Identification of brain target neurons using a fluorescent conjugate of corticotropin-releasing factor

In vivo uptake of a fluorescent conjugate of melanin-concentrating hormone in the rat brain

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide synthesized by posterior hypothalamic and incerto-hypothalamic neurons that project throughout the central nervous system. The MCHergic system modulates several important functions such as feeding behavior, mood and sleep. MCH exerts its biological functions through interaction with the MCHR-1 receptor, the only functional receptor present in rodents. The internalization process of MCHR-1 triggered by MCH binding was described in vitro in non-neuronal heterologous systems with over-expression of MCHR-1. Reports of in vivo MCHR-1 internalization dynamics are scarce, however, this is an important process to explore based on the critical functions of the MCHergic system. We had previously determined that 60 min after intracerebroventricular (i.c.v.) microinjections of MCH conjugated with fluorophore rhodamine (R-MCH), the dorsal and median raphe nucleus presented R-MCH positive labeled neurons. In the present work, we further studied the in vivo uptake process focusing on the distribution and time-dependent pattern of R-MCH positive cells 10, 20 and 60 min (T10, T20 and T60, respectively) after i.c.v. microinjection of R-MCH. We also explored this uptake process to see whether it was receptor- and clathrin-dependent and examined the phenotype of R-MCH positive cells and their proximity to MCHergic fibers. We found a great number of R-MCH positive cells with high fluorescence intensity in the lateral septum, nucleus accumbens and hippocampus at T20 and T60 (but not at T10), while a lower number with low intensity was observed in the dorsal raphe nucleus. At T20, in rats pre-treated with a MCHR-1 antagonist (ATC-0175) or with phenylarsine oxide (PAO), a clathrin endocytosis inhibitor, a robust decrease (> 50 %) of R-MCH uptake occurred in these structures. The R-MCH positive cells were identified as neurons (NeuN positive, GFAP negative) and some MCHergic fibers run in the vicinities of them. We concluded that neurons localized at structures that were close to the ventricular surfaces could uptake R-MCH in vivo through a receptor-dependent and clathrin-mediated process. Our results support volume transmission of MCH through the cerebrospinal fluid to reach distant targets. Finally, we propose that R-MCH would be an effective tool to study MCH-uptake in vivo.

Intranasally administered neuropeptide S (NPS) exerts anxiolytic effects following internalization into NPS receptor-expressing neurons

Experiments in rodents revealed neuropeptide S (NPS) to constitute a potential novel treatment option for anxiety diseases such as panic and post-traumatic stress disorder. However, both its cerebral target sites and the molecular underpinnings of NPS-mediated effects still remain elusive. By administration of fluorophore-conjugated NPS, we pinpointed NPS target neurons in distinct regions throughout the entire brain. We demonstrated their functional relevance in the hippocampus. In the CA1 region, NPS modulates synaptic transmission and plasticity. NPS is taken up into NPS receptor-expressing neurons by internalization of the receptor-ligand complex as we confirmed by subsequent cell culture studies. Furthermore, we tracked internalization of intranasally applied NPS at the single-neuron level and additionally demonstrate that it is delivered into the mouse brain without losing its anxiolytic properties. Finally, we show that NPS differentially modulates the expression of proteins of the glutamatergic system involved inter alia in synaptic plasticity. These results not only enlighten the path of NPS in the brain, but also establish a non-invasive method for NPS administration in mice, thus strongly encouraging translation into a novel therapeutic approach for pathological anxiety in humans.

Brainstem neuronal and behavioral activation by corticotropin-releasing hormone depend on the behavioral state of the animal

Central administration of corticotropin-releasing hormone (CRH) is known to enhance locomotion across a wide range of vertebrates, including the roughskin newt, Taricha granulosa. The present study aimed to identify the CRH effects on locomotor-controlling medullary neurons that underlie the peptide's behavioral stimulating actions. Single neurons were recorded from the rostral medullary reticular formation before and after intraventricular infusion of CRH in freely behaving newts and newts paralyzed with a myoneural blocking agent. In behaving newts, most medullary neurons showed increased firing 3-23 min after CRH infusion. Decreases in firing were less common. Of particular importance was the finding that in behaving newts, medullary neurons showed a cyclic firing pattern that was strongly associated with an increase in the incidence of walking bouts, an effect blocked by pretreatment with the CRH antagonist, alpha-helical CRH and not seen following vehicle administration. In contrast, the majority of medullary neurons sampled in immobilized newts lacked temporal cyclicity in their firing patterns following intraventricular infusion of CRH. That is, there was no evidence for a fictive locomotor activity pattern. Our results indicate that the actual expression of locomotion is a critical factor in regulating the behavior-activating effects of CRH and underscore the importance of using an awake, unrestrained animal for analysis of a hormone's neurobehavioral actions.

Brainstem reticulospinal neurons are targets for corticotropin-releasing factor-Induced locomotion in roughskin newts

Stress-induced release or central administration of corticotropin-releasing factor (CRF) enhances locomotion in a wide range of vertebrates, including the roughskin newt, Taricha granulosa. Although CRF's stimulatory actions on locomotor behavior are well established, the target neurons through which CRF exerts this effect remain unknown. To identify these target neurons, we utilized a fluorescent conjugate of CRF (CRF-TAMRA 1) to track this peptide's internalization into reticulospinal and other neurons in the medullary reticular formation (MRF), a region critically involved in regulating locomotion. Epifluorescent and confocal microscopy revealed that CRF-TAMRA 1 was internalized by diverse MRF neurons, including reticulospinal neurons retrogradely labeled with Cascade Blue dextran. In addition, we immunohistochemically identified a distinct subset of serotonin-containing neurons, located throughout the medullary raphé, that also internalized the fluorescent CRF-TAMRA 1 conjugate. Chronic single-unit recordings obtained from microwire electrodes in behaving newts revealed that intracerebroventricular (icv) administration of CRF-TAMRA 1 increased medullary neuronal firing and that appearance of this firing was associated with, and strongly predictive of, episodes of CRF-induced locomotion. Furthermore, icv administered CRF-TAMRA 1 produced behavioral and neurophysiological effects identical to equimolar doses of unlabeled CRF. Collectively, these findings provide the first evidence that CRF directly targets reticulospinal and serotonergic neurons in the MRF and indicate that CRF may enhance locomotion via direct effects on the hindbrain, including the reticulospinal system.