GABA- and glycine-mediated inhibitory postsynaptic potentials in neonatal rat rostral ventrolateral medulla neurons in vitro

Effects of centrally administered glucagon-like peptide-2 on blood pressure and barosensitive neurons in spontaneously hypertensive rats

The central administration of glucagon-like peptide-2 (GLP-2) decreases blood pressure in rats. In the present study, we investigated the hypotensive effects of GLP-2 using spontaneously hypertensive rats (SHRs), an animal model of hypertension. The central administration of GLP-2 (0.6 μg) decreased mean arterial pressure (MAP) in SHRs (-24.1 ± 4.5%; P < 0.05), but not in normotensive Wistar-Kyoto (WKY) rats (-10.6 ± 7.4%; P > 0.05), whereas GLP-2 (6 μg) decreased MAP in WKY rats (-23.5 ± 4.2%; P < 0.05) and SHRs (-46.7 ± 11.6%; P < 0.01) under anesthesia with urethane and α-chloralose. Histological analyses revealed that the central administration of GLP-2 (6 μg) induced Fos immunoreactivity (Fos-IR) in the hypothalamic and medullary areas in WKY rats and SHRs. However, the distribution of Fos-IR in GABAergic neurons in the rostral ventrolateral medulla (RVLM) differed between WKY rats and SHRs. GLP-2 directly modulated the excitability of RVLM neurons in brainstem slices from SHRs, but not WKY rats. These results suggest that neuronal activity through the activation of GLP-2 receptors in the RVLM contributes to lowering blood pressure in SHRs.

Expression and functions of β1- and β2-adrenergic receptors on the bulbospinal neurons in the rostral ventrolateral medulla

The expression and effects of β-adrenergic receptors (β-ARs) on the neurons of the bulbospinal rostral ventrolateral medulla (RVLM) have been limitedly examined to date. The objective of this study was to examine the expression of β1- and β2-ARs on the bulbospinal RVLM neurons electrophysiologically and histologically. To directly investigate whether RVLM neurons display sensitivity to metoprolol (a β1-AR antagonist), dobutamine (a β1-AR agonist), butoxamine (a β2-AR antagonist), and salbutamol (a β2-AR agonist), we examined changes in the membrane potentials of the bulbospinal RVLM neurons using the whole-cell patch-clamp technique during superfusion of these drugs. During metoprolol superfusion, 16 of the 20 RVLM neurons were hyperpolarized, and 5 of the 6 RVLM neurons were depolarized during dobutamine superfusion. During butoxamine superfusion, 11 of the 16 RVLM neurons were depolarized, and all of the 8 RVLM neurons were hyperpolarized during salbutamol superfusion. These results suggest the expression of β1- and β2-ARs on the RVLM neurons. To determine the presence of β1- and β2-ARs histologically, immunofluorescence examination was performed. Five metoprolol-hyperpolarized neurons were examined for β1-AR and tyrosine hydroxylase (TH) immunoreactivity. All of the neurons displayed β1-AR immunoreactivity, whereas three of the neurons displayed TH immunoreactivity. All of the five RVLM neurons that became depolarized during metoprolol superfusion and hyperpolarized during butoxamine superfusion displayed β1- and β2-AR immunoreactivity. Our findings suggest that β1-AR antagonists or β2-AR agonists may decrease blood pressure through decreasing the activity of the bulbospinal RVLM neurons.

Developmental changes in GABAergic neurotransmission to presympathetic and cardiac parasympathetic neurons in the brainstem

Cardiovascular function is regulated by a dynamic balance composed of sympathetic and parasympathetic activity. Sympathoexcitatory presympathetic neurons (PSNs) in the rostral ventrolateral medulla project directly to cardiac and vasomotor sympathetic preganglionic neurons in the spinal cord. In proximity to the PSNs in the medulla, there are preganglionic cardiac vagal neurons (CVNs) within the nucleus ambiguus, which are critical for parasympathetic control of heart rate. Both CVNs and PSNs receive GABAergic synaptic inputs that change with challenges such as hypoxia and hypercapnia (H/H). Autonomic control of cardiovascular function undergoes significant changes during early postnatal development; however, little is known regarding postnatal maturation of GABAergic neurotransmission to these neurons. In this study, we compared changes in GABAergic inhibitory postsynaptic currents (IPSCs) in CVNs and PSNs under control conditions and during H/H in postnatal day 2-5 (P5), 16-20 (P20), and 27-30 (P30) rats using an in vitro brainstem slice preparation. There was a significant enhancement in GABAergic neurotransmission to both CVNs and PSNs at age P20 compared with P5 and P30, with a more pronounced increase in PSNs. H/H did not significantly alter this enhanced GABAergic neurotransmission to PSNs in P20 animals. However, the frequency of GABAergic IPSCs in PSNs was reduced by H/H in P5 and P30 animals. In CVNs, H/H elicited an inhibition of GABAergic neurotransmission in all ages studied, with the most pronounced inhibition occurring at P20. In conclusion, there are critical development periods at which significant rearrangement occurs in the central regulation of cardiovascular function.

Importance of rostral ventrolateral medulla neurons in determining efferent sympathetic nerve activity and blood pressure

Accentuated sympathetic nerve activity (SNA) is a risk factor for cardiovascular events. In this review, we investigate our working hypothesis that potentiated activity of neurons in the rostral ventrolateral medulla (RVLM) is the primary cause of experimental and essential hypertension. Over the past decade, we have examined how RVLM neurons regulate peripheral SNA, how the sympathetic and renin-angiotensin systems are correlated and how the sympathetic system can be suppressed to prevent cardiovascular events in patients. Based on results of whole-cell patch-clamp studies, we report that angiotensin II (Ang II) potentiated the activity of RVLM neurons, a sympathetic nervous center, whereas Ang II receptor blocker (ARB) reduced RVLM activities. Our optical imaging demonstrated that a longitudinal rostrocaudal column, including the RVLM and the caudal end of ventrolateral medulla, acts as a sympathetic center. By organizing and analyzing these data, we hope to develop therapies for reducing SNA in our patients. Recently, 2-year depressor effects were obtained by a single procedure of renal nerve ablation in patients with essential hypertension. The ablation injured not only the efferent renal sympathetic nerves but also the afferent renal nerves and led to reduced activities of the hypothalamus, RVLM neurons and efferent systemic sympathetic nerves. These clinical results stress the importance of the RVLM neurons in blood pressure regulation. We expect renal nerve ablation to be an effective treatment for congestive heart failure and chronic kidney disease, such as diabetic nephropathy.

Identification of neurotransmitters and co-localization of transmitters in brainstem respiratory neurons

Identifying the major ionotropic neurotransmitter in a respiratory neuron is of critical importance in determining how the neuron fits into the respiratory system, whether in producing or modifying respiratory drive and rhythm. There are now several groups of respiratory neurons whose major neurotransmitters have been identified and in some of these cases, more than one transmitter has been identified in particular neurons. This review will describe the physiologically identified neurons in major respiratory areas that have been phenotyped for major ionotropic transmitters as well as those where more than one transmitter has been identified. Although the purpose of the additional transmitter has not been elucidated for any of the respiratory neurons, some examples from other systems will be discussed.

RVLM glycine receptors mediate GABAA and GABAB)independent sympathoinhibition from CVLM in rats

The caudal ventrolateral medulla (CVLM) provides tonic inhibitory and also excitatory inputs to the rostral ventrolateral medulla (RVLM). These experiments evaluated the role of RVLM gamma-amino butyric acid (GABA) receptor subtypes and glycine receptors in mediating CVLM sympathoinhibition. In Inactin anesthetized female rats, the CVLM and RVLM were functionally defined by pressor and depressor responses to microinjected GABA (500 pmol, 50 nl). Although reduced, pressor and sympathoexcitatory responses due to inhibition of the CVLM with GABA persisted following ipsilateral RVLM GABA(A) receptor blockade (bicuculline, BIC, 400 pmol, 100 nl; n=12) in rats with contralateral nucleus tractus solitarius (NTS) lesion. In the presence of either ipsilateral (+contralateral NTS lesion; n=8) or bilateral (n=6) GABA(A) and GABA(B) receptor blockade of the RVLM (400 pmol BIC+400 pmol CGP35348, 100 nl), inhibition of the CVLM still increased MAP and renal sympathetic nerve activity (RSNA). Thus neither GABA(B) receptors nor a contralateral CVLM to RVLM GABAergic pathway explains residual responses to CVLM blockade. The addition of strychnine (300 pmol, 100 nl) to the RVLM eliminated responses to CVLM inhibition, suggesting that a GABA(A) and GABA(B) independent sympathoinhibitory influence from CVLM to RVLM is mediated by glycine receptors. Decreases in MAP and RSNA due to activation of the CVLM with glutamate (500 pmol, 50 nl) were reversed to increases in the presence of RVLM GABA(A) receptor blockade (n=7). Thus, a sympathoexcitatory pathway from the CVLM can be activated in the presence of RVLM GABA receptor blockade, but sympathoinhibitory influences from the CVLM predominate.

Cardiovascular effects produced by activation of GABA receptors in the rostral ventrolateral medulla of conscious rats

The rostral ventrolateral medulla (RVLM) has been proposed as a region playing a major role in the tonic and reflex control of sympathetic vasomotor activity and blood pressure. Pharmacological activation of GABA(A) receptors with muscimol in the RVLM of anesthetized rats results in a large fall in mean arterial pressure (MAP), heart rate (HR) and sympathetic activity. In this study we evaluated the effects of activation of GABA receptors in the RVLM of conscious, freely moving rats. Bilateral microinjections of muscimol into the RVLM of conscious rats produced a large fall in MAP (-38+/-4 mm Hg, n=7) when compared with saline injections (NaCl 0.9%, 7+/-1 mm Hg, n=4). The decrease in MAP evoked by muscimol was accompanied by a significant increase in HR (muscimol 69+/-13 bpm vs. vehicle -33+/-12 bpm, P<0.05), an effect that was completely abolished by beta1 adrenergic receptor blockade. Conversely, bilateral microinjections of GABA(B) agonist, baclofen, evoked a pressor response, but in this case, the increase was not significantly different from that evoked by vehicle injections. These results 1) indicate that GABA(A) receptors have a powerful influence on the resting activity of RVLM neurons in conscious rats; 2) indicate that a compensatory sympathetic-mediated tachycardia is present after inhibition of RVLM neurons in conscious rats; 3) confirm and extend previous findings showing that RVLM neurons are critical for blood pressure maintenance even in normal non-anesthetized conditions.

Acute tolerance to ethanol inhibition of NMDA-induced responses in rat rostral ventrolateral medulla neurons

The present study was performed to examine the effects of acute ethanol exposure on N-methyl-D-aspartate (NMDA)-induced responses and the development of acute tolerance in rat rostral ventrolateral medulla (RVLM) in vivo and in vitro. Repeated microinjections of NMDA (0.14 nmol) into the RVLM every 30 min caused reproducible increases in mean arterial pressure in urethane-anesthetized rats weighing 325-350 g. Intravenous injections of ethanol (0.16 or 0.32 g, 1 ml) inhibited NMDA-induced pressor effects in a blood-concentration-dependent and reversible manner. The inhibitory effect of ethanol was reduced over time during continuous infusion of ethanol or on the second injection 3.5 h after prior injection of a higher dose of ethanol (0.32 g). A high dose of ethanol (0.32 g) had no significant effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, gamma-aminobutyric acid and glycine-induced changes in blood pressure. In vitro studies showed that ethanol (10- 100 mM) dose-dependently inhibited inward currents elicited by pressure ejection of NMDA (10 mM) in RVLM neurons of neonatal brainstem slice preparations. When the superfusion time of ethanol (100 mM) was increased to 50 min, its inhibitory effect decreased gradually after 30-40 min in 60% of RVLM neurons examined. These data suggested that ethanol inhibition and subsequent tolerance development is associated with changed sensitivity to NMDA in the RVLM, which may play important roles in the ethanol regulation of cardiovascular function.

Detection of amino acid and peptide transmitters in physiologically identified brainstem cardiorespiratory neurons

Most of the CNS neurons that regulate circulation and respiration reside in regions of the brain characterized by extreme cellular heterogeneity (nucleus of the solitary tract, reticular formation, parabrachial nuclei, periaqueductal gray matter, hypothalamus, etc.). The chemical neuroanatomy of these regions is correspondingly complex and teasing out specific circuits in their midst remains a problem that is usually very difficult if not impossible to solve by conventional tract-tracing methods, Fos methodology or electrophysiology in slices. In addition, identifying the type of amino acid or peptide transmitter used by electrophysiologically recorded neurons has been until recently an especially difficult task either for lack of a specific marker or because such markers (many peptides for example) are exported to synaptic terminals and thus undetectable in neuronal cell bodies. In this review, we describe a general purpose method that solves many of these problems. The approach combines juxtacellular labeling in vivo with the histological identification of mRNAs that provide definitive neurochemical phenotypic identification (e.g. vesicular glutamate transporter 1 or 2, glutamic acid decarboxylase). The results obtained with this method are discussed in the general context of amino acid transmission in brainstem cardiorespiratory pathways. The presence of markers of amino acid transmission in specific aminergic pre-sympathetic neurons is especially emphasized as is the extensive co-localization of markers of GABAergic and glycinergic transmission in the brainstem reticular formation.

Maintenance of sympathetic tone by a nickel chloride-sensitive mechanism in the rostral ventrolateral medulla of the adult rat

In urethane-anaesthetised artificially ventilated Sprague-Dawley rats, bilateral microinjection of the divalent cation nickel chloride (Ni(2+); 50 mM, 50 nl) into the rostral ventrolateral medulla elicited a dramatic inhibition of splanchnic sympathetic nerve activity (-44+/-6%) and a marked depressor response (-35+/-7 mmHg). Selective blockade of high-voltage activated Ca(2+) channels with omega-agatoxin IVA (P/Q-type), omega-conotoxin GVIA (N-type) and nifedipine (L-type) did not decrease arterial pressure or splanchnic sympathetic nerve activity when injected separately into the rostral ventrolateral medulla, or combined with kynurenate. Injection of caesium chloride or ZD 7288, a blocker of the hyperpolarization-activated cation current, into the rostral ventrolateral medulla had no effect on arterial pressure or splanchnic sympathetic nerve activity. Bilateral microinjection of nickel chloride into the caudal ventrolateral medulla/pre-Bötzinger complex elicited small increases in splanchnic sympathetic nerve activity (+17+/-13%) and arterial pressure (+12+/-4 mmHg). These were substantially smaller than those evoked by blockade of glutamatergic receptors or high-voltage activated Ca(2+) channels in this area. Injection of kynurenate or high-voltage activated Ca(2+) channel blocker, but not Ni(2+), in this area evoked respiratory termination. The results indicate the existence of a distinct mechanism maintaining the tonic activity of rostral ventrolateral medulla presympathetic neurons that is different from that maintaining the tonic activity in the caudal ventrolateral medulla/pre-Bötzinger region. We conclude that ion channels that are sensitive to Ni(2+), but are insensitive to high-voltage activated (L, P/Q, N) Ca(2+) channel blockers, and are located postsynaptically on the presympathetic rostral ventrolateral medulla neurons are responsible for the tonic activity of the presympathetic neurons in rostral ventrolateral medulla. These channels could well be the low-voltage-activated (or T-type) Ca(2+) channels although other conductances cannot be conclusively excluded.

Glutamate receptor subunit immunoreactivity in neurons of the rat rostral ventrolateral medulla

Immunohistochemical studies were conducted to assess the subunits of ionotropic and metabotropic glutamate receptor present in the rostral ventrolateral medulla (RVLM) of the rat. Double labeling the medullary sections with polyclonal GluR1, GluR2/3, GluR4, NMDAR1, NMDAR2A/B, mGluR1alpha, and mGluR2/3 antiserum and monoclonal tyrosine hydroxylase (TH) antiserum revealed nearly all TH immunoreactive (irTH) cells and many TH-negative neurons were immunoreactive to GluR2/3 (irGluR2/3), NMDAR1 (irNMDAR1), and NMDAR2A/B (irNMDAR2A/B). A few RVLM neurons were immunoreactive to GluR1 (irGluR1) and GluR4 (irGluR4), but they were generally TH-negative. Immunoreactivity to mGluR1alpha (irmGluR1alpha) appeared to be localized exclusively to fiber-like elements in the RVLM area. Our results show that neurons in the RVLM, including irTH, are endowed mainly with GluR2/3 and NMDAR1 or NMDAR2A/B ionotropic receptor subunits, and that irmGluR1alpha splice variant appears to be located on nerve fibers ramifying within the RVLM. Moreover, TH-negative neurons in the RVLM appear to bear similar subunits of ionotropic glutamate receptors.

Evidence for a tonic GABA-ergic inhibition of excitatory respiratory-related afferents to presympathetic neurons in the rostral ventrolateral medulla

The effect of blockade of ionotropic GABA and glutamate receptors in the rostral ventrolateral medulla (RVLM) on the relationship between phrenic nerve, splanchnic sympathetic nerve and lumbar sympathetic nerve activities was examined in urethane anesthetized, paralyzed and vagotomized Sprague-Dawley rats. Bilateral microinjection of the GABA-A receptor antagonist, bicuculline (4 mM, 100 nl), into the RVLM dramatically, and almost exclusively, increased the post-inspiratory related discharge in both splanchnic sympathetic nerve and lumbar sympathetic nerve activities and elicited hypertension with fluctuations of arterial pressure phase locked to the discharge of the phrenic nerve. Subsequent bilateral microinjection of kynurenate, a non-selective ionotropic excitatory amino acid receptor antagonist (50 mM, 100 nl), into the RVLM significantly attenuated the sympathoexcitation and hypertension evoked by injection of bicuculline. This was accompanied by an abolition of the post-inspiratory related burst discharge of splanchnic sympathetic nerve and lumbar sympathetic nerve activities. These data suggest that the GABAergic inputs to RVLM tonically inhibit glutamatergic inputs from central respiratory neurons that normally act to increase the firing of presympathetic neurons in the RVLM. Inputs from post-inspiratory neurons appear to be an especially potent excitatory synaptic drive to the presympathetic neurons in the absence of the GABAergic inhibition.

Orexins: a role in medullary sympathetic outflow

Orexin A and B, also known as hypocretin 1 and 2, are two recently isolated hypothalamic peptides. As orexin-containing neurons are strategically located in the lateral hypothalamus, which has long been suspected to play an important role in feeding behaviors, initial studies were focused on the involvement of orexins in positive food intake and energy metabolism. Recent studies implicate a more diverse biological role of orexins, which can be manifested at different level of the neuraxis. For example, canine narcolepsy, a disorder with close phenotypic similarity to human narcolepsy, is caused by a mutation of hypocretin receptor 2 gene. Results from our immunohistochemical and functional studies, which will be summarized here, suggest that the peptide acting on neurons in the rostral ventrolateral medulla augment sympathoexcitatory outflow to the spinal cord. This finding is discussed in the context of increased sympathetic activity frequently associated with obesity.

Three types of putative presympathetic neurons in the rostral ventrolateral medulla studied with rat brainstem-spinal cord preparation

To study the electrophysiological properties of presympathetic neurons in the rostral ventrolateral medulla (RVLM), intracellular recordings were performed by the whole-cell patch-clamp technique. We utilized the neonatal rat brainstem-spinal cord preparation, in which the sympathetic neuronal network is thought to be preserved, unlike in slice preparation. In response to stimulation in the ipsilateral Th2 spinal segment including intermediolateral cell column (IML), 33 of 151 non-respiratory RVLM neurons showed antidromic action potentials with a constant latency of 45 ms, and can be considered as presympathetic neurons. We classified and characterized the RVLM presympathetic neurons into three types: 'regularly firing neurons (n=7)', which showed ramp depolarization and frequent action potentials (4.2+/-0.9 spikes/s) with rare excitatory postsynaptic potentials (EPSPs); 'irregularly firing neurons (n=21)', which exhibited many EPSPs that modulated the firing rate; and 'silent-type neurons (n=5)', which discharged action potentials only during current-induced depolarization. Lucifer-Yellow staining showed that the irregularly firing neurons were significantly larger and had more dendrites than the regularly firing neurons. All regularly firing neurons retained their discharges during low-Ca2+ -high-Mg2+ superfusion that blocks synaptic input, whereas the discharges in 11 of 16 irregularly firing neurons were abolished, suggesting that the regularly firing neurons discharged independently of synaptic input. Seven of 31 RVLM neurons were hyperpolarized by stimulation of vagal afferent nerves. In summary, three types of RVLM presympathetic neurons were characterized by the patch-clamp technique in the brainstem-spinal cord preparation, in which the connection was preserved from vagal afferent to the Th2 spinal segment through the RVLM. Since antidromic action potentials were demonstrated by stimulation in the Th2 spinal segment in 33 neurons of all three types, all types of RVLM neurons constitute a part of the sympathetic neuronal network.

Serotonin modulates synaptic transmission in immature rat ventrolateral medulla neurons in vitro

Patch-clamp recordings in whole-cell configuration were made from ventrolateral medulla neurons of brainstem slices from 8-12-day-old rats. 5-Hydroxytryptamine (3-30 microM) concentration-dependently suppressed excitatory and inhibitory postsynaptic currents evoked by focal stimulation. An augmentation of inhibitory synaptic currents by 5-hydroxytryptamine was noted in a small number of neurons. 5-Hydroxytryptamine depressed synaptic currents with or without causing a significant change in holding currents and membrane conductances; the inward or outward currents induced by exogenously applied glutamate or GABA/glycine were also not significantly changed by 5-hydroxytryptamine. In paired-pulse paradigms designed to evaluate a presynaptic site of action, 5-hydroxytryptamine suppressed synaptic currents but enhanced the paired-pulse facilitation. 5-Hydroxytryptamine reduced the frequency of miniature excitatory postsynaptic currents without significantly affecting the amplitude. 5-Carboxamidotryptamine, 8-hydroxy-2(di-n-propylamino)tetralin, sumatriptan and N-(3-trifluoromethylphenyl)piperazine which exhibit 5-hydroxytryptamine1 receptor agonist activity, depressed synaptic currents with different potencies, with 5-carboxamidotryptamine being the most potent. The non-selective 5-hydroxytryptamine1 receptor antagonist pindolol attenuated the presynaptic effect of 5-hydroxytryptamine, whereas the 5-hydroxytryptamine1A antagonist pindobind-5-hydroxytryptamine1A and 5-hydroxytryptamine2 receptor antagonist ketanserin were ineffective. Our results indicate that 5-hydroxytryptamine suppressed synaptic transmission in ventrolateral medulla neurons by activating presynaptic 5-hydroxytryptamine1 receptors, probably the 5-hydroxytryptamine1B/5-hydroxytryptamine1D subtype. In addition, 5-hydroxytryptamine augmented inhibitory synaptic currents in a small number of neurons the site and mechanism of this potentiating action are not known.

5-HT modulates multiple conductances in immature rat rostral ventrolateral medulla neurones in vitro

1. Whole-cell patch-clamp recordings were made from rostral ventrolateral medulla (RVLM) neurones of brainstem slices from 8- to 12-day-old rats. In the presence of tetrodotoxin (0.5 microM), 5-HT (50 microM) elicited an outward current (I5-HT,outward) (10/44 neurones) associated with an increase in membrane conductance, and an inward current (I5-HT,inward) (29/44 neurones) accompanied by a decrease or no significant change in membrane conductance. 2. The steady-state I-V relationship of I5-HT,outward showed an inward rectification; the 5-HT-induced current, which reversed at -87.9 +/- 3.0 mV, was suppressed by 0.1 mM Ba2+. 3. Two types of steady-state I-V relationship for I5-HT,inward were noted: type I I5-HT,inward was characterized by a significant decrease in membrane conductance and reversed at a potential close to or negative to the theoretical K+ equilibrium potential (EK), -94 mV, in 8/17 neurones; type II I5-HT,inward was not associated with a significant change in membrane conductance and was relatively independent of membrane potential. 4. Both type I and type II I5-HT,inward were significantly reduced in a low [Na+]o solution. In this solution, I5-HT,inward decreased with hyperpolarization and had a linear steady-state I-V relationship with a reversal potential of approximately -110 mV. The reversal potential of type I I5-HT,inward shifted to about -80 mV as the [K+]o was increased from 3.1 to 7.0 mM in low [Na+]o solution. The type II I5-HT,inward did not reverse at the estimated EK in the same solution. 5. While not affected by externally applied Cs+ (1 mM), I5-HT,inward was significantly smaller in RVLM neurones patched with Cs+-containing electrodes; the current reversed at -11.9 +/- 6.4 mV in 8/15 responsive neurones. 6. It may be concluded that in rat RVLM neurones 5-HT increases an inwardly rectifying K+ conductance which may underlie the I5-HT, outward and that a combination of varying degrees of K+ conductance decrease and a Cs+-insensitive, non-selective cation conductance increase may account for the two types of conductance change associated with I5-HT,inward.

5-Hydroxytryptamine responses in immature rat rostral ventrolateral medulla neurons in vitro

Whole cell patch recordings were made from rostral ventrolateral medullar (RVLM) neurons of brain-stem slices from 8- to 12-day-old rats. By superfusion or pressure ejection to RVLM neurons, 5-hydroxytryptamine (5-HT) elicited three types of membrane potential changes: a slow hyperpolarization (5-HTH), a slow depolarization (5-HTD) and a biphasic response, which persisted in a tetrodotoxin (TTX, 0.3 microM)-containing solution. 5-HTH were accompanied by a decrease of input resistance in the majority of responsive neurons. Hyperpolarization reduced and depolarization increased the 5-HTH; the mean reversal potential was -92.3 mV in 3.1 mM and shifted to -69.3 mV in 7 mM [K+]o. Barium (Ba2+, 0.1 mM) but not tetraethylammonium (TEA, 10 mM) suppressed 5-HTH. The 5-HT1A receptor agonist (+/-)-8-hydroxy-dipropylamino-tetralin (8-OH-DPAT; 5-50 microM) hyperpolarized RVLM neurons. The 5-HT1A antagonist pindobind-5-HT1A (PBD; 1-3 microM) and the 5-HT2/5-HT1 receptor antagonist spiperone (1-10 microM) suppressed 5-HTH and the hyperpolarizing phase of biphasic responses; the 5-HT2 receptor antagonist ketanserin (3 microM) was without significant effect. 5-HTD were associated with an increase or no apparent change of input resistance in RVLM neurons. Hyperpolarization of the membrane decreased or caused no apparent change in 5-HTD. 5-HTD were reduced in an elevated [K+]o (7.0 mM) solution and > 60% in a low Na+ (26 mM) solution and were not significantly changed in a low Cl- (6.7 mM) or Ca(2+)-free/high Mg2+ (10.9 mM) solution. The 5-HT2 receptor agonist alpha-methyl-5-HT (50 microM) depolarized RVLM neurons, and the 5-HT2 antagonist ketanserin (1-10 microM) attenuated the 5-HTD and the depolarizing phase of biphasic responses, whereas the 5-HT1A receptor antagonist PBD (2 microM) was without effect. Inclusion of the hydrolysis resistant guanine nucleotide GDP-beta-S in patch solution significantly reduced the 5-HTH as well as the 5-HTD. The present study shows that, in the immature rat RVLM neurons, 5-HT causes a slow hyperpolarization and depolarization probably by interacting with 5-HT1A and 5-HT2 receptors, which are G-proteins coupled. 5-HTH may involve an increase of an inwardly rectifying K+ conductance, and 5-HTD appear to be caused by a decrease of K+ conductance and/or increase of nonselective cation conductance.