Presynaptic inhibition by noradrenaline of the EPSC evoked in neonatal rat sympathetic preganglionic neurons

Presynaptic Adrenoceptors

Presynaptic α2-adrenoceptors are localized on axon terminals of many noradrenergic and non-noradrenergic neurons in the peripheral and central nervous systems. Their activation by exogenous agonists leads to inhibition of the exocytotic release of noradrenaline and other transmitters from the neurons. Most often, the α2A-receptor subtype is involved in this inhibition. The chain of molecular events between receptor occupation and inhibition of the exocytotic release of transmitters has been determined. Physiologically released endogenous noradrenaline elicits retrograde autoinhibition of its own release. Some clonidine-like α2-receptor agonists have been used to treat hypertension. Dexmedetomidine is used for prolonged sedation in the intensive care; It also has a strong analgesic effect. The α2-receptor antagonist mirtazapine increases the noradrenaline concentration in the synaptic cleft by interrupting physiological autoinhibion of release. It belongs to the most effective antidepressive drugs. β2-Adrenoceptors are also localized on axon terminals in the peripheral and central nervous systems. Their activation leads to enhanced transmitter release, however, they are not activated by endogenous adrenaline.

Noradrenergic effects in rat sacral autonomic nucleus using in vitro slice patch-clamp recordings

Noradrenergic modulation has been frequently discussed in the context of neural activities that are related to pelvic organs. The sacral preganglionic nucleus (SPN) is a spinal nucleus containing parasympathetic preganglionic neurons that send fibers to pelvic nerves. In spite of the abundant presence of noradrenergic fibers around the SPN, the effects of noradrenaline (NA) remain obscure. To explore this issue, NA (50 μM) was applied to parasympathetic preganglionic neurons in the SPN during whole-cell patch clamp recording. The SPN was labeled with the retrograde tracer, DiI. These neurons demonstrated two classes of firing patterns (delayed and regular) in terms of initiation of firing. Independent of these firing patterns, NA induced inward (56%) or outward (32%) currents in labeled SPN neurons. Phenylephrine, an α1 receptor agonist, induced an inward current, and clonidine, an α2 receptor agonist, induced an outward current, indicating the existence of both α1 and α2 adrenoreceptors in DiI-labeled SPN neurons. NA also modulated synaptic currents according to the firing patterns. In delayed firing neurons, NA inhibited both spontaneous excitatory post-synaptic currents (sEPSCs) and spontaneous inhibitory post-synaptic currents (sIPSCs). Hence, NA facilitated sEPSCs and sIPSCs in about a half of regular firing neurons. Bath application of phenylephrine facilitated sEPSCs and sIPSCs, and clonidine inhibited them. These results support the hypothesis of multiple effects of NA in the SPN, and may suggest functional differences among SPN neurons.

Modulation of sympathetic preganglionic neuron activity via adrenergic receptors

The sympathetic preganglionic neurons (SPNs) play a key role in the sympathetic nervous system. Previous reports have suggested that norepinephrine (NE) directly affects SPNs via both inhibitory hyperpolarization interactions mediated by α2 receptors and excitatory depolarization interactions mediated by α1 receptors. It remains poorly understood, however, whether the excitability of SPNs can be inhibited indirectly (presynaptically) as well as directly (postsynaptically). We intracellularly recorded 41 SPNs using the whole-cell patch-clamp technique in spinal cord slice preparations of neonatal rats. We examined the effects of NE or dexmedetomidine hydrochloride (Dxm) (α2-adrenergic receptor agonist) on SPNs by analyzing the excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). EPSPs were dominant in 15 SPNs (EPSP-SPNs) and IPSPs were dominant in 7 SPNs (IPSP-SPNs) at baseline. We were unable to analyze the postsynaptic potentials in the other 19 SPNs, due to high frequency of action potential firings (firing-SPNs). At baseline, the membrane potentials and resistances of each type of SPN were similar. NE (1  μM) gradually depolarized the EPSP-SPNs and IPSP-SPNs (P   < 0.001) and NE significantly increased the EPSP frequency of the EPSP-SPNs (P   < 0.05). Dxm (10  nM) after application of NE decreased the EPSP frequency of the EPSP-SPNs (P  <  0.001) and the EPSP voltage and IPSP voltage of the IPSP-SPNs (P  <  0.05). In 5 of the 19 firing-SPNs, NE induced membrane hyperpolarization (P   < 0.05) and completely inhibited firings. Dxm had no effect in these neurons. The SPNs received inhibitory modulation through α2-adrenergic receptors. Some SPNs can be directly inhibited via effects independent of the α2 receptors.

The α2A -adrenoceptor suppresses excitatory synaptic transmission to both excitatory and inhibitory neurons in layer 4 barrel cortex

KEY POINTS

The effects of noradrenaline on excitatory synaptic transmission to regular spiking (excitatory) cells as well as regular spiking non-pyramidal and fast spiking (both inhibitory) cells in cortical layer 4 were studied in thalamocortical slice preparations, focusing on vertical input from thalamus and layer 2/3 in the mouse barrel cortex. Excitatory synaptic responses were suppressed by noradrenaline. However, currents induced by iontophoretically applied glutamate were not suppressed. Further, paired pulse ratio and coefficient of variation analysis indicated the site of action was presynaptic. Pharmacological studies indicated that the suppression was mediated by the α_2- adrenoceptor. Consistent with this, involvement of α_2A -adrenoceptor activation in the synaptic suppression in excitatory and inhibitory cells was confirmed by the use of α_2A -adrenoceptor knockout mice.

ABSTRACT

The mammalian neocortex is widely innervated by noradrenergic (NA) fibres from the locus coeruleus. To determine the effects of NA on vertical synaptic inputs to layer 4 (L4) cells from the ventrobasal thalamus and layer 2/3 (L2/3), thalamocortical slices were prepared and whole-cell recordings were made from L4 cells. Excitatory synaptic responses were evoked by electrical stimulation of the thalamus or L2/3 immediately above. Recorded cells were identified as regular spiking, regular spiking non-pyramidal or fast spiking cells through their firing patterns in response to current injections. NA suppressed (∼50% of control) excitatory vertical inputs to all cell types in a dose-dependent manner. The presynaptic site of action of NA was suggested by three independent studies. First, responses caused by iontophoretically applied glutamate were not suppressed by NA. Second, the paired pulse ratio was increased during NA suppression. Finally, a coefficient of variation (CV) analysis was performed and the resultant diagonal alignment of the ratio of CV^-2 plotted against the ratio of the amplitude of postsynaptic responses suggests a presynaptic mechanism for the suppression. Experiments with phenylephrine (an α₁ -agonist), prazosin (an α₁ -antagonist), yohimbine (an α₂ -antagonist) and propranolol (a β-antagonist) indicated that suppression was mediated by the α₂ -adrenoceptor. To determine whether the α_2A -adrenoceptor subtype was involved, α_2A -adrenoceptor knockout mice were used. NA failed to suppress EPSCs in all cell types, suggesting an involvement of the α_2A -adrenoceptor. Altogether, we concluded that NA suppresses vertical excitatory synaptic connections in L4 excitatory and inhibitory cells through the presynaptic α_2A -adrenoceptor.

Noradrenergic Mechanisms Controlling Urethral Smooth and Striated Muscle Function in Urethral Continence Reflex in Rats

OBJECTIVES

To investigate the role of noradrenergic pathways in the urethral continence reflex during abdominal compression in rats.

METHODS

Under urethane anesthesia, urethral baseline pressure (UBP) and urethral pressure response (UPR) during momentary abdominal compression using a 100 g weight was measured using a transurethral microtransducer-tipped catheter placed at the middle urethra in Sprague-Dawley female rats. Following intravenous (i.v.) application of hexamethonium or α-bungarotoxin to block urethral smooth or striated muscle function, respectively, the effects of terazosin, an α1 -adrenoceptor (AR) antagonist (0.3 mg/kg, i.v.), medetomidine, an α2 -AR agonist (0.3 mg/kg, i.v.) or nisoxetine, a norepinephrine reuptake inhibitor (1 mg/kg, i.v.) followed by terazosin on UBP and UPR were examined.

RESULTS

After hexamethonium pretreatment, terazosin did not alter UBP or UPR, whereas medetomidine significantly decreased UPR by 28% without UBP changes. Nisoxetine significantly increased UPR by 64%, which was eliminated by terazosin, but UBP was not altered by nisoxetine. After α-bungarotoxin pretreatment, UBP and UPR were significantly decreased by terazosin or medetomidine. Nisoxetine induced significant increases in UBP and UPR by 16 and 15%, respectively, which were antagonized by terazosin.

CONCLUSION

These results suggest that: the baseline activity and reflex contraction of urethral smooth muscle are decreased by α1 -AR inhibition or α2 -AR stimulation; the reflex contraction of urethral striated muscle is decreased by α2 -AR stimulation, but not by α1 -AR inhibition; and nisoxetine increases baseline and reflex activity of smooth muscle in addition to striated muscle reflex activity by α1 -AR stimulation. These findings will be useful to understand nerve-mediated urethral closure mechanisms.

Non-additive modulation of synaptic transmission by serotonin, adenosine, and cholinergic modulators in the sensory thalamus

The thalamus relays sensory information to the cortex. Oscillatory activities of the thalamocortical network are modulated by monoamines, acetylcholine, and adenosine, and could be the key features characteristic of different vigilance states. Although the thalamus is almost always subject to the actions of more than just one neuromodulators, reports on the modulatory effect of coexisting neuromodulators on thalamic synaptic transmission are unexpectedly scarce. We found that, if present alone, monoamine or adenosine decreases retinothalamic synaptic strength and short-term depression, whereas cholinergic modulators generally enhance postsynaptic response to presynaptic activity. However, coexistence of different modulators tends to produce non-additive effect, not predictable based on the action of individual modulators. Acetylcholine, acting via nicotinic receptors, can interact with either serotonin or adenosine to abolish most short-term synaptic depression. Moreover, the coexistence of adenosine and monoamine, with or without acetylcholine, results in robustly decreased synaptic strength and transforms short-term synaptic depression to facilitation. These findings are consistent with a view that acetylcholine is essential for an "enriched" sensory flow through the thalamus, and the flow is trimmed down by concomitant monoamine or adenosine (presumably for the wakefulness and rapid-eye movement, or REM, sleep states, respectively). In contrast, concomitant adenosine and monoamine would lead to a markedly "deprived" (and high-pass filtered) sensory flow, and thus the dramatic decrease of monoamine may constitute the basic demarcation between non-REM and REM sleep. The collective actions of different neuromodulators on thalamic synaptic transmission thus could be indispensable for the understanding of network responsiveness in different vigilance states.

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.

Emotional enhancement of memory: how norepinephrine enables synaptic plasticity

Changes in synaptic strength are believed to underlie learning and memory. We explore the idea that norepinephrine is an essential modulator of memory through its ability to regulate synaptic mechanisms. Emotional arousal leads to activation of the locus coeruleus with the subsequent release of norepineprine in the brain, resulting in the enhancement of memory. Norepinephrine activates both pre- and post-synaptic adrenergic receptors at central synapses with different functional outcomes, depending on the expression pattern of these receptors in specific neural circuitries underlying distinct behavioral processes. We review the evidence for noradrenergic modulation of synaptic plasticity with consideration of how this may contribute to the mechanisms of learning and memory.

Noradrenergic modulation of neuronal responses to glutamate in the vestibular complex

Increases in firing rate induced in secondary vestibular neurons by microiontophoretic application of glutamate were studied during long-lasting applications of noradrenaline (NA) and/or its antagonists and agonists. Sixty-nine percent of the tested neurons, scattered through all nuclei of the vestibular complex, modified their responsiveness to glutamate in the presence of NA. The effects were depressive in a majority (40%) and enhancing in a minority (29%) of cases. NA application depressed responses to glutamate more often than it increased them in lateral, medial and superior vestibular nuclei, while the reverse was true for the spinal nucleus. The mean intensities of NA-evoked effects were comparable in the various nuclei. The enhancing effects of NA were antagonized by application of the alpha2 receptor antagonist yohimbine, and their depressive effects were enhanced by the beta receptor antagonist timolol. It is concluded that NA exerts a control on the processing of vestibular information and that this modulation is exerted by at least two mechanisms involving alpha2 and beta noradrenergic receptors.

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.

Role of alpha2-adrenoceptors and glutamate mechanisms in the external urethral sphincter continence reflex in rats

PURPOSE

We investigated the role of alpha(2)-adrenoceptors and glutamate mechanisms in the urethral continence reflex in response to abdominal pressure increases.

MATERIALS AND METHODS

Under urethane anesthesia external urethral sphincter electromyogram activity was evaluated in spinal cord transected (T8-T9) female rats during lower abdominal wall compression before and after intravenous application of test drugs. The effects of the N-methyl-D-aspartate glutamate receptor antagonist MK-801 (Sigma) or the alpha(2)-adrenoceptor agonist medetomidine (Tocris Cookson, Ellisville, Missouri) (each 0.03, 0.3 and 3 mg/kg intravenously) on external urethral sphincter activity were examined. A 0.3 mg/kg intravenous dose of the alpha(2)-adrenoceptor antagonist idazoxan (Sigma) was then administered before or after the application of 1 mg/kg MK-801 intravenously. In addition, 0.3 mg/kg idazoxan were administered intravenously following the application of 1 mg/kg of the serotonin/norepinephrine reuptake inhibitor duloxetine (Kemprotec, Middlesbrough, United Kingdom) intravenously.

RESULTS

MK-801 and medetomidine dose dependently decreased external urethral sphincter activity. Idazoxan significantly increased external urethral sphincter activity by 64% but the increase in activity after idazoxan was abolished by MK-801. On the other hand, idazoxan did not reverse the inhibitory effects of MK-801. In addition, idazoxan significantly potentiated the duloxetine effects on external urethral sphincter activity by 120%.

CONCLUSIONS

These results indicate that 1) glutamate is a major excitatory neurotransmitter in the urethral continence reflex response to abdominal pressure increases, 2) alpha(2)-adrenoceptor activation suppresses external urethral sphincter activity, probably via presynaptic inhibition of glutamate release and 3) the effects of serotonin/norepinephrine reuptake inhibitors are enhanced by alpha(2)-adrenoceptor inhibition. Therefore, alpha(2)-adrenoceptor antagonists could be beneficial for treating stress urinary incontinence.

Developmental enhancement of alpha2-adrenoceptor-mediated suppression of inhibitory synaptic transmission onto mouse cerebellar Purkinje cells

Noradrenaline (NA) modulates glutamatergic and GABAergic transmission in various areas of the brain. It is reported that some alpha2-adrenoceptor subtypes are expressed in the cerebellar cortex and alpha2-adrenoceptors may play a role in motor coordination. Our previous study demonstrated that the selective alpha2-adrenoceptor agonist clonidine partially depresses spontaneous inhibitory postsynaptic currents (sIPSCs) in mouse cerebellar Purkinje cells (PCs). Here we found that the inhibitory effect of clonidine on sIPSCs was enhanced during postnatal development. The activation of alpha2-adrenoceptors by clonidine did not affect sIPSCs in PCs at postnatal days (P) 8-10, when PCs showed a few sIPSCs and interneurons in the molecular layer (MLIs) did not cause action potential (AP). In the second postnatal week, the frequency of sIPSCs increased temporarily and reached a plateau at P14. By contrast, MLIs began to fire at P11 with the firing rate gradually increasing thereafter and reaching a plateau at P21. In parallel with this rise in the rate of firing, the magnitude of the clonidine-mediated inhibition of sIPSCs increased during postnatal development. Furthermore, the magnitude of the clonidine-mediated firing suppression in MLIs, which seemed to be mediated by a reduction in amplitude of the hyperpolarization-activated nonselective cation current, I(h), was constant across development. Both alpha2A- and alpha2B-, but not alpha2C-, adrenoceptors were strongly expressed in MLIs at P13, and P31. Therefore, the developmental enhancement of the clonidine-mediated inhibition of sIPSCs is attributed to an age-dependent increase in AP-derived sIPSCs, which can be blocked by clonidine. Thus, presynaptic activation of alpha2-adrenoceptors inhibits cerebellar inhibitory synaptic transmission after the second postnatal week, leading to a restriction of NA signaling, which is mainly mediated by alpha1- and beta2-adrenoceptors in the adult cerebellar neuronal circuit.

Action of dexmedetomidine on the substantia gelatinosa neurons of the rat spinal cord

Dexmedetomidine is a highly specific, potent and selective alpha(2)-adrenoceptor agonist. Although intrathecal and epidural administration of dexmedetomidine has been found to produce analgesia, whether this analgesia results from an effect on spinal cord substantia gelatinosa (SG) neurons remains unclear. Here, we investigated the effects of dexmedetomidine on postsynaptic transmission in SG neurons of rat spinal cord slices using the whole-cell patch-clamp technique. In 92% of the SG neurons examined (n = 84), bath-applied dexmedetomidine induced outward currents at -70 mV in a concentration-dependent manner, with the value of effective concentration producing a half-maximal response (0.62 microM). The outward currents induced by dexmedetomidine were suppressed by the alpha(2)-adrenoceptor antagonist yohimbine, but not by prazosin, an alpha(1)-, alpha(2B)- and alpha(2C)-adrenoceptor antagonist. Moreover, the dexmedetomidine-induced currents were partially suppressed by the alpha(2C)-adrenoceptor antagonist JP-1302, while simultaneous application of JP-1302 and the alpha(2A)-adrenoceptor antagonist BRL44408 abolished the current completely. The action of dexmedetomidine was mimicked by the alpha(2A)-adrenoceptor agonist oxymetazoline. Plots of the current-voltage relationship revealed a reversal potential at around -86 mV. Dexmedetomidine-induced currents were blocked by the addition of GDP-beta-S [guanosine-5'-O-(2-thiodiphosphate)] or Cs+ to the pipette solution. These findings suggest that dexmedetomidine hyperpolarizes the membrane potentials of SG neurons by G-protein-mediated activation of K+ channels through alpha(2A)- and alpha(2C)-adrenoceptors. This action of dexmedetomidine might contribute, at least in part, to its antinociceptive action in the spinal cord.

Noradrenergic modulation of XII motoneuron inspiratory activity does not involve α2-receptor inhibition of the Ih current or presynaptic glutamate release

Norepinephrine has powerful and diverse modulatory effects on hypoglossal (XII) motoneuron activity, which is important in maintaining airway patency. The objective was to test two hypotheses that α2-adrenoceptor-mediated, presynaptic inhibition of glutamatergic inspiratory drive (Selvaratnam SR, Parkis MA, and Funk GD. Brain Res 805: 104–115, 1998) and postsynaptic inhibition of the hyperpolarization-activated inward current ( Ih) (Parkis MA and Berger AJ. Brain Res 769: 108–118, 1997) modulate XII inspiratory activity. Nerve and whole cell recordings were applied to rhythmic medullary slice preparations from neonatal rats ( postnatal days 0–4) to monitor XII inspiratory burst amplitude and motoneuron properties. Application of an α2-receptor agonist (clonidine, 1 mM) to the XII nucleus reduced inspiratory burst amplitude to 71 ± 3% of control but had no effect on inspiratory synaptic currents. It also reduced the Ih current by ∼40%, but an Ih current blocker (ZD7288), at concentrations that blocked ∼80% of Ih, had no effect on inspiratory burst amplitude. The clonidine inhibition was unaffected by the GABAA antagonist (+)bicuculline but attenuated by the α2-antagonist rauwolscine and the imidazoline 1 (I1) antagonist efaroxan. The I1 agonist rilmenidine, but not the α2-agonist UK14304, inhibited XII output. Clonidine also reduced action potential amplitude or impaired repetitive firing. Although a contribution from α2, and in particular I1, receptors remains possible, results demonstrate that 1) noradrenergic modulation of XII inspiratory activity is unlikely to involve α2-receptor-mediated presynaptic inhibition of glutamate release or modulation of Ih; 2) inhibition of repetitive firing is a major factor underlying the inhibition of XII output by clonidine; and 3) Ih is present in neonatal XII motoneurons but does not contribute to shaping their inspiratory activity.

Functional organization of brain pathways subserving the baroreceptor reflex: studies in conscious animals using immediate early gene expression

1. This paper reviews studies carried out in our laboratory in which we have used the c-fos functional mapping method, in combination with other methods, to determine the functional organization of central baroreceptor pathways as they operate in the conscious rabbit. 2. First, we showed that periods of induced hypertension or hypotension each result in a specific and reproducible pattern of activation of neurons in the brainstem and forebrain. In particular, hypotension (but not hypertension) results in the activation of catecholamine neurons in the medulla and pons and vasopressin-synthesizing neurons in the hypothalamus. 3. The activation of medullary cell groups in response to induced hypertension or hypotension in the conscious rabbit is almost entirely dependent on inputs from arterial baroreceptors, while the activation of hypothalamic vasopressin-synthesising neurons in response to hypotension is largely dependent on baroreceptors, although an increase in circulating angiotensin also appears to contribute. 4. Discrete groups of neurons in the rostral ventrolateral medulla (RVLM) and A5 area in the pons are the major groups of spinally projecting neurons activated by baroreceptor unloading. In contrast, spinally projecting neurons in the paraventricular nucleus in the hypothalamus appear to be largely unaffected by baroreceptor signals. 5. Direct afferent inputs to RVLM neurons in response to increases or decreases in arterial pressure originate primarily from other medullary nuclei, particularly neurons located in the caudal and intermediate levels of the ventrolateral medulla (CVLM and IVLM), as well as in the nucleus tractus solitarius (NTS). 6. There is also a direct projection from barosensory neurons in the NTS to the CVLM/IVLM region, which is activated by baroreceptor inputs. 7. Collectively, the results of our studies in conscious animals indicate that baroreceptor signals reach all levels of the brain. With regard to the baroreceptor reflex control of sympathetic activity, our studies are consistent with previous studies in anesthetized animals, but in addition reveal other previously unrecognized pathways that also contribute to this reflex regulation.

Functional organisation of central cardiovascular pathways: studies using c-fos gene expression

Until about 10 years ago, knowledge of the functional organisation of the central pathways that subserve cardiovascular responses to homeostatic challenges and other stressors was based almost entirely on studies in anaesthetised animals. More recently, however, many studies have used the method of the expression of immediate early genes, particularly the c-fos gene, to identify populations of central neurons that are activated by such challenges in conscious animals. In this review we first consider the advantages and limitations of this method. Then, we discuss how the application of the method of immediate early gene expression, when used alone or in combination with other methods, has contributed to our understanding of the central mechanisms that regulate the autonomic and neuroendocrine response to various cardiovascular challenges (e.g., hypotension, hypoxia, hypovolemia, and other stressors) as they operate in the conscious state. In general, the results of studies of central cardiovascular pathways using immediate early gene expression are consistent with previous studies in anaesthetised animals, but in addition have revealed other previously unrecognised pathways that also contribute to cardiovascular regulation. Finally, we briefly consider recent evidence indicating that immediate early gene expression can modify the functional properties of central cardiovascular neurons, and the possible significance of this in producing long-term changes in the regulation of the cardiovascular system both in normal and pathological conditions.

Alpha-2 adrenoceptor mediating the facilitatory effect of norepinephrine on the glycine response in the spinal dorsal horn neuron of the rat

Effects of norepinephrine (NE) on the glycine-mediated inhibitory response were investigated in neurons acutely dissociated from the rat spinal dorsal horn, using nystatin perforated patch recording mode under voltage-clamp conditions. NE reversibly and concentration dependently facilitated Cl(-) current induced by 3 x 10(-5) M glycine. NE neither changed the reversal potential of the glycine response nor affected the affinity of glycine to its receptor. This effect could be mimicked by clonidine (10(-7) M) and blocked by yohimbine (10(-6) M), respectively. N-[2(methylamino)ethyl]-5-isoquinoline sulfonamide dihydrochloride (H-89), an inhibitor of protein kinase A, effectively mimicked the effect of NE on glycine response, whereas chelerythrine (an inhibitor of protein kinase C) failed. NE further enhanced glycine response even in the presence of chelerythrine or stearoylcarnitine chloride (another inhibitor of protein kinase C) or chelerythrine together with stearoylcarnitine chloride. The present results suggest that alpha2-adrenoceptor is involved in the potentiation of NE on glycine response in freshly isolated spinal dorsal horn neurons. Activation of alpha2-adrenoceptor down-regulates the activity of protein kinase A that results in the potentiation of the glycinergic inhibitory effects within the spinal dorsal horn.

alpha 2-Adrenoceptor-mediated presynaptic modulation of GABAergic transmission in mechanically dissociated rat ventrolateral preoptic neurons

The ventrolateral preoptic nucleus (VLPO) is a key nucleus involved in the homeostatic regulation of sleep-wakefulness. Little is known, however, about the cellular mechanisms underlying its role in sleep regulation and how the neurotransmitters, such as GABA and noradrenaline (NA), are involved. In the present study we investigated GABAergic transmission to acutely dissociated VLPO neurons using an enzyme-free, mechanical dissociation procedure in which functional terminals remained adherent and we investigated how this GABAergic transmission was modulated by NA. As previously reported in slices, NA hyperpolarized multipolar VLPO neurons and depolarized bipolar VLPO neurons. NA also inhibited the release of GABA onto multipolar VLPO neurons but had no effect on GABAergic transmission to bipolar neurons. The inhibition of release was mediated by presynaptic alpha(2) adrenoceptors coupled to N-ethylmaleimide (NEM)-sensitive G-proteins which appeared to act via inhibition of adenylate cyclase and subsequent decreases in protein kinase A activity. The inhibition of GABA release did not, however, involve an inhibition of external Ca(2+) influx. The results indicate that all VLPO neurons contain GABAergic inputs and that the different morphological subgroups of VLPO neurons are correlated not only to different postsynaptic responses to NA but also to different presynaptic NA responses. Furthermore our results demonstrate an additional mechanism by which NA can modulate the excitability of multipolar VLPO neurons which may have important implications for its role in regulating sleep/wakefulness.

Fos expression in spinally projecting neurons after hypotension in the conscious rabbit

Hypotension produces a reflex increase in the activity of sympathetic vasomotor nerves. Studies in anaesthetised animals have established that neurons in the rostral ventrolateral medulla (RVLM) that project directly to sympathetic vasomotor preganglionic neurons in the spinal cord are a critical component of the central pathways mediating this reflex response. There are also neurons in supramedullary regions (the A5 area in the pons and the paraventricular nucleus (PVN) in the hypothalamus), however, that project directly to the sympathetic vasomotor outflow. The aim of this study was to identify and map neurons within the A5 area and PVN, as well as in the RVLM, which may contribute to the reflex sympathoexcitatory response to a hypotensive challenge in conscious rabbits. In a preliminary operation, a retrogradely transported tracer was injected into a site centred on the intermediolateral cell column in the upper lumbar spinal cord. After a waiting period of at least 1 week, a moderate hypotension (decrease in arterial pressure of approximately 20 mm Hg) was induced in conscious rabbits for 60 min by continuous infusion of sodium nitroprusside. In confirmation of previous studies, hypotension resulted in the expression of Fos in the RVLM, the A5 area and PVN. There were also retrogradely labelled neurons in all these regions. In both the RVLM and A5 area, approximately 40% of the retrogradely labelled neurons were also immunoreactive for Fos. In contrast, in the PVN the proportion of retrogradely labelled neurons that were also Fos-positive was much less (approximately 6%). This study has demonstrated that, in the conscious rabbit, a significant proportion of spinally projecting neurons within discrete regions in the RVLM and A5 area are activated by hypotension (as indicated by Fos expression). In the PVN, only a very small proportion of spinally projecting neurons are activated by hypotension, and thus these neurons appear to be regulated primarily by inputs other than baroreceptor inputs.

Inhibition of glutamatergic synaptic input to spinal lamina II(o) neurons by presynaptic alpha(2)-adrenergic receptors

Activation of spinal alpha(2)-adrenergic receptors by the descending noradrenergic system and alpha(2)-adrenergic agonists produces analgesia. However, the sites and mechanisms of the analgesic action of spinally administered alpha(2)-adrenergic receptor agonists such as clonidine are not fully known. The dorsal horn neurons in the outer zone of lamina II (lamina II(o)) are important for processing nociceptive information from C-fiber primary afferents. In the present study, we tested a hypothesis that activation of presynaptic alpha(2)-adrenergic receptors by clonidine inhibits the excitatory synaptic input to lamina II(o) neurons. Whole cell voltage-clamp recordings were performed on visualized lamina II(o) neurons in the spinal cord slice of rats. The miniature excitatory postsynaptic currents (mEPSCs) were recorded in the presence of tetrodotoxin, bicuculline, and strychnine. The evoked EPSCs were obtained by electrical stimulation of the dorsal root entry zone or the attached dorsal root. Both mEPSCs and evoked EPSCs were abolished by application of 6-cyano-7-nitroquinoxaline-2,3-dione. Clonidine (10 microM) significantly decreased the frequency of mEPSCs from 5.8 +/- 0.9 to 2.7 +/- 0.6 Hz (means +/- SE) without altering the amplitude and the decay time constant of mEPSCs in 25 of 27 lamina II(o) neurons. Yohimbine (2 microM, an alpha(2)-adrenergic receptor antagonist), but not prazosin (2 microM, an alpha(1)-adrenergic receptor antagonist), blocked the inhibitory effect of clonidine on the mEPSCs. Clonidine (1-20 microM, n = 8) also significantly attenuated the peak amplitude of evoked EPSCs in a concentration-dependent manner. The effect of clonidine on evoked EPSCs was abolished in the presence of yohimbine (n = 5). These data suggest that clonidine inhibits the excitatory synaptic input to lamina II(o) neurons through activation of alpha(2)-adrenergic receptors located on the glutamatergic afferent terminals. Presynaptic inhibition of glutamate release from primary afferents onto lamina II(o) neurons likely plays an important role in the analgesic action produced by activation of the descending noradrenergic system and alpha(2)-adrenergic agonists.

Modulatory actions of serotonin, norepinephrine, dopamine, and acetylcholine in spinal cord deep dorsal horn neurons

The deep dorsal horn represents a major site for the integration of spinal sensory information. The bulbospinal monoamine transmitters, released from serotonergic, noradrenergic, and dopaminergic systems, exert modulatory control over spinal sensory systems as does acetylcholine, an intrinsic spinal cord biogenic amine transmitter. Whole cell recordings of deep dorsal horn neurons in the rat spinal cord slice preparation were used to compare the cellular actions of serotonin, norepinephrine, dopamine, and acetylcholine on dorsal root stimulation-evoked afferent input and membrane cellular properties. In the majority of neurons, evoked excitatory postsynaptic potentials were depressed by the bulbospinal transmitters serotonin, norepinephrine, and dopamine. Although, the three descending transmitters could evoke common actions, in some neurons, individual transmitters evoked opposing actions. In comparison, acetylcholine generally facilitated the evoked responses, particularly the late, presumably N-methyl-D-aspartate receptor-mediated component. None of the transmitters modified neuronal passive membrane properties. In contrast, in response to depolarizing current steps, the biogenic amines significantly increased the number of spikes in 14/19 neurons that originally fired phasically (P < 0.01). Together, these results demonstrate that even though the deep dorsal horn contains many functionally distinct subpopulations of neurons, the bulbospinal monoamine transmitters can act at both synaptic and cellular sites to alter neuronal sensory integrative properties in a rather predictable manner, and clearly distinct from the actions of acetylcholine.

Noradrenergic modulation of calcium currents and synaptic transmission in the olfactory bulb of Xenopus laevis tadpoles

Norepinephrine (NE) has various modulatory roles in both the peripheral and the central nervous systems. Here we investigate the function of the locus coeruleus efferent fibres in the olfactory bulb of Xenopus laevis tadpoles. In order to distinguish unambiguously between mitral cells and granule cells of the main olfactory bulb and the accessory olfactory bulb, we used a slice preparation. The two neuron types were distinguished on the basis of their location in the slice, their typical branching pattern and by electrophysiological criteria. At NE concentrations lower than 5 microM there was only one effect of NE upon voltage-gated conductances; NE blocked a high-voltage-activated Ca(2+)-current in mitral cells of both the main and the accessory olfactory bulbs. No such effect was observed in granule cells. The effect of NE upon mitral cell Ca(2+)-currents was mimicked by the alpha(2)-receptor agonists clonidine and alpha-methyl-NE. As a second effect, NE or clonidine blocked spontaneous synaptic activity in granule cells of both the main and the accessory olfactory bulbs. NE or clonidine also blocked the spontaneous synaptic activity in mitral cells of either olfactory bulb. The amplitude of glutamate-induced currents in granule cells was modulated neither by clonidine nor by alpha-methyl-NE. Taken together, the main effect of the noradrenergic, presynaptic, alpha(2)-receptor-mediated block of Ca(2)+-currents in mitral cells appeared to be a wide-spread disinhibition of mitral cells in the accessory olfactory bulb as well as in the main olfactory bulb.

Evidence for central alpha(2)-adrenergic modulation of rat pineal melatonin synthesis

This study was performed to distinguish central and peripheral alpha(2)-adrenoceptors in the inhibition of rat pineal melatonin synthesis. The rats received lipo- or hydrophilic alpha(2)-adrenoceptor ligand injections at middark; after 1 or 2 h the pineal melatonin contents were measured. The lipophilic agonist medetomidine (100 microg/kg s.c.) suppressed the melatonin contents significantly, while the hydrophilic agonists ST-91 and p-aminoclonidine (10 or 100 microg/kg i.v.) did not. The suppression by medetomidine was counteracted by the lipophilic antagonist yohimbine (0.3-3.0 mg/kg i.p.) but not by the hydrophilic antagonist L-659,066 (1-10 mg/kg i.v.). In conclusion, the suppression of nocturnal melatonin synthesis by alpha(2)-adrenoceptor agonists is mainly of central origin.

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.