A behavioural model for the study of facial nociception and the effects of descending modulatory systems in the rat

Cortical pain induced by optogenetic cortical spreading depression: from whole brain activity mapping

BACKGROUND

Cortical spreading depression (CSD) is an electrophysiological event underlying migraine aura. Traditional CSD models are invasive and often cause injuries. The aim of the study was to establish a minimally invasive optogenetic CSD model and identify the active networks after CSD using whole-brain activity mapping.

METHODS

CSD was induced in mice by light illumination, and their periorbital thresholds and behaviours in the open field, elevated plus-maze and light-aversion were recorded. Using c-fos, we mapped the brain activity after CSD. The whole brain was imaged, reconstructed and analyzed using the Volumetric Imaging with Synchronized on-the-fly-scan and Readout technique. To ensure the accuracy of the results, the immunofluorescence staining method was used to verify the imaging results.

RESULTS

The optogenetic CSD model showed significantly decreased periorbital thresholds, increased facial grooming and freezing behaviours and prominent light-aversion behaviours. Brain activity mapping revealed that the somatosensory, primary sensory, olfactory, basal ganglia and default mode networks were activated. However, the thalamus and trigeminal nucleus caudalis were not activated.

CONCLUSIONS

Optogenetic CSD model could mimic the behaviours of headache and photophobia. Moreover, the optogenetic CSD could activate multiple sensory cortical regions without the thalamus or trigeminal nucleus caudalis to induce cortical pain.

Inhibited Expression of α4β2 Nicotinic Acetylcholine Receptor in Blood Leukocytes of Chinese Patients with Vascular Dementia and in Blood Leukocytes as Well as the Hippocampus of Brain from Ischemic Rats

Our present aim was to investigate whether changes in the expression of α₄β₂ nicotinic acetylcholine receptor (nAChR) in patients with vascular dementia (VaD) and ischemic rats are related to cognitive scores. Blood leukocytes for 59 Chinese patients with VaD (diagnosed on the basis of clinical guidelines) and 31 cases as age-matched controls were examined, and the animal model established employing Pulsinelli's four-vessel occlusion. The levels of α₄ and β₂ subunit mRNA in leukocytes and the hippocampus were analyzed by real-time PCR, and the protein level in the hippocampus by Western blotting. The mini-mental state examination was utilized to characterize the intellectual capacity of the patients with reference to the DSM IV diagnosis and Hachinski Ischemic Scale score, and the Morris Water Maze test to assess the ability of learning and memory of the rats. In patients, the level of α₄ mRNA, but not β₂, in blood leukocytes was clearly lowered, which was significantly correlated to their clinical cognitive test scores. Smoking exerted no impact on the level of α₄ mRNA in the present study. In the blood leukocytes and the hippocampus of the brains of the ischemic rats, the levels of both α₄ and β₂ mRNA were lowered, and the proteins of these subunits in the hippocampus were decreased. The changes of α₄ and β₂ mRNA in blood leukocytes, and their protein levels in the hippocampus were significantly correlated with impaired learning and memory. These findings indicate that alterations in expression of the α₄β₂ subtype of nAChR may be involved in the molecular mechanism(s) underlying the cognitive deficit associated with VaD.

A novel operant-based behavioral assay of mechanical allodynia in the orofacial region of rats

BACKGROUND

Detecting behaviors related to orofacial pain in rodent models often relies on subjective investigator grades or methods that place the animal in a stressful environment. In this study, an operant-based behavioral assay is presented for the assessment of orofacial tactile sensitivity in the rat.

NEW METHODS

In the testing chamber, rats are provided access to a sweetened condensed milk bottle; however, a 360° array of stainless steel wire loops impedes access. To receive the reward, an animal must engage the wires across the orofacial region. Contact with the bottle triggers a motor, requiring the animal to accept increasing pressure on the face during the test. To evaluate this approach, tolerated bottle distance was measured for 10 hairless Sprague Dawley rats at baseline and 30 min after application of capsaicin cream (0.1%) to the face. The experiment was repeated to evaluate the ability of morphine to reverse this effect.

RESULTS

The application of capsaicin cream reduced tolerated bottle distance measures relative to baseline (p<0.05). As long as morphine did not cause reduced participation due to sedation, subcutaneous morphine dosing reduced the effects of capsaicin (p<0.001). Comparison with existing method: For behavioral tests, experimenters often make subjective decisions of an animal's response. Operant methods can reduce these effects by measuring an animal's selection in a reward-conflict decision. Herein, a method to measure orofacial sensitivity is presented using an operant system.

CONCLUSIONS

This operant device allows for consistent measurement of heightened tactile sensitivity in the orofacial regions of the rat.

Opioidergic orofacial antinociception induced by electroacupuncture at acupoint St36

The participation of opioids in the antinociceptive effect of electroacupuncture was evaluated in terms of nociception produced by thermal stimuli applied to the face of male Wistar rats, weighing 180-230 g. Electrical stimulation (bipolar and asymmetric square wave with 0.5 mA intensity for 20 min) of acupoint St36, located in the anterior tibial muscle 10 mm distal to the knee joint, induced antinociception in the present model, which was maintained for 150 min. Acupoint LI4, located in the junction of the first and second metacarpal bones, did not achieve antinociception at any frequency studied (5 Hz: 1.7 +/- 0.1; 30 Hz: 1.8 +/- 0.1; 100 Hz: 1.7 +/- 0.1 vs 1.4 +/- 0.2). The antinociception obtained by stimulation of acupoint St36 was only achieved when high frequency 100 Hz (3.0 +/- 0.2 vs 1.0 +/- 0.1) was used, and not with 5 or 30 Hz (1.2 +/- 0.2 and 0.7 +/- 0.1 vs 1.0 +/- 0.1). The antinociceptive effect of acupuncture occurred by opioid pathway activation, since naloxone (1 and 2 mg/kg, subcutaneously) antagonized it (1.8 +/- 0.2 and 1.7 +/- 0.2 vs 3.0 +/- 0.1).

Nitric oxide/cGMP pathway mediates orofacial antinociception induced by electroacupuncture at the St36 acupoint

The aim of the present study was to test the hypothesis that electroacupuncture (EA) at acupoint St36 induces antinociception by activation of the L-arg/NO/cGMP pathway. Nociception was produced by thermal stimuli applied to the face of Wistar rats and latency was measured by face withdrawal. Electric stimulation of acupoint St36 for 20 min induced antinociception in this model, which was maintained for 150 min. For comparison, a so-called dry needle group (DN) was used, which received needling at the same point without stimulation. The antinociception obtained by stimulation of acupoint St36 was only achieved when high frequency (100 Hz) was used, whereas low frequencies (5 and 30 Hz) were not capable of achieving this effect. Subcutaneous administration of both inhibitors of NO synthase (N-nitro-L-arginine) and guanylyl cyclase (ODQ) and intraperitoneal administration of specific antagonists of neuronal NO synthase (L-NNA) and inductible NO synthase (aminoguanidine) antagonized the antinociception induced by St36 stimulation. The results of this paper suggest that stimulation of acupoint St36 at high frequency induces antinociception, which seems to be related to L-arg/NO/cGMP pathway activation.

The orofacial formalin test

The subcutaneous injection of formalin into the rat upper lip generates behavioral responses that last several minutes. The time course of the response is similar to what is observed following formalin injection into the paw, i.e. biphasic, with an early and short-lasting first phase followed, after a quiescent period by a second, prolonged (tonic) phase. The applied chemical stimulus (formalin) can be qualified as noxious since it produces tissue injury, activates Adelta and C nociceptors as well as trigeminal and spinal nociceptive neurons and is felt as painful in man. In addition, increasing the concentration of formalin causes a parallel aggravation of histological signs of tissue inflammation and injury. The measured behavioral response (face rubbing) is a relevant end-point: prolonged face rubbing is evoked by formalin but not saline injection and a positive relationship between the amplitude of the response and the formalin concentration is observed, at least up to 2.5%. At higher formalin concentrations, the use of other or additional end-points should be considered. Finally, the behavioral response in the orofacial formalin test is sensitive to various opioid and non-opioid analgesics. The orofacial formalin test can then be considered as a reliable way of producing and quantifying nociception in the trigeminal region of the rat.

Orofacial deep and cutaneous tissue inflammation and trigeminal neuronal activation. Implications for persistent temporomandibular pain

A rat model has been developed to characterize the responses of brainstem trigeminal neurons to orofacial deep and cutaneous tissue inflammation and hyperalgesia. Complete Freund's adjuvant (CFA) was injected unilaterally into the rat temporomandibular joint (TMJ) or perioral (PO) skin to produce inflammation in deep or cutaneous tissues, respectively. The TMJ and PO inflammation resulted in orofacial behavioral hyperalgesia and allodynia that peaked within 4-24 h and persisted for at least 2 weeks. Compared to cutaneous CFA injection, the injection of CFA into the TMJ produced a significantly stronger inflammation associated with a selective upregulation of preprodynorphin mRNA in the trigeminal spinal complex, an enhanced medullary dorsal horn hyperexcitability, and a greater trigeminal Fos protein expression, a marker of neuronal activation. The Fos-LI induced by TMJ inflammation persisted longer, was more intense, particularly in the superficial laminae, and more widespread rostrocaudally. Thus, the inflammatory irritant produces a stronger effect in deep than in cutaneous orofacial tissue. As there is heavy innervation of the TMJ by unmyelinated nerve endings, a strong nociceptive primary afferent barrage is expected following inflammation. An increase in TMJ C-fiber input after inflammation and strong central neuronal activation may initiate central hyperexcitability and contribute to persistent pain associated with temporomandibular disorders. Since deep inputs may be more effective in inducing central neuronal excitation than cutaneous inputs, greater sensory disturbances may occur in pain conditions involving deep tissues than in those involving cutaneous tissues.

An improved method for assessing mechanical allodynia in the rat

A method has been developed for assessing mechanical nociceptive threshold and allodynia in the rat. The animal was habituated to stand on its hind paws while leaning against an experimenter's hand. The rat was unrestrained, but remained in the position throughout the test session. The Semmes-Weinstein monofilaments were used to probe body areas such as the lateral edge of the hind paw and the orofacial skin. The median withdrawal response threshold was found to be 62.9 g for both hind paws (95 % confidence interval, CL, 61.4-66.7 g, n = 69). The median response thresholds of the orofacial skin were 62.9 g (95% CL: 55.7-68.7 g, n = 45), and 25.6 g (95% CL: 22.6-36.3 g, n = 45), for the skin above the temporomandibular joint (PT) and the perioral (PO) skin, respectively. The injection of an inflammatory agent, complete Freund's adjuvant, into the hind paw, peritemporomandibular joint tissue (PTMJ), or PO skin resulted in significant mechanical allodynia. The median response thresholds were reduced to 0.09 g (p < 0.01, n = 6), 5.60 g (p < 0.001, n = 9), and 3.24 g (p < 0.001, n = 9), after hind paw, PTMJ, and PO inflammation, respectively. The allodynia persisted for at least 2 weeks in all cases. This testing paradigm eliminates confounding factors related to weight bearing, and offers a simple, objective, and reliable approach to assess mechanical sensitivity in rats. The method will be useful for studying the central mechanisms of mechanical allodynia. Importantly, this method can also be used in the orofacial region, and will facilitate studies on the mechanisms of persistent orofacial pain in animals.

A pharmacological study of the modulation of neuronal and behavioural nociceptive responses in the rat trigeminal region

Electrical stimulation of the brain, particularly in the periventricular grey areas, caused long-lasting increases in behavioural escape thresholds to heating and mechanical stimuli applied to the facial region of the rat. The brain stimulation selectively suppressed responses to noxious stimuli. Responses to non-noxious stimuli, evoked by low threshold brush, were unaffected. The same animals that were studied in the behavioural tests were then anaesthetized with urethane and the inhibitory effect of the same brain stimulation was studied in single neurones recorded in the caudal trigeminal nucleus. A clear correlation (rs = 0.63) emerged between degree of behavioural antinociception and the amount of inhibition seen in nociceptive neurones. In addition the mean duration of the inhibition (6 min) was similar to the mean duration of the antinociceptive effect (7.3 min). Other classes of non-nociceptive neurones were unaffected by the stimulation. The neurones were also studied using iontophoretically applied monoamine candidates for the inhibitory neurotransmitter noradrenaline (NA) and 5-hydroxytryptamine (5-HT). The profile of the effects of NA most closely fitted that of the inhibitory neurotransmitter. This profile was expressed in terms of depression and excitation of different classes of neurones, and by the duration of effects. The depressants effects could be antagonized by iontophoretic idazoxan. In addition clonidine induced long-lasting depression of firing. 5-HT was more likely than NA to excite nociceptive neurones and to depress non-nociceptive neurons. Only NA consistently elevated thermal response thresholds in a similar manner to that produced by brain stimulation. These results provide some support for the hypothesis that selective descending inhibition of nociceptive responses in neurones of the rat caudal trigeminal nucleus is mediated by NA, possibly by an action at alpha2-adrenoceptors.

The orofacial formalin test in rats: effects of different formalin concentrations

In this study of the orofacial formalin test in rats, the effects of different formalin concentrations (0.2%, 0.5%, 1.5%, 2.5%, 5% and 10%) on the behavioural nociceptive response (face rubbing) was investigated. The histological responses of the skin were also evaluated. Increasing the concentration of formalin caused a parallel aggravation of histological signs of tissue inflammation and injury. All concentrations provoked an early phase of nociceptive response, but its intensity was not concentration-dependent. The 2nd phase of response to formalin only occurred for concentrations of 1.5% and higher. A positive relationship between the formalin concentration and the amplitude of the rubbing activity measured between 12 and 45 min after injection could be observed until 2.5% but with the highest concentrations (5 and 10%), the amplitude of the response decreased. Our findings indicate that the orofacial formalin test should be carried out using concentration between 0.5 and 2.5%. This is essential to assess increase as well as decrease in pain intensity. Moreover, this will have the effect of minimizing the suffering of the experimental animal.

Sensory responses of caudal trigeminal neurons to thermal and mechanical stimuli and their behavioural correlates in the rat

Behavioural experiments in the freely moving rat were carried out to determine thermal and mechanical nociceptive thresholds to ramp stimuli applied to the face. The mean thermal escape threshold was 43.5 degrees C, and the mean mechanical escape threshold was 179.2 g/mm2. In a parallel set of experiments recordings were made from single neurons in the caudal trigeminal nucleus of the anaesthetized rat. Neurons were classified according to their responses to a range of thermal and mechanical stimuli applied to the face. Three classes of neuron responded exclusively to mechanical stimuli and four classes responded to thermal stimuli (usually in addition to responding to mechanical stimuli). The mean thermal threshold of neurons responsive to warming stimulation was 44.4 degrees C. Neurons responsive to innocuous warming were located in deeper laminae. Many of the neurons responsive to noxious heat appeared to show an exponential relation between temperature and firing rate. An argument is made for a direct role of exponentially responding neurons in thermal nociception. The distribution of all neuronal response thresholds was left-skewed compared with a normal distribution, whereas the behavioural escape thresholds approximated a normal distribution.

Response of brainstem trigeminal neurons to electrical stimulation of the dura

The extracellular response of medullary trigeminal neurons to electrical stimulation of the dura was studied in anesthetized cats. Fifty-six medullary trigeminal units were excited by stimulation sites near major dural vessels with an average latency of 11.0 ms. Many units also responded to infraorbital nerve shock and had cutaneous receptive fields that included the ipsilateral periorbital region. These cutaneous responses were either wide dynamic range or nociceptive specific in type. Electrical stimulation of the midbrain periaqueductal gray region suppressed the response of medullary trigeminal units to either dural stimulation or infraorbital nerve shock. Medullary trigeminal neurons that receive convergent inputs from dura and facial skin may provide a physiological substrate for the cutaneous referral of dural sensation.

Medial hypothalamic stimulation produces analgesia to facial heating in unrestrained rats

Gradual heating of the face in unrestrained conscious rats produced a behavioural response at a mean threshold temperature of 41.9 degrees C (S.E.M. +/- 0.02, 174 tests). This temperature did not change with tests repeated at 5-min intervals over one hour. During stimulation of the preoptic area of the hypothalamus the response temperature was consistently raised by about 3 degrees C but returned to the control level within 5 min.

Pontomedullary raphe neurons: monosynaptic excitation from midbrain sites that suppress the jaw opening reflex

The intracellular responses of pontomedullary raphe neurons to midbrain stimulation were studied in chloralose anesthetized cats. Electrical stimulation of the periaqueductal gray region at sites that suppressed the nociceptive jaw opening reflex evoked monosynaptic excitatory postsynaptic potentials in the majority of raphe neurons recorded. Intracellular labeling of studied cells demonstrated that recordings were obtained primarily from large and medium sized neurons in nucleus raphe magnus and the rostral nucleus raphe obscurus. These results provide further evidence that analgesia induced by periaqueductal gray stimulation may result in part from the direct activation of caudal raphe neurons.