The use of heat-induced hydrolysis in immunohistochemistry on angiotensin II (AT1) receptors enhances the immunoreactivity in paraformaldehyde-fixed brain tissue of normotensive Sprague–Dawley rats

Maternal separation induces alterations in reversal learning and brain-derived neurotrophic factor expression in adult rats

AIMS

Early postnatal maternal and/or sibling separation (MS) can play an important role in the development of psychopathologies during ontogeny. The purpose of the present study is to investigate the effects of repeated MS on the cognitive and brain-derived neurotrophic factor (BDNF) function of rats.

METHODS

We investigated the effects of repeated MS that lasted 3 h/day during postnatal days 1-21 on spatial learning and reversal learning in Morris water maze tests in male rats. The rats were tested in 4 trials. Moreover, we examined the effects of MS on BDNF protein expression in the medial prefrontal cortex (mPFC), the nucleus accumbens, and the hippocampus via immunohistochemistry measurements.

RESULTS

We found that repeated MS modestly disrupted reversal learning performance in the Morris water maze and decreased BDNF protein expression in the mPFC.

CONCLUSION

The present study enhances our understanding of the neurobiological and behavioral consequences of repeated episodes of MS in rats to some degree.

Brief social isolation in early adolescence affects reversal learning and forebrain BDNF expression in adult rats

Isolation rearing produces significant behavioral and neurochemical dysregulations in rodents. However, few studies have examined the effects of short-term isolation rearing during puberty compared to chronic social isolation from weaning to adulthood. In this study, we subjected weaning rats to a brief two-week social isolation and then re-socialized them until adulthood. We found that early isolation rearing affected reversal learning without interfering with spatial learning in the Morris water maze. We also found that brain-derived neurotrophic factor (BDNF) protein expression was increased in the medial prefrontal cortex (mPFC) but was decreased in the nucleus accumbens (NAc), CA1 and dentate gyrus of the hippocampus in isolation-reared rats. Together, our findings support the use of adolescent social isolation as a rodent model to study brain and behavior abnormalities induced by early environmental interruptions.

Hypothalamic oxytocin mediates adaptation mechanism against chronic stress in rats

Accumulation of continuous life stress (chronic stress) often causes gastric symptoms. Although central oxytocin has antistress effects, the role of central oxytocin in stress-induced gastric dysmotility remains unknown. Solid gastric emptying was measured in rats receiving acute restraint stress, 5 consecutive days of repeated restraint stress (chronic homotypic stress), and 7 consecutive days of varying types of stress (chronic heterotypic stress). Oxytocin and oxytocin receptor antagonist were administered intracerebroventricularly (icv). Expression of corticotropin-releasing factor (CRF) mRNA and oxytocin mRNA in the paraventricular nucleus (PVN) of the hypothalamus was evaluated by real-time RT-PCR. The changes of oxytocinergic neurons in the PVN were evaluated by immunohistochemistry. Acute stress delayed gastric emptying, and the delayed gastric emptying was completely restored after 5 consecutive days of chronic homotypic stress. In contrast, delayed gastric emptying persisted following chronic heterotypic stress. The restored gastric emptying following chronic homotypic stress was antagonized by icv injection of an oxytocin antagonist. Icv injection of oxytocin restored delayed gastric emptying induced by chronic heterotypic stress. CRF mRNA expression, which was significantly increased in response to acute stress and chronic heterotypic stress, returned to the basal levels following chronic homotypic stress. In contrast, oxytocin mRNA expression was significantly increased following chronic homotypic stress. The number of oxytocin-immunoreactive cells was increased following chronic homotypic stress at the magnocellular part of the PVN. Icv injection of oxytocin reduced CRF mRNA expression induced by acute stress and chronic heterotypic stress. It is suggested that the adaptation mechanism to chronic stress may involve the upregulation of oxytocin expression in the hypothalamus, which in turn attenuates CRF expression.

Osmosensation in vasopressin neurons: changing actin density to optimize function

The proportional relation between circulating vasopressin concentration and plasma osmolality is fundamental for body fluid homeostasis. Although changes in the sensitivity of this relation are associated with pathophysiological conditions, central mechanisms modulating osmoregulatory gain are unknown. Here, we review recent data that sheds important light on this process. The cell autonomous osmosensitivity of vasopressin neurons depends on cation channels comprising a variant of the transient receptor potential vanilloid 1 (TRPV1) channel. Hyperosmotic activation is mediated by a mechanical process where sensitivity increases in proportion with actin filament density. Moreover, angiotensin II amplifies osmotic activation by a rapid stimulation of actin polymerization, suggesting that neurotransmitter-induced changes in cytoskeletal organization in osmosensory neurons can mediate central changes in osmoregulatory gain.

Neuronal and glial cell expression of angiotensin II type 1 (AT1) and type 2 (AT2) receptors in the rat retina

The bio-active peptide, angiotensin II (Ang II), has been suggested to exert a neuromodulatory effect on inner retinal neurons. In this study, we examined the distribution of angiotensin receptors (ATRs) in the developing and mature rat retina and optic nerve using immunofluorescence immunocytochemistry. Double-labeling experiments were performed with established markers to identify different retinal cell populations. In adult retinae, ATRs were observed on neurons involved in "ON" pathways of neurotransmission. Angiotensin II type 1 receptors (AT(1)Rs) were expressed by a sub-population of "ON" cone bipolar cells that also labeled for G alpha(0) and islet-1. Extra-neuronal expression of AT(1)Rs was evident on retinal astrocytes, Müller cells and blood vessels. Immunoreactivity for the angiotensin II type 2 receptor (AT(2)R) was observed on conventional and displaced GABAergic amacrine cells. Co-localization studies showed that AT(2)R-expressing amacrine cells constituted at least two separate sub-populations. Cell counts revealed that all wide-field amacrine cells expressing protein kinase C-alpha were also AT(2)R-positive; a further subset of amacrine cells expressing AT(2)Rs and stratifying in sublamina "b" of the inner plexiform layer (IPL) was identified. Developmental expression of AT(1)Rs was dynamic, involving multiple inner neuronal classes. At postnatal day 8 (P8), AT(1)R immunoreactivity was observed on putative ganglion cells. The characteristic bipolar cell labeling observed in adults was not evident until P13. In contrast, AT(2)Rs were detected as early as P2 and localized specifically to amacrine cells from this age onward. These data provide further evidence for the potential role of angiotensin II in the modulation of retinal neurons and glia. The differential pattern of expression of these receptors across these cell types is similar to that observed in the brain and suggests that a similar functional role for Ang II may also exist within the retina.