Tachykinin receptor inhibition and c-Fos expression in the rat brain following formalin-induced pain

Abstract Recent pharmacological evidence has implicated substance P and neurokinin A, natural ligands for neurokinin-1 and neurokinin-2 receptors, respectively, as neurotransmitters in brain neuronal circuits activated upon noxious stimulation. The expression of the inducible transcription factor, c-Fos, was used to identify areas in the brain activated by a noxious stimulus (the subcutaneous injection of formalin), and to investigate the effects of intracerebroventricular administration of selective, non-peptide antagonists for neurokinin-1 and neurokinin-2 tachykinin receptors on the neural activity in these areas and on the behavioural response to formalin-induced pain. Formalin (5%, 50 μl), injected subcutaneously through a chronically implanted catheter in the region of the lower hindlimb, increased c-Fos expression in a number of brain areas related to nociceptive transmission or the integration of stress responses. Grooming behaviour, licking and biting directed to the injected site, was the most frequent behavioural response. Intracerebroventricular pretreatment of rats with either RP 67580 (500 pmol), the active enantiomer of a neurokinin-1 receptor antagonist, or with SR 48968 (500 pmol), the active enantiomer of a neurokinin-2 receptor antagonist, reduced the formalin-induced c-Fos staining in the prefrontal cortex, dorsomedial and ventromedial nuclei of the hypothalamus, the locus coeruleus and the periaqueductal gray. The neurokinin-1, but not the neurokinin-2, receptor antagonist attenuated the formalin-induced activation of c-Fos in the paraventricular nucleus of the hypothalamus. Simultaneous intracerebroventricular pretreatment with both neurokinin-1 and neurokinin-2 receptor antagonists did not produce any additional inhibitory effect on the post-formalin c-Fos expression. None of the tachykinin receptor antagonists had an effect on the formalin-induced c-Fos expression in the septohypothalamic nucleus, medial thalamus, parabrachial nucleus and central amygdaloid nucleus, indicating that neurotransmitters other than neurokinins are most probably responsible for the activation of these areas in respose to noxious stimulation. While both tachykinin receptor antagonists reduced the grooming behaviour to formalin, the neurokinin-1 receptor antagonist was clearly more effective than the neurokinin-2 receptor antagonist. Intracerebroventricular pretreatment of rats with the inactive enantiomers of the tachykinin receptor antagonists, RP 68651 and SR 48965, was without effect. Our results show that (i) the modified formalin test elicited an intense grooming behaviour and expression of c-Fos in numerous forebrain and brainstem areas, (ii) both tachykinin receptor antagonists were able to attenuate the behavioural response to pain and to reduce the formalin-induced c-Fos expression in some, but not all, brain areas, and (iii) the neurokinin-1 antagonist, RP 67580, was more effective in inhibiting the behavioural response to formalin and the pain-induced activation of c-Fos than the antagonist for neurokinin-2 receptors, SR 48968, indicating that neurokinin-1 receptors are preferentially activated in neurokinin-containing pathways responding to noxious stimuli. Our results demonstrate that blockade of brain tachykinin receptors, especially of the neurokinin-1 receptor, reduces the behavioural response to pain and the pain-induced c-Fos activation in distinct brain areas which are intimately linked with nociceptive neurotransmission and the initiation and integration of central stress responses. Together with the previous findings of the inhibition of hypertensive and tachycardic responses to pain, the present data indicate that tachykinin receptor antagonists can effectively inhibit the generation of an integrated cardiovascular and behavioural response pattern to noxious stimuli.