Linking photosynthesis and yield reveals a strategy to improve light use efficiency in a climbing bean breeding population

Photosynthesis drives plant physiology, biomass accumulation, and yield. Photosynthetic efficiency, specifically the operating efficiency of PSII (Fq'/Fm'), is highly responsive to actual growth conditions, especially to fluctuating photosynthetic photon fluence rate (PPFR). Under field conditions, plants constantly balance energy uptake to optimize growth. The dynamic regulation complicates the quantification of cumulative photochemical energy uptake based on the intercepted solar energy, its transduction into biomass, and the identification of efficient breeding lines. Here, we show significant effects on biomass related to genetic variation in photosynthetic efficiency of 178 climbing bean (Phaseolus vulgaris L.) lines. Under fluctuating conditions, the Fq'/Fm' was monitored throughout the growing period using hand-held and automated chlorophyll fluorescence phenotyping. The seasonal response of Fq'/Fm' to PPFR (ResponseG:PPFR) achieved significant correlations with biomass and yield, ranging from 0.33 to 0.35 and from 0.22 to 0.31 in two glasshouse and three field trials, respectively. Phenomic yield prediction outperformed genomic predictions for new environments in four trials under different growing conditions. Investigating genetic control over photosynthesis, one single nucleotide polymorphism (Chr09_37766289_13052) on chromosome 9 was significantly associated with ResponseG:PPFR in proximity to a candidate gene controlling chloroplast thylakoid formation. In conclusion, photosynthetic screening facilitates and accelerates selection for high yield potential.

Influence of Increase in Phosphorus Supply on Agronomic, Phenological, and Physiological Performance of Two Common Bean Breeding Lines Grown in Acidic Soil under High Temperature Stress Conditions

Many common bean (Phaseolus vulgaris L.) plants cultivated in areas of the world with acidic soils exhibit difficulties adapting to low phosphorus (P) availability, along with aluminum (Al) toxicity, causing yield loss. The objective of this study was to evaluate the influence of an increase in P supply level on the agronomic, phenological, and physiological performance of two common bean breeding lines grown in acidic soil, with low fertility and under high temperature conditions, in a screenhouse. A randomized complete block (RCB) design was used under a factorial arrangement (five levels of P × 2 genotypes) for a total of 10 treatments with four replications. The factors considered in the experiment were: (i) five P supply levels (kg ha-1): four levels of P0, P15, P30, and P45 through the application of rock phosphate (RP), and one P level supplied through the application of organic matter (PSOM) corresponding to 25 kg P ha-1 (P25); and (ii) two advanced bean lines (BFS 10 and SEF10). Both bean lines were grown under the combined stress conditions of high temperatures (day and night maximum temperatures of 42.5 °C/31.1 °C, respectively) and acidic soil. By increasing the supply of P, a significant effect was found, indicating an increase in the growth and development of different vegetative organs, as well as physiological efficiency in photosynthesis and photosynthate remobilization, which resulted in higher grain yield in both bean lines evaluated (BFS 10 and SEF10). The adaptive responses of the two bean lines were found to be related to phenological adjustments (days to flowering and physiological maturity; stomatal development), as well as to heat dissipation strategies in the form of heat (NPQ) or unregulated energy (qN) that contributed to greater agronomic performance. We found that, to some extent, increased P supply alleviated the negative effects of high temperature on the growth and development of the reproductive organs of bean lines. Both bean lines (BFS 10 and SEF 10) showed adaptive attributes suited to the combined stress conditions of high temperature and acidic soil, and these two lines can serve as useful parents in a bean breeding program to develop multiple stress tolerant cultivars.

Bioactive Compounds and Antioxidant Activity in Seeds of Bred Lines of Common Bean Developed from Interspecific Crosses

Knowledge is limited about the level of bioactive compounds and antioxidant activity of seeds from bred lines of common beans developed from interspecific crosses using four different Phaseolus species (P. vulgaris L., P. coccineus L., P. acutifolius A. Gray. Gray., and P. dumosus). In this study, differences in the nutritional quality of seeds among 112 bean genotypes were evaluated by measuring the levels of phenolic compounds, pigments, antioxidant activity, and sugars. The bean genotypes were grown under high temperatures and acid soil conditions in the Amazon region of Colombia. Five typology groups of bean genotypes were identified based on the level of bioactive compounds and their functional capacity: (1) highly bioactive and functional (HBF); (2) moderately bioactive and functional (MBF); (3) moderate antioxidant content with pigment influence (MACP); (4) moderately antinutritional with limited antioxidant potential (MALAP); and (5) antinutritional, low bioactive, and functional (ALBF). We developed a nutritional quality index (NQI) with values ranging from 0 to 1 based on the nutritional and anti-nutritional balance of each genotype and the higher values of the NQI of a genotype indicating greater nutritional quality. We found three interspecific bred lines (SER 212, SER 213, and RRA 81), with NQI values higher than 0.8. These three lines belong to the typology group of HBF. The superior nutritional quality of these three interspecific bred lines is attributed to a greater level of bioactive compounds and antioxidant capacity. These three bred lines may serve as useful parents to develop nutritionally superior and stress-resilient beans from bean breeding programs. Further research is needed to explore the role of testa color in improving the nutritional quality of seeds of common bean genotypes grown under different climatic conditions.

High Temperature Tolerance in a Novel, High-Quality Phaseolus vulgaris Breeding Line Is Due to Maintenance of Pollen Viability and Successful Germination on the Stigma

The common bean (Phaseolus vulgaris L.) is an important nutritional source globally but is sensitive to high temperatures and thus particularly vulnerable to climate change. Derived from a breeding program at CIAT (Colombia), a heat-tolerant breeding line, named heat-tolerant Andean-type 4 (HTA4), was developed by a series of crosses of parents with a small-bean tepary genotype (Phaseolus acutifolius L.) in their pedigree, which might be the donor of heat stress (HS) tolerance. Importantly, in HTA4, the large, commercially desirable Andean-type beans was restored. To assess underlying tolerance mechanisms, HTA4, together with a heat-sensitive Colombian variety (Calima), was exposed to HS (31 °C/24 °C HS vs. 26 °C/19 °C day/night) under controlled environment conditions. Vegetative growth and photosynthetic performance were not negatively impacted by HS in either genotype, although senescence was delayed in Calima. HS during the reproductive stage caused an increase in pod number in Calima but with few fully developed seeds and many pods aborted and/or abscised. In contrast, HTA4 maintained a similar filled pod number under HS and a higher seed weight per plant. Pollen showed high sterility in Calima, with many non-viable pollen grains (24.9% viability compared to 98.4% in control) with a thicker exine and fewer starch granules under HS. Calima pollen failed to adhere to the stigma and germinate under HS. In HTA4, pollen viability was significantly higher than in Calima (71.1% viability compared to 95.4% under control), and pollen successfully germinated and formed pollen tubes in the style under HS. It is concluded that HTA4 is heat tolerant and maintains a high level of reproductive output due to its ability to produce healthy pollen that is able to adhere to the stigma.

Interspecific common bean population derived from Phaseolus acutifolius using a bridging genotype demonstrate useful adaptation to heat tolerance

Common bean (Phaseolus vulgaris L.) is an important legume crop worldwide and is a major nutrient source in the tropics. Common bean reproductive development is strongly affected by heat stress, particularly overnight temperatures above 20°C. The desert Tepary bean (Phaseolus acutifolius A. Gray) offers a promising source of adaptative genes due to its natural acclimation to arid conditions. Hybridization between both species is challenging, requiring in vitro embryo rescue and multiple backcrossing cycles to restore fertility. This labor-intensive process constrains developing mapping populations necessary for studying heat tolerance. Here we show the development of an interspecific mapping population using a novel technique based on a bridging genotype derived from P. vulgaris, P. Acutifolius and P. parvifolius named VAP1 and is compatible with both common and tepary bean. The population was based on two wild P. acutifolius accessions, repeatedly crossed with Mesoamerican elite common bush bean breeding lines. The population was genotyped through genotyping-by-sequencing and evaluated for heat tolerance by genome-wide association studies. We found that the population harbored 59.8% introgressions from wild tepary, but also genetic regions from Phaseolus parvifolius, a relative represented in some early bridging crosses. We found 27 significative quantitative trait loci, nine located inside tepary introgressed segments exhibiting allelic effects that reduced seed weight, and increased the number of empty pods, seeds per pod, stem production and yield under high temperature conditions. Our results demonstrate that the bridging genotype VAP1 can intercross common bean with tepary bean and positively influence the physiology of derived interspecific lines, which displayed useful variance for heat tolerance.

Impact of Web Blight on Photosynthetic Performance of an Elite Common Bean Line in the Western Amazon Region of Colombia

Disease stress caused by plant pathogens impacts the functioning of the photosynthetic apparatus, and the symptoms caused by the degree of severity of the disease can generally be observed in different plant parts. The accurate assessment of plant symptoms can be used as a proxy indicator for managing disease incidence, estimating yield loss, and developing genotypes with disease resistance. The objective of this work was to determine the response of the photosynthetic apparatus to the increased disease severity caused by web blight Thanatephorus cucumeris (Frank) Donk on the common bean (Phaseolus vulgaris L.) leaves under acidic soil and the humid tropical conditions of the Colombian Amazon. Differences in chlorophyll fluorescence parameters, including F_v/F_m, Y(II), Y(NPQ), Y(NO), ETR, qP, and qN in leaves with different levels of severity of web blight in an elite line (BFS 10) of common bean were evaluated under field conditions. A significant effect of web blight on the photosynthetic apparatus was found. A reduction of up to 50% of energy use dedicated to the photosynthetic machinery was observed, even at the severity scale score of 2 (5% surface incidence). The results from this study indicate that the use of fluorescence imaging not only allows for the quantifying of the impact of web blight on photosynthetic performance, but also for detecting the incidence of disease earlier, before severe symptoms occur on the leaves.

Water Use, Leaf Cooling and Carbon Assimilation Efficiency of Heat Resistant Common Beans Evaluated in Western Amazonia

In our study, we analyzed 30years of climatological data revealing the bean production risks for Western Amazonia. Climatological profiling showed high daytime and nighttime temperatures combined with high relative humidity and low vapor pressure deficit. Our understanding of the target environment allows us to select trait combinations for reaching higher yields in Amazonian acid soils. Our research was conducted using 64 bean lines with different genetic backgrounds. In high temperatures, we identified three water use efficiency typologies in beans based on detailed data analysis on gasometric exchange. Profligate water spenders and not water conservative accessions showed leaf cooling, and effective photosynthate partitioning to seeds, and these attributes were found to be related to higher photosynthetic efficiency. Thus, water spenders and not savers were recognized as heat resistant in acid soil conditions in Western Amazonia. Genotypes such as BFS 10, SEN 52, SER 323, different SEFs (SEF 73, SEF 10, SEF 40, SEF 70), SCR 56, SMR 173, and SMN 99 presented less negative effects of heat stress on yield. These genotypes could be suitable as parental lines for improving dry seed production. The improved knowledge on water-use efficiency typologies can be used for bean crop improvement efforts as well as further studies aimed at a better understanding of the intrinsic mechanisms of heat resistance in legumes.

Adaptation of Interspecific Mesoamerican Common Bean Lines to Acid Soils and High Temperature in the Amazon Region of Colombia

Knowledge of the physiological basis for improved genetic adaptation of common bean (Phaseolus vulgaris L.) lines to acid soils and high temperature conditions in the Amazon region of Colombia is limited. In this study, we evaluated the differences among 41 common bean lines in energy use, leaf cooling, photosynthate partitioning to pod formation and grain filling, and grain yield over two seasons under acid soil and high temperature stress in the Amazon region of Colombia. Common bean lines evaluated included medium and large seeded interspecific lines of Mesoamerican and Andean gene pools with different levels of adaptation to abiotic stress conditions and some lines are improved for iron and zinc (biofortified) concentration in seeds. We found three bean lines (GGR 147, SMG 21 and SMG 12) that were superior in their photosynthetic response, leaf cooling, photosynthate partitioning ability to pod formation and grain filling, resulting in grain yields exceeding 1900 kg ha-1 under acid soil and high temperature stress conditions. The superior photosynthetic performance was attributed to the efficient use of absorbed energy on the electron level in thylakoids, which is mainly oriented to a higher quantum yield of PSII (ΦII), lower energy dissipation in the form of heat (ΦNPQ), high linear electron flow (LEF) and high fraction of PSI centers in open state (PSIopen). We speculate that these photosynthetic and photosynthate partitioning responses of superior bean lines are part of the genetic adaptation to acidic soils and high temperature stress conditions. Among the evaluated bean lines, three lines (GGR 147, SMG 21 and SMG 12) combined the desirable attributes for genetic improvement of stress tolerance and biofortification. These lines can serve as parents to further improve traits (energy use efficiency and multiple stress resistance) that are important for bean production in the Amazon region.

Morphological and Molecular Characterization of Selected Chilean Runner Bean (Phaseolus coccineus L.) Genotypes Shows Moderate Agronomic and Genetic Variability

The runner bean is the third most economically important Phaseolus species, which is cultivated on small-scale agriculture for the production of immature pods or to obtain dry seeds. However, despite the economic importance and agronomic potential of this species, the runner bean has been little studied from the genetic standpoint. Therefore, the main objective of this study was to characterize ten selected lines of runner bean obtained from Central (Santiago) and Southern (Valdivia and Villarica) Chile based on morphological and agronomic traits. In addition, the genetic variability of these lines was determined using 12 Inter-Simple Sequence Repeat (ISSR) markers to evaluate the potential of this germplasm for breeding and commercial purposes. As a result, the lines from Central Chile were characterized, and had a higher number of pods per plant compared to the Southern lines, although the size and weight of their seeds were lower. Moreover, a low level of genetic diversity (He = 0.251) was encountered in this population. Finally, this is one of the first studies that generate relevant and novel information on the morphological, agronomic and genetic characterization of the P. coccineus germplasm present in Chile.

The Resistance of Oilseed Rape Microspore-Derived Embryos to Osmotic Stress Is Associated With the Accumulation of Energy Metabolism Proteins, Redox Homeostasis, Higher Abscisic Acid, and Cytokinin Contents

The present study aims to investigate the response of rapeseed microspore-derived embryos (MDE) to osmotic stress at the proteome level. The PEG-induced osmotic stress was studied in the cotyledonary stage of MDE of two genotypes: Cadeli (D) and Viking (V), previously reported to exhibit contrasting leaf proteome responses under drought. Two-dimensional difference gel electrophoresis (2D-DIGE) revealed 156 representative protein spots that have been selected for MALDI-TOF/TOF analysis. Sixty-three proteins have been successfully identified and divided into eight functional groups. Data are available via ProteomeXchange with identifier PXD024552. Eight selected protein accumulation trends were compared with real-time quantitative PCR (RT-qPCR). Biomass accumulation in treated D was significantly higher (3-fold) than in V, which indicates D is resistant to osmotic stress. Cultivar D displayed resistance strategy by the accumulation of proteins in energy metabolism, redox homeostasis, protein destination, and signaling functional groups, high ABA, and active cytokinins (CKs) contents. In contrast, the V protein profile displayed high requirements of energy and nutrients with a significant number of stress-related proteins and cell structure changes accompanied by quick downregulation of active CKs, as well as salicylic and jasmonic acids. Genes that were suitable for gene-targeting showed significantly higher expression in treated samples and were identified as phospholipase D alpha, peroxiredoxin antioxidant, and lactoylglutathione lyase. The MDE proteome profile has been compared with the leaf proteome evaluated in our previous study. Different mechanisms to cope with osmotic stress were revealed between the genotypes studied. This proteomic study is the first step to validate MDE as a suitable model for follow-up research on the characterization of new crossings and can be used for preselection of resistant genotypes.

SpaTemHTP: A Data Analysis Pipeline for Efficient Processing and Utilization of Temporal High-Throughput Phenotyping Data

The rapid development of phenotyping technologies over the last years gave the opportunity to study plant development over time. The treatment of the massive amount of data collected by high-throughput phenotyping (HTP) platforms is however an important challenge for the plant science community. An important issue is to accurately estimate, over time, the genotypic component of plant phenotype. In outdoor and field-based HTP platforms, phenotype measurements can be substantially affected by data-generation inaccuracies or failures, leading to erroneous or missing data. To solve that problem, we developed an analytical pipeline composed of three modules: detection of outliers, imputation of missing values, and mixed-model genotype adjusted means computation with spatial adjustment. The pipeline was tested on three different traits (3D leaf area, projected leaf area, and plant height), in two crops (chickpea, sorghum), measured during two seasons. Using real-data analyses and simulations, we showed that the sequential application of the three pipeline steps was particularly useful to estimate smooth genotype growth curves from raw data containing a large amount of noise, a situation that is potentially frequent in data generated on outdoor HTP platforms. The procedure we propose can handle up to 50% of missing values. It is also robust to data contamination rates between 20 and 30% of the data. The pipeline was further extended to model the genotype time series data. A change-point analysis allowed the determination of growth phases and the optimal timing where genotypic differences were the largest. The estimated genotypic values were used to cluster the genotypes during the optimal growth phase. Through a two-way analysis of variance (ANOVA), clusters were found to be consistently defined throughout the growth duration. Therefore, we could show, on a wide range of scenarios, that the pipeline facilitated efficient extraction of useful information from outdoor HTP platform data. High-quality plant growth time series data is also provided to support breeding decisions. The R code of the pipeline is available at https://github.com/ICRISAT-GEMS/SpaTemHTP.

Sensitivity of leaflet growth rate to drought predicts yield in common bean (Phaseolus vulgaris)

Although drought limits yield by decreasing photosynthesis and therefore biomass accumulation, biomass is not the strongest predictor of yield under drought in common beans (Phaseolus vulgaris L.). Instead, resource partitioning from pod walls into seeds is a stronger correlate. Our aim was to determine whether growth rates of developing leaflets and pods, as independent indicators of sink strength, predict resource partitioning into seeds. Using 20 field-grown genotypes, we paired biomass, yield, and resource partitioning data with leaflet and pod growth rates under well-watered and droughted conditions. We hypothesised that genotypes with faster growing leaflets and pods under drought would fill seeds better. However, we found that leaflet and pod growth rates did not predict partitioning to seeds; rather, sensitivity of leaflet growth rate to drought was a good predictor of yield reduction. Further, plants with rapidly growing leaves under well-watered conditions were most vulnerable to decreases in leaflet growth rate under drought. This suggests that lines that inherited a conservative growth strategy were better able to maintain yield by allocating resources to seeds. Our findings indicate that inherent sensitivity of leaflet growth rate to drought may be used as a predictor of partitioning and yield in common beans.

Enhanced Leaf Cooling Is a Pathway to Heat Tolerance in Common Bean

Common bean is the most consumed legume in the world and an important source of protein in Latin America, Eastern, and Southern Africa. It is grown in a variety of environments with mean air temperatures of between 14°C and 35°C and is more sensitive to high temperatures than other legumes. As global heating continues, breeding for heat tolerance in common bean is an urgent priority. Transpirational cooling has been shown to be an important mechanism for heat avoidance in many crops, and leaf cooling traits have been used to breed for both drought and heat tolerance. As yet, little is known about the magnitude of leaf cooling in common bean, nor whether this trait is functionally linked to heat tolerance. Accordingly, we explore the extent and genotypic variation of transpirational cooling in common bean. Our results show that leaf cooling is an important heat avoidance mechanism in common bean. On average, leaf temperatures are 5°C cooler than air temperatures, and can range from between 13°C cooler and 2°C warmer. We show that the magnitude of leaf cooling keeps leaf temperatures within a photosynthetically functional range. Heat tolerant genotypes cool more than heat sensitive genotypes and the magnitude of this difference increases at elevated temperatures. Furthermore, we find that differences in leaf cooling are largest at the top of the canopy where determinate bush beans are most sensitive to the impact of high temperatures during the flowering period. Our results suggest that heat tolerant genotypes cool more than heat sensitive genotypes as a result of higher stomatal conductance and enhanced transpirational cooling. We demonstrate that it is possible to accurately simulate the temperature of the leaf by genotype using only air temperature and relative humidity. Our work suggests that greater leaf cooling is a pathway to heat tolerance. Bean breeders can use the difference between air and leaf temperature to screen for genotypes with enhanced capacity for heat avoidance. Once evaluated for a particular target population of environments, breeders can use our model for modeling leaf temperatures by genotype to assess the value of selecting for cooler beans.

Plant Abiotic Stress Proteomics: The Major Factors Determining Alterations in Cellular Proteome

HIGHLIGHTS: Major environmental and genetic factors determining stress-related protein abundance are discussed.Major aspects of protein biological function including protein isoforms and PTMs, cellular localization and protein interactions are discussed.Functional diversity of protein isoforms and PTMs is discussed. Abiotic stresses reveal profound impacts on plant proteomes including alterations in protein relative abundance, cellular localization, post-transcriptional and post-translational modifications (PTMs), protein interactions with other protein partners, and, finally, protein biological functions. The main aim of the present review is to discuss the major factors determining stress-related protein accumulation and their final biological functions. A dynamics of stress response including stress acclimation to altered ambient conditions and recovery after the stress treatment is discussed. The results of proteomic studies aimed at a comparison of stress response in plant genotypes differing in stress adaptability reveal constitutively enhanced levels of several stress-related proteins (protective proteins, chaperones, ROS scavenging- and detoxification-related enzymes) in the tolerant genotypes with respect to the susceptible ones. Tolerant genotypes can efficiently adjust energy metabolism to enhanced needs during stress acclimation. Stress tolerance vs. stress susceptibility are relative terms which can reflect different stress-coping strategies depending on the given stress treatment. The role of differential protein isoforms and PTMs with respect to their biological functions in different physiological constraints (cellular compartments and interacting partners) is discussed. The importance of protein functional studies following high-throughput proteome analyses is presented in a broader context of plant biology. In summary, the manuscript tries to provide an overview of the major factors which have to be considered when interpreting data from proteomic studies on stress-treated plants.

Quantitative analysis of proteome extracted from barley crowns grown under different drought conditions

Barley cultivar Amulet was used to study the quantitative proteome changes through different drought conditions utilizing two-dimensional difference gel electrophoresis (2D-DIGE). Plants were cultivated for 10 days under different drought conditions. To obtain control and differentially drought-treated plants, the soil water content was kept at 65, 35, and 30% of soil water capacity (SWC), respectively. Osmotic potential, water saturation deficit, (13)C discrimination, and dehydrin accumulation were monitored during sampling of the crowns for proteome analysis. Analysis of the 2D-DIGE gels revealed 105 differentially abundant spots; most were differentially abundant between the controls and drought-treated plants, and 25 spots displayed changes between both drought conditions. Seventy-six protein spots were successfully identified by tandem mass spectrometry. The most frequent functional categories of the identified proteins can be put into the groups of: stress-associated proteins, amino acid metabolism, carbohydrate metabolism, as well as DNA and RNA regulation and processing. Their possible role in the response of barley to drought stress is discussed. Our study has shown that under drought conditions barley cv. Amulet decreased its growth and developmental rates, displayed a shift from aerobic to anaerobic metabolism, and exhibited increased levels of several protective proteins. Comparison of the two drought treatments revealed plant acclimation to milder drought (35% SWC); but plant damage under more severe drought treatment (30% SWC). The results obtained revealed that cv. Amulet is sensitive to drought stress. Additionally, four spots revealing a continuous and significant increase with decreasing SWC (UDP-glucose 6-dehydrogenase, glutathione peroxidase, and two non-identified) could be good candidates for testing of their protein phenotyping capacity together with proteins that were significantly distinguished in both drought treatments.

Comparison of the Antinociceptive Profiles of Gabapentin and 3-Methylgabapentin in Rat Models of Acute and Persistent Pain: Implications for Mechanism of Action

The anticonvulsant gabapentin (GBP) has been shown effective for the treatment of neuropathic pain, although its mechanism of action remains unclear. A recent report has suggested that binding to the alpha(2)delta subunit of voltage-gated calcium channels contributes to its antinociceptive effect, based on the stereoselective efficacy of two analogs: (1S,3R)3-methylgabapentin (3-MeGBP) (IC(50) = 42 nM), which is effective in neuropathic pain models; and (1R,3R)3-MeGBP (IC(50) > 10,000 nM), which is ineffective (Field et al., 2000). The present study was designed to further examine the profiles of GBP and 3-MeGBP in rat models of acute and persistent pain. Systemic administration of GBP or (1S,3R)3-MeGBP inhibited tactile allodynia in the spinal nerve ligation model of neuropathic pain, whereas (1R,3R)3-MeGBP was ineffective. The antiallodynic effect of GBP, but not (1S,3R)3-MeGBP, was blocked by i.t. injection of the GABA(B) receptor antagonist [3-[[(3,4-dichlorophenyl)methyl]amino]propyl](diethoxymethyl)phosphinic acid (CGP52432). Systemic GBP or (1S,3R)3-MeGBP also inhibited the second phase of formalin-evoked nociceptive behaviors, whereas (1R,3R)3-MeGBP was ineffective. However, both (1S,3R)3-MeGBP and (1R,3R)3-MeGBP, but not GBP, inhibited first phase behaviors. In the carrageenan model of inflammatory pain, systemic GBP or (1R,3R)3-MeGBP failed to inhibit thermal hyperalgesia, whereas (1S,3R)3-MeGBP had a significant, albeit transient, effect. Systemic (1S,3R)3-MeGBP, but not GBP or (1R,3R)3-MeGBP, also produced an antinociceptive effect in the warm water tail withdrawal test of acute pain. These data demonstrate that GBP and 3-MeGBP display different antinociceptive profiles, suggesting dissimilar mechanisms of antinociceptive action. Thus, the stereoselective efficacy of 3-MeGBP, presumably related to alpha(2)delta binding, likely does not completely account for the mechanism of action of GBP.

Role of metabotropic glutamate receptor subtype 5 (mGluR5) in the maintenance of cold hypersensitivity following a peripheral mononeuropathy in the rat

The present series of experiments were designed to examine the contribution of metabotropic glutamate receptor subtype 5 (mGluR5) to neuropathic pain by determining the effects of the selective mGluR5 antagonist MPEP (2-methyl-6-(phenylethynyl)-pyridine) on neuropathy-induced cold hypersensitivity. Unilateral chronic constriction injury (CCI) to the sciatic nerve in rats produced an increase in the number of hind paw withdrawals from a cold surface (4 +/- 2 degrees C) which was dose-dependently inhibited by systemic (i.p.) injection of MPEP (ID(50) = 11.3 mg/kg). In vivo brain mGluR5 receptor occupancy following systemic (i.p.) MPEP revealed that >90% occupancy is required for behavioral efficacy. Intracerebroventricular (i.c.v.) injection of MPEP dose-dependently inhibited CCI-induced cold hypersensitivity (ID(50) = 123.5 nmol), while microinjection of MPEP directly into the rostral ventromedial medulla (RVM) potently inhibited this hypersensitivity (ID(50) = 1.3 pmol). A role for mGluR5 in the RVM was further supported by the observation that intra-RVM injection of the mGluR5 agonist CHPG (10 nmol; 2-chloro-5-hydroxyphenylglycine) produced cold hypersensitivity in naïve rats that was blocked by pretreatment with intra-RVM MPEP (3 nmol). Intrathecal (500 nmol; i.t.) or intraplantar (300 nmol; i.pl.) injection of MPEP was ineffective in reversing CCI-induced cold hypersensitivity. These results demonstrate that mGluR5 contributes to cold hypersensitivity following peripheral neuropathy exclusively at supraspinal sites in the CNS. Additionally, mGluR5 in the RVM significantly contributes to the maintenance of cold hypersensitivity, likely via activation of descending nociceptive facilitatory systems.

The role of CNS NMDA receptors and nitric oxide in visceral hyperalgesia

The studies summarized here document the role of NMDA receptors and nitric oxide in the lumbosacral spinal cord and rostral ventromedial medulla in the maintenance of visceral hyperalgesia. Experiments were conducted in rats in which drugs were administered into either the lumbosacral intrathecal space or directly into the rostral ventromedial medulla. The visceral stimulus was noxious colorectal distension, administered before and 3 h after intracolonic instillation of either saline or 25% zymosan. The visceromotor response to colonic distension was quantified and found to be significantly enhanced in rats in which the colon had previously been treated with zymosan. Enhanced responses to distension were attenuated dose-dependently by intrathecal administration of the NMDA receptor channel blocker MK-801 and by inhibition of the neuronal isoform of nitric oxide synthase (nNOS). In corresponding studies wherein drugs were administered directly into the rostral ventromedial medulla, NMDA receptor antagonism and NOS inhibition dose-dependently attenuated exaggerated responses to colonic distension. Taken together, these data suggest that zymosan-produced visceral hyperalgesia is influenced both at the level of the spinal cord and rostral ventromedial medulla, and that descending facilitatory influences from the rostral ventromedial medulla are important to the maintenance of visceral hyperalgesia.

Blockade of opioid receptors in rostral ventral medulla prevents antihyperalgesia produced by transcutaneous electrical nerve stimulation (TENS)

Although transcutaneous electrical nerve stimulation (TENS) is used extensively in inflammatory joint conditions such as arthritis, the underlying mechanisms are unclear. This study aims to demonstrate an opiate-mediated activation of descending inhibitory pathways from the rostral ventral medulla (RVM) in the antihyperalgesia produced by low- (4 Hz) or high-frequency (100 Hz) TENS. Paw withdrawal latency to radiant heat, as an index of secondary hyperalgesia, was recorded before and after knee joint inflammation (induced by intra-articular injection of 3% kaolin and carrageenan) and after TENS/no TENS coadministered with naloxone (20 microg/1 microl), naltrindole (5 microg/1 microl), or vehicle (1 microl) microinjected into the RVM. The selectivity of naloxone and naltrindole doses was tested against the mu-opioid receptor agonist [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO) (20 ng, 1 microl) and the delta2-opioid receptor agonist deltorphin (5 microg, 1 microl) in the RVM. Naloxone microinjection into the RVM blocks the antihyperalgesia produced by low frequency (p < 0.001), but not that produced by high-frequency TENS (p > 0.05). In contrast, naltrindole injection into the RVM blocks the antihyperalgesia produced by high-frequency (p < 0.05), but not low-frequency (p > 0.05) TENS. The analgesia produced by DAMGO and deltorphin is selectively blocked by naloxone (p < 0.05) and naltrindole (p < 0.05), respectively. Thus, the dose of naloxone and naltrindole used in the current study blocks mu- and delta-opioid receptors, respectively. Hence, low-frequency and high-frequency TENS produces antihyperalgesia by activation of mu- and delta-opioid receptors, respectively, in the RVM.

Supraspinal contributions to hyperalgesia

Tissue injury is associated with sensitization of nociceptors and subsequent changes in the excitability of central (spinal) neurons, termed central sensitization. Nociceptor sensitization and central sensitization are considered to underlie, respectively, development of primary hyperalgesia and secondary hyperalgesia. Because central sensitization is considered to reflect plasticity at spinal synapses, the spinal cord has been the principal focus of studies of mechanisms of hyperalgesia. Not surprisingly, glutamate, acting at a spinal N-methyl-D-aspartate (NMDA) receptor, has been implicated in development of secondary hyperalgesia associated with somatic, neural, and visceral structures. Downstream of NMDA receptor activation, spinal nitric oxide (NO.), protein kinase C, and other mediators have been implicated in maintaining such hyperalgesia. Accumulating evidence, however, reveals a significant contribution of supraspinal influences to development and maintenance of hyperalgesia. Spinal cord transection prevents development of secondary, but not primary, mechanical and/or thermal hyperalgesia after topical mustard oil application, carrageenan inflammation, or nerve-root ligation. Similarly, inactivation of the rostral ventromedial medulla (RVM) attenuates hyperalgesia and central sensitization in several models of persistent pain. Inhibition of medullary NMDA receptors or NO. generation attenuates somatic and visceral hyperalgesia. In support, topical mustard oil application or colonic inflammation increases expression of NO. synthase in the RVM. These data suggest a prominent role for the RVM in mediating the sensitization of spinal neurons and development of secondary hyperalgesia. Results to date suggest that peripheral injury and persistent input engage spinobulbospinal mechanisms that may be the prepotent contributors to central sensitization and development of secondary hyperalgesia.

Biphasic modulation of visceral nociception by neurotensin in rat rostral ventromedial medulla

A potential role for neurotensin in the rostral ventromedial medulla (RVM) in modulation of visceral nociceptive transmission was examined in this study. Microinjection of neurotensin (3-3000 pmol) into the RVM of awake rats produced a dose-dependent inhibition of the visceromotor response (VMR) to noxious colorectal distension (CRD) that lasted 30 to 120 min. Additionally, intra-RVM injection of neurotensin (300 pmol) significantly reduced the slope of the stimulus-response function to graded CRD (20-80 mm Hg), whereas the greatest dose of neurotensin (3000 pmol) completely inhibited the VMR at all intensities of CRD. General motor function was unaffected after intra-RVM injection of neurotensin (3000 pmol). Intra-RVM injection of lesser doses of neurotensin (0.03-0.30 pmol) resulted an enhancement of the VMR to noxious CRD that had a short duration (18-30 min), and produced a leftward shift of the stimulus-response function to graded CRD without a change in the slope of the function. Additionally, intra-RVM injection of the neurotensin-receptor antagonist SR48692 (0.3-300 fmol) in naive animals produced dose-dependent inhibition of VMR to noxious CRD, whereas a lesser dose (0.03 fmol) enhanced the VMR. These data support a role for neurotensin in the RVM in biphasic modulation of visceral nociception. The results obtained with SR48692 suggest that endogenous neurotensin in the RVM modulates VMR to noxious CRD via a prominent interaction with neurotensin receptors that mediate facilitatory influences and a lesser interaction with neurotensin receptors that mediate masked inhibitory influences.

Descending facilitatory influences from the rostral medial medulla mediate secondary, but not primary hyperalgesia in the rat

Prolonged nociceptive input following peripheral injury results in hyperalgesia (enhanced response to a noxious stimulus), which is thought to occur as a consequence of sensitization of primary afferent nociceptors and enhanced excitability of spinal dorsal horn nociceptive neurons (central sensitization). Since there is often an expansion of hyperalgesia to tissue adjacent, and even distant from the site of injury (secondary hyperalgesia), it is thought that this phenomenon primarily involves mechanisms of central modulation/plasticity. In contrast, hyperalgesia observed at the site of tissue injury (primary hyperalgesia) involves peripheral mechanisms. In the current study, we examined the relative contribution of descending nociceptive facilitatory systems from the rostral medial medulla to enhanced behavioral nociceptive responses in models of primary and secondary hyperalgesia in awake rats. The effect of bilateral rostral medial medulla lesions produced by the soma-selective neurotoxin ibotenic acid was determined in three different models of cutaneous thermal hyperalgesia following peripheral inflammation: (i) intraplantar injection of carrageenan into the hindpaw (model of primary hyperalgesia); (ii) intra-articular injection of carrageenan/kaolin into the knee of the hind leg (model of secondary hyperalgesia); and (iii) topical application of mustard oil to the hind leg (model of secondary hyperalgesia). Compared with sham lesion animals, a bilateral lesion of the rostral medial medulla completely blocked thermal hyperalgesia in the two models of secondary hyperalgesia (intra-articular carrageenan/kaolin injection into the knee and topical mustard oil application to the hind leg), but was ineffective in blocking facilitation of the thermal paw withdrawal response in the model of primary hyperalgesia (intraplantar carrageenan injection into the hindpaw). These results suggest that primary and secondary hyperalgesia are differentially modulated in the CNS, and support the notion that descending nociceptive facilitatory influences from the rostral medial medulla significantly contribute to secondary, but not primary, hyperalgesia.

Central mechanisms in pain

Nociceptive input into the central nervous system is not simply passively received but rather is subject to modulation through spinal cord neuroplasticity and descending influences from supraspinal sites activated by a variety of environmental signals, including the acute or persistent nociceptive input itself and behavioral and emotional stimuli. The significant role of NMDA receptors and production of NO. in central sensitization, hyperalgesia, and chronic pain has been demonstrated in numerous models of peripheral injury. It has been shown that persistent nociceptive input is also subject to centrifugal descending modulation through activation of both prominent facilitatory and masked inhibitory influences from supraspinal sites (e.g., RVM) likely involving a spino-bulbar-spinal loop. These descending modulatory influences from the RVM appear to contribute selectively to hyperalgesia observed in uninjured tissue, distant from the site of insult (secondary hyperalgesia), and involve mechanisms similar to those found in the spinal cord (i.e., NMDA receptors and production of NO.). The significant role that modulatory influences in the central nervous system have in the development and maintenance of chronic pain and hyperalgesia clearly supports continued investigation into the physiologic mechanisms contributing to these events.

Involvement of excitatory amino acid receptors and nitric oxide in the rostral ventromedial medulla in modulating secondary hyperalgesia produced by mustard oil

We have recently reported a model of secondary hyperalgesia in which facilitation of the thermal nociceptive tail-flick reflex following topical mustard oil is largely dependent on descending influences from the rostral ventromedial medulla (RVM). The current study was designed to examine a potential role for excitatory amino acid receptors and nitric oxide in the RVM in modulating this hyperalgesia. Topical application of mustard oil (100%) to the lateral surface of the hind leg of awake rats produced a short-lived (60 min) facilitation of the tail-flick reflex that was dose-dependently attenuated by microinjection of the selective N-methyl-D-aspartate (NMDA) receptor antagonist APV (1-100 fmol) into the RVM. Microinjection of a greater dose of APV (1000 fmol) into the RVM produced a significant inhibition of the tail-flick reflex in the presence, but not absence, of mustard oil. In contrast, microinjection of the non-NMDA receptor antagonist DNQX (10 nmol) into the RVM further enhanced the magnitude and duration of the hyperalgesic response, and produced a facilitation of the tail-flick reflex following injection into the RVM of naive animals. Similar to APV, microinjection of the nitric oxide synthase inhibitor L-NAME (100-1000 nmol) into the RVM attenuated mustard oil hyperalgesia, while the greatest dose (1000 nmol) produced a significant inhibition of the tail-flick reflex in the presence, but not absence, of mustard oil. A role for nitric oxide synthase in the RVM in mustard oil hyperalgesia was further demonstrated by a significant increase in the number of NADPH-d labeled cells in the RVM at the time of maximal hyperalgesia. Involvement of NMDA receptors and nitric oxide in the RVM in descending nociceptive facilitation was supported by the observation that microinjection of either NMDA or the NO* donor GEA 5024 into the RVM of naive animals dose-dependently facilitated the tail-flick reflex. The hyperalgesia produced by NMDA injection into the RVM was blocked by prior intra-RVM injection of either APV or L-NAME. These results support the notion that secondary hyperalgesia produced by mustard oil involves concurrent activation of dominant descending facilitatory, as well as masked inhibitory systems from the RVM. Additionally, the data suggest that descending facilitation involves activation of NMDA receptors and production NO* in the RVM, whereas inhibition involves activation of non-NMDA receptors in the RVM.

Characterization of biphasic modulation of spinal nociceptive transmission by neurotensin in the rat rostral ventromedial medulla

Modulation of spinal nociceptive transmission by neurotensin microinjected in the rostral ventromedial medulla (RVM) was examined in anesthetized, paralyzed rats. Forty-three spinal dorsal horn neurons in the L3-L5 spinal segments responding to mechanical and noxious thermal stimulation (50 degrees C) of the plantar surface of the ipsilateral hind foot were studied. Spinal units were classified as either wide dynamic range or nociceptive specific and were located in spinal laminae I-V. Microinjection of neurotensin (0.03 pmol/0.2 microl) into the RVM produced a significant facilitation (135% of control) of spinal unit responses to noxious thermal stimulation (50 degrees C) that lasted approximately 12 min. In contrast, injection of greater doses of neurotensin (300 or 3,000 pmol) produced an inhibition of spinal unit responses to noxious heat (51.7 and 10.6% of control, respectively) that had a longer duration (60-120 min). The effects of neurotensin on wide-dynamic-range and nociceptive-specific neuron responses to noxious heat were qualitatively and quantitatively similar. Spinal unit responses to graded heating of the skin (42-50 degrees C) were completely inhibited after microinjection of 3,000 pmol of neurotensin into the RVM. Injection of a lesser dose of neurotensin (300 pmol), however, resulted in a partial inhibition of spinal unit responses and significantly reduced the slope of the stimulus-response function to graded heating of the skin. Transection of either the ipsilateral or contralateral dorsolateral funiculus (DLF) significantly reduced the inhibition of spinal nociceptive transmission produced by neurotensin (3,000 pmol) in the RVM, whereas bilateral transection of the DLFs completely blocked the effect. In contrast, bilateral transection of the DLFs had no effect on facilitation of spinal nociception by neurotensin (0.03 pmol) in the RVM. The inhibition of spinal nociceptive transmission by neurotensin (3,000 pmol) in the RVM was completely blocked by injection of the nonpeptide neurotensin receptor antagonist SR48692 (30 fmol) into the RVM 10 min before neurotensin. To confirm a specific block of neurotensin-receptor-mediated effects by the antagonist, a subsequent injection of L-glutamate into the RVM was performed. L-Glutamate (100 nmol) was found to inhibit the nociceptive responses of those spinal units whose responses were no longer inhibited by neurotensin. In contrast, injection of SR48692 (30 fmol) into the RVM failed to block the facilitation of spinal unit responses to noxious heat produced by a subsequent injection of neurotensin (0.03 pmol) into the same site. The present series of experiments demonstrate a specific role for neurotensin in the RVM in the modulation of spinal nociceptive transmission, because the peptide was found to both facilitate and inhibit spinal neuron responses to noxious thermal stimulation. Additionally, the facilitatory and inhibitory effects of neurotensin appear to occur via interaction with multiple neurotensin receptors in the RVM that activate independent systems that descend in the ventrolateral funiculi and DLFs, respectively. The results from these experiments are consistent with prior studies demonstrating that the RVM both facilitates and inhibits spinal nociceptive transmission, and they complement previous work showing that neurotensin in the RVM modulates spinal nociceptive behavioral responses.

Dose-dependent pain-facilitatory and -inhibitory actions of neurotensin are revealed by SR 48692, a nonpeptide neurotensin antagonist: influence on the antinociceptive effect of morphine

Neurotensin has bipolar (facilitatory and inhibitory) effects on pain modulation that may physiologically exist in homeostasis. Facilitation predominates at low (picomolar) doses of neurotensin injected into the rostroventral medial medulla (RVM), whereas higher doses (nanomolar) produce antinociception. SR 48692, a neurotensin receptor antagonist, discriminates between receptors mediating these responses. Consistent with its promotion of pain facilitation, the minimal antinociceptive responses to a 30-pmol dose of neurotensin microinjected into the RVM were markedly enhanced by prior injection of SR 48692 into the site (detected using the tail-flick test in awake rats). SR 48692 had a triphasic effect on the antinociception from a 10-nmol dose of neurotensin. Antinociception was attenuated by femtomolar doses, attenuation was reversed by low picomolar doses (corresponded to those blocking the pain-facilitatory effect of neurotensin) and the response was again blocked, but incompletely, by higher doses. The existence of multiple neurotensin receptor subtypes may explain these data. Physiologically, pain facilitation appears to be a prominent role for neurotensin because the microinjection of SR 48692 alone causes some antinociception. Furthermore, pain-facilitatory (i.e., antianalgesic) neurotensin mechanisms dominate in the pharmacology of opioids; the response to morphine administered either into the PAG or systemically was potentiated only by the RVM or systemic injection of SR 48692. On the other hand, reversal of the enhancement of antinociception occurred under certain circumstances with SR 48692, particularly after its systemic administration.

Participation of central descending nociceptive facilitatory systems in secondary hyperalgesia produced by mustard oil

The present series of experiments were designed to examine a potential role for central descending pain facilitatory systems in mediating secondary hyperalgesia produced by topical application of mustard oil and measuring the nociceptive tail-flick reflex in awake rats. Topical application of mustard oil (100%) to the lateral surface of the hind leg produced a facilitation of the tail-flick reflex that was significantly reduced in spinal transected animals. Mustard oil hyperalgesia was also inhibited in animals that had received electrolytic lesions in the rostral ventromedial medulla (RVM). Intrathecal (i.t.) administration of the non-selective cholecystokinin (CCK) receptor antagonist proglumide (10 micrograms) prior to mustard oil application completely blocked both the lesser and greater hyperalgesic responses observed in spinal transected and normal animals, respectively, and produced an inhibition of the tail-flick reflex in normal animals. Administration of the selective CCKB receptor antagonist L-365260 i.t. dose-dependently inhibited mustard oil hyperalgesia (ID50 = 364 ng) at doses approximately 5-fold less than the CCKA receptor antagonist devazepide (ID50 = 1760 ng). Similar to spinal proglumide, microinjection of the neurotensin antagonist SR48692 (3.5 micrograms) into the RVM blocked mustard oil hyperalgesia and inhibited the tail-flick reflex. These data suggest that secondary hyperalgesia produced by mustard oil is mediated largely by a central, centrifugal descending pain facilitatory system which involves neurotensin in the RVM and spinal CCK (via CCKB receptors). The inhibition of the tail-flick reflex produced by mustard oil following spinal or supraspinal administration of receptor antagonists suggests concurrent activation of central descending facilitatory and inhibitory systems.

Involvement of spinal cholecystokininB receptors in mediating neurotensin hyperalgesia from the medullary nucleus raphe magnus in the rat

Neurotensin microinjection into the medullary nucleus raphe magnus (RMg) has been shown to both inhibit and facilitate the spinal nociceptive tail-flick reflex in a dose-dependent manner. Our study was designed to determine a potential involvement of spinal cholecystokinin octapeptide (CCK) in mediating neurotensin hyperalgesia from the RMg. Microinjection of neurotensin (50 ng) into the RMg of awake rats produced a facilitation of the tail-flick reflex that was completely inhibited by intrathecal (i.t.) administration of the nonselective CCK receptor antagonist proglumide (100 ng). Conversely, injection of a greater dose of neurotensin (5 micrograms) into the RMg produced an inhibition of the tail-flick reflex that was enhanced by i.t. proglumide. Intrathecal administration of the selective CCKB receptor antagonist L-365260 dose-dependently inhibited neurotensin hyperalgesia from the RMg (ID50 = 0.42 ng) at doses approximately 1000-fold less than that observed with the selective CCKA receptor antagonist devazepide (ID50 = 646 ng). Injection of CCK alone i.t. produced a biphasic response on the tail-flick reflex as lesser doses (0.1-0.3 ng) inhibited the reflex although greater doses (30-100 ng) facilitated it. Similar to supraspinal neurotensin hyperalgesia, the hyperalgesia observed with i.t. CCK (30 ng) was inhibited by i.t. L-365260 (ID50 = 0.59 ng) at doses approximately 1000-fold less than that observed with i.t. devazepide (ID50 = 630 ng). These data indicate that spinal CCK can both inhibit and facilitate spinal nociceptive responses. The facilitation of nociception observed with spinal CCK appears to involve CCKB receptors, which is consistent with the data in our study suggesting that spinal CCKB receptors mediate neurotensin hyperalgesia from the RMg via descending neuronal projections.

Nuclei within the rostral ventromedial medulla mediating morphine antinociception from the periaqueductal gray

The relative contributions of nuclei within the rostral ventromedial medulla (RVM) involved in mediating morphine induced antinociception from the periaqueductal gray (PAG) were examined. Lidocaine injections (4%) at the time of morphine's maximal response were used to provide a localized neural block and were administered in the nucleus raphe magnus/reticularis gigantocellularis pars alpha (RMg/GiA; commonly referred to as RMg), reticularis gigantocellularis (Gi) and reticularis paragigantocellularis lateralis (LPGi). Microinjection of morphine (6 nmol; 0.5 microliter) into the PAG of awake rats produced an inhibition of the tail-flick reflex that was maximal after 30 min. This response was unaffected by a single medial lidocaine injection (0.5 microliter) into the RMg/GiA or Gi, bilateral injections into the Gi or LPGi or triple injections that included both the RMg/GiA and LPGi. A partial, non-significant block of morphine's response was observed either by bilateral injections (0.5 microliter) into both the Gi and LPGi (% inhibition = 16.4 +/- 24.8) or by bilateral injections in the LPGi and a single medial injection into the Gi (% inhibition = 41.5 +/- 29.8). However, injection of a greater volume of lidocaine (1 microliter) into the RMg/GiA or bilaterally into the LPGi affected adjacent medial and lateral tissue, and completely inhibited morphine's response. Furthermore, triple injections of lidocaine (0.5 microliter) into the Gi or bilateral injections (0.5 microliter) into the Gi and a single medial injection into the RMg/GiA completely blocked morphine's antinociceptive response. These results indicate that morphine antinociception from the PAG is mediated by a large volume of tissue in the RVM containing nuclei located both medially and laterally. Additionally, the principal nuclei involved in this response appear to be the Gi and RMg/GiA.

Localization of the antinociceptive and antianalgesic effects of neurotensin within the rostral ventromedial medulla

Triple microinjections of neurotensin (10 nmol each), which occupied a large volume of tissue within the nucleus reticularis gigantocellularis (Gi), produced an inhibition of the tail-flick reflex in awake rats. This effect was less than that previously observed by a single injection (10 nmol) into the nucleus raphe magnus (RMg) (see ref. [25]). Bilateral injections of neurotensin (10 nmol each) into the nucleus reticularis paragigantocellularis lateralis (LPGi) had no effect. The neurotensin antagonist [D-Trp11]-neurotensin (3 pmol) was previously found to enhance morphine, but not beta-endorphin antinociception from the periaqueductal gray (PAG) when injected into the RMg. A similar enhancement of morphine, but not beta-endorphin antinociception from the PAG was observed in the current study by [D-Trp11]-neurotensin injections into the bilateral LPGi, bilateral Gi, or medial Gi. These data suggest that neurotensinergic projections from the PAG function in an antianalgesic manner throughout the RVM during morphine, but not beta-endorphin antinociception. The antinociceptive effect of neurotensin, on the other hand, is more localized.

Role of neurotensin in the nucleus raphe magnus in opioid-induced antinociception from the periaqueductal gray

These studies examined the role of the neurotensinergic projections extending from the periaqueductal gray (PAG) to the nucleus raphe magnus (NRM) on the inhibition of the tail-flick reflex produced by microinjection of morphine or beta-endorphin in the PAG. Neurotensin (3-30 nmol) or the partial agonist [D-Trp11] neurotensin (100 and 300 pmol) microinjected into the NRM of awake rats produced a dose-dependent inhibition of the tail-flick response lasting 90 to 150 min. Lower doses of neurotensin (0.03-0.3 nmol) produced a hyperreflexive tail-flick response 10 min after injection, which correlated with a decreased hot plate latency. Additionally, a dose of [D-Trp11]neurotensin (3 pmol) that had no intrinsic activity antagonized both the antinociceptive as well as hyperreflexive responses of neurotensin. Morphine (6 nmol) injected into the PAG produced an inhibition of the tail-flick response that was enhanced by injection of [D-Trp11]neurotensin (3 pmol) into the NRM. In contrast, injection of [D-Trp11]neurotensin (3 pmol) into the NRM had no effect on the inhibition of the tail-flick produced by beta-endorphin (10 nmol) in the PAG. Antineurotensin antiserum yielded results similar to those obtained with [D-Trp11]neurotensin. Although neurotensin was found to produce changes in tail skin temperature, it was possible to dissociate these effects from changes in tail-flick latency. These data suggest that neurotensin produces both antinociceptive and hyperalgesic responses when injected into the NRM.(ABSTRACT TRUNCATED AT 250 WORDS)