Cardiovascular responses to intrathecal administration of L- and D-baclofen in the rat

Derivatization of γ-Amino Butyric Acid Analogues for Their Determination in the Biological Samples and Pharmaceutical Preparations: A Comprehensive Review

γ-Aminobutyric acid (GABA) plays an important role in regulating neuronal excitability. Four structurally related drugs to GABA including pregabalin (PGB), gabapentin (GBP), vigabatrin (VGB), and baclofen are used for the treatment of central nervous system disorders. These drugs are small aliphatic molecules having neither fluorescent nor strong absorbance in the ultraviolet/visible region; therefore, direct determination of these analytes by optical methods is difficult. Additionally, their high boiling point makes gas chromatography impossible. Accordingly, the amine or acid moiety in these drugs is derivatized in order to improve their selectivity and sensitivity during determination in the biological samples. This review focuses on derivatization based methods and their different reactions for determination of PGB, GBP, VGB, and baclofen in the biological samples and pharmaceutical preparations reported between 1980 and 2020. High-performance liquid chromatography methods coupled with different detectors are a commonly used methods for determination of GABA analogs after derivatization. These methods cover 38.89% of all developed methods for determination of GABA analogs.

GABAB receptors expressed in human aortic endothelial cells mediate intracellular calcium concentration regulation and endothelial nitric oxide synthase translocation

GABA_B receptors regulate the intracellular Ca²⁺ concentration ([Ca²⁺]i) in a number of cells (e.g., retina, airway epithelium and smooth muscle), but whether they are expressed in vascular endothelial cells and similarly regulate the [Ca²⁺]i is not known. The purpose of this study was to investigate the expression of GABA_B receptors, a subclass of receptors to the inhibitory neurotransmitter γ-aminobutyric acid (GABA), in cultured human aortic endothelial cells (HAECs), and to explore if altering receptor activation modified [Ca²⁺]i and endothelial nitric oxide synthase (eNOS) translocation. Real-time PCR, western blots and immunofluorescence were used to determine the expression of GABA_B1 and GABA_B2 in cultured HAECs. The effects of GABA_B receptors on [Ca²⁺]i in cultured HAECs were demonstrated using fluo-3. The influence of GABA_B receptors on eNOS translocation was assessed by immunocytochemistry. Both GABA_B1 and GABA_B2 mRNA and protein were expressed in cultured HAECs, and the GABA_B1 and GABA_B2 proteins were colocated in the cell membrane and cytoplasm. One hundred μM baclofen caused a transient increase of [Ca²⁺]i and eNOS translocation in cultured HAECs, and the effects were attenuated by pretreatment with the selective GABA_B receptor antagonists CGP46381 and CGP55845. GABA_B receptors are expressed in HAECs and regulate the [Ca²⁺]i and eNOS translocation. Cultures of HAECs may be a useful in vitro model for the study of GABA_B receptors and vascular biology.

Microwave-assisted synthesis and reversed-phase high-performance liquid chromatographic separation of diastereomers of (R,S)-baclofen using ten chiral derivatizing reagents designed from trichloro-s-triazine

Four dichloro-s-triazine (DCT) and five monochloro-s-triazine (MCT) chiral derivatizing reagents (CDRs) were synthesized by incorporating amino acid amide moieties as chiral auxiliaries in trichloro-s-triazine and its 6-methoxy derivative, respectively. Another MCT reagent was synthesized by substitution of two chlorine atoms with two different amino acid amides in trichloro-s-triazine. These reagents were used for synthesis of diastereomers of (R,S)-baclofen under microwave irradiation (i.e. 60 s at 85% power using DCT reagents and 90 s at 85% power using MCT reagents). The diastereomers were separated on a reversed-phase C18 column using mixtures of methanol with aqueous trifluoroacetic acid (TFA) with UV detection at 230 nm. The separation behavior in terms of retention times and resolutions obtained for the two sets of diastereomers prepared with DCT and MCT reagents were compared among themselves and among the two groups. Longer retention times and better resolutions were observed with DCT reagents as compared to MCT reagents. The calibration curves were linear for both (R)- and (S)-baclofen in the concentration range 50-500 μg/ml. The average regression was 0.999 for both (R)- and (S)-baclofen. The RSD for (R)-baclofen was 0.40-0.86% for intra-day precision and 0.60-1.40% for inter-day precision and these values for (S)-baclofen were 0.52-0.75% and 0.64-1.32%, respectively. The recovery was 97.2-98.9% for (R)- and 97.0-98.9% for (S)-baclofen. The limit of detection was 1.63 ng/ml and 1.52 ng/ml for (R)- and (S)-baclofen, respectively.

Intrathecal baclofen for autonomic instability due to spinal cord injury

Autonomic dysreflexia may occur following spinal cord injury above mid-thoracic level, commonly developing in the early posttraumatic period. Cardiovascular dysregulation is the most prominent feature, characterized by paroxysmal high blood pressure attacks, which are precipitated by distension of urinary bladder or bowels, skin wounds, or increased spastic muscle tone. Severe drops in blood pressure may occur in orthostatic conditions. Baclofen is effective for treating spasticity. While orally administered baclofen often fails to alleviate severe spasticity adequately, intrathecal baclofen (ITB) is more effective and thus is being used increasingly. A 61-year-old male sustained a cervical spinal cord injury, subsequently developing severe spastic tetraparesis, predominantly in the legs. Some 30 years later he experienced marked spasms of the muscles of the abdominal wall, leading to extreme fluctuations of blood pressure. After positive evaluation with ITB the patient underwent implantation of a pump-catheter-system for continuous ITB application. Abdominal wall spasms ceased entirely with a daily dose of 190 microg ITB, accompanied by a sustained normotensive blood pressure profile. However, spasms reoccurred after inadvertent reduction of ITB flow when increasing the pump's ITB concentration but subsided again when the optimal antispastic dose was reestablished. Baclofen per se has the potential of lowering blood pressure. In this patient, however, ITB treatment enabled permanent stabilization of insidious blood pressure fluctuations. It would appear that suppression of abdominal spasms prevented the triggering of dysautonomic crises. This case demonstrates that ITB administration may help to stabilize autonomic dysreflexia and orthostatic hypotension in patients with spinal cord lesions.

Ketamine attenuates sympathetic activity through mechanisms not mediated by N-methyl-D-aspartate receptors in the isolated spinal cord of neonatal rats

Ketamine is believed to have sympathomimetic effects, although the central mechanism remains unclear. Using an in vitro splanchnic nerve-spinal cord preparation from neonatal rats, our previous investigations have demonstrated that tonic sympathetic activity is spontaneously generated from the thoracic spinal cord. We designed this study to investigate whether applications of ketamine to the cord would augment sympathetic activity and whether this action was dependent on N-methyl-d-aspartate receptors. Bath application of ketamine significantly reduced sympathetic activity in a concentration-dependent manner. Ketamine in 10, 20, 40, 80, and 120 microM reduced the sympathetic activity to 82.6% +/- 4.4% (P < 0.05), 61.7% +/- 5.1%, 42.8% +/- 4.2%, 24.9% +/- 4.4%, and 9.2% +/- 2.7% of the control value, respectively (P < 0.01, n = 8 for each test). The 50% inhibitory concentration of ketamine on sympathetic activity was 32 muM. Pretreatment with DL-2-amino-5-phosphonovaleric acid, a selective competitive N-methyl-d-aspartate receptor antagonist, did not alter ketamine-induced depression of sympathetic activity. These results suggest that ketamine reduces sympathetic activity by mechanisms that are independent of N-methyl-d-aspartate receptor activity.

Inhibition of nociceptive responses of spinal cord neurones during hypertension involves the spinal GABAergic system and a pain modulatory center located at the caudal ventrolateral medulla

The mechanisms of hypertension-induced hypoalgesia were studied in a model of hypertension induced by adenosine receptors blockade with the non-selective antagonist 1,3-dipropyl-8-sulfophenylxanthine (DPSPX) during 7 days. Based on the positive correlation between pain thresholds and noxious-evoked expression of the c-fos protooncogene in spinal cord neurones, we used this marker of nociceptive activation of spinal neurones to evaluate the involvement of the spinal GABAergic system and the caudal ventrolateral medulla (VLM), an important inhibitory component of the supraspinal endogenous pain modulatory system. In DPSPX-treated animals, a 20% increase in blood pressure was achieved along with a decrease in Fos expression in the superficial (laminae I-II) and deep (laminae III-VII) dorsal horn. In these animals, lower percentages of neurones labeled for GABAB receptors that expressed Fos were obtained in the superficial dorsal horn. Lesioning the VLMlat with quinolinic acid prevented the decrease in Fos expression at the spinal cord of DPSPX-hypertensive rats whereas in normotensive animals, no changes in Fos expression were detected. The present results support previous findings that hypertension is associated with a decrease of nociceptive activation of spinal cord neurones, through descending inhibition exerted by the VLMlat. This study further shows that during hypertension a decrease in the expression of GABAB receptors in nociceptive spinal neurones occurs, probably due to changes in the local GABAergic inhibitory system.

GABAB-receptor-mediated suppression of sympathetic outflow from the spinal cord of neonatal rats

Using a splanchnic nerve-spinal cord preparation in vitro that could spontaneously generate sympathetic nerve discharge (SND), we investigated the roles of intraspinal GABAB receptors in the regulation of SND. Despite an age-dependent difference in sensitivity, bath applications of baclofen (Bac; GABAB-receptor agonist) consistently reduced SND in a concentration-dependent manner. The drug specificity of Bac in activation of GABAB receptors was verified by application of its antagonist saclofen (Sac) or CGP-46381 (CGP). Sac or CGP alone did not change SND. However, in the presence of Sac or CGP, the effects of Bac on SND inhibition were reversibly attenuated. The splanchnic sympathetic preganglionic neuron (SPN) was recorded by blind whole cell, patch-clamp techniques. We examined Bac effects on electrical membrane properties of SPNs. Applications of Bac reduced excitatory synaptic events, induced membrane hyperpolarizations, and inhibited SPN firing. In the presence of 12 mM Mg2+ or 0.5 μM TTX to block Ca2+- or action potential-dependent synaptic transmissions, applications of Bac induced an outward baseline current that reversed at −29 ± 6 mV. Because the K+ equilibrium potential in our experimental conditions was −100 mV, the Bac-induced currents could not simply be attributed to an alteration of K+ conductance. On the other hand, applications of Bac to Cs+-loaded SPNs reduced Cd2+-sensitive and high-voltage-activated inward currents, indicating an inhibition of voltage-gated Ca2+ currents. Our results suggest that the activation of intraspinal GABAB receptors suppresses SND via a mixture of ion events that may link to a change in Ca2+ conductance.

Optimization of the chiral resolution of baclofen by capillary electrophoresis using beta-cyclodextrin as the chiral selector

The chiral resolution of baclofen was achieved by capillary electrophoresis using a fused-silica capillary (60 cm x 75 microm ID). The background electrolyte (BGE) was phosphate buffer (pH 7.0, 50 mM)-acetonitrile (95:5 v/v) containing 10 mM beta-cyclodextrin. The applied voltage was 15 kV. The values of alpha and R(s) were 1.06 and 1.00, respectively. The electrophoretic conditions were optimized varying the pH and the ionic strength of the BGE, concentrations of beta-cyclodextrin and acetonitrile and the applied voltage.

Evidence for a GABA(B) receptor component in the spinal action of Substance P (SP) on arterial blood pressure in the awake rat

1 The activation of tachykinin NK(1) receptors in the rat spinal cord produced a transient drop in arterial blood pressure followed by a more prolonged pressor effect which is mediated by the stimulation of the sympatho-adrenal system. This study aims at characterizing the spinal mechanism of that initial hypotension occurring in awake unrestrained rats. 2 The initial hypotension (-18+/-2.0 mmHg at 1 min) and the tachycardia (110+/-10 b.p.m.) produced by the intrathecal (i.t.) injection of the stable NK(1) receptor agonist [Sar(9), Met(O(2))(11)]-SP (Sar9, 0.65 nmol) at T-9 spinal cord level was inhibited by the prior injection of 65 nmol LY306740 or LY303870 (NK(1) receptor antagonists). No inhibition was seen when a similar dose of antagonists was given intravenously. 3 The prior i.t. injection of the GABA(B) receptor antagonist CGP52432 (100 nmol) reduced the hypotension evoked by Sar9 (0.65 nmol) and by the GABA(B) receptor agonist baclofen (100 nmol). The GABA(A) receptor antagonist bicuculline (25 nmol, i.t.) was without effect against Sar9, and the GABA(A) agonist muscimol (100 nmol, i.t.) had no cardiovascular effect. 4 The putative involvement of other mediators (dopamine, serotonine, glycine and glutamate) in Sar9-induced hypotension was made unlikely on the basis of various pharmacological treatments. Thus data, suggest that the transient hypotension which occurs upon the activation of NK(1) receptors in the spinal cord is due to the release of GABA which in turn activates GABA(B) receptors to inhibit sympathetic pre-ganglionic fibres. This mechanism may have a physiological significance in the spinal reflex autonomic control of arterial blood pressure.

Mediation of the cardiovascular response to spinal gamma-aminobutyric acid(B) receptor stimulation by adenosine A(1) receptors in anesthetized rats

Cardiovascular inhibitory effects induced by intrathecal (i.t.) administration of adenosine A(1) receptor agonist and its modulation by gamma-aminobutyric acid(B) (GABA(B)) receptor was suggested by our previous report. In this experiment, we examined the mediation of cardiovascular effects of GABA(B) receptor stimulation by adenosine A(1) and A(2) in the spinal cord. I.t. administration of GABA(B) receptor agonist, baclofen (30, 60 and 100 nmol) produced a dose dependent decrease of blood pressure and heart rate. Pretreatment with adenosine A(1) receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine (50 nmol), attenuated the depressor and bradycardiac effects of baclofen (100 nmol), but not with adenosine A(2) receptor antagonist, 3, 7-dimethyl-1-propargylxanthine (25 nmol). These results suggest that GABA(B) receptors in the spinal cord play an inhibitory role in the central cardiovascular regulation and that the depressor and bradycardiac actions are mediated by adenosine A(1) receptors.

Modification of cardiovascular responses to spinal GABA(B) receptor stimulation by cAMP and by K(ATP) channel blockade in anaesthetized rats

1. Intrathecal (i.t.) injection of baclofen (30, 60 and 100 nmol), a GABA(B) receptor agonist, produced a dose-dependent decrease in blood pressure (BP) and heart rate (HR). 2. Pretreatment with 5-aminovaleric acid (50 nmol), a GABA(B) receptor antagonist, blocked the depressor and bradycardic effects of baclofen (100 nmol). 3. Pretreatment with 8-bromo-cAMP (10 nmol), a cAMP analogue, attenuated the depressor and bradycardic effects of baclofen (100 nmol), but not with 8-bromo-cGMP (10 nmol), a cGMP analogue. 4. In addition, pretreatment with glipizide (20 nmol), an ATP-sensitive K+ channel (K(ATP)) blocker, attenuated the depressor and bradycardic effects of baclofen (100 nmol). These results suggest that GABA(B) receptors in the spinal cord have an inhibitory role in the central cardiovascular regulation and that these depressive and bradycardic actions are modified by cAMP and by K(ATP) channel blockade.

GABAergic and glutaminergic modulation of centrally evoked arrhythmias in rats

A standard electrical stimulus applied to the posterior hypothalamus evoked cardiac arrhythmogenic responses in the spontaneously hypertensive rat. Isolated premature ventricular beats or doublets and nonsustained ventricular tachycardic salvos were observed. This effect was associated with a large rise in blood pressure (79 +/- 3 mm Hg). The same stimulus in normotensive Wistar-Kyoto rats produced no significant cardiac arrhythmias, and the rise in blood pressure was smaller (36 +/- 2 mm Hg). We investigated the influence of baclofen, a GABAB receptor agonist, and two N-methyl-D-aspartate receptor antagonists on the arrhythmogenic response to hypothalamic stimulation. Intravenous baclofen (3 mg/kg) had no effect in the normotensive Wistar-Kyoto rats, but in the spontaneously hypertensive rats it enhanced the adjusted mean value of the number of extrasystoles from 0.5 +/- 0.5 to 18 +/- 1 (P < .001). This value was also increased (from 3 +/- 1 to 17 +/- 1, P < .001) by an intracisternal injection of baclofen (1 micrograms/kg). This facilitatory effect of baclofen was prevented by treatment with atenolol (0.5 mg/kg). Two glutamate receptor antagonists, ketamine (7.5 mg/kg IV) and kynurenic acid (200 micrograms/kg intracerebroventricularly), prevented both the arrhythmogenic response to the hypothalamic stimulation and its facilitation by baclofen. The study confirms that hypothalamic stimulation facilitates the development of arrhythmias through a sympathetic drive and that these arrhythmias are easier to induce in spontaneously hypertensive rats than in normotensive Wistar-Kyoto rats. Both the central GABAergic and the glutamatergic systems are implicated in the development of these ventricular arrhythmias, since baclofen could disinhibit the glutamatergic central pathway. These results could account for the ability of the spontaneously hypertensive rats to develop ventricular arrhythmias of central origin.

Evidence for a spinal origin of the effect of baclofen on the myocardial oxygen demand indexes

In a previous study in anaesthetized rabbits we observed that electrical stimulation of the hypothalamic paraventricular nucleus (PVN) elicited substantial rises in the maximum rate of change of left ventricular pressure (dP/dtmax) and in myocardial oxygen demand indexes (rate-pressure product and triple product), similar to the changes observed during stress or physical effort. Baclofen, a selective GABA(B) receptor agonist, injected intravenously prevented these responses. In the present study, we show that low doses of baclofen (0.1, 0.3 and 1 microgram/kg), injected intrathecally (i.t.) at the T9 level, reduced the myocardial oxygen demand during PVN stimulation. After 0.3 microgram/kg baclofen i.t., the peak value of the triple product during stimulation was 140 +/- 20 compared with 193 +/- 20 before treatment. An i.t. injection (500 micrograms/kg), of saclofen a selective GABA(B) receptor antagonist, did not modify the resting haemodynamics significantly but attenuated the inhibitory effects of baclofen (3 mg/kg i.v.). These results suggest that the main site of the effects of baclofen is located within the spinal cord and that GABA(B) receptors probably mediate these effects by modulating the central control of cardiac function. In conclusion, baclofen might be a useful tool to prevent the centrally evoked increases of myocardial oxygen demand.

Glutamate, NMDA and NMDA receptor antagonists: cardiovascular effects of intrathecal administration in the rat

Selected excitatory amino acids and antagonists were tested for their effects on arterial pressure and heart rate when administered intrathecally at the second (T2) or ninth (T9) thoracic spinal levels in urethane-anesthetized Sprague-Dawley rats with spontaneous or artificial respiration. Intrathecal administration of glutamate (1 mumol) and N-methyl-D-aspartic acid (NMDA; 2 nmol) at T9 increased arterial pressure and heart rate. The response began within 1 min, peaked at 2-3 min and persisted for 8-15 min. The maximum changes were 20-25 mm Hg for arterial pressure and 40-50 beats/min for heart rate. These responses were prevented by systemic administration of hexamethonium (10 mg/kg). Responses to administration of NMDA at the two spinal levels were essentially the same. Effects elicited by NMDA but not by glutamate were blocked by pretreatment with the NMDA receptor antagonists, D,L-2-amino-5-phosphonovaleric acid (APV; 10 nmol, intrathecal administration) and ketamine (7 mg/kg, i.v.). Intrathecal administration of APV (10, 50 and 200 nmol) at T2 produced dose-dependent decreases in arterial pressure without changing heart rate. The results support the hypothesis that NMDA receptors are involved in regulation of sympathetic output at the spinal level. They also indicate that in this preparation there is a tonic activation of NMDA receptors in sympathetic pathways to the vessels but not to the heart. Finally, the persistence of the response to glutamate in the presence of NMDA receptor antagonists suggests the involvement of non-NMDA receptors in spinal control of sympathetic output.

NMDA receptor antagonists block cardiovascular responses to intrathecal administration of D-baclofen in the rat

In previous studies we found that D and L-baclofen have different effects on sympathetic output when administered intrathecally, yet the actions of both enantiomers are blocked by intrathecal administration of phaclofen. The present experiments were done to determine the mechanism by which D-baclofen expresses its effects. In urethane-anaesthetized Sprague-Dawley rats, when D-baclofen was given intrathecally at the T9 spinal level following pretreatment with 2 nmol of the NMDA receptor antagonist, DL-2-amino-5-phosphonovaleric acid (APV), it increased systolic and diastolic arterial pressures (n = 7), as in the previous studies. However, after intrathecal administration of 10 nmol of APV, administration of D-baclofen had no effect on these parameters (n = 7). Intravenous administration of ketamine (7.5 mg/kg), another NMDA receptor antagonist, also blocked the effect of D-baclofen (n = 6) but it had no effect on the pressor responses produced by intrathecal administration of carbachol (27.4 nmol; n = 6). In additional experiments, L-baclofen (70 nmol) had no effect on the increases in heart rate and arterial pressure produced by N-methyl-D-aspartic acid (NMDA) (2 nmol; n = 8). These results indicate that D-baclofen increases arterial pressure via an NMDA receptor-mediated mechanism, perhaps by provoking the release of an endogenous ligand which activates these receptors.

Phaclofen-reversible effects of GABA in the spinal cord of the rat

Our earlier observation that intrathecal administration of L- and D-baclofen had different effects on sympathetic output regulating arterial pressure and heart rate in the rat prompted the present study which was designed to determine whether intrathecal administration of GABA elicits a phaclofen-reversible effect on arterial pressure and/or heart rate and whether this effect mimics that of L-baclofen or that of D-baclofen. Following intrathecal administration of the GABAA antagonist, bicuculline (10 nmol), at the T9 level, administration of GABA at a dose of 5 mumol (n = 8) decreased arterial pressure and heart rate by about 25 mm Hg and 45 bpm, respectively. The responses started at 1-2 min and lasted 3-20 min; comparison was made with rats given NaCl (5 mumol; n = 5), which was without effect on arterial pressure and heart rate. In rats pretreated with both bicuculline and the GABAB antagonist, phaclofen (5 mumol intrathecally; n = 9), the effect of GABA on arterial pressure was attenuated and the effect of GABA on heart rate was absent; comparisons were made with rats given bicuculline, phaclofen and NaCl (n = 5) and with rats given bicuculline, NaCl and GABA (n = 6). These data suggest that there is a phaclofen-reversible effect of GABA in spinal pathways regulating sympathetic output and that this effect of GABA resembles that L-baclofen reported in our earlier study.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of phaclofen and the enantiomers of baclofen on cardiovascular responses to intrathecal administration of L- and D-baclofen in the rat

In a previous study it was found that i.t. administration of L-baclofen decreased arterial pressure and heart rate while D-baclofen differentially increased arterial pressure. The objective of the present study was to determine which of these effects was blocked by prior administration of the GABAB receptor antagonist, phaclofen, and whether the effect of one enantiomer of baclofen could be blocked by prior administration of the other. The decreases in systolic and diastolic arterial pressures and in heart rate produced by i.t. administration of 70 nmol of L-baclofen were unaffected by i.t. administration of 7, 70 or 700 nmol of D-baclofen 10 min prior to administration of L-baclofen, but were blocked by administration of 5 mumol of phaclofen given 3-5 min prior to L-baclofen. On the other hand, the increases in systolic and diastolic arterial pressures induced by i.t. administration of 700 nmol of D-baclofen were blocked by 70 nmol but not by 7 nmol of L-baclofen, as well as by 2.5 mumol of phaclofen; the effect of L-baclofen cannot be attributed to a desensitization of D-baclofen-sensitive receptors as two successive doses of D-baclofen given 7 min apart had quantitatively similar effects. Phaclofen alone increased systolic and diastolic arterial pressures and heart rate. The results are interpreted as indicating that D-baclofen is not an antagonist of L-baclofen in this paradigm; rather, they suggest that L-baclofen reduces the effects of D-baclofen.(ABSTRACT TRUNCATED AT 250 WORDS)