Trigeminal receptors

Birds may have a trigeminal receptor for o-aminoacetophenone and methyl anthranilate, analogous to the capsaicin receptor in mammals (Clark and Shah, 1994). 

Tucker, D. Olfactory, vomernasal and trigeminal receptor responses to odorants. In Y. Zotterman, Ed., "Olfaction and Taste" Pergamon Press Oxford, 1963 pp. 45-69. 

TK NKxr displays a broad distribution in both peripheral tissues and in the central nervous system (CNS). In both CNS and enteric neurons, NKxr stimulation increase their excitability, whereas in trigeminal ganglion neurons SP has no intrinsic electrophysio-logical effects but is capable to enhance the amplitude of the inward current induced by the stimulation of serotonin 5-HT3 recqrtors. This enhancement dqjends on the activation of PKC via the stimulation of NKX recqrtors. This is an interesting case of receptor cross talk. Other functions of NKxr have been also highlighted. 

TRPVl, also known as the capsaicin- or vanilloid-receptor, is a nonselective cation channel expressed e.g., in neurons of the dorsal root and trigeminal ganglions, which integrates multiple pain-producing stimuli including heat, protons, capsaicin, and resiniferatoxin. In addition, TRPVl currents can be activated by ananda-mide, protein kinase C (PKC), and by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2). 

Direct attenuation of the excitability of neurons in the trigeminal nuclei, as 5-HTib/ 5-HTid receptors on pain transmission neurons in the trigeminal nucleus caudalis and in the upper cervical cord, are activated. Stimulation of these receptors is caused by second-generation triptans that cross the blood-brain barrier such as zolmitriptan, naratriptan, rizatriptan and eletriptan. 

Sahara, Y., Noro, N., Iida, Y., Soma, K., and Nakamura, Y. (1997) Glutamate receptor subunits GluR5 and KA-2 are coexpressed in rat trigeminal ganglion neurons. J. Neurosci. 17, 6611-6620. 

However, both sensory and pulmonary irritation can be expected with sufficient exposure. Sensory irritation is characterized by immediate eye and nose irritation that may increase to sensations of burning and pain and is due to interaction between the substance and receptors in the trigeminal nerve. Vapors reaching the lower respiratory tract as well as the lungs may interact with the nerves in these regions, causing dyspnea and breathlessness or pulmonary irritation. 

Baclofen, labeled as a skeletal muscle relaxant binds to GABA receptors and depresses excitation. It is also useful in the treatment of paroxysms of trigeminal neuralgia. Baclofen is effective in patients with carbamazepine-resistant pain and has been used successfully to relieve attacks in patients previously unresponsive to carbamazepine or phenytoin. 

HT3 receptors are located exclusively on neurons and are widely distributed throughout the peripheral and central nervous systems. In the periphery, 5-HT3 receptors are found on autonomic, sensory, and enteric neurons (Fozard 1984). In the central nervous system, 5-HT3 receptors are labeled in cortex, hippocampus, caudate hypothalamus, brain stem, midbrain, and cerebellum, with the highest density in discrete nuclei of the lower brain stem (e.g., dorsal vagal complex and spinal trigeminal nucleus), the area postrema, and substantia gelatinosa at all levels of the spinal cord (Palacios et al. 1991 Waeber et al. 1989). 

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