Sustained inhibition of hippocampal networks in perinatal development

The main inhibitory neurotransmitter in the mature mammalian brain is the amino acid γ-aminobutyric acid (GABA), which is released from interneuronal axon terminals. GABA-induced inhibition is mediated by GABAA and GABAB receptors (GABAARs and GABABRs, respectively). GABA has an important role in shaping synchronous oscillations in the brain. Sustained inhibition of brain networks may be induced, for example, by the tonic activation extrasynaptic GABAARs or the persistent increase in interneuronal firing. Exacerbated sustained inhibition has been implicated as a pathophysiological mechanism involved in various neuropathological disorders, such as stroke, epilepsy, and autism. In the immature brain GABA exerts depolarizing actions due to high intracellular Cl- concentrations brought up by the activity of the Na+-K+-2Cl- transporter 1 (NKCC1). This Thesis comprises of two studies on the role and mechanisms of sustained inhibition of rodent hippocampal networks during perinatal development, a period when the brain is not only highly plastic, but also particularly vulnerable as disruption of early-stage plasticity may have life-long effects on brain function. In Study I of this Thesis, we provide evidence that during birth, the neuropeptide arginine vasopressin (AVP) strongly activates CA3 startum lucidum-radiatum interneurons in the rodent hippocampus, leading to sustained inhibition of the hippocampal network, as seen in the suppression of giant depolarizing potentials (GDPs). The AVP-mediated persistent activation of interneurons leads to a concomitant decrease in synchronous GABAergic depolarizing drive needed to facilitate the generation of GDPs. This action of sustained inhibition is independent of the polarity of GABAAR current as shown by replicating the above results in perinatal guinea pigs, where GABAAR-mediated signals are already more mature-like, hyperpolarizing. In Study II, we demonstrate that in the perinatal rat and mouse hippocampus the activity of K+-Cl- cotransporter 2 (KCC2) restrains GABAAR-mediated depolarizing drive in CA3 pyramidal neurons, brought about by NKCC1-mediated neuronal uptake of Cl-. We show for the first time that transport-functional KCC2 is expressed in the perinatal rodent hippocampus, where its activity leads to powerful sustained inhibition of CA3 pyramidal neuron excitability and the enhanced spike synchronization during the rising phase of GDPs. In summary, this Thesis examines novel mechanisms of sustained inhibition during perinatal brain development in rodents. The work included describes previously uncharacterized physiological mechanisms of sustained hippocampal inhibition during perinatal development in rodents.GABAergisellä signaloinnilla on tärkeä rooli synkronisen verkkotason aktiivisuuden säätelyssä aivoissa. Pitkäkestoista hermoverkkojen hillitsemistä voi ilmentyä niin toonisena sypasien ulkopuolisten GABAA reseptorien aktivoinnin tai GABAergisten hermosolujen pitkäkestoisen aktivoinnin seurauksena. Pitkäkestoisella hermoverkkojen hillitsemisellä on myös osuutta neuropatologisten sairauksien syntymekanismeissa. Tässä väitöskirjassa tutkin pitkäkestoisen hermoverkkojen hillinnän mekanismeja kehittyvien jyrsijöiden aivoissa. Väitöskirjassa esittelen aikaisemmin tuntemattomia fysiologisia mekanismeja, joilla pitkäkestoinen hermoverkkojen hillintä säätelee aivojen kehitystä perinataaleissa jyrsijöissä. Väitöskirja koostuu kahdesta osatyöstä. Ensimmäisessä osatyössä osoitamme miten arginiini vasopressiini aktivoi pitkäkestoisesti GABAergisiä hermosoluja hippokampuksen CA3 alueella, joka johtaa paikallisen verkkotason aktiivisuuden pitkäkestoiseen hillintään. Toisessa osatyössä osoitamme, että kloridikuljettaja KCC2:lla on tärkeä rooli jo perinataalisen hippokampuksen verkkotason aktiivisuuden pitkäkestoisessa hillitsemisessä

KCC2-Mediated Cl- Extrusion Modulates Spontaneous Hippocampal Network Events in Perinatal Rats and Mice

It is generally thought that hippocampal neurons of perinatal rats and mice lack transport-functional K-Cl cotransporter KCC2, and that Cl- regulation is dominated by Cl- uptake via the Na-K-2Cl cotransporter NKCC1. Here, we demonstrate a robust enhancement of spontaneous hippocampal network events (giant depolarizing potentials [GDPs]) by the KCC2 inhibitor VU0463271 in neonatal rats and late-gestation, wildtype mouse embryos, but not in their KCC2-null littermates. VU0463271 increased the depolarizing GABAergic synaptic drive onto neonatal CA3 pyramidal neurons, increasing their spiking probability and synchrony during the rising phase of a GDP. Our data indicate that Cl- extrusion by KCC2 is involved in modulation of GDPs already at their developmental onset during the perinatal period in mice and rats.Peer reviewe

Vasopressin excites interneurons to suppress hippocampal network activity across a broad span of brain maturity at birth

During birth in mammals, a pronounced surge of fetal peripheral stress hormones takes place to promote survival in the transition to the extrauterine environment. However, it is not known whether the hormonal signaling involves central pathways with direct protective effects on the perinatal brain. Here, we show that arginine vasopressin specifically activates interneurons to suppress spontaneous network events in the perinatal hippocampus. Experiments done on the altricial rat and precocial guinea pig neonate demonstrated that the effect of vasopressin is not dependent on the level of maturation (depolarizing vs. hyperpolarizing) of postsynaptic GABA(A) receptor actions. Thus, the fetal mammalian brain is equipped with an evolutionarily conserved mechanism well-suited to suppress energetically expensive correlated network events under conditions of reduced oxygen supply at birth.Peer reviewe

Quantitative Changes in the Mitochondrial Proteome of Cerebellar Synaptosomes From Preclinical Cystatin B-Deficient Mice

Progressive myoclonus epilepsy of Unverricht-Lundborg type (EPM1) is a neurodegenerative disorder caused by loss-of-function mutations in the cystatin B (CSTB) gene. Progression of the clinical symptoms in EPM1 patients, including stimulus-sensitive myoclonus, tonic-clonic seizures, and ataxia, are well described. However, the cellular dysfunction during the presymptomatic phase that precedes the disease onset is not understood. CSTB deficiency leads to alterations in GABAergic signaling, and causes early neuroinflammation followed by progressive neurodegeneration in brains of a mouse model, manifesting as progressive myoclonus and ataxia. Here, we report the first proteome atlas from cerebellar synaptosomes of presymptomatic Cstb-deficient mice, and propose that early mitochondrial dysfunction is important to the pathogenesis of altered synaptic function in EPM1. A decreased sodium- and chloride dependent GABA transporter 1 (GAT-1) abundance was noted in synaptosomes with CSTB deficiency, but no functional difference was seen between the two genotypes in electrophysiological experiments with pharmacological block of GAT-1. Collectively, our findings provide novel insights into the early onset and pathogenesis of CSTB deficiency, and reveal greater complexity to the molecular pathogenesis of EPM1.Peer reviewe

Habitat complexity modulates phenotype expression through developmental plasticity in the threespine stickleback

The expression of alternative phenotypes within a single species is often considered to be the result of ontogenetic processes and specifically phenotypic plasticity responses to exposure to different environmental conditions. In fish, which have been widely used to test such questions, exposure to different diets is the most frequently described initiator of plastic responses. The effect of physical characteristics of the habitat on fish morphology has not been fully explored. In the present study, a clear effect of habitat complexity on fish shape was found. Threespine sticklebacks were exposed to two different habitat treatments, simple and complex, over a 17-week period. The exposure to the habitats resulted in the expression of very significant differences in body and head morphologies and spine position, showing that the physical environment can modulate the expression of traits through phenotypic plasticity during ontogen