinal tract, the cardiovascular system, the sensory system, thermoregulation, nutritional impacts, and an overview of the five series is also detailed.

Cholecystokinin-octapeptide-induced hypothermia in rats: dose-effect and structure-effect relationships, effect of ambient temperature, pharmacological interactions and tolerance

Subcutaneous injection of cholecystokinin octapeptide (CCK-8) (0.005-1.25 mg/kg) elicited dose-dependent hypothermia in rats. The threshold of the response was between 0.01 and 0.05 mg/kg and the dose-response curve levelled off at doses larger than 0.2-0.5 mg/kg. Warm and cold ambient temperatures decreased and increased the response, respectively. Pretreatment with capsaicin, morphine, naloxone, atropine, haloperidol or propranolol did not affect the response to CCK-8, whereas pretreatment with phenoxybenzamine and a large dose of proglumide, an antagonist for CCK-receptors, attenuated the hypothermia. It seems that neither capsaicin-sensitive thermal and non-thermal afferents, nor opiate mechanisms are involved in the response, but alpha-adrenoceptors might be of some importance in the hypothermia. Non-sulphated-CCK-8, the C-terminal tetrapeptide and hexapeptide, [D-Ala4]-CCK-8 and [D-Met6]-CCK-8 were ineffective. Chronic treatment with CCK-8 resulted in the development of tolerance to the thermoregulatory effect, while the hypothermic responses to apomorphine and capsaicin were not affected. It seems that the tolerance cannot be attributed to conditioned homeostatic reactions.

Reduced responsiveness of locus coeruleus neurons to cutaneous thermal stimuli in capsaicin-treated rats

Recent electrophysiological experiments have shown that brain norepinephrine (NE) neurons in the locus coeruleus (LC) are activated by cutaneous thermal stimuli of both non-noxious and noxious character. In the present study the LC neuronal response to thermal stimuli was used to evaluate cutaneous thermal sensitivity in capsaicin-treated rats, a treatment that is described to cause impaired thermoregulation. Capsaicin treatment, of neonates as well as of adult rats, caused a reduced responsiveness of brain LC neurons to thermal stimuli. The results suggest that a reduction in peripheral thermal afferent transmission may be one mechanism underlying the capsaicin-induced thermoregulatory dysfunction.

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.