Diagonal band projection towards the hypothalamic supraoptic nucleus: Light and electron microscopic observations in the rat

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Abstract
Supraoptic nucleus (SON) neurons receive a prominent gamma‐aminobutyric acid (GABA) input. This study evaluated the hypothesis, partly on the basis of recent electrophysiological data, that this innervation might arise from GABAergic neurons located in the ventral diagonal band of Broca area. For retrograde transport studies, pentobarbital‐anesthetized male Long‐Evans rats received 0.03–0.20‐μl injections of a suspension of rhodamine tagged latex microspheres into the SON. In two cases where such injections were confined to the SON, less than 60 retrogradely labeled neurons were detected in the ipsilateral diagonal band. In three animals where injections extended into the perinuclear zone around the SON, more than 2,000 retrogradely labeled cells were counted in the ipsilateral diagonal band. For anterograde transport studies, another group of animals received either 30% horseradish peroxidase (HRP) in 0.5% poly‐L‐ornithine (0.05–0.10 μl injections) or Phaseolus vulgarus (iontophoresed from a 2% solution) into the diagonal band. After survivals of 18–24 hours (HRP) or 5 days (PHAL‐L) labeled axon terminals invested the perinuclear zone above the SON. The presence of just a single fiber within the nucleus indicated a minor projection to the SON itself. The HRP‐injected material was processed for ultrastructural examination and revealed dense HRP‐labeled axon terminals in this perinuclear zone, most often (98%) forming axodendritic appositions. A postembedding colloidal gold technique to visualize GABA‐synthesizing terminals revealed that fewer than 5% of these perinuclear HRP‐labeled terminals also exhibited GABA‐like immunoreactivity. Within the SON, where GABAergic axon terminals are abundant, few (≤5%) GABAergic terminals contained HRP. Therefore, only a minor component of this prominent GABAergic innervation of SON neurons appears to arise from GABAergic diagonal band neurons. It is proposed that interneurons in the perinuclear zone serve as a relay station for information transfer from the ventral diagonal band to the SON.