Deletion of presynaptic adenosine A1 receptors impairs the recovery of synaptic transmission after hypoxia.

Adenosine protects neurons during hypoxia by inhibiting excitatory synaptic transmission and preventing NMDA receptor activation. Using an adeno-associated viral (AAV) vector containing Cre recombinase, we have focally deleted adenosine A(1) receptors in specific hippocampal regions of adult mice. Recently, we found that deletion of A(1) receptors in the CA1 area blocks the postsynaptic responses to adenosine in CA1 pyramidal neurons, and deletion of A(1) receptors in CA3 neurons abolishes the presynaptic effects of adenosine on the Schaffer collateral input [J Neurosci 23 (2003) 5762]. In the current study, we used this technique to delete A(1) receptors focally from CA3 neurons to investigate whether presynaptic A(1) receptors protect synaptic transmission from hypoxia. We studied the effects of prolonged (1 h) hypoxia on the evoked field excitatory postsynaptic potentials (fEPSPs) in the CA1 region using in vitro slices. Focal deletion of the presynaptic A(1) receptors on the Schaffer collateral input slowed the depression of the fEPSPs in response to hypoxia and impaired the recovery of the fEPSPs after hypoxia. Delayed responses to hypoxia linearly correlated with impaired recovery. These findings provide direct evidence that the neuroprotective role of adenosine during hypoxia depends on the rapid inhibition of synaptic transmission by the activation of presynaptic A(1) receptors.

Phenotypic effects of balanced X-autosome translocations in females: a retrospective survey of 104 cases reported from UK laboratories.

Females with balanced X-autosome translocations are a clinically heterogeneous group of patients in which X breakpoint position and replication behaviour may influence phenotypic outcome. This study reviewed all cases reported by UK cytogenetics laboratories over a 15-year period (1983-1997). Publication bias was avoided by reviewing all reported cases. One hundred and four female carriers were identified, 62 of who were probands. By reason for referral, these were: multiple congenital abnormalities and/or developmental delay (MCA/DD): 26 (42%); gonadal dysfunction: 22 (35%); phenotypically normal with or without recurrent miscarriage (NRM): 9 (15%); recognized X-linked syndrome: 5 (8%). The information obtained was compared with published data and with data from the authors' own laboratories of female patients with balanced autosome-autosome translocations (n=115). We concluded that: (1) MCA/DD cases were significantly over-represented compared to previous published data (P<0.005) and were more common than in female probands with balanced autosome-autosome translocations (P<0.05). (2) MCA/DD cases showed random breakpoint distribution along the X chromosome (P>0.05). MCA/DD cases with subtelomeric breakpoints at Xp22 or Xq28 were not always associated with deviation from the expected pattern of X-inactivation where this was known. De novo cases were significantly more likely to be assigned as MCA/DD than any other category (P<0.005). (3) Gonadal dysfunction (GD) was invariably associated with a 'critical region' breakpoint, Xq13-q26, (20/22 probands). However, 7/44 (16%) of patients surveyed had breakpoints within Xq13-Xq26 and proven fertility. (4) Recognized 'X-linked syndrome' cases were significantly under-represented (P<0.001) compared to previous published data.

Modified genetic response to X-irradiation of mouse spermatogonial stem cells surviving treatment with TEM.

Earlier studies have shown that the genetic response to X-irradiation of mouse spermatogonial stem-cell populations that are recovering from a previous radiation exposure may differ from that of a normal, unirradiated stem-cell population. Similar modified responses to X-irradiation have now been observed in stem spermatogonia that are recovering from treatment with the chemical mutagen, TEM. (1) In contrast to a normal response to 900 R, high translocation yields were obtained when this dose was administered 24 h after a TEM treatment. (2) A small but non-significant increase above a normal response to 500 R was obtained when this dose followed 24 h after TEM treatment and a normal response to 500 R was obtained from the reverse order treatment (500 R + TEM). (3) When given 4 days after a TEM treatment, the translocation yield from 500 R was only about half that normally obtained. Unexpectedly, a similar low response was obtained from the reverse order treatment which, if verified, would suggest selective cell killing by the TEM. THe chemical administered alone, was almost totally ineffective in producing recoverable translocations in stem spermatogonia. Since it is unlikely that TEM and X-rays should similarly synchronize the cell cycle of surviving stem spermatogonia it is concluded that the modified genetic responses obtained result from a different cause. Depletion of the stem-cell population is suggested as the common mediating factor. This may 'trigger' the radio-resistant, long cycling stem cells into a more active cycle such that, 24--48 h later, survivors may be highly 'synchronized' into either a sensitive cell-cycle stage or transient state preparatory to entering a shorter cell cycle to achieve repopulation. The low translocations yields obtained with longer intervals between treatments may typify the genetic response of the repopulating stem cells.

Translocation yield from mouse spermatogonial stem cells following unequal-sized x-ray fractionations: evidence of radiation-induced loss of heterogeneity.

Previous work has shown that a high yield of genetic damage can be recovered from stem spermatogonia exposed to a high (900 R) X-ray dose, despite extensive cell killing, when this follows 24 h after a smaller (100 R) radiation exposure. This differs from the response of the normal stem-cell population and has been interpreted to mean that the more radio-resistant cells surviving the first exposure become sensitive both to radiation-induced killing and genetic damage after this time interval and, as a consequence, lose the heterogeneity in radio-sensitivity that typifies a normal stem-cell population. Similar results have now been obtained with doses of 600 and 800 R given in fractions of 100 + 500 R and 100 + 700 R 24 h apart. Yields of translocations among spermatocytes were higher than obtained with the single doses and responses consistent with the fractions acting additively were obtained when the fractions were given in reverse order. Further analyses of the data provided support for the concept that 24 h after a radiation exposure there is a loss of heterogeneity in radio-sensitivity in the surviving stem-cell population.