Retinal Innervation of Calbindin-D28K Cells in the Hamster Suprachiasmatic Nucleus: Ultrastructural Characterization

Abstract

The authors have described a subregion of the hamster hypothalamic suprachiasmatic nucleus (SCN) containing cells that are immunopositive for the cytosolic calcium-binding protein, Calbindin-D 28K (CaBP). Several lines of evidence indicate that this region may constitute the site of the pacemaker cells that are responsible for the regulation of circadian locomotor rhythms. First, 79% of the CaBP-immunoreactive (ir) neurons express Fos in response to photic stimulation, indicating that they are close to or part of the input pathway to pacemakers. Second, at the light microscopy level, retinal terminals innervate the CaBP subnucleus. Finally, destruction of this subnucleus renders animals arrhythmic in locomotor activity. In this study, the authors examined the ultrastructural relationship between cholera toxin (CTβ) labeled retinal fibers and the CaBP-ir subregion within the hamster SCN. CTβ-ir retinal terminals make primarily axo-somatic, symmetric, synaptic contacts with CaBP-ir perikarya. In addition, retinal terminals form synapses with CaBP processes as well as with unidentified profiles. There are also complex interactions between retinal terminals, CaBP perikarya, and unidentified profiles. Given that axo-somatic synaptic input has a more potent influence on a cell's electrical activity than does axo-dendritic synaptic input, cells of the CaBP subregion of the SCN are ideally suited to respond rapidly to photic stimulation to reset circadian pacemakers.Keywords calcium-binding proteins; calbindin-D 28K ; CaBP; pacemaker; suprachiasmatic nucleus; retinal terminals; electron microscopy; circadian rhythmsThe suprachiasmatic nucleus (SCN) of the hypothalamus is the site of the putative circadian clock in mammals (see Klein et al., 1991). The circadian system regulates the periodicity of many rhythmic responses in mammals including rest-wake cycles (see Moore-Ede et al., 1982), electrical activity, and neuropeptide levels (Inouye, 1996), to name a few. In the absence of exogenous stimuli, the circadian clock oscillates with a periodicity that is close to 24 h. The circadian timing system also has the capacity to entrain or synchronize the organism to the exogenous geo-solar cycle. The primary neural substrate responsible for the © 2000 Sage Publications, Inc.Correspondence to: Maria-Teresa Romero. transduction of exogenous photic stimuli is the retinohypothalamic tract (RHT), a monosynaptic pathway projecting from the retina to the SCN (Card and Moore, 1991;Moore, 1996). A secondary pathway, the geniculo-hypothalamic tract, also provides photic input to the circadian pacemaker. Photic information is integrated in the SCN to produce a stable phase relationship between physiological and behavioral activity and the environmental light-dark cycle (Meijer, 1991). While it is clear that the SCN is the site of the circadian pacemaker, the precise identity of retinorecipient cells that mediate entrainment remains to be elucidated. NIH Public AccessWe recently described cells immunopositive for Calbindin-D 28K (...

Expression of 5-HT7 receptor mRNA in the hamster brain: Effect of aging and association with calbindin-D28K expression

“…This region is crucial for the generation of circadian rhythmicity, as shown by the loss of circadian rhythms in behavior (locomotor activity, drinking, gnawing) and circulating concentrations of hormones (melatonin and cortisol) in hamsters after discrete lesions of this area (LeSauter and Silver, 1999;Kriegsfeld et al, 2004b). Also, the calbindin subnucleus receives direct retinal innervation (Bryant et al, 2000) and mediates photic phase shifts, as demonstrated by the finding that intracerebroventricular administration of calbindin antisense oligonucleotides attenuates light induction of Per1 mRNA expression in the SCN and circadian phase shifts (Hamada et al, 2003). The present identification of 5-HT 7 receptor mRNA within the calbindin subnucleus supports other findings that post-synaptic 5-HT 7 receptors in the SCN modulate the effects of serotonergic agonists on responses to light or to glutamate, the major neurotransmitter released by the retinohypothalamic tract that innervates the SCN (Ying and Rusak, 1997;Quintero and McMahon, 1999).…”

Calbindin-D28K cells selectively contact intra-SCN neurons

“…7. This figure indicates that the retinal (Silver et al, 1996;Bryant et al, 2000), NPY and 5-HT afferent pathways, as well as the ventral VIP cells, contact all or nearly all the CalB cells.A caveat in the present study is that only somatic appositions were counted. While the varicosities terminating onto the cell bodies, on large fibers, or axon hillocks near the cell bodies are quite clear, it is very difficult to determine whether varicosities seen on fine fibers are appositions, fibers crossing each other, or small artifacts resulting from the immunocytochemistry procedure.…”

“…All these cells may be part of the light entrainment mechanism in hamsters. CalB (Bryant et al, 2000) and GRP (Aioun et al, 1998) cells receive retinal terminals and presentation of a light pulse during the night induces c-Fos in about 80% of the CalB cells (Silver et al, 1996), 78% of the GRP cells and 60% of the VIP cells (Aioun et al, 1998). It is also known that administration of GRP (McArthur et al, 2000) or coadministration of VIP, peptide histidine isoleucine and GRP (Albers et al, 1991) induce phase-dependent resetting of the SCN that resembles the lightinduced phase response curve.…”

“…The CalB cells are part of the entrainment pathway. They receive synaptic contacts from the retinal fibers (Bryant et al, 2000). Most (79%) of the CalB cells within the CalB subnucleus express Fos in response to a light pulse (Silver et al, 1996).…”

“…These cells are densely packed, receive direct retinal synaptic input (Bryant et al, 2000) and are responsive to photic stimuli, with about 80% of the cells showing Fos-like immunoreactivity (ir) following a light pulse given during subjective night (Silver et al, 1996). The foregoing evidence indicates these CalB cells are on the input pathway for photic stimuli reaching the SCN.…”