On the possible roles of noradrenaline, adenosine 5'-triphosphate and neuropeptide Y as sympathetic cotransmitters in the mouse vas deferens

Genesis and regulation of the heart automaticity

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.

Neuropeptide Y inhibits sympathetic neurotransmission in ipsilaterally innervated but not contralaterally reinnervated superior tarsal smooth muscle of the rat

The superior tarsal smooth muscle (STM), which elevates the upper eyelid, normally is innervated by sympathetic neurons from the ipsilateral superior cervical ganglion that are not neuropeptide Y-immunoreactive (NPY-ir). Following neonatal ganglionectomy, this target is reinnervated by sympathetic nerves from the contralateral superior cervical ganglion that are strongly NPY-ir. We examined the effects of exogenously administered NPY on STM tone, response to norepinephrine, and sympathetic neurotransmission in ipsilaterally innervated and contralaterally reinnervated STMs. NPY (2-10 micrograms/kg iv) increased blood pressure but did not alter STM tone. Similarly, contractile responses to co-administered norepinephrine were not affected. These findings imply an absence of direct and indirect postjunctional actions of NPY on STM. Contractions elicited by stimulation of the cervical sympathetic nerve (1.5 Hz) were not affected by NPY on the contralaterally reinnervated side; however, ipsilateral contractions were decreased in a dose-dependent fashion, with an inhibition of about 40% at 10 micrograms/kg. We conclude that while the STM is unresponsive to exogenously administered NPY, this peptide exerts selective inhibitory effects on the ipsilateral NPY-ir-negative but not the contralateral NPY-ir-positive innervation. This suggests that the neonatally denervated STM is reinnervated by contralateral fibers that are functionally different from the normal ipsilateral innervation in being devoid of functional prejunctional NPY receptors.

Sprouting of aberrant neuropeptide Y-immunoreactive sympathetic nerves into neonatally denervated smooth muscle

Sympathetic nerves normally project ipsilaterally to lateral cranial targets. Following unilateral superior cervical ganglionectomy in neonatal rats, however, neurons from the contralateral superior cervical ganglion sprout into the denervated region. In the present study we examined neuropeptide Y immunoreactivity (NPY-ir) of neurons comprising ipsilateral (control) and denervation-induced contralateral pathways to the superior tarsal smooth muscle of the eyelid. Fluoro-Gold injection of the control muscle retrogradely labelled 133 +/- 18 neurons in the ipsilateral superior cervical ganglion; of these, 21 +/- 3% displayed detectable NPY-ir. Fluoro-Gold injections of the reinnervated muscle labelled 20 +/- 4 neurons in the contralateral superior cervical ganglion, of which 85 +/- 3% contained detectable NPY-ir. Examination of the control tarsal muscle revealed DBH-ir noradrenergic nerves throughout the muscle and vasculature, while NPY-ir nerves were present primarily around blood vessels. In the reinnervated preparation, NPY-ir fibers innervated both blood vessels and tarsal muscle in a pattern similar to that of DBH-ir innervation. Acute excision of the remaining superior cervical ganglion eliminated all DBH-ir fibers bilaterally; NPY-ir was reduced markedly in the reinnervated preparations, though some fibers remained. We conclude that, following neonatal denervation, the tarsal muscle is reinnervated by a subpopulation of sympathetic neurons that differs in neuropeptide phenotype from that of the normal ipsilateral innervation.

Neuropeptide Y neuromodulation of sympathetic co-transmission in the guinea-pig vas deferens

1. We examined the neuromodulatory effects of neuropeptide Y (NPY) on purinergic and adrenergic co-transmission in the guinea-pig vas deferens. 2. In superfused vas deferens preparations, NPY (0.3 microM) inhibited the stimulus-evoked overflow of both ATP and [3H]-noradrenaline ([3H]-NA) at 2 Hz, but only the stimulus-evoked release of [3H]-NA at 20 Hz. 3. Postjunctionally, NPY greatly enhanced responses to alpha,beta-methylene ATP and to a lesser extent to exogenous NA. 4. Preparations stimulated in organ baths showed frequency-dependent contractions to field stimulation. NPY abolished responses to field stimulation at low frequency and a small number of pulses. At high frequency (20 Hz), NPY abolished responses elicited by 10 pulses, inhibited responses by 50% at 20 pulses and had little effect on preparations stimulated for 240 pulses. 5. Our study suggests that NPY neuromodulates co-transmission in the vas deferens by inhibiting the release of ATP and NA and that these effects predominate over the postjunctional enhancement by NPY. These results also show that the physiological effect of NPY will be determined both by the frequency at which the nerves are discharging and the duration of their firing.

Angiotensin neuromodulation of adrenergic and purinergic co-transmission in the guinea-pig vas deferens

1. The effects of angiotensin II (AII) and angiotensin III (AIII) on the isolated vas deferens of the guinea-pig were studied via three parameters: the overflow of adenosine 5'-triphosphate (ATP) and tritiated noradrenaline (NA), the mechanical response to field stimulation and the mechanical response to exogenous NA and alpha, beta-methylene ATP (alpha, beta-mATP). 2. At 2 Hz, AII enhanced the overflow of ATP and NA, whereas at 20 Hz, AII enhanced the overflow of NA but was without significant effect on ATP overflow. AIII, at 2 Hz, inhibited the overflow of ATP, but enhanced NA overflow, whereas at 20 Hz ATP overflow was unaffected, but NA overflow was still enhanced. 3. At 2 Hz, AII enhanced both phases of the response to field stimulation and at 20 Hz the overall response. AIII at 2 Hz enhanced the adrenergic response, but was without effect on the purinergic response to field stimulation. At 20 Hz, AIII was without effect on the overall response. 4. AII enhanced responses to exogenous NA and alpha, beta-mATP, whereas AIII was without effect. 5. These results provide evidence that both ATP and NA release are capable of being modulated by angiotensins. Furthermore, modulation of ATP release is frequency-dependent, whereas [3H]-NA release is not. These results raise questions about the mechanisms of storage and release of the sympathetic co-transmitters NA and ATP; they also show that angiotensin receptors in the guinea-pig vas deferens are not a homogeneous population.

Interactions of calcium antagonists and the calcium channel agonist Bay K 8644 on neurotransmission of the mouse isolated vas deferens

1. The present study compares the effects of verapamil and Bay K 8644 on twitches of the mouse vas deferens induced by field stimulation at 0.1 Hz. The influence of interactions between these drugs and nifedipine on neurotransmission was also investigated. 2. Bay K 8644 (0.1 nM-3 microM) and verapamil (1-100 microM) potentiated twitches maximally by about 1000% (EC50 17.3 nM) and 300% (EC50 17.5 microM), respectively. Nifedipine (0.1 nM-1 microM) only reduced twitch magnitude (IC50 7.7 nM). All effects were reversed following washout. 3. Yohimbine (1-100 microM) reversed twitch potentiation caused by verapamil but not by Bay K 8644. Prazosin (1 microM) did not reduce basal twitch tension nor antagonize twitch potentiation by verapamil. 4. Twitch inhibition by nifedipine was unaltered by previous incubation with verapamil (30 microM), but Bay K 8644 (1 microM) shifted the curve to nifedipine 120 fold to the right. Previous incubation with nifedipine (1 microM) blocked potentiation induced by verapamil but did not modify responsiveness to Bay K 8644. 5. Previous addition of verapamil (30 microM) markedly enhanced twitch potentiation caused by Bay K 8644 in a supra-additive fashion. In experiments conducted in the reversed condition, Bay K 8644 (1 nM but not 10 nM) potentiated the effect of verapamil in a similar manner but to a lesser extent. 6. It is concluded that verapamil, in contrast to nifedipine, markedly enhances neurally-evoked twitches of the mouse vas deferens. Bay K 8644 produces essentially the same effect as verapamil, but its potency is 1000 fold and its maximal effect about 3 fold greater than that observed for verapamil. It is suggested that the mechanism of twitch potentiation by verapamil is different from that of Bay K 8644 and may involve an increased release of non-adrenergic co-transmitter(s).

Central administration of neuropeptide Y induces hypothermia in mice. Possible interaction with central noradrenergic systems

Neuropeptide Y (0.24 and 1.17 nmol icv) and clonidine (0.025, 0.05 and 0.1 mg/Kg ip) induced a slight decrease of short duration of the rectal temperature in mice in a dose-dependent manner. While pretreatment with yohimbine (0.5 mg/Kg sc), was without effect on neuropeptide Y-induced hypothermia, it attenuated the hypothermic effect of clonidine. The association of neuropeptide Y (0.05 and 0.24 nmol icv) with clonidine (0.0125, 0.025, 0.05 and 0.1 mg/Kg ip) induced a synergistic effect, but it only was significant when neuropeptide Y 0.05 and 0.24 nmol icv was associated with clonidine 0.1 mg/Kg ip and when neuropeptide Y 0.05 nmol icv was associated with clonidine 0.05 mg/Kg ip. These results suggest that the effect of neuropeptide Y is not mediated by an interaction on alpha 2-adrenoceptor, but in accordance with these results, the existence of a collaborative mechanism between both neuropeptide Yergic and noradrenergic systems cannot be ruled out.

Integration of gut function by sympathetic reflexes

1. The spinal sympathetic outflow that innervates the gastrointestinal tract, including its blood vessels, has an impressive representation quantitatively, yet little is known about the functions of this system and its peripheral or central organization. Electrical stimulation or section of the splanchnic nerves have variable effects on the GI tract and does not, therefore, lead to a deeper understanding of the system. 2. The targets of the sympathetic supply of the GI tract are blood vessels, nonvascular (sphincteric) smooth musculature, myenteric neurones, submucous neurones and gut associated lymphoid tissues. The corresponding functions associated with these targets are regulation of blood flow (particularly through the mucosa) and resistance to flow, of motility, of secretion and absorption and of immune responses. Little is known about the effects of the sympathetic nervous system on the latter function. 3. The sympathetic postganglionic neurones are (at least in the guinea-pig) neurochemically characterized with respect to the targets. Neurones projecting to blood vessels contain neuropeptide Y in addition to noradrenaline, while neurones projecting to the submucous plexus contain somatostatin. No neuropeptide has been detected to date in neurones projecting to the myenteric plexus. 4. Transmission through guinea-pig prevertebral ganglia in vitro have been studied electrophysiologically. The following functions have been demonstrated: (a) transmission and distribution of preganglionic impulse activity to the targets in a relay-like fashion; (b) mediation of peripheral intestinointestinal reflexes between different sections of the GI tract; (c) integration of activity from the spinal cord and from various peripheral sources. The first function may apply particularly to the sympathetic pathway innervating blood vessels. Whether the second function occurs in vivo is questionable. The third function is the most important one for pathways involved in the regulation of motility and probably secretion and absorption. 5. Only limited information is available on preganglionic neurones projecting to prevertebral ganglia. Neurones regulating blood vessels are probably located in the intermediolateral cell column, and non-vascular visceral preganglionic neurones are situated medial to this cell column in the intermediate zone of the spinal cord. Vascular (vasoconstrictor) neurones exhibit a reflex pattern which is largely dependent on the brain stem. Spinal cord transection rostral to the sympathetic outflow causes an immediate abolition of basal and reflex activity in these neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Biochemistry of information storage in the nervous system

The use of molecular biological approaches has defined new mechanisms that store information in the mammalian nervous system. Environmental stimuli alter steady-state levels of messenger RNA species encoding neurotransmitters, thereby altering synaptic, neuronal, and network function over time. External or internal stimuli alter impulse activity, which alters membrane depolarization and selectively changes the expression of specific transmitter genes. These processes occur in diverse peripheral and central neurons, suggesting that information storage is widespread in the neuraxis. The temporal profile of any particular molecular mnemonic process is determined by specific kinetics of turnover and by the geometry of the neuron resulting in axonal transport of molecules to different synaptic arrays at different times. Generally, transmitters, the agents of millisecond-to-millisecond communication, are subject to relatively long-lasting changes in expression, ensuring that ongoing physiological function is translated into information storage.