Analysis of the efferent projections of the lateral geniculate nucleus with special reference to the innervation of the subcommissural organ and related areas

Photic induction of c-Fos in enkephalin neurons of the rat intergeniculate leaflet innervated by retinal PACAP fibres

The brain's biological clock, located in the suprachiasmatic nucleus (SCN), is synchronised with the cyclic environment by photic and non-photic cues. Photic information to the SCN is mediated by pituitary adenylate-cyclase-activating polypeptide (PACAP)-containing retinal ganglion cells (RGCs), whereas non-photic input originates primarily from neuropeptide Y (NPY) cells in the ipsilateral thalamic intergeniculate leaflet (IGL). RGCs also seem to project to the IGL, indicating a role for this structure in the integration of photic and non-photic inputs related to the resetting of the biological clock. In the present study, we have used anterograde tracing from both eyes, bilateral eye enucleation, double-immunofluorescence histochemistry, high-resolution confocal laser scanning microscopy and three-dimensional computer analysis to show that (1) PACAP-containing RGCs project to the IGL and are the only source for the PACAP-immunoreactive fibres in the IGL; (2) a few NPY-containing neurons in the IGL are innervated by PACAP-containing retinal nerve fibres and the contacts are both axodendritic and axosomatic; (3) most enkephalin-immunoreactive neurons in the IGL are innervated by PACAP-containing retinal afferents and the contacts are mainly axodendritic; (4) light stimulation at various time points activates (as evidenced by c-Fos induction) enkephalin-positive neurons but not NPY-immunoreactive neurons. The findings suggest that PACAP-immunoreactive retinal afferents in the IGL primarily innervate enkephalin-immunoactive neurons and that the enkephalin-containing neurons, which project locally and to the contralateral IGL, are activated by light independent of diurnal time.

Identification of NPY-induced c-Fos expression in hypothalamic neurones projecting to the dorsal vagal complex and the lower thoracic spinal cord

Neuropeptide Y exerts profound effects on body weight and glucose homeostasis. We have investigated the effect of centrally administered neuropeptide Y on the activity of descending neurones of the hypothalamic paraventricular nucleus by combining retrograde tract tracing with c-Fos immunocytochemistry. Male rats were injected with True Blue into the dorsal vagal complex and with FluoroGold into the intermediolateral column of the lower thoracic spinal cord. One week after the last surgical procedure, animals were injected centrally with an orexigenic dose of neuropeptide Y (5 microg) and sacrificed 60 to 240 minutes following this injection. Temporal analysis of NPY-induced c-Fos expression showed a peak at 90 minutes, which was nearly returned to basal levels between 120 and 240 minutes. Expression of c-Fos was prominent in several of the subnuclei of the paraventricular nucleus and in the adjacent perifornical nucleus. Neurones projecting to the spinal cord were prominent in the dorsal, lateral, and ventral portion of the medial parvicellular subnuclei of the PVN. About 15% of IML projecting neurones of the medial parvicellular subnucleus were Fos-positive, whereas less than 5% of IML projecting neurones from other subnuclei were Fos-positive. Hardly any PVN neurones projecting to the dorsal vagal complex were concomitantly Fos-positive. A considerably larger (>10%) proportion of perifornical neurones projecting to the nucleus of the solitary tract were c-Fos-immunopositive. In conclusion, NPY induces c-Fos in paraventricular neurones projecting to intermediolateral column of the spinal cord and in neurones of the perifornical nucleus projecting to the dorsal vagal complex.

Neural input and neural control of the subcommissural organ

The neural control of the subcommissural organ (SCO) has been partially characterized. The best known input is an important serotonergic innervation in the SCO of several mammals. In the rat, this innervation comes from raphe nuclei and appears to exert an inhibitory effect on the SCO activity. A GABAergic innervation has also been shown in the SCO of the rat and frog Rana perezi. In the rat, GABA and the enzyme glutamate decarboxylase are involved in the SCO innervation. GABA is taken up by some secretory ependymocytes and nerve terminals, coexisting with serotonin in a population of synaptic terminals. Dopamine, noradrenaline, and different neuropeptides such as LH-RH, vasopressin, vasotocin, oxytocin, mesotocin, substance P, alpha-neoendorphin, and galanin are also involved in SCO innervation. In the bovine SCO, an important number of fibers containing tyrosine hydroxylase are present, indicating that in this species dopamine and/or noradrenaline-containing fibers are an important neural input. In Rana perezi, a GABAergic innervation of pineal origin could explain the influence of light on the SCO secretory activity in frogs. A general conclusion is that the SCO cells receive neural inputs from different neurotransmitter systems. In addition, the possibility that neurotransmitters and neuropeptides present in the cerebrospinal fluid may also affect the SCO activity, is discussed.

Connections of the precommissural nucleus

The connections of the precomissural nucleus (PRC) have been examined with anterograde and retrograde axonal tracing methods in the rat. Experiments with cholera toxin B subunit (CTb) indicate that the PRC shares a number of common afferent sources with the dorsolateral periaqueductal gray (PAG). Thus, we have shown that the nucleus receives substantial inputs from the prefrontal cortex, specific domains of the rostral part of the lateral septal nucleus, rostral zona incerta, perifornical region, anterior hypothalamic nucleus, ventromedial hypothalamic nucleus, dorsal premammillary nucleus, medial regions of the intermediate and deep layers of the superior colliculus, and cuneiform nucleus. Moreover, the PRC also receives inputs from several PAG regions and from neural sites involved in the control of attentive or motivational state, including the laterodorsal tegemental nucleus and the ventral tegmental area. The efferent projections of the PRC were analyzed by using the Phaseolus vulgaris-leucoagglutinin (PHA-L) method. Notably, the PRC presents a projection pattern that resembles in many ways the pattern described previously for the rostral dorsolateral PAG in addition to projections to a number of targets that also are innervated by neighboring pretectal nuclei, including the rostrodorsomedial part of the lateral dorsal thalamic nucleus, the ventral part of the lateral geniculate complex, the medial pretectal nucleus, the nucleus of the posterior commissure, and the ventrolateral part of the subcuneiform reticular nucleus. Overall, the results suggest that the PRC might be viewed as a rostral component of the PAG, and the possible functional significance of the nucleus is discussed in terms of its connections.

The ventral lateral geniculate nucleus and the intergeniculate leaflet: interrelated structures in the visual and circadian systems

The ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL) are retinorecipient subcortical nuclei. This paper attempts a comprehensive summary of research on these thalamic areas, drawing on anatomical, electrophysiological, and behavioral studies. From the current perspective, the vLGN and IGL appear closely linked, in that they share many neurochemicals, projections, and physiological properties. Neurochemicals commonly reported in the vLGN and IGL are neuropeptide Y, GABA, enkephalin, and nitric oxide synthase (localized in cells) and serotonin, acetylcholine, histamine, dopamine and noradrenalin (localized in fibers). Afferent and efferent connections are also similar, with both areas commonly receiving input from the retina, locus coreuleus, and raphe, having reciprocal connections with superior colliculus, pretectum and hypothalamus, and also showing connections to zona incerta, accessory optic system, pons, the contralateral vLGN/IGL, and other thalamic nuclei. Physiological studies indicate species differences, with spectral-sensitive responses common in some species, and varying populations of motion-sensitive units or units linked to optokinetic stimulation. A high percentage of IGL neurons show light intensity-coding responses. Behavioral studies suggest that the vLGN and IGL play a major role in mediating non-photic phase shifts of circadian rhythms, largely via neuropeptide Y, but may also play a role in photic phase shifts and in photoperiodic responses. The vLGN and IGL may participate in two major functional systems, those controlling visuomotor responses and those controlling circadian rhythms. Future research should be directed toward further integration of these diverse findings.