Somatosensory systems and the milk-ejection reflex in the rat. I. Lesions of the mesencephalic lateral tegmentum disrupt the reflex and damage mesencephalic somatosensory connections

Bilateral electrolytic lesions and unilateral tracer injections were performed in lactating rats in order to study the participation of the mesencephalic lateral tegmentum in the milk-ejection reflex. The release of oxytocin was detected as a rise in intramammary pressure during each milk ejection. In animals with lesions, the lateral part of the deep grey layers of the superior colliculus, the intercollicular area and the rostromedial portion of the external nucleus of the inferior colliculus were destroyed. The mesencephalic lateral tegmentum of animals in which the milk-ejection reflex was blocked sustained a larger damage than in rats where the frequency of the milk-ejection response was only slowed down. Solutions of True Blue, horseradish peroxidase or horseradish peroxidase coupled to wheat germ agglutinin were injected in the mesencephalic lateral tegmentum of rats with and without lesions. Retrogradely labelled cells were found in several nuclei of the somatosensory pathways: the principal sensory and spinal parts of the trigeminal complex, the cuneate and gracile nuclei, the lateral cervical nucleus and the nucleus proprius of the spinal cord. Labelled cells were also found in the ventral nucleus of the lateral lemniscus, the ventral parabrachial nucleus, the gigantocellular reticular nucleus, the lateral nucleus of the substantia nigra, the prerubral nucleus of the thalamus, the hypothalamic ventromedial nucleus, the zona incerta and in the anterior and lateral hypothalamic areas. Labelled fibres and "terminal-like" labelling were found in the anterior pretectal area, in the thalamic parafascicular nucleus, in the posterior nucleus and the ventroposterior complex, in the zona incerta and in the fields of Forel, but none were observed in the supraoptic or paraventricular nuclei. Injections made in the area of the lateral cervical nucleus and in the cuneate and gracile nuclei labelled fibres and "terminal-like" fields in the external nucleus of the inferior colliculus, the intercollicular area, the deep grey layers of the superior colliculus and in the mesencephalic lateral tegmentum. After injections in the posterior nucleus and ventroposterior complex of the thalamus, retrogradely labelled cells were found in the lateral tegmentum, the intercollicular area and the external nucleus of the inferior colliculus. These results indicate that bilateral lesioning of the mesencephalic lateral tegmentum, which disrupts the milk-ejection response, could damage somatosensory projections originating from the dorsal horn of the spinal cord, the lateral cervical nucleus, the dorsal column nuclei and the sensory and spinal trigeminal nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)