Stimulation of the mesencephalic central gray increases blood prolactin in ovariectomized rats

Reversal by estrogen of the effect of dorsal raphe stimulation on release of LH but not prolactin

Electrical stimulation of the dorsal raphe nucleus (DRN) was administered to rats 2-4 weeks after ovariectomy and at least 7 days following implantation of a bipolar electrode. During stimulation the concentration of luteinizing hormone (LH) decreased whereas that of prolactin (Prl) increased in serial plasma samples obtained from the unanesthetized animals via a cannula chronically implanted in the right external jugular vein. Administration of estradiol benzoate (5 micrograms/100 g BW) for 2 days prior to the experiment reversed the effect of stimulation on LH but augmented the Prl response, suggesting modulation by estrogen of the relative effectiveness of opposing pathways ascending from DRN to influence LH release. It is also apparent that LH and Prl release are controlled by distinct mechanisms since estrogen exerted opposite influences on the responses of these two hormones to stimulation of DRN.

An autoradiographic study of projections ascending from the midbrain central gray, and from the region lateral to it, in the rat

Ascending projections from the midbrain central gray (CG) and from the region lateral to it were traced in the rat using tritiated amino acid autoradiography. Leucine or a cocktail of amino acids (leucine, proline, lysine, histidine, and tyrosine) were used as tracers. In addition to projections within the midbrain, ascending fibers follow three trajectories. The ventral projection passes through the ventral tegmental region of Tsai and the medial forebrain bundle to reach the hypothalamus, preoptic area, caudoputamen, substantia innominata, stria terminalis, and amygdala. There are labeled fibers in the diagonal bands of Broca and medial septum, and terminal labeling in the lateral septum, nucleus accumbens, olfactory tubercle, and frontal cortex. The dorsal periventricular projection terminates in the midline and intralaminar thalamic nuclei. The ventral periventricular projection follows the ventral component of the third ventricle into the hypothalamus, passing primarily through the dorsal hypothalamic area and labeling the rostral hypothalamus and preoptic area. Projections from the region lateral to the CG are similar, but exhibit stronger proximal, and weaker distal, projections. Rostral levels of the CG send heavier projections to the fields of Forel and the zona incerta, but fewer fibers through the supraoptic decussation, than do caudal levels. Ascending projections from the CG are both strong and widespread. Strong projections to the limbic system and the intralaminar thalamic nuclei provide an anatomical substrate for CG involvement in nociception and affective responses.

The midbrain periaqueductal gray in the rat. I. Nuclear volume, cell number, density, orientation, and regional subdivisions

The midbrain periaqueductal gray is a functionally heterogeneous region which plays an important role in pain modulation. Despite the heterogeneity considerable controversy exists regarding the presence or absence of morphological subdivisions within the region. The present study was designed to evaluate the possibility of morphological subdivisions within the rat periaqueductal gray by using a statistical cluster analysis system. In addition both qualitative and quantitative data concerning neuronal size, shape, and density were obtained. On the basis of measurements of over 12,000 neurons in two planes of section, the mean neuronal length of cell bodies in this region was 14.82 microns and the mean neuronal area was 95.59 microns squared . The mean neuronal density was found to be 16,284 cells per mm3. Neuronal density decreased from rostral to caudal in the periaqueductal gray. The data obtained from cluster maps suggest the presence of four subdivisions within this midbrain region. The medial subdivision contains the smallest neurons and exhibits the lowest cell density. The dorsolateral and ventrolateral divisions contain the largest neurons while the dorsal division displays the highest packing density. These results are discussed in light of recent receptor binding and immunohistochemical studies of this region.

The periaqueductal gray projections to the rat spinal trigeminal, raphe magnus, gigantocellular pars alpha and paragigantocellular nuclei arise from separate neurons

Possible collateral branches of periaqueductal gray axons which distribute to the nucleus raphe magnus, nucleus reticularis paragigantocellularis, nucleus reticularis gigantocellularis pars alpha and the spinal trigeminal nucleus were analyzed with the double fluorescent retrograde tracer technique. With the exception of a small number of double-labeled neurons observed in the periaqueductal gray following injections of fluorescent dyes into the nuclei reticularis paragigantocellularis and gigantocellularis pars alpha, no double-labeled cells were found in this midbrain region following injections of tracers into various combinations of the above 4 nuclear groups. The results of this investigation indicate that these 4 brainstem nuclei are innervated predominantly by separate neuronal populations within the periaqueductal gray.

Inhibition of pulsatile luteinizing hormone release by morphine microinjected into mesencephalic dorsal raphe

Blood samples were collected via jugular catheters from ovariectomized rats at 10-minute intervals for one hour before and two hours after microinjection of -.5 microliter of either saline vehicle or morphine sulfate (10 micrograms) into the dorsal raphe nucleus (DRN) or adjacent periaqueductal gray by means of chronically-implanted guide cannulae. LH was measured by radioimmunoassay and mean pre-injection and mean post-injection values were compared for each rat (t test) as well as for each treatment group (paired t test). Neither saline in DRN nor morphine at other sites significantly altered circulating LH. A significant decrease in LH was observed following injection of morphine into DRN. This effect of morphine was prevented by pre treatment of the animals with the narcotic antagonist naltrexone (10 mg/kg i.v.), indicating the involvement of opiate receptors. These results indicate that DRN is one site at which systemically-administered morphine might act, and suggest the possibility of participation of this mechanism in modulation of LH release by endogenous opioids.