Postsynaptic Action Of Baclofen Research Articles

Distribution and depression of the GABA B receptor in the spinal dorsal horn of adult rat

γ-Aminobutyric acid (GABA) is a principal inhibitory neurotransmitter in vertebrate nervous system. The metabotropic receptor for GABA, GABA B receptor, is characterized as a G protein-coupled receptor subtype. In the present study, GABA B receptor-like immunoreactivity (GABA BR-LI) in the rat spinal cord and dorsal root ganglion (DRG), as well as GABA B receptor-mediated depression in the spinal dorsal horn were examined by using immunohistochemistry and whole-cell voltage-clamp recording technique, respectively. Under light microscope, GABA BR-LI was densely found in laminae I and II of the dorsal horn. DRG cells of various diameters also showed GABA BR-LI. Electron microscopy further revealed that GABA BR-LI was also localized in terminals of myelinated, unmyelinated fibers as well as the somatodendritic sites of dorsal horn neurons. Bath application of a GABA B receptor agonist, baclofen (10 μM, 30 s), induced a slow outward (inhibitory) current in dorsal horn neurons. This slow current was depressed when the postsynaptic G protein-coupled receptor was inhibited, indicating the postsynaptic action of baclofen. Under the condition of postsynaptic GABA B receptor being inhibited, baclofen (10 μM, 60 s) depressed large (Aβ) and fine (C, Aδ) afferent fiber-evoked monosynaptic excitatory postsynaptic currents, indicating presynaptic inhibition of GABA B receptor on elicited neurotransmitter release. Taken together, the results suggest that baclofen-sensitive GABA B receptor is expressed pre- and postsynaptically on primary afferent fibers and neurons in the spinal dorsal horn; activation of GABA B receptor in the dorsal horn postsynaptically hyperpolarizes dorsal horn neurons and presynaptically inhibits primary afferents.

Baclofen inhibits more effectively C-afferent than Aδ-afferent glutamatergic transmission in substantia gelatinosa neurons of adult rat spinal cord slices

Although intrathecal administration of baclofen, a selective GABA B-receptor agonist, is known to have an antinociceptive effect on various pain models, the role of presynaptic GABA B receptors in antinociception is not well characterized. In the present study, the action of baclofen on primary afferent-evoked glutamatergic excitatory transmission was examined in substantia gelatinosa (SG) neurons of an adult rat spinal cord slice with an attached dorsal root, prepared from the lumbar segment, by use of the blind whole-cell patch-clamp technique. Under the condition where a postsynaptic action of baclofen was inhibited, baclofen (1 μM) reduced the amplitudes of excitatory postsynaptic currents (EPSCs; V H=−70 mV) which were monosynaptically evoked by stimulating primary-afferent C- and/or Aδ-fibers and which were remarkably depressed by CNQX (10 μM). The identification of the C-fiber or Aδ-fiber EPSC was based on antidromic action potentials recorded from neurons of isolated dorsal root ganglia. The C-fiber EPSC was depressed in peak amplitude by baclofen (1 μM) to a larger extent than the Aδ-fiber EPSC (20 and 45% of control, respectively). Each of the baclofen actions was suppressed by a selective GABA B-receptor antagonist, CGP 35348 (50 μM). Baclofen (1 μM) did not affect a response of SG neurons to bath-applied AMPA (10 μM). These results indicate that baclofen inhibits the release of L-glutamate from Aδ and C primary-afferent terminals in the SG through the activation of GABA B receptor; this action is more effective to C-fiber than Aδ-fiber transmission. Considering that the SG is the main part of termination of Aδ- and C-fibers transmitting nociceptive information, the present finding would account for at least a part of the inhibitory action of baclofen on pain transmission.

Pharmacology and physiological function of gamma-aminobutyric acid B type receptor

Baclofen, a beta-chlorophenyl derivative of gamma-aminobutyric acid (GABA), depresses neuronal excitability in various parts of the central nervous system. The site of action for this drug had once been considered to be distinct from GABA recognition sites. In addition to the classical GABA recognition site (GABAA site), a new class of GABA receptor (GABAB site) has been characterized. GABAB sites are mainly present on nerve terminals and, when activated, result in diminished transmitter release, probably through a reduction in Ca2+ influx. Baclofen was shown to be a selective agonist for this novel GABAB recognition. Baclofen also directly hyperpolarizes the membrane of mammalian brain neurons, in addition to its presynaptic action. This postsynaptic action of baclofen was shown to result from an increase in K+ conductance when studied in hippocampal pyramidal neurons through postsynaptic GABAB receptors. Thus, the inhibitory neurotransmitter GABA activates two receptor subtypes that can be distinguished by their physiological and pharmacological properties. GABAA receptors mediate rapid alterations in the distribution of Cl- across the membrane. GABAA receptors are linked directly to an ion channel, thus contributing to the prompt inhibition of cellular excitability. On the contrary, the GABAB receptor does not contain an integral ion channel and is thus responsible for slower responses through receptor-G-protein-effector complexes. G-protein may be directly coupled to K+ or Ca2+ channels. In addition, G-protein may modulate a variety of regulatory proteins or second messengers, thus contributing to the slower alteration of cellular excitability or to the modulation of neurotransmitter release.

Carbachol reduces IK,Baclofen, but not IK,GABA in guinea pig hippocampal slices

In the presence of bicuculline and/or picrotoxin (−)-baclofen and γ-aminobutyric acid (GABA) induced outward currents ( I Bac and I GABA) at holding potentials of −55 to −75 mV in guinea pig CA3 neurones of hippocampal slices. Zero potentials for these currents were at the K-equilibrium potential indicating that they were carried by K-ions ( I K,Bac, I K,GABA). I K,Bac was strongly reduced by carbachol (Cch) in low concentration (0.1–0.3 μM), while I K,GABA was not affected by Cch concentrations even up to 20 μM. The K-dependent late inhibitory postsynaptic potential (IPSP) was reduced significantly by Cch concentrations higher than 1 μM, but with these concentrations the early, Cl-dependent IPSP was reduced as well. The baclofen-derivative phaclofen, considered a selective antagonist of both the postsynaptic action of baclofen and the bicuculline — and picrotoxin — resistant action of GABA, exhibited, in our hands, partial agonistic effects and effects on non-transmitter gated K-currents. Our findings cast some doubt on the assumption that I K,Bac, I K,GABA and the late IPSP are all mediated by the same receptor and generated by the same mechanism.

Pertussis toxin pretreatment discriminates between pre- and postsynaptic actions of baclofen in rat dorsal raphe nucleus in vitro

Intracellular recordings were made from rat dorsal raphe neurons in vitro. Baclofen (30 μM) and 5-carboxamidotryptamine (5-CT, 300 nM to 1 μM) hyperpolarized these neurons by 10 and 13 mV, respectively. Depolarizing synaptic potentials (DSPs) were evoked by single shocks: baclofen reduced the amplitude of the DSP by 81%, but 5-CT reduced it by only 23%. The somatic response to iontophoretically applied glutamate pulses was reduced by 12% by baclofen, and 23% by 5-CT. In slices from rats pretreated with intracerebroventricular pertussis toxin (PTX), the ability of baclofen to reduce the DSP was almost unchanged, although the hyperpolarizing action of baclofen, and both actions of 5-CT were virtually eliminated. We conclude that it is possible to distinguish the pre- and postsynaptic actions of baclofen with PTX, and that the actions of 5-CT are both blocked.

Pre-and postsynaptic actions of baclofen: blockade of the late synaptically-evoked hyperpolarization of CA1 hippocampal neurones.

Using intracellular recording techniques, the effects of beta-p-chlorophenyl-GABA (baclofen) on passive membrane properties and postsynaptic potentials of CA1 pyramidal neurones were investigated. In experiments where only the hyperpolarizing action of baclofen was precluded by conventional current clamp techniques, 20 microM ( +/- ) baclofen blocked the early GABA-mediated IPSP and also a late hyperpolarization which, since it could be evoked by orthodromic stimulation subthreshold for spike firing, would not be expected to be produced by a Ca2+-activated increase in potassium conductance (AHP), but to be a transmitter-mediated event. In addition the conductance increase associated with this late IPSP evoked by subthreshold stimulation and also that associated with the AHP produced by spike activation were abolished. Baclofen also appeared to increase the duration of EPSPs, an event possibly related to loss of IPSPs. The hyperpolarization produced by baclofen was associated with an increased conductance of the resting membrane, an event possibly associated with an elevated potassium flux. To preclude this postsynaptic effect as a cause of reduced synaptic responses, tetraethylammonium chloride (TEA), a compound which decreases conductance and depolarizes the membrane of CA1 pyramidal neurones by a reduction of a 'leak' or resting potassium conductance (gK), was added to the bathing medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Postsynaptic actions of baclofen associated with its antagonism of bicuculline-induced epileptogenesis in hippocampus.

The effect of baclofen on bicuculline-induced epileptogenesis was investigated in the CA3 region of hippocampal slices taken from rats 9 days to 8 weeks of age. Bath application of baclofen blocked all spontaneous epileptiform activity and raised the stimulus strength required for orthodromic induction of epileptiform discharges. Baclofen was equally effective in antagonizing depolarization shift generation in mature and immature rat slices. The duration of afterdischarges recorded in immature hippocampus was unaltered, yet these events were eliminated when the proceeding depolarization shift was blocked. Baclofen hyperpolarized all CA3 pyramidal cells studied with an associated decrease in membrane resistance. These effects were produced by a direct postsynaptic action.