Columnar organization of estrogen receptor-alpha immunoreactive neurons in the periaqueductal gray projecting to the nucleus para-retroambiguus in the caudal brainstem of the female golden hamster.

In the hamster brainstem estrogen receptor-alpha-immunoreactive neurons (ER-alpha-IR) are present in the nucleus para-retroambiguus (NPRA), located in the caudal ventrolateral medulla (CVLM) ventrolaterally to the nucleus retroambiguus (NRA). NPRA neurons project mainly to the thoracic and upper lumbar cord and are probably involved in the autonomic adaptations during the estrous cycle. The periaqueductal gray (PAG), projecting to the CVLM, also contains ER-alpha-IR neurons. This raises the questions: how are these projections organized and are ER-alpha-IR neurons in PAG and NPRA linked directly? Combined retro- and anterograde tracing techniques, using wheat germ agglutinin-horseradish peroxidase (WGA-HRP), were carried out to demonstrate neuronal relationships between PAG and NPRA and/or NRA. Finally, a double-label immunostaining was performed in ovariectomized hamsters combining anti-ER-alpha antibody immunocytochemistry with cholera toxin B injections into the CVLM, to differentiate between ER-alpha-IR projections from the PAG to either NRA or NPRA. The experiments showed that retrograde labeling from the NRA mainly occurred in the rostral and intermediate ipsilateral PAG, while injections involving both NRA and NPRA resulted in numerous labeled neurons in the ipsilateral rostral, intermediate and especially the caudal PAG. The anterograde tracing studies confirmed these projections: from the rostral PAG almost exclusively to the NRA and from the caudal PAG to the NPRA, while the intermediate lateral PAG projects to both NPRA and NRA. Our double-immunostudies revealed that ER-alpha-IR projections descend only towards the NPRA and mainly originate from the ipsilateral caudal PAG. Retrogradely labeled ER-alpha-IR neurons in the PAG were observed in two separate columns, laterally and ventrolaterally in the caudal half of the PAG. The results provide evidence for the existence of differentiated PAG-CVLM projections to NRA and NPRA, respectively, originating from discrete longitudinal "PAG-columns." Only the projection to the NPRA is estrogen receptive, supporting the hypothesis that the NPRA is involved in the adaptive changes in autonomic control during successive phases of the estrous cycle.