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Expression of Somatostatin Subtype 1 Receptor in the Rabbit Retina

¹ From the Dipartimento di Fisiologia e Biochimica "G. Moruzzi," Università di Pisa, Italy; ² Dipartimento di Scienze Ambientali, Università della Tuscia, Viterbo, Italy; and ³ Institut National de la Santé et de la Recherche Médicale, Paris, France.

PURPOSE. To detect mRNAs for somatostatin (somatotropin release-inhibiting factor [SRIF]) receptor subtypes 1 to 5 (sst₁ through sst₅) in rabbit retinas by reverse transcription–polymerase chain reaction (RT-PCR) and to investigate the distribution of sst₁ by single- and double-label immunocytochemistry.

METHODS. Semiquantitative RT-PCR using sst-specific primers from mouse sequences was performed. sst₁ was localized using a polyclonal antiserum directed to human sst₁ in cryostat sections of retinas from either normal or optic nerve–transected animals. Immunolabeled cell sizes and densities were measured in wholemounted retinas using computer-assisted image analysis. Double-label immunofluorescence was performed using the sst₁ antiserum in conjunction with monoclonal antibodies directed to SRIF, tyrosine hydroxylase (TH), parvalbumin (PV), or -aminobutyric acid (GABA).

RESULTS. With RT-PCR it was found that all five sst mRNAs were expressed in the rabbit retina, with highest levels of sst₁ mRNA. sst₁ immunolabeling was localized to amacrine cells in the proximal inner nuclear layer (INL) of all retinal regions and to displaced amacrine cells in the ganglion cell layer (GCL) of the ventral retina. Some large sst₁-immunoreactive (IR) somata were also present in the GCL. They were not observed after optic nerve transection. Double-label immunofluorescence showed sst₁ expression by all TH-IR amacrine cells and by other amacrine cells that were neither PV-IR nor GABA-IR. In addition, sst₁ was expressed by all SRIF-containing displaced amacrine cells.

CONCLUSIONS. All five sst mRNAs are expressed in the rabbit retina. The localization of sst₁ suggests that it may mediate SRIF actions onto amacrine (including dopaminergic) and sparse ganglion cells. sst₁ expression in SRIF-IR cells suggests that this receptor may also act as an autoreceptor.

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