Alpha2-adrenoceptor-mediated presynaptic inhibition in bulbospinal neurons of rostral ventrolateral medulla

Optogenetic identification of hypothalamic orexin neuron projections to paraventricular spinally projecting neurons

Orexin neurons, and activation of orexin receptors, are generally thought to be sympathoexcitatory; however, the functional connectivity between orexin neurons and a likely sympathetic target, the hypothalamic spinally projecting neurons (SPNs) in the paraventricular nucleus of the hypothalamus (PVN) has not been established. To test the hypothesis that orexin neurons project directly to SPNs in the PVN, channelrhodopsin-2 (ChR2) was selectively expressed in orexin neurons to enable photoactivation of ChR2-expressing fibers while examining evoked postsynaptic currents in SPNs in rat hypothalamic slices. Selective photoactivation of orexin fibers elicited short-latency postsynaptic currents in all SPNs tested (n = 34). These light-triggered responses were heterogeneous, with a majority being excitatory glutamatergic responses (59%) and a minority of inhibitory GABAergic (35%) and mixed glutamatergic and GABAergic currents (6%). Both glutamatergic and GABAergic responses were present in the presence of tetrodotoxin and 4-aminopyridine, suggesting a monosynaptic connection between orexin neurons and SPNs. In addition to generating postsynaptic responses, photostimulation facilitated action potential firing in SPNs (current clamp configuration). Glutamatergic, but not GABAergic, postsynaptic currents were diminished by application of the orexin receptor antagonist almorexant, indicating orexin release facilitates glutamatergic neurotransmission in this pathway. This work identifies a neuronal circuit by which orexin neurons likely exert sympathoexcitatory control of cardiovascular function.NEW & NOTEWORTHY This is the first study to establish, using innovative optogenetic approaches in a transgenic rat model, that there are robust heterogeneous projections from orexin neurons to paraventricular spinally projecting neurons, including excitatory glutamatergic and inhibitory GABAergic neurotransmission. Endogenous orexin release modulates glutamatergic, but not GABAergic, neurotransmission in these pathways.

Myocardial infarction sensitizes medial prefrontal cortex to inhibitory effect of locus coeruleus stimulation in rats

RATIONALE

Anxiety is a common comorbidity that develops after myocardial infarction and is now an established independent risk factor for cardiovascular mortality.

OBJECTIVE

Here, we assessed anxiety and mapped neural activity of forebrain regions that regulate anxiety in a rat model of myocardial infarction in order to identify sites of dysregulation.

METHODS

Anxiety responses to novel (open field) or aversive stimuli (discriminative auditory fear conditioning) were assessed in rats subjected to coronary artery ligation (CAL) or sham ligation. Forebrain metabolic activity was measured by cytochrome oxidase (CO) histochemistry. Changes in CO activity and the incidence of ventricular arrhythmias were also assessed during modulation of fear circuitry induced by electrical stimulation of the locus coeruleus.

RESULTS

Coronary artery ligation had negligible effects on open-field behavior, but increased expression of learned fear and impaired fear cue discrimination. Cytochrome oxidase activity was increased in the medial prefrontal cortex and in the lateral amygdala after CAL. Locus coeruleus stimulation reduced CO activity in the infralimbic medial prefrontal cortex only in rats subjected to CAL. Stimulation of the LC also elicited new ventricular arrhythmias in rats subjected to CAL.

CONCLUSION

Coronary artery ligation sensitizes the infralimbic medial prefrontal cortex to the inhibitory effects of locus coeruleus stimulation. Suppression of infralimbic medial prefrontal cortical activity may impair the ability of rats subjected to CAL to discriminate between cues that signal aversive and neutral events which, in turn, may promote excessive sympathetic activation of the cardiovascular system in response to innocuous stimuli.

α1- and α2-adrenergic receptors in the retrotrapezoid nucleus differentially regulate breathing in anesthetized adult rats

Norepinephrine (NE) is a potent modulator of breathing that can increase/decrease respiratory activity by α1-/α2-adrenergic receptor (AR) activation, respectively. The retrotrapezoid nucleus (RTN) is known to contribute to central chemoreception, inspiration, and active expiration. Here we investigate the sources of catecholaminergic inputs to the RTN and identify respiratory effects produced by activation of ARs in this region. By injecting the retrograde tracer Fluoro-Gold into the RTN, we identified back-labeled catecholaminergic neurons in the A7 region. In urethane-anesthetized, vagotomized, and artificially ventilated male Wistar rats unilateral injection of NE or moxonidine (α2-AR agonist) blunted diaphragm muscle activity (DiaEMG) frequency and amplitude, without changing abdominal muscle activity. Those inhibitory effects were reduced by preapplication of yohimbine (α2-AR antagonist) into the RTN. Conversely, unilateral RTN injection of phenylephrine (α1-AR agonist) increased DiaEMG amplitude and frequency and facilitated active expiration. This response was blocked by prior RTN injection of prazosin (α1-AR antagonist). Interestingly, RTN injection of propranolol (β-AR antagonist) had no effect on respiratory inhibition elicited by applications of NE into the RTN; however, the combined blockade of α2- and β-ARs (coapplication of propranolol and yohimbine) revealed an α1-AR-dependent excitatory response to NE that resulted in increase in DiaEMG frequency and facilitation of active expiration. However, blockade of α1-, α2-, or β-ARs in the RTN had minimal effect on baseline respiratory activity, on central or peripheral chemoreflexes. These results suggest that NE signaling can modulate RTN chemoreceptor function; however, endogenous NE signaling does not contribute to baseline breathing or the ventilatory response to central or peripheral chemoreceptor activity in urethane-anesthetized rats.

Adrenoreceptor modulation of oromotor pathways in the rat medulla

Regulation of feeding behavior involves the integration of multiple physiological and neurological pathways that control both nutrient-seeking and consummatory behaviors. The consummatory phase of ingestion includes stereotyped oromotor movements of the tongue and jaw that are controlled through brain stem pathways. These pathways encompass not only cranial nerve sensory and motor nuclei for processing feeding-related afferent signals and supplying the oromotor musculature but also reticular neurons for orchestrating ingestion and coordinating it with other behaviors that utilize the same musculature. Based on decerebrate studies, this circuit should be sensitive to satiety mechanisms mediated centrally by A2 noradrenergic neurons in the caudal nucleus of the solitary tract (cNST) that are potently activated during satiety. Because the first observable phase of satiety is inhibition of oromotor movements, we hypothesized that norepinephrine (NE) would act to inhibit prehypoglossal neurons in the medullary reticular formation. Using patch-clamp electrophysiology of retrogradely labeled prehypoglossal neurons and calcium imaging to test this hypothesis, we demonstrate that norepinephrine can influence both pre- and postsynaptic properties of reticular neurons through both α1- and α2-adrenoreceptors. The α1-adrenoreceptor agonist phenylephrine (PE) activated an inward current in the presence of TTX and increased the frequency of both inhibitory and excitatory miniature postsynaptic currents. The α2-adrenoreceptor agonist dexmedetomidine (DMT) inhibited cNST-evoked excitatory currents as well as spontaneous and miniature excitatory currents through presynaptic mechanisms. The diversity of adrenoreceptor modulation of these prehypoglossal neurons may reflect their role in a multifunctional circuit coordinating both ingestive and respiratory lingual function.

Synaptic and extrasynaptic transmission of kidney-related neurons in the rostral ventrolateral medulla

The rostral ventrolateral medulla (RVLM) is a critical component of the sympathetic nervous system regulating homeostatic functions including arterial blood pressure. Using the transsynaptic retrograde viral tracer PRV-152, we identified kidney-related neurons in the RVLM. We found that PRV-152-labeled RVLM neurons displayed an unusually large persistent, tonic current to both glutamate, via N-methyl-d-aspartate (NMDA) and 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA)/kainate receptors, and to γ-aminobutyric acid (GABA), via GABAA receptors, in the absence of large-scale phasic neurotransmission with whole cell patch-clamp recordings. A cocktail of potent NMDA and AMPA/kainate ionotropic glutamate receptor antagonists AP-5 (50 μM) and CNQX (10 μM) revealed a two-component somatic tonic excitatory current with an overall amplitude of 42.6 ± 13.4 pA. Moreover, application of the GABAA receptor blockers gabazine (15 μM) and bicuculline (30 μM) revealed a robust somatic tonic inhibitory current with an average amplitude of 196.3 ± 39.3 pA. These findings suggest that the tonic current plays a role in determining the resting membrane potential, input resistance, and firing rate of RVLM neurons. The magnitude of the tonic inhibitory current demonstrates that GABAergic inhibition plays a critical role in regulation of kidney-related RVLM neurons. Our results indicate that the GABAergic tonic current may determine the basal tone of firing activity in kidney-related 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.

Presynaptic inhibition of glutamate transmission by α2 receptors in the VTA

The ventral tegmental area (VTA) forms part of the mesocorticolimbic system and plays a pivotal role in reward and reinforcing actions of drugs of abuse. Glutamate transmission within the VTA controls important aspects of goal-directed behavior and motivation. Noradrenergic receptors also present in the VTA have important functions in the modulation of neuronal activity. Here we studied the effects of α2 noradrenergic receptor activation in the alteration of glutamate neurotransmission in VTA dopaminergic neurons from male Sprague-Dawley rats. We used whole-cell patch-clamp recordings from putative VTA dopaminergic neurons and measured excitatory postsynaptic currents. Clonidine (40 μm) and UK 14,304 (40 μm), both α2 receptor agonists, reduced (approximately 40%) the amplitude of glutamate-induced excitatory postsynaptic currents. After clonidine administration, there was a dose-dependent reduction over the concentration range of 15-40 μm. Using yohimbine (20 μm) and two other α2 adrenergic receptor antagonists, idaxozan (40 μm) and atipemazole (20 μm), we demonstrated that the inhibitory action is specifically mediated by α2 receptors. Moreover, by inhibiting protein kinases with H-7 (75 μm), Rp-adenosine 3',5'-cyclic (11 μm) and chelerythrine (1 μm) it was shown that the clonidine-induced inhibition seems to involve a selective activation of the protein kinase C intracellular pathway. Increased paired-pulse ratios and changes in spontaneous and miniature excitatory postsynaptic current frequencies but not amplitudes indicated that the effect of the α2 agonist was presynaptically mediated. It is suggested that the suppression of glutamate excitatory inputs onto VTA dopaminergic neurons might be relevant in the regulation of reward and drug-seeking behaviors.

Anandamide centrally depresses the respiratory rhythm generator of neonatal mice

Endogenous cannabinoid receptors are widely distributed throughout the CNS, including the brainstem, and modulate a variety of functions, including breathing. In adult rats, activation of cannabinoid 1 receptors has been shown to depress breathing. Here in neonatal mice, we used in vitro electrophysiology, pharmacology, and immunohistochemistry to analyse the central effects of the endocannabinoid anandamide (AEA) on the activity of the medullary respiratory rhythm generator (RRG). First of all, in vitro electrophysiology on medullary preparations has revealed that bath application of AEA (30 μM, 15 min) significantly depressed respiratory activity. Secondly, applying pre-treatments with alpha-1 (Prazosin, 5 μM, 10 min) and alpha-2 (Yohimbine, 5 μM, 10 min) adrenoceptor antagonists prior to AEA application abolished the AEA-induced depression of the RRG. Finally, immunostaining revealed a dense network of fibres positive for the cannabinoid 1 receptor in the ventrolateral medulla (VLM), a region known to contain both the RRG and the modulatory A1/C1 catecholaminergic group. Moreover, cannabinoid 1 receptor positive fibres were found in close apposition with A1/C1 catecholaminergic cells, identified by the presence of tyrosine hydroxylase. In regard of our electrophysiological, pharmacological and immunostaining results, we conclude that AEA has a central depressive effect on the neonatal RRG, probably via the medullary A1/C1 catecholaminergic neurons which are already known to modulate the respiratory rhythm generator.

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.

Effects of verapamil and amiodarone on sympathoadrenal system and balance of excitatory and inhibitory amino acids in rat medulla oblongata

Local injection of verapamil into ventrolateral region of the medulla oblongata triggered the release of epinephrine. Verapamil increased the total content of norepinephrine and epinephrine by 560% and decreased the content of serotonin by 46%. Verapamil had no effect on norepinephrine/epinephrine and norepinephrine/(norepinephrine+epinephrine) ratios in normal rats. Blockade of K+-channels in the medulla oblongata by local injection of 0.001 mg amiodarone did not change the levels of epinephrine and norepinephrine and norepinephrine/epinephrine and norepinephrine/(norepinephrine+epinephrine) ratios. In the medulla oblongata, verapamil proportionally increased the levels of norepinephrine, dopamine, and L-DOPA. Similarly, amiodarone increased the levels of L-DOPA and dopamine by 2.6 and 3.2 times, respectively. Amiodarone shifted the ratio of neuroactive amino acids towards inhibitory transmitters.

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.

Interaction between clonidine and N-methyl-D-aspartate receptors in the caudal ventrolateral medulla of rats

It is known that the caudal ventrolateral medulla (CVLM) plays an important role in controlling blood pressure and mediating the cardiovascular effects of centrally acting antihypertensive drugs such as clonidine. Recently, the effect of clonidine was believed to be related to the functional states of N-methyl-D-aspartate (NMDA) receptors. The present work was designed to observe the interactions between clonidine and NMDA receptor in the CVLM. Unilaterally injected clonidine (6 nmol) into the CVLM not only produced a pressor action, but also effectively (P<0.01, n=8) antagonized the decreases in both mean arterial pressure (MAP) (-22.3+/-5.0 to -7.9+/-2.3 mmHg) and heart rate (HR) (-31.9+/-5.9 to -10.3+/-2.7 beats/min) evoked by L-glutamate in the CVLM. Unilaterally injected NMDA receptor antagonist MK801 (200 pmol) into the CVLM significantly increased MAP by 26.5+/-3.7 mmHg and HR by 37.1+/-7.6 beats/min, and completely (P<0.01, n=10) abolished the pressor effect (16.1+/-6.6 to 1.5+/-2.8 mmHg) of clonidine in the CVLM. In conclusion, these findings show that NMDA receptors within the CVLM contribute to clonidine-induced action, and suggest that the CVLM plays an important role in the interaction between clonidine and NMDA receptors.

Autonomic control after blockade of the norepinephrine transporter: a model of orthostatic intolerance

Orthostatic intolerance is a debilitating syndrome characterized by tachycardia on assumption of upright posture. The norepinephrine (NE) transporter (NET) has been implicated in a genetic form of the disorder. We assessed the combined central and peripheral effects of pharmacological NET blockade on cardiovascular regulation and baroreflex sensitivity in rats. NE reuptake was blocked chronically in female Sprague-Dawley rats by the NET antagonist desipramine (DMI). Treated animals demonstrated an elevated supine heart rate, reduced tyramine responsiveness, and a reduced plasma ratio of the intraneuronal NE metabolite dihydroxyphenylglycol relative to NE, all of which are consistent with observations in human NET deficiency. Spectral analysis revealed a dramatic decrease in low-frequency spectral power after DMI that was consistent with decreased sympathetic outflow. Stimulation of the baroreflex with the vasodilator nitroprusside revealed an attenuated tachycardia in DMI-treated animals. This indicated that the DMI-induced sympathoinhibitory effects of increased NE in the brain stem predominates over the functional elevation of NE stimulation of peripheral targets. Thus attenuated baroreflex function and reduced sympathetic outflow may contribute to the orthostatic intolerance of severe NET deficiency.

Role of presympathetic C1 neurons in the sympatholytic and hypotensive effects of clonidine in rats

The rostral ventrolateral medulla (RVLM) may play an important role in the sympatholytic and hypotensive effects of clonidine. The present study examined which type of presympathetic RVLM neuron is inhibited by clonidine, and whether the adrenergic presympathetic RVLM neurons are essential for clonidine-induced sympathoinhibition. In chloralose-anesthetized and ventilated rats, clonidine (10 microg/kg iv) decreased arterial pressure (116 +/- 6 to 84 +/- 2 mmHg) and splanchnic nerve activity (93 +/- 3% from baseline). Extracellular recording and juxtacellular labeling of barosensitive bulbospinal RVLM neurons revealed that most cells were inhibited by clonidine (26/28) regardless of phenotype [tyrosine hydroxylase (TH)-immunoreactive cells: 48 +/- 7%; non-TH-immunoreactive cells: 42 +/- 5%], although the inhibition of most neurons was modest compared with the observed sympathoinhibition. Depletion of most bulbospinal catecholaminergic neurons, including 76 +/- 5% of the rostral C1 cells, by microinjection of saporin anti-dopamine beta-hydroxylase into the thoracic spinal cord (levels T2 and T4, 42 ng. 200 nl(-1). side(-1)) did not alter the sympatholytic or hypotensive effects of clonidine. These data show that although clonidine inhibits presympathetic C1 neurons, bulbospinal catecholaminergic neurons do not appear to be essential for the sympatholytic and hypotensive effects of systemically administered clonidine. Instead, the sympatholytic effect of clonidine is likely the result of a combination of effects on multiple cell types both within and outside the RVLM.

Differential cardiorespiratory control elicited by activation of ventral medullary sites in mice

We studied the respiratory and blood pressure responses to chemical stimulation of two regions of the ventral brainstem in mice: the rostral and caudal ventrolateral medulla (RVLM and CVLM, respectively). Stimulation of the RVLM by microinjections of the excitatory amino acid L-glutamate induced increases in diaphragm activity and breathing frequency, elevation of blood pressure (BP), and a slight increase in heart rate (HR). However, activation of the CVLM induced a decrease in breathing frequency, mainly due to prolongation of expiratory time (TE), and hypotension associated with a slight slowing of HR. Because adrenergic mechanisms are known to participate in the control of respiratory timing, we examined the role of alpha(2)-adrenergic receptors in the RVLM region in mediating these inhibitory effects. The findings demonstrated that blockade of the alpha(2)-adrenergic receptors within the RVLM by prior microinjection of SKF-86466 (an alpha(2)-adrenergic receptor blocker) significantly reduced changes in TE induced by CVLM stimulation but had little effect on BP responses. These results indicate that, in mice, activation of the RVLM increases respiratory drive associated with an elevation of BP, but stimulation of CVLM induces prolongation of TE via an alpha(2)-adrenergic signal transduction pathway.