Inward-rectifying K+ (Kir2) leak conductance dampens the excitability of lamina I projection neurons in the neonatal rat

Spinal lamina I projection neurons serve as a major conduit by which noxious stimuli detected in the periphery are transmitted to nociceptive circuits in the brain, including the parabrachial nucleus (PB) and the periaqueductal gray (PAG). While neonatal spino-PB neurons are more than twice as likely to exhibit spontaneous activity compared to spino-PAG neurons, the underlying mechanisms remain unclear since nothing is known about the voltage-independent (i.e. 'leak') ion channels expressed by these distinct populations during early life. To begin identifying these key leak conductances, the present study investigated the role of classical inward-rectifying K⁺ (K_ir2) channels in the regulation of intrinsic excitability in neonatal rat spino-PB and spino-PAG neurons. The data demonstrate that a reduction in K_ir2-mediated conductance by external BaCl₂ significantly enhanced intrinsic membrane excitability in both groups. Similar results were observed in spino-PB neurons following K_ir2 channel block with the selective antagonist ML133. In addition, voltage-clamp experiments showed that spino-PB and spino-PAG neurons express similar amounts of K_ir2 current during the early postnatal period, suggesting that the differences in the prevalence of spontaneous activity between the two populations are not explained by differential expression of K_ir2 channels. Overall, the results indicate that K_ir2-mediated conductance tonically dampens the firing of multiple subpopulations of lamina I projection neurons during early life. Therefore, K_ir2 channels are positioned to tightly shape the output of the immature spinal nociceptive circuit and thus regulate the ascending flow of nociceptive information to the developing brain, which has important functional implications for pediatric pain.

Importance of Adult Dmbx1 in Long-Lasting Orexigenic Effect of Agouti-Related Peptide

Dmbx1 is a brain-specific homeodomain transcription factor expressed primarily during embryogenesis, and its systemic disruption (Dmbx1(-/-)) in the ICR mouse strain resulted in leanness associated with impaired long-lasting orexigenic effect of agouti-related peptide (AgRP). Because spatial and temporal expression patterns of Dmbx1 change dramatically during embryogenesis, it remains unknown when and where Dmbx1 plays a critical role in energy homeostasis. In the present study, the physiological roles of Dmbx1 were examined by its conditional disruption (Dmbx1(loxP/loxP)) in the C57BL/6 mouse strain. Although Dmbx1 disruption in fetal brain resulted in neonatal lethality, its disruption by synapsin promoter-driven Cre recombinase, which eliminated Dmbx1 expression postnatally, exempted the mice (Syn-Cre;Dmbx1(loxP/loxP) mice) from lethality. Syn-Cre;Dmbx1(loxP/loxP) mice show mild leanness and impaired long-lasting orexigenic action of AgRP, demonstrating the physiological relevance of Dmbx1 in the adult. Visualization of Dmbx1-expressing neurons in adult brain using the mice harboring tamoxifen-inducible Cre recombinase in the Dmbx1 locus (Dmbx1(CreERT2/+) mice) revealed Dmbx1 expression in small numbers of neurons in restricted regions, including the lateral parabrachial nucleus (LPB). Notably, c-Fos expression in LPB was increased at 48 hours after AgRP administration in Dmbx1(loxP/loxP) mice but not in Syn-Cre;Dmbx1(loxP/loxP) mice. These c-Fos-positive neurons in LPB did not coincide with neurons expressing Dmbx1 or melanocortin 4 receptor but did coincide with those expressing calcitonin gene-related peptide. Accordingly, Dmbx1 in the adult LPB is required for the long-lasting orexigenic effect of AgRP via the neural circuitry involving calcitonin gene-related peptide neurons.

[Distribution of GABAergic neurons in pneumotaxic center nuclei in the early postnatal period in norm and in prenatal deficiency of serotoninergic system in rats]

The aim of this study was to determine the distribution of GABAergic neurons in pneumotaxic center structures (parabrachial complex medial subnucleus and Kölliker-Fuse nucleus) in norm and in deficiency of serotoninergic system during the prenatal period of development in Wistar rats. Reduction of endogenous serotonin levels in fetal rats was achieved by tryptophan hydroxylase inhibition with para-chlorophenylalanine (PCPA), which was administered to female rats on Day 16 of gestation. Material was obtained from the area of the pons from experimental and control (intact) rat pups at early postnatal (Days 5, 10 and 12) and juvenile (Day 20) periods. At each time point, 5-6 animals were studied from both experimental and control groups. To demonstrate GABAergic neurons, antibodies against glutamate decarboxylase (GAD-67), the enzyme involved in its synthesis, were used. The results have shown that Kölliker-Fuse nucleus contained a population of GABAergic neurons at early postnatal period, the size of which was preserved until juvenile age. In parabrachial complex medial subnucleus during the early postnatal period, a small number of GABAergic neurons was detected, which was somewhat increased by juvenile age. Serotonin deficiency in pneumotaxic center structures lead to a reduction of the numbers of GABAergic neurons, GABAergic synapses and their clusters. A reduction of serotonin levels during the prenatal period may cause the disturbances in the inhibitory afferent signaling of the pneumotaxic center nuclei and lead to the changes of local inhibitory GABAergic networks in its nuclei, resulting in the disturbances of the inhibitory processes in the center structures.