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R Huerta

Publications and source records attributed to R Huerta.

At least 19 recordsLinked to original sources
PubMed2002

Dynamical model of long-term synaptic plasticity.

Long-term synaptic plasticity leading to enhancement in synaptic efficacy (long-term potentiation, LTP) or decrease in synaptic efficacy (long-term depression, LTD) is widely regarded as underlying learning and memory in nervous systems. LTP and LTD at excitatory neuronal synapses are observed to be induced by precise timing of pre- and postsynaptic events. Modification of synaptic transmission in long-term plasticity is a complex process involving many pathways; for example, it is also known that both forms of synaptic plasticity can be induced by various time courses of Ca(2+) introduction into the postsynaptic cell. We present a phenomenological description of a two-component process for synaptic plasticity. Our dynamical model reproduces the spike time-dependent plasticity of excitatory synapses as a function of relative timing between pre- and postsynaptic events, as observed in recent experiments. The model accounts for LTP and LTD when the postsynaptic cell is voltage clamped and depolarized (LTP) or hyperpolarized (LTD) and no postsynaptic action potentials are evoked. We are also able to connect our model with the Bienenstock, Cooper, and Munro rule. We give model predictions for changes in synaptic strength when periodic spike trains of varying frequency and Poisson distributed spike trains with varying average frequency are presented pre- and postsynaptically. When the frequency of spike presentation exceeds approximately 30-40 Hz, only LTP is induced.

Long-Term Potentiation↗
PubMed2001

Dynamical encoding by networks of competing neuron groups: winnerless competition.

Following studies of olfactory processing in insects and fish, we investigate neural networks whose dynamics in phase space is represented by orbits near the heteroclinic connections between saddle regions (fixed points or limit cycles). These networks encode input information as trajectories along the heteroclinic connections. If there are N neurons in the network, the capacity is approximately e(N-1)!, i.e., much larger than that of most traditional network structures. We show that a small winnerless competition network composed of FitzHugh-Nagumo spiking neurons efficiently transforms input information into a spatiotemporal output.

Action Potentials↗
PubMed2001

Topology selection by chaotic neurons of a pyloric central pattern generator.

The pyloric Central Pattern Generator (CPG) in the lobster has an architecture in which every neuron receives at least one connection from another member of the CPG. We call this a "non-open" network topology. An "open" topology, where at least one neuron does not receive synapses from any other CPG member, is found neither in the pyloric nor in the gastric mill CPG. Here we investigate a possible reason for this topological structure using the ability to perform a biologically functional task as a measure of the efficacy of the network. When the CPG is composed of model neurons that exhibit regular membrane voltage oscillations, open topologies are as able to maximize this functionality as non-open topologies. When we replace these models by neurons which exhibit chaotic membrane voltage oscillations, the functional criterion selects non-open topologies. As isolated neurons from invertebrate CPGs are known in some cases to undergo chaotic oscillations, this suggests that there is a biological basis for the class of non-open network topologies that we observe.

Animals↗
PubMed2001

Altered autophosphorylation of the insulin receptor in the ovary of a woman with polycystic ovary syndrome.

OBJECTIVE: To determine whether the tyrosine autophosphorylation of the insulin receptor (IR) in the ovary of a woman with polycystic ovary syndrome (PCOS) was reduced compared to normal. DESIGN: Experimental study. SETTING: Tertiary care medical center. PATIENT(S): One woman with PCOS and one healthy control, both of whom underwent a hysterectomy and bilateral salpingo-oophorectomy. INTERVENTION(S): Plasma membrane fraction of ovarian tissue was isolated, and the IR was purified and concentrated. MAIN OUTCOME MEASURE(S): Western blots of the IR, which had been incubated with and without insulin, were prepared. Colorimetric and chemiluminescent methods were used to detect the presence of the IR beta-subunit and IR tyrosine autophosphorylation, respectively. RESULT(S): The presence of the beta-subunit of the IR was identified in both ovarian samples. The degree of insulin-stimulated IR tyrosine autophosphorylation, reflected by the mean (+/-SD) relative optical density of the 95 kd band, was 4.3-fold higher in the normal ovary compared to the PCOS ovary (0.56 +/- 0.18 optical density vs. 0.13 +/- 0.10 optical density, respectively). CONCLUSION(S): Tyrosine autophosphorylation of the IR may be decreased in the ovaries of women with PCOS, similar to the findings in other tissues. The mechanisms through which insulin acts to produce an excess in ovarian androgen production in the face of a decrease in ovarian IR autophosphorylation remain to be determined.

Adult↗
PubMed2001

Model of transient oscillatory synchronization in the locust antennal lobe.

Transient pairwise synchronization of locust antennal lobe (AL) projection neurons (PNs) occurs during odor responses. In a Hodgkin-Huxley-type model of the AL, interactions between excitatory PNs and inhibitory local neurons (LNs) created coherent network oscillations during odor stimulation. GABAergic interconnections between LNs led to competition among them such that different groups of LNs oscillated with periodic Ca(2+) spikes during different 50-250 ms temporal epochs, similar to those recorded in vivo. During these epochs, LN-evoked IPSPs caused phase-locked, population oscillations in sets of postsynaptic PNs. The model shows how alternations of the inhibitory drive can temporally encode sensory information in networks of neurons without precisely tuned intrinsic oscillatory properties.

Animals↗
PubMed2001

Facial and abdominal hair growth in hirsutism: a computerized evaluation.

Methods of objectively assessing the growth rate of hairs in hirsute women have generally required some form of shaving and have focused on studying hairs affecting the face, which has reduced the number of patients willing or able to participate in such studies. A possible solution is to assess the terminal hairs on the lower abdomen (ie, the male escutcheon) because these two body areas are the most frequently affected with excess hair growth in hirsute patients. Nonetheless, it is unclear how the growth characteristics (density, diameter, and growth rate) of the hairs on the abdomen and face differ in these patients. We hypothesize that the growth characteristics of terminal hairs on the abdomen and face are similar and that evaluation of either area may be sufficient in assessing the hair growth rate of these patients. To objectively evaluate hair growth in the face and abdomen in hirsute patients, we developed a computer-aided image analysis system capable of measuring several growth parameters. Twenty hirsute women (12 white and 8 black), aged 31.2 +/- 6.1 years, were studied. Facial and abdominal skin areas were shaved, and 3 to 5 days later the areas were photographed through a calibrated glass plate and 5 terminal hairs were plucked from each area. The daily hair growth rate (assessed by photography and by direct measurement of the plucked hair), the density of hairs (number of hairs per surface area assessed by photography), and hair diameter (of the plucked hairs) were determined. The extent of hirsutism was also measured, albeit subjectively, by a modification of the Ferriman-Gallwey method, with each area given a score of 0 (no terminal hairs seen) to 4 (terminal hairs in a pattern similar to that of a very hirsute man). Facial, abdominal, and total Ferriman-Gallwey scores were 1.3 +/- 0.6, 1.8 +/- 0.9, and 12.5 +/- 5.4, respectively. Our results indicated that facial hairs were distributed in greater density and had a greater diameter than abdominal hairs (15.6 +/- 14.2 hairs/cm(2) vs 5.4 +/- 1.9 hairs/cm(2), and 84.5 +/- 19.5 microm and 66.2 +/- 17.5 microm, respectively, P <.005). Alternatively, the growth rates of facial and abdominal hairs were similar, whether determined photographically (0.36 +/- 0.18 mm/day vs 0.43 +/- 0.19 mm/day, respectively) or from plucked hairs (1.2 +/- +0.2 mm/d vs 1.4 + 0.4 mm/d, respectively). We conclude that although the density and diameter of facial hairs are greater than that of lower abdominal hairs, these areas have very similar growth rates. Hence evaluation of either of the body areas, using an objective method of assessing hair growth, should provide equivalent results.

Abdomen↗
PubMed2001

Global dynamics of a network of stochastic neurons maximizes local mutual information.

We define a stochastic neuron as an element that increases its internal state with probability p until a threshold value is reached; after that its internal state is set back to the initial value. We study the local information of a stochastic neuron between the message arriving from the input neurons and the response of the neuron. We study the dependence of the local information on the firing probability alpha of the synaptic inputs in a network of such stochastic neurons. The values of alpha obtained in the simulations are the same as those obtained theoretically by maximization of local mutual information. We conclude that the global dynamics maximizes the local mutual information of single units, which means that the self-selected parameter value of the population dynamics is such that each neuron behaves as an optimal encoder.

Animals↗
PubMed2001

[Infertility treatment before assisted reproductive techniques].

The prevalence of infertility has increased in recent years, but the medical services to treat these problems are not available for most of the affected couples. The prognosis for fertility is important in order to determine the therapeutic capacity of each service, and to select the couples that could be treated at a primary level, or to send them to more advanced levels of reproductive technology. In practice, the infertility is treated in primary medical levels and the assisted reproductive technology is available only to a limited sector of the poblation. In general, the managed-care plans do not compensate directly for infertility treatments, but they are indirectly paying some therapeutic procedures for fertility.

Adolescent↗
PubMed2000

Fast response and temporal coherent oscillations in small-world networks.

We have investigated the role that different connectivity regimes play in the dynamics of a network of Hodgkin-Huxley neurons by computer simulations. The different connectivity topologies exhibit the following features: random topologies give rise to fast system response yet are unable to produce coherent oscillations in the average activity of the network; on the other hand, regular topologies give rise to coherent oscillations, but in a temporal scale that is not in accordance with fast signal processing. Finally, small-world topologies, which fall between random and regular ones, take advantage of the best features of both, giving rise to fast system response with coherent oscillations.

Animals↗
PubMed2000

A central pattern generator to control a pyloric-based system.

A central pattern generator (CPG) is built to control a mechanical device (plant) inspired by the pyloric chamber of the lobster. Conductance-based models are used to construct the neurons of the CPG. The plant has an associated function that measures the amount of food flowing through it per unit of time. We search for the best set of solutions that give a high positive flow of food in the maximization function. The plant is symmetric and the model neurons are identical to avoid any bias in the space of solutions. We find that the solution is not unique and that three neurons are sufficient to produce positive flow. We propose an effective principle for CPGs (effective on-off connectivity) and a few predictions to be corroborated in the pyloric system of the lobster.

Animals↗
PubMed2000

Modeling observed chaotic oscillations in bursting neurons: the role of calcium dynamics and IP3.

Chaotic bursting has been recorded in synaptically isolated neurons of the pyloric central pattern generating (CPG) circuit in the lobster stomatogastric ganglion. Conductance-based models of pyloric neurons typically fail to reproduce the observed irregular behavior in either voltage time series or state-space trajectories. Recent suggestions of Chay [Biol Cybern 75: 419-431] indicate that chaotic bursting patterns can be generated by model neurons that couple membrane currents to the nonlinear dynamics of intracellular calcium storage and release. Accordingly, we have built a two-compartment model of a pyloric CPG neuron incorporating previously described membrane conductances together with intracellular Ca2+ dynamics involving the endoplasmic reticulum and the inositol 1,4,5-trisphosphate receptor IP3R. As judged by qualitative inspection and quantitative, nonlinear analysis, the irregular voltage oscillations of the model neuron resemble those seen in the biological neurons. Chaotic bursting arises from the interaction of fast membrane voltage dynamics with slower intracellular Ca2+ dynamics and, hence, depends on the concentration of IP3. Despite the presence of 12 independent dynamical variables, the model neuron bursts chaotically in a subspace characterized by 3-4 active degrees of freedom. The critical aspect of this model is that chaotic oscillations arise when membrane voltage processes are coupled to another slow dynamic. Here we suggest this slow dynamic to be intracellular Ca2+ handling.

Action Potentials↗
PubMed2000

Adrenocortical hyperresponsivity to adrenocorticotropic hormone: a mechanism favoring the normal production of cortisol in 21-hydroxylase-deficient nonclassic adrenal hyperplasia.

OBJECTIVE: To test the hypothesis that patients with nonclassic adrenal hyperplasia (NCAH) exhibit a generalized exaggeration in their response to ACTH stimulation that favors the normal production of F. Patients with 21-hydroxylase (21-OH)-deficient NCAH do not demonstrate cortisol (F) deficiency. DESIGN: Prospective controlled study. SETTING: Tertiary university clinic. PATIENT(S): Twenty-four untreated patients with NCAH diagnosed by a 17 alpha-hydroxyprogesterone (17-HP) level of >30.3 nmol/L (>10 ng/mL), and 37 age- and body mass-matched healthy eumenorrheic nonhirsute controls. INTERVENTION(S): All study subjects underwent a 60 minute acute stimulation using 0.25 mg of ACTH-(1-24) i.v. MAIN OUTCOME MEASURE(S): Basal and stimulated serum levels of pregnenolone (PREG), 17-hydroxypregnenolone (17-HPREG), dehydroepiandrosterone (DHA), progesterone (P4), 17-HP, androstenedione (A4), 11-deoxycortisol (S), and cortisol (F). RESULT(S): The median basal (i.e., Steroid(0)) or ACTH-stimulated (i. e., Steroid(60)) serum levels of PREG, 17-HPREG, DHA, P4, 17-HP, A4 and, most importantly, S were higher in NCAH patients than in controls. In contrast, the levels of F at either 0 minute or 60 minutes of stimulation were similar between NCAH and control women. The proportion of NCAH patients with stimulated steroids levels of >the 95th percentile of controls were as follows: 84.21% for PREG(60), 87.5% for 17-HPREG(60), 95.8% for DHA(60), 89.5% for P4(60), 100% for 17-HP(60), 91.7% for A4(60), 29.2% for S(60), and 4. 1% for F(60). CONCLUSION(S): A generalized adrenocortical hyperresponsivity to ACTH stimulation seems to be present in patients with 21-OH-deficient NCAH, with an exaggerated production of S evident in approximately 30%. The excess production of S in these NCAH patients may, in part, account for their normal F production.

17-alpha-Hydroxypregnenolone↗
PubMed1999

11beta-hydroxyandrostenedione and delta5-androstenediol as markers of adrenal androgen production in patients with 21-hydroxylase-deficient nonclassic adrenal hyperplasia.

OBJECTIVE: To determine the sensitivity of 11beta-hydroxyandrostenedione (11-OHA4) and delta5-androstenediol (ADIOL) as markers of excessive adrenal androgen production. DESIGN: Prospective study. SETTING: Academic medical centers. PATIENT(S): Thirteen women with untreated 21-hydroxylase-deficient nonclassic adrenal hyperplasia (NCAH) and 18 healthy, eumenorrheic, nonhirsute controls matched for age and body mass index. INTERVENTION(S): All subjects were studied before and after acute adrenal stimulation with 0.25 mg of IV ACTH-(1-24). MAIN OUTCOME MEASURE(S): Basal levels of total testosterone, sex hormone-binding globulin, DHEAS, and free testosterone were measured. Levels of androstenedione (A4), DHEA, 11-OHA4, and ADIOL were determined before (Steroid0) and 60 minutes after (Steroid60) acute ACTH-(1-24) stimulation. RESULT(S): Patients with NCAH had higher median basal levels of DHEAS and total and free testosterone than controls. Patients with NCAH had higher median A4(0), A460, DHEA(0), DHEA60, 11-OHA4(0), ADIOL0, and ADIOL60 levels but similar 11-OHA4(60) levels compared with controls. Among patients with NCAH, 30%, 54%, 15%, and 85% had 11-OHA4(0), ADIOL0, 11-OHA4(60), and ADIOL(60) levels, respectively, above the 95th percentile of controls. CONCLUSION(S): Overall, serum levels of 11-OHA4 did not appear to be a very sensitive marker of excessive adrenal androgen production, at least in patients with NCAH. Although ACTH-stimulated ADIOL levels were elevated in 85% of the patients studied, they did not appear to have any advantage over the measurement of A4 or DHEA levels.

Adrenal Hyperplasia, Congenital↗
PubMed1999

Origin of coherent structures in a discrete chaotic medium.

Using as an example a large lattice of locally interacting Hindmarsh-Rose chaotic neurons, we disclose the origin of ordered structures in a discrete nonequilibrium medium with fast and slow chaotic oscillations. The origin of the ordering mechanism is related to the appearance of a periodic average dynamics in the group of chaotic neurons whose individual slow activity is significantly synchronized by the group mean field. Introducing the concept of a "coarse grain" as a cluster of neuron elements with periodic averaged behavior allows consideration of the dynamics of a medium composed of these clusters. A study of this medium reveals spatially ordered patterns in the periodic and slow dynamics of the coarse grains that are controlled by the average intensity of the fast chaotic pulsation.

Journal Article↗
PubMed1999

Dynamic control of irregular bursting in an identified neuron of an oscillatory circuit.

In the oscillatory circuits known as central pattern generators (CPGs), most synaptic connections are inhibitory. We have assessed the effects of inhibitory synaptic input on the dynamic behavior of a component neuron of the pyloric CPG in the lobster stomatogastric ganglion. Experimental perturbations were applied to the single, lateral pyloric neuron (LP), and the resulting voltage time series were analyzed using an entropy measure obtained from power spectra. When isolated from phasic inhibitory input, LP generates irregular spiking-bursting activity. Each burst begins in a relatively stereotyped manner but then evolves with exponentially increasing variability. Periodic, depolarizing current pulses are poor regulators of this activity, whereas hyperpolarizing pulses exert a strong, frequency-dependent regularizing action. Rhythmic inhibitory inputs from presynaptic pacemaker neurons also regularize the bursting. These inputs 1) reset LP to a similar state at each cycle, 2) extend and further stabilize the initial, quasi-stable phase of its bursts, and 3) at sufficiently high frequencies terminate ongoing bursts before they become unstable. The dynamic time frame for stabilization overlaps the normal frequency range of oscillations of the pyloric CPG. Thus, in this oscillatory circuit, the interaction of rhythmic inhibitory input with intrinsic burst properties affects not only the phasing, but also the dynamic stability of neural activity.

Animals↗
PubMed1999

Age at menopause in women with type 2 diabetes mellitus.

OBJECTIVE: Considering that chronic diseases such as diabetes mellitus (DM) may determine premature ovarian failure by various mechanisms, we studied the age at menopause in women without diabetes and in women with type 2 DM. DESIGN: We studied 409 women without diabetes and 404 patients with type 2 DM, selected from 45 to 55 years of age, for analysis with the status quo method. The age at menopause was calculated with a logistic regression on the proportions of menopausal women for each age group. RESULTS: In the groups, 172 women without diabetes and 207 women with diabetes had menopause. The regression procedure gave a median age of 49.7 +/- SD 1.7 years for the whole group, 49.6 +/- 1.6 years for the nondiabetic group, and 49.8 +/- 1.7 years for women with diabetes. Women without diabetes were 1.4 years younger, but this factor did not have an influence on the results. Smoking habits, vegetarianism, and somatometric variables were similar in both groups, except for waist/hip and abdomen/hip ratios, larger in the group of women with diabetes. The mean for years since diagnosis in patients < 45 years of age was 4.9 years. For older patients, the figure increased to 8.9 years. CONCLUSIONS: No difference for age at menopause was found between women without diabetes and women with type 2 diabetes who were 5 to 8 years since the diagnosis was made.

Age Distribution↗
PubMed1998

Basic principles for generating motor output in the stomatogastric ganglion.

The lobster stomatogastric ganglion contains 30 neurons and when modulated can produce two distinct rhythmic motor patterns--the gastric mill and the pyloric. The complete neural circuitry underlying both patterns is well known. Without modulatory input no patterns are produced, and the neurons fire tonically or are silent. When neuromodulators are released into the ganglion from specific neurons or are delivered as hormones, the properties of the neurons and synapses change dramatically and modulator-specific gastric mill and pyloric patterns are produced. In general the rhythmicity derives from the induced burstiness of the neurons, and the pattern from the strengths of the electrical and chemical synapses. The organized activity can be traced to a marked reduction of chaotic activity in individual neurons when they shift from the unmodulated to the modulated state.

Animals↗