Ovarian hormone‐induced reorganization of oxytocin‐labeled dendrites and synapses lateral to the hypothalamic ventromedial nucleus in female rats

Abstract

Central oxytocin (OT) modulates many social behaviors, including female rat sexual receptivity, quantified as the copulatory stance known as lordosis. The expression of the lordosis response is modulated by OT action in the ventromedial nucleus of the hypothalamus (VMH), as demonstrated by behavioral pharmacology experiments. However, the subcellular localization of OT in this brain region has been unclear. We tested the hypothesis that ovarian hormones reorganize OT-labeled pre- or postsynaptic elements in the fiber complex lateral to the VMH by using immunoelectron microscopy. OT immunolabeling occurred in axonal boutons identified by the presence of small, clear synaptic vesicles and double labeling with the presynaptic markers synaptophysin and vesicular glutamate transporter 2. OT immunoreactivity also was observed in dendritic profiles, verified with double labeling for the dendrite-specific marker microtubule-associated protein 2. Ovarian hormones did not alter the density of axonal boutons; however, estradiol treatment reduced the density of dendritic profiles by 34%. This effect was reversed when progesterone was given subsequent to estradiol. The effect of estradiol treatment was specific to dendrites that lacked OT immunostaining; the density of OT-labeled dendritic profiles remained constant during estradiol treatment. With the estradiol-induced exit of non-OT-labeled dendritic profiles, the remaining OT-labeled dendritic profiles experienced an increase in their number of synaptic contacts. Thus, hormone treatments that mimic the 4- day rat estrous cycle provoke a chemically coded reorganization of dendrite innervation in the fiber plexus lateral to the VMH that may underlie the hormone-specific effect of OT on reproductive behavior.

Orphanin FQ in the mediobasal hypothalamus facilitates sexual receptivity through the deactivation of medial preoptic nucleus mu-opioid receptors

“…One possible mechanism is β-END neurons may have dendritic projections extending dorsolateral beyond the boundaries of the ARH into portions of the central VMH (VMHc) and ventrolateral VMH (VMHvl) that may be regulated by steroid exposure (Millhouse, 1973a, b; van den Pol and Cassidy, 1982). This possibility is supported by estradiol priming increasing the area of ORL-1 immunoreactivity in the VMH (Sinchak et al, 2007) and the ability of steroid priming to rapidly alter dendritic length in the VMH (Flanagan-Cato et al, 2006; Griffin et al, 2010; Griffin and Flanagan-Cato, 2008, 2009, 2011). The second viable mechanism for OFQ/N acting in the VMH to regulate ARH POMC neurons is through VMH-ARH microcircuits (Sternson et al, 2005).…”

Glutamatergic phenotype of glucagon-like peptide 1 neurons in the caudal nucleus of the solitary tract in rats

“…Both procedures eliminated GLP1 immunolabeling in rat brain tissue sections. The specificity of mouse anti-VGLUT2 has been characterized and reported (Griffin et al 2010). …”

Assembling the Puzzle: Pathways of Oxytocin Signaling in the Brain

“…4 However, in some structures, such as the ventromedial hypothalamic nucleus, no OT axons have been detected, even after physiological stimulation. Despite that, OT-containing axons make axo-dendritic synapses at the external border of the nucleus (46). Due to the fact that OT neurons are glutamatergic in nature, they can modulate activity of the ventromedial hypothalamic nucleus in a classical transsynaptic fashion.…”