Transient expression of alpha-1B adrenoceptor messenger ribonucleic acids in the rat superior cervical ganglion during postnatal development

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.

Baroreceptor-mediated activation of sympathetic nerve activity to salivary glands

Salivary gland function is regulated by both the sympathetic and parasympathetic nervous systems. Previously we showed that the basal sympathetic outflow to the salivary glands (SNA(SG)) was higher in hypertensive compared to normotensive rats and that diabetes reduced SNA(SG) discharge at both strains. In the present study we sought to investigate how SNA(SG) might be modulated by acute changes in the arterial pressure and whether baroreceptors play a functional role upon this modulation. To this end, we measured blood pressure and SNA(SG) discharge in Wistar-Kyoto rats (WKY-intact) and in WKY submitted to sinoaortic denervation (WKY-SAD). We made the following three major observations: (i) in WKY-intact rats, baroreceptor loading in response to intravenous infusion of the phenylephrine evoked an increase in SNA(SG) spike frequency (81%, p<0.01) accompanying the increase mean arterial pressure (ΔMAP: +77 ± 14 mmHg); (ii) baroreceptor unloading with sodium nitroprusside infusion elicited a decrease in SNA(SG) spike frequency (17%, p<0.01) in parallel with the fall in arterial blood pressure (ΔMAP: -30 ± 3 mmHg) in WKY-intact rats; iii) in the WKY-SAD rats, phenylephrine-evoked rises in the arterial pressure (ΔMAP: +56 ± 6 mmHg) failed to produce significant changes in the SNA(SG) spike frequency. Taken together, these data show that SNA(SG) increases in parallel with pharmacological-induced pressor response in a baroreceptor dependent way in anaesthetised rats. Considering the key role of SNA(SG) in salivary secretion, this mechanism, which differs from the classic cardiac baroreflex feedback loop, strongly suggests that baroreceptor signalling plays a decisive role in the regulation of salivary gland function.

Sympathetic nerve activity in the superior cervical ganglia increases in response to imposed increases in arterial pressure

Sympathetic vasoconstriction of cerebral vessels has been proposed to be a protective mechanism for the brain, limiting cerebral perfusion and microcirculatory pressure during transient increases in arterial pressure. To furnish direct neural evidence for this proposition, we aimed to develop a method for recording cerebral sympathetic nerve activity (SNA) from the superior cervical ganglion (SCG). We hypothesized that SNA recorded from the SCG increases during imposed hypertension, but not during hypotension. Lambs (n = 11) were anesthetized (alpha-chloralose, 20 mg.kg(-1).h(-1)) and ventilated. SNA was measured using 25-microm tungsten microelectrodes inserted into the SCG. Arterial blood pressure (AP) was pharmacologically raised (adrenaline, phenylephrine, or ANG II, 1-50 microg/kg iv), mechanically raised (intravascular balloon in the thoracic aorta), or lowered (sodium nitroprusside, 1-50 microg/kg iv). In response to adrenaline (n = 10), mean AP increased 135 +/- 10% from baseline (mean +/- SE), and the RMS value of SNA (Square Root of the Mean of the Squares, SNA(RMS)) increased 255 +/- 120%. In response to mechanically induced hypertension, mean AP increased 43 +/- 3%, and SNA(RMS) increased 53 +/- 13%. Generally, (9 of 10 animals), SNA(RMS) did not increase, as AP was lowered with sodium nitroprusside. Using a new model for direct recording of cerebral SNA from the SCG, we have demonstrated that SNA increases in response to large induced rises, but not falls, in AP. These findings furnish direct support for the proposed protective role for sympathetic nerves in the cerebral circulation.

Crosstalk between presynaptic angiotensin receptors, bradykinin receptors and alpha 2-autoreceptors in sympathetic neurons: a study in alpha 2-adrenoceptor-deficient mice

1. In mouse atria, angiotensin II and bradykinin lose much or all of their noradrenaline release-enhancing effect when presynaptic alpha(2)-autoinhibition does not operate either because of stimulation with very brief pulse trains or because of treatment with alpha(2) antagonists. We now studied this operational condition in alpha(2)-adrenoceptor-deficient mice. Release of (3)H-noradrenaline was elicited by electrical stimulation. 2. In tissues from wild-type (WT) mice, angiotensin II and bradykinin increased the overflow of tritium evoked by 120 pulses at 3 Hz. This enhancement did not occur or was much reduced when tissues were stimulated by 120 pulses at 3 Hz in the presence of rauwolscine and phentolamine, or when they were stimulated by 20 pulses at 50 Hz. 3. In tissues from mice lacking the alpha(2A)-adrenoceptor (alpha(2A)KO) or the alpha(2B)-adrenoceptor (alpha(2B)KO), the concentration-response curves of angiotensin II and bradykinin (120 pulses at 3 Hz) were unchanged. In tissues from mice lacking the alpha(2C)-adrenoceptor (alpha(2C)KO) or both the alpha(2A)- and the alpha(2C)-adrenoceptor (alpha(2AC)KO), the concentration-response curves were shifted to the same extent downwards. 4. As in WT tissues, angiotensin II and bradykinin lost most or all of their effect in alpha(2A)KO and alpha(2AC)KO tissues when rauwolscine and phentolamine were present or trains consisted of 20 pulses at 50 Hz. 5. Rauwolscine and phentolamine increased tritium overflow evoked by 120 pulses at 3 Hz up to seven-fold in WT and alpha(2B)KO tissues, three-fold in alpha(2A)KO and alpha(2C)KO tissues, and two-fold in alpha(2AC)KO tissues. 6. Results confirm that angiotensin II and bradykinin require ongoing alpha(2)-autoinhibition for the full extent of their release-enhancing effect. Specifically, they require ongoing alpha(2C)-autoinhibition. The peptide effects that remain in alpha(2C)-autoreceptor-deficient mice seem to be because of alpha(2B)-autoinhibition. The results hence also suggest that in addition to alpha(2A)- and alpha(2C)- mouse postganglionic sympathetic neurons possess alpha(2B)-autoreceptors.

Expression of mRNA and functional alpha(1)-adrenoceptors that suppress the GIRK conductance in adult rat locus coeruleus neurons

1. Locus coeruleus neurons in adult rats express binding sites and mRNA for alpha(1)-adrenoceptors even though the depolarizing effect of alpha(1)-adrenoceptor agonists on neonatal neurons disappears during development. 2. In this study intracellular microelectrodes were used to record from locus coeruleus neurons in brain slices of adult rats and reverse transcription-polymerase chain reaction (RT - PCR) was used to investigate the mRNA expression of alpha(1)- and alpha(2)-adrenoceptors in juvenile and adult rats. 3. The alpha(1)-adrenoceptor agonist phenylephrine had no effect on the membrane conductance of locus coeruleus neurons (V(hold) -60 mV) but decreased the G protein coupled, inward rectifier potassium (GIRK) conductance induced by alpha(2)-adrenoceptor or mu-opioid agonists. The GIRK conductance induced by noradrenaline was increased in amplitude when alpha(1)-adrenoceptors were blocked with prazosin. 4. RT - PCR of total cellular RNA isolated from microdissected locus coeruleus tissue demonstrated strong mRNA expression of alpha(1a)-, alpha(1b)- and alpha(1d)-adrenoceptors in both juvenile and adult rats. However, only mRNA transcripts for the alpha(1b)-adrenoceptors were consistently detected in cytoplasmic samples taken from single locus coeruleus neurons of juvenile rats, suggesting that this subtype may be responsible for the physiological effects seen in juvenile rats. 5. Juvenile and adult locus coeruleus tissue expressed mRNA for the alpha(2a)- and alpha(2c)-adrenoceptors while the alpha(2b)-adrenoceptor was only weakly expressed in juveniles and was not detected in adults. 6. The results of this study show that alpha(1)-adrenoceptors expressed in adult locus coeruleus neurons function to suppress the GIRK conductance that is activated by mu-opioid and alpha(2)-adrenoceptors.

Development of peripheral autonomic synapses: neurotransmitter receptors, neuroeffector associations and neural influences

1. The functional innervation of autonomic target tissues occurs early during development, at a time when both the nerves and post-synaptic target tissues are still differentiating. 2. Physiological responses appear soon after the arrival of the first fibres when uptake and release mechanisms within the nerves are already functional. Initial responses differ from those in the mature animal, both in the form and, frequently, in the subtypes of receptors involved. 3. Results of a number of studies suggest that the initial expression of neurotransmitter receptors during development is largely independent of neural influences. Changes recorded in neurotransmitter receptor expression during development appear to be similarly independent of neural influences. 4. While signal transduction pathways coupling adrenergic neurotransmitter receptors to effector responses appear to develop independently of the nerves, the efficient coupling of muscarinic receptors often requires the action of the neurotransmitter, acetylcholine. 5. During the period of synapse formation, the neural plexus continues to expand. While developing varicosities can release the neurotransmitter, the capacity for neurotransmitter retention appears to be restricted. Developmental changes in the neurotransmitters that produce functional responses, while well known in the sweat glands, may also be seen in more subtle forms in other target tissues. 6. Ultrastructural studies suggest that close physical associations between the membranes of the release sites of the developing nerves and the target cells may form early during development when physiological responses are still immature. These close associations could enable more specific reciprocal interactions between nerves and target cells involving known and novel growth factors, neuropeptides and cytokines important in shaping the mature synaptic characteristics.

Marsupial retinocollicular system shows differential expression of messenger RNA encoding EphA receptors and their ligands during development

The protracted development of the wallaby (Macropus eugenii) has allowed study of messenger RNAs encoding Eph receptors EphA3 and EphA7 and ligands ephrin-A2 and -A5 in the retina and superior colliculus at intervals throughout the development of the retinocollicular projection: from birth, before retinal innervation, to postnatal day 95, when the projection is mature. Reverse transcription-polymerase chain reaction showed messenger RNAs for both receptors and ligands were expressed at all ages. EphA7 was expressed more highly in the rostral superior colliculus. Ephrin-A2 and -A5 were expressed more highly in the caudal colliculus. EphA3 was expressed in a complementary manner, more highly in temporal than in nasal retina. There are higher levels of expression of the ligands when the projection is only coarsely topographically organised. This suggests a role for them and their receptor EphA3 in this stage, by repulsive interactions which restrict temporal axons to rostral superior colliculus. This is the first account in a marsupial mammal of the appearance of this molecular family, substantiating its ubiquitous role in topographically organised neuronal connections. Nevertheless, expression is not the same as in the mouse, suggesting differences in the details of topographic coding between species.

Presynaptic adrenergic facilitation of parasympathetic neurotransmission in sympathectomized rat smooth muscle

1. Parasympathetic innervation of rat eyelid tarsal smooth muscle normally inhibits sympathetic neurotransmission prejunctionally without significant direct postjunctional effects. Following surgical sympathectomy, parasympathetic stimulation elicits smooth muscle contraction. This study examined the relative contributions of cholinergic and adrenergic mechanisms mediating these contractions. 2. Electrical stimulation of the superior salivatory nucleus, which activates tarsal muscle parasympathetic nerves, elicited large contractions at 2 days postsympathectomy, which were abolished by atropine and were decreased by 65 % by alpha1-adrenoceptor blockade or spinal cord transection. 3. Contractions in response to direct cholinergic stimulation by bethanechol at 2 days postsympathectomy were increased following spinal cord transection (C2) and suppressed by the alpha1-adrenoceptor agonist phenylephrine, indicating that adrenoceptors on smooth muscle attenuate cholinergic contractions. However, phenylephrine infusion enhanced contractile responses to parasympathetic stimulation. 4. Reverse transcription-polymerase chain reaction revealed alpha1D-adrenoceptor mRNA within pterygopalatine ganglia. 5. At 5 weeks and 14 months postsympathectomy, adrenergic facilitation was significantly less than at 2 days, whereas prazosin-insensitive muscarinic contraction was increased. 6. We conclude that degeneration of sympathetic innervation is followed rapidly by adrenoceptor-mediated prejunctional enhancement of parasympathetic nerve-smooth muscle neurotransmission, which occurs prior to neuroeffector junction formation as determined previously by electron microscopy. Subsequently, noradrenergic enhancement is diminished as cholinergic neurotransmission becomes established.

Age and region-dependent contraction to alpha-adrenoceptor agonists in rat and guinea-pig isolated trachea

1. The influence of age and of region on alpha-adrenoceptor-mediated contraction to (-)-adrenaline and (-)-noradrenaline was examined in rat (4-136 weeks) and guinea-pig (2-156 weeks) isolated tracheal ring preparations with particular emphasis on the early (up to 12 weeks) maturation phase. 2. In rat tracheal rings, significant regional variation was observed with respect to maximal (-)-adrenaline-induced contraction, such that the greatest activity was seen in ring preparations from the laryngeal end of the trachea. Tracheal rings from the carinal end responded very poorly or were unresponsive to (-)-adrenaline, depending on animal age. These regional differences were seen across the age range. The potencies of (-)-adrenaline and (-)-noradrenaline remained unchanged with respect to animal age, but the maximum contractile tension that developed in response to these agonists increased with increasing animal age in all regions of the trachea. 3. In guinea-pig isolated tracheal tissue, maximum contractile responses (Emax) to (-)-adrenaline and (-)-noradrenaline remained unchanged with increasing animal age. In addition, there was no evidence for a region-dependence in the responsiveness of tracheal tissue to alpha-adrenoceptor-mediated contraction in this species. 4. In both guinea-pig and rat isolated tracheal tissue, alpha-adrenoceptor-mediated contraction appeared to involve the activation of alpha1-adrenoceptors.