Identification and terminal distribution of the tubero-hypophyseal monoamine fibre systems in the rat by means of stereotaxic and microspectrofluorimetric techniques

Abstract

Summary The monoamine-containing fibres to the median eminence and the neurointermediate lobe have been studied by fluorescence microscopy in combination with microknife lesions. The origin and the course of the fibres were evaluated on the basis of monoamine accumulation proximal to the lesion and monoamine disappearance distal to the lesion. The nature of the monoamines stored in the various fibre systems was characterized with microspectrofluorimetry. Five groups of monoamine-containing axons were seen to enter the median eminence: (a) A large group of dopamine axons which originated in cells of the arcuate nuclei and the ventral parts of the anterior periventricular nuclei; they entered the fibre layer of the median eminence close to the infundibular recess. (b) A large group of noradrenaline axons which originated in areas outside the mediobasal hypothalamus; they entered the anterior median eminence from the lateral side, close to the brain surface, and intermingled with the arcuato-hypophyseal dopamine fibres in the internal layer and the deeper part of the external layer of the median eminence. (c) Two minor groups of catecholamine axons: one reached the median eminence from the anterior side, with an antero-posterior direction, probably mingled with the axons of the supraoptico-hypophyseal tract; the other ran into the arcuate nucleus with a dorso-ventral course along and close to the third ventricle; the fibres could not be traced further. (d) A small group of scattered axons containing an unidentified fluorigenic substance which differed microspectrofluorimetrically from the catecholamines and 5-hydroxytryptamine. Judging from the disappearance of monoamines distal to the lesion and from the chemical and microspectrofluorimetric analyses of the catecholamines in the median eminence and the neuro-intermediate lobe, the following conclusions concerning the terminal ramifications of the catecholamine fibre systems could be drawn: The arcuato-hypophyseal dopamine neurones give rise to one part of the terminals in the zona externa of the median eminence and to most, possibly all, terminals in the zona interna. The tubero-hypophyseal noradrenaline fibres also have their terminals in the zona externa, and they contribute to the catecholamine innervation of the neuro-intermediate lobe. Many studies on the functional role of aminergic mechanisms in the hypothalamic regulation of the pituitary functions have focussed on the presence of dopamine in the median eminence. The present investigation has demonstrated two prominent neurone systems in the hypothalamo-pituitary complex: one containing dopamine, the other containing noradrenaline. Moreover, the results suggest the involvement of a third neurone system, probably containing an indole derivative.