Gabor Tuboly

Selective disturbance of pain sensitivity after social isolation.

The effects of social isolation or NMDA-receptor antagonists on pain sensitivity have repeatedly been described. However, the mechanisms underlying the alterations of pain perception in these models still remain a matter of debate. Thus, we aimed to determine the long-lasting effects of subchronic ketamine treatment and social isolation on the C- and Adelta-fiber-mediated nociception. Wistar rats after weaning (21-23 days old) were either housed individually or grouped for 21 days. The animals were treated daily for 14 days with either ketamine (30 mg/kg) or saline. On the 21st day, tail-flick latency was determined at 48 degrees C (C-fiber activation) and 52 degrees C (affects mainly Adelta-fibers), and rats were rehoused. Tail-flick test was repeated 2 and 4 weeks later. On the 5th week, carrageenan-induced heat hyperalgesia was determined on paw-withdrawal test before and after morphine treatment (1, 2 or 3 mg/kg). Regarding tail-flick latencies at 48 degrees C, juvenile isolation, but not ketamine resulted in a significantly enhanced pain threshold (p<0.001) throughout the investigation period, while the changes at 52 degrees C were not significant. In addition, both isolation and ketamine treatments enhanced the antihyperalgesic effect of 2 mg/kg morphine. In summary, juvenile isolation exerts a long-lasting effect on acute heat pain sensitivity, disturbing primarily the C-fiber-linked pain pathways, suggesting a selective disruption in the parallel sensory pathways. Since both social isolation and NMDA treatment are well-known animal models of schizophrenia, our results showed that juvenile isolation but not ketamine administration can simulate hypoalgesia associated with this disease.

The peripheral antinociceptive effects of endomorphin-1 and kynurenic acid in the rat inflamed joint model

BACKGROUND: Several data suggest that both opioid and N-methyl-d-aspartate (NMDA) receptors are localized at the peripheral level, and drugs acting on these receptors may produce antinociception after topical administration; however, the antinociceptive effect of endogenous ligands at these receptors is poorly clarified. Our goal in this study was to determine the antinociceptive potency of the endogenous opioid peptide, endomorphin-1 (EM1), and the endogenous NMDA receptor antagonist, kynurenic acid (KYNA), and their interaction at the peripheral level in the rat inflamed joint model. METHODS: Mechanical hypersensitivity was produced by injection of carrageenan (300 &mgr;g/20 &mgr;L) into the tibiotarsal joint of the right hind leg. The mechanical pain threshold was assessed by von Frey filaments (0.064-110 g). EM1 (30, 100, and 200 &mgr;g), KYNA (30, 100, 200, and 400 &mgr;g), and their combinations in a fixed-dose ratio (1:1) were injected into the inflamed joint, and the pain threshold was determined repeatedly for 75 min after the drug administrations. RESULTS: Neither EM1 nor KYNA administered to the inflamed joint influenced the pain threshold at the noninflamed side. Both ligands produced dose-dependent antihyperalgesia, and the highest doses caused a prolonged effect. EM1 had higher potency (30% effective dose [ED30] and 50% effective dose [ED50] values were 112 &mgr;g [confidence interval {CI}: 80-146] and 167 &mgr;g [CI: 135-220], respectively) compared with KYNA (ED30 and ED50 values were 204 &mgr;g [CI: 160-251] and 330 &mgr;g [CI: 280-407], respectively). The antinociceptive effect of EM1 was prevented by subcutaneous naltrexone pretreatment. The coadministration of EM1 with KYNA caused an enhanced and/or prolonged antinociceptive effect. The ED30 and ED50 values of the combination were 141 &mgr;g [CI: 83-182] and 231 &mgr;g [CI: 190-293], respectively, which did not differ significantly from the theoretically additive values (ED30 and ED50 values were 145 &mgr;g [CI: 68-237] and 220 &mgr;g [CI: 144-230], respectively), thus the interaction between these ligands is additive. None of the treatments caused any sign of side effects. CONCLUSION: Peripherally administered endogenous opioid agonist and NMDA receptor antagonist ligands might be beneficial in inflammatory pain. Because both drugs barely cross the blood-brain barrier, their local administration causes no central side effects.

Characterization of gene–environment interactions by behavioral profiling of selectively bred rats: The effect of NMDA receptor inhibition and social isolation

Gene-environment interactions have an important role in the development of psychiatric disorders. To generate and validate a new substrain of rats with signs related to schizophrenia, we used selective breeding after postweaning social isolation and chronic ketamine treatment through several generations of animals and compared the subsequent strain to naive rats that were not genetically manipulated. We further investigated whether social isolation and ketamine treatment augmented the appearance of schizophrenic-like signs in these rats. Four experimental groups were studied (n=6-15 rats/group): naive rats without any treatment (NaNo); naive rats with postweaning social isolation and ketamine treatment (NaTr); 15th generation of selectively bred animals without any treatment (SelNo) or selectively bred rats with both isolation and ketamine treatment (SelTr). The startle reaction, tail-flick and novel object recognition tests were used to classify the animals into low- or high-risk for schizophrenia. Reduced pain sensitivity, higher degree of the startle reaction, disturbed prepulse inhibition, altered motor activity and decreased differentiation index in the memory test were observed in the 15th generation of the substrain, along with enhanced grooming behavior. Five functional indices (TF latency, startle reaction, prepulse inhibition, differentiation index, and grooming activity) were rated from 0 to 2, and the analysis of the summarized score revealed that the NaNo group had the lowest overall indication of schizophrenic-like signs, while the SelTr animals scored the highest, suggesting that both heritable and environmental factors were important in the generation of the behavioral alterations. We assume that further breeding after this complex treatment may lead to a valid and reliable animal model of schizophrenia.

Somatostatin and cognitive function in neurodegenerative disorders

During the past 40 years, somatostatin (SST) has been a subject of intensive research. Apart from its substantial role in the neuroendocrine system, due to its dense localization in various areas in the brain, its functions as a neuromodulator have also been thoroughly investigated. Increasing evidence suggests that SST plays a crucial role in memory and cognition. Synthetic forms, biologically active peptide sequences, SST receptor agonists and SST depleting agents have been applied in animal models and in human studies of a number of neuropsychiatric disorders. The translation of experimental data into clinical use could provide novel therapies in neurodegenerative disorders involving cognitive dysfunctions. However in view of the controversial data reported concerning the different roles of the SST receptor subtypes, and the lack of SST analogs that are able to cross diffusion barriers and act selectively at these receptor subtypes, broader clinical use of SST analogs as cognitive enhancers is limited. This review covers the whole range of available experimental results relating to the behavioral effects of SST, and highlights the potential for further investigations.

Peripheral antinociceptive effect of 2-arachidonoyl-glycerol and its interaction with endomorphin-1 in arthritic rat ankle joints

1. Both cannabinoid and opioid receptors are localized at the peripheral level, and drugs acting on these receptors may produce antinociception after topical administration; however, the effect of endogenous ligands at these receptors is poorly understood. Our goal was to determine the antinociceptive potency of the endogenous cannabinoid 2‐arachidonoyl‐glycerol (2‐AG), and its interaction with endomorphin‐1 (EM1) at joint level in the rat inflammation model.

Antinociceptive interactions of triple and quadruple combinations of endogenous ligands at the spinal level.

A very interesting and rapidly developing field of pain research is related to the roles of different endogenous ligands. This study determined the antinociceptive interactions of triple and quadruple combinations of different endogenous ligands (endomorphin-1, adenosine, agmatine and kynurenic acid) on carrageenan-induced inflammatory pain model at the spinal level. Intrathecal infusion (60 min) of these drugs alone, in double, triple or quadruple combinations, was followed by a 60-min observation period. During the infusion, antihyperalgesic effect of 0.3 microg/min endomorphin-1 was higher in the triple combinations than those in the double combinations. After cessation of drug administration, only the combination of 0.3 microg/min endomorphin-1, 1 microg/min agmatine, and 0.3 microg/min adenosine was more effective than the double combinations. In quadruple combinations, the antinociceptive effects of both 0.1 and 0.3 microg/min endomorphin-1 were significantly potentiated by the otherwise ineffective triple combination of adenosine, agmatine, and kynurenic acid. No side effects could be observed at these doses. These results demonstrate that triple and quadruple combinations of these endogenous ligands caused more effective antihyperalgesia compared with double combinations. Accordingly, the doses of these substances could be further reduced, thus, reinforcing the view that complex activation and/or inhibition of different systems can be sufficiently effective in blocking nociception without adverse effects. Because all of these drugs had effects on various receptors and systems, the possible types of these interactions were discussed.

Long-lasting, distinct changes in central opioid receptor and urinary bladder functions in models of schizophrenia in rats

Ketamine treatments and social isolation of rats reflect certain features of schizophrenia, among them altered pain sensitivity. To study the underlying mechanisms of these phenomena, rats were either housed individually or grouped for 33 days after weaning, and treated with either ketamine or saline for 14 days. After one month re-socialization, the urinary bladder capacity by ultrasound examination in the anesthetized animals, and changes of μ-opioid receptors by saturation binding experiments using a specific μ-opioid agonist [(3)H]DAMGO were determined. G-protein signaling was investigated in DAMGO-stimulated [(35)S]GTPγS functional assays. Ketamine treatment significantly decreased the bladder volume and isolation decreased the receptor density in cortical membranes. Among all groups, the only change in binding affinity was an increase induced by social isolation in the cortex. G-protein signaling was significantly decreased by either ketamine or social isolation in this tissue. Ketamine treatment, but not housing, significantly increased μ-opioid receptor densities in hippocampal membranes. Both ketamine and isolation increased the efficacy, while the potency of signaling was decreased by any treatment. Ketamine increased the receptor density and G-protein activation; while isolation decreased the efficacy of G-protein signaling in hippocampal membranes. The changes in the co-treated group were similar to those of the isolated animals in most tests. The distinct changes of opioid receptor functioning in different areas of the CNS may, at least partially, explain the augmented nociceptive threshold and morphine potency observed in these animals. Changes in the relative urinary bladder suggest a detrusor hyperreflexia, another sign of schizophrenia.

ANTINOCICEPTIVE INTERACTIONS BETWEEN ANANDAMIDE AND ENDOMORPHIN‐1 AT THE SPINAL LEVEL

1 Although it is well known that the combined administration of synthetic or plant‐originated opioids with cannabinoids (CB) results in synergistic antinociception, the effects of combined administration of endogenous ligands acting at µ‐opioid and CB receptors are not known. The aim of the present study was to determine the interaction between anandamide (AEA; a CB1 receptor agonist) and endomorphin‐1 (EM‐1; a µ‐opioid receptor agonist) after intrathecal administration. 2 Nociception was assessed by the paw‐withdrawal test after carrageenan‐induced inflammation in male Wistar rats. 3 Endomorphin‐1 (16.4 pmol to 16.4 nmol) and AEA (4.3–288 nmol) alone dose‐dependently decreased carrageenan‐induced thermal hyperalgesia, although the highest dose of AEA also exhibited pain‐inducing potential. The potency of AEA was approximately 59‐fold lower than that of EM‐1 (35% effective dose (ED35) 194.4 vs 3.3 nmol, respectively). Coadministration of these ligands revealed that combinations of 16.4 pmol EM‐1 plus 28.8 or 86.5 nmol AEA were more effective than either drug alone, but other combinations were no more effective than the administration of EM‐1 itself. Therefore, coadministration of AEA did not significantly shift the dose–response curve to EM‐1. 4 The results of the present study indicate that the coadministration of AEA and EM‐1 results in potentiated antihyperalgesia only for a combination of specific doses. Because AEA activates other receptor types (e.g. TRPV1) in addition to CB1 receptors, the results of the present suggest that, after the coadministration of EM‐1 and AEA, complex interactions ensue that may lead to different outcomes compared with those seen following the injection of exogenous ligands.

The inimitable kynurenic acid: The roles of different ionotropic receptors in the action of kynurenic acid at a spinal level

Kynurenic acid (KYNA) is a neuroactive metabolite that interacts with NMDA, AMPA/kainate and alpha 7 nicotinic receptors. The goal of this study was to clarify the roles of these receptors in the action of KYNA at a spinal level by using highly specific receptor antagonists alone or in triple combinations. Chronic osteoarthritis-like joint pain was induced with monosodium-iodoacetate in male Wistar rats. Mechanical allodynia and motor function were quantified. In the first series we determined the dose-response and time course effects of intrathecally administered KYNA (10-100 μg), D-(-)-2-amino-5-phosphonopentanoic acid (AP5; an NMDA receptor antagonist; 10-200 μg), methyllycaconitine (MLA; an alpha 7 nicotinic receptor antagonist; 100-200 μg) and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzoquinoxaline-7-sulfonamide (NBQX; an AMPA/kainate receptor antagonist; 1-20 μg). In the second series, four different triple combinations of MLA, AP5 and NBQX were investigated. Intrathecal administration of KYNA caused a dose-dependent motor impairment and antinociception. The highly specific NMDA receptor antagonist AP5 caused a motor impairment and antinociception with lower potency. High doses of NBQX resulted in significant antinociception with a slight motor impairment, while only the highest dose of MLA gave rise to significant antinociception with a slight motor impairment. After the coadministration of these ligands as combinations, no potentiation was observed. It may be supposed that the effects of KYNA are primarily due to the inhibition of NMDA receptors at both glycine and phencyclidine (PCP) binding sites, and not to the interactions at the different ionotropic receptors, but the mechanisms behind its high bio-efficiency are still unknown.

The effects of juvenile capsaicin desensitization in rats: Behavioral impairments

Capsaicin desensitization leads to behavioral changes, some of which are related to schizophrenia, but investigations into these effects have been scarce. The goal of this study was to characterize the consequences of juvenile capsaicin desensitization on different functions: acute and inflammation-induced thermal and mechanical sensitivity, urinary bladder capacity and thermoregulation, and also on the potentially schizophrenia-related impairments in sensory-motor gating, motor activity and cognitive functioning. Male Wistar rats desensitized with increasing doses of subcutaneous capsaicin after weaning were investigated. Heat and mechanical pain sensitivity did not change significantly; however, morphine produced a prolonged decrease in the nociceptive response to inflammation in desensitized animals. Ultrasound examination of the bladder revealed enhanced bladder volume in treated animals. Capsaicin-treated animals had higher body temperature at 22 °C in both dark and light periods, and they also showed prolonged hyperthermia in new environmental circumstances. Warm environment induced a profound impairment of thermoregulation in desensitized animals. The treated animals also showed higher levels of activity during the active phase and at both cool and warm temperatures. The amplitude of the responses to auditory stimuli and prepulse inhibition did not differ between the two groups, but the desensitized animals showed learning impairments in the novel object recognition test. These results suggest that juvenile capsaicin desensitization leads to sustained changes in several functions that may be related to schizophrenia. We propose that capsaicin desensitization, together with other interventions, may lead to an improved chronic animal model of schizophrenia.