Hydrogen sulphide: an endogenous stimulant of capsaicin-sensitive primary afferent neurons?

Hydrogen sulphide (H(2)S) is a gas best known for its rotten egg smell. The toxic effects of high concentrations of H(2)S have been extensively investigated. It is known that H(2)S is generated in mammalian systems, but little is known of its effects in physiological concentrations. In the present issue of this journal, Patacchini et al. present evidence that H(2)S stimulates capsaicin-sensitive primary afferent neurons to release tachykinins in the rat urinary bladder. The possible significance of this finding is discussed in this commentary.

Effect of chronic intermittent immobilization stress on Fos-like immunoreactivity in rat brain and adrenal medulla.

The present study examined the influence of short- and long-term chronic intermittent immobilization stress throughout the brain and on the adrenal medulla of intact rats using Fos-like immunoreactivity (Fos-LI) as a marker of cellular activation. The effect of adreno-medullectomy on the central nervous system (CNS) response to chronic immobilization stress was also examined. It was found that control unoperated, unstressed rats had no Fos-LI cells in the brain or in the adrenal medulla. In intact rats, neither short term (1 week) nor long term (4 weeks) chronic intermittent immobilization stress produced significant increases in Fos-LI in the CNS compared with control animals. However, marked increase in the number of Fos-LI cells was observed in the adrenal medulla of animals stressed for 4 weeks compared with control, unstressed animals or those stressed for 1 or 2 weeks. In adreno-medullectomised rats, 4 weeks, but not 1 week, chronic immobilization stress produced significant increases in numbers of Fos-LI neurons in the paraventricular hypothalamic and supraoptic nuclei and the medial amygdala compared with intact animals stressed for a similar period of time. It is concluded that long term stress produces chronic Fos-LI in the adrenal medulla and that adreno-medullectomy increases the Fos response of the PVN, supraoptic nucleus and medial amygdala to long term stress.

Increased tachykinin NK(1) receptor immunoreactivity in the prefrontal cortex in schizophrenia.

BACKGROUND: Changes in levels of substance P and substance P-binding sites have been implicated in schizophrenia. However, no studies have used receptor-specific antibodies to directly investigate the substance P (neurokinin 1) receptor in schizophrenia. METHODS: We used an antibody directed against the human neurokinin-1 receptor to compare the distribution of neurokinin-1 receptors in the prefrontal cortices from six subjects with schizophrenia and six control subjects, matched for age, gender, and postmortem interval. RESULTS: In control tissue, dots of neurokinin-1 receptor immunoreactivity were observed in layer I to upper/mid layer III only. In contrast, dots of neurokinin-1 receptor immunoreactivity were observed in all layers of the prefrontal cortex in subjects with schizophrenia, and the density of dots was significantly greater than in control subjects. CONCLUSIONS: This is the first report of increased neurokinin-1 receptor immunoreactivity in the prefrontal cortex in subjects with schizophrenia. These changes may have implications for understanding the pathophysiology of the prefrontal cortex in schizophrenia and for the treatment of this disorder.

Localization of NK1 and NK3 receptors in guinea-pig brain.

In this study the localizations of tachykinin neurokinin-1 (NK1) and neurokinin-3 (NK3) receptors in the guinea-pig brain are described. In agreement with studies in rat and human brain, the neurons that exhibited the most marked NK1 receptor immunoreactivity were found in the dorsomedial caudate putamen. NK1 receptors were also widely distributed in diencephalic structures and in the mid and hind brain. NK3 receptors were distributed in both superficial and deep layers of the cortex and many appeared to be located on cells with astrocyte-like morphology in the glia limitans. In several regions including the thalamus, hypothalamus, amygdala, periaqueductal gray, substantia nigra and area postrema, both NK1 and NK3 immunoreactivity were found. The present study revealed that tachykinin receptors are widely distributed in the guinea-pig central nervous system.

Localization of tachykinin receptors and Fos-like immunoreactivity induced by substance P in guinea-pig brain.

1. In the present study, a comparison was made between the distribution of tachykinin NK1 and NK3 receptor immunoreactivity and the distribution of Fos-like immunoreactivity induced by the tachykinin agonist substance P (SP) in the guinea-pig brain. 2. In agreement with results from previous studies in rat brain, NK1 receptor-immunoreactive neurons were found to be widely distributed throughout the brain in the striatum and in diencephalic and mesencephalic structures, while NK3 receptor-immunoreactive neurons were mainly in telencephalic structures. Considerable overlap was observed between NK1 and NK3 receptor distributions. 3. Substance P induced Fos-like immunoreactivity (Fos-LI) in extensive areas of the guinea-pig brain. The induction of Fos-LI was markedly inhibited in many areas by pretreatment with the NK1 receptor antagonist SR 140333. The NK3 receptor antagonist SR 142801 reduced Fos-LI staining in fewer areas, although a reduction was observed in the cortex, striatum and hypothalamus. 4. In general, tachykinin receptors were located at sites corresponding to areas of functional activation by SP, as shown by Fos-LI. These results extend previous studies by adding a functional dimension to tachykinin receptor localization studies.

Tachykinin NK1 and NK3 receptors in the prefrontal cortex of the human brain.

1. The tachykinins are neuropeptides found in both the central and peripheral nervous systems that play a role in inflammation and pain mechanisms and some autonomic reflexes and behaviours. 2. Although the distribution of the tachykinin receptors has been described in the brains of various animal species, little is known about the distribution of the NK1 and NK3 receptors in the human brain. 3. The present paper examines the distribution of the NK1 and NK3 receptors in the prefrontal cortex of formalin-fixed postmortem human brain tissue by immunohistochemical techniques. 4. The majority of NK1 receptor immunoreactivity appeared as a thin band of punctate staining at the pial surface, with dark brown dots of NK1 receptor immunoreactivity predominantly scattered across the mid to upper cortical layers. 5. The NK3 receptor immunoreactivity was found in the glia limitans at the pial surface, where astrocytes and beaded fibres were intensely stained. Dots of NK3 receptor immunoreactivity were scattered across all cortical layers. In the white matter, astrocytes and beaded fibres displayed NK3 receptor immunoreactivity, particularly in areas surrounding blood vessels.

Localisation of tachykinin NK1 and NK3 receptors in the human prefrontal and visual cortex.

The distribution of tachykinin NK(1) and NK(3) receptors in the prefrontal (Brodmann area 9) and visual cortex (Brodmann area 17) of formalin-fixed postmortem human brain tissue was studied by immunohistochemistry. NK(1)-like immunoreactivity (NK(1)-LI) was observed as a thin band at the cortical surface and dots of NK(1)-LI localised on small non-pyramidal cells and in the neuropil (layers I-III). NK(3)-LI was found in beaded fibres and cells with astrocyte-like morphology in the superficial cortical layers and white matter. Dots of NK(3)-LI were prominent in the neuropil and on pyramidal (layers III/V) and non-pyramidal (layers V/VI) cells. The NK(3)-LI was more abundant and widespread than the NK(1)-LI. This is the first report of the distribution of the NK(1) receptor in the prefrontal and visual cortex of the human brain by immunohistochemistry.

Distribution of Fos-like immunoreactivity in guinea-pig brain following administration of the neurokinin-1 receptor agonist, [SAR9,MET(O2)11]substance P.

The tachykinins are a family of peptides with putative neurotransmitter roles in the nervous system. They mediate their effects via neurokinin-1, neurokinin-2 and neurokinin-3 receptors. There has been increasing interest in the therapeutic application of the tachykinin neurokinin-1 receptor antagonists in the treatment of pain and emesis, and more recently in depression. However, the central role of neurokinin-1 receptors is not well understood. The aims of the present study were to determine the behavioural responses of guinea-pigs, and the distribution of Fos-like immunoreactivity in the guinea-pig brain, following intracerebroventricular administration of the neurokinin-1 receptor-selective agonist, [Sar9,Met(O2)11]substance P. The effects of pretreatment with the neurokinin-1 receptor antagonist, SR 140333, were also investigated. Administration of [Sar9,Met(O2)11]substance P induced increased locomotor activity, as well as face washing, grooming and wet-dog shake behaviours, all of which were inhibited by the neurokinin-1 receptor antagonist, SR 140333, indicating the involvement of neurokinin-1 receptors. In order to localize the brain regions activated by [Sar9,Met(O2)11]substance P, the distribution of neurons expressing Fos-like immunoreactivity was examined. [Sar9,Met(O2)11]substance P induced increased Fos-like immunoreactivity in widespread areas, including the frontal cortex, hippocampus, amygdala, thalamus, hypothalamus, periaqueductal gray, area postrema and nucleus of the solitary tract. SR 140333 reduced Fos-like immunoreactivity induced by [Sar9,Met(O2)11]substance P in most areas. Thus, brain regions associated with emotion, sensation, learning and memory, autonomic regulation and emesis were activated by stimulation of neurokinin-1 receptors. The present data have added a functional domain to previous neurokinin-1 receptor localization studies by describing the extensive regions of the CNS that may be activated by stimulation of these receptors, and the potential of neurokinin-1 receptor antagonists to inhibit activation of these regions.

Fos-like immunoreactivity in tyrosine hydroxylase and substance P-like immunoreactive neurones in guinea-pig brain following intracerebroventricular injection of morphine and U50,488H.

Opioid drugs such as morphine have powerful reinforcing effects which lead to drug-seeking behaviour. Both dopamine- and substance P-containing neurones have been implicated in reward. In the present study twocolour immunohistochemistry was used to investigate whether Fos protein was induced in dopaminergic (tyrosine hydroxylase) and substance P-containing neurones of guinea-pig brain following intracerebroventricular administration of the predominantly mu-receptor agonist, morphine, and the kappa-receptor agonist, U50,488H, which have been reported to produce rewarding and aversive effects, respectively. The present study has shown that of the large number of neurones showing Fos-like immunoreactivity following a single injection of morphine or U50,488H, few were tyrosine hydroxylase-positive (dopaminergic) but a larger number were substance Plike immunoreactive. These results support the proposal that substance P plays a role in reward and reinforcement.

Molecular biological investigations into the role of the NMDA receptor in the pathophysiology of schizophrenia.

OBJECTIVE: There is increasing acceptance that schizophrenia is associated with a generalised disorder in cortical neurodevelopment. The aim of this paper is to review the evidence that this disorder may be accounted for by abnormalities in mechanisms mediated by the main family of excitatory neuroreceptors in cortical brain systems, the N-methyl-D-aspartate (NMDA) glutamatergic receptors. METHOD: The neurobiological evidence is presented for an abnormality in cortical development related to synaptic pathology in schizophrenia. The unique functions of the NMDA receptor in information processing are described, especially its role in learning and memory, and in neural plasticity and brain development. It is argued that the cellular and molecular mechanisms which underlie learning and memory also govern normal brain development. Studies examining abnormalities in glutamatergic transmission in schizophrenia are reviewed. RESULTS: There is a substantial literature in support of the possibility that NMDA receptor abnormalities may be involved in the neurodevelopmental predisposition to schizophrenia, as well as in symptom production. CONCLUSIONS: Research to determine the role of the NMDA receptor in the pathophysiology of schizophrenia is warranted and now feasible. To be successful, this research will require the application of molecular biology techniques to postmortem brain tissue studies, in addition to traditional histochemical approaches.

Localization of Fos-like immunoreactivity induced by the NK3 tachykinin receptor agonist, senktide, in the guinea-pig brain.

1. The effects of intracerebroventricular (i.c.v.) administration of the NK3 tachykinin receptor agonist, senktide (10 nmol each side), in guinea-pigs pretreated with the selective NK3 tachykinin receptor antagonist, SR142801 (3 mg kg(-1) subcutaneous, s.c., 30 min before senktide), or its less active enantiomer, SR142806 (3 mg kg(-1) s.c. 30 min before senktide), on behaviour and on the distribution of Fos-like immunoreactivity (Fos-LI) in central neurones were investigated. Guinea-pigs were chosen for the study since they possess NK3 tachykinin receptors with pharmacological characteristics similar to those in man. 2. Wet-dog shakes, but not locomotor activity, elicited by senktide i.c.v. were significantly reduced by SR142801 but not by SR142806, confirming the involvement of NK3 tachykinin receptors in wet-dog shake behaviour. 3. Senktide induced increased numbers of Fos-LI neurones in the following brain areas: frontal, parietal and piriform cortex, the lateral septum, the CA1, CA2, subiculum and dentate gyrus of the hippocampus, most areas in the amygdala, thalamus and hypothalamus, medial geniculate nucleus and the ventral cochlear nucleus. Pretreatment with SR142801, but not with SR142806, before administration of senktide inhibited Fos-LI expression in the cingulate cortex, dentate gyrus of the hippocampus, some regions of the thalamus, hypothalamus and amygdala and the ventral cochlear nucleus. 4. The present results are the first demonstration that senktide induces Fos-LI in widespread areas of the guinea-pig brain. It is proposed that NK3 tachykinin receptors may play a more extensive role in the control of diverse brain functions, including cortical processing, learning and memory, neuroendocrine and behavioural regulation, than is currently recognized.

Acute effects of morphine on Substance P concentrations in microdissected regions of guinea-pig brain.

The present study investigated the effects of acute morphine treatment and of naloxone-induced morphine withdrawal on Substance P (SP) concentrations in microdissected regions of the guinea-pig brain. Guinea-pigs, which were treated with a single dose of morphine sulphate (15mg/kg s.c.), received naloxone hydrochloride (15mg/kg s.c.) after 2h. Control animals received injections of saline, saline and naloxone, or morphine and saline. Locomotor and behavioural activities were measured throughout the experiments. Animals were killed 0.5h after naloxone administration, brains were removed and SP-like immunoreactivity (SP-LI) was measured in microdissected regions using radioimmunoassay. Morphine significantly increased the concentration of SP-LI in the central nucleus of the amygdala, but reduced SP-LI overall in the mesencephalon. Guinea-pigs pretreated with morphine and then given naloxone to precipitate withdrawal showed no change in SP-LI concentrations in any brain region, compared with control animals, despite an increase in locomotor activity and other behaviours characteristic of opioid withdrawal in guinea-pigs.

Induction of Fos-like immunoreactivity by opioids in guinea-pig brain.

In the present study the effects of intracerebroventricular (i.c.v.) administration of 100 nmol of morphine, the selective mu-receptor agonist DAMGO, the delta-receptor agonist DPDPE and the kappa-receptor agonist U50,488H, on the induction of Fos-like immunoreactivity (Fos-LI) in the guinea-pig brain were investigated using immunohistochemical techniques. Guinea-pigs given i.c.v. injection of opioids showed marked increases in the number of Fos-LI nuclei within a large number of brain regions, several of which, including hypothalamic nuclei, paraventricular thalamic nucleus, the amygdala, periaqueductal gray, superior and inferior colliculi, the piriform and entorhinal cortices, have been shown to be activated under stressful or aversive conditions. Pretreatment with the opioid antagonist, naltrexone, before administration of morphine or U50,488H, inhibited Fos-LI induction indicating that the effects of the opioids were mediated by opioid receptors. U50,488H administration resulted in higher numbers of Fos-LI stained neurons compared to morphine in most regions other than the nucleus accumbens and interpeduncular nucleus. Morphine and DAMGO produced significantly higher numbers of Fos-LI neurons in the nucleus accumbens shell region than U50,488H, which may reflect the more powerful reinforcing/rewarding effects of mu-receptor agonists. Thus the present study supports a critical role for the nucleus accumbens shell region in the reinforcing/rewarding effects of opioids.

Distribution of c-Fos in guinea-pig brain following morphine withdrawal.

The distribution of the immediate-early gene and transcription factor protein, c-Fos, was examined in the brains of guinea-pigs following treatment with morphine, naloxone or naltrexone, or the induction of morphine withdrawal by these opioid antagonists. Guinea-pigs were given subcutaneous injections of morphine sulphate or tartrate three times per day in increasing doses for three days (total dose 690 mg/kg as base). Control animals received saline injections. Naloxone hydrochloride (30 mg/kg), naltrexone hydrochloride (15 mg/kg) or saline was administered subcutaneously 1 h after the last dose of morphine or saline, and the animals killed 1.5 h later by perfusion-fixation under deep sodium pentobarbitone anaesthesia. In the animals that were treated with morphine and withdrawn with either naloxone or naltrexone, c-Fos was expressed in neurons in many brain areas, including the frontal and cingulate cortices, olfactory tubercles, ventral pallidum, nucleus accumbens, habenular, paraventricular thalamic nucleus, septal and arcuate nuclei, lateral and posterior hypothalamic areas, ventral tegmental area, central grey, dorsal raphe nucleus, locus coeruleus, raphe magnus, lateral paragigantocellular nucleus and solitary tract nucleus. In contrast, only low levels of c-Fos were found in brains of animals that had been treated for three days with morphine followed by saline, or with saline followed by naltrexone or naloxone. The widespread distribution of c-Fos induced by morphine withdrawal reflects the complexity of the accompanying behavioural and autonomic responses.

The kappa-opioid receptor agonist U50,488H induces acute physical dependence in guinea-pigs.

The present study was undertaken to determine whether acute physical dependence occurred in guinea-pigs in vivo and guinea-pig isolated ileum following a single dose of the kappa-opioid receptor agonist U50,488H. Administration of naloxone hydrochloride, 15 and 30 mg/kg s.c., to guinea-pigs treated 1 h before with U50,488H, 10 mg/kg s.c., induced increased locomotor activity accompanied by behavioural responses which differed from those previously found in this species with morphine withdrawal. Nor-binaltorphimine, 10 mg/kg s.c., given 1 h after administration of U50,488H, 10 mg/kg s.c., produced a small but significant increase in locomotor activity but no other withdrawal behaviours. The morphine withdrawal response was not significantly affected by U50,488H, 1 or 10 mg/kg s.c. On the guinea-pig isolated ileum, nor-binaltorphimine, 1 microM, produced a withdrawal contracture following 2 min contact of the ileum with U50,488H 1 microM. U50,488H, 1 microM, abolished the [Met5]enkephalin withdrawal response of the ileum. It is concluded that dependence occurs following activation of kappa-opioid receptors, which is largely non-morphine-like in the central nervous system, but which is morphine-like in the enteric nervous system.

Enhancement of the opiate withdrawal response by antipsychotic drugs in guinea-pigs is not mediated by sigma binding sites.

Haloperidol, a 'typical' neuroleptic, with affinity for both dopamine (DA) receptors and sigma binding sites, exacerbates morphine withdrawal in guinea-pigs. In this study, the effects of sigma ligands (+)- and (-)-SKF 10,047 (1 and 10 mg/kg s.c.), pentazocine (20 mg/kg s.c.) and DTG (1 and 10 mg/kg s.c.), non-competitive NMDA antagonists ketamine hydrochloride (20 mg/kg s.c.) and MK-801 (0.025, 0.1 and 1 mg/kg s.c.), 'atypical' neuroleptic drugs with (remoxipride 25 mg/kg s.c.) and without (raclopride 10 mg/kg s.c.; clozapine 25 mg/kg s.c.) affinity for sigma sites, and atropine sulphate (20 mg/kg s.c.), were investigated on the opiate withdrawal response induced by naloxone hydrochloride (15 mg/kg s.c.) in guinea-pigs treated 2 h before with a single dose of morphine sulphate (15 mg/kg s.c.). (+)- and (-)-SKF 10,047, pentazocine, ketamine and MK-801, given 0.5 h before naloxone, significantly attenuated the increased locomotor activity and other behaviours associated with morphine withdrawal in guinea-pigs. The selective sigma ligand DTG, and remoxipride had no effect on the withdrawal response but raclopride, clozapine, and atropine exacerbated the response. It is concluded that exacerbation of the morphine withdrawal response by neuroleptics is not related to sigma activity but to other mechanisms. Furthermore NMDA but not sigma mechanisms might play a role in the morphine withdrawal response.