Evidence from pharmacological studies has implicated substance P (SP), a natural ligand of tachykinin NK(1) receptors which can also interact with NK(2) receptors, in the generation of pressor and tachycardic responses to stress. Using selective blockade of brain NK(1) and NK(2) receptors, we tested in conscious rats the hypothesis that SP initiates, within the neuronal brain circuits, the sympathoadrenal, hypothalamic-pituitary-adrenal (HPA) and behavioural responses to noxious stimuli. Formalin injected s.c. through a chronically implanted catheter in the area of the lower leg was used as a pain stimulus. Rats were pretreated i.c.v. with vehicle or the selective, nonpeptide antagonists of tachykinin NK(1) and NK(2) receptors, RP 67580 and SR 48968, respectively. Ten minutes thereafter, formalin was injected s.c. and the cardiovascular responses were recorded, plasma concentrations of catecholamines, adrenocorticotrophic hormone (ACTH) and corticosterone were determined and the expression of the inducible transcription factor c-Fos in the paraventricular (PVN) and supraoptic nuclei was detected to identify neurones which were activated during pain stimulation. Blockade of NK(1) and NK(2) receptors attenuated the formalin-induced increases in mean arterial pressure and heart rate, adrenaline and ACTH concentrations in plasma, and completely abolished the pain-induced c-Fos expression in corticotrophin-releasing hormone neurones localised in the parvocellular division of the PVN. The results obtained provide pharmacological evidence that tachykinins, most probably SP, act as mediators within the neuronal circuits linked to the initiation and control of the cardiovascular, sympathoadrenal, HPA and behavioural responses to pain stimuli and provide an excitatory input to corticotrophin-releasing hormone neurones in the PVN to activate the HPA axis. Our data demonstrating the inhibition of the complex response pattern to noxious stimuli and stress are consistent with the proposed anxiolytic and antidepressant activity of NK(1) and NK(2) receptor antagonists.

The formalin test for nociception, predominantly used with rodents, is characterized by continuous pain due to tissue injury induced by formalin. In addition to the pain generated by formalin itself, the classical formalin test for nociception comprises a number of additional strong stressful events such as restraint and injury caused by needle insertion. These events have hampered its use as a model of stress employing continuous pain as a stress stimulus. We describe here a new, simple method of a subcutaneous application of formalin in conscious rats without restraint and needle insertion. Formalin or physiological saline (controls) was injected subcutaneously through a chronically implanted catheter in the region of the lower leg. In most animals, the cardiovascular and behavioural responses to subcutaneously injected formalin were biphasic. The early phase was quantitatively characterized. Formalin (2.5%, 50 microL) induced a marked increase in mean arterial pressure and heart rate. Maximal increases occurred during the first 3 min after formalin injection and were followed by a gradual decline of both cardiovascular parameters to levels higher than the preinjection baseline. Subcutaneous saline injection did not induce any changes in mean arterial pressure and heart rate. The behavioural response to formalin featured intensive licking and biting of the injection site. The behavioural response to subcutaneous saline did not differ from the spontaneous behavioural activity recorded in noninjected rats. Subcutaneously injected formalin induced an immediate increase in plasma corticotrophin (ACTH), corticosterone, noradrenaline, and adrenaline levels, and all hormones remained increased during the whole observation period (60 min). Adrenaline and noradrenaline levels in plasma were slightly, but significantly, elevated during the initial 5 min after subcutaneous saline application but returned to basal values after 15 min. The magnitude of the hormonal responses characterizes the described formalin test as a moderate stress stimulus. The complex response pattern to formalin injected through the subcutaneous catheter is brought about exclusively by pain generated by formalin-induced tissue injury. The described technique of subcutaneous formalin injection represents a new tool to study mechanisms activating brain neuronal circuits that generate the cardiovascular, endocrine, and behavioural responses of the reaction to pain.

The formalin test for nociception, predominantly used with rodents, is characterized by continuous pain due to tissue injury induced by formalin. In addition to the pain generated by formalin itself, the classical formalin test for nociception comprises a number of additional strong stressful events such as restraint and injury caused by needle insertion. These events have hampered its use as a model of stress employing continuous pain as a stress stimulus. We describe here a new, simple method of a subcutaneous application of formalin in conscious rats without restraint and needle insertion. Formalin or physiological saline (controls) was injected subcutaneously through a chronically implanted catheter in the region of the lower leg. In most animals, the cardiovascular and behavioural responses to subcutaneously injected formalin were biphasic. The early phase was quantitatively characterized. Formalin (2.5%, 50 microL) induced a marked increase in mean arterial pressure and heart rate. Maximal increases occurred during the first 3 min after formalin injection and were followed by a gradual decline of both cardiovascular parameters to levels higher than the preinjection baseline. Subcutaneous saline injection did not induce any changes in mean arterial pressure and heart rate. The behavioural response to formalin featured intensive licking and biting of the injection site. The behavioural response to subcutaneous saline did not differ from the spontaneous behavioural activity recorded in noninjected rats. Subcutaneously injected formalin induced an immediate increase in plasma corticotrophin (ACTH), corticosterone, noradrenaline, and adrenaline levels, and all hormones remained increased during the whole observation period (60 min). Adrenaline and noradrenaline levels in plasma were slightly, but significantly, elevated during the initial 5 min after subcutaneous saline application but returned to basal values after 15 min. The magnitude of the hormonal responses characterizes the described formalin test as a moderate stress stimulus. The complex response pattern to formalin injected through the subcutaneous catheter is brought about exclusively by pain generated by formalin-induced tissue injury. The described technique of subcutaneous formalin injection represents a new tool to study mechanisms activating brain neuronal circuits that generate the cardiovascular, endocrine, and behavioural responses of the reaction to pain.

Meperidine has potent antishivering properties. The underlying mechanisms are not fully elucidated, but recent investigations suggest that alpha2-adrenoceptors are likely to be involved. We performed the current study to investigate the effects of meperidine on nonshivering thermogenesis in a model of thermoregulation in mice. After injection (0.1 mL/kg intraperitoneally) of saline, meperidine (20 mg/kg), the specific alpha2-adrenoceptor antagonist atipamezole (2 mg/kg), plus saline or atipamezole plus meperidine, respectively, mice were positioned in a Plexiglas chamber. Rectal temperature and mixed expired carbon dioxide were measured after provoking thermoregulatory effects by whole body cooling. Maximum response intensity of nonshivering thermogenesis and the thermoregulatory threshold for nonshivering thermogenesis, which was defined as the temperature at which a sustained increase in expiratory carbon dioxide can be measured, were investigated. Meperidine significantly decreased the threshold of nonshivering thermogenesis (36.6 degrees C +/- 0.7 degrees C) versus saline (37.9 degrees C +/- 0.6 degrees C) and versus atipamezole plus saline (37.8 degrees C +/- 0.4 degrees C; P <0.01). This effect was abolished after administration of meperidine combined with atipamezole (37.7 degrees C +/- 0.6 degrees C; P <0.05). Meperidine did not decrease the maximum intensity of nonshivering thermogenesis. The results suggest a major role of alpha2-adrenoceptors in the inhibition of thermoregulation by meperidine in mice.