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NEUROPHARMACOLOGY

Modulation of Synaptic Transmission to Second-Order Peripheral Chemoreceptor Neurons in Caudal Nucleus Tractus Solitarius by ₁-Adrenoreceptors

Department of Pharmacology, The University of Texas Health Science Center at San Antonio, San Antonio, Texas

Received September 14, 2006; accepted October 31, 2006.

	Abstract

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Norepinephrine (NE) is an important neurotransmitter in central autonomic regulation. Peripheral chemoreceptor stimulation activates central noradrenergic structures. These structures innervate and therefore could modulate neurons in caudal nucleus tractus solitarius (cNTS), which receives the first central projections from peripheral chemoreceptors. However, the role of ₁-adrenoreceptors in synaptic transmission of peripheral chemoreceptor inputs in cNTS is unknown. We investigated the responses to activation of ₁-adrenoreceptors on glutamatergic and GABAergic inputs in NTS slices using whole-cell recording. Second-order neurons were identified by 1,1'-dilinoleyl-3,3,3',3'-tetra-methylindocarbocyanine, 4-chlorobenzenesulphonate (DiA) labeling of carotid bodies. Electrical stimulation of ipsilateral tractus solitarius was used to evoke excitatory postsynaptic currents (eEPSCs), whereas inhibitory postsynaptic currents were evoked (eIPSCs) by electrically stimulating NTS near the recorded neuron. Application of ₁-adrenoreceptor agonist phenylephrine (PE) at 20 µM significantly decreased amplitudes of eEPSCs (78 ± 1% of control; n = 16; p < 0.01), and it increased amplitudes of eIPSCs (120 ± 13% of control; n = 7; p < 0.01). Both effects were blocked by the ₁-adrenoreceptor antagonist prazosin at 10 µM. PE did not change holding current, input resistance, and current-voltage relationship in cNTS neurons. PE significantly changed paired pulse ratios of eEPSC/eIPSCs, increased the frequency of miniature IPSCs (329 ± 10% of control; n = 6; p < 0.05), but it decreased that of miniature EPSCs (69 ± 6% of control; n = 5; p < 0.01). PE-induced inhibition of eEPSCs was independent of N-methyl-D-aspartate or GABA_B receptors. These results suggest that activation of ₁-adrenoreceptors reduces excitatory and enhances inhibitory inputs to second-order peripheral chemoreceptor neurons in cNTS via a presynaptic mechanism. These actions result in the inhibition of synaptic transmission and could play a role in the autonomic responses to hypoxia.

The caudal nucleus tractus solitarius (cNTS), where peripheral chemoreceptor afferents and other visceral afferents make their first central synapses (Mifflin, 1992), has intense anatomical connections with central noradrenergic neural structures (Loewy, 1990). The cNTS also contains noradrenergic neurons, i.e., the A2 cell group. Both ₁- and ₂-adrenoreceptors exist throughout the NTS (Young and Kuhar, 1980; Dashwood et al., 1985; Jones et al., 1985; Day et al., 1997). These data suggest that NE, acting through various adrenoreceptor subtypes, could function as an important neuromodulator of synaptic transmission of peripheral chemoreceptor inputs in cNTS. However, the synaptic mechanisms whereby NE might modulate cardiorespiratory afferent integration by NTS neurons remain to be clarified.

A few studies have examined the involvement of NTS ₂-adrenoreceptors in peripheral chemoreflexes. Microinjection of an ₂-adrenoreceptor antagonist in cNTS attenuated the cardiorespiratory responses to peripheral chemoreceptor stimulation with potassium cyanide (Hayward, 2001). Stimulating locus coeruleus inhibits NTS neuronal discharge evoked by peripheral chemoreceptor stimulation via ₂-adrenoreceptors (Perez et al., 1998). To our knowledge, no studies have investigated modulation of synaptic transmission of peripheral chemoreceptor inputs in cNTS by ₁-adrenoreceptors. Activation of ₁-adrenoreceptors inhibited neuronal discharge in medial NTS, which primarily receives baroreceptor inputs (Feldman and Moises, 1988; Feldman and Felder, 1989). It is therefore hypothesized that activation of ₁-adrenoreceptors will inhibit synaptic transmission of peripheral chemoreceptor inputs in cNTS. Using in vitro whole-cell recording, we studied the effect of ₁-adrenoreceptor activation on synaptic transmission of both excitatory glutamatergic and inhibitory GABAergic synaptic inputs to second-order peripheral chemoreceptor neurons in cNTS. We further investigated whether ₁-adrenoreceptor modulation of synaptic transmission occurs via a pre- and/or postsynaptic mechanism.

	Materials and Methods

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All experimental protocols were approved by the Institutional Animal Care and Use Committee at The University of Texas Health Science Center at San Antonio.

Surgical Preparation for Labeling Carotid Body. Male Sprague-Dawley rats (100–125 g) were anesthetized with a combination of ketamine (75 mg/kg i.p.; Fort Dodge Laboratories, Fort Dodge, IA) and medetomidine (0.5 mg/kg i.p.; Pfizer, Inc., New York, NY). Under aseptic conditions, crystals of anterograde fluorescent dye DiA were gently applied unilaterally to the carotid body region. DiA dissolves in the nerve axons and diffuses centrally, permitting visualization of chemoreceptor synaptic terminals and neurons receiving these synaptic contacts as described previously for an aortic nerve study (Mendelowitz et al., 1992). The area was then embedded with silicone adhesive (Kwik-Sil; WPI, Sarasota, FL). Anesthesia was terminated by atipamezole (1 mg/kg i.p.; Pfizer, Inc.) at the conclusion of the surgical procedures. Postoperative analgesics (nubaine i.m.) were available as needed. The rats were allowed to recover for 7 to 10 days before the experimental protocols.

Brain Slice Preparation. Rats were anesthetized with isoflurane, and the brainstem was rapidly removed and placed in ice-cold, high-sucrose artificial cerebrospinal fluid (aCSF) that contained 3 mM KCl, 1 mM MgCl₂, 1 mM CaCl₂, 2 mM MgSO₄, 1.25 mM NaH₂PO₄, 26 mM NaHCO₃, 10 mM glucose, and 206 mM sucrose, pH 7.4, when continuously bubbled with 95% O₂, 5% CO₂. Brainstem horizontal slices (250 µm in thickness) were cut with a sapphire knife (Delaware Diamond Knives, Wilmington, DE) and mounted in a vibrating microtome (VT1000E; Leica Microsystems, Bannockburn, IL). Then, the slices were incubated for at least 1 h in normal aCSF that contained 124 mM NaCl, 3 mM KCl, 2 mM MgSO₄, 1.25 mM NaH₂PO₄, 26 mM NaHCO₃, 10 mM glucose, and 2 mM CaCl₂, pH 7.4, when continuously bubbled with 95% O₂, 5% CO₂.

Electrophysiological Recording. A single slice was transferred into the recording chamber on an upright epifluorescent microscope (Olympus BX50WIl; Olympus, Tokyo, Japan) equipped with infrared-differential interference contrast and an optical filter set for visualization of DiA. The slice was held in place with a nylon mesh, submerged in normal aCSF equilibrated with 95% O₂, 5% CO₂, and perfused at a rate of approximately 2 ml/min. All images were captured with a charge-coupled device camera (IR-1000, CCD-100; Dage-MTI, Michigan City, IN), displayed on a TV monitor, and stored in a PC computer. Patch pipettes were pulled from borosilicate glass capillaries with an inner filament (0.90 mm i.d., 1.2 mm o.d.; WPI) on a pipette puller (model P-2000; Sutter Instrument Company, Novato, CA), and they were filled with a solution of the following composition: 145 mM potassium gluconate (replaced with KCl when recording IPSCs), l mM MgCl₂, 10 mM HEPES, 1.1 mM EGTA, 2 mM Mg₂ATP, and 0.3 mM Na₃GTP. The pH was adjusted to 7.3 with KOH. With this pipette solution, the junction potential was 15.5 mV at 24°C (3.6 mV for KCl-based pipette solution) and was not corrected in subsequent analysis. The pipette resistance ranged from 3 to 6 M. A seal resistance of at least 1 G or above and an access resistance <20 M, which changed <15% during recording, were considered acceptable. Series resistance was optimally compensated. Cells were clamped at a membrane potential of –60 mV. Input resistances of cells were monitored by frequently applying a 10-mV hyperpolarizing voltage step (100-ms duration) from a holding potential of –60 mV.

Recordings of postsynaptic currents began 5 min later, after the whole-cell access was established and the holding current reached a steady state. Recordings were made with an AxoPatch 200B patch-clamp amplifier and pClamp software version 8 (Molecular Devices, Sunnyvale, CA). Whole-cell currents were filtered at 2 kHz, digitized at 10 kHz with the DigiData 1200 interface (Molecular Devices), and stored in a PC computer for off-line analysis. All experiments were performed at room temperature.

Whole-cell voltage-clamp recordings were performed on second-order NTS peripheral chemoreceptor neurons labeled with fluorescent DiA. Evoked excitatory EPSCs (eEPSCs) were elicited by electrical stimulation of the ipsilateral solitary tract (ST) using concentric bipolar electrodes (FHC, Bowdoinham, ME) with a tip diameter of 200 µm. Square electric pulses of 0.1-ms duration with a frequency of 0.2 Hz were delivered through a stimulus isolator A360 (WPI), in series with a programmable stimulator (Master8; AMPI, Jerusalem, Israel). When recording evoked IPSCs (eIPSCs), the electrode was positioned in the NTS ipsilateral to the recording neuron and medial to the ST. Electrical stimuli were delivered at 0.1 Hz. Stimulus intensity was 50 to 300 µA. To determine the effect of phenylephrine (PE) on paired pulse stimulation, two synaptic responses (A1 and A2) were evoked by a pair of stimuli given at short intervals (40 ms for eEPSCs and 50 ms for eIPSCs). Paired pulse ratio (PPR) was expressed as the amplitude ratio of the second synaptic response to the first synaptic response (A2/A1). Bath application of drugs typically lasted approximately 3 to 5 min before beginning electrophysiological recordings.

Recordings of glutamatergic EPSCs were performed in the presence of the GABA_A receptor antagonist (–)-bicuculline methiodide (BIC; 30 µM). Recordings of the GABAergic IPSCs were performed in the presence of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 µM). Miniature EPSCs (mEPSCs) and IPSCs (mIPSCs) were recorded in the presence of the sodium channel blocker tetrodotoxin (TTX; 1 µM) and 30 µM BIC or 10 µM CNQX, respectively.

To test whether PE has a postsynaptic effect and changes current-voltage (I-V) relationships of second-order peripheral chemoreceptor neurons, a voltage-step protocol was performed in the presence of 1 µM TTX, 30 µM BIC, 50 µM DL-2-amino-5-phosphonopentanoic acid (AP-5), and 10 µM CNQX. Membrane potential was changed from –130 to –30 mV in 10-mV steps. The duration of each step was 500 ms and voltage steps were applied every 2 s.

Data Analysis. Data are presented as mean ± S.E.M. Peak amplitudes of averaged evoked postsynaptic currents (10 sweeps) were calculated as the difference from the baseline measured several milliseconds before the stimulation artifacts. Differences in drug effects were tested by one-way repeated-measures analysis of variance or paired t test. The threshold value for detecting miniature IPSCs/EPSCs was set as four times the root-mean-square baseline noise, and all miniature events detected by the software were visually checked to minimize errors. Cumulative distributions of miniature synaptic current amplitudes and frequencies were averaged over the 5-min period during control, during PE application, and after 15-min washout. All miniature events were detected with MiniAnalysis software, version 6.0 (Synaptosoft, Fort Lee, NJ). Cumulative distributions of miniature synaptic current amplitudes and frequencies were compared using Kolmogorov-Smirnov (K-S) nonparametric analysis. Averaged data were compared with paired t test. Statistics were performed using SigmaStat, version 2.03 (SPSS Inc., Chicago, IL), and graphs were made in SigmaPlot, version 8.0 (SPSS Inc.). Values of p < 0.05 were considered significant.

View larger version (34K): [in this window] [in a new window]   Fig. 1. Effect of PE on eEPSCs. A, whole-cell patch-clamp recording of second-order peripheral chemoreceptor neurons in cNTS. The left photograph shows the position of stimulating electrode (arrow) covering the ST and recording site at the tip of a recording pipette (*) in cNTS. Two cells are shown in the right photograph with infrared-differential contrast, but only one of cell is labeled with DiA when viewed with fluorescence set (indicated by arrows). B, example of 10 sweeps of eEPSCs. The response was abolished by 10 µM CNQX. Notice little variability of response latency. C, dose-dependent inhibition of eEPSC amplitude by PE application. Each tracing represents the average of 10 sweeps. D, group data showing PE dose-dependent inhibition of the amplitudes of eEPSCs in four NTS neurons that received all three doses of PE. E, group data showing PE dose-dependently inhibits the amplitudes of eEPSCs in NTS neurons. Numbers by each point indicate the number of cells treated at each concentration. F, effect of 20 µM PE on the PPR of eEPSCs. G, group data showing that 20 µM PE significantly increased the PPR of eEPSCs (n = 6). All experiments were performed in the presence of 30 µM BIC. Stimulus artifacts were truncated for clear view. Data are presented as means ± S.E. *, p < 0.05 versus control.

	Results

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Data were obtained from second-order neurons in cNTS, identified by presence of DiA-labeled somatic appositions as shown in Fig. 1A. These fluorescently labeled boutons usually formed along the outline of the soma and proximal processes of the neurons, leaving the center largely empty. Whole-cell patch-clamp recordings were performed on 135 DiA-labeled NTS cells from 59 rats (282 ± 5 g). Labeled NTS neurons displayed an average resting membrane potential of 57.5 ± 0.5 mV and input resistance of 705.6 ± 31.0 M.

Direct Effect of PE on Labeled NTS Neurons. Activation of postsynaptic ₁-adrenoreceptors decreases potassium conductance and depolarizes neurons in CNS (Aghajanian, 1985). Therefore, a postsynaptic effect of PE would be expressed as increased input resistance and/or inward current at a holding potential of –60 mV, thus the change of I-V relationship. In the presence of 1 µM TTX, 20 µM PE did not significantly alter the I-V relationship (curve slope, 0.405±0.057 versus 0.410 ± 0.061; n = 17; p > 0.05), indicating no significant change in potassium conductance. There was no significant change in input resistance after bath application of 20 µM PE (688.9 ± 45.3 versus 693.7 ± 52.0 M; n = 60; p > 0.05). PE did not cause discernible alteration in holding current, except that in one cell bath application of 20 µM PE caused a 93.7-pA inward current. To further confirm the lack of direct postsynaptic effect, we tested responses to 100 µM PE. This concentration of PE elicited inward current in only one of 15 cells. PE did not significantly change action potential discharge when applied to cells with spontaneous discharge (n = 2). These data suggest that activation of ₁-adrenoreceptors does not have a postsynaptic effect on second-order neurons of peripheral chemoreceptors in cNTS.

Effect of PE on Evoked EPSCs in Labeled NTS Neurons. To examine the effect of PE on glutamatergic synaptic inputs to second-order neurons in cNTS, eEPSCs were isolated at a holding potential of –60 mV and in the presence of 30 µM BIC. As described previously (Doyle and Andresen, 2001), eEPSCs elicited from ST stimulation were all-or-none responses with little recruitment at suprathreshold levels (Fig. 1B). The average latency of eEPSCs was 4.6 ± 0.2 ms (n = 61). As calculated from response latencies of 10 eEPSCs sweeps, the standard deviation of onset latency ranged from 30.2 µs to 175.0 µs with a median value of 96.6 µs, further suggesting these neurons receive monosynaptic inputs from the tractus (Doyle and Andresen, 2001). The eEPSCs were eliminated in the presence of 10 µM CNQX (Fig. 1B). PE (10–40 µM) decreased the peak amplitudes of eEPSCs in a concentration-dependent manner without significant effect on response onset latencies (Fig. 1, C–E). At the same time, application of PE did not cause discernible change of holding current. The eEPSCs in two of 29 labeled NTS cells were not altered by 20 or 40 µM PE.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Effect of PE on mEPSCs. A, recordings of mEPSCs before (control) and during 20 µM PE perfusion. B, cumulative probabilities of the interevent interval (left) and amplitude (right) before and during PE in the same neuron as illustrated in A. C and D, group data (n = 5) confirm that PE significantly decreased the mEPSC frequency (C) but not the amplitude (D), suggesting a presynaptic mechanism. All experiments were performed in the presence of 1 µM TTX and 30 µM BIC. Data are presented as means ± S.E. *, p < 0.05 versus control.

We further tested presynaptic mechanisms of ₁-adrenoreceptor activation on eEPSCs. The PPR of evoked postsynaptic currents was examined during the application of PE (Fig. 1, F and G). PE at 20 µM significantly increased the PPR of eEPSCs (p < 0.01; n = 6).

Effect of PE on mEPSCs of Labeled NTS Neurons. The presynaptic effect of PE on glutamatergic synaptic inputs to labeled NTS neurons was further examined by analysis of mEPSCs (Fig. 2A). The mEPSCs were recorded in the presence of 1 µM TTX and 30 µM BIC. PE at a concentration of 20 µM significantly decreased the frequency of mEPSCs (p < 0.001; n = 5; Fig. 2, B and C) without altering amplitude (Fig. 2, B and D). K-S testing of records from individual neurons also showed significant decreases in frequencies (p < 0.001) but not amplitudes (p > 0.05). Bath application of 10 µM CNQX abolished mEPSCs (data not shown).

Effect of PE on Evoked IPSCs in Labeled NTS Neurons. Second-order neurons in the NTS receive both glutamatergic and GABAergic inputs. The effect of PE on eIPSCs on labeled NTS neurons was examined. The eIPSCs were isolated at a holding potential of –60 mV and in the presence of 10 µM CNQX. The eIPSCs were abolished by 30 µM BIC (Fig. 3A). PE at 20 µM significantly increased the peak amplitude of eIPSCs (p < 0.001; n = 7; Fig. 3, B and C), with no significant effect on response latencies (2.9 ± 0.3 versus 3.0 ± 0.3 ms) and holding current. In two labeled NTS neurons, prazosin at 10 µM did not significantly change the amplitudes of eIPSCs (93.4 ± 1.7% of control), but it abolished the effect of 20 µM PE on eIPSCs (95.3 ± 12.8% of control).

View larger version (21K): [in this window] [in a new window]   Fig. 3. Effect of PE on eIPSCs. A, example of 10 sweeps of eIPSCs. The response was abolished by 30 µM BIC. B, example showing that 20 µMPE increased amplitudes of eIPSCs. C, group data showing that 20 µM PE significantly increased the amplitude of eIPSCs (n = 7). D, effect of 20 µMPE on the PPR of eIPSCs. E, group data showing that 20 µM PE significantly decreased the PPR of eIPSCs (n = 6). All experiments were performed in the presence of 10 µM CNQX. The tracing represents the average of 10 sweeps. Stimulus artifacts were truncated for clear view. Data are presented as means ± S.E. *, p < 0.05 versus control.

Effect of PE on mIPSCs of Labeled NTS Neurons. To further determine the presynaptic effect of PE on GABAergic synaptic inputs, we examined the effect of PE on mIPSCs in DiA-labeled NTS neurons (Fig. 4A). The mIPSCs were recorded in the presence of 1 µM TTX and 10 µM CNQX. PE at 20 µM significantly increased the frequency of mIPSCs (p < 0.05; n = 6) without significantly altering the amplitude (Fig. 4, B–D). K-S testing of records from individual neurons also showed significant increases in frequencies in six of six neurons (p < 0.001) and amplitudes in two of six neurons (p < 0.01). Bath application of 30 µM BIC abolished mIPSCs (data not shown).

View larger version (20K): [in this window] [in a new window]   Fig. 4. Effect of PE on mIPSCs. A, recordings of mIPSCs before (control) and during 20 µM PE perfusion. B, cumulative probabilities of the interevent interval (left) and amplitude (right) before and during PE in the same neuron as illustrated in A. C and D, group data (n = 6) confirm that PE significantly increased the mIPSC frequency (C) but not the amplitude (D), suggesting a presynaptic mechanism. All experiments were performed in the presence of 1 µM TTX and 10 µM CNQX. Data are presented as means ± S.E. *, p < 0.05 versus control.

Role of GABA_B Receptors in PE-Induced Inhibition on eEPSCs. The GABA_B receptor-selective antagonist SCH-50911 at 20 µM did not significantly change the amplitudes of eEPSCs (–190.3 ± 25.1 versus –193.5 ± 26.6 pA; p > 0.05; n = 10), suggesting no tonic effect of GABA_B receptor activation on excitatory synaptic transmission in cNTS in our preparation. Previous work in our laboratory has shown this concentration blocked outward currents evoked by 10 µM GABA_B receptor agonist baclofen (our unpublished observation). Coapplication of 20 µM SCH-50911 did not significantly alter 40 µM PE inhibition of the amplitude of eEPSCs (39 ± 3 versus 34 ± 5%; p > 0.05; n = 6).

Role of NMDA Receptors in PE-Induced Inhibition on eEPSCs. NMDA receptors did not mediate the eEPSCs during low-frequency tractus stimulation in our preparation. The NMDA receptor-selective antagonist AP-5 at 50 µM did not significantly change the amplitudes of eEPSCs (–189.4 ± 19.6 versus –185.5 ± 17.2 pA; p > 0.05; n = 10). Previous work in our laboratory has shown this concentration blocked currents evoked by NMDA (P. M. de Paula and S. W. Mifflin, unpublished observation). Furthermore, this concentration of AP-5 abolished NMDA receptor-mediated neuronal responses evoked by ST stimulation (Aylwin et al., 1997). During coapplication of 50 µM AP-5 and 40 µM PE, the amplitude of eEPSCs was not significantly different from that measured during application of PE alone (32 ± 4 versus 33 ± 5%; p > 0.05; n = 5).

	Discussion

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The effects of activation of ₁-adrenoreceptors on synaptic transmission to cNTS neurons were examined. Second-order neurons in cNTS relay chemoreceptor inputs to other neural structures throughout the CNS, and they modulate autonomic, respiratory, and hormonal responses to hypoxia. The results showed that activation of ₁-adrenoreceptors inhibits glutamatergic excitatory inputs and increases GABAergic inhibitory inputs to the second-order neurons in cNTS. Both effects of PE were blocked by the ₁-adrenoreceptor antagonist prazosin. The effect of PE on synaptic transmission seems to be mediated primarily via presynaptic mechanisms. Alterations in ₁-adrenoreceptor inhibition of excitatory glutamatergic synaptic transmission were independent of GABA_B and NMDA receptors. The results suggest that the overall effect of activation of ₁-adrenoreceptors is the inhibition of peripheral chemoreceptor inputs to cNTS neurons by a combined decrease in excitatory inputs and increase in inhibitory inputs.

This is the first study of the role of ₁-adrenoreceptors in synaptic transmission of peripheral chemoreceptor inputs in cNTS. In many central neural structures, activation of ₁-adrenoreceptors has been shown to cause a depolarization through the inhibition of a resting potassium conductance (Aghajanian, 1985). The ₁-adrenoreceptor is a G protein-coupled receptor. Activation of ₁-adrenoreceptors promotes phospholipase C activation and increases the level of biologically available calcium in the cytoplasm. Therefore, a direct effect of ₁-adrenoreceptor activation would be expected to increase neuronal excitability in cNTS. However, the present results strongly suggest that in second-order neurons in cNTS the primary target of ₁-adrenoreceptor activation is at a presynaptic site. This is supported by the lack of any significant changes in input resistance, holding current, or the I-V relationship after PE application. The effect of ₁-adrenoreceptor activation in cNTS is mediated by modulating the balance between excitatory and inhibitory inputs to the second-order neurons, rather than directly affecting membrane properties of the neurons.

The neuronal circuits that mediate peripheral chemoreflexes rely on glutamatergic transmission from afferent terminals to second-order neurons in cNTS (Zhang and Mifflin, 1993; Sapru, 1996). Our results suggest that activation of ₁-adrenoreceptors reduces excitatory inputs to second-order neurons by decreasing the release of glutamate from afferent terminals as demonstrated by reduced eEPSCs amplitudes after PE application. This result suggests that ₁-adrenoreceptors are localized on primary afferent terminal since activation of ₁-adrenoreceptors decreased monosynaptic eEPSCs. The presynaptic mechanism was further confirmed by the findings of a reduced PPR of eEPSCs and reduced frequency of mEPSCs. There are reports of presynaptic inhibition of glutamate release by ₁-adrenoreceptors (Scanziani et al., 1993; Kirkwood et al., 1999). The neural mechanisms are still not clear.

Several studies suggest that indirect mechanisms may be involved in PE-mediated inhibition of excitatory inputs. In hypothalamic hypocretin neurons, PE inhibited spontaneous discharge without affecting resting membrane potential, which was suggested to be due to increased bicuculline-sensitive inhibition (Li and van den Pol, 2005). In the current study, we demonstrated that activation of ₁-adrenoreceptors increased release of GABA from GABAergic terminals in cNTS. However, decreased excitatory synaptic inputs were observed in the presence of BIC, indicating that noradrenergic modulation of GABA_A receptor-mediated transmission was not involved in the reduction of excitatory inputs. Increased extracellular GABA in the NTS could spread to afferent terminals and activate presynaptic GABA_B receptors thus inhibiting the release of glutamate (Isaacson et al., 1993). Our data with the GABA_B receptor antagonist did not support this possibility and further demonstrated that peripheral chemoreceptor synaptic transmission in cNTS was not under the tonic influence of GABA_B receptors in our preparation. Activation of ₁-adrenoreceptors in visual cortex elicited a long-term synaptic depression which was completely blocked by NMDA receptor blocker AP-5 (Kirkwood et al., 1999). We found that NMDA receptors did not mediate PE-induced inhibition of eEPSCs. A few studies reported that activation of ₁-adrenoreceptors can attenuate Ca²⁺ currents (Calcagnotto and Baraban, 2003; Li and van den Pol, 2005). Since release of neurotransmitters relies on presynaptic Ca²⁺ entry, this may explain PE-mediated inhibition on EPSCs in our preparation. We examined receptor systems most likely to mediate indirect PE effects based on work in other systems: GABA and NMDA. It is well beyond the scope of this study to analyze every potential neurotransmitter and neuromodulator receptor system that could alter synaptic inputs to NTS neurons (e.g., adenosine, metabotropic glutamate, serotonin, tachykinin, vasopressin, and oxytocin). Future studies will be needed to elucidate the mechanisms of the PE-mediated inhibition observed in current project.

We further demonstrated that in cNTS activation of ₁-adrenoreceptors increased amplitudes of eIPSCs and the frequency of mIPSCs and decreased the PPR of eIPSCs, suggesting the activation of local GABAergic neurons and inhibitory GABAergic inputs to second-order neurons. This PE-induced increase in GABA release was not action potential-dependent since mIPSCs recordings were observed in the presence of TTX. This increase in synaptic inhibition has been observed in other sites in CNS (McCormick and Wang, 1991; Alreja and Liu, 1996; Li and van den Pol, 2005). Activation of ₁-adrenoreceptors usually depolarizes neurons and increases neuronal excitability (Aghajanian, 1985). These GABAergic terminals could originate from local interneurons or from inputs to NTS from other brain structures. Therefore, ₁-adrenoreceptor-mediated activation of GABAergic neurons could provide increased inhibitory inputs to second-order neurons, and damp excitatory inputs from peripheral chemoreceptors.

This study focused on the role of ₁-adrenoreceptors in synaptic modulation of peripheral chemoreceptor afferent input integration in the NTS. It is not yet clear where the exact source of NE that activates NTS adrenoreceptors originates. Various central noradrenergic neural structures in the brain, including A1, A2, A5, and A6 noradrenergic cell groups, are activated by peripheral chemoreceptor stimulation (Erickson and Millhorn, 1994; Smith et al., 1995; Teppema et al., 1997; Buller et al., 1999). The noradrenergic projections from these neural structures or other brain sites, including paraventricular nucleus, could target primary afferent localized adrenoreceptors and modulate chemoreceptor afferent input integration in the NTS.

The results of this study place the NTS ₁-adrenoreceptors in a unique position. Unlike ₁-adrenoreceptors, activation of ₂-adrenoreceptors inhibits both excitatory and inhibitory inputs to second-order peripheral chemoreceptors neurons in cNTS (our unpublished observation). Thus, the final effect of ₂-adrenoreceptor activation on synaptic transmission depends on the balance between these two inputs to a given neuron at any point in time. Activation of ₁-adrenoreceptors seems capable of biasing this balance toward inhibition. Therefore, we would predict activation of ₁-adrenoreceptors in cNTS should attenuate chemoreflex responses, similar to the effect of PE in medial NTS (Feldman and Moises, 1988; Feldman and Felder, 1989). The physiological roles of NTS ₁-adrenoreceptors in peripheral chemoreflexes have yet to be explored. Long-term activation of peripheral chemoreceptors could change the role of different subtypes of adrenoreceptors in NE-induced responses. Chronic intermittent cold exposure significantly increased ₁-adrenoreceptor-mediated responses in the paraventricular nucleus of the hypothalamus without significant changes in extracellular NE levels, suggesting enhanced ₁-adrenoreceptor sensitivity (Ma and Morilak, 2005). Chronic sustained hypoxia increases the activity and expression of tyrosine hydroxylase in the NTS (Soulier et al., 1992; Schmitt et al., 1994). If this results in increased NE release within the NTS, ₁-adrenoreceptor-mediated inhibition could provide neuroprotection to second-order neurons receiving increased excitatory afferent inputs from peripheral chemoreceptors. However, a reduced sensitivity of ₁-adrenoreceptor or receptor number in cNTS could enhance chemoreflexes. Further studies will be needed to investigate the long-term effects of chronic hypoxia on NTS ₁-adrenoreceptor function.

In summary, activation of ₁-adrenoreceptors inhibits synaptic transmission of peripheral chemoreceptor inputs in cNTS. This inhibitory effect is achieved by reducing glutamatergic excitatory inputs and increasing GABAergic inhibitory inputs to second-order peripheral chemoreceptor neurons in cNTS. These results suggest that activation of ₁-adrenoreceptors acts via presynaptic mechanisms to bias afferent inputs to NTS neurons receiving arterial chemoreceptor inputs toward inhibition by decreasing excitation and increasing inhibition. This inhibitory biasing could lead to reduced or normalized reflex responses to hypoxia. The modulation of ₁-adrenoreceptor function in pathological conditions could affect the physiological responses to hypoxia.

	Acknowledgements

 
We acknowledge expert technical assistance from Jaci Castania, Myrna Herrera-Rosales, and Melissa Vitela.

	Footnotes

 
This work was supported by National Institutes of Health Grant HL-41894.

Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.

doi:10.1124/jpet.106.114033.

ABBREVIATIONS: NE, norepinephrine; CNS, central; nervous system; cNTS, caudal nucleus tractus solitarius; NTS, nucleus tractus solitarius; DiA, 1,1'-dilinoleyl-3,3,3',3'-tetra-methylindocarbocyanine, 4-chlorobenzenesulphonate; aCSF, artificial cerebrospinal fluid; IPSC, inhibitory postsynaptic current; EPSC, excitatory postsynaptic current; eIPSC, evoked inhibitory postsynaptic current; eEPSC, evoked excitatory postsynaptic current; ST, solitary tract; PE, phenylephrine; PPR, paired pulse ratio; NMDA, N-methyl-D-aspartic acid; CNQX, 6-cyano-7-nitroquinoxaline-2,3-dione; mEPSC, miniature, excitatory postsynaptic current; mIPSC, miniature inhibitory postsynaptic current; TTX, tetrodotoxin; I-V, current-voltage; AP-5, phosphonopentanoic acid; K-S, Kolmogorov-Smirnov; SCH-50911, (+)-5,5-dimethyl-2-morpholineacetic acid hydrochloride.

Address correspondence to: Dr. Steve W. Mifflin, Department of Pharmacology, MSC 7764, The University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Dr., San Antonio, TX 78229-3900. E-mail: mifflin{at}uthscsa.edu

	References

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