Oxytocin is released from neurons in the paraventricular hypothalamic nucleus (PVN) in mothers upon suckling and during adult social interactions. However, neuronal pathways that activate oxytocin neurons in social contexts are not yet established. Neurons in the posterior intralaminar complex of the thalamus (PIL), which contain tuberoinfundibular peptide 39 (TIP39) and are activated by pup exposure in lactating mothers, provide a candidate projection. Innervation of oxytocin neurons by TIP39 neurons was examined by double labeling in combination with electron microscopy and retrograde tract-tracing. Potential classic neurotransmitters in TIP39 neurons were investigated by in situ hybridization histochemistry. Neurons activated after encounter with a familiar conspecific female in a familiar environment were mapped with the c-Fos technique. PVN and the supraoptic nucleus oxytocin neurons were closely apposed by an average of 2.0 and 0.4 TIP39 terminals, respectively. Asymmetric (presumed excitatory) synapses were found between TIP39 terminals and cell bodies of oxytocin neurons. In lactating rats, PIL TIP39 neurons were retrogradely labeled from the PVN. TIP39 neurons expressed vesicular glutamate transporter 2 but not glutamic acid decarboxylase 67. PIL contained a markedly increased number of c-Fos-positive neurons in response to social encounter with a familiar conspecific female. Furthermore, the PIL received ascending input from the spinal cord and the inferior colliculus. Thus, TIP39 neurons in the PIL may receive sensory input in response to social interactions and project to the PVN to innervate and excite oxytocin neurons, suggesting that the PIL-PVN projection contributes to the activation of oxytocin neurons in social contexts.

Nursing has important consequences on mothers. To separate the prolactin-mediated and the neuronally-mediated actions of nursing, neurons directly affected by prolactin were visualized using pSTAT5 immunohistochemistry in relation to Fos-expressing neurons in suckled mother mice. In response to pup exposure following 22-h pup deprivation, we found a markedly elevated number of pSTAT5-containing neurons in several brain regions, including the lateral septum, medial amygdaloid nucleus, subparafascicular area, caudal periaqueductal gray, dorsal raphe, lateral parabrachial nucleus, nucleus of the solitary tract, and the periventricular, medial preoptic, paraventricular, arcuate and ventromedial nuclei of the hypothalamus. Pup exposure also induced Fos expression in all of these brain regions except the arcuate and ventromedial hypothalamic nuclei. Bromocriptine treatment known to reduce prolactin levels eliminated pSTAT5 from most brain regions while it did not affect Fos activation following suckling. The degree of colocalization for pSTAT5 and Fos ranged from 8 to 80% in the different brain regions suggesting that most neurons responding to pup exposure in mother mice are driven either by prolactin or direct neuronal input from the pups, while the number of neurons affected by both types of inputs depends on the examined brain area. In addition, both pSTAT5 and Fos were also double-labeled with estrogen receptor alpha (ERα) in mother mice, which revealed a very high degree of colocalization between pSTAT5 and ERα with much less potential interaction between Fos- and ERα-containing neurons suggesting that estrogen-sensitive neurons are more likely to be affected by prolactin than by direct neuronal activation.

Experiments on human samples and on genetic animal models of Huntington's disease (HD) suggest that a number of neuroactive metabolites in the kynurenine (KYN) pathway (KP) of the tryptophan (TRP) catabolism may play a role in the development of HD. Our goal in this study was to assess the concentrations of TRP, KYN, kynurenic acid and 3-hydroxykynurenine (3-OHK) in the serum and brain of 5-month-old C57Bl/6 mice in the widely used 3-nitropropionic acid (3-NP) toxin model of HD. We additionally investigated the behavioral changes through open-field, rotarod and Y-maze tests. Our findings revealed an increased TRP catabolism via the KP as reflected by elevated KYN/TRP ratios in the striatum, hippocampus, cerebellum and brainstem. As regards the other examined metabolites of KP, we found only a significant decrease in the 3-OHK level in the cerebellum of the 3-NP-treated mice. The open-field and rotarod tests demonstrated that treatment with 3-NP resulted in a reduced motor ability, though this had almost totally disappeared a week after the last injection, similarly as observed previously in most murine 3-NP studies. The relevance of the alterations observed in our biochemical and behavioral analyses is discussed. We propose that the identified biochemical alterations could serve as applicable therapeutic endpoints in studies of drug effects on delayed-type neurodegeneration in a relatively fast and cost-effective toxin model of HD.