Marked increase in cholecystokinin B receptor messenger RNA levels in rat dorsal root ganglia after peripheral axotomy

Role of Spinal Cholecystokinin Octapeptide, Nociceptin/Orphanin FQ, and Hemokinin-1 in Diabetic Allodynia

A complication of diabetes is neuropathic pain, which is difficult to control with medication. We have confirmed that neuropathic pain due to mechanical allodynia in diabetic mice is mediated by a characteristic neuropeptide in the spinal cord. We evaluated the strength of mechanical allodynia in mice using von Frey filaments. When mice were intravenously injected with streptozotocin, mechanical allodynia appeared 3 days later. Antibodies of representative neuropeptides were intrathecally (i.t.) administered to allodynia-induced mice 7 days after the intravenous administration of streptozotocin, and allodynia was reduced by anti-cholecystokinin octapeptide antibodies, anti-nociceptin/orphanin FQ antibodies, and anti-hemokinin-1 antibodies. In contrast, i.t.-administered anti-substance P antibodies, anti-somatostatin antibodies, and anti-angiotensin II antibodies did not affect streptozotocin-induced diabetic allodynia mice. Mechanical allodynia was attenuated by the i.t. administration of CCK-B receptor antagonists and ORL-1 receptor antagonists. The mRNA level of CCK-B receptors in streptozotocin-induced diabetic allodynia mice increased in the spinal cord, but not in the dorsal root ganglion. These results indicate that diabetic allodynia is caused by cholecystokinin octapeptide, nociceptin/orphanin FQ, and hemokinin-1 released from primary afferent neurons in the spinal cord that transmit pain to the brain via the spinal dorsal horn.

CCK-8 enhances acid-sensing ion channel currents in rat primary sensory neurons

Cholecystokinin (CCK) is a peptide that has been implicated in pain modulation. Acid sensitive ion channels (ASICs) also play an important role in pain associated with tissue acidification. However, it is still unclear whether there is an interaction between CCK signaling and ASICs during pain process. Herein, we report that a functional link between them in rat dorsal root ganglion (DRG) neurons. Pretreatment with CCK-8 concentration-dependently increased acid-evoked ASIC currents. CCK-8 increased the maximum response of ASICs to acid, but did not changed their acid sensitivity. Enhancement of ASIC currents by CCK-8 was mediated by the stimulation of CCK2 receptor (CCK2R), rather than CCK1R. The enhancement of ASIC currents by CCK-8 was prevented by application of either G-protein inhibitor GDP-β-S or protein kinase C (PKC) inhibitor GF109203×, but not by protein kinase A (PKA) inhibitor H-89 or JNK inhibitor SP600125. Moreover, CCK-8 increased the number of action potentials triggered by acid stimuli by activating CCK2R. Finally, CCK-8 dose-dependently exacerbated acid-induced nociceptive behavior in rats through local CCK2R. Together, these results indicated that CCK-8/CCK2R activation enhanced ASIC-mediated electrophysiological activity in DRG neurons and nociception in rats. The enhancement effect depended on G-proteins and intracellular PKC signaling rather than PKA and JNK signaling pathway. These findings provided that CCK-8/CCK2R is an important therapeutic target for ASIC-mediated pain.

Blockade of Cholecystokinin Type 2 Receptors Prevents the Onset of Vincristine-Induced Neuropathy in Mice

Vincristine (VCR) is responsible for the onset of the VCR-induced peripheral neuropathy (VIPN), associated with neuropathic pain. Several reports have strongly linked the cholecystokinin type 2 receptor (CCK2R) to nociceptive modulation. Thus, our aim was to evaluate the effect of CCK2R blockade on the onset of VIPN, as well as its interaction on VCR anticancer efficacy. VCR was administrated in mice for 8 days (100 µg/kg/d, i.p.). Transcriptomic analysis of the dorsal root ganglia (DRG) was performed at day 7 in VCR and control mice. Proglumide (30 mg/kg/d), a CCK1R and CCK2R antagonist, and Ly225910 (1 mg/kg/d), a selective CCK2R antagonist, were administrated one day before and during VCR treatment. Tactile sensitivity was assessed during treatments. Immunofluorescence and morphological analyses were performed on the skin, DRG and sciatic nerve at day 7. The cytotoxicity of VCR in combination with proglumide/Ly225910 was evaluated in human cancer cell lines. Cck2r was highly upregulated in the DRG of VCR mice. Proglumide accelerated the recovery of normal sensitivity, while Ly225910 totally prevented the onset of allodynia and nerve injuries induced by VCR. Proglumide or Ly225910 in combination with VCR did not affect the cytotoxicity of VCR. Targeting CCK2R could therefore be an effective strategy to prevent the onset of VIPN.

The Cholecystokinin Type 2 Receptor, a Pharmacological Target for Pain Management

Over the past decades, accumulating evidence has demonstrated a pivotal role of cholecystokinin type 2 receptor (CCK2R) in pain modulation. The established role of CCK2R activation in directly facilitating nociception has led to the development of several CCK2R antagonists, which have been shown to successfully alleviate pain in several rodent models of pain. However, the outcomes of clinical trials are more modest since they have not demonstrated the expected biological effect obtained in animals. Such discordances of results between preclinical and clinical studies suggest reconsidering our knowledge about the molecular basis of the pharmacology and functioning of CCK2R. This review focuses on the cellular localization of CCK2R specifically in the sensory nervous system and discusses in further detail the molecular mechanisms and signal transduction pathways involved in controlling pain perception. We then provide a comprehensive overview of the most successful compounds targeting CCK2R and report recent advances in pharmacological strategies used to achieve CCK2R modulation. We purposely distinguish between CCK2R benefits obtained in preclinical models and outcomes in clinical trials with different pain etiologies. Lastly, we emphasize the biological and clinical relevance of CCK2R as a promising target for the development of new treatments for pain management.

Gene expression changes in dorsal root ganglia following peripheral nerve injury: roles in inflammation, cell death and nociception

Subsequent to a peripheral nerve injury, there are changes in gene expression within the dorsal root ganglia in response to the damage. This review selects factors which are well-known to be vital for inflammation, cell death and nociception, and highlights how alterations in their gene expression within the dorsal root ganglia can affect functional recovery. The majority of studies used polymerase chain reaction within animal models to analyse the dynamic changes following peripheral nerve injuries. This review aims to highlight the factors at the gene expression level that impede functional recovery and are hence are potential targets for therapeutic approaches. Where possible the experimental model, specific time-points and cellular location of expression levels are reported.

Behavioral and anatomical characterization of the bilateral sciatic nerve chronic constriction (bCCI) injury: correlation of anatomic changes and responses to cold stimuli

Background

Unilateral constrictive sciatic nerve injury (uCCI) is a common neuropathic pain model. However, the bilateral constrictive injury (bCCI) model is less well studied, and shows unique characteristics. In the present study, we sought to correlate effects of bCCI on nocifensive responses to cold and mechanical stimuli with selected dorsal horn anatomic markers. bCCI or sham ligation of both rat sciatic nerves were followed up to 90 days of behavioural testing. Additional rats sacrificed at 15, 30 and 90 days were used for anatomic analyses. Behavioural tests included hindpaw withdrawal responses to topical acetone, cold plate testing, an operant thermal preference task and hindpaw withdrawal thresholds to mechanical probing.

Results

All nocifensive responses to cold increased and remained enhanced for >45 days. Mechanical withdrawal thresholds decreased for 25 days only. Densitometric analyses of immunoperoxidase staining in the superficial dorsal horn at L4-5 revealed decreased cholecystokinin (CCK) staining at all times after bCCI, decreased mu opiate receptor (MOR) staining, maximal at 15 days, increased neuropeptide Y (NPY) staining only at days 15 and 30, and increased neurokinin-1 receptor (NK-1R) staining at all time points, maximal at 15 days. Correlation analyses at 45 days post-bCCI, were significant for individual rat nocifensive responses in each cold test and CCK and NK-1R, but not for MOR or NPY.

Conclusions

These results confirm the usefulness of cold testing in bCCI rats, a new approach using CCI to model neuropathic pain, and suggest a potential value of studying the roles of dorsal horn CCK and substance P in chronic neuropathic pain. Compared to human subjects with neuropathic pain, responses to cold stimuli in rats with bCCI may be a useful model of neuropathic pain.

Roles of gastro-oesophageal afferents in the mechanisms and symptoms of reflux disease

Oesophageal pain is one of the most common reasons for physician consultation and/or seeking medication. It is most often caused by acid reflux from the stomach, but can also result from contractions of the oesophageal muscle. Different forms of pain are evoked by oesophageal acid, including heartburn and non-cardiac chest pain, but the basic mechanisms and pathways by which these are generated remain to be elucidated. Both vagal and spinal afferent pathways are implicated by basic research. The sensitivity of afferent fibres within these pathways may become altered after acid-induced inflammation and damage, but the severity of symptoms in humans does not necessarily correlate with the degree of inflammation. Gastro-oesophageal reflux disease (GORD) is caused by transient relaxations of the lower oesophageal sphincter, which are triggered by activation of gastric vagal mechanoreceptors. Vagal afferents are therefore an emerging therapeutic target for GORD. Pain in the absence of excess acid reflux remains a major challenge for treatment.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

Modulatory effect of CCK-8S on GABA-induced depolarization from rat dorsal root ganglion

CCK is a brain-gut peptide that is abundantly distributed in both gastrointestinal tract and mammalian brain. The sulfated octapeptide fragment of cholecystokinin (CCK-8S) has been shown to be involved in numerous physiological functions such as behavior, anxiety, learning/memory processes and neuropathic pain. CCK-8S is one of the strongest endogenous anti-opioid substances and suppresses opioid peptides-mediated 'pre-synaptic inhibition' of gamma-aminobutyric acid (GABA) release. Here we provide evidence that CCK-8S modulates GABA-evoked membrane depolarization in rat dorsal root ganglion (DRG) neurons using intracellular recording technique. Bath application CCK-8S-induced membrane depolarization in most of the rat DRG neurons. The depolarization was blocked by prolumide but not LY225910. Pretreatment with CCK-8S suppressed the GABA-evoked depolarization in a concentration-dependent manner. The CCK-8S inhibition was also time-dependent and reached the peak at about 2 min. The inhibitory effect of CCK-8S was strongly suppressed by pre-incubation of CCK-B receptor antagonist LY225910, phospholipase C inhibitor U73122, protein kinase C inhibitor chelerythrine and calcium chelator BAPTA-AM, respectively. The protein kinase A inhibitor H-89 did not affect CCK-8S effect. The results suggest that CCK-8S inhibits GABA-A receptor function by activation of CCK-B receptor followed by activation of intracellular PLC-Ca(2+)-PKC cascade. Thus, CCK-8S might enhance nociceptive information transmission through inhibition of the "pre-synaptic inhibition" evoked by GABA, which may explain its role in modulation of primary sensory information (especially pain).

Involvement of cholecystokinin in peripheral nociceptive sensitization during diabetes in rats as revealed by the formalin response

The possible pronociceptive role of peripheral cholecystokinin (CCK-8) as well as CCK(A) and CCK(B) receptors in diabetic rats was assessed. Subcutaneous injection of 0.5% formalin induced a greater nociceptive behavior in diabetic than in non-diabetic rats. Moreover, local peripheral injection of CCK-8 (0.1-100 microg) significantly increased 0.5% formalin-induced nociceptive activity in diabetic, but not in non-diabetic, rats. This effect was restricted to the formalin-injected paw as administration of CCK-8 into the contralateral paw was ineffective. Local peripheral administration of CCK-8, in the absence of formalin injection, produced a low level of, but significant increase in, flinching behavior in diabetic compared to non-diabetic rats. In addition, local peripheral administration of the non-selective CCK receptor antagonist proglumide (1-100 microg), CCK(A) receptor antagonist lorglumide (0.1-100 microg) or CCK(B) receptor antagonist CR-2945 (0.1-100 microg), but not vehicle or contralateral administration of CCK receptor antagonists, significantly reduced 0.5% formalin-induced flinching in diabetic rats. CR-2945 was the most effective drug in this condition. These effects were not observed in non-diabetic rats. The local peripheral pronociceptive effect of CCK-8 (100 microg) was significantly reduced by proglumide (100 microg), lorglumide (100 microg), and CR-2945 (100 microg). Results suggest that diabetes-induced peripheral sensitization could be due to a local peripheral release of CCK-8, which in turn would act on CCK(B), mainly but also in CCK(A), receptors located on the primary afferent neurons.

Pharmacological strategies in relieving neuropathic pain

The pharmacological treatment of neuropathic pain relies, to a large extent, on drugs belonging to a small number of defined classes. Opioids, tricyclic antidepressants, antiepileptic drugs and membrane stabilisers form the current basis of treatment. Varying levels of evidence support the use of individual members of these classes and overall show no indication that one class of drug, or individual drug has universal effectiveness. More refined knowledge of the modes of action of these agents used to treat neuropathic pain should lead to a more logical approach to the management of this difficult series of conditions. A number of drugs currently licensed for a different indication have recently had an analgesic effect in neuropathic pain attributed to them. In addition, a number of novel compounds are undergoing investigation and provide hope of dicovering more efficacious treatment options in the future.

Opioid hyperalgesia and tolerance versus 5-HT1A receptor-mediated inverse tolerance

In addition to analgesia, opioids also produce paradoxical hyperalgesic effects following acute and chronic treatment. In this article, we review the occurrence of this hyperalgesia under several conditions, and discuss the potential mechanisms and clinical implications. We also review recent evidence that paradoxical analgesia and inverse tolerance induced by stimulation of 5-HT(1A) receptors, which is a mirror image of opioid-induced hyperalgesia and tolerance, might achieve clinically significant analgesia in chronic pain.

A randomised, double blind, placebo controlled crossover study of the cholecystokinin 2 antagonist L-365,260 as an adjunct to strong opioids in chronic human neuropathic pain

The aim of this study was to establish if the cholecystokinin (CCK) 2 antagonist L-365,260 augments the analgesic effect of morphine in human subjects with chronic neuropathic pain. This is a randomised, double blind, placebo controlled study of 40 adult subjects taking morphine for neuropathic pain. Each received placebo, L-365,260 30 mg and L-365,260 120 mg in three divided doses daily separated by a washout period in random order. Pain, activity, sedation, sleep and side effects were recorded along with 12 lead ECGs, renal and liver function tests and full blood pictures. L-365,260 failed to augment the analgesic effect of morphine at any of the dose levels used. Side effects were minor. There were no changes in ECGs and biochemical indices were unaltered with its use. The CCK 2 antagonist L-365,260 does not augment the analgesic effect of morphine in subjects with chronic neuropathic pain. L-365,260 was well tolerated and side effects from its use were minor.

The role of spinal cholecystokinin in chronic pain states

It is well established that cholecystokinin (CCK) reduces the antinociceptive effect of opioids. The level of CCK and CCK receptors, as well as CKK release, exhibits considerable plasticity after nerve injury and inflammation, conditions known to be associated with chronic pain. Such altered CCK release coupled in some situation with changes in CCK receptor levels may underlie the clinical phenomenon of varying opioid sensitivity in different clinical pain conditions. In particular, neuropathic pain after injury to the peripheral and central nervous system does not respond well to opioids, which is likely to be caused by increased activity in the endogenous CCK system. CCK receptor antagonists may thus be useful as analgesics in combination with opioids to treat neuropathic pain.

Replicate high-density rat genome oligonucleotide microarrays reveal hundreds of regulated genes in the dorsal root ganglion after peripheral nerve injury

BACKGROUND

Rat oligonucleotide microarrays were used to detect changes in gene expression in the dorsal root ganglion (DRG) 3 days following sciatic nerve transection (axotomy). Two comparisons were made using two sets of triplicate microarrays, naïve versus naïve and naïve versus axotomy.

RESULTS

Microarray variability was assessed using the naïve versus naïve comparison. These results support use of a P < 0.05 significance threshold for detecting regulated genes, despite the large number of hypothesis tests required. For the naïve versus axotomy comparison, a 2-fold cut off alone led to an estimated error rate of 16%; combining a >1.5-fold expression change and P < 0.05 significance reduced the estimated error to 5%. The 2-fold cut off identified 178 genes while the combined >1.5-fold and P < 0.05 criteria generated 240 putatively regulated genes, which we have listed. Many of these have not been described as regulated in the DRG by axotomy. Northern blot, quantitative slot blots and in situ hybridization verified the expression of 24 transcripts. These data showed an 83% concordance rate with the arrays; most mismatches represent genes with low expression levels reflecting limits of array sensitivity. A significant correlation was found between actual mRNA differences and relative changes between microarrays (r2 = 0.8567). Temporal patterns of individual genes regulation varied.

CONCLUSIONS

We identify parameters for microarray analysis which reduce error while identifying many putatively regulated genes. Functional classification of these genes suggest reorganization of cell structural components, activation of genes expressed by immune and inflammatory cells and down-regulation of genes involved in neurotransmission.

Neuropeptides in neuropathic and inflammatory pain with special emphasis on cholecystokinin and galanin

Neuropeptides present in primary afferents and the dorsal horn of the spinal cord have an important role in the mediation of nociceptive input under normal conditions. Under pathological conditions, such as chronic inflammation or following peripheral nerve injury, the production of peptides and peptide receptors is dramatically altered, leading to a number of functional consequences. In this review, the role of two neuropeptides that undergo such altered expression under pathological conditions, cholecystokinin (CKK) and galanin, is reviewed.

Increased level of cholecystokinin in cerebrospinal fluid is associated with chronic pain-like behavior in spinally injured rats

Cholecystokinin (CCK) is a physiological antagonist of opioid-mediated antinociception and may be involved in some chronic pain states where opioids have reduced effect. We have previously shown in a rat model of central neuropathic pain after spinal cord injury that blockade of CCK-B receptors lead to marked pain relief. In the present study, we showed that spinally injured rats exhibiting chronic pain-like behaviors (aversive reaction to innocuous mechanical and cold stimulation) had significantly elevated level of CCK-like immunoreactivity in cerebrospinal fluid compared to normal rats or spinally injured rats which did not exhibit pain-like behaviors. The increased level of circulating CCK in the cerebrospinal fluid may thus contribute to the maintenance of chronic pain in these rats by reducing the endogenous inhibitory tone provided by opioid peptides and may be involved in the phenomenon of opioid insensitivity.

Morphine-induced in vivo release of spinal cholecystokinin is mediated by delta-opioid receptors--effect of peripheral axotomy

Morphine and other opioid agonists induce spinal in vivo release of cholecystokinin (CCK), a neuropeptide with anti-opioid properties. However, so far the opioid receptor subtype responsible for this effect has not been determined. In the present in vivo microdialysis study, the morphine-induced release of cholecystokinin-like immunoreactivity (CCK-LI) in the dorsal horn was completely blocked by the delta-opioid antagonist naltrindole (10 microM in the perfusion fluid). Neither the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr amide (CTOP; 10 microM in the perfusion fluid), nor the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI); 10 microM in the perfusion fluid) had any significant effect in this respect. In addition, systemic administration of the delta-opioid receptor agonist BW373U86 (1 mg/kg, s.c.) and spinal administration of the delta(2)-opioid receptor agonist, Tyr-D-Ala-Phe-Glu-Val-Val-Gly amide ([D-Ala(2)] deltorphin II) (1 microM in the perfusion fluid) induced a significant increase of the CCK-LI level. The effect of BW373U86 on spinal CCK-LI release was completely blocked by spinal administration of naltrindole. The mu-opioid receptor agonist [D-ala(2)-N-Me-Phe(4)-Gly(5)-ol]-enkephalin (DAMGO) (1 microM in the perfusion fluid or 1 mg/kg, s.c.) failed to alter the CCK-LI level. Peripheral nerve lesions have previously been shown to down-regulate mu- and delta-opioid receptors in the dorsal horn, to increase the gene-expression of CCK and CCK-receptor mRNA in dorsal root ganglion neurons and to alter the potassium-induced spinal CCK-LI release. After complete sciatic nerve transection, administration of the two selective delta-opioid receptor agonists induced a significant release of CCK-LI, which was comparable to controls. In contrast, neither systemic nor spinal administration of morphine and DAMGO altered the spinal CCK-LI release in axotomized animals. The present data indicate that the delta-opioid receptor mediates morphine-induced CCK-LI release in the spinal cord.

Expression and regulation of cholecystokinin and cholecystokinin receptors in rat nodose and dorsal root ganglia

Cholecystokinin (CCK) is an important satiety factor, acting via the vagus nerve to influence central feeding centers. CCK binding sites have been demonstrated in the vagal sensory nodose ganglion and within the nerve proper. Using in situ hybridization, expression of the CCK(A) and (B) receptors (Rs), as well as of CCK itself, was studied in the normal nodose ganglion (NG), and after vagotomy, starvation and high-fat diet. CCK(A)-R mRNA expression in dorsal root ganglia (DRGs) was also explored. In the NG, 33% of the neuron profiles (NPs) contained CCK(A)-R mRNA and in 9% we observed CCK(B)-R mRNA. CCK mRNA was not found in normal NGs. Peripheral vagotomy decreased the number of CCK(A)-R mRNA-expressing NPs, dramatically increased the number of CCK(B)-R mRNA, and induced CCK mRNA and preproCCK-like immunoreactivity in nodose NPs. No significant differences in the number of NPs labelled for either mRNA species were detected following 48 h food deprivation or in rats fed a high-fat content diet. In DRGs, 10% of the NPs expressed CCK(A)-R mRNA, a number that was not affected by either axotomy or inflammation. This cell population was distinct from neurons expressing calcitonin gene-related peptide mRNA. These results demonstrate that the CCK(A)-R is expressed by both viscero- and somatosensory primary sensory neurons, supporting a role for this receptor as a mediator both of CCK-induced satiety and in sensory processing at the spinal level. The stimulation of CCK and CCK(B)-R gene expression following vagotomy suggests a possible involvement in the response to injury for these molecules.

Neuropeptides--an overview

The present article provides a brief overview of various aspects on neuropeptides, emphasizing their multitude and their wide distribution in both the peripheral and central nervous system. Interestingly, neuropeptides are also expressed in various types of glial cells under normal and experimental conditions. The recent identification of, often multiple, receptor subtypes for each peptide, as well as the development of peptide antagonists, have provided an experimental framework to explore functional roles of neuropeptides. A characteristic of neuropeptides is the plasticity in their expression, reflecting the fact that release has to be compensated by de novo synthesis at the cell body level. In several systems peptides can be expressed at very low levels normally but are upregulated in response to, for example, nerve injury. The fact that neuropeptides virtually always coexist with one or more classic transmitters suggests that they are involved in modulatory processes and probably in many other types of functions, for example exerting trophic effects. Recent studies employing transgene technology have provided some information on their functional role, although compensatory mechanisms in all probability could disguise even a well defined action. It has been recognized that both 'old' and newly discovered peptides may be involved in the regulation of food intake. Recently the first disease-related mutation in a peptidergic system has been identified, and clinical efficacy of a substance P antagonist for treatment of depression has been reported. Taken together it seems that peptides may play a role particularly when the nervous system is stressed, challenged or afflicted by disease, and that peptidergic systems may, therefore, be targets for novel therapeutic strategies.

Effect of morphine on cholecystokinin and mu-opioid receptor-like immunoreactivities in rat spinal dorsal horn neurons after peripheral axotomy and inflammation

In order to further investigate the interaction between the octapeptide cholecystokinin and opioid analgesia in the spinal cord we used double-colour immunofluorescence to examine the anatomical distribution of cholecystokinin and mu-opioid receptors in the dorsal horn, as well as the effect of morphine on cholecystokinin- and mu-opioid receptor-like immunoreactivities following peripheral nerve injury and inflammation. Mu-opioid receptor-like immunoreactivity was present in 65.6% of cholecystokinin-positive neurons in laminae I and II of rat spinal cord. Conversely, 40.4% of mu-opioid receptor-positive neurons contained cholecystokinin-like immunoreactivity. Systemic application of morphine (1, 3 or 10 mg/kg; i.v.) after sciatic nerve section significantly, but reversibly, decreased mu-Opioid receptor-like immunoreactivity in the medial half of lamina II in segment L5 of the ipsilateral dorsal horn, and cholecystokinin-like immunoreactivity was also markedly reduced in the same region. These effects were dose- and time-dependent and could be prevented by naloxone preadministration. In contrast, no significant change in the pattern of distribution or intensity of mu-opioid receptor- and cholecystokinin-like immunoreactivities was observed in intact rats or during peripheral inflammation. These results provide a cellular basis for the interaction of mu-opioid receptors and cholecystokinin at the spinal level by showing a high degree of co-existence of these two molecules in local interneurons, and also show that morphine can induce rapid and short lasting effects on mu-opioid receptors after peripheral nerve injury. The results contribute to our understanding of how endogenous cholecystokinin reduces the analgesic effect of morphine.

Increased spinal cholecystokinin activity after systemic resiniferatoxin: electrophysiological and in situ hybridization studies

The present study assessed the effect of a single subcutaneous injection of resiniferatoxin (RTX), an ultrapotent capsaicin analogue, on the activity of spinal cholecystokinin (CCK) systems, by using electrophysiological and in situ hybridization techniques. Subcutaneous RTX at 0.3 mg/kg, but not vehicle, produced marked thermal hypoalgesia in rats on the hot plate and tail flick tests. Partial recovery from hypoalgesia occurred in some (<50%), but not all, RTX-treated rats after 2 weeks. The flexor reflex in response to activation of high threshold afferents was recorded 15-35 days after RTX- or vehicle-treatment. There was no obvious difference between RTX- and vehicle-treated rats in the baseline flexor reflex. Intravenous morphine at 1 mg/kg caused a depression of the flexor reflex in vehicle- and in RTX-treated rats. The reflex depressive effect of morphine was significantly briefer in RTX-treated, non-recovered rats than vehicle-treated rats. Furthermore, CI-988, a high affinity antagonist of CCKB receptors, caused a minor depression of the reflex in vehicle- and RTX-treated rats that had partially recovered, whereas the reflex depressive effect of CI-988 was significantly enhanced in RTX-treated, non-recovered rats. In situ hybridization showed that RTX treatment caused a marked and significant increase in the number of dorsal root ganglion (DRG) neurone profiles expressing CCKB receptor mRNA, whereas only a small increase was observed for CCKA receptor mRNA expressing neurone profiles. Significantly more DRG neurone profiles expressed CCKB receptor mRNA in RTX-treated, non-recovered rats compared to partially recovered rats. RTX-treatment did not influence the expression of CCK mRNA in DRGs. Since CCK functions as a physiological antagonist of morphine, it is suggested that RTX treatment enhances the activity of spinal CCK systems, leading to the reduced effect of morphine and increased effect of the CCKB receptor antagonist CI-988. This may mainly be due to upregulation of CCKB receptors in DRG neurones.

Cholecystokinin/opioid interactions

Cholecystokinin (CCK) acts as an anti-opioid peptide. The mechanisms of CCK-opioid interaction under normal and pathological conditions were examined with various techniques. Nerve injury induces upregulation of CCK mRNA and CCK2 receptors in sensory neurons. The involvement of CCK in spinal nociception in normal and axotomized rats was examined. The CCK2 receptor antagonist CI-988 did not reduce spinal hyperexcitability following repetitive C-fiber stimulation in normal or axotomized rats, suggesting that CCK is probably not released from injured primary afferents. With in vivo microdialysis intravenous (i.v.) or intrathecal (i.t.) morphine increased the extracellular level of CCK in the dorsal horn in a naloxone reversible manner. Morphine also released CCK after axotomy, but not during carrageenan-induced inflammation. In contrast, K(+)-stimulation failed to increase extracellular levels of CCK in axotomized rats, but did so in inflamed rats. Double-coloured immunofluorescence technique revealed partial co-localization between CCK-like immunoreactivity (LI) and mu-opioid receptor (MOR)-LI in superficial dorsal horn neurons. The presence of MOR in CCK containing neurons suggests a possible direct influence of opioids on CCK release in the spinal cord. Axotomy, but not inflammation, induced a moderate decrease in CCK- and MOR-LI in the dorsal horn. I.v. morphine further temporarily reduced CCK- and MOR-LIs in axotomized, but not in normal or inflamed, rats. While the effect of morphine on CCK-LI can be interpreted as the result of increased CCK release, the effect on MOR-LI may be related to changes in the microenvironment of the dorsal horn induced by nerve injury.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Building blocks of pain: the regulation of key molecules in spinal sensory neurones during development and following peripheral axotomy

The pathways, synapses and molecules involved in pain processing in the newborn are not only required to trigger repair and recuperation but are also involved in the process of forming a mature nervous system. Sensory neurons in the dorsal root ganglion and dorsal horn express a phenomenal array of molecules which contribute to their structural and functional characteristics and many of these are developmentally regulated both pre- and postnatally. In order to understand nociceptive signalling and pain in the neonate we need a clear picture of that regulation. This review concentrates on the changing expression of selected key molecules, receptors and channels in the embryo, neonate and adult, which both characterise the sensory neuron and contribute to its response to painful stimuli in normal and pathological conditions.

Effects of peripheral axotomy on cholecystokinin neurotransmission in the rat spinal cord

Because cholecystokinin (CCK) acts as a "functional" endogenous opioid antagonist, it has been proposed that changes in central CCKergic neurotransmission might account for the relative resistance of neuropathic pain to the analgesic action of morphine. This hypothesis was addressed by measuring CCK-related parameters 2 weeks after unilateral sciatic nerve section in rats. As expected, significant decreases (-25-38%) in the tissue concentrations and in vitro release of both substance P and calcitonin gene-related peptide were noted in the dorsal quadrant of the lumbar spinal cord on the lesioned side. In contrast, the tissue levels and in vitro release of CCK were unchanged in the same area in lesioned rats. Measurements in dorsal root ganglia at L4-L6 levels revealed no significant changes in proCCK mRNA after the lesion. However, sciatic nerve section was associated with a marked ipsilateral increase in both CCK-B receptor mRNA levels in these ganglia (+70%) and the autoradiographic labeling of CCK-B receptors by [3H]pBC 264 (+160%) in the superficial layers of the lumbar dorsal horn. Up-regulation of CCK-B receptors rather than CCK synthesis and release probably contributes to increased spinal CCKergic neurotransmission in neuropathic pain.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Peripheral axotomy influences the in vivo release of cholecystokinin in the spinal cord dorsal horn-possible involvement of cholecystokinin-B receptors

An increased expression of cholecystokinin (CCK) messenger RNA (mRNA) as well as CCK-B receptor mRNA in dorsal root ganglion (DRG) cells following peripheral axotomy has previously been demonstrated. In the present in vivo microdialysis study, the effect of unilateral sciatic nerve section on basal and potassium-induced release of CCK-like (CCK-LI) immunoreactivity in the rat dorsal horn was investigated. We also compared the effects of the CCK-B receptor antagonist CI988 on basal and potassium-stimulated CCK-LI release in intact animals and in chronically axotomized rats. Perfusion of the microdialysis probe with KCl (100 mM) induced a more than 6-fold increase of the extracellular level of CCK-LI in control animals. In contrast, following unilateral sciatic nerve section the same KCl stimulation failed to evoke a release of CCK-LI ipsilaterally. However, after systemic administration of CI988 (1 mg kg-1, i.v.), 100 mM KCl induced a significant increase of the extracellular CCK-LI level in axotomized rats, similar to that observed in control animals. In control animals no effect of CI988 on KCl-stimulated CCK-LI release could be detected. CI988 by itself had no influence on the extracellular CCK-LI level in either nerve injured or control animals. The present data suggest that axotomy reduces the release of CCK-like immunoreactivity in the spinal cord by a mechanism involving the CCK-B receptor binding site.

Differential release of cholecystokinin by morphine in rat spinal cord

The analgesic efficacy of opioids is reduced in neuropathic pain states and increased in inflammation. Since the neuropeptide cholecystokinin (CCK) plays a role in the modulation of opiate-induced analgesia, the morphine-mediated release of CCK in the spinal cord of rats was compared with in vivo microdialysis in normals and different pain models. The effect of systemic and intrathecal (i.t.) morphine on the extracellular level of CCK was analyzed in the spinal cord dorsal horn of halothane-anaesthetized normal rats as well as during peripheral neuropathy and inflammation. No difference was found in basal CCK level among groups. However, morphine significantly increased extracellular CCK concentration after both systemic and spinal application in intact as well as axotomized rats and this effect was naloxone-reversible in non-lesioned animals. Similar results were seen in axotomized rats. In contrast, morphine did not induce CCK release during carrageenan-induced inflammation. These data provide evidence that the ability of opiates to release CCK under different pain states may play a key role in their analgesic efficacy.

Localization and regulation of the delta-opioid receptor in dorsal root ganglia and spinal cord of the rat and monkey: evidence for association with the membrane of large dense-core vesicles

Using immunohistochemistry and immunoelectron microscopy, the localization and regulation of delta-opioid receptor-like immunoreactivity were studied in dorsal root ganglia and spinal cord of normal rat and monkey, and after peripheral axotomy. Delta-opioid receptor-like immunoreactivity was observed in many small dorsal root ganglion neurons, and in the rat most of them contained substance P and calcitonin gene-related peptide. At the ultrastructural level, delta-opioid receptor-like immunoreactivity was localized in the Golgi complex, on the membrane of the large dense-core vesicles and on the membrane of and/or inside a type of large vesicle with an interior of low electron density. The latter vesicles were often in contact with multivesicular bodies. In the superficial dorsal horn of the spinal cord, most delta-opioid receptor-positive nerve fibers contain substance P and/or calcitonin gene-related peptide, both in rat and monkey. Also, in these nerve endings delta-opioid receptor-like immunoreactivity was found on the membrane of large dense-core vesicles and on the membrane of, or in, the lucent vesicles. Occasionally, delta-opioid receptor-like immunoreactivity was observed on the plasmalemma of the terminals, particularly when the vesicles were in exocytotic contact with the plasmalemma. Peripheral axotomy induced a decrease in delta-opioid receptor-like immunoreactivity both in cell bodies in the dorsal root ganglia and in terminals in the dorsal horn. These data suggest that the delta-opioid receptor may be a constituent of the membrane of large dense-core vesicles storing and releasing neuropeptides. It is suggested that upon exocytotic release of substance P and calcitonin gene-related peptide from large dense-core vesicles, there is a transient modification of the surface of the primary afferent terminals which leads to exposure of the receptor protein so that enkephalin released from adjacent terminals can activate the receptor. The decrease in delta-opioid receptors after axotomy indicates that delta-opioid receptor-mediated inhibitory effects are attenuated at the spinal level both in the rat and monkey.

On the role of cholecystokinin in the mediation of spinal reflex excitability in intact and axotomized rats

The effects of intrathecal (i.t.) administration of cholecystokinin (CCK) on the hindlimb flexor withdrawal reflex were compared in decerebrate, spinalized, unanaesthetized rats with intact sciatic nerves or 14-26 days after unilateral transection of the sciatic nerve. The effect of CI-988, a CCK-B receptor antagonist, on the facilitation of the flexor reflex induced by conditioning stimulation (CS) of C-afferents was also examined. Intrathecal CCK induced facilitation of the flexor reflex over the dose range of 10 ng-1 micrograms in rats with intact and sectioned sciatic nerves. However, the facilitation induced by CCK in axotomized rats was significantly reduced compared with nerve intact animals. CI-988 did not block the facilitation of the flexor reflex induced by the CS and the increases in reflex magnitude during the CS train (wind-up) in rats with intact and sectioned sciatic nerves. The present results indicate that the excitatory effect of CCK on the spinal cord is reduced after axotomy, suggesting that nerve section may induce transsynaptic changes in the spinal cord leading to hyposensitivity to CCK. Furthermore, it is suggested that CCK may not play a significant role in the phenomenon of wind-up and central sensitization after repetitive C-fiber input under normal conditions or after axotomy, when expression of CCK and CCK-B receptors are increased in sensory neurons.

Down-regulation of mu-opioid receptors in rat and monkey dorsal root ganglion neurons and spinal cord after peripheral axotomy

To understand the role of opioids and their receptors in chronic pain following peripheral nerve injury, we have studied the mu-opioid receptor in rat and monkey lumbar 4 and 5 dorsal root ganglion neurons and the superficial dorsal horn of the spinal cord under normal circumstances and after peripheral axotomy. Our results show that many small neurons in rat and monkey dorsal root ganglia, and some medium-sized and large neurons in rat dorsal root ganglia, express mu-opioid receptor-like immunoreactivity. Most of these neurons contain calcitonin gene-related peptide. The mu-opioid receptor was closely associated with the somatic plasmalemma of the dorsal root ganglion neurons. Both mu-opioid receptor-immunoreactive nerve fibers and cell bodies were observed in lamina II of the dorsal horn. The highest intensity of mu-opioid receptor-like immunoreactivity was observed in the deep part of lamina II. Most mu-opioid receptor-like immunoreactivity in the dorsal horn originated from spinal neurons. A few mu-opioid receptor-positive peripheral afferent terminals in the rat and monkey dorsal horn were calcitonin gene-related peptide-immunoreactive. In addition to pre- and post-junctional receptors in rat and monkey dorsal horn neurons, mu-opioid receptors were localized on the presynaptic membrane of some synapses of primary afferent terminals in the monkey dorsal horn. Peripheral axotomy caused a reduction in the number and intensity of mu-opioid receptor-positive neurons in the rat and monkey dorsal root ganglia, and of mu-opioid receptor-like immunoreactivity in the dorsal horn of the spinal cord. The decrease in mu-opioid receptor-like immunoreactivity was more pronounced in the monkey than in the rat dorsal root ganglia and spinal cord. It is probable that there was a parallel trans-synaptic down-regulation of mu-opioid-like immunoreactivity in local dorsal horn neurons of the monkey. These data suggest that one factor underlying the well known insensitivity of neuropathic pain to opioid analgesics could be due to a marked reduction in the number of mu-opioid receptors in the axotomized sensory neurons and in interneurons in the dorsal horn of the spinal cord.

Electrophysiological studies on rat dorsal root ganglion neurons after peripheral axotomy: changes in responses to neuropeptides

The effect of three peptides, galanin, sulfated cholecystokinin octapeptide, and neurotensin (NT), was studied on acutely extirpated rat dorsal root ganglia (DRGs) in vitro with intracellular recording techniques. Both normal and peripherally axotomized DRGs were analyzed, and recordings were made from C-type (small) and A-type (large) neurons. Galanin and sulfated cholecystokinin octapeptide, with one exception, had no effect on normal C- and A-type neurons but caused an inward current in both types of neurons after sciatic nerve cut. In normal rats, NT caused an outward current in C-type neurons and an inward current in A-type neurons. After sciatic nerve cut, NT only caused an inward current in both C- and A-type neurons. These results suggest that (i) normal DRG neurons express receptors on their soma for some but not all peptides studied, (ii) C- and A-type neurons can have different types of receptors, and (iii) peripheral nerve injury can change the receptor phenotype of both C- and A-type neurons and may have differential effects on these neuron types.

Inducible receptors

While regulation of receptor function is known to occur at many levels (e.g. transcriptional, post-translational), it is generally perceived that a tissue either expresses or does not express a particular receptor in an all-or-none fashion. Many pathological (e.g. tissue injury) and physiological (e.g. angiogenesis) processes have, however, been shown to be associated with the transcriptional induction of specific receptors. Induced receptors are not confined to any particular class, but range from G protein-coupled receptors to receptor tyrosine kinases. The potential implications of de novo receptor expression are profound with respect to potential novel therapeutic targets in specific disease states. Further, this observation may explain unexpected side-effects in the pharmacotherapy of existing disease states. In this article Lucy Donaldson, Michael Hanley and Amparo Villablanca discuss circumstances under which de novo receptor induction has been described, potential mechanisms of induction and the implications for pharmacology.

Immunofluorescence in situ hybridization (IFISH) in neurones retrogradely labelled with rhodamine latex microspheres

The method of non-radioactive in situ hybridization was developed as an alternative to radioactive assay because of the difficulties and disadvantages of the safety measures required, extensive time required for autoradiography (especially with 3H-labelled probes) and limited cellular resolution obtained using 32P- and 35S-labelled probes. This method holds great potential for studying functional anatomy of specific neuronal pathways if it can be used in conjunction with conventional tract tracing techniques. In this article we describe a simple method by which immunofluorescence in situ hybridization (IFISH) was jointly used with rhodamine latex microspheres (RLM) to trace the origin of the thalamic cholecystokininergic input in rat. RLM is a widely used retrograde fluorescence tracer and seems ideal for IFISH because: (1) it lacks aversive effect on the hybridization and immunocytochemical reactions, (2) it is resistant to the rather harsh tissue treatment required for IFISH, and (3) both the RLM and mRNA hybrids give fluorescence signals; therefore, the extent of signal co-localization can be conveniently and more accurately verified under an epifluorescence microscope. Success of the IFISH-RLM combination is chiefly limited by the quantity and availability of mRNA signals in the tissue. In our case, we used a digoxigenin (DIG)-labelled oligonucleotide probe, which through immunological amplification significantly enhanced the sensitivity of mRNA detection.

Cholecystokinin octapeptide reverses the kappa-opioid-receptor-mediated depression of calcium current in rat dorsal root ganglion neurons

Although the cholecystokinin octapeptide (CCK-8) is reported to antagonize the kappa-opioid-receptor-mediated analgesic effect in spinal cord, its mechanism and sites of action remain obscure. In the present study, the whole-cell patch-clamp recording technique was employed to examine the effect of kappa-opioid agonist U50488H on voltage-gated calcium channels and the interaction between the CCK-8 and U50488H in acutely isolated rat dorsal root ganglion neurons. The results indicate that the calcium currents elicited in dorsal root ganglion neurons can be depressed by U50488H, an effect readily reversed by the kappa-opioid receptor antagonist Nor-BNI or by the antiopioid peptide CCK-8. The effect of the CCK-8 can be abolished by the CCK-B receptor antagonist, L365,260. While CCK-8 showed a potent opioid-reversal effect, it by itself exerted a slight inhibitory effect on calcium current. This novel observation in the dorsal root ganglion neurons indicates that CCK-8 can antagonize the kappa-opioid-receptor-mediated depressant effect on voltage-gated calcium current, and this antagonizing effect appears to be mediated via CCK-B receptor.

Homolateral cerebrocortical changes in neuropeptide and receptor expression after minimal cortical infarction

A cortical infarct of 2 mm diameter was obtained in the parietal cortex after a craniotomy, disruption of the dura mater and topical application of 3 M KCl. It has been shown previously that the presence of a small cortical infarct induces an increase in immediate early gene messenger RNA expression followed by an increase in neuropeptide and glutamic acid decarboxylase messenger RNA expression. Glutamate, acting at N-methyl-D-aspartate receptors, is held responsible for these changes, since they are blocked by pretreatment with dizocilpine. In the present study, we have analysed the consequences of the dramatic changes in messenger RNA expression on the level of immediate early gene products c-fos and zif 268, and on that of neuropeptides by using immunohistochemistry. After just 1 h, an increase in c-fos- and zif 268-like immunoreactivity is observed in the entire cortical hemisphere homolateral to the infarct, and is no longer detected after 6 h. An increase in cholecystokinin octapeptide-, substance P-, neuropeptide Y- and somatostatin-like immunoreactivity is observed in the entire cortical hemisphere homolateral to the infarct after three days, and is no longer detected after 30 days. To investigate if these dramatic increases in neuropeptide immunoreactivities may have functional consequences, we studied the level of cholecystokinin receptors by autoradiographic binding using [125I]cholecystokinin-8S and in situ hybridization for the detection of cholecystokinin-b receptor messenger RNA. A decrease in cholecystokinin binding sites and cholecystokinin-b receptor messenger RNA is observed in the entire cortical hemisphere homolateral to the infarct after three days, and is no longer detected after nine days. This study shows that a topical stimulation has diffuse effects, reaching regions far from the site of the lesion, and some of them are still strongly present after nine days. The increase in neuropeptide messenger RNAs is followed by an increase in the protein products of these genes, which may modify the neurotransmission. As a corollary to this, a decrease in cholecystokinin binding sites occurs. This may have further consequences on signal transduction pathways. This decrease in cholecystokinin binding sites is associated with a decrease in the cholecystokinin-b receptor messenger RNA, and this is the first example of a decrease in messenger RNA levels in this experimental model.

Changes in hypothalamic cholecystokininA and cholecystokininB receptor subtypes and associated neuropeptide expression in response to salt-stress in the rat and mouse

This study demonstrates cholecystokinin receptor plasticity in response to salt-loading in the rat and mouse hypothalamus. It identifies, for the first time, the cholecystokinin receptor subtypes involved, firstly by receptor autoradiography and secondly by in situ hybridization. Both species showed increases in hypothalamic [125I]Bolton Hunter-cholecystokinin-8 binding. Co-incubation with the specific cholecystokininA and cholecystokininB antagonists, devazepide and CI-988, indicated that in the rat cholecystokininB receptor binding markedly increased, with a small increase in cholecystokininA receptor binding. In the mouse the response was comprised solely of cholecystokininA receptors. In situ hybridization studies were carried out on a range of peptide messenger ribonucleic acids after salt-loading. In the rat large increases in hypothalamic gene expression were detected for oxytocin, vasopressin, corticotrophin-releasing factor and preprocholecystokinin. In the mouse only vasopressin messenger ribonucleic acid increased, whilst hypothalamic oxytocin, preprocholecystokinin and corticotropin-releasing factor remained unchanged. However, corticotrophin-releasing factor messenger ribonucleic acid increased in the mouse amygdala. In situ hybridization was performed using oligonucleotide probes specific for either the cholecystokininA or cholecystokininB receptor messenger ribonucleic acid, and this showed good agreement with the receptor autoradiography. CholecystokininB receptor expression was upregulated in the rat hypothalamus along with a small but significant increase in cholecystokininA receptors. In the mouse only cholecystokininA receptor expression was increased. In addition to these molecular changes rats lost about 25% of their body weight during six days of salt-challenge, whilst mice continued to grow in line with controls. This work demonstrates differential changes in cholecystokinin receptor subtype binding between the rat and the mouse. It represents the first report of differential changes in cholecystokininA and cholecystokininB receptor messenger ribonucleic acids within the brain, and shows that cholecystokinin receptors within the rodent hypothalamus are capable of plastic responses to chronic osmotic stress.