Serotonin potentiates the response of neurons of the superficial laminae of the rat spinal dorsal horn to γ-aminobutyric acid

doi:10.1016/S0361-9230(00)00297-5

Brain Research Bulletin

Volume 52, Issue 6, August 2000, Pages 559-565

https://doi.org/10.1016/S0361-9230(00)00297-5 Get rights and content

Abstract

Employing the Nystatin-perforated whole-cell patch-clamp recording technique, the modulatory effects of serotonin (5-HT) on γ-aminobutyric acid (GABA)-activated whole-cell currents were investigated in neurons acutely dissociated from the superficial laminae (laminae I and II) of the rat spinal dorsal horn. The results showed: (1) GABA acted on GABA_A receptors and elicited inward Cl⁻ currents (I_GABA) at a holding potential (V_H) of −40 mV; (2) 5-HT potentiated GABA-induced Cl⁻ current without affecting the reversal potential of I_GABA and the apparent affinity of GABA to its receptor; (3) α-methyl-5-HT, a selective agonist of 5-HT₂ receptor, mimicked the potentiation effect of 5-HT on I_GABA, whereas ketanserine, an antagonist of 5-HT₂ receptor, blocked the potentiation effect of 5-HT; (4) Chelerythrine, an inhibitor of protein kinase C, reduced the potentiation effect of 5-HT on I_GABA. The present results indicate: (1) The potentiation of 5-HT on I_GABA is mediated by 5-HT₂ receptor and through a protein kinase-dependent transduction pathway; (2) The interactions between 5-HT and GABA might play an important role in the modulation of nociceptive information transmission at spinal cord level.

Introduction

The superficial laminae (laminae I and II) of the spinal dorsal horn, especially lamina II (SG) which is composed mainly of small interneurons, receive primary afferent C fibers, which transmit peripheral noxious information, and descending projecting fibers from endogenous antinociceptive system 6, 9, 39. Serotonin (5-HT) is one of the major transmitters involved in the supraspinal control of nociception 6, 9, 39. It is well known that spinal dorsal horn is heavily innervated with 5-HT-like immunoreactive (5-HT-LI) fibers, which mainly originate from the rostral ventromedial medulla (RVM), including the nucleus raphe magnus (NRM) and its surrounding reticular formation 13, 16, 30. 5-HT-LI terminals make axo-somatic and axo-dendritic synapses with nociceptive thalamic-projection neurons and local circuit neurons in the spinal dorsal horn 3, 18, 20. In the spinal dorsal horn, 5-HT depresses the exciting effects evoked by glutamate and nociceptive pinch stimulation [38] and decreases the reaction to nociceptive stimulation [8]. γ-Aminobutyric acid (GABA) is a putative inhibitory neurotransmitter in the vertebrate nervous system 17, 33, 34. Immunohistochemical studies have shown that GABA-like immunoreactive (GABA-LI) neurons and terminals and GABA_A receptors are intensely located in the superficial laminae of the spinal dorsal horn, especially in the SG 1, 4, 12, 24. Several lines of evidence suggest that GABA plays a key role in the processing and modulation of sensory inputs in the spinal dorsal horn 2, 5, 17, 33, 34.

Previous studies have demonstrated that 5-HT potentiates the inhibitory effects of GABA and glycine on rat sacral dorsal commissural neurons 42, 44 and the inhibitory effects of glycine at the superficial laminae neurons of the rat spinal dorsal horn [14]. In the superficial laminae of the spinal dorsal horn, 5-HT alone dose not induce noticeable currents 14, 42, 44. On the other hand, 5-HT-LI terminals, 5-HT receptor expressing neurons and GABA-containing neurons are heavily located in the superficial laminae of the spinal dorsal horn. Some of these 5-HT receptor expressing neurons also exhibit GABA-like immunoreactivity, and 5-HT-LI terminals contact 5-HT receptor and GABA co-localized neurons [19]. It is inferred that 5-HT might act on 5-HT receptor expressing neurons, which also contain GABA, to indirectly influence the activities of other neurons in the superficial laminae of spinal dorsal horn. In the present study, in order to examine whether 5-HT modulates GABA response and the related intracellular signal transduction mechanism, whole-cell patch-clamp recording technique was used to investigate the facilitatory effects of 5-HT on GABA-activated whole-cell currents in the neurons acutely dissociated from the superficial laminae of the rat spinal dorsal horn.

Section snippets

Preparation

Two-week-old Sprague–Dawley rats were used in the present experiments. All protocols described bellow have been approved by the Committee of Animal Use for Research and Education of The Fourth Military Medical University (Xi’an, P. R. China). All surgical procedures were performed under general anaesthesia with sodium pentobarbital (40 mg/kg b. wt., intraperitoneal [i.p.]). When the back or the tail was pinched with saw-tooth nippers, the anaesthetized rat showed almost no response, then the

GABA-induced inward current

Neurons in the superficial laminae of the spinal dorsal horn were voltage-clamped at a holding potential (V_H) of −40 mV. The application of GABA evoked an inward current (I_GABA) in all tested neurons. The current became detectable at a concentration of 3 × 10⁻⁷ M and increased in a concentration-dependent manner (Fig. 1A), leading to a Sigmoid-shaped concentration-response curve. The concentration-response relationship of I_GABA is summarized in Fig. 1B. The GABA concentrations for threshold,

Discussion

It is well known that GABA depresses the electrical activities of neurons in lamina II (SG) of the spinal cord 40, 47. 5-HT is mainly released from the serotoninergic descending fibers coming from the RVM, including the NRM and its surrounding reticular formation 13, 16, 30. 5-HT also has distinct depressive effects on neuronal activities of the spinal dorsal horn 30, 40. The extensive serotoninergic and GABAergic innervations of the spinal dorsal horn by fibers descending from RVM or local

Acknowledgements

This study was supported in part by the grants from the National Natural Science Foundation of China (No. 39625011; No. 39970239) and National Basic Research Program of China (No. 1999054000) to Yun-Qing Li.

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ArticleSerotonin potentiates the response of neurons of the superficial laminae of the rat spinal dorsal horn to γ-aminobutyric acid

Abstract

Introduction

Section snippets

Preparation

GABA-induced inward current

Discussion

Acknowledgements

Neuroscience

Neuropharmacology

Prog. Neurobiol.

Brain Res.

Brain Res.

Brain Res.

Pain

Brain Res.

Neurosci. Lett.

Brain Res.

GABAergic terminals are presynaptic to primary afferent terminals in the substantia gelatinosa of the rat spinal cord

Brain Res.

Bulbospinal projections in the primatesA light and electromicroscopic study of a pain modulation system

J. Comp. Neurol.

Laminar compartmentalization of GABAA receptor subtypes in the spinal cordAn immunohistochemial study

J. Neurosci.

GABA-immunoreactive terminals synapse on primate spinothalamic tract cells

J. Comp. Neurol.

Processing of sensory information in the superficial dorsal horn of the spinal cord

Protein kinase C reduces Mg2+ block of NMDA-receptor channels as a mechanism of modulation

Nature

Inhibition by 5-hydroxytryptamine and noradrenaline in substantia gelatinosa of guinea-pig spinal trigeminal nucleus

J. Physiol.

Actions of opiates, substance P and serotonin on the excitability of primary afferent terminals and observations on interneuronal activity in the neonatal rat’s dorsal horn in vitro

Neuroscience

GABAergic neurons in the mouse superficial dorsal horn with special emphasis on their relation to primary afferent central terminals

Arch. Histol. Cytol.

Termination patterns of serotoninergic medullary raphe-spinal fibres in the rat lumbar spinal cordAn anterograde immunohistochemical study

J. Comp. Neurol.

Enhancement of glycine-activated whole-cell currents by serotonin in the superficial laminae neurons of the rat spinal dorsal horn

Chin. J. Neuroanat.

Dopamine enhances excitatory amino acid-gated conductances in cultured retinal horizontal cells

Nature

Article
Serotonin potentiates the response of neurons of the superficial laminae of the rat spinal dorsal horn to γ-aminobutyric acid

Laminar compartmentalization of GABA_A receptor subtypes in the spinal cordAn immunohistochemial study

Protein kinase C reduces Mg²⁺ block of NMDA-receptor channels as a mechanism of modulation