Calcitonin gene-related peptide content, basal outflow and electrically-evoked release from monoarthritic rat spinal cord in vitro

In this study, Freund's adjuvant-induced monoarthritis in the rat hind paw was used to induce chronic pain and inflammation. In order to compare the basal outflow, electrically-evoked release and total content of calcitonin gene-related peptide like immunoreactivity (CGRP-LI) with previously reported changes in substance P (SP-LI), the lumbar enlargement of monoarthitic (complete Freund's adjuvant-treated, CFA rat) and control (incomplete Freund's adjuvant-treated, IFA rat) spinal cords were used. During the 4-wk period after injection, neither the basal nor the evoked release of CGRP-LI from CFA cords differed from controls. By contrast, we have previously reported that SP-LI release from CFA rat spinal cords was significantly higher than from controls, 21 days after inoculation with Freund's adjuvant. Electrically-evoked CGRP-LI release from 21-day CFA rat spinal cord slices was not modified by superfusion with a GABAB antagonist, CGP 36742 (100 microM) which could greatly increase SP-LI release. However, the release of both peptides was significantly increased to the same extent in IFA and normal tissue but to a lesser extent in CFA cords, by superfusion with the opioid antagonist naloxone (1 microM). In conclusion, CGRP-LI, unlike SP-LI, did not appear to be susceptible to any changes in the lumbar enlargement of the rat spinal cord during inflammation of the hind paw. In addition, CGRP-LI release was increased by antagonism of opiate but not GABAB receptors, suggesting that during chronic inflammation of one hind paw, the GABAB ergic system, unlike the opioid system, might be activated to selectively inhibit the enhanced SP-LI release but not CGRP-LI release which is not changed.

GABA, glutamate and substance P-like immunoreactivity release: effects of novel GABAB antagonists

1. The effects of various GABA receptor ligands on the electrically-evoked release of endogenous GABA, glutamate and substance P-like immunoreactivity from the dorsal horn of rat isolated spinal cord were examined. 2. Exogenous GABA (10-300 microM) significantly decreased the evoked, but not basal, release of endogenous glutamate in a concentration-dependent manner. The GABAA agonist, isoguvacine (1-100 microM), failed to decrease the release of glutamate although it did reduce the release of GABA. Baclofen (0.1-1000 microM), the GABAB agonist, reduced the release of GABA and glutamate in a stereospecific and concentration-dependent manner. 3. The actions of five GABAB antagonists on these release systems were compared. CGP36742, CGP52432, CGP55845A and CGP57250A significantly increased the evoked release of GABA and glutamate. They also reversed the effects of (-)-baclofen in a concentration-dependent manner. On the other hand, while CGP56999A had no effect on glutamate release, it was an effective antagonist of the baclofen-induced inhibition of GABA and substance P release. 4. These results suggest that GABAB receptors on nerve terminals within the dorsal horn spinal cord may be heterogeneous. However, this is based solely on the data obtained with CGP56999A which affected only GABA and substance P, but not glutamate, release.

Tetanus toxin-induced effects on extracellular amino acid levels in rat hippocampus: an in vivo microdialysis study

Tetanus toxin is a potent neurotoxin that is widely considered to produce its effect through impairment of inhibitory neurotransmission. We report the effect of a single unilateral intrahippocampal injection of tetanus toxin on extracellular levels of neuroactive amino acids in freely moving rats, at times ranging between 1 and 7 days posttreatment. Tetanus toxin treatment did not alter extracellular levels of aspartate, glutamate, and taurine at any time during the study. However, although extracellular GABA levels were unaffected by toxin injection 1, 2, and 3 days after treatment, they were reduced (45 +/- 8% of contralateral vehicle-injected level) at day 7. Challenge with a high K+ concentration, 7 days after treatment, produced elevations in extracellular levels of taurine and GABA in both vehicle- and toxin-injected hippocampi, with evoked levels of GABA being lower in the toxin-treated side (39 +/- 16% of contralateral vehicle-injected level). Aspartate and glutamate levels were not increased by high-K+ infusion. These findings are discussed in relation to the possible role that an imbalance in excitatory/inhibitory tone may play in the production of tetanus toxin-induced neurodegeneration.

The pharmacology of adenylyl cyclase modulation by GABAB receptors in rat brain slices

GABAB receptor activation inhibits forskolin-stimulated adenylyl cyclase activity but augments noradrenaline-stimulated adenylyl cyclase activity. The present study investigated the pharmacology of these two GABAB receptor mediated responses. In a cross-chopped rat cortical slice preparation, it was confirmed that (-)baclofen inhibited forskolin-stimulated adenylyl cyclase activity and augmented noradrenaline-stimulated adenylyl cyclase activity. The potency of five further agonists was investigated (SKF97541, CGP47656, CGP44533, 3-APA and CGP44532). Of these agonists two compounds were significantly more potent as inhibitors of forskolin-stimulated adenylyl cyclase than as augmenters of noradrenaline-stimulated adenylyl cyclase activity, these were (-)baclofen (pEC50 = 6.07 +/- 0.29 and 5.04 +/- 0.17, respectively (p < 0.05)), and CGP47656 (pEC50 = 6.44 +/- 0.05 and 4.48 +/- 0.26, respectively (p < 0.05)). It is possible to explain this difference in potency by proposing that these compounds have low intrinsic efficacy, and the augmentation of noradrenaline-stimulated adenylyl cyclase has a low receptor reserve. In addition six antagonists (CGP49311A, CGP46381, CGP45024, CGP45397, CGP36742) were also tested for their ability to antagonize 10 microM (-)baclofen in these two assays. These antagonists ranged in potency as inhibitors of forskolin-stimulated adenylyl cyclase activity from CGP49311A (pEC50 = 5.45 +/- 0.30) to CGP36742 (pEC50 = 3.87 +/- 0.16). Each antagonist had similar potency in the two assays, suggesting that these two responses are mediated by pharmacologically similar receptors.

Evidence for release of glutamic acid, aspartic acid and substance P but not gamma-aminobutyric acid from primary afferent fibres in rat spinal cord

In vitro superfusion release experiments and autoradiography were carried out on spinal cords of neonatally capsaicin-treated rats. Electrical and chemical stimulations significantly increased the release of aspartate, glutamate and gamma-aminobutyric acid (GABA) from hemisected dorsal horn slices of vehicle-treated animals. In capsaicin-treated rats, the evoked release of aspartate, glutamate and substance P but not GABA, were significantly lower. Capsaicin (1 microM) stimulated the release of aspartate and glutamate, as reported for substance P, in control slices but this effect was not as apparent in tissues from capsaicin-treated rats. Evoked GABA release was not affected in either case. alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate, dizocilpine and GABAB binding sites were highly localised in the substantia gelatinosa. Capsaicin treatment did not affect the affinity of the binding sites in all four cases but significantly reduced the density of kainate, dizocilpine and GABAB binding sites. The data suggest that capsaicin-sensitive primary afferent fibres release aspartate, glutamate and Substance P following high-intensity stimulations and that this release might be modulated by presynaptic glutamate and GABAB receptors present on these terminals.

Effect of interleukin-1 beta on the release of substance P from rat isolated spinal cord

Superfusion of rat spinal cord slices with rat interleukin-1 beta resulted in a significant enhancement of electrically evoked substance P-like immunoreactivity with a maximal effect (> 2-fold increase) at 0.1 ng/ml, whereas higher concentration (10-50 ng/ml) of the cytokine inhibited (approximately 50%) the release of the neuropeptide. Interleukin-1 beta (0.1 ng/ml) potentiation of substance P-like immunoreactivity release was abrogated by co-perfusion with interleukin-1 receptor antagonist (10-100 ng/ml) or with indomethacin (1 microM). Superfusion of spinal cord with interleukin-1 beta inhibited electrically evoked calcitonin gene-related peptide-like immunoreactivity release. Modulation of substance P-like immunoreactivity release from the spinal cord by interleukin-1 beta may represent a mechanism responsible for the hyperalgesic action of the cytokine characteristic of the inflammatory response.

Comparative effects of the GABA uptake inhibitors, tiagabine and NNC-711, on extracellular GABA levels in the rat ventrolateral thalamus

The primary mechanism by which the action of synaptically released GABA is thought to be terminated is by re-uptake into neurones and glial cells, and the pharmacological inhibition of this uptake may be beneficial in conditions where decreased GABAergic transmission has been implicated, such as epilepsy. We have compared the effects of two of these uptake inhibitors, tiagabine and NNC-711, on extracellular GABA levels in the thalamus of the rat, after both systemic and local administration. Both compounds produced dose-dependent increases in GABA concentration irrespective of the route of administration, but the concentrations required to produce increased extracellular GABA levels were considerably higher than those known to be effective for anticonvulsant purposes. These data suggest that, initially at least, alternative GABA transporters, not susceptible to inhibition by the compounds used, may still be able to remove synaptically released GABA from the extracellular space.

Effect of lamotrigine on the electrically-evoked release of endogenous amino acids from slices of dorsal horn of the rat spinal cord

The novel anti-epileptic, lamotrigine (LTG) has been shown to exhibit antinociceptive effects in the rat. In the present study, the effect of LTG on the electrically-evoked release of endogenous amino acids from rat isolated spinal dorsal horn slices with intact dorsal roots has been examined and compared with those of morphine in the same preparation. LTG (0.1-300 microM) inhibited the release of aspartate, glutamate and GABA in a concentration-dependent manner. The lowest concentrations of morphine (0.001-0.01 microM) enhanced the stimulated release of aspartate and glutamate while the higher concentrations inhibited their release. Stimulated GABA release was reduced in a concentration-dependent manner. The anticonvulsant was more potent at inhibiting the release of glutamate (IC50 = 20 microM) than that of GABA (IC50 = 44 microM) supporting the previous suggestion that lamotrigine is a selective inhibitor of glutamate release. This suggests that the reduction in glutamate release could be one of the mechanisms by which lamotrigine exerts its antinociceptive effect.

Extracellular GABA in the ventrolateral thalamus of rats exhibiting spontaneous absence epilepsy: a microdialysis study

There is compelling evidence that excessive GABA-mediated inhibition may underlie the abnormal electrical activity, initiated in the thalamus, associated with epileptic absence seizures. In particular, the GABAB receptor subtype seems to play a critical role, because its antagonists are potent inhibitors of absence seizures, whereas its agonists exacerbate seizure activity. Using a validated rat model of absence epilepsy, we have previously found no evidence of abnormal GABAB receptor density or affinity in thalamic tissue. In the present study, we have used in vivo microdialysis to monitor changes in levels of extracellular GABA and other amino acids in this brain region. We have shown that basal extracellular levels of GABA and, to a lesser extent, taurine are increased when compared with values in nonepileptic controls. However, modifying GABAergic transmission with the GABAB agonist (-)-baclofen (2 mg/kg i.p.), the GABAB antagonist CGP-35348 (200 mg/kg i.p.), or the GABA uptake inhibitor tiagabine (100 microM) did not produce any further alteration in extracellular GABA levels, despite the ability of these compounds to increase (baclofen and tiagabine) or decrease (CGP-35348) seizure activity. These findings suggest that the increased basal GABA levels observed in this animal model are not simply a consequence of seizure activity but may contribute to the initiation of absence seizures.

Possible therapeutic application of GABAB receptor agonists and antagonists

After their discovery within the mammalian periphery in 1981, gamma-aminobutyric acid-B (GABAB) receptors have been characterized also in the central nervous system (CNS). The highest concentrations of GABAB binding sites appear to be in the cerebellum, frontal cortex, and thalamic nuclei, where they are located on pre- and postsynaptic neurons. On activation, the primary effects appear to be membrane hyperpolarization, suppression of transmitter release, and changes in the levels of cyclic nucleotides. GABAB receptors have been implicated in a variety of neurological phenomena and, as a consequence, receptor agonists and antagonists may well have therapeutic potential. This article is an introduction to GABAB receptor pharmacology and reviews the future of the receptor ligands. Particular attention is given to the role of spinal cord GABAB receptors in chronic pain.

Complex changes of GABAA and GABAB receptor binding in the spinal cord dorsal horn following peripheral inflammation or neurectomy

Chronic peripheral inflammation or peripheral neurectomy cause changes in GABA levels and GABA immunoreactivity in the spinal cord dorsal horn. The present study aimed to investigate if such changes are accompanied by alterations in GABA receptor binding. Neurectomy of the sciatic nerve caused an ipsilateral down-regulation of GABAB receptor binding in lamina II of the spinal cord 2-4 weeks after the nerve injury. Since approximately 50% of GABAB receptor binding in that region is located on primary afferent endings, degenerative changes of such endings caused by the nerve lesion can explain the observed reduction. In contrast, GABAA binding was substantially enhanced following neurectomy, which may be due to an up-regulation of the receptors issued by the concomitant decrease of endogenous GABA. In rats bearing unilateral chronic peripheral inflammation induced by intraarticular injection of complete Freund's adjuvant we found a reduction of GABAB binding in the superficial dorsal horn. This effect, which was maximal at 3-4 weeks after adjuvant injection, was attributed to an enhanced release of GABA by spinal interneurons. GABAA receptor binding was not changed in this experimental model. Together, these results suggest that the two receptor types may be located at different loci and are differently affected by variations in sensory input.

Effect of tetanus toxin on basal and evoked release of 5-hydroxytryptamine and dopamine in rat hippocampus in vivo

Tetanus toxin is a potent neurotoxin which produces seizures and usually death in mammals. In the present study, we have investigated the effect of tetanus toxin on extracellular basal and evoked release of 5-hydroxytryptamine (5-HT), dopamine (DA) and their metabolites in vivo, 7 days after toxin injection into the hippocampus of rats. Tetanus toxin decreased both basal and evoked release of 5-HT and DA. The 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) was not significantly decreased in the extracellular space. Of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) concentration was decreased while that of homovanillic acid (HVA) was unchanged. These findings are considered in light of the possible contributory role that a decrease in monoamine concentration in the hippocampus might have in the effects of tetanus toxin.

Chronic (-)baclofen or CGP 36742 alters GABAB receptor sensitivity in rat brain and spinal cord

Administration of the GABAB receptor agonist, (-)-baclofen 10 mg kg-1, i.p. daily for 21 days to rats prevented (-)-baclofen-induced hyperpolarizing responses and synaptically-evoked late inhibitory potentials (IPSPs) in olfactory cortical neurones recorded intracellularly from 450 microns brain slices. In contrast, pre-treatment with CGP 36742 induced a significant increase in (-)-baclofen-mediated post-synaptic responses and late IPSP amplitude. In the spinal cord, the potency of (-)-baclofen in inhibiting electrically-evoked substance P-like immunoreactivity or amino acid release was significantly reduced or increased in slices from rats pre-treated with the GABAB agonist or antagonist, respectively. These data suggest that functional responses to GABAB receptor activation in the mammalian central nervous system can be up- or down-regulated.

Spinal cord SP release and hyperalgesia in monoarthritic rats: involvement of the GABAB receptor system

1. Monoarthritis was induced in Lewis rats by interdermal injection in the left hind paw of a suspension of Mycobacterium tubercolusis in mineral oil (500 micrograms 100 microliters-1). Controls were injected with 100 microliters mineral oil. 2. Withdrawal latencies to thermal stimuli of the inflamed paw, the contralateral and both paws of control rats were measured at daily intervals after injection by the plantar test. 3. After detection of the pain threshold, rat spinal cords were removed and horizontal dorsal slices were mounted in a 3-compartment bath to measure electrically-evoked release of substance P-like immunoreactivity (SP-LI). 4. The inflamed paw of monoarthritic rats exhibited a lower pain threshold to thermal stimuli than the contralateral paw of the same animals and both paws of control rats. Inflamed paw hyperalgesia was maximal two days after injection, and declined gradually between 7 to 21 days with no evidence of excitability of withdrawal reflexes after 28 days. 5. During the 28 days study, monoarthritic rats gained less weight than control rats. 6. Electrical stimulation of the dorsal roots attached to rat isolated spinal cord slices induced a significant increase (174 +/- 18% of basal outflow which was 30.3 fmol 8 ml-1, n = 5) in SP-LI release. 7. One-week after induction of inflammation no differences in the amount of SP-LI released from the spinal cord of incomplete Freund's adjuvant-treated rats (IFA) and Freund's adjuvant-treated rats (CFA) were detected. Two weeks after, CFA spinal cord tended to release more SP-LI than IFA cords and, 21 days after injection, the spinal cord of CFA rats released significantly more peptide than IFA rats (17.8 +/- 2.8 fmol ml-1, n = 12 and 6.9 +/- 3.2 fmol ml-1, n = 9, respectively).8. Twenty-one days after treatment, the evoked release from monoarthritic rat spinal cords was increased by 263 + 42% (n = 3) in the presence of the GABAB receptor antagonist, CGP 36742 (100 micro M)which also significantly potentiated monoarthritis-induced hyperalgesia up to 45 min after injection(100 mgkg-1, i.p.).9. These findings may provide a basis for a novel approach to chronic pain therapy but also an explanation for the lack of analgesia produced by the GABAB agonist, baclofen, in chronic as compared to acute pain.

Effect of the tachykinin NK1 receptor antagonists, RP 67580 and SR 140333, on electrically-evoked substance P release from rat spinal cord

1. The effects of the non-peptide tachykinin NK1 receptor antagonists, RP 67580, SR 140333, CP-96,345 and CP-99,994 have been investigated on electrically-evoked release of substance P-like immunoreactivity (SP-LI) from rat spinal cord slices. 2. RP 67580 (10 nM) and SR 140333 (1 nM), perfused 5 min prior to and during 8 min stimulation of the dorsal roots (20 V, 0.5 ms, 1 Hz), significantly enhanced SP-LI release by 213 +/- 43 (n = 8) and 203 +/- 31 (n = 5) % of control evoked release (187 +/- 16% of basal outflow, n = 22) respectively. Neither compound modified basal outflow of SP-LI (15.3 +/- 2.5 fmol/8 ml, n = 10). 3. RP 67580 (10 nM) did not modify electrically-evoked release of calcitonin gene-related peptide-LI from rat spinal cord slices. 4. CP-96,345 (10 nM) and CP-99,994 (1 and 10 nM) did not alter electrically-evoked SP-LI release; however, they both inhibited release at 1 microM. Inhibition was also induced by 1 microM RP 67580 but not 1 microM SR 140333. 5. The effect of the NK1 receptor agonists, [Sar9 Met (O2)11]SP and [Sar9]SP, could not be tested on SP-LI release due to interference with the substance P radioimmunoassay (RIA). The other NK1 receptor agonists used, GR 73632, [Pro9]SP and septide, which did not interfere with the RIA, increased SP-LI basal outflow by 1807 +/- 713% (n = 3), 1259 +/- 160% (n = 3) and 620 +/- 69% (n = 3) at 10 nM, 1 nM and 1 microM, respectively. At the same concentrations, the three agonists also enhanced electrically evoked SP-LI release by 204 +/- 38% (n = 6), 753 +/- 40% (n = 3) and 504 +/- 97% (n = 3), respectively. The GR 73632 (10 nM)-induced increase in electrically-evoked SP-LI release, was not prevented by SR140333 (100 nM). None of the agonists inhibited SP-LI release at lower concentrations (0.1 nM GR73632; 0.01 and 0.1 nM [Pro9]SP and 1-100 nM septide).6 NKA and NKB, at concentrations up to 10 nM which did not interfere with the RIA, did not modify electrically-evoked release of SP-LI.7 The ability of NKI receptor antagonists to enhance electrically-evoked SP-LI release supports the concept of an NK1 autoreceptor control mechanism at substance P nerve terminals within the dorsal horn of the rat spinal cord.

Increased sensitivity to the antinociceptive activity of (+/-)-baclofen in an animal model of chronic neuropathic, but not chronic inflammatory hyperalgesia

Modulation of sensory afferent inputs to the spinal cord by GABA appears to be an important physiological mechanism and may provide an antinociceptive control system. In the present study we have evaluated the antinociceptive activity of the GABAB receptor agonist, (+/-)-baclofen, in rats with unilateral chronic inflammatory or neuropathic hyperalgesia. (+/-)-Baclofen was antinociceptive in untreated control animals and both animal models. In the neuropathic model the sensitivity to (+/-)-baclofen was significantly increased by 3-fold in the ipsilateral limb. By contrast, in animals with chronic inflammation no difference in sensitivity between ipsilateral and contralateral limbs to (+/-)-baclofen was observed. Receptor autoradiographic analysis in spinal cord sections revealed no increase in the density of GABAB receptor binding sites and no change in receptor affinity in the neuropathic model.

Comparative receptor autoradiography of ex vivo and in vitro [3H]dizocilpine binding in mouse brain after middle cerebral artery occlusion

In the present study the in vitro and ex vivo distributions of [3H]dizocilpine binding sites in mouse brain after middle cerebral artery occlusion (MCA-O) were compared using receptor autoradiography. The distribution patterns of [3H]dizocilpine binding sites obtained in vitro and ex vivo in normal mouse brain were the same with the highest densities occurring in the hippocampus and cerebral cortex. MCA-O had little or no effect on the in vitro binding density for at least 24 hr post-ischaemia. However after 2-3 days binding densities in the region of infarct were significantly reduced compared to the contralateral cerebral cortex. Further reductions occurred after 5-7 days. By contrast ex vivo [3H]dizocilpine binding was reduced in the infarcted area by 78.7 +/- 4% within 2 hr of the ischaemic insult and at all subsequent times binding was reduced by more than 75%. Ex vivo binding after ischaemia was always less than 30% of in vitro binding and this decrease was apparent within 2 hr of the ischaemic insult whereas in vitro binding was maintained at control levels for at least 24 hr. The neuroprotective activity of the NMDA antagonists dizocilpine and CGP 37849 in this model at different times after MCA-O was assessed. The time scale for receptor access following MCA-O is discussed and it is suggested that although the population of NMDA receptors is maintained in the infarct region for some days access to them in vivo may be sufficiently impaired within 2 or 4 hr of ischaemic insult to reduce the neuroprotective activity of NMDA antagonists after this time.

Solubilization and characterization of GABAB receptor binding sites from porcine brain synaptic membranes

1. The characteristics of membrane bound GABAB receptors in pig brain are similar to those in rat brain as judged by in vitro binding experiments and sensitivity to GTP. The rank order of affinity of GABAB receptor ligands was CGP 54626 > GABA approximately (-)-baclofen >> CGP 35348 = CGP 36742 > (+)-baclofen in membranes from both species. 2. For solubilization of GABAB receptors from pig brain, washed membranes were preincubated with 5 mM MgSO4 and subsequently incubated with various detergents. 3-[(3-Cholamidopropyl)dimethyl-ammoniol]-1-propane sulphonate (CHAPS) (0.5%) proved to be the most successful, solubilizing 22.7 +/- 4.7% (mean +/- s.e. mean, n = 6) of GABAB receptors. 3. Binding of [3H]-GABA to GABAB receptors solubilized with 0.5% CHAPS exhibited similar characteristics to the binding at membrane bound receptors since, firstly, the Kd and Bmax values (around 30 nM and 450 fmol mg-1 protein, respectively) were comparable; secondly, stereospecific binding for baclofen was obtained in both forms; thirdly, the affinity for the agonists GABA and (-)-baclofen and the antagonists CGP 35348, CGP 36742 and CGP 54626 were the same; fourthly, comparable sensitivity to Ca2+ (2.5 mM) was observed and finally, a similar sensitivity to GTP was apparent. 4. Saturation experiments performed with the GABAB antagonist, [3H]-CGP 54626, indicated a higher Kd value and a lower Bmax value for solubilized (7.7 +/- 2.6 nM and 1033 +/- 41 fmol mg-1 protein, mean +/- s.e. mean, n = 3) than for membrane bound receptors (1.35 +/- 0.08 nM, 1171 +/- 20 fmol mg-1 protein, n = 3).

Plasticity of GABAB receptor in rat spinal cord detected by autoradiography

Prolonged administration of (+/-)-baclofen induces tolerance to its antinociceptive effect in mice. In the present study we compare the effects of 21 days administration of (-)-baclofen or GABAB (gamma-aminobutyric acidB) receptor antagonists, with saline on the density of GABAB receptor binding sites in rat spinal cord using receptor autoradiography. Treatment with (-)-baclofen significantly reduced the number of silver grains (71%) whilst GABAB receptor antagonists, CGP 36742 (3-aminopropyl-n-butyl-phosphinic acid) and CGP 46381 (3-aminopropyl-cyclohexylmethylphosphinic acid), increased the grain count by 114% and 89%, respectively. These data indicate that GABAB receptors in the spinal cord can be influenced by prolonged administration of receptor ligands.

Repeated administration of desipramine and a GABAB receptor antagonist, CGP 36742, discretely up-regulates GABAB receptor binding sites in rat frontal cortex

1. GABAB receptor binding site densities within laminar regions of the rat frontal cortex were examined autoradiographically following repeated administration (21 days) of the antidepressants desipramine, paroxetine and amitriptyline in addition to the GABAB receptor antagonists, CGP 35348 and CGP 36742. beta 1-Adrenoceptor autoradiography was studied in parallel with that for GABAB receptor sites. 2. The effects of these compounds were examined concomitantly on the GABAB receptor-mediated inhibition of forskolin- and enhancement of noradrenaline-stimulated cyclic AMP production. 3. GABAB receptor binding was increased by both desipramine (20 mg kg-1, p.o. and 10 mg kg-1, i.p.) and CGP 36742 (100 mg kg-1, i.p.) in the outer laminar region of the frontal cortex by around 50% above control levels. Conversely, no significant changes were mediated by paroxetine, amitriptyline, CGP 35348 or the GABAB receptor agonist, baclofen. 4. With the exception of paroxetine, all compounds down-regulated the total beta-adrenoceptor population throughout frontal cortical laminae which was attributable to the beta 1-adrenoceptor subtype. In contrast, the reduction in beta-adrenoceptors mediated by CGP 35348 and CGP 36742 did not occur as a consequence of reduced beta 1-adrenoceptor numbers. 5. Protracted treatment with CGP 35348, failed to influence forskolin-stimulated cyclic AMP production; however, a significant increase in the accumulation of cyclic AMP produced in response to forskolin was seen after treatment with CGP 36742. 6. Such discretely localized changes in GABAB receptor densities induced by desipramine and CGP 36742 may provide an explanation for the discrepancies reported in membrane binding studies and possibly implicate a role for GABAB receptor antagonists in antidepressant therapy.

Gamma-aminobutyric acidB, but not gamma-aminobutyric acidA receptor activation, inhibits electrically evoked substance P-like immunoreactivity release from the rat spinal cord in vitro

Substance P (SP) is believed to be a neuromodulator of primary afferent neurons involved in nociception. Because baclofen alters nociception at the level of the spinal cord and the receptor it activates (gamma-aminobutyric acidB; GABAB) is located on presynaptic terminals, we examined whether this agent and GABA could influence the electrically evoked release of SP from rat spinal cord in vitro. The calcium- and tetrodotoxin-dependent release of SP was inhibited completely by GABA (IC50, 165 +/- 17.8 microM) and (-)-baclofen (IC50, 0.8 +/- 0.2 microM) in a dose-dependent manner. The effect of baclofen was stereospecific, (+)-baclofen being approximately 1000 times weaker then the (-)-isomer. The GABAA agonist, isoguvacine (10-100 microM), did not reduce SP release but, if anything, tended to increase SP release. GABA- and (-)-baclofen-induced inhibition of electrically evoked SP release was antagonized by the GABAB antagonists, CGP 35348 and CGP 36742 (10-100 microM). Bicuculline (300 microM) did not affect GABA-inhibition of SP release. These observations suggest that GABAB receptors are likely to mediate the effect of GABA and baclofen on primary afferent terminals. In view of the presence of GABAB receptors in the dorsal horn of the rat spinal cord on slow conducting primary afferent terminals which contain SP, we suggest that the inhibition of the neuropeptide release may be one mechanism to explain baclofen-induced antinociception within the spinal cord.

GABAB receptor-mediated inhibition of forskolin-stimulated cyclic AMP accumulation in rat spinal cord

GABA (30-1000 microM) and the GABAB agonist, (-)baclofen (10-100 microM), but not (+)baclofen, inhibited forskolin-induced cAMP formation in rat spinal cord slices. In contrast, GABA and (-)baclofen failed to enhance the stimulation of cAMP induced by noradrenaline in the same tissue, even though they both increased the response to noradrenaline in cerebral cortex slices. Neonatal capsaicin treatment, which reduces the density of GABAB binding sites in the cord, did not modify the effect of GABA or (-)baclofen on the forskolin-induced elevation of cAMP. (-)Baclofen and GABA inhibition of forskolin effects were insensitive to the GABAB antagonists CGP 35348 and CGP 36742 in the spinal cord. Since CGP 35348 antagonizes baclofen-induced antinociception, it seems unlikely that this effect stems from any change in cAMP within the spinal cord.

Age-related regional sensitivity to pertussis toxin-mediated reduction in GABAB receptor binding in rat brain

GABAB binding was performed in rat brain membranes incubated with pertussis toxin (PTX; 7-15 micrograms/mg protein) or vehicle during postnatal development. In peripubertal rats, GABAB binding was reduced by PTX in corpus striatum and hippocampus but not in cortex or cerebellum, while in sexually mature adults binding was reduced in all areas except the corpus striatum. These data may indicate regional differences in the postnatal development of GABAB receptor-G protein linkage.

GABAB receptor pharmacology

In conclusion, GABAB receptors appear to be of major importance in synaptic processing within the brain and are present at both post- and presynaptic sites. Their activation can hyperpolarize neurones and diminish neurotransmitter release from presynaptic terminals. We already know that drugs, i.e. baclofen, that mimic this activation are therapeutically useful, although the full significance of their use both inside and outside the brain has yet to be realized. Drugs that interfere with GABAB receptor activation should also prove to be important therapeutic agents. A number of suggestions have been proposed but it will be many years before the potential effects can be consolidated or refuted in humans. Only now are brain-penetrating GABAB antagonists being discovered, due largely to the expertise of the research group at CIBA-Geigy, Basel. The emergence of such compounds makes future studies an exciting prospect. In particular, the discovery that GABAB antagonism can suppress absence seizures in rats has provided an important therapeutic target. It is now just over ten years since we first designated the term GABAB. Since then a wealth of information has been obtained, but perhaps the best is still to come.

Inhibition of [3H]-(+)-MK 801 binding to rat brain sections by CPP and 7-chlorokynurenic acid: an autoradiographic analysis

1. The regional binding of [3H]-(+)-5-methyl-10,11-dihydro-5H-dibenzo (a,d)cyclohepten-5,10-imine maleate ([3H]-(+)-MK 801) to sections of rat brain was measured by an in vitro quantitative autoradiographic technique. A heterogeneous distribution of binding sites was observed. 2. High values of binding were detected in the hippocampal formation and cerebral cortex, while very low binding was found in cerebellum. [3H]-(+)-MK 801 binding was not detectable in white matter tracts or in the brain stem. 3. [3H]-(+)-MK 801 binding was inhibited by increasing concentrations of both 7-chlorokynurenate (1-1000 microM) and ((+)-2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP) (0.1-100 microM). High concentrations of both drugs were able to inhibit completely specific [3H]-(+)-MK 801 binding. 4. IC50 values calculated for both 7-chlorokynurenate and CPP-induced [3H]-(+)-MK 801 binding inhibition were similar in all brain regions analyzed. 5. The inhibitory action of 7-chlorokynurenate and that of CPP on [3H]-(+)-MK 801 binding were reversed by addition of glycine and glutamate respectively. 6. It is concluded that activation of glycine and N-methyl-D-aspartate (NMDA) receptors is obligatory for the binding of [3H]-(+)-MK 801 to occur in all of the brain regions examined in the present study. Furthermore, on the basis of the similar regional sensitivities of [3H]-(+)-MK 801 binding to the inhibitory action of 7-chlorokynurenate and CPP, a single pharmacological classification of the NMDA receptor complex in brain is suggested. The cerebellum was not included in the study due to the very low level of [3H]-(+)-MK 801 binding detected under the experimental conditions used.