Innervation of the pineal gland in the rat: an HRP study

Transsynaptic activity-dependent regulation of axon branching and neurotrophin expression in vivo

The two major classes of activity-dependent neuroplasticity predict different consequences of activity alteration on circuit response. Hebbian plasticity (positive feedback) posits that alteration of neuronal activity causes a parallel response within a circuit. In contrast, homeostatic plasticity (negative feedback) predicts that altering neuronal activity results in compensatory responses within a circuit. The relative roles of these modes of plasticity in vivo are unclear, since neuronal circuits are difficult to manipulate in the intact organism. In this study, we tested the in vivo effects of activity deprivation in the superior cervical ganglion-pineal circuit of adult rats, which can be noninvasively silenced by exposing animals to constant light. We demonstrated that total deprivation of sympathetic activity markedly decreased the presence of axonal proteins in the pineal and reduced the density and thickness of sympathetic axonal arbors. In addition, we demonstrated that sympathetic inactivity eliminated pineal function and markedly decreased pineal expression of neurotrophins. Administration of β-adrenergic agonist restored the expression of presynaptic and postsynaptic proteins. Furthermore, compensatory axonal growth through collateral sprouting, normally seen following unilateral denervation of the pineal, was profoundly impaired in the absence of neural activity. Thus, these data suggest that sympathetic axonal terminals are maintained by neural activity that induces neurotrophins, which may act through a retrograde mechanism to preserve the integrity of axonal arbors via a positive feedback loop. Conversely, by using Hebbian-like neuroplasticity, silent yet intact circuits enter a hibernation mode marked by reduction of presynaptic axonal structures and dramatically reduced postsynaptic expression of neurotrophins.

Pineal microdialysis in freely moving rats

We describe a surgical technique to implant the guide cannula for in vivo microdialysis in the rat pineal gland. This technique has the following features and advantages: (a) does not require ligation of the superior or transverse sinus, (b) minimizes bleeding from the dural veins, (c) does not disturb the sympathetic innervation originating from superior cervical ganglia, which is essential for pineal function. This new technique makes it possible to carry out chronic pineal microdialysis of freely moving rats.

Direct projections from the anterior and tuberal regions of the lateral hypothalamus to the rostral part of the pineal complex of the rat. An anterograde neuron-tracing study by using Phaseolus vulgaris leucoagglutinin

The anterograde tracer, Phaseolus vulgaris leucoagglutinin, was injected into the lateral hypothalamic division of the Wistar rat. After 7 days of survival the animals were fixed by perfusion, and cryostat sections processed for visualization of the tracer by immunohistochemistry. Injections into the anterior and tuberal regions of the lateral hypothalamic division labeled neurons which projected caudally to the rostral part of the pineal complex, e.g. the deep pineal gland and the pineal stalk. Labeled fibers were in this study not observed in the superficial pineal gland. Due to the direct innervation of the lateral hypothalamic region from the retina and the involvement of this area in circadian rhythmicity, the projections, demonstrated in this study, from the lateral hypothalamic region to the pineal gland are suggested to transmit impulses which modulate the circadian activity of the rat pineal gland.

A direct neural projection from the intergeniculate leaflet of the lateral geniculate nucleus to the deep pineal gland of the rat, demonstrated with Phaseolus vulgaris leucoagglutinin

The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into different subregions of the rat lateral geniculate nucleus. After a survival for 5-10 days, the rats were fixed by perfusion with 4% paraformaldehyde, whereafter the brains were cut in a cryostat and the tracer was localized by immunohistochemistry. After deposits of PHA-L involving the intergeniculate leaflet, a high number of PHA-L-immunoreactive fibers were observed to project directly into the posterior commissure. From the posterior commissure, some nerve fibers turned dorsally and entered into the deep pineal gland, a part of the pineal complex located in between the posterior and the habenular commissure. A few PHA-L-immunoreactive fibers were observed in the pineal stalk, but no fibers were detected in the superficial pineal gland. In cases where the injections were placed in the dorsal or ventral subnuclei, no immunoreactive nerve fibers were observed to enter the pineal complex. These results indicate that the intergeniculate leaflet of the lateral geniculate nucleus, a nucleus considered to be involved in circadian rhythmicity, might influence the pineal gland, via a neural projection to the rostral part of the pineal complex.

Characterization of the light response in the pineal gland of intact and sympathectomized rats

Electrophysiological recordings were performed in the pineal gland of pigmented rats BD9 to investigate the possible contribution of a direct neural connection of the gland with the central nervous system in the transmission of photic information. Extracellular potentials were recorded during brief photic stimulation of the eyes before and after bilateral sympathectomy. Two types of responses could be distinguished in intact as well as in sympathectomized rats: spontaneously active units which were unresponsive to light flashes and units responding to photic stimulation with ON and ON/OFF discharges. Spectral sensitivity curves recorded from the pineal organ of dark adapted rats showed a maximum at 500 nm. Interruption of the sympathetic innervation by bilateral denervation or removal of the superior cervical ganglia did not alter the spectral properties of pineal units. Additionally, response-intensity curves and response threshold (about 0.0017 microW/cm2) were not changed after sympathectomy. These results suggest that the pineal gland of the rat receives projections from the visual system via a central pinealopetal innervation.

Increase of paradoxical sleep episodes after electrical stimulation of the lateral and third ventricles in the rat

Electrical stimulation (0.3 mA in intensity, 80 Hz in frequency, 0.7 ms in signal duration) applied at 17.00 h in the lateral and third ventricles induced a significant increase of nocturnal paradoxical sleep (PS). This effect persisted for two consecutive days in both cases. Non-specific aspects involved in this effect are discussed and the results are commented on in terms of humoral processes and possible control of pineal gland activity.

Histochemical and electron microscopical demonstration of the sympathetic nerve fibers joining to the fourth and the sixth cranial nerves in rats

The localization of sympathetic fibers on the floor of the cranium was studied in rats using amine fluorescence histochemistry, neuropeptide-Y (NPY) immunohistochemistry, and electron microscopy. The vast majority of amine fluorescent fibers joined the abducent nerve and were localized in the peripheral zone under the perineurium. After advancing along this nerve for some distance, the fibers diverged into many bundles that converged to form the cavernous plexus at a rostral end of the trigeminal ganglion. On the dorsal surface of the trigeminal ganglion, one or two medium-calibered fluorescent bundles ran inside or in close proximity to the trochlear nerve, while many small-calibered, brightly fluorescent bundles also extended longitudinally in the epidural connective tissue. In rats that had undergone nerve severance, NPY-immunoreactive fibers were detected at the cut ends of the abducent and trochlear nerve. The differing amounts of NPY accumulated at the rostral and the caudal stumps indicated the direction of the NPY-bearing fibers. Electron microscopy confirmed the presence of unmyelinated fibers in both the abducent and trochlear nerves.

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

Fast blue (FB), rhodamine microspheres (RH), horseradish peroxidase (HRP), and wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) were used as retrograde tracers to study the innervation of the rat superficial pineal gland (SP). One of the tracers was injected into the gland of each animal. All four retrograde tracers injected into the gland always labeled neurons in the superior cervical ganglia (SCG). No retrograde labeling was ever seen in the suprachiasmatic nuclei, paraventricular hypothalamic nuclei, lateral hypothalamus, habenular nuclei, amygdalar nuclei, or superior salivatory nuclei. Retrograde labeling was seen in the anterior hypothalamic nuclei, anterior thalamic nuclei, lateral geniculate bodies, and midbrain tectal structures when a tracer spread from the injection site to the overlying cortex, tectum, or commissures. Control studies included injection of tracer into the subarachnoid space around the SP or into structures adjacent to the SP. Only the injection of FB or WGA-HRP into the subarachnoid space labeled neurons in the SCG. This labeling was probably due to the spread of tracer to the choroid plexus. These results agree with recent work confirming the existence of a direct projection of the SCG into the interstitium around pinealocytes. The evidence does not substantiate an innervation originating in the habenular nuclei; the superior salivatory nuclei; or any other diencephalic, midbrain, pontine, or medullary structure.

Sympathetic projections to the epithalamus

Injections of horseradish peroxidase were made into surgically exposed superior cervical ganglia of Sprague-Dawley rats weighing 250 to 320 g. Anterogradely labeled postganglionic fibers were observed in the nervus conarii, pineal gland, pineal stalk, and the choroid plexus. Terminal fields were identified in lamina intercalaris and the medial habenular nuclei. These findings support a previous study indicating that the sole innervation of the pineal gland is via the postganglionic sympathetics.