[Association between dietary glycemic load during first trimester and the risk of gestational diabetes mellitus: a prospective study]

Objective: To explore the effects of dietary glycemic load (GL) during first trimester on the risk of gestational diabetes mellitus (GDM). Methods: A prospective study was conducted among healthy women with singleton pregnancy at 8-14 weeks of gestation in a maternity out-patient clinic of maternal-and-child health care institution in Chengdu, Sichuan province. Information on dietary intake during the first trimester was collected through a 3-day 24-hour dietary recall. Glycemic index (GI) values were obtained from China Food Composition Tables (Standard Edition) and International Tables of Glycemic Index and Glycemic Load Values (2008). Dietary GL and GLs of staple foods were calculated based on GI values and the amount of carbohydrate consumed per day. Diagnostic criteria of GDM was followed the Guidelines for Diagnosis and Treatment of Pregnancy Diabetes in China (2014), and used on participants who underwent an oral glucose tolerant test during 24-28 weeks of gestation. Log-binomial regression models were used to explore the associations between both quartiles of dietary GL, GLs of staple foods and the risks of GDM,respectively. Results: The medians of dietary GL and GL of staple foods were 145.70 (113.23-180.85) and 121.05 (89.08-155.70), respectively. The median GL of both rice and tubers were 73.14 (43.89-107.50) and 3.43 (0.00-9.84), respectively. After adjusting for the age at pregnancy, pre-pregnancy body mass index and other confounding factors, results of log-binomial regressions analysis showed that when compared with the lowest quartile of dietary GL group, the third and highest quartiles of dietary GL groups increased the risk of GDM (RR=1.47, 95%CI: 1.20-1.80; RR=1.31, 95%CI: 1.04-1.64), respectively. Compared with the lowest quartile of GL of staple foods, the third and highest quartiles of GL of staple foods groups also increased the risk of GDM (RR=1.28, 95%CI: 1.04-1.58; RR=1.27, 95%CI: 1.02-1.60), respectively. The third and highest quartiles of GL of rice groups increased the risk of GDM (RR=1.30, 95%CI: 1.06-1.59; RR=1.28, 95%CI: 1.03-1.59), respectively, than the lowest quartile of GL of rice group. When compared with the lowest quartile of GL of tubers group, the highest quartile of GL of tubers group increased the risk of GDM (RR=1.30, 95%CI: 1.09-1.54). However, we did not notice the effects of wheat GL and coarse grain GL on the risk of GDM. Conclusions: A positive association was found between dietary glycemic load and the risk of GDM. Higher dietary glycemic load, especially in rice and tubers during first trimester, seemed to have increased the risk of GDM.

[Association between gestational weight gain and adverse pregnancy outcomes: a prospective study]

Objective: To explore the association between gestational weight gain (GWG) and adverse pregnancy outcomes. Methods: A prospective study was conducted among 1 220 healthy singleton pregnant women in the first trimester of pregnancy, from Chengdu city, Sichuan province. Pre-gestational body mass and other basic information were collected through a set of questionnaires. Weight at the last week before delivery was measured and GWG was classified by IOM criteria (2009). Related information on pregnancy outcomes was collected after delivery, through the hospital information system. Multiple non-conditional logistic regression models were used to test the association between GWG and adverse pregnancy outcomes. Results: In total, data on 1 045 pregnant women were analyzed. Compared with adequate GWG, excessive GWG was associated with the increased risks of cord entanglement and large for gestational age (OR=1.641, 95%CI: 1.197-2.252; OR=1.678, 95%CI: 0.132-2.488), respectively. Additionally, when compared with the adequate GWG, insufficient GWG was associated with the increased risk of preterm delivery (OR=3.189, 95%CI: 1.604-6.341). Conclusions: Both excessive and insufficient GWG appeared associated with the pregnancy outcomes. Weight monitoring should be strengthened for pregnant women to reduce related risks on adverse pregnancy outcomes.

[Assessment of comprehensive nutritional status and eating behavior among 311 middle aged and aged women with osteoporosis in Chengdu]

Objective: To investigate the comprehensive nutritional status and diet behavior of middle aged and elderly women with osteoporosis, and thereby to explore the relationship between diet behavior and comprehensive nutritional status. Methods: 311 middle-aged and elderly women with osteoporosis in Chengdu were included in this study. Mini Nutritional Assessment (MNA) was applied to assess their comprehensive nutritional status. Information of social-demographic characteristics and diet behavior (about meals, snacks and water drinking, etc.) of the subjects was collected by questionnaire. Chi square test was used to assess the differences in nutritional status among patients who have different eating behaviors. Logistic regression analysis was performed to evaluate the relationship between diet behaviors and comprehensive nutritional status. Results: The mean MNA score of subjects was 25.8±2.5. 20.3% (63/311) of the subjets were at risk of potential malnutrition, but there was no malnourished subjects found. 46.9% (46/311) of the subjects were in good appetite. 95.2% (296/311) of them had a fixed food intake each meal. 65.8% (198/311) of them had snacks every day, and the most common choice was fruit (86.4% (248/287)). 54.8% (165/311) of them had initiative drinking water habits, and the most common choice was plain boiled water (79.9%, 246/308). 76.5% (238/311) of them had daily portable water less than 1 500 ml. After adjusting the effects of age, occupation and education level, bad appetite (OR=3.50, 95%CI: 1.18-10.62), unfixed food intake (OR=7.27, 95%CI: 1.40-35.83), and seldom or never intake of snack (OR=3.71, 95%CI: 1.42-9.72) were risk factors for malnutrition risk, while tea drinking was protective factor(OR=0.31, 95%CI: 0.11-0.93). Conclusion: Risk of potential malnutrition and unhealthy diet behavior among the middle aged and elderly women with osteoporosis should be paid more attention. Unhealtghy diet behavior has a negative effect on their comprehensive nutritional status.

Potential applications of nicotinic ligands in the laboratory and clinic

The nicotinic acetylcholine receptor (nAChR) is a receptor, ion channel complex composed of five polypeptide subunits. There are many different nAChR subtypes constructed from a variety of different subunit combinations. This structural diversity contributes to the varied roles of nAChRs in the peripheral and central nervous system, and this diversity offers an excellent opportunity for chemists who are producing ligands. Subunit specific ligands could have wide and varied effects in the laboratory as experimental tools and in the clinic as therapeutic agents. Because presynaptic nAChRs have been shown to enhance the release of many neurotransmitters, new nicotinic ligands that potentiate nAChR activity would be very useful. Such ligands could enhance the release of various neurotransmitters during degenerative diseases that cause neurotransmitter systems to decrease their output. For example, boosting the release from cholinergic neurons would help patients with Alzheimer's disease, and boosting the release from dopaminergic neurons would help patients with Parkinson's disease.

Endogenous nicotinic cholinergic activity regulates dopamine release in the striatum

Dopamine is vital for coordinated motion and for association learning linked to behavioral reinforcement. Here we show that the precise overlap of striatal dopaminergic and cholinergic fibers underlies potent control of dopamine release by ongoing nicotinic receptor activity. In mouse striatal slices, nicotinic antagonists or depletion of endogenous acetylcholine decreased evoked dopamine release by 90%. Nicotine at the concentration experienced by smokers also regulated dopamine release. In mutant mice lacking the beta2 nicotinic subunit, evoked dopamine release was dramatically suppressed, and those mice did not show cholinergic regulation of dopamine release. The results offer new perspectives when considering nicotine addiction and the high prevalence of smoking in schizophrenics.

Dopamine D2 receptor-mediated presynaptic inhibition of olfactory nerve terminals

Olfactory receptor neurons of the nasal epithelium project via the olfactory nerve (ON) to the glomeruli of the main olfactory bulb, where they form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the olfactory bulb, and with juxtaglomerular interneurons. The glomerular layer contains one of the largest population of dopamine (DA) neurons in the brain, and DA in the olfactory bulb is found exclusively in juxtaglomerular neurons. D2 receptors, the predominant DA receptor subtype in the olfactory bulb, are found in the ON and glomerular layers, and are present on ON terminals. In the present study, field potential and single-unit recordings, as well as whole cell patch-clamp techniques, were used to investigate the role of DA and D2 receptors in glomerular synaptic processing in rat and mouse olfactory bulb slices. DA and D2 receptor agonists reduced ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells. Spontaneous and ON-evoked spiking of mitral cells was also reduced by DA and D2 agonists, and enhanced by D2 antagonists. DA did not produce measurable postsynaptic changes in juxtaglomerular cells, nor did it alter their responses to mitral/tufted cell inputs. DA also reduced 1) paired-pulse depression of ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells and 2) the amplitude and frequency of spontaneous, but not miniature, excitatory postsynaptic currents in juxtaglomerular cells. Taken together, these findings are consistent with the hypothesis that activation of D2 receptors presynaptically inhibits ON terminals. DA and D2 agonists had no effect in D2 receptor knockout mice, suggesting that D2 receptors are the only type of DA receptors that affect signal transmission from the ON to the rodent olfactory bulb.

Synaptic plasticity and nicotine addiction

Nicotine, the main addictive component of tobacco, activates and desensitizes nicotinic acetylcholine receptors (nAChRs). In that way, nicotine alters normal nicotinic cholinergic functions. Among the myriad of psychopharmacological effects that underlie the addiction process, nicotine influences nAChR participation in synaptic plasticity. This influence has particular importance in the mesocorticolimbic dopamine system, which serves during the reinforcement of rewarding behaviors.

Nicotinic acetylcholine receptor-mediated synaptic potentials in rat neocortex

In the neocortex, fast excitatory synaptic transmission can typically be blocked by using excitatory amino acid (EAA) receptor antagonists. In recordings from layer II/III neocortical pyramidal neurons, we observed an evoked excitatory postsynaptic potential (EPSP) or current (EPSC) in the presence of EAA receptor antagonists (40-100 microM D-APV+20 microM CNQX, or 5 mM kynurenic acid) plus the GABA(A)-receptor antagonist bicuculline (BIC, 20 microM). This EAA-antagonist resistant EPSC was observed in about 70% of neurons tested. It had a duration of approximately 20 ms and an amplitude of 61.5+/-6.8 pA at -70 mV (n=35). The EAA-antagonist resistant EPSC current-voltage relation was linear and reversed near 0 mV (n=23). The nonselective nicotinic acetylcholine receptor (nAChR) antagonists dihydro-beta-erythroidine (DH beta E, 100 microM) or mecamylamine (50 microM) reduced EPSC amplitudes by 42 (n=20) and 33% (n=9), respectively. EPSC kinetics were not significantly changed by either antagonist. Bath application of 10 microM neostigmine, a potent acetylcholinesterase inhibitor, prolonged the EPSC decay time. EAA-antagonist resistant EPSCs were observed in the presence of antagonists of metabotropic glutamate, serotonergic (5-HT(3)) and purinergic (P2) receptors. The EAA-antagonist resistant EPSC appears to be due in part to activation of postsynaptic nAChRs. These results suggest the existence of functional synaptic nAChRs on pyramidal neurons in rat neocortex.

Tonic and synaptically evoked presynaptic inhibition of sensory input to the rat olfactory bulb via GABA(B) heteroreceptors

Olfactory receptor neurons of the nasal epithelium send their axons, via the olfactory nerve (ON), to the glomeruli of the olfactory bulb (OB), where the axon terminals form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the OB, and with juxtaglomerular (JG) interneurons. Many JG cells are GABAergic. Here we show that, despite the absence of conventional synapses, GABA released from JG cells activates GABA(B) receptors on ON terminals and inhibits glutamate release both tonically and in response to ON stimulation. Field potential recordings and current-source density analysis, as well as intracellular and whole cell recording techniques were used in rat OB slices. Baclofen (2-5 microM), a GABA(B) agonist, completely suppressed ON-evoked synaptic responses of both mitral/tufted cells and JG cells, with no evidence for postsynaptic effects. Baclofen (0.5-1 microM) also reversed paired-pulse depression (PPD) of mitral/tufted cell responses to paired-pulse facilitation (PPF), and reduced depression of JG cell excitatory postsynaptic currents (EPSCs) during repetitive ON stimulation. These results suggest that baclofen reduced the probability of glutamate release from ON terminals. The GABA(B) antagonists CGP35348 or CGP55845A increased mitral/tufted cell responses evoked by single-pulse ON stimulation, suggesting that glutamate release from ON terminals is tonically suppressed via GABA(B) receptors. The same antagonists reduced PPD of ON-evoked mitral/tufted cell responses at interstimulus intervals 50-400 ms. This finding suggests that a single ON impulse evokes sufficient GABA release, presumably from JG cells, to activate GABA(B) receptors on ON terminals. Thus GABA(B) heteroreceptors on ON terminals are activated by ambient levels of extrasynaptic GABA, and by ON input to the OB. The time course of ON-evoked, GABA(B) presynaptic inhibition suggests that neurotransmission to M/T cells and JG cells will be significantly suppressed when ON impulses arrive in glomeruli at 2.5-20 Hz. GABA(B) receptor-mediated presynaptic inhibition of sensory input to the OB may play an important role in shaping the activation pattern of the OB glomeruli during olfactory coding.

Activation of serotonin receptors modulates synaptic transmission in rat cerebral cortex

The cerebral cortex receives an extensive serotonergic (5-hydroxytryptamine, 5-HT) input. Immunohistochemical studies suggest that inhibitory neurons are the main target of 5-HT innervation. In vivo extracellular recordings have shown that 5-HT generally inhibited cortical pyramidal neurons, whereas in vitro studies have shown an excitatory action. To determine the cellular mechanisms underlying the diverse actions of 5-HT in the cortex, we examined its effects on cortical inhibitory interneurons and pyramidal neurons. We found that 5-HT, through activation of 5-HT(2A) receptors, induced a massive enhancement of spontaneous inhibitory postsynaptic currents (sIPSCs) in pyramidal neurons, lasting for approximately 6 min. In interneurons, this 5-HT-induced enhancement of sIPSCs was much weaker. Activation of 5-HT(2A) receptors also increased spontaneous excitatory postsynaptic currents (sEPSCs) in pyramidal neurons. This response desensitized less and at a slower rate. In contrast, 5-HT slightly decreased evoked IPSCs (eIPSCs) and eEPSCs. In addition, 5-HT via 5-HT(3) receptors evoked a large and rapidly desensitizing inward current in a subset of interneurons and induced a transient enhancement of sIPSCs. Our results suggest that 5-HT has widespread effects on both interneurons and pyramidal neurons and that a short pulse of 5-HT is likely to induce inhibition whereas the prolonged presence of 5-HT may result in excitation.

Dopamine modulation of membrane and synaptic properties of interneurons in rat cerebral cortex

Dopamine (DA) is an endogenous neuromodulator in the mammalian brain. However, it is still controversial how DA modulates excitability and input-output relations in cortical neurons. It was suggested that DA innervation of dendritic spines regulates glutamatergic inputs to pyramidal neurons, but no experiments were done to test this idea. By recording individual neurons under direct visualization we found that DA enhances inhibitory neuron excitability but decreases pyramidal cell excitability, through depolarization and hyperpolarization, respectively. Accordingly, DA also increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs). In the presence of TTX, DA did not affect the frequency, amplitude, or kinetics of miniature IPSCs and excitatory postsynaptic currents in inhibitory interneurons or pyramidal cells. Our results suggest that DA can directly excite cortical interneurons, but there is no detectable DA gate to regulate spontaneous GABA and glutamate release or the properties of postsynaptic GABA and glutamate receptors in neocortical neurons.

AMPA receptor-mediated EPSCs in rat neocortical layer II/III interneurons have rapid kinetics

The properties of spontaneous and miniature (m) AMPA receptor-mediated excitatory postsynaptic currents (EPSCs) were studied in rat neocortical layer II/III fast spiking interneurons. Under optimal recording conditions, averaged mEPSCs had a 10-90% rise time of about 0.3 ms. The decay of averaged mEPSCs was double exponential with time constants of about 1 and 4 ms. Kinetics were observed to slow as series resistance increased. The amplitudes of mEPSCs were much smaller at +50 mV than at -50 mV indicating that the currents were inwardly rectifying. These results suggest that synaptic AMPA receptors on neocortical inhibitory interneurons have a deactivation time constant less than 1 ms which largely determines the decay of the synaptic currents. The receptors appear to lack GluR-2 subunits and may be Ca2+ permeable.

Metabotropic glutamate receptor enhancement of spontaneous IPSCs in neocortical interneurons

Metabotropic glutamate receptor enhancement of spontaneous IPSCs in neocortical interneurons. J. Neurophysiol. 78: 2287-2295, 1997. Using neocortical layer I neurons as a model for GABAergic interneurons, we have studied gamma-aminobutyric acid-A (GABAA) receptor-mediated spontaneous inhibitory postsynaptic currents (IPSCs) and modulation by metabotropic glutamate receptors (mGluRs). In the presence of 0.5 mu M tetrodotoxin (TTX) and ionotropic glutamate receptor antagonists and under symmetrical Cl- conditions, the mean amplitude of miniature IPSCs (mIPSCs) was approximately 50 pA at a holding potential of -70 mV with individual events ranging from 10 to 400 pA. Averaged mIPSCs had a 10-90% rise time of approximately 0.6 ms. The decay was double exponential. The fast component had a time constant of approximately 4 ms and comprised approximately 40% of the total amplitude. The slow component had a time constant of approximately 22 ms. The frequency of spontaneous IPSCs (sIPSCs), recorded in the absence of TTX, was increased by bath application of the mGluR agonist 1S,3R-1-aminocyclopentane-1, 3-dicarboxylic acid (ACPD; 10-100 mu M) or the group I mGluR selective agonist quisqualic acid (Quis; 0.5-1 mu M). Under identical conditions, mIPSCs were not affected. The kinetics of sIPSCs and mIPSCs were not altered by ACPD or Quis. Quis (1 mu M) induced an inward current of approximately 70 pA at a holding potential of -70 mV, whereas ACPD (40-200 mu M) induced a smaller inward current. This current was linear over the voltage range -70 to +30 mV and reversed polarity near 0 mV. In current-clamp recordings, both Quis and ACPD induced a depolarization and action potential firing in layer I and deeper layer interneurons. We conclude that neocortical layer I neurons receive GABAA receptor-mediated inhibitory synaptic inputs. Activation of mGluRs, possibly mGluR1 and/or mGluR5, causes an enhancement of inhibitory synaptic transmission by directly depolarizing corticalGABAergic interneurons through the opening of nonselective cation channels.

Rapid kinetics and inward rectification of miniature EPSCs in layer I neurons of rat neocortex

With the use of the whole cell patch-clamp technique combined with visualization of neurons in brain slices, we studied the properties of miniature excitatory postsynaptic currents (mEPSCs) in rat neocortical layer I neurons. At holding potentials (-50 to -70 mV) near the resting membrane potential (RMP), mEPSCs had amplitudes of 5-100 pA and were mediated mostly by alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptors. Amplitude histograms were skewed toward large events. An N-methyl-D-aspartate (NMDA) component was revealed by depolarization to -30 mV or by the use of a Mg2+-free bathing solution. At RMP, averaged AMPA mEPSCs had a 10-90% rise time of approximately 0.3 ms (uncorrected for instrument filtering). The decay of averaged mEPSCs was best fit by double-exponential functions in most cases. The fast, dominating component had a decay time constant of approximately 1.2 ms and comprised approximately 80% of the total amplitude. A small slow component had a decay time constant of approximately 4 ms. Positive correlations were found between rise and decay times of both individual and averaged mEPSCs, indicative of dendritic filtering. Some large-amplitude mEPSCs and spontaneous EPSCs (recorded in the absence of tetrodotoxin) had slower kinetics, suggesting a role of asynchronous transmitter release in shaping EPSCs. The amplitudes of mEPSCs were much smaller at +60 mV than at -60 mV, indicating that synaptic AMPA-receptor-mediated currents were inwardly rectifying. These results suggest that neocortical layer I neurons receive both NMDA- and AMPA-receptor-mediated synaptic inputs. The rapid decay of EPSCs appears to be largely determined by AMPA receptor deactivation. The observed rectification of synaptic responses suggests that synaptic AMPA receptors in layer I neurons may lack GluR-2 subunits and may be Ca2+ permeable.

Morphological properties of intracellularly labeled layer I neurons in rat neocortex

The morphology of neurons in layer I of rat neocortex, including Cajal-Retzius (CR) cells, was studied by using intracellular biocytin staining in brain slices obtained from rats during the first 22 postnatal days. Within the first postnatal week, horizontal bipolar neurons or CR cells were prominent in layer I. Typically, CR cells had one main dendrite and one axon originating from opposite poles of the somata. Even though the main dendrites and axons could be quite long, complex dendritic or axonal arbors were not observed. Starting around postnatal day 6 (PN 6), CR cells were less frequently observed. From PN 10 to PN 21, nonpyramidal neurons with diverse morphologies became the main neuronal component in layer I. The somata of layer I nonpyramidal neurons were quite variable in size and shape. Dendrites were smooth or sparsely spiny, and the dendritic trees were mainly restricted to layer I, covering an area with a diameter of about 200 microns. Axon collaterals of these cells formed elaborate arbors with diameters of around 700 microns in layer I and extending, in many cases, to layer II/III and even layer IV. This extensive axonal plexus provides a rich anatomical base on which layer I neurons, functioning as local circuit elements, may interact with each other and with neurons in other layers.

Layer I neurons of rat neocortex. I. Action potential and repetitive firing properties

1. Whole cell patch-clamp techniques, combined with direct visualization of neurons, were used to study action potential (AP) and repetitive firing properties of layer I neurons in slices of rat neocortex. 2. Layer I neurons had resting membrane potentials (RMP) of -59.8 +/- 4.7 mV (mean +/- SD) and input resistances (RN) of 592 +/- 284 M Omega. Layer II/III pyramidal neurons had RMPs and RNs of -61.5 +/- 5.6 mV and 320 +/- 113 M omega, respectively. A double exponential function was needed to describe the charging curves of both neuron types. In layer I neurons, tau(0) was 45 +/- 22 ms and tau(1) was 5 +/- 3.3 ms whereas in layer II/III pyramidal neurons, tau(0) was 41 +/- 11 ms and tau(1) was 3 +/- 2.6 ms. Estimates of specific membrane resistance (Rm) for layer I and layer II/III cells were 45 +/- 22 and 41 +/- 11 k omega cm2, respectively (Cm was assumed to be 1 microF/cm2). 3. AP threshold was -41 +/- 2 mV in layer I neurons. Spike amplitudes, measured from threshold to peak, were 90.6 +/- 7.7 mV. AP durations, measured both at the base and half maximal amplitude, were 2.5 +/- 0.4 and 1.1 +/- 0.2 ms, respectively. AP 10-90% rise and repolarization times were 0.6 +/- 0.1 and 1.1 +/- 0.2 ms, respectively. In layer II/III pyramidal neurons, AP threshold was -41 +/- 2.5 mV and spike amplitude was 97 +/- 9.7 mV. AP duration at base and half maximal amplitude was 5.4 +/- 1.1 ms and 1.8 +/- 0.2 ms, respectively. AP 10-90% rise and decay times were 0.6 +/- 0.1 ms and 2.8 +/- 0.6 ms, respectively. 4. Layer I neurons were fast spiking cells that showed little frequency adaptation, a large fast afterhyperpolarization (fAHP), and no slow afterhyperpolarization (sAHP). Some cells had a medium afterhyperpolarization (mAHP) and a slow afterdepolarization (sADP). All pyramidal cells in layer II/III and "atypical" pyramidal neurons in upper layer II showed regular spiking behavior, prominent frequency adaptation, and marked sAHPs. 5. In both layer I neurons and layer II/III pyramidal neurons, changes in membrane potential did not greatly alter AP properties. The duration of APs evoked from -50 to -60 mV was only slightly longer, from -80 to -90 mV. The latency to first spike also was not solely dependent on membrane potential. 6. During repetitive firing, APs broadened in both layer I neurons and layer II/III pyramidal neurons. This was most prominent in pyramidal cells. Broadening was dependent on spike frequency and appeared to result from partial inactivation of both outward potassium and inward sodium currents. 7. In layer I neurons, removing Ca2+ from the bathing solution slightly prolonged spike duration and modestly increased AP firing frequency. These results indicate minimal involvement of Ca2+-dependent K+ currents in AP repolarization. fAHPs were reduced whereas sADPs were abolished. In layer II/III pyramidal neurons, removing Ca2+ reduced or blocked mAHPs and sAHPs and decreased or abolished frequency adaptation. 8. Low concentrations (50 microM) of 4-aminopyridine (4-AP) prolonged APs and induced burst-like firing in layer I neurons. In the presence of 4-AP, the spiking behavior of layer I neurons resembled that of regular spiking layer II/III pyramidal cells. At high concentrations (4 mM), 4-AP could induce a delayed depolarization (DD) after each spike in layer I neurons and in a minority of pyramidal neurons. 9. All layer I neurons had a prominent fAHP that was absent or very small in layer II/III pyramidal neurons. fAHP amplitude was inversely related to AP duration. The reduction of fAHPs by 4-AP or during repetitive firing was accompanied by AP prolongation, suggesting that the current underlying fAHP played an essential role in AP repolarization. The fAHP of layer I neurons could be effectively blocked by 4-AP but only slightly reduced by removing Ca2+ from bathing solution, indicating that the fAHP was mediated primarily by a voltage-dependent transient outward current.(ABSTRACT TRUNCATED)

Layer I neurons of the rat neocortex. II. Voltage-dependent outward currents

1. Whole cell patch-clamp techniques, combined with direct visualization of neurons, were used to study voltage-dependent potassium currents in layer 1 neurons and layer II/III pyramidal cells. 2. In the presence of tetrodotoxin, step depolarizations evoked an outward current. This current had a complex waveform and appeared to be a composite of early and late components. The early peak of the composite K+ outward current was larger in layer I neurons. 3. In both layer I and pyramidal cells, the composite outward K+ current could be separated into two components based on kinetic and pharmacological properties. The early component was termed I(A) because it was a transient outward current activating rapidly and then decaying. I(A) was more sensitive to blocking by 4-aminopyridine (4-AP) than tetraethylammonium (TEA). The second component, termed the delayed rectifier or I(DR), activated relatively slowly and did not decay significantly during a 200-ms test pulse. I(DR) was insensitive to blocking by 4-AP at concentrations up to 4 mM and blocked by > 60% by 40-60 mM TEA. 4. I(A) kinetics were examined in the presence of 40-60 mM TEA. Under these conditions, I(A) began to activate between -40 and -30 mV. Half-maximal activation occurred around 0 mV. In both layer I and pyramidal cells, the half-inactivation potential (Vh-inact) was around or more positive than -50 mV. At -60 mV, > 70% of I(A) conductance was available. I(A) decayed along a single exponential time course with a time constant of approximately 15 ms. This decay showed little voltage dependence. 5. In both layer I and pyramidal cells, I(DR) was studied in the presence of 4 mM 4-AP to block I(A) and in saline containing 0.2 mM Ca2+ and 3.6 mM Mg2+ to reduce contributions from Ca2+-dependent K+ currents. Under these conditions, I(DR) began to activate at -35 to -25 mV with Vh-act of 3.6 +/- 4.5 mV (mean +/- SD). The 10-90% rise time of I(DR) was 15 ms at 30 mV. At 2.2 ms after the onset of the command potential to +30 mV, I(DR) could reach a significant amplitude (approximately 1.5 nA in layer I neurons and 2.2 nA in pyramidal cells depending on the cell size). When long test pulses (> or = 1,000 ms) were used, a decay time constant approximately 800 ms at +40 mV was observed. In both layer I and pyramidal cells, steady state inactivation of I(DR) was minimal. 6. These results indicate that I(A) and I(DR) are the two major hyperpolarizing currents in layer I and pyramidal cells. The kinetics and pharmacological properties of I(A) and I(DR) were not significantly different in fast-spiking layer I neurons and regular-spiking layer II/III pyramidal cells. The relatively positive activation threshold (more than or equal to -40 mV) of both I(A) and I(DR) suggest that they do not play a role in neuronal behavior below action potential (AP) threshold and that their properties are more suitable to repolarize AP. The greater density of I(A) in layer I neurons appears responsible for fast spike generation.

Postnatal development of membrane properties of layer I neurons in rat neocortex

Using whole-cell patch-clamp techniques in brain slices, we studied the postnatal development of electrophysiological properties of rat neocortical layer I neurons during the first three weeks of postnatal life. Neurons, including Cajal-Retzius cells, were visualized under Nomarski optics before recording. In the first postnatal week, all layer I neurons, including Cajal-Retzius cells, had low resting membrane potentials (-40 to -55 mV), high input resistances (1-5 G omega), and long membrane time constants (80-130 msec). Action potentials (APs) of layer I neurons early in postnatal development were lower in amplitude and longer in duration. The threshold for APs also was more depolarized than in older neurons. A medium after-hyperpolarization already was present at postnatal day 0 (PN0), but fast afterhyperpolarizations were not seen until PN10. At all postnatal ages, layer I neurons were capable of repetitive firing, displayed little or no frequency adaptation, and did not display slow afterhyperpolarizations. Early in development, layer I neurons had a prominent hyperpolarization-activation depolarizing sag that decreased with age. These results suggest that the membrane properties of rat neocortical layer I neurons mature rapidly during the first two postnatal weeks. Cajal-Retzius cells had electrical properties similar to other layer I neurons and did not show an earlier maturation of membrane properties.

Zinc enhances GABAergic transmission in rat neocortical neurons

1. Intracellular recordings were made in layer II-III neurons of rat neocortical slices maintained in vitro. The effect of bath application of zinc (50-300 microM) on evoked synaptic activity and passive membrane properties was examined. 2. Excitatory postsynaptic potentials (EPSPs) mediated by N-methyl-D-aspartate (NMDA) and non-NMDA receptors were recorded in response to electrical stimulation. Zinc did not affect either type of EPSP. Resting membrane potential, repetitive firing properties, and input resistance were not altered by zinc. 3. Inhibitory postsynaptic potentials (IPSPs) were enhanced after zinc application. Zinc also induced generation of large amplitude spontaneous gamma-aminobutyric acid-A (GABAA)- and GABAB-mediated IPSPs. Postsynaptic responses to iontophoretically applied GABA were unaffected. In the presence of zinc, GABAergic synaptic potentials could result in generation of action potentials. 4. Directly evoked IPSPs recorded in the presence of the excitatory amino acid receptor blockers 6-cyano-7-nitroquinoxaline-2,3-dione and 2-amino-5-phosphonovaleric acid were enhanced by zinc. Under these conditions spontaneous IPSPs with superimposed action potentials were present. Baclofen, in the presence of zinc, reduced the amplitude of evoked IPSPs. 5. These results indicate that zinc may be an endogenously occurring neuromodulator. Zinc appears to enhance GABAergic IPSPs by increasing the excitability of inhibitory interneurons, thus resulting in increased GABA release.