Stretch injury selectively enhances extrasynaptic, GluN2B-containing NMDA receptor function in cortical neurons.

Alterations in the function and expression of NMDA receptors are observed after in vivo and in vitro traumatic brain injury. We recently reported that mechanical stretch injury in cortical neurons transiently increases the contribution of NMDA receptors to network activity and results in an increase in calcium-permeable AMPA (CP-AMPA) receptor-mediated transmission 4 h postinjury (Goforth et al. 2011). Here, we evaluated changes in the function of synaptic vs. extrasynaptic GluN2B-containing NMDA receptors after injury. We also determined whether postinjury treatment with the GluN2B-selective antagonist Ro 25-6981 or memantine prevents injury-induced increases in CP-AMPA receptor activity. We found that injury increased extrasynaptic, GluN2B-containing NMDA receptor-mediated whole cell currents. In contrast, we found no differences in synaptic NMDA receptor-mediated transmission after injury. Furthermore, treatment with Ro 25-6981 or memantine after injury prevented injury-induced increases in CP-AMPA receptor-mediated activity. Together, our data suggest that increased NMDA receptor activity after injury is predominantly due to alterations in extrasynaptic, GluN2B-containing NMDA receptors and that activation of these receptors may contribute to the appearance of CP-AMPA receptors after injury.

Differential effect of insulin-like growth factor I on in vitro gonadotropin (I and II) and growth hormone secretions in rainbow trout (Oncorhynchus mykiss) at different stages of the reproductive cycle.

The short-term effect of insulin-like growth factor I (IGF-I) on GTH I (FSH-like), GTH II (LH-like), and GH production by cultured rainbow trout pituitary cells was studied in immature fish of both sexes, at early gametogenesis and in spermiating and periovulatory animals. IGF-I had no effect on basal GTH I and GTH II release, whereas it always inhibited basal GH, showing decreasing intensity with the gonad maturation. In absence of IGF-I, GTH I and GTH II cells were always responsive to GnRH, whereas no response was observed for GH cells whatever the sexual stage. The action of IGF-I on the sensitivity to GnRH differs between GTH and GH cells. The former requires a coincubation with IGF-I (10(-6) M)/GnRH to show an increase in sensitivity, independent of the sexual stage. To be responsive to GnRH, the GH cells require longer exposure to IGF-I, the efficiency of which decreases with gonad maturation. The action of IGF-I (10(-6) M) on GTH cell sensitivity to GnRH does not seem to be related to a mitogenic effect or to an improvement in cell survival. It seems to be IGF-I specific, not passing via the insulin receptor. Certain hypotheses on the putative role of IGF-I and GnRH as a link between growth and puberty are suggested.

Gonadotropin-I and -II subunit gene expression of male striped bass (Morone saxatilis) after gonadotropin-releasing hormone analogue injection: quantitation using an optimized ribonuclease protection assay.

In fish, both gonadotropin (GtH)-I and -II are involved in the spermatogenic process, but the differential regulation of these hormones by GnRH is still poorly understood. To gain further insight into the GnRH regulation of GtH-I and -II gene expression in the male striped bass, we have developed and optimized a ribonuclease protection assay for the simultaneous measurement of all GtH subunit mRNAs in a single pituitary gland. The RNA extraction protocol enables the determination of GtH protein content in the same sample, thus enhancing the power of the method. Maturing striped bass males were injected intramuscularly with [D-Ala6,Pro9Net]-LHRH (GnRHa) and sampled at 6 and 24 h postinjection. The mRNA levels of the alpha subunit and GtH-IIbeta increased after 6 h (4- and 6-fold, respectively), while the GtH-Ibeta mRNA levels increased only 2-fold after 24 h. Interestingly, GnRHa stimulation caused a significant increase in beta-actin mRNA levels. GnRHa treatment also resulted in a 2-fold decrease in pituitary GtH-II content, associated with a dramatic increase of plasma GtH-II levels from undetectable levels (< 0.2 ng/ml) to 13+/-2 ng/ml after 6 h. These results demonstrate that both GtH-Ibeta and -Ilbeta are expressed during striped bass spermatogenesis and that the two genes are subjected to differential regulation by GnRHa.

Role of IGF-I in the control of GH secretion in rainbow trout (Oncorhynchus mykiss).

To study the role of IGF-I on GH secretion, we looked firstly for IGF-I binding sites in the central nervous system. IGF-I presents a single class of binding sites on brain membranes and pituitary extracts. Their affinity constants (Ka) were 11.44 +/- 4.66 and 4.42 +/- 1.37 x 10(9) M-1, respectively and their capacity (Bmax) were 119.83 +/- 46.21 and 73.65 +/- 20.87 fmoles/mg, respectively. In a second step IGF-I and bGH action on GH release was tested in vitro and in vivo. IGF-I inhibited GH release by pituitary cell cultures while bGH did not, suggesting direct action of IGF at the pituitary level and indirect action of GH, possibly mediated by IGF-I. IGF-I injected into catheterization fish induced a rapid inhibition of GH release, while bGH induced a delayed one. This timing supports a direct effect of IGF-I on GH release and the indirect effect of bGH. So peripheral IGF-I can play a role on GH secretion, perhaps as a mediator of GH action. This could explain the delayed fluctuation of GH and IGF-I plasma levels previously observed. This strong relationship between plasma IGF and GH secretion in trout seems to be different from that in mammals in which systemic IGF does not seem to play a predominant role in regulating GH secretion.

Growth hormone (GH) and reproduction: a review.

Interaction between growth and reproduction occurs in many vertebrates and is particularly obvious at certain stages of the life cycle in fish. Endocrine interactions between the gonadotropic axis and the somatotropic axis are described, the potential role of GH being emphasised. A comparative analysis of these phenomena in mammals, amphibians and fish, suggests a specific role of GH in the physiology of puberty, gametogenesis and fertility. It also shows the original contribution made by studies on the fish model in this field of investigations.

Effects of starvation on valine and alanine transport across the intestinal mucosal border in sea bass, Dicentrarchus labrax.

Kinetics of intestinal transport of L-alanine and L-valine (substrates of the A-system and the L-system, respectively, in mammals) across the brush-border membrane in sea bass, Dicentrarchus labrax, were studied on intact mucosa using a short-term uptake technique. When fish were starved for 4-8 weeks, total influx (mucosa-to-cell) of valine fell owing to disappearance or modification of the diffusion component. The maximum influx rate of saturable component increased but its affinity (reflected by the Michaelis constant) decreased. Alanine transport by Na(+)-dependent and diffusion pathways was unchanged after starvation. Fasting also induced an almost 20% decrease in the length of intestinal microvilli.