GABA uptake inhibitors produce a greater antinociceptive response in the mouse tail-immersion assay than other types of GABAergic drugs

Antinociceptive effect of intrathecal P7C3 via GABA in a rat model of inflammatory pain

The recently identified molecule P7C3 has been highlighted in the field of pain research. We examined the effect of intrathecal P7C3 in tissue injury pain evoked by formalin injection and determined the role of the GABA system in the activity of P7C3 at the spinal level. Male Sprague-Dawley rats with intrathecal catheters implanted for experimental drug delivery were studied. The effects of intrathecal P7C3 and nicotinamide phosphoribosyltransferase (NAMPT) administered 10 min before the formalin injection were examined. Animals were pretreated with bicuculline, a GABA-A receptor antagonist; saclofen, a GABA-B receptor antagonist; L-allylglycine, a glutamic acid decarboxylase (GAD) blocker; and CHS 828, an NAMPT inhibitor; to observe involvement in the effects of P7C3. The effects of P7C3 alone and the mixture of P7C3 with GABA receptor antagonists on KCl-induced calcium transients were examined in rat dorsal root ganglion (DRG) neurons. The expression of GAD and the concentration of GABA in the spinal cord were evaluated. Intrathecal P7C3 and NAMPT produced an antinociceptive effect in the formalin test. Intrathecal bicuculline, saclofen, L-allylglycine, and CHS 828 reversed the antinociception of P7C3 in both phases. P7C3 decreased the KCl-induced calcium transients in DRG neurons. Both bicuculline and saclofen reversed the blocking effect of P7C3. The levels of GAD expression and GABA concentration decreased after formalin injection and were increased by P7C3. These results suggest that P7C3 increases GAD activity and then increases the GABA concentration in the spinal cord, which in turn may act on GABA receptors causing the antinociceptive effect against pain evoked by formalin injection.

GABA pharmacology: the search for analgesics

Decades of research have been devoted to defining the role of GABAergic transmission in nociceptive processing. Much of this work was performed using rigid, orthosteric GABA analogs created by Povl Krogsgaard-Larsen and his associates. A relationship between GABA and pain is suggested by the anatomical distribution of GABA receptors and the ability of some GABA agonists to alter nociceptive responsiveness. Outlined in this report are data supporting this proposition, with particular emphasis on the anatomical localization and function of GABA-containing neurons and the molecular and pharmacological properties of GABAA and GABAB receptor subtypes. Reference is made to changes in overall GABAergic tone, GABA receptor expression and activity as a function of the duration and intensity of a painful stimulus or exposure to GABAergic agents. Evidence is presented that the plasticity of this receptor system may be responsible for the variability in the antinociceptive effectiveness of compounds that influence GABA transmission. These findings demonstrate that at least some types of persistent pain are associated with a regionally selective decline in GABAergic tone, highlighting the need for agents that enhance GABA activity in the affected regions without compromising GABA function over the long-term. As subtype selective positive allosteric modulators may accomplish these goals, such compounds might represent a new class of analgesic drugs.

Sex-specific differences in GABA(A) -benzodiazepine receptor availability: relationship with sensitivity to pain and tobacco smoking craving

Sex differences exist in tobacco smoking behaviors. Nicotine, the primary addictive ingredient in tobacco smoke, indirectly affects γ-amino butyric acid (GABA) function. Previous studies reported sex-by-smoking interactions in brain GABA levels. The goal of the present study was to evaluate if there is a sex-by-smoking interaction at the GABA(A)-benzodiazepine receptors (GABA(A)-BZRs), as well as relationships between GABA(A)-BZR availability and behavioral variables before and after 1 week of smoking cessation. Twenty-six women (8 non-smokers, age 36.0 ± 13.4 years; 19 smokers, age 34.6 ± 8.9 years) and 25 men (8 non-smokers, age 37.9 ± 13.8 years; 17 smokers, 34.1 ± 12.4 years) were imaged using [123I]iomazenil and single-photon emission computed tomography. Smokers were imaged at baseline 7 hours after the last cigarette. A significantly great number of men were able to abstain from smoking for 1 week (P = 0.003). There were no significant differences in nicotine dependence and cigarette craving, mood or pain sensitivity between male and female smokers. There was a significant effect of gender across all brain regions (frontal, parietal, anterior cingulate, temporal and occipital cortices, and cerebellum; P < 0.05), with all women (smokers and non-smokers combined) having a higher GABA(A)-BZR availability than all men. There was a negative correlation between GABA(A)-BZR availability and craving (P ≤ 0.02) and pain sensitivity (P = 0.04) in female smokers but not male smokers. This study provides further evidence of a sex-specific regulation of GABA(A)-BZR availability in humans and demonstrates the potential for GABA(A)-BZRs to mediate tobacco smoking craving and pain symptoms differentially in female and male smokers.

GABAA-benzodiazepine receptor availability in smokers and nonsmokers: relationship to subsyndromal anxiety and depression

Many smokers experience subsyndromal anxiety symptoms while smoking and during acute abstinence, which may contribute to relapse. We hypothesized that cortical gamma aminobutyric acid(A)-benzodiazepine receptor (GABA(A)-BZR) availability in smokers and nonsmokers might be related to the expression of subsyndromal anxiety, depressive, and pain symptoms. Cortical GABA(A)-BZRs were imaged in 15 smokers (8 men and 7 women), and 15 healthy age and sex-matched nonsmokers, and 4 abstinent tobacco smokers (3 men; 1 woman) using [(123)I]iomazenil and single photon emission computed tomography (SPECT). Anxiety and depressive symptoms were measured using the Spielberger's State-Trait Anxiety Index (STAI) and the Center for Epidemiology Scale for Depressive Symptoms (CES-D). The cold pressor task was administered to assess pain tolerance and sensitivity. The relationship between cortical GABA(A)-BZR availability, smoking status, and subsyndromal depression and anxiety symptoms, as well as pain tolerance and sensitivity, were evaluated. Surprisingly, there were no statistically significant differences in overall GABA(A)-BZR availability between smokers and nonsmokers or between active and abstinent smokers; however, cortical GABA(A)-BZR availability negatively correlated with subsyndromal state anxiety symptoms in nonsmokers but not in smokers. In nonsmokers, the correlation was seen across many brain areas with state anxiety [parietal (r = -0.47, P = 0.03), frontal (r = -0.46, P = 0.03), anterior cingulate (r = -0.47, P = 0.04), temporal (r = -0.47, P = 0.03), occipital (r = -0.43, P = 0.05) cortices, and cerebellum (r = -0.46, P = 0.04)], trait anxiety [parietal (r = -0.72, P = 0.02), frontal (r = -0.72, P = 0.02), and occipital (r = -0.65, P = 0.04) cortices] and depressive symptoms [parietal (r = -0.68; P = 0.02), frontal (r = -0.65; P = 0.03), anterior cingulate (r = -0.61; P = 0.04), and temporal (r = -0.66; P = 0.02) cortices]. The finding that a similar relationship between GABA(A)-BZR availability and anxiety symptoms was not observed in smokers suggests that there is a difference in GABA(A)-BZR function, but not number, in smokers. Thus, while subsyndromal anxiety and depressive symptoms in nonsmokers may be determined in part by GABA(A)-BZR availability, smoking disrupts this relationship. Aberrant regulation of GABA(A)-BZR function in vulnerable smokers may explain why some smokers experience subsyndromal anxiety and depression.

In vivo characterization of the effects of human hemokinin-1 and human hemokinin-1(4-11), mammalian tachykinin peptides, on the modulation of pain in mice

Human hemokinin-1 (h HK-1) and its truncated form h HK-1(4-11) are mammalian tachykinin peptides encoded by the recently identified TAC4 gene in human, and the biological functions of these peptides have not been well investigated. In the present study, an attempt has been made to investigate the effects and mechanisms of action of h HK-1 and h HK-1(4-11) in pain modulation at the supraspinal level in mice using the tail immersion test. Intracerebroventricular (i.c.v.) administration of h HK-1 (0.3, 1, 3 and 6 nmol/mouse) produced a dose- and time-related antinociceptive effect. This effect was significantly antagonized by the NK(1) receptor antagonist SR140333, but not by the NK(2) receptor antagonist SR48968, indicating that the analgesic effect induced by i.c.v. h HK-1 is mediated through the activation of NK(1) receptors. Interestingly, naloxone, beta-funaltrexamine and naloxonazine, but not naltrindole and nor-binaltorphimine, could also block the analgesic effect markedly, suggesting that this effect is related to descending mu opioidergic neurons (primary mu(1) subtype). Human HK-1(4-11) could also induce a dose- and time-dependent analgesic effect after i.c.v. administration, however, the potency of analgesia was less than h HK-1. Surprisingly, SR140333 could not modify this analgesic effect, suggesting that this effect is not mediated through the NK(1) receptors like h HK-1. SR48968 could modestly enhance the analgesic effect induced by h HK-1(4-11), indicating that a small amount of h HK-1(4-11) may bind to NK(2) receptors. Furthermore, none of the opioid receptor (OR) antagonists could markedly block the analgesia of h HK-1(4-11), suggesting that the analgesic effect is not mediated through the descending opioidergic neurons. Blocking of delta ORs significantly enhanced the analgesia, indicating that delta OR is a negatively modulatory factor in the analgesic effect of h HK-1(4-11). It is striking that bicuculline (a competitive antagonist at GABA(A) receptors) effectively blocked the analgesia induced by h HK-1(4-11), suggesting that this analgesic effect is mediated through the descending inhibitory GABAergic neurons. The novel mechanism involved in the analgesic effect of h HK-1(4-11), which is different from that of h HK-1, may pave the way for a new strategy for the investigation and control of pain.

The Role of GABA in the Mediation and Perception of Pain

A great deal of effort has been expended in attempting to define the role of GABA in mediating the transmission and perception of pain. Pursuit of this question has been stimulated by the fact that GABAergic neurons are widely distributed throughout the central nervous system, including regions of the spinal cord dorsal horn known to be important for transmitting pain impulses to the brain. In addition, GABA neurons and receptors are found in supraspinal sites known to coordinate the perception and response to painful stimuli and this neurotransmitter system has been shown to regulate control of sensory information processing in the spinal cord. The discovery that GABA receptor agonists display antinociceptive properties in a variety of animal models of pain has provided an impetus for developing such agents for this purpose. It has been shown that GABA receptor agonists, as well as inhibitors of GABA uptake or metabolism, are clinically effective in treating this symptom. However, even with an enhanced understanding of the relationship between GABAergic transmission and pain, it has proven difficult to exploit these findings in designing novel analgesics that can be employed for the routine management of pain. Work in this area has revealed a host of reasons why GABAergic drugs have, to date, been of limited utility in the management of pain. Chief among these are the side effects associated with such agents, in particular sedation. These limitations are likely due to the simultaneous activation of GABA receptors throughout the neuraxis, most of which are not involved in the transmission or perception of pain. This makes it difficult to fully exploit the antinociceptive properties of GABAergic drugs before untoward effects intervene. The discovery of molecularly and pharmacologically distinct GABAA receptors may open the way to developing subtype selective agents that target those receptors most intimately involved in the transmission and perception of pain. The more limited repertoire of GABAB receptor subunits makes it more difficult to develop subtype selective agents for this site. Nonetheless, a GABAB agonist, CGP 35024, has been identified that induces antinociceptive responses at doses well below those that cause sedation (Patel et al., 2001). It has also been reported that, unlike baclofen, tolerance to antinociceptive responses is not observed with CGP 44532, a more potent GABAB receptor agonist (Enna et al., 1998). While the reasons for these differences in responses to members of the same class remain unknown, these findings suggest it may be possible to design a GABAB agonist with a superior clinical profile than existing agents. Besides the challenges associated with identifying subtype selective GABAA and GABAB receptor agonists, the development of GABA analgesics has been hindered by the fact that the responsiveness of these receptor systems appear to vary with the type and duration of pain being treated and the mode of drug administration. Further studies are necessary to more precisely define the types of pain most amenable to treatment with GABAergic drugs. Inasmuch as the antinociceptive responses to these agents in laboratory animals are mediated, at least in part, through activation or inhibition of other neurotransmitter and neuromodulator systems, it is conceivable that GABA agonists will be most efficacious as analgesics when administered in combination with other agents. The results of anatomical, biochemical, molecular, and pharmacological studies support the notion that generalized activation of GABA receptor systems dampens the response to painful stimuli. The data leave little doubt that, under certain circumstances, stimulation of neuroanatomically discreet GABA receptor sites could be of benefit in the management of pain. Continued research in this area is warranted given the limited choices, and clinical difficulties, associated with conventional analgesics.

Gamma-aminobutiric acid immunostaining in trigeminal, nodose and spinal ganglia of the cat

Gamma-aminobutyric acid (GABA) is a principal inhibitory neurotransmitter in the vertebrate nervous system. It is found mainly in local circuit neurons, but it has also been described in sensory organs and dorsal root ganglia (DRG). The present study describes the presence of GABA in primary afferent neurons of feline sensory ganglia: trigeminal ganglia (TrG), nodose ganglia (NG), and DRG. Quantitative analysis revealed that approximately 20% of the cells in the TrG, NG and DRG are GABAergic. GABA-expressing neurons varied in size. GABA-containing neuronal fibres were also observed in the neuropil. Some of these were in close apposition to both GABA-positive and GABA-negative ganglionic neuronal perikarya. The localization of GABA in small primary afferent neurons, which are considered to be nociceptors, suggests that the amino acid may function as a pain transmitter or modulator, whereas processing of other sensory modalities, such as somatosensory and proprioceptive, may also be affected by GABA.

Increased expression of gamma-aminobutyric acid transporters GAT-1 and GAT-3 in the spinal trigeminal nucleus after facial carrageenan injections

The present study aimed to elucidate the distribution of gamma-aminobutyric acid (GABA) transporters in the spinal trigeminal nucleus after carrageenan injections. Dense GAT-1 and GAT-3 but very little GAT-2 immunoreactivity was observed in the normal rat spinal trigeminal nucleus. The GAT-1-positive glial cells in the normal rat spinal trigeminal nucleus contained dense bundles of glial filaments and had features of astrocytes. Some GAT-3-positive cells contained dense bundles of glial filaments and had features of astrocytes, whilst others lacked glial filaments, and contained dense marginated heterochromatin, and had features of oligodendrocyte precursor cells. An increase in immunoreactivity to both transporters was observed on the injected but not the contralateral side 3 days after facial carrageenan injections. In rats given three further weekly injections of carrageenan and killed 3 days after the fourth injection, further increases in GAT-1 and GAT-3 immunoreactivities were observed. Electron microscopy showed that transporter immunoreactivity in the spinal trigeminal nucleus of carrageenan-injected rats was predominantly present in glial processes, showing that the increase in the number of processes observed at light microscopy was due to increased immunoreactivity in glial processes. An increased expression of GABA transporters in the carrageenan-injected spinal trigeminal nucleus could therefore result in a faster removal of GABA from the synaptic cleft of GABAergic axon terminals compared to normal rats. This could result in reduced inhibition/increased activity of the trigeminothalamic neurons in the spinal trigeminal nucleus, and could contribute to hyperalgesia after carrageenan injections.

gamma-Aminobutyric acid transporters in the cat periaqueductal gray: a light and electron microscopic immunocytochemical study

The gamma-aminobutyric acid (GABA) plasma membrane transporters (GATs) mediate GABA uptake into presynaptic axon terminals and glial processes, thus contributing to the regulation of the magnitude and duration of the action of GABA at the synaptic cleft. The aim of the present study was to investigate the expression of three high-affinity GABA transporters (GAT-1, GAT-2, and GAT-3) in the periaqueductal gray matter (PAG) of adult cats by using immunocytochemistry with affinity-purified antibodies. Light microscopic observations revealed GAT-1 immunoreactivity in punctate structures, particularly dense in the lateral portion of the dorsolateral PAG column. Weak GAT-2-immunopositive puncta were homogeneously distributed in the PAG. GAT-3 immunoreactivity was detected in each column of the PAG but was more intense in the dorsolateral PAG column and around the aqueduct. Electron microscopic studies showed GAT-1 immunoreactivity in distal astroglial processes, in unmyelinated and small myelinated axons, and in axon terminals making symmetric synapses on both PAG neurons and dendrites. GAT-2 immunoreactivity was present mostly in the form of patches of different sizes in the cytoplasm of neuronal elements like the perikarya and dendrites of PAG neurons, in myelinated and unmyelinated axons, and in the axon terminals forming both symmetric and asymmetric synapses. Labeling was also observed in nonneuronal elements. Astrocytic cell bodies and their distal processes as well as the ependymal cells lining the wall of the aqueduct showed patches of GAT-2 immunoreactivity. Electron microscopic observation revealed GAT-3 immunoreactivity exclusively in distal astrocytic processes adjacent to the somata of PAG neurons and in axon terminals making both symmetric and asymmetric synapses. The present results suggest that three types of termination systems of GABAergic transmission are present in the cat periaqueductal gray matter.

Role of arginine in immunonutrition

Arginine plays an important role in many physiologic and biologic processes beyond its role as a protein-incorporated amino acid. Dietary supplementation of arginine can enhance wound healing, regulate endocrine activity and potentiate immune activity. Under normal unstressed conditions the arginine requirement of adult humans is fulfilled by endogenous sources, however this is compromised during times of stress, especially in critical illness. These finding have led to use of arginine supplementation as part of an immune-enhancing dietary regimen to help combat the immune suppression seen in such patients. Though the results from studies examining the use of this type of immunonutrition in critically ill patients are far from definitive, they are promising that this mode of therapy may be of some advantage. A better understanding of the in vivo biology of arginine and its metabolism is necessary to truly define a benefit from arginine supplementation.

Tiagabine antinociception in rodents depends on GABA(B) receptor activation: parallel antinociception testing and medial thalamus GABA microdialysis

The effects of a new antiepileptic drug, tiagabine, (R)-N-[4,4-di-(3-methylthien-2-yl)but-3-enyl] nipecotic acid hydrochloride, were studied in mice and rats in antinociceptive tests, using three kinds of noxious stimuli: mechanical (paw pressure), chemical (abdominal constriction) and thermal (hot plate). In vivo microdialysis was performed in parallel in awake, freely moving rats in order to evaluate possible alterations in extracellular gamma-aminobutyric acid (GABA) levels in a pain-modulating region, the medial thalamus. Systemic administration of tiagabine, 30 mg kg(-1) i.p., increased nearly twofold the extracellular GABA levels in rats and increased significantly the rat paw pressure nociceptive threshold in a time-correlated manner. Dose-related significant tiagabine-induced antinociception was also observed at the doses of 1 and 3 mg kg(-1) i.p. in the mouse hot plate and abdominal constriction tests. The tiagabine antinociception was completely antagonised by pretreatment with the selective GABA(B) receptor antagonist, CGP 35348, (3-aminopropyl-diethoxy-methyl-phosphinic acid) (2.5 microg/mouse or 25 microg/rat i.c.v.), but not by naloxone (1 mg kg(-1) s.c.), both administered 15 min before tiagabine. Thus, it is suggested that tiagabine causes antinociception due to raised endogenous GABA levels which in turn activate GABA(B) receptors.

Localization of messenger RNAs encoding three GABA transporters in rat brain: an in situ hybridization study

Localization of the messenger RNAs encoding three gamma-aminobutyric acid (GABA) transporters, termed GAT-1, GAT-2, and GAT-3, has been carried out in rat brain using radiolabeled oligonucleotide probes and in situ hybridization histochemistry. Hybridization signals for GAT-1 mRNA were observed over many regions of the rat brain, including the retina, olfactory bulb, neocortex, ventral pallidum, hippocampus, and cerebellum. At the microscopic level, this signal appeared to be restricted to neuronal profiles, and the overall distribution of GAT-1 mRNA closely paralleled that seen in other studies with antibodies to GABA. Areas containing hybridization signals for GAT-3 mRNA included the retina, olfactory bulb, subfornical organ, hypothalamus, midline thalamus, and brainstem. In some regions, the hybridization signal for GAT-3 seemed to be preferentially distributed over glial cells, although hybridization signals were also observed over neurons, particularly in the retina and olfactory bulb. Notably, hybridization signal for GAT-3 mRNA was absent from the neocortex and cerebellar cortex, and was very weak in the hippocampus. In contrast to the parenchymal localization obtained for GAT-1 and GAT-3 mRNAs, hybridization signals for GAT-2 mRNA were found only over the leptomeninges (pia and arachnoid). The differential distribution of the three GABA transporters described here suggests that while each plays a role in GABA uptake, they do so via distinct cellular populations.

Desensitization of GABAB receptors and antagonism by CGP 35348, prevent bicuculline- and picrotoxin-induced antinociception

The effect of the GABAA antagonists, bicuculline and picrotoxin, in the hot plate and writhing tests in mice and the paw-pressure test in rats was assessed. Subconvulsant doses of bicuculline (1.3-4 mumol kg-1, s.c.) or picrotoxin (0.8-2.5 mumol kg-1, s.c.) induced a dose-related increase in latency of licking in the hot plate test in mice, whereas subconvulsant doses of strychnine and thiosemicarbazide (0.9 and 6 mg kg-1, s.c. respectively), did not modify the threshold to thermal stimuli in mice. The effects of bicuculline and picrotoxin were not modified by naloxone (3 mg kg-1, i.p., a dose which inhibited the antinociceptive effect of morphine) or by atropine (5 mg kg-1, i.p., a dose which prevented oxotremorine-induced antinociception) but were antagonized by the GABAB antagonist CGP 35348 (2.5 micrograms, i.c.v., a dose which prevented (+/-)baclofen-induced antinociception). Mice, rendered tolerant to baclofen-induced antinociception by twice daily injection of increasing doses of baclofen (5-18 mg kg-1, s.c.), were unresponsive to the antinociceptive effects of bicuculline and picrotoxin but still responded to morphine. Bicuculline and picrotoxin, in the same range of doses which affected the three models of antinociception used, inhibited pentobarbital-induced hypnosis. Large doses of bicuculline and picrotoxin (4 and 2.5 mumol kg-1, s.c. respectively), reduced locomotor activity and impaired rota-rod performance in mice. The changes in response to noxious stimuli, induced by bicuculline and picrotoxin, are interpreted as an antinociceptive effect. It is then suggested that this effect might depend on an indirect activation of GABAB receptors through release of GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

GABAergic and cholinergic mediation in the antinociceptive action of homotaurine

1. The role of GABAergic and cholinergic mechanisms in the antinociceptive effect of homotaurine (22.25-111.24 mg/kg i.p.) in chemical (acetic acid) and thermal (tail flick, tail immersion) tests has been studied by means of the interaction with baclofen and anticholinergic drugs. 2. Baclofen (2 mg/kg p.o.) and scopolamine sulfate (2.5 mg/kg i.p.) potentiate the antinociceptive effect of the amino acid in the chemical test. 3. Bicuculline (1 mg/kg i.p.) pretreatment does not modify the antinociceptive effect of homotaurine in the tail immersion and tail flick tests. 4. Scopolamine sulfate and methylnitrate (1 mg/kg i.p.) antagonise the effect of homotaurine (111.24 mg/kg i.p.) in the tail flick test. 5. The above results imply that peripheral GABAB and central cholinergic mechanisms play a role in the antinociceptive effect of homotaurine.

Anticonvulsant profiles of the potent and orally active GABA uptake inhibitors SK&F 89976-A and SK&F 100330-A and four prototype antiepileptic drugs in mice and rats

The anticonvulsant profiles of two potent and orally active gamma-aminobutyric acid (GABA) uptake inhibitors, 1-(4,4-diphenyl-3-butenyl)-3-piperidine-carboxylic acid hydrochloride (SK&F 89976-A) and 1-(4,4-diphenyl-3-butenyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride (SK&F 100330-A), were determined with a battery of well-standardized tests in mice and rats and compared with the profiles of phenytoin (PHT), carbamazepine (CBZ), valproate (VPA) and clonazepam (CZP) when subjected to the same tests. ED50 values were calculated and compared with TD50 values for minimal motor impairment to provide protective indexes (PI = TD50/ED50). The anticonvulsant profiles of SK&F 89976-A and SK&F 100330-A were similar and suggest that these compounds raise the threshold for seizure initiation rather than inhibit seizure spread. Like intraperitoneal (i.p.) PHT, CBZ, VPA, and CZP, SK&F 89976-A and SK&F 100330-A inhibited seizures in corneally kindled rats. The profiles of SK&F 89976-A and SK&F 100330-A were most similar to that of CZP and virtually opposite to that of PHT. Intraperitoneal SK&F 100330-A provided complete protection against pentylenetetrazol-induced seizures [subcutaneous (s.c.) PTZ] in mice but was ineffective against seizures induced by maximal electroshock (MES) at doses slightly greater than its TD50. SK&F 100330-A provided complete protection against picrotoxin-induced seizures (s.c. Pic) and against both clonus and forelimb tonic extension induced by NMDA N-methyl-D-aspartate [intracerebral ventricular (i.c.v.)-NMDA] in mice; however, SK&F 100330-A was ineffective against seizures induced by bicuculline (s.c. Bic) and strychnine (s.c. Strych) at doses slightly greater than its TD50. SK&F 89976-A was similar but provided partial protection against NMDA-induced clonus.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of homotaurine in experimental analgesia tests

1. The possible antinociceptive action of GABA A receptor agonist homotaurine, has been studied through a battery of chemical (acetic acid) and thermal (hot plate, tail flick and tail immersion) tests in rats and mice. 2. The aminoacid was used at the following doses 22.25; 55.62 and 111.24 mg/kg i.p. and 50-100 micrograms i.c.v.; and measurements were made at the time of and at 5, 15 and 30 min after drug administration. 3. Homotaurine exhibited a significantly antinociceptive effect in all the above mentioned test except hot plate and when administered i.c.v. in the tail flick test. 4. The antinociceptive effect in the chemical test was dose and time dependent. 5. In the tail immersion test, latency time for withdrawal of the tail was significantly increased with the dose of 55.62 mg/kg at 15 min and 111.24 mg/kg at 30 min. 6. In the tail flick test the antinociceptive effect was dose dependent at 15 and 30 min. 7. From the above results the implication of peripheral and spinal mechanisms in the antinociceptive effect of homotaurine may be concluded.

Increased inhibition in dentate gyrus granule cells following exposure to GABA-uptake blockers

Rats anesthetized with urethane had stimulating and recording electrodes placed in the perforant pathway and in the dentate gyrus. They were then exposed to increasing doses of either the vehicle control dimethylsulfoxide (DMSO) or one of two gamma-aminobutyric acid (GABA)-uptake blockers (SKF-100330A or SKF-89976A). Analysis of evoked field potentials from dentate granule cells indicated that the only effect of the GABA uptake blockers was to increase the threshold for evoking the field population spikes (PS). No other measure of excitatory postsynaptic potentials (EPSPs) or PS's were significantly affected. The lack of effect on evoked EPSP by these drugs suggests no direct effect on transmitter release at this synapse, while the increase in PS threshold suggests a slight decrease in granule cell excitability. The effects of the two GABA-uptake blockers on synaptically mediated facilitation and inhibition was tested by using paired-pulse paradigms. Both GABA-uptake blockers increased early GABA-mediated inhibition to a greater extent than they reduced synaptically mediated facilitation. Neither GABA uptake blocker appeared to effect the late inhibition seen at paired-pulse intervals of 400-1000 ms which is presumably associated with calcium-activated increases in potassium conductance. These effects on granule cell responses occurred at doses found previously not to be associated with side effects and yet to be anticonvulsant in unanesthetized rats. These data confirm in vivo that SKF-100330A and SKF-89976A increase GABA-mediated inhibition. The effect on granule cell excitability and late inhibition are minimal. Although facilitation was reduced by exposure to these drugs, the mechanism of this reduction (direct or prolongation of early inhibition) cannot be determined.

Injection of baclofen into the ventromedial hypothalamus stimulates gastric motility in the rat

Injection of 2 micrograms (+/-)-baclofen into the ventromedial hypothalamus (VMH) of urethane-anaesthetized rats resulted in an increase in gastric tone and amplitude of contractions. This effect was curtailed by administration of atropine methyl nitrate (20 mg/kg i.p.) or bilateral cervical vagotomy. These results provide evidence for a hypothalamic modulation of gastric motility by the vagus. Baclofen, possibly acting on receptors insensitive to gamma-aminobutyric acid (GABA), may be mimicking a vagal activation system, located within the ventro-medial hypothalamus.

The GABA agonist THIP, attenuates antinociception in the mouse by modifying central cholinergic transmission

The effect of THIP, a direct-acting gamma-aminobutyric acid (GABA) receptor agonist, on the antinociceptive response to a variety of agents was examined using the mouse tail-immersion assay. Alone, THIP produced an antinociceptive response in smaller doses (5 mg/kg) but was ineffective at doses exceeding 10 mg/kg. Treatment with THIP (15 mg/kg) was found to block the antinociceptive response to an inhibitor of the uptake of GABA, an inhibitor of GABA-transaminase, a direct-acting GABA receptor agonist and to a cholinesterase inhibitor. In contrast, THIP had no effect on the antinociceptive responses to morphine, clonidine or oxotremorine. The results indicate that large doses of THIP reduce cholinergic activity in a pathway important for mediating the antinociceptive action of GABAergic drugs and physostigmine.

GABAergic mechanisms of analgesia: an update

Both directly acting (GABAA and GABAB agonists) and indirectly acting GABAergic agents (GABA uptake inhibitors and GABA-transaminase inhibitors) produce analgesia in a variety of animal test systems. Analgesia produced by GABAA agonists is probably due to a supraspinal action, although spinal sites may also play a role. GABAA agonist analgesia is insensitive to naloxone, bicuculline, picrotoxin and haloperidol, but is blocked by atropine, scopolamine and yohimbine suggesting a critical role for central cholinergic and noradrenergic pathways in this action. The lack of blockade by the GABAA antagonist bicuculline is difficult to explain. Both bicuculline and picrotoxin have intrinsic analgesia actions which may not necessarily be mediated by GABA receptors. The GABAB agonist baclofen produces analgesia by actions at both spinal and supraspinal sites. Baclofen analgesia is insensitive to naloxone, bicuculline and picrotoxin, and blockade by cholinergic antagonists occurs only under limited conditions. Catecholamines are important mediators of baclofen analgesia because analgesia is potentiated by reserpine, alpha-methyl-p-tyrosine, phentolamine, ergotamine, haloperidol and chlorpromazine. A role for serotonergic mechanisms is less well defined. Methylxanthines, which produce a clonidine-sensitive increase in noradrenaline (NA) turnover, increase baclofen analgesia by a clonidine-sensitive mechanism. Both ascending and descending NA pathways are implicated in the action of baclofen because dorsal bundle lesions, intrathecal 6-hydroxydopamine and medullary A1 lesions markedly decrease baclofen analgesia. However, simultaneous depletion of NA in ascending and descending pathways by locus coeruleus lesions potentiates baclofen analgesia suggesting a functionally important interaction between the two aspects. Baclofen analgesia within the spinal cord may be mediated by a distinct baclofen receptor because GABA does not mimic the effect of baclofen and the rank order of potency both of close structural analogs of baclofen as well as antagonists differs for analgesia and GABAB systems. The spinal mechanism may involve an interaction with substance P (SP) because SP blocks baclofen analgesia, and desensitization to SP alters the spinal analgesic effect of baclofen. GABA uptake inhibitors produce analgesia which is similar to that produced by GABAA agonists because it is blocked by atropine, scopolamine and yohimbine. Analgesia produced by GABA-transaminase inhibitors is similar to that produced by GABAA agonists because it can be blocked by atropine, but it is potentiated by haloperidol while THIP analgesia is not.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of nociceptive responses of laminae V-VII dorsal horn neurones by stimulation of mixed and muscle nerves, in the cat

Low frequency (1-3 Hz) stimulation of muscle nerves (ipsilateral medial and lateral gastrocnemius nerves) had to be applied at intensities which would recruit C-fibre afferents, to produce inhibition of nociceptive responses of cat lumbar spinal cord laminae V-VII units. Inhibitory responses evoked by muscle nerve stimulation and also by mixed nerve stimulation could be blocked by microionophoretically applied bicuculline. No effects on these inhibitions were obtained with naloxone.

The parafasciculus thalami as a site for mediating the antinociceptive response to GABAergic drugs

Electrophysiological (single cell) experiments were undertaken to examine whether neurons in the rat parafasciculus thalami (PF) are involved in mediating the antinociceptive response to GABAergic drugs. The results indicated that: noxious stimuli excite most PF neurons; microiontophoretic application of morphine, GABA or the GABA agonist, THIP, attenuated the spontaneous firing rate of PF neurons; morphine, THIP and GABA reduced the neuronal excitation induced by noxious stimuli; application of the GABA receptor antagonist, bicuculline, prevented the effects of THIP and GABA on PF activity; while naloxone blocked the response to morphine on PF neurons, it failed to influence the actions of GABA and THIP; and the injection of THIP or GABA into the PF produced an antinociceptive response as assessed by the rat tail-immersion assay, whereas pentobarbital was inactive. The findings suggest that GABA receptors located in the PF may mediate the antinociceptive response to GABAergic drugs, and that the action of these agents is unrelated to opiate receptors.