Radioimmunoassay of detirelix ([ N-Ac-D-Nal(2)1,D-p-Cl-Phe2,D-Trp3, D-hArg(Et)6(2),D-Ala10]-luteinizing hormone-releasing hormone) in plasma or serum

A procedure for the radioimmunoassay (RIA) of detirelix in plasma or serum at concentrations as low as 0.15 ng/ml is described. Antiserum was produced by deacetylation of the N-terminus amino groups of detirelix and coupling this analog to bovine serum albumin with a carbodiimide and immunizing rabbits with the resultant conjugate. For RIA, 125I-labeled detirelix was used as the tracer and a double antibody procedure was used to separate the free and bound fractions. No purification of samples was required prior to RIA. Accuracy of the method was assessed by adding known quantities of detirelix to detirelix-free plasma and determining the ratio of measured to added analyte. Linear regression analysis for the concentration range 0.15-150.0 ng/ml yielded a regression equation of y = 0.88 X +1.46 and a correlation coefficient of 0.996. Additional validation was obtained from an in vivo study in which [14C]detirelix was administered to monkeys and plasma clearance profiles were determined by RIA and an HPLC-radiochemical method. The RIA results were in good agreement with those obtained by the HPLC method.     

Decorrelation of retinal response to natural scenes by fixational eye movements

Under natural viewing conditions the input to the retina is a complex spatiotemporal signal that depends on both the scene and the way the observer moves. It is commonly assumed that the retina processes this input signal efficiently by taking into account the statistics of the natural world. It has recently been argued that incessant microscopic eye movements contribute to this process by decorrelating the input to the retina. Here we tested this theory by measuring the responses of the salamander retina to stimuli replicating the natural input signals experienced by the retina in the presence and absence of fixational eye movements. Contrary to the predictions of classic theories of efficient encoding that do not take behavior into account, we show that the response characteristics of retinal ganglion cells are not sufficient in themselves to disrupt the broad correlations of natural scenes. Specifically, retinal ganglion cells exhibited strong and extensive spatial correlations in the absence of fixational eye movements. However, the levels of correlation in the neural responses dropped in the presence of fixational eye movements, resulting in effective decorrelation of the channels streaming information to the brain. These observations confirm the predictions that microscopic eye movements act to reduce correlations in retinal responses and contribute to visual information processing.Much effort has been devoted to understanding how the neural code of the retina and downstream neurons can represent visual information efficiently given the statistical structure of the natural world (1–6). Although these theories have contributed tremendously to current understanding of early visual processing, they do not consider the observer’s motor activity but rather rely on the simplifying assumption that the input to the retina is a stationary image. However, even during fixation on a single point, small movements of the eye, head, and other parts of the body continually modulate visual input signals. Experiments have shown that elimination of retinal image motion leads to fading of vision (7, 8). Therefore, eye movements are essential for the normal functioning of the visual system.It has been proposed that, rather than simply preventing adaptation in neural responses, fixational eye movements are a critical stage of information processing, in which predictable spatial correlations are discarded to enable encoding of luminance discontinuities by synchronous neural activity (9, 10). Thus, fixational eye movements counterbalance the spectral density of natural scenes and yield temporal modulations with equalized power over a broad range of spatial frequencies. Because spectral equalization is equivalent to decorrelation in space, this theory predicts that fixational eye movements should attenuate correlations in the responses of the retinal ganglion cells. Modeling results have provided support to this hypothesis (9, 10).     

Pharmacokinetics of norethindrone: Effect of particle size

In 24 healthy women between the ages of 19 and 35 years who had not used oral contraceptive preparations for at least 60 days, it was found that the smaller the particle size of norethindrone (NET) administered, the higher was the plasma NET level obtained. Three different preparations having particle sizes of NET smaller than 250 μm, 44 μm or 10 μm were tested in a crossover pattern. The time required to reach maximum plasma concentration (T_max) became shorter with decreasing particle size, 1.69 hr, 1.52 hr and 1.06 hr, respectively. As particle size was reduced, the maximum NET plasma concentration (C_max) increased for the 3 different 1 mg NET preparations, i.e. 8.66 ng/ml, 10.53 ng/ml and 15.73 ng/ml. A trial with a 2 mg NET preparation made with NET utilizing the 44 μm size material displayed a T _maxsimilar to the 1 mg NET preparation having the same particle size while the C_max reached a level of 17.56 ng/ml. The area under the plasma concentration versus time curve from 0–24 hrs and the extrapolated total area under the curve, increased with decreasing particle size. The use of a smaller particle size allows for more rapid dissolution of oral contraceptive tablets when measured in vitro; however, there is no evidence that such faster dissolution leads to a significant difference in efficacy. Oral contraceptive tablets have, since their inception, utilized both large and small NET particle size material in various preparations.     

A Radioimmunoassay Procedure for the Determination of the Antiviral Nucleoside DHPG (9-[(l,3-Dihydroxy-2-propoxy)-methyl]guanine) in Plasma or Serum

A procedure is described that is suitable for the radioimmunoassay (RIA) of 9-[(l,3-dihydroxy-2-propoxy)-methyl]guanine (DHPG) in plasma or serum at concentrations as low as 0.7 ng/ml (2.75 × 10^–9 M). Antiserum was prepared by coupling DHPG monohemisuccinate to bovine serum albumin and immunizing rabbits with the resulting conjugate. The antibodies did not show significant cross-reactivities with structurally related endogenous compounds. For RIA, tritium-labeled DHPG was used as the tracer and charcoal–dextran was used to separate the free and bound fractions. No purification of samples was required prior to RIA. The accuracy of the method was assessed by adding known quantities of DHPG to DHPG-free plasma and determining the ratio of measured to added analyte. Linear regression analysis for the concentration range 0.0007 to 15.0 µg/ml yielded the following equation; y = 0.90 x + 0.033 (r = 0.999). Additional validation was obtained from studies in which DHPG was administered to a monkey, mice, dogs, and rats, and plasma-clearance profiles were determined by RIA and high-performance liquid chromatography (HPLC). The results obtained by RIA were in good agreement with those obtained by HPLC.     

A non-chromatographic radioimmunoassay of testosterone in serum

Convenient methodology based on separation of testosterone from non-alcoholic neutral steroids by means of a sulfation procedure has been developed for the radioimmunoassay (RIA) of testosterone in male and in female serum. When coordinated with our previously published non-chromatographic procedure (1) for the RIA of estrone and 17 beta-estradiol in serum all 3 steroids can be determined in the same specimen. With only minor modification (14) progesterone also can be determined. Validation of the procedure was based on: 1. agreement between results obtained using TLC and sulfation to fractionate testosterone (r=0.99; b=1.03), 2. accurate recovery of different quatities of testosterone added to serum, 3. independence of the concentration of testosterone and volume of serum used for assay, 4. low procedural blanks (1.1+/-0.7 pg), 5. low intra-assay (4.7-4.8%) and interassay (4.8-8.8%) variability and 6. correspondence of observed values for testosterone in male serum (7.00+/-2.03 ng/ml) and in femal serum (420+/-115 pg/ml) with those reported previously by others.     

Identification of serotonergic neurons in human brain by a monoclonal antibody binding to all three aromatic amino acid hydroxylases

A monoclonal antibody, PH8, has been isolated and shown by immunocytochemistry to bind to serotonergic and catecholaminergic neurons in sections of the rat and human brain. In human brain, obtained at autopsy, particular fixation and embedding conditions eliminate the labelling of catecholaminergic neurons while leaving intact the labelling of serotonergic neurons. This property makes the antibody of potential use for structural studies of serotonergic neurons in the normal and diseased human brain. PH8 was raised to pure monkey phenylalanine hydroxylase and has been shown to bind to the 50,000 mol. wt. phenylalanine hydroxylase polypeptide. Immunocytochemical and immunochemical evidence is presented in support of the hypothesis that the labelling of serotonergic and catecholaminergic neurons results from the binding of PH8 to tryptophan and tyrosine hydroxylase, respectively.