Opioidergic modulation of excitability of rat trigeminal root ganglion neuron projections to the superficial layer of cervical dorsal horn

• Published in: Neuroscience Vol. 125; no. 4; pp. 995 - 1008
• Main Authors: Takeda, M, Tanimoto, T, Ikeda, M, Kadoi, J, Nasu, M, Matsumoto, S
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 2004; Elsevier
• Subjects: Analgesics, Opioid - pharmacology; Animals; Biological and medical sciences; Cervical Vertebrae; Dose-Response Relationship, Drug; Enkephalin, Ala-MePhe-Gly- - pharmacology; Fundamental and applied biological sciences. Psychology; Immunohistochemistry; Male; Membrane Potentials - drug effects; Neurons - metabolism; Patch-Clamp Techniques; Posterior Horn Cells - cytology; Posterior Horn Cells - metabolism; Potassium Channel Blockers - pharmacology; Potassium Channels, Voltage-Gated - drug effects; Potassium Channels, Voltage-Gated - metabolism; Rats; Receptors, Opioid, mu - agonists; Receptors, Opioid, mu - drug effects; Receptors, Opioid, mu - metabolism; retrograde-labeling; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger - analysis; RT-PCR; Somatostatin - analogs & derivatives; Somatostatin - pharmacology; Trigeminal Ganglion - drug effects; Trigeminal Ganglion - physiology; trigeminal root ganglion; Vertebrates: nervous system and sense organs; voltage-dependent K + channel; μ-opioid receptor; FG; DAMGO; TRG; Kv; CTOP; RT-PCR; TTX; immunohistochemistry; trigeminal root ganglion; PBS; DTx; I A; EDTA; I D; PIPES; SpVc; μ-opioid receptor; EGTA; retrograde-labeling; 4-AP; TEA; voltage-dependent K + channel; I K; HEPES; SpVi; DRG; GAPDH; Immunohistochemistry; Spinal cord; Rat; Rodentia; Central nervous system; μ Opioid receptor; Excitability; Projection; Ionic channel; Posterior spinal horn; Polymerase chain reaction; Ganglion neuron; Vertebrata; Mammalia; Animal; voltage-dependent K+ channel; Trigeminal ganglion; Potassium
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/j.neuroscience.2004.02.029

The aim of the present study was to investigate the effect of a μ-opioid receptor agonist DAMGO (Tyr- d-Ala-Gly-NMe-Phe-Gly-ol) on the excitability of trigeminal root ganglion (TRG) neurons, projecting onto the superficial layer of the cervical dorsal horn, by using the perforated-patch technique and to determine whether TRG neurons show the expression of mRNA or functional protein for μ-opioid receptors by using reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. TRG neurons projecting onto the superficial layer of the cervical dorsal horn were retrogradely labeled with Fluorogold (FG). The cell diameter of FG-labeled TRG neurons was small (<30 μm). Under voltage-clamp (V h=−60 mV), voltage-dependent K + currents were recorded in the TRG neurons and isolated by blocking Na + and Ca 2+ currents with appropriate ion replacement. Separation of the K + current components was achieved by the response to variation in the conditioning voltage. Two distinct K + current components, a transient (I A) and sustained (I K), were identified. DAMGO significantly increased I A by 57% (20 μM) and in a dose-dependent manner (1–50 μM). Similarly, I K was also enhanced by DAMGO administration (42%, 20 μM). The augmentation of both I A and I K was antagonized by a μ-opioid receptor antagonist, CTOP ( d-Phe-Cys-Thr- d-Trp-Orn-Thr-Pen-Thr-NH 2). Hyperpolarization of the membrane potential was elicited by DAMGO (20 μM) and the response was associated with a decrease in the input resistance. DAMGO induced hyperpolarization was blocked by CTOP. DAMGO-sensitive I A and I K currents were antagonized by K + channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). In the presence of both 4-AP and TEA, no significant changes in membrane potential induced by DAMGO application were observed. In the presence of BaCl 2, DAMGO evoked hyperpolarization with decreased resistance was observed. The firing rate of action potentials and the first spike duration induced by depolarizing step pulses were decreased in the presence of DAMGO. RT-PCR analysis demonstrated the expression of mRNA for μ-opioid receptors in the trigeminal ganglia. The μ-opioid receptor immunoreactivity was expressed in the small diameter FG-labeled TRG neurons. These results suggest that the activation of μ-opioid receptors inhibits the excitability of rat small diameter TRG neurons projecting on the superficial layer of the cervical dorsal horn and this inhibition is mediated by potentiation of voltage-dependent K + currents. We therefore concluded that modulation of nociceptive transmission in the trigeminal system, resulting in the functional activation of μ-opioid receptors, occurs at the level of small TRG cell bodies and/or their primary afferent terminals, which contribute to opioid analgesia in the trigeminal pain.