Release of immunoreactive substance P in the trigeminal brain stem nuclear complex evoked by chemical stimulation of the nasal mucosa and the dura mater encephali--a study with antibody microprobes.

In order to study a possible involvement of substance P in the processing of chemonociceptive input from the nasal mucosa and the dura mater encephali in the spinal trigeminal, the release of immunoreactive substance P was measured in the trigeminal brain stem nuclear complex in anaesthetized rats. Microprobes coated with antibody to substance P were inserted into the lateral area of the brain stem up to 1 mm posterior to the obex corresponding to the trigeminal subnucleus caudalis. When the nasal mucosa was stimulated by topical administration of mustard oil (1% and 5%) into the nostrils, immunoreactive substance P was mainly detected in the dorsal region of the trigeminal brain stem nuclear complex with a maximum in the superficial gray matter. When the dura mater encephali was stimulated by topical administration of Tyrode's solution (pH 6.2), immunoreactive substance P was mainly released in the ventral region of the trigeminal brain stem nuclear complex; with pH 5.5 the release was more diffuse extending from the ventral to the dorsal part of the spinal trigeminal nucleus. Release was maximal rather after than during the administration of the stimuli, and it considerably outlasted the stimulation periods. These data suggest that substance P plays an important role in the processing of chemonociceptive inputs from the nasal mucosa and the dura mater encephali in the trigeminal brain stem nuclear complex. Substance P may be important, therefore, in the generation of those headaches that are caused by affections of the nasal mucosa and the dura mater encephali. Since enhanced levels of immunoreactive substance P were present for considerable time periods beyond the administration of the stimuli, substance P and neurokinin-1 receptors may be involved in long-lasting neuronal events following noxious stimulation.

Responses of rat medullary dorsal horn neurons following intranasal noxious chemical stimulation: effects of stimulus intensity, duration, and interstimulus interval.

1. Most quantitative examinations of nociception are performed with thermal or mechanical stimuli. Because nociceptive processing mechanisms may depend on the modality of the stimuli, comparable studies on chemonociception are necessary. 2. We examined the activity of chemonociceptive medullary dorsal horn neurons in halothane-anesthetized rats. For controlled noxious chemical stimulation, defined CO2 pulses were applied to the nasal mucosa. The effects of stimulus intensity, duration, and interstimulus interval (ISI) were tested by performing three different CO2 stimulation protocols (see below). 3. The recorded neurons were characterized by intranasal and facial stimuli of different modalities. The cells received input from intranasal A delta- and/or C-fibers. All tested neurons also responded to other intranasally applied irritants, e.g., mustard oil. Furthermore, the units were sensitive to intranasal high-threshold mechanical stimulation and to facial mechanical stimulation. According to the properties of their facial mechanoreceptive fields, the units were classified as wide dynamic range (WDR) or nociceptive specific (NS) neurons. The majority of the cells also responded to facially applied noxious heat stimuli, so that most of the recorded neurons were found to be multimodal. Some of the neurons, in addition, had convergent input from primary afferents innervating the maxillary tooth pulps or the cornea and periorbital structures. 4. In the first stimulation protocol we presented four different CO2 concentrations (25, 50, 75, and 100%; stimulus duration 2 s). In total, each concentration was applied 10 times (2 trains of 5 stimuli). Stimulus response functions (SRFs) were computed with average responses to identical stimuli. All but 2 of the 23 tested neurons displayed enhanced responses after stimulation with increasing intensities. In general, WDR cells (n = 15) discharged more vigorously to the same CO2 concentration than NS cells (n = 8). WDR neurons discriminated more reliably between stimulus intensities in the low to moderate range (25-50% CO2) than NS cells. Both categories of neurons, however, discriminated equally well in the moderate- to high-intensity range (50-75% CO2). The discriminatory capacity of WDR and NS neurons was reduced in the highest concentration range (75-100% CO2). The proportion of NS neurons significantly discriminating between these intensities tended to be higher compared with WDR neurons when stimuli were applied with long ISIs (120 s). 5. To examine the effects of the duration of the ISI, identical test sequences were performed with ISIs of 30 and 120 s. (ABSTRACT TRUNCATED AT 400 WORDS)

Controlled noxious chemical stimulation: responses of rat trigeminal brainstem neurones to CO2 pulses applied to the nasal mucosa.

The nasal mucosa of halothane-anesthetized rats was stimulated with defined CO2 pulses. Recordings were performed from single trigeminal brainstem neurons to characterize their responses to this controlled chemical irritation. All cells examined with this stimulus displayed graded discharges to increasing concentrations of CO2. Enhanced responses were obtained in a group of neurons when the duration of the interstimulus interval was increased. The application of chemical irritants, notably mustard oil or acetic acid induced vigorous ongoing discharges in all cells tested. The CO2 stimulation method described here thus provides an ideal model for the quantitative physiological and pharmacological examination of chemically induced nociception.

c-FOS-like immunoreactivity in rat brainstem neurons following noxious chemical stimulation of the nasal mucosa.

It has previously been shown that noxious and non-noxious peripheral stimuli induce c-fos expression in spinal dorsal horn neurons. In the present study we have examined the expression of c-fos in brainstem neurons following noxious chemical stimulation of the respiratory region of the nasal mucosa. In urethane-anaesthetized rats we injected mustard oil or applied CO2 pulses to the right nasal cavity. In control animals we applied paraffin oil or a continuous flow of air. A further group of control animals was anaesthetized and not subjected to any experimental treatment. Two hours after the first stimulus the rats were perfused with 4% phosphate-buffered paraformaldehyde. Brainstem sections were incubated with primary antiserum against the FOS protein and processed according to the ABC method. Only the mustard oil-treated rats had obvious signs of rhinitis and displayed FOS-positive cells in laminae I and II of the subnucleus caudalis and in the subnucleus interpolaris of the trigeminal brainstem nuclear complex as well as in the medullary lateral reticular nucleus. These areas are known to be involved in the processing of nociceptive information. Although CO2 pulses applied to the nasal mucosa are known to evoke pain sensations in man we did not observe any FOS-positive neurons in trigeminal and reticular brainstem areas of CO2-treated rats. This lack of c-fos expression probably results from the fact that unlike mustard oil, CO2 did not induce any apparent inflammatory reactions. In all animals c-fos expression was found in the nucleus of the solitary tract and in the area postrema. Staining in these areas might partly result from factors related to anaesthesia, changed respiration parameters and stress. Since the mustard oil-treated rats displayed the highest levels of immunoreactivity in the nucleus of the solitary tract and in the area postrema, additional effects specifically related to nociceptive input are very likely.

Central projections of trigeminal primary afferents innervating the nasal mucosa: a horseradish peroxidase study in the rat.

The respiratory region of the nasal mucosa is innervated by the ethmoidal nerve. Chemical nociceptive stimulation of this area leads to upper airway reflexes that prevent access of noxious substances to the respiratory tract and the lungs. In the present study we examined the localization of the cell bodies of the respective primary afferent fibres within the trigeminal ganglion, as well as their central projections. In 25 rats a horseradish peroxidase-wheat germ agglutinin gel was applied to the right nasal cavity. The animals were killed after 48-72 h. For visualization of the tracer the tissue was processed according to the tetramethylbenzidine method. In the trigeminal ganglion almost all labelled cell bodies were localized in a medial band immediately caudal to the entrance of the ophthalmomaxillary branch. Transganglionic projections to the trigeminal brainstem nuclear complex were only localized in the superficial laminae of the subnucleus caudalis and in the subnucleus interpolaris, areas known to be involved in processing of nociceptive information. An additional labelled terminal field was observed in the interstitial subnucleus of the nucleus tractus solitarius, which is involved in respiratory control. These results are in favour of the hypothesis that the ethmoidal nerve in rat constitutes the afferent limb of protective upper airway reflexes since it transmits mainly nociceptive information.