We report here that BDNF-KIV mice slept less overall than WT mice. This decrease was accounted for by less sleep in the later parts of the active (dark) cycle and a delay in entering the rest phase. With progression of the active cycle or in response to SD, both Bdnf and Cort expression increase as sleep pressure builds. Consistent with this notion, we observed a rise in both Bdnf and Cort gene expression in WT animals after exposure to SD. However, in BDNF-KIV animals the levels of BDNF mRNA and protein as well as Cort gene expression were impaired. The genetic manipulation in the BDNF-KIV animals results in complete disruption of promoter IV-derived BDNF and down-regulation of additional Bdnf transcripts, resulting in a complete loss of the SD-induced increase in BDNF protein expression. Interestingly, transcription driven by promoter I was the most highly regulated in response to SD. In comparison to the 2-fold increase in promoter IV-driven transcription, promoter I driven transcription was elevated 12-fold. This difference is quite striking compared to the effects of ECS, which enhanced promoter IV activity by ~3-fold and promoter I activity by ~2-fold. Indeed, following SD, exon I-containing transcripts became the most highly expressed in the mPFC, with exon I-containing transcripts being expressed 4-fold more abundantly than exon IV-containing transcripts. Since the large SD-induced increase in exon I-driven transcription is significantly attenuated in addition to the complete disruption of promoter IV-driven activity, the BDNF-KIV mutant animals provide a very useful tool for studying the effects of activity-dependent BDNF expression on sleep homeostasis and behavior.
BDNF has been implicated in the development and function of the cortical inhibitory system at multiple levels [16
]. Several studies have suggested that BDNF plays an important role in regulating inhibitory interneuron migration in the cerebral cortex [46
]. BDNF also plays a key role in inhibitory interneuron differentiation and maintenance later in neurodevelopment [16
]. Substantial evidence has shown that BDNF signaling through TrkB is critical in promoting inhibitory interneuron synaptogenesis and development of a mature GABAergic phenotype by inducing the expression of markers of GABAergic cells including GAD67, GAT1, calcium-binding proteins including parvalbumin as well as various neuropeptides [16
]. Functionally, the impact of BDNF signaling on promoting maturation of cortical inhibition results in regulation of the critical period for plasticity in the visual cortex [16
]. The onset of the critical period for visual development coincides with the initiation of non-REM sleep homeostasis, and monocular deprivation experiments have shown that sleep enhances synaptic remodeling during the critical period of visual cortex development [49
]. It is of interest that the developmental induction of cortistatin expression at the end of the second postnatal week in rodent life coincides with rapidly rising cortical BDNF levels, initiation of the critical period for visual cortical plasticity, and onset of non-REM sleep [16
]. The present study demonstrates a role for activity-dependent BDNF expression in regulation of sleep behavior, possibly via effects on a subset of cortical GABAergic interneurons. These results may have implications in critical period plasticity in visual cortex as well as other cortical areas.
In the adult brain cortistatin expression is correlated with sleep need and its administration promotes slow-wave activity (SWA) [27
gene expression is highest at the end of the circadian active period and increases dramatically in response to SD [27
]. Functionally, it has been shown that intracerebroventricular administration of cortistatin leads to induction of SWA [27
]. Interestingly, the increase in SWA observed after SD only occurs in rats after postnatal day 20, coinciding with the time when SD begins to induce cortical BDNF expression [52
]. This time period also coincides with electrophysiological maturation of the inhibitory interneuron system and attainment of maximal slow wave delta power [50
It has been reported that enhancing BDNF expression via exploratory activity during waking is causally linked to homeostatic sleep mechanisms [9
]. Some studies have suggested that BDNF protein, which increases during sustained waking, leads to heightened synaptic potentiation and increased cell-to-cell coupling [6
]. This in turn, leads to increased cortical synchrony and, subsequently, increased SWA power [6
]. Our results suggest that a subset of GABAergic interneurons expressing several neuropeptide markers, including cortistatin, are highly sensitive to activity-dependent BDNF signaling and sleep pressure. Expression of Crhbp
, a binding protein that acts to sequester corticotropin releasing hormone (CRH) may also define this subset of interneurons. Crhbp
expression is also misregulated in the BDNF-KIV mice, is induced following sleep deprivation, and has been co-localized with NPY-positive interneurons [54
]. Recent studies have shown that CRH may also be capable of modulating sleep homeostasis [55
]. Future experiments should directly examine whether regulation of sleep homeostasis by activity-dependent BDNF expression is mediated by this subpopulation of cortistatin-positive GABAergic interneurons.