Differential maturation of motoneurons innervating ankle flexor and extensor muscles in the neonatal rat

The first postnatal week is a critical period for the development of posture in the rat. The use of ankle extensor muscles in postural reactions increases during this period. Changes in excitability of motoneurons are probably an important factor underlying this maturation. The aim of this study was to identify whether variations in the maturation exist between motor pools innervating antagonistic muscles. Intracellular recordings in the in vitro brain stem-spinal cord preparation of neonatal rats (from postnatal day 0-5) were used to examine the developmental changes in excitability of motoneurons innervating the ankle flexors (F-MNs) and the antigravity ankle extensors (E-MNs). No significant difference in resting potential, action potential threshold, input resistance or rheobase was observed at birth. The age-related increase in rheobase was more pronounced for F-MNs than for E-MNs. The development of discharge properties of E-MNs lagged behind that of F-MNs. More F-MNs than E-MNs were able to fire repetitively in response to current injection at birth. F-MNs discharged at a higher frequency than E-MNs at all ages. Differences in the duration of action potential afterhyperpolarization accounted, at least partly, for the differences in discharge frequency between E-MNs and F-MNs at birth, and for the age-related increase in firing rate. These results suggest that E-MNs are more immature at birth than F-MNs and that there is a differential development of motoneurons innervating antagonistic muscles. This may be a critical factor in the development of posture and locomotion.     

Differential behavioural sensitization to intermittent morphine treatment in alcohol-preferring AA and alcohol-avoiding ANA rats: role of mesolimbic dopamine

Alcohol-preferring AA (Alko Alcohol) and alcohol-avoiding ANA (Alko Non-Alcohol) rats have well-documented differences in their voluntary ethanol consumption and brain opioidergic systems. The aim of the present study was to investigate whether these rat lines differ in their susceptibility to morphine-induced behavioural and neurochemical sensitization. The rats were given 15 injections of morphine (10 mg/kg, s.c.) or saline every other day. Locomotor activity and release of dopamine in the nucleus accumbens were monitored after a challenge with additional morphine injections (10 mg/kg) 1 and 5 weeks after withdrawal from the repeated treatment. Morphine increased locomotion more in the previously morphine-treated rats than in the saline-treated controls. Furthermore, AA rats were more sensitive to this effect of morphine than ANA rats. Accumbal morphine-induced dopamine release was significantly higher in the morphine-treated AA than ANA rats after the first challenge injection 1 week from withdrawal, but no differences were observed after the second challenge. The brain and plasma concentrations of morphine were similar among the lines suggesting that the differences in the effects of morphine cannot be explained in terms of differential pharmacokinetics of morphine in these lines. These data show that AA rats are more susceptible to morphine-induced behavioural sensitization than ANA rats. Furthermore, it suggests that mesolimbic dopamine has at best only a transient role in the expression of opioid-induced behavioural sensitization. The relationship between the mechanisms underlying the differential sensitivity of these rat lines to the effects of repeated morphine and voluntary ethanol drinking remains to be determined.     

Contribution of bone marrow hematopoietic stem cells to adult mouse inner ear: mesenchymal cells and fibrocytes

Bone marrow (BM)-derived stem cells have shown plasticity with a capacity to differentiate into a variety of specialized cells. To test the hypothesis that some cells in the inner ear are derived from BM, we transplanted either isolated whole BM cells or clonally expanded hematopoietic stem cells (HSCs) prepared from transgenic mice expressing enhanced green fluorescent protein (EGFP) into irradiated adult mice. Isolated GFP(+) BM cells were also transplanted into conditioned newborn mice derived from pregnant mice injected with busulfan (which ablates HSCs in the newborns). Quantification of GFP(+) cells was performed 3-20 months after transplant. GFP(+) cells were found in the inner ear with all transplant conditions. They were most abundant within the spiral ligament but were also found in other locations normally occupied by fibrocytes and mesenchymal cells. No GFP(+) neurons or hair cells were observed in inner ears of transplanted mice. Dual immunofluorescence assays demonstrated that most of the GFP(+) cells were negative for CD45, a macrophage and hematopoietic cell marker. A portion of the GFP(+) cells in the spiral ligament expressed immunoreactive Na, K-ATPase, or the Na-K-Cl transporter (NKCC), proteins used as markers for specialized ion transport fibrocytes. Phenotypic studies indicated that the GFP(+) cells did not arise from fusion of donor cells with endogenous cells. This study provides the first evidence for the origin of inner ear cells from BM and more specifically from HSCs. The results suggest that mesenchymal cells, including fibrocytes in the adult inner ear, may be derived continuously from HSCs.     

Contribution of spinal 5-HT1A and 5-HT7 receptors to locomotor-like movement induced by 8-OH-DPAT in spinal cord-transected mice

Growing evidence from in vitro studies suggests that spinal serotonin (5-HT) receptor subtypes 5-HTR(1A) and 5-HTR(7) are associated with an induction of central pattern generator activity. However, the possibility of a specific role for these receptor subtypes in locomotor rhythmogenesis in vivo remains unclear. Here, we studied the effects of a single dose (1 mg/kg, i.p.) of 8-hydroxy-2-(di-N-propylamino)-tetralin (8-OH-DPAT), a potent and selective 5-HTR(1A/7) agonist, in mice spinal cord transected at the low-thoracic level (Th9/10). The results show that 8-OH-DPAT acutely induced, within 15 min, hindlimb movements that share some characteristics with normal locomotion. Paraplegic mice pretreated with the selective 5-HTR(1A) antagonists, WAY100,135 or WAY100,635, displayed significantly less 8-OH-DPAT-induced movement. A similar reduction of 8-OH-DPAT-induced movements was found in animals pretreated with SB269970, a selective 5-HTR(7) antagonist. Moreover, a near complete blockade of 8-OH-DPAT-induced movement was obtained in wild-type mice pretreated with 5-HTR(1A) and 5-HTR(7) antagonists, and in 5-HTR(7)-/- mice pretreated with 5-HTR(1A) antagonists. Overall, these results clearly demonstrate that 8-OH-DPAT potently induces locomotor-like movement in the previously paralysed hindlimbs of low-thoracic-transected mice. The results, with selective antagonists and knockout animals, provide compelling evidence of a specific contribution of both receptor subtypes to spinal locomotor rhythmogenesis in vivo.     

Contribution of feedforward thalamic afferents and corticogeniculate feedback to the spatial summation area of macaque V1 and LGN

Neurons in the primary visual cortex (V1) respond best to oriented gratings of optimal size within their receptive field (RF) and are suppressed by larger gratings involving the nonclassical RF surround. A V1 neuron's optimal stimulus size is larger at lower stimulus contrast. A central question in visual neuroscience is what circuits generate the size tuning of V1 cells. We recently demonstrated that V1 horizontal connections integrate signals within a region of the RF center corresponding to the V1 neuron's optimal stimulus size at low contrast; extrastriate feedback connections to V1, instead, are longer range and can integrate signals from the most distant regions of the V1 cell's RF surround. Here, we have determined the contribution of geniculocortical feedforward and corticogeniculate feedback connections to the size-tuning of macaque V1 and lateral geniculate (LGN) neurons, respectively. Specifically, we have quantitatively compared the visuotopic extent of geniculate feedforward afferents to V1 with the size of the RF center and surround of neurons in the V1 input layers and the visuotopic extent of V1 feedback connections to the LGN with the RF size of cells in V1 layer 6, where these connections originate. We find geniculate feedforward connections to provide visuotopic information to V1 that is spatially coextensive with the V1 neuron's optimal stimulus size measured with high-contrast gratings. V1 feedback connections restrict their influence to an LGN region visuotopically coextensive with the size of the minimum response field (or classical RF) of V1 layer 6 cells and commensurate with the LGN region from which they receive feedforward connections.     

Differential contributions of microglial and neuronal IKKβ to synaptic plasticity and associative learning in alert behaving mice

Microglia are CNS resident immune cells and a rich source of neuroactive mediators, but their contribution to physiological brain processes such as synaptic plasticity, learning, and memory is not fully understood. In this study, we used mice with partial depletion of IκB kinase β, the main activating kinase in the inducible NF‐κB pathway, selectively in myeloid lineage cells (mIKKβKO) or excitatory neurons (nIKKβKO) to measure synaptic strength at hippocampal Schaffer collaterals during long‐term potentiation (LTP) and instrumental conditioning in alert behaving individuals. Resting microglial cells in mIKKβKO mice showed less Iba1‐immunoreactivity, and brain IL‐1β mRNA levels were selectively reduced compared with controls. Measurement of field excitatory postsynaptic potentials (fEPSPs) evoked by stimulation of the CA3‐CA1 synapse in mIKKβKO mice showed higher facilitation in response to paired pulses and enhanced LTP following high frequency stimulation. In contrast, nIKKβKO mice showed normal basic synaptic transmission and LTP induction but impairments in late LTP. To understand the consequences of such impairments in synaptic plasticity for learning and memory, we measured CA1 fEPSPs in behaving mice during instrumental conditioning. IKKβ was not necessary in either microglia or neurons for mice to learn lever‐pressing (appetitive behavior) to obtain food (consummatory behavior) but was required in both for modification of their hippocampus‐dependent appetitive, not consummatory behavior. Our results show that microglia, through IKKβ and therefore NF‐κB activity, regulate hippocampal synaptic plasticity and that both microglia and neurons, through IKKβ, are necessary for animals to modify hippocampus‐driven behavior during associative learning. GLIA 2015;63:549–566     

Differential contribution of Ih to the integration of excitatory synaptic inputs in substantia nigra pars compacta and ventral tegmental area dopaminergic neurons

The selective vulnerability of substantia nigra pars compacta (SNc) dopaminergic (DA) neurons is an enigmatic trait of Parkinson's disease (PD), especially if compared to the remarkable resistance of closely related DA neurons in the neighboring ventral tegmental area (VTA). Overall evidence indicates that specific electrophysiological, metabolic and molecular factors underlie SNc vulnerability, although many pieces of the puzzle are still missing. In this respect, we recently demonstrated that 1‐methyl‐4‐phenylpyridinium (MPP+), the active metabolite of the parkinsonizing toxin 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP), alters the electrophysiological properties of SNc DA neurons in vitro by inhibiting the hyperpolarization‐activated current (Ih). Here, we present an electrophysiological investigation of the functional role of Ih in the integration of synaptic inputs in identified SNc and VTA DA neurons, comparatively, in acute midbrain slices from TH‐GFP mice. We show that pharmacological suppression of Ih increases the amplitude and decay time of excitatory postsynaptic potentials, leading to temporal summation of multiple excitatory potentials at somatic level. Importantly, these effects are quantitatively more evident in SNc DA neurons. We conclude that Ih regulates the responsiveness to excitatory synaptic transmission in SNc and VTA DA neurons differentially. Finally, we present the hypothesis that Ih loss of function may be linked to PD trigger mechanisms, such as mitochondrial failure and ATP depletion, and act in concert with SNc‐specific synaptic connectivity to promote selective vulnerability.