Targeted microlesions reveal novel organization of the hamster suprachiasmatic nucleus

Abstract

The role of the suprachiasmatic nuclei (SCN) in generating circadian rhythms in physiology and behavior is well established. Recent evidence based on clock gene expression indicates that the rodent SCN are composed of at least two functional subdivisions. In Syrian hamsters (Mesocricetus auratus), cells in a subregion of the caudal SCN marked by calbindin-D(28K) (CalB) express light-induced, but not rhythmic, clock genes (Per1, Per2, and Per3). In the SCN region marked by vasopressinergic cells and fibers, clock gene expression is rhythmic. Importantly, lesions of the CalB subregion that spare a significant portion of the SCN abolish rhythms in locomotor behavior. One possibility is that the CalB subregion is required to maintain SCN function necessary to support all behavioral and physiological rhythms. Alternatively, this subregion may control circadian rhythms in locomotor behavior, whereas other circadian responses in physiology and behavior are sustained by different SCN compartments. The present study sought to distinguish between these possibilities by examining the role of the CalB subregion in a battery of rhythms within an individual animal. The results indicate that lesions of the CalB subregion of the SCN abolish circadian rhythms in behavior (locomotion, drinking, gnawing), physiology (body temperature, heart rate), and hormone secretion (melatonin, cortisol), even when other SCN compartments are spared. Together, these findings suggest a novel fundamental property of SCN organization, with a subset of cells being critical for the maintenance of SCN function manifest in circadian rhythms in physiology and behavior.

Figure 1.

Actograms depicting locomotor (top), drinking (middle), and gnawing (bottom) behavior of hamster 14-1, an animal with a missed SCN lesion. Spectral analyses are presented to the right of each actogram. Black vertical bars to the right of the actograms indicate the days on which the analyses were performed.

Figure 2.

Actograms depicting locomotor (top), drinking (middle), and gnawing (bottom) behavior of animals housed in DD. Hamster 41-1 (left) had an SCN lesion that spared the CalB subregion unilaterally (see photomicrographs in Fig. 3). In contrast, hamster 21-4 (right) had an SCN lesion abolishing the CalB subregion bilaterally (see photomicrographs in Fig. 3). Spectral analyses are presented to the right of each actogram. Black vertical bars to the right of actograms indicate the days on which the analyses were performed.

Figure 3.

An example of three hamsters with lesions that either missed the SCN (left) or partial SCN lesions sparing (middle) or eliminating (right) the CalB subregion. Hamster 41-1 has a comparatively large SCN lesion eliminating vasopression staining, while sparing CalB and VIP, and this animal remained rhythmic in all measures. In contrast, hamster 21-4 had a small SCN lesion eliminating CalB bilaterally, while sparing vasopressinergic and VIPergic cells and fibers. The phenotypes of these same animals are depicted in Figures 1 and 3, 4, 5. The insets depict high-power photomicrographs of outlined regions, and arrows point to cell bodies.

Figure 4.

Records of heart rate and body temperature of a hamster (hamster 14-1) maintained in DD. This animal received a lesion that missed the SCN. Spectral analyses for the data presented are shown to the right of each record.

Figure 5.

Records of heart rate and body temperature of animals maintained in DD that either had an SCN lesion that spared the CalB subregion unilaterally (top; see photomicrographs in Fig. 2) or abolished the CalB subregion bilaterally (bottom; see photomicrographs in Fig. 2). Spectral analyses for the data presented are shown to the right of each record.

Figure 6.

ETV (in cubic millimeters) of hamsters held in DD for 8-10 weeks. Each point depicts the ETV of individual hamsters. Animals received a lesion that missed the SCN or either spared or abolished the CalB subregion of this nucleus.

Figure 7.

Serum cortisol and melatonin concentrations over the course of the day of an unlesioned control hamster implanted with a jugular catheter. The hamster was maintained in DD during the collection of blood samples.

Figure 8.

Serum cortisol (top) and melatonin (bottom) concentrations over the course of the day of animals receiving an SCN lesion either eliminating vasopression staining, while sparing CalB and VIP (hamster 41-1; left), or eliminating CalB bilaterally, while sparing vasopressinergic and VIPergic cells and fibers (hamster 21-4; right). Hamsters were maintained in DD during the collection of blood samples. Because there are not reference time points for arrhythmic animals held in DD, precluding averaging group data, hormone data from individual animals are shown.

Figure 9.

Coronal section through the caudal SCN stained immunohistochemically for CalB. Note that CalB fibers course dorsomedially toward the SCN shell.