A small yet comprehensive subset of human experimental pain models emerging from correlation analysis with a clinical quantitative sensory testing protocol in healthy subjects.

BACKGROUND: Picturing the complexity of pain in human experimental settings has increased the predictivity for clinical pain but requires increasingly complex test batteries. This raises problems in studies in which time is objectively limited, for example by the course of action of an analgesic drug. We addressed the selection of a small yet comprehensive set of pain tests for the use in such a situation. METHOD: Nineteen different pain measures from 'classical' pain models (n = 9) and a clinically established QST-pain test battery (n = 10), were obtained from 72 healthy volunteers (34 men). The nonparametric correlation structure among the various pain measures was analysed using Ward clustering. RESULTS: Four clusters emerged, each consisting of highly correlated pain measures. The pain model groups emerged comprised (I) pain thresholds and tolerances to blunt pressure or electrical pain; (II) pain thresholds to thermal stimuli; (III) pain measures obtained following application of punctate mechanical, intranasal CO2 chemical or cutaneous laser heat stimuli; and (IV) detection thresholds to thermal stimuli. The first three clusters agreed with an immediate mechanistic interpretation as reflecting C-fibre mediated pain, thermal pain and Aδ-fibre mediated pain, respectively, whereas the last cluster contained non-painful measures and was disregarded. CONCLUSIONS: When basing a selection of a small comprehensive set of pain models on the assumption that highly correlated pain measures account for redundant results and therefore, one member of each group suffices an economic yet comprehensive pain study, results suggest inclusion of established C-fibre, Aδ-fibre mediated and thermal pain measures.

Differences between D- and L-aspartate binding to the Na+-dependent binding sites on glutamate transporters in frozen sections of rat brain.

The Na+-dependent, "high-affinity" transport of L-glutamate (GluT) in brain tissue has become a significant focus of interest, particularly since it has been revealed that abnormalities of GluT may be associated with serious neurological disorders. Using quantitative autoradiography on 3H-sensitive films, we have studied, in thaw-mounted sections of rat brain, the distribution and pharmacology of radioligand binding to sites with characteristics of the substrate-recognition/binding locus on GluT. The technique makes it possible to determine not only the intensity of binding in brain regions but, with a high level of precision, pharmacological constants such as IC50 or nH. [3H]L-aspartate and [3H]D-aspartate are two classical radioligands used in studies of GluT. We have determined IC50 values for the inhibition of [3H]L- and [3H]D-aspartate binding by their non-radioactive counterparts in the cerebral neocortex. hippocampus, striatum, septal nuclei and the cerebellar cortex. The two radioligands did not appreciably differ from each other in their interactions with the binding sites in the forebrain, consistent with all Na+-dependent GluT binding sites in that region having no stereoselectivity for aspartate enantiomers. In the cerebellar cortex, however, the data indicated the presence of a GluT binding site that preferred L- over D-aspartate. These findings contrast with many previous observations and suggest that the pharmacological characteristics of the ligand binding sites on GluT in the mammalian cerebellar cortex may have to be re-assessed and/or a possibility of an existence of (a) hitherto unknown molecule(s) with properties of a glutamate transporter be considered.

Quantitative autoradiography of Na+-dependent [3H]L-aspartate binding to L-glutamate transporters in rat brain: structure-activity studies using L-trans-pyrrolidine-2,4-dicarboxylate (L-t-PDC) and 2-(carboxycyclopropyl)-glycine (CCG).

Sodium-dependent binding of [3H]L-aspartate was studied in thaw-mounted horizontal sections of fresh-frozen (i.e. not fixed) rat brain. After the incubation with [3H]L-aspartate, the sections were exposed against a 3H-sensitive film and the resulting autoradiograms were evaluated by quantitative densitometry. Effects of several inhibitors were examined and their potency expressed as IC50 and nH. Together with previously published data, the present study supports the view that [3H]L-aspartate binding to fresh-frozen sections of rat brain represents interaction of the radioligand with the substrate-binding sites on glutamate transporters. The most potent inhibitors were (2S,3S,4R)-2-(carboxycyclopropyl)-glycine (L-CCG III) and (2S,4R)-4-methylglutamate. In contrast, L-anti,endo-3,4-methanopyrrolidine dicarboxylate (L-a,e-MPDC) was about an order of magnitude less potent. Only subtle regional variations in the characteristics of inhibitors of [3H]L-aspartate binding were detected. It is not certain whether these differences reflect regional variations in the distribution of individual glutamate transporters or regional peculiarities in their pharmacological characteristics. In particular, (2S,4R)-4-methylglutamate, shown previously to differentiate between GLT-1 (principal glutamate transporter in the forebrain) and GLAST (expressed mainly in the cerebellum), did not strongly differentiate between the binding of [3H]L-aspartate in forebrain and cerebellum. Computer-assisted molecular modelling using selected glutamate analogues with restricted conformation (L-trans-pyrrolidine-2,4-dicarboxylate and four isomers of 2-(carboxycyclopropyl)-glycine: L- and D-CCG I, L-CCG III and L-CCG IV) identified at least one area of unfavourable steric interaction. We conclude that the quantitative autoradiographic studies using [3H]L-aspartate or other transporter-specific ligands, will be a useful tool to study the pharmacology of substrate binding sites on glutamate transporters in the mammalian brain in situ.