Innervation of the superficial pineal of the rat using retrograde tracing methods

NGF expression in the aged rat pineal gland does not correlate with loss of sympathetic axonal branches and varicosities

The factors that determine the ability of some, but not all neurons, to sustain their axonal projections during aging remain largely unknown. Because sympathetic neurons remain responsive to nerve growth factor (NGF) in old age, it has been proposed that the selective decrease observed in the sympathetic innervation to some targets in aged rats may be the result of a deficit in target-derived NGF. In this study we utilized two different techniques to demonstrate decreased target innervation by sympathetic fibers in the aged rat pineal gland, which is an appropriate and relevant model for examining mechanisms of neuron-target interactions in aging. Tyrosine hydroxylase immunoreactive profiles were quantified in pineal glands of young and aged male Sprague-Dawley rats. The density of tyrosine hydroxylase-immunoreactive fibers was 30% lower in aged pineals, although the remaining fibers contained 20% more tyrosine hydroxylase-immunoreactivity. Othograde tracing of the pineal sympathetic innervation using biotinylated dextran revealed that average axon length, varicosity numbers, branch point numbers, and numbers of terminations were all decreased by approximately 50% in aged tissues, indicating possible functional deficits. These findings suggest that whole branches, along with their associated varicosities were lost in old age. A sensitive quantitative ribonuclease protection assay and a two-site ELISA assay were used to examine whether reduced NGF availability might correlate with sympathetic nerve atrophy. No significant differences were detected in either NGF mRNA or NGF protein levels when comparing young and aged pineal glands, suggesting that atrophy in aged sympathetic neurons is not causally related to reduced availability of NGF at the target. Our results indicate that mechanisms other than NGF expression need to be explored in order to explain the age-related axonal regression observed in this target.

Functionally antagonistic interactions between the TrkA and p75 neurotrophin receptors regulate sympathetic neuron growth and target innervation

In this report, we provide evidence that NGF and BDNF have functionally antagonistic actions on sympathetic neuron growth and target innervation, with NGF acting via TrkA to promote growth and BDNF via p75NTR to inhibit growth. Specifically, in cultured sympathetic neurons that themselves synthesize BDNF, exogenous BDNF inhibits and function-blocking BDNF antibodies enhance process outgrowth. Both exogenous and autocrine BDNF mediate this effect via p75NTR because (1) BDNF does not inhibit growth of neurons lacking p75NTR, (2) function-blocking p75NTR antibodies enhance NGF-mediated growth, and (3) p75NTR-/- sympathetic neurons grow more robustly in response to NGF than do their wild-type counterparts. To determine the physiological relevance of this functional antagonism, we examined the pineal gland, a well defined sympathetic target organ. BDNF is present in the pineal gland during target innervation, and incoming sympathetic axons are p75NTR positive. Moreover, the pineal glands of BDNF+/- and BDNF-/- mice are hyperinnervated with sympathetic fibers, and tyrosine hydroxylase (TH) levels are elevated. Increased tyrosine hydroxylase is also observed in the BDNF+/- carotid artery, another sympathetic neuron target. Thus, BDNF, made by sympathetic neurons and/or their target organs, acts via p75NTR to antagonize NGF-mediated growth and target innervation, suggesting that sympathetic target innervation is determined by the balance of positively and negatively acting neurotrophins present in developing and potentially mature targets.

Direct link from the suprachiasmatic nucleus to hypothalamic neurons projecting to the spinal cord: a combined tracing study using cholera toxin subunit B and Phaseolus vulgaris-leucoagglutinin

By combining retrograde and anterograde tracing, evidence for a bineuronal connection from the suprachiasmatic nucleus (SCN) to the intermediolateral cell column in the spinal cord (IML) was obtained. The retrograde tracer cholera toxin subunit B (ChB) was pressure-injected into the spinal cord and the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was iontophoretically injected into the SCN. The two tracers were visualized simultaneously by a double immunohistochemical procedure. In the hypothalamus, ChB injections gave rise to retrogradely labeled cell bodies in the paraventricular nucleus, retrochiasmatic area, perifornical region, lateral hypothalamic area, and the posterior hypothalamic area. The SCN were found to project to all of these areas. Furthermore, spinal-projecting neurons were found in the brain stem, but no efferents from the SCN were observed to innervate these areas. In the most sparsely innervated areas, the lateral hypothalamic area and the perifornical region, only occasionally a PHA-L fiber in close apposition to a ChB-ir cell body was observed. This was also the case in the retrochiasmatic area and posterior hypothalamic area, although these areas received a moderate number-immunoreactive (ir) PHA-L-ir fibers. The highest number of closely apposed PHA-L-ir fibers and ChB-ir cell bodies was observed in the dorsal parvicellular and in the ventral division of the medial parvicellular paraventricular nucleus, which were also the areas receiving the densest input from the SCN. By anterograde tracing from the paraventricular nucleus of the hypothalamus, the exact topography of the terminal field formed by descending paraventricular neurons was established. Thus, it was confirmed that the paraventricular nucleus of the hypothalamus predominantly innervates the IML. The present study suggests the existence of a bineuronal link between the SCN and the IML, possibly involved in transmission of circadian signals from the endogenous clock to the pineal gland and other organs receiving sympathetic afferents.

Morphological assessment in pinealectomy and foetal pineal gland transplantation in rats: Part I

The effects of pineal gland, an endocrine organ known to affect the physiology of various organs and systems including the hypothalamo-hypophyseal axis, on the morphological characteristics of target organs were investigated in rats after pinealectomy and foetal pineal gland transplantation to a subpial cortical area close to the pinealectomized region. It was demonstrated that weight gain was significantly slower in pinealectomized male rats (p < 0.01); pituitary gland weight was lower in the pinealectomized group (p < 0.01) and transplantation had no effect on the weight of this organ; weight of adrenal gland and testis were lower after pinealectomy (p < 0.01) but restored back to control levels after transplantation; the mitotic activity in seminiferous tubules increased with pinealectomy and returned back to control indexes after transplantation. Research on hypothalamic catecholaminergic content revealed a diminished histofluorescence in pinealectomized rats which showed a potentiated restoration after transplantation. The innervation of the pineal gland was studied by anterograde and retrograde injections of Wheat Germ Agglutinin Horse Radish Peroxidase from superior cervical ganglion and pineal gland, respectively. It was demonstrated that cell bodies were traced both at the transplanted tissue and hypothalamus. In view of these data, the multifunctional and a probable high level homeostatic harmony regulator essence of pineal gland is discussed.