Elsevier

Neuroscience

Volume 78, Issue 4, 14 April 1997, Pages 1059-1068
Neuroscience

Chemical sensory deafferentation abolishes hypothalamic pituitary activation induced by noxious stimulation or electroacupuncture but only decreases that caused by immobilization stress. A c-fos study

https://doi.org/10.1016/S0306-4522(96)00661-6Get rights and content

Abstract

We have shown in previous c-fos studies that noxious stimulation or electroacupuncture in deeply anaesthetized rats activate the hypothalamic–pituitary corticotrope axis in a specific way. C-fos expression was more pronounced in the arcuate than the paraventricular hypothalamic nuclei, and none occurred in the pituitary intermediate lobe. The absence of the usual autonomic responses to psychological stress, such as tachycardia or blood pressure elevation, suggested a specific action of the somatosensory input on the hypothalamic–pituitary axis. To prove this hypothesis, c-fos expression was examined in the paraventricular, arcuate and other hypothalamic nuclei, the pituitary gland, and the A1 and A2 medullary catecholaminergic cell groups of animals deprived of nociceptive primary afferent input by neonatal capsaicin. After noxious stimulation or electroacupuncture, no c-fos enhancement occurred in any of those sites in capsaicin-treated animals, and there was no increased plasma release of adrenocorticotropic hormone. In contrast, the hypothalamic–pituitary c-fos activation provoked by immobilization stress though markedly decreased, was not abolished by capsaicin, whereas plasma release of adrenocorticotropic hormone remained undiminished.

These findings suggest that noxious stimulation or electroacupuncture act on the hypothalamic–pituitary corticotrope axis through an exclusively physical effect depending on the noxious signal elicited in the somatosensory pathway. They also demonstrate the occurrence of a minor somatosensory physical component after forced immobilization, acting on the hypothalamic–pituitary axis probably together with the prevalent component of emotional arousal elicited by this form of stress.

Section snippets

Experimental procedures

The regulations of local authorities in handling laboratory animals, and the guidelines of the European Communities Council Directive of 24 November 1986 (86/609/EEC), were followed.

Animal behaviour

There were no differences between intact, vehicle- or capsaicin-treated animals during restraint trials, both being quiet during the first minutes of restraint and later struggling intermittently against the forced immobilization. In animals anaesthetized before somatosensory stimulation, there were no motor or vocal reactions to stimulation. As usual, electrical pulses during acupuncture caused gentle rhythmic contractions of local hindleg muscles.

Central nervous system

In unstimulated controls, Fos-IR cells

Neurotoxic action of capsaicin

Neonatal subcutaneous capsaicin, at a dose of 50 mg/kg, causes a massive reduction (95%) of the number of unmyelinated C-fibres in rat dorsal roots[36]extending to less than 40% of the thin myelinated Aδ-fibres.[37]This is accompanied by the virtual disappearance of substance P (SP) and other peptides[35]from lamina I–II of the superficial dorsal horn in which most unmyelinated afferents terminate.[29]In accordance with the fact that noxious input is mostly conveyed by C-fibres,[47]a long-term

Conclusions

The results show that activation of the HPA by noxious stimuli in anaesthetized animals depends entirely on the nociceptive input elicited in primary somatosensory afferents. The effects of EA on the HPA were similar to those of noxious stimulation, and like them totally blocked by capsaicin. Therefore EA action on the HPA also depends on the excitation of nociceptive primary afferents. The somatosensory input provoked by both types of stimulation was likely to reach the HPA through the caudal

Acknowledgements

The authors are grateful to Prof. M. D. Madeira, the Anatomy Department of the Faculty of Medicine of Oporto, for help with the stereological evaluation of the PVN, Dr Maria Eugenia Azevedo, Radio-Isotope Laboratory, for the hormone assays, and Prof. Altamiro Costa-Pereira, the Biostatistics and Medical Informatics Department, for help with the statistical work. Dr B. Pan was the recipient of a scholarship from the PRAXIS XXI Program, Lisbon. This work was supported by JNICT research grant

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    On leave from the Institute of Acupuncture, China Academy of Traditional Chinese Medicine, Beijing, China

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