Scn Cells Expressing mt1 Receptor mRna Coexpress Avp mRna In Syrian And Siberian Hamsters

C. K. Song,13 T. J. Bartness,1'2'3'4* S. L. Petersen,5 and E. L. Bittman5

'Department of Biology 2Department of Psychology 3Department of Neurobiology

Neuropsychology and Behavioral Neuroscience Programs Georgia State University Atlanta, Georgia 30303 5Department of Biology Neurosciences and Behavior Program University of Massachusetts Amherst, Massachusetts 01003

Many animals, especially mammals living in temperate zones, confine life history events to specific times of the year. For example, species of hamsters and voles bred in the long days of spring/summer, whereas species of sheep and deer breed in the short days (SDs) of fall/winter (1). The most noise-free seasonal cue in their environment is the photoperiod (10). The photoperiod triggers, seasonal responses, including changes in reproductive status mentioned above, as well as cycles of body mass and fat and pelage (1). This photic information is transmitted to the pineal gland via a multi-synaptic circuit beginning with the retina and concluding with the pinealocytes (5). Among the central nervous system structures included in this circuit is the suprachiasmatic nucleus (SCN) of the hypothalamus, the predominant biological clock. The pineal gland, through its primary hormone melatonin (MEL), triggers these and many other seasonal responses (1). MEL is synthesized and released only during the night, therefore, it faithfully codes the duration of the dark portion of the photoperiod. Thus, the durational characteristic of nightly MEL secretion induces these seasonally appropriate responses (3). The SCN not only participates in the transmission of photic information to the pineal gland, but it also appears to be important for the reception of the

*Address all correspondence to Dr. Timothy J. Bartness.

durational MEL signals. That is, in pinealectomized Siberian hamsters (Phodopus sun-gorus), lesions of the SCN (SCNx) block the reception of SD MEL signals generated exogenously via S.C. infusions of the hormone (2,9). Similar experiments in SCNx Syrian hamsters (Mesocricetus auratus) have shown that an intact SCN is not necessary for the induction of SD-type reproductive responses to exogenously generated SD-like MEL signals (7), but it does seem that the SCN is important for the interpretation of series of SD MEL signals (4).

Binding of the melatonin analog 2-[125I] MEL (IMEL) to the SCN of Siberian hamsters supports the apparent critical nature of this structure in the reception of MEL signals in this species (6,12). Moreover, distribution of the mRNA for the MEL receptor that is functionally responsible for the photoperiodic responses (i.e., the mt1 receptor [a.k.a. MEL1A receptor]) shows its expression in the same areas that also exhibit IMEL binding, including the SCN, as well as the paraventricular and reunions nuclei of the thalamus in Siberian hamsters (8,11). Microscopic resolution of cells expressing the mt1 receptor has not been reported, however. In addition, the neurochemical phe-notype of neurons expressing the mt1 receptor also is unknown. Therefore, we attempted to identify the neurons within the SCN that express the mt1 receptor in both Siberian and Syrian hamsters, and to begin to typify their neurochemical identity in the former species.

We first used single label in situ hybridization for the mt1 receptor mRNA (probe courtesy of Drs. Steven Reppert and David Weaver). Densely labeled cells in the SCN, as well as other hypothalamic, thalamic and brainstem structures were found using emulsion autoradiography. Specifically for the SCN, cells expressing the mt1 receptor were distributed throughout the nucleus in Siberian hamsters, with less extensive

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Figure 1. Photomicrograph of Siberian hamster SCN. mt, mRNA expressing cells of the SCN also contained AVP mRNA (curved arrow). Single-labeled mt, mRNA (triangle) and AVP mRNA (hollow arrows) also are shown.

Figure 1. Photomicrograph of Siberian hamster SCN. mt, mRNA expressing cells of the SCN also contained AVP mRNA (curved arrow). Single-labeled mt, mRNA (triangle) and AVP mRNA (hollow arrows) also are shown.

Figure 2. Photomicrograph of Syrian hamster SCN. Co-expression of mt, and AVP mRNAs are indicated by curved arrows.

labeling in the Syrian hamster SCN. Then we tested for co-expression of the mt, receptor with messages for either arginine vasopressin (AVP), somatostatin (SS) or retinoid Z receptor P (RZRP; a putative nuclear mt, receptor) in Siberian hamster brain only. Cells in the SCN expressing the mt, receptor also contained mRNA for AVP (Figures 1 and 2) and RZRP, but not for SS.

In conclusion, this preliminary report is the first to demonstrate cellular resolution of the mt, receptor mRNA in Siberian and Syrian hamster brains. In addition, mt, receptor mRNA is co-expressed with AVP and RZRP mRNA in the SCN of Siberian hamsters. These data suggest that seasonal photoperiodic MEL signals may be received directly by vasopressinergic cells within the SCN.

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