Amino acid release at the spinomedullary junction after inflammation of the TMJ region in male and female rats.

Temporomandibular joint (TMJ) disorders are painful conditions that are more prevalent in women than men. This study tested the hypothesis that acute inflammation of the TMJ region evoked sex-related changes in amino acid transmitter concentrations at the trigeminal subnucleus/upper cervical cord (Vc/C2) junction, the major terminal zone for TMJ sensory afferents. Microdialysis samples were collected in male, intact and ovariectomized (OvX) female rats after injection of mustard oil into the TMJ region (TMJ-MO) under barbiturate anesthesia. Males displayed increases in glutamate, aspartate and serine at 5 min and secondary increases 40-45 min after TMJ-MO. Intact and OvX females given low dose estrogen (LE2) displayed increases in glutamate, aspartate and serine at 5 min but no secondary increase at 40 min, while OvX females given high dose estrogen (HE2) revealed no increases after TMJ-MO. Glycine increased 20 min after TMJ-MO in males and cycling females, but not in OvX rats. Perfusion of high potassium through the probe evoked similar increases in glutamate, aspartate and glycine in all groups. In separate experiments, perfusion of the glutamate-aspartate reuptake inhibitor, L-trans-2,4-pyrrolidine dicarboxylate (PDC), through the probe caused a prompt elevation in glutamate that was significantly greater in HE2 than LE2 females or males. These results suggested sex hormone status affects glutamatergic neurotransmission at the Vc/C2 junction by acting, in part, through modulation of glutamate reuptake. Altered amino acid transmitter release and/or availability at the Vc/C2 junction may contribute to differential processing of sensory input from the TMJ region in males and females.

Sex differences in amino acid release from rostral trigeminal subnucleus caudalis after acute injury to the TMJ region.

The neurological basis for painful temporomandibular disorders (TMD) and the higher prevalence of TMD pain in women than men is not known. To better define the circuitry and neurochemical mechanisms in the lower brainstem associated with noxious sensory inputs from the temporomandibular joint (TMJ) region a microdialysis method was used to measure the release of amino acid transmitters from the ventral trigeminal subnucleus interpolaris/caudalis transition region (Vi/Vc-vl). The irritant chemical, mustard oil, was injected into the TMJ region (TMJ-MO) under barbiturate anesthesia in males and normal cycling female rats. Males displayed significant increases in glutamate, serine, and glycine within 15 min after TMJ-MO and increases in citrulline occurred after a delay of 15-30 min. TMJ-MO did not enhance amino acid release in diestrus or proestrus females. GABA release was not affected by TMJ-MO in males or females. Pretreatment with morphine (3 mg/kg, i.v.) prevented the increase in amino acid release seen after TMJ-MO in males. Amino acid release at the Vi/Vc-vl transition region evoked by TMJ-MO also was prevented by prior microinjection of the GABA(A) receptor agonist, muscimol, into the most caudal portion of Vc suggesting this region acted as a critical relay for nociceptive inputs from the TMJ region. These results suggest that glutamatergic mechanisms acting at the Vi/Vc-vl transition region contribute to processing of nociceptive signals that arise from the TMJ region. These results also are consistent with the hypothesis that central neural mechanisms that integrate nociceptive inputs from deep craniofacial tissues are different in males and females.

Encoding of corneal input in two distinct regions of the spinal trigeminal nucleus in the rat: cutaneous receptive field properties, responses to thermal and chemical stimulation, modulation by diffuse noxious inhibitory controls, and projections to the parabrachial area.

To determine whether corneal input is processed similarly at rostral and caudal levels of the spinal trigeminal nucleus, the response properties of second-order neurons at the transition between trigeminal subnucleus interpolaris and subnucleus caudalis (Vi/Vc) and at the transition between subnucleus caudalis and the cervical spinal cord (Vc/C1) were compared. Extracellular single units were recorded in 68 Sprague-Dawley rats under chloralose or urethan/chloralose anesthesia. Neurons that responded to electrical stimulation of the cornea at the Vi/Vc transition region (n = 61) and at laminae I/II of the Vc/C1 transition region (n = 33) were classified regarding 1) corneal mechanical threshold; 2) cutaneous mechanoreceptive field, if present; 3) electrical input characteristics (A and/or C fiber); 4) response to thermal stimulation; 5) response to the small-fiber excitant, mustard oil (MO), applied to the cornea; 6) diffuse noxious inhibitory controls (DNIC); and 7) projection status to the contralateral parabrachial area (PBA). On the basis of cutaneous receptive field properties, neurons were classified as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), nociceptive specific (NS), or deep nociceptive (D). All neurons recorded at the Vc/C1 transition region were either WDR (n = 19) or NS (n = 14). In contrast, 54% of the Vi/Vc neurons had no cutaneous receptive field. Of those Vi/Vc neurons that had a cutaneous receptive field, 57% were LTM, 25% were WDR, and 18% were D. All Vc/ C1 neurons responded to noxious thermal and MO stimulation. Only 22 of 47 and 13 of 19 Vi/Vc corneal units responded to thermal or MO stimulation, respectively. At the Vc/C1 transition region, 12 of 17 neurons demonstrated DNIC, whereas at the Vi/Vc transition region, DNIC was present in only 4 of 26 neurons. Of 15 Vc/C1 corneal units, 12 could be antidromically activated from the contralateral PBA (average latency 6.29 ms, range 1.8-26 ms). None of 22 Vi/Vc corneal units tested could be antidromically activated from the PBA. These findings suggest that neurons in laminae I/II at the Vc/C1 transition and at the Vi/Vc transition process corneal input differently. Neurons in laminae I/II at the Vc/C1 transition process corneal afferent input consistent with that from other orofacial regions. Corneal-responsive neurons at the Vi/Vc transition region may be important in motor reflexes or in recruitment of descending antinociceptive controls.

Excitatory amino release within spinal trigeminal nucleus after mustard oil injection into the temporomandibular joint region of the rat.

Inflammation of the temporomandibular joint (TMJ) region evokes pain and hyperalgesia as well as causing persistent changes in the response properties of central trigeminal neurons. To determine if excitatory amino acids have a role in TMJ-induced responses, extracellular concentrations were measured in microdialysate samples from probes positioned in the spinal trigeminal nucleus (Vsp) near the transition region between subnucleus interpolaris and subnucleus caudalis (Vi/Vc) in chloralose-anesthetized rats. Injection of the selective small fiber excitant, mustard oil (20 microliters, 20% solution), into the ipsilateral TMJ region caused a transient (by 10 min) increase in glutamate (from 0.48 +/- 0.16 to 1.94 +/- 0.78 microM, P < 0.005) and aspartate (from 0.29 +/- 0.11 to 1.78 +/- 0.82 microM, P < 0.025) among sites located at the ventrolateral pole of the Vi/Vc transition region (n = 6). Samples from probes located within the ventral Vsp, but outside this Vi/Vc transition region (n = 9), did not show significant changes in amino acid concentrations. Glutamate and aspartate also increased after mustard oil injections into the contralateral TMJ region. Dialysate concentrations of serine and taurine did not change significantly after mustard oil injections. Addition of high potassium (150 mM) to the perfusate solution caused increases in glutamate and aspartate regardless of probe location. The transient and selective release of glutamate and aspartate within the Vi/Vc transition after acute irritation of the TMJ region is consistent with a proposed role for excitatory amino acids in mediating noxious sensory input from deep orofacial structures. Together with previous reports of c-fos expression, these results suggest that neurons within the ventrolateral portion of the Vi/Vc transition may serve as a relay site for the integration of sensory or reflex responses to acute inflammation of the TMJ region.

Central trigeminal effects of somatostatin and etorphine on adrenal and autonomic function in the cat.

The influence of somatostatin and the potent mu-opiate receptor agonist etorphine on adrenal and autonomic responses mediated by trigeminal neurons was examined in chloralose-anesthetized cats. Microinjections of somatostatin (100 pmol) into laminae I-II of trigeminal subnucleus caudalis (Vc) evoked increases in the adrenal secretion of catecholamines and adrenal blood flow without affecting arterial pressure, heart rate, or plasma adrenocorticotropic hormone. Injections of somatostatin into laminae III-IV of Vc had no effect. Microinjections of etorphine (500 pmol) into laminae I-II of Vc had no effect, whereas injections into laminae III-IV caused small increases in total adrenal blood flow and peripheral concentrations of norepinephrine. To assess local effects of these drugs on peripheral trigeminal nociceptor-evoked autonomic responses, corneal stimulation was presented before and after topical application of somatostatin or etorphine on the dorsal brain stem surface. Somatostatin did not affect the adrenal and autonomic responses evoked by noxious thermal or chemical irritant stimulation of the cornea. Topical etorphine blocked completely the increase in adrenal blood flow, arterial pressure, and heart rate after noxious thermal stimulation of the cornea. These results suggest that somatostatin and mu-opiate receptor agonists act by different mechanisms at the level of the spinal trigeminal complex to affect adrenal and autonomic function.

Caudal portions of the spinal trigeminal complex are necessary for autonomic responses and display Fos-like immunoreactivity after corneal stimulation in the cat.

Corneal input to the spinal trigeminal nucleus (Vsp) was assessed by examining Fos-like immunoreactivity (Fos-LI) after chemical irritant stimulation by mustard oil in chloralose-anesthetized cats. The distribution of Fos-LI within the ipsilateral Vsp was bimodal: a dominant group of cells within the superficial laminae at caudal levels of subnucleus caudalis and a second group of cells within the ventrolateral pole of Vsp at obex levels and within the adjacent interstitial islands. Few Fos-positive cells were seen within the Vsp rostral to the mid-portion of subnucleus interpolaris or within the contralateral Vsp. To assess the involvement of caudal portions of the Vsp in mediating the adrenal and autonomic responses to corneal stimulation, mustard oil was applied before and after lidocaine blockade of the Vsp at obex levels in a second group of cats. Corneal stimulation alone increased significantly (P < 0.001) the adrenal secretion of catecholamines, adrenal blood flow, mean arterial pressure and heart rate. With the exception of heart rate, the adrenal and autonomic responses to mustard oil were greatly attenuated or abolished by lidocaine blockade of the ipsilateral Vsp at the level of the obex, a region that displayed a high number of Fos-positive cells after corneal stimulation. These results indicate that neurons within the Vsp at or more caudal than the level of the obex process chemical irritant input from the cornea and are necessary for corneal-evoked changes in adrenal and autonomic function.

Microinjections of calcitonin gene-related peptide within the trigeminal subnucleus caudalis of the cat affects adrenal and autonomic function.

To assess the role of calcitonin gene-related peptide (CGRP) within the trigeminal subnucleus caudalis (Vc) on adrenal and autonomic function, microinjections were directed at different laminae of Vc in chloralose-anesthetized cats. Microinjections of CGRP (5 pmol, 100 nl) into laminae I-II increased significantly the adrenal secretion of epinephrine, adrenal blood flow, adrenal vascular conductance, mean arterial pressure and heart rate. Injections of CGRP into laminae V-VI decreased significantly the adrenal secretion of epinephrine, however, other measured variables were not affected. To examine if CGRP interacts with substance P within Vc to modify adrenal and autonomic function, subthreshold doses of each peptide were injected alone and simultaneously. Combined subthreshold doses of CGRP and substance P injected into laminae V-VI, but not into laminae I-II or III-IV, evoked increases in arterial pressure and in heart rate that exceeded the responses seen after injection of either peptide alone. The adrenal secretion of catecholamines was not affected by individual or combined subthreshold doses of either peptide, regardless of the laminar site of injection. These data suggest that release of CGRP within laminae I-II of Vc alters adrenal and autonomic function via mechanisms separate from those that mediate substance P-evoked responses. In contrast, CGRP and substance P may act, at least in part, through a common neural substrate within the deeper laminae of Vc to modify arterial pressure and heart rate. Thus, multiple subpopulations of peptide-responsive neurons in the medullary dorsal horn likely contribute to the reflex adrenal and autonomic responses that often accompany nociception.

Thermal nociception potentiates the release of ACTH and norepinephrine by blood loss.

Neuroendocrine and autonomic responses were assessed in chloralose-anesthetized cats after nociceptor activation and after acute blood loss, two of the sensory components that accompany injury. Plasma adrenocorticotropic hormone (ACTH), catecholamines (peripheral and adrenal), and cardiovascular responses were examined after corneal thermal stimulation (52 degrees C), after hemorrhage (10 ml/kg), and after simultaneous presentation of these two stimuli. Corneal heat during hemorrhage evoked an exaggerated increase in plasma ACTH (+185 +/- 76 pg/ml, P less than 0.01) and in peripheral plasma norepinephrine (+0.41 +/- 0.11 ng/ml, P less than 0.01) compared with the responses seen after either stimulus alone. A peripheral origin of norepinephrine was indicated, since the adrenal secretion of catecholamines increased similarly after all stimuli. Extracellular levels of norepinephrine, collected by push-pull perfusion, within raphe pallidus increased after corneal heat but not after blood loss and did not reflect the interaction between these two stimuli. Results indicated that physiological stimulation of nociceptive and cardiovascular receptors interact to potentiate the release of ACTH and norepinephrine into peripheral blood, effects that were not predicted from the responses to either sensory input alone.