GABA B receptor-mediated effects on expression of c-Fos in rat trigeminal nucleus following high- and low-intensity afferent stimulation

Attenuation of Sensory Transmission Through the Rat Trigeminal Ganglion by GABA Receptor Activation

While the trigeminal ganglion is often considered a passive conduit of sensory transmission, neurons and satellite glial cells (SGCs) within it can release neurotransmitters and express neuroreceptors. Some trigeminal ganglion neurons contain the neurotransmitter γ-aminobutyric acid (GABA) and express GABA receptors. There is behavioral evidence that increased GABA levels in the trigeminal ganglion decreases nociception, while a loss of GABA receptors results in hyperalgesia, although the neural mechanisms for this remain to be investigated. In this study, the expression of GABA receptors by trigeminal ganglion neurons that innervate rat labial skin and masseter muscle was compared using immunohistochemistry. The effect of intraganglionic administration of GABA receptor agonists was investigated by single unit recording of trigeminal brainstem and ganglion neuron responses to stimulation of the labial skin and/or masseter muscle in anesthetized rats. The mean frequency of expression of GABA_A and GABA_B receptors by masseter and labial skin ganglion neurons was 62.5% and 92.7%, and 55.4% and 20.3%, respectively. The expression of both GABA receptors was significantly greater in skin ganglion neurons. Masticatory muscle evoked brainstem trigeminal neuron responses were significantly attenuated by intraganglionic injection of muscimol (GABA_A) but not baclofen (GABA_B). The mechanical sensitivity of slow and fast conducting masticatory muscle afferent fibers was decreased and increased, respectively, by intraganglionic injection of both muscimol and baclofen. Activation of GABA_A receptors may exert a gating effect on sensory transmission through the trigeminal ganglion by decreasing putative nociceptive input and enhancing innocuous sensory input.

Peripheral and central alterations of pituitary adenylate cyclase activating polypeptide-like immunoreactivity in the rat in response to activation of the trigeminovascular system

Pituitary adenylate cyclase activating polypeptide (PACAP) is present in the cranial arteries and trigeminal sensory neurons. We therefore examined the alterations in PACAP-like immunoreactivity (PACAP-LI) in a time-dependent manner in two rat models of trigeminovascular system (TS) activation. In one group chemical stimulation (CS) was performed with i.p. nitroglycerol (NTG), and in the other one the trigeminal ganglia (TRG) were subjected to electrical stimulation (ES). The two biologically active forms, PACAP-38 and PACAP-27, were determined by means of radioimmunoassay (RIA) and mass spectrometry (MS) in the plasma, the cerebrospinal fluid (CSF), the trigeminal nucleus caudalis (TNC), the spinal cord (SC) and the TRG. The tissue concentrations of PACAP-27 were 10 times lower than those of PACAP-38 in the TNC and SC, but about half in the TRG. PACAP-38, but not PACAP-27, was present in the plasma. Neither form could be identified in the CSF. PACAP-38-LI in the plasma, SC and TRG remained unchanged after CS, but it was increased significantly in the TNC 90 and 180 min after NTG injection. In response to ES of the TRG, the level of PACAP-38 in the plasma and the TNC was significantly elevated 90 and 180 min later, but not in the SC or the TRG. The alterations in the levels of PACAP-27 in the tissue homogenates in response to both forms of stimulation were identical to those of PACAP-38. The selective increases in both forms of PACAP in the TNC suggest its important role in the central sensitization involved in migraine-like headache.

The analgesic effects of the GABAB receptor agonist, baclofen, in a rodent model of functional dyspepsia

BACKGROUND

The amino acid γ-aminobutyric acid (GABA) is an important modulator of pain but its role in visceral pain syndromes is just beginning to be studied. Our aims were to investigate the effect and mechanism of action of the GABA(B) receptor agonist, baclofen, on gastric hypersensitivity in a validated rat model of functional dyspepsia (FD).

METHODS

10-day-old male rats received 0.2 mL of 0.1% iodoacetamide in 2% sucrose daily by oral gavages for 6 days. Control group received 2% sucrose. At 8-10 weeks rats treated with baclofen (0.3, 1, and 3 mg kg(-1) bw) or saline were tested for behavioral and electromyographic (EMG) visceromotor responses; gastric spinal afferent nerve activity to graded gastric distention and Fos protein expression in dorsal horn of spinal cord segments T8-T10 to noxious gastric distention.

KEY RESULTS

Baclofen administration was associated with a significant attenuation of the behavioral and EMG responses (at 1 and 3 mg kg(-1)) and expression of Fos in T8 and T9 segments in neonatal iodoacetamide sensitized rats. However, baclofen administration did not significantly affect splanchnic nerve activity to gastric distention. Baclofen (3 mg kg(-1)) also significantly reduced the expression of spinal Fos in response to gastric distention in control rats to a lesser extent than sensitized rats.

CONCLUSIONS & INFERENCES

Baclofen is effective in attenuating pain associated responses in an experimental model of FD and appears to act by central mechanisms. These results provide a basis for clinical trials of this drug in FD patients.

Conditional gene deletion reveals functional redundancy of GABAB receptors in peripheral nociceptors in vivo

BACKGROUND

gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter which mainly mediates its effects on neurons via ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. GABA(B) receptors are widely expressed in the central and the peripheral nervous system. Although there is evidence for a key function of GABA(B) receptors in the modulation of pain, the relative contribution of peripherally- versus centrally-expressed GABA(B) receptors is unclear.

RESULTS

In order to elucidate the functional relevance of GABA(B) receptors expressed in peripheral nociceptive neurons in pain modulation we generated and analyzed conditional mouse mutants lacking functional GABA(B1) subunit specifically in nociceptors, preserving expression in the spinal cord and brain (SNS-GABA(B1)-/- mice). Lack of the GABA(B1) subunit precludes the assembly of functional GABA(B) receptor. We analyzed SNS-GABA(B1)-/- mice and their control littermates in several models of acute and neuropathic pain. Electrophysiological studies on peripheral afferents revealed higher firing frequencies in SNS-GABA(B1)-/- mice compared to corresponding control littermates. However no differences were seen in basal nociceptive sensitivity between these groups. The development of neuropathic and chronic inflammatory pain was similar across the two genotypes. The duration of nocifensive responses evoked by intraplantar formalin injection was prolonged in the SNS-GABAB(1)-/- animals as compared to their control littermates. Pharmacological experiments revealed that systemic baclofen-induced inhibition of formalin-induced nociceptive behaviors was not dependent upon GABA(B1) expression in nociceptors.

CONCLUSION

This study addressed contribution of GABA(B) receptors expressed on primary afferent nociceptive fibers to the modulation of pain. We observed that neither the development of acute and chronic pain nor the analgesic effects of a systematically-delivered GABA(B) agonist was significantly changed upon a specific deletion of GABA(B) receptors from peripheral nociceptive neurons in vivo. This lets us conclude that GABA(B) receptors in the peripheral nervous system play a less important role than those in the central nervous system in the regulation of pain.

Mechanisms of orofacial pain control in the central nervous system

Recent advances in the study of pain have revealed somatotopic- and modality-dependent processing and the integration of nociceptive signals in the brain and spinal cord. This review summarizes the uniqueness of the trigeminal sensory nucleus (TSN) in structure and function as it relates to orofacial pain control. The oral nociceptive signal is primarily processed in the rostral TSN above the obex, the nucleus principalis (Vp), and the subnuclei oralis (SpVo) and interpolaris (SpVi), while secondarily processed in the subnucleus caudalis (SpVc). In contrast, the facial nociceptive signal is primarily processed in the SpVc. The neurons projecting to the thalamus are localized mostly in the Vp, moderately in the SpVi, and modestly in the ventrolateral SpVo and the SpVc. Orofacial sensory inputs are modulated in many different ways: by interneurons in the TSN proper, through reciprocal connection between the TSN and rostral ventromedial medulla, and by the cerebral cortex. A wide variety of neuroactive substances, including substance P, gamma-aminobutyric acid, serotonin and nitric oxide (NO) could be involved in the modulatory functions of these curcuits. The earliest expression of NO synthase (NOS) in the developing rat brain is observed in a discrete neuronal population in the SpVo at embryonic day 15. NOS expression in the SpVc is late at postnatal day 10. The neurons receiving intraoral signals are intimately related with the sensorimotor reflexive function through the SpVo. In summary, a better understanding of the trigeminal sensory system--which differs from the spinal system--will help to find potential therapeutic targets and lend to developing new analgesics for orofacial-specific pain with high efficacy and fewer side effects.

Activaton of GABAB receptor inhibits the excitability of rat small diameter trigeminal root ganglion neurons

A selective GABA(B) receptor agonist, baclofen, is known to suppress neuropathic pain. In the present study, we investigated the effect of baclofen on the excitability of trigeminal root ganglion (TRG) neurons by using the whole cell and perforated patch-clamp recording techniques. Under voltage-clamp (V(h)=-60 mV), voltage-dependent K(+) currents were recorded in the small diameter TRG neurons (<30 microm) 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 a sustained (I(k)), were identified. Baclofen significantly increased I(A) by 74.8% (50 microM) and in a dose-dependent manner (1-50 microM). Similarly, I(K) was also enhanced by baclofen administration (41.8%: 50 microM). The relative amplitude of potentiation of I(A) was significantly higher than that of I(K) (P<0.05). Baclofen-sensitive I(A) and I(K) currents were antagonized by K(+) channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). The augmentation of K(+) currents was antagonized by 3-amino-2-(4-chlorophenyl)-2-hydroxypropylsulfonic acid (saclofen; GABA(B) antagonist). In the current clamp mode, the resting membrane potential was -62+/-1.6 mV (n=24). Hyperpolarization of the membrane potential was elicited by baclofen (10-50 microM), and the response was associated with a decrease in the input resistance. Baclofen induced-hyperpolarization was blocked by saclofen (100 microM). In the presence of both 4-AP and TEA, no significant changes in membrane potential induced by baclofen application were observed. In the presence of BaCl(2), baclofen-evoked hyperpolarization with decreased resistance was observed. During application of baclofen, the firing rate of the action potentials by depolarizing step pulses was decreased. Application of baclofen reduced action potential duration evoked by a depolarization current pulse.These results indicated that activation of GABA(B) receptors inhibits the excitability of rat small diameter TRG neurons and this inhibitory action is mediated by potentiation of voltage-dependent K(+) currents. We therefore concluded that modification of nociceptive transmission in the trigeminal system by activation of GABA(B) receptors occurs at the level of small TRG neuron cell bodies and/or their primary afferent terminals, which are potential targets of analgesia by baclofen.

Localization of trigeminal, spinal, and reticular neurons involved in the rat blink reflex

Electrical stimulation of the supraorbital nerve (SO) induces eyelid closure by activation of orbicularis oculi muscle motoneurons located in the facial motor nucleus (VII). Neurons involved in brainstem central pathways implicated in rat blink reflex were localized by analyzing c-Fos protein expression after SO stimulation in conjunction with tracing experiments. A retrograde tracer (gold-horseradish peroxidase [HRP]) was injected into the VII. The distribution patterns of activated c-Fos-immunoreactive neurons and of neurons exhibiting both c-Fos immunoreactivity and gold-HRP labeling were determined in the sensory trigeminal complex (STC), the cervical spinal cord (C1), and the pontomedullary reticular formation. Within the STC, c-Fos immunoreactivity labeled neurons in the ipsilateral ventral part of the principal nucleus, the pars oralis and interpolaris, and bilaterally in the pars caudalis. Colocalization of gold-HRP and c-Fos immunoreactivity was observed in neurons of ventral pars caudalis layers I-IV and ventral pars interpolaris. In C1, SO stimulation revealed c-Fos neurons in laminae I-V. After additional injections in VII, the double-labeled c-Fos/gold-HRP neurons were concentrated in laminae IV and V. Although c-Fos neurons were found throughout the pontomedullary reticular formation, most appeared rostrally around the motor trigeminal nucleus and in the ventral parvocellular reticular nucleus medial to the fiber bundles of the seventh nerve. Caudally, c-Fos neurons were in the lateral portion of the dorsal medullary reticular field. In addition, these reticular areas contained double-labeled neurons in electrically stimulated rats that had received gold-HRP injections in the VII. The presence of double-labeled neurons in the STC, C1, and the reticular formation implies that these neurons receive sensory information from eyelids and project to the VII. These double-labeled neurons could then be involved in di- or trisynaptic pathways contributing to the blink reflex.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.