Distribution of choline acetyltransferase in cerebral and extracerebral cranial arteries of the cat. Its relationship to neurogenic atropine-sensitive dilation

Microsphere embolism-induced cortical cholinergic deafferentation and impairments in attentional performance

Ischemic events have been hypothesized to play a critical role on the pathogenesis of dementia and the acceleration of cognitive impairments. This experiment was designed to determine the consequences of microvascular ischemia on the cortical cholinergic input system and associated attention capacities. Injections of microspheres ( approximately 50 microm diameter; approximately 5000 microspheres/100 microL) into the right common carotid artery of rats served as a model of microvascular ischemia and resulted in decreases in the density of cholinergic fibers in the ipsilateral medial prefrontal cortex and frontoparietal areas. Furthermore, dense astrogliosis, indicated by glial fibrillary acidic protein (GFAP) immunohistochemistry, was observed in the globus pallidus, including the areas of origin of cholinergic projections to the cortex. Fluoro-Jade B staining indicated that loss of neurons in the cortex was restricted to areas of microsphere-induced infarcts. Attentional performance was assessed using an operant sustained attention task; performance in this task was previously demonstrated to reflect the integrity and activity of the cortical cholinergic input system. Embolized animals' performance was characterized by a decrease in the animals' ability to detect signals. Their performance in non-signal trials remained unaffected. The residual density of cholinergic axons in prefrontal and frontoparietal areas correlated with the animals' performance. The present data support the hypothesis that microvascular ischemia results in loss of cortical cholinergic inputs and impairs associated attentional performance. Microsphere embolism represents a useful animal model for studying the role of interactions between microvascular disorder and impaired forebrain cholinergic neurotransmission in the manifestation of cognitive impairments.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Inhibition of acetylcholinesterase modulates the autoregulation of cerebral blood flow and attenuates ischemic brain metabolism in hypertensive rats

We designed the present study to examine whether or not the inhibition of acetylcholinesterase modulates cerebral microcirculation in hypotension and improves brain metabolism in ischemia induced by bilateral carotid artery occlusion in hypertensive rats. Blood flow to the parietal cortex was determined by the H2 clearance method. Lactate, pyruvate, and ATP were estimated by enzymatic methods. Acetylcholinesterase inhibitor (AChEI, ENA-713), at 0.05, 0.1, or 0.5 mg/kg, was intravenously injected 10 min before either hemorrhagic hypotension or cerebral ischemia. The levels of acetylcholine in the control were 29.3 +/- 8.1 (mean +/- SD) and 39.5 +/- 8.1 pmol/mg in the cortex and hippocampus, respectively, and they were significantly decreased by 15-19% after 60 min of ischemia in the vehicle-treated rats. AChEI preserved the levels to 93-98% of the control (p < 0.05 versus vehicle). The lower limit of autoregulation was 74 +/- 9% of the resting values. The administration of AChEI helped preserve blood flow and lowered the limit to 64 +/- 6% (p < 0.05 versus control). After 60 min of ischemia, lactate increased 6.5-fold and ATP decreased to 64% of the control value. The administration of AChEI dose-dependently reduced the lactate level 1.9- to 3.9-fold and well preserved the ATP level to 94-97% of the control. The inhibition of acetylcholinesterase activity may preserve cerebral autoregulation during hypotension and protect cerebral metabolism against ischemic insult.

Cholinergic and vasoactive intestinal polypeptidergic innervation of the cerebral arteries

Acetylcholine and vasoactive intestinal polypeptide are not only two vasoactive agonists that predominantly induce a vasodilatation of the cerebral arteries, but also correspond to neurotransmitters that innervate the various anatomical segments of the cerebral vasculature. The distinct patterns of the cerebrovascular cholinergic and vasoactive intestinal polypeptidergic innervation, their neurochemistry, in vitro and in vivo pharmacology, as well as the putative pathophysiological implications of these neurotransmission systems are critically summarized on the basis of the most recently published literature.

Presynaptic muscarinic receptor subtypes involved in the inhibition of acetylcholine and noradrenaline release in bovine cerebral arteries

Experiments were performed in bovine cerebral arteries preincubated with [3H]-choline or [3H]-noradrenaline to analyze the presynaptic muscarinic receptors involved in inhibition of acetylcholine and noradrenaline release induced by electrical stimulation (4 Hz, 200 mA, 0.3 ms, 1 min). For this purpose, the actions of several muscarinic receptor antagonists on the 3H overflow and on the carbachol-induced inhibition of this overflow were assessed. The evoked [3H]-acetylcholine release and [3H]-noradrenaline release were markedly reduced by the presence of tetrodotoxin, Ca(2+)-free medium, and the inhibitor of both choline transport and choline acetyltransferase, AF64A. Chemical sympathetic denervation with 6-hydroxydopamine (6-OHDA) decreased the uptake of [3H]-noradrenaline, and AF64A reduced mainly the uptake of [3H]-choline, but also of [3H]-noradrenaline. Carbachol reduced the evoked [3H]-noradrenaline and [3H]-acetylcholine release; the IC50 values were 0.37 and 0.43 mumol/l, respectively. Atropine and 4-DAMP, but not AF-DX 116, methoctramine or pirenzepine, increased the evoked [3H]-acetylcholine release. However, these muscarinic antagonists failed to modify the evoked [3H]-noradrenaline release. Carbachol inhibited the release of both acetylcholine and noradrenaline. The inhibition was blocked by the antagonists. The rank orders of potency (based on plC50 values) were, in the case of [3H]-acetylcholine release, atropine greater than 4-DAMP greater than AF-DX 116 greater than or equal to pirenzepine greater than or equal to methoctramine, and, in the case of [3H]-noradrenaline release, atropine greater than 4-DAMP greater than AF-DX 116 greater than or equal to methoctramine greater than or equal to pirenzepine.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine levels and choline acetyltransferase activity in rat cerebrovascular bed after uni- or bilateral sphenopalatine ganglionectomy

Endogenous acetylcholine (ACh) levels and choline acetyltransferase (ChAT) activity were measured in several vascular segments (major cerebral arteries, cortical pial vessels, and peripheral arteries) and nervous tissues [including the sphenopalatine ganglion (SPG)] in the rat. The effects of uni- or bilateral surgical ablation of the SPG, a putative origin of the cholinergic cerebrovascular innervation, were investigated on these two specific cholinergic markers at various postoperative times. ChAT activity and ACh levels were enriched in the cerebral as compared to the peripheral arteries. Among the cerebrovascular tissues tested, ACh levels were particularly high in the circle of Willis and the vertebrobasilar segments and, to a lesser extent, in the middle cerebral artery. Lower levels were found in the small pial vessels and choroid plexus. Overall, ChAT activity measured in different arterial beds paralleled the distribution of ACh. Following uni- or bilateral removal of the SPG, slight reductions (18-36%, statistically not significant) were observed in ChAT activity in rostral cerebral arteries and pial vessels overlying the frontal cortex. Similarly, bilateral ganglionectomy resulted in minor decreases (11-22%, not significant) in the cerebrovascular contents of ACh in these same vascular segments. These results clearly show that the SPG does not or only partly contributes to the cholinergic fibers that supply the cerebrovascular bed.

Small pial vessels, but not choroid plexus, exhibit specific biochemical correlates of functional cholinergic innervation

In an attempt to provide the biochemical foundations for a putative cholinergic innervation of small pial vessels and choroid plexus, we have assessed their ability to specifically accumulate choline, synthesize and release acetylcholine (ACh) in response to depolarization. Our results show that both small pial vessels and choroid plexus avidly accumulate choline via a sodium-dependent mechanism which could be inhibited by hemicholinium-3 (IC50 in pial vessels = 47.8 microM). Light microscopic examination of radioautographs from vessels incubated with [3H]choline revealed two distinct sites of accumulation in the vessel wall. One site probably corresponded to nerve terminals and the other was closely associated with the endothelial cells. In small pial vessels, a major proportion (60%-70%) of the choline acetyltransferase (ChAT) activity could be inhibited by 4-naphthylvinylpyridine (4-NVP), a potent inhibitor of neuronal ChAT; and, following either K+ or veratridine depolarization, a Ca2(+)-dependent release of authentic [3H]ACh could be measured. In contrast, the choroid plexus exhibited a rather low ChAT activity which was not inhibited by 4-NVP and no release of ACh could be detected in this tissue following depolarization. Altogether, the results of the present study show that (1) small pial vessels exhibit all the most selective biochemical markers that are characteristic of cholinergic nerves; (2) [3H]choline in pial vessels can be accumulated in non-neuronal elements which probably correspond to the endothelial cells; and (3) the choroid plexus failed to exhibit convincing biochemical markers that would attest in favor of a functional cholinergic innervation.(ABSTRACT TRUNCATED AT 250 WORDS)

Origins and pathways of choline acetyltransferase-positive parasympathetic nerve fibers to cerebral vessels in rat

The presence of cholinergic nerve fibers in the brain vasculature has been a matter of controversy, partly due to the lack of a reliable histochemical marker. Accordingly, no distinct information about the origin and pathways for such fibers has been available. In the present study on the rat pial vasculature, utilizing a choline acetyltransferase (ChAT) antibody, which is able to demonstrate this enzyme in peripheral nervous tissue, evidence was obtained for an innervation by cholinergic fibers of large pial arteries. Vasoactive intestinal polypeptide (VIP) was present in or in close association with these fibers. By the aid of the retrograde axonal tracer True Blue (TB) applied to the middle cerebral arterial wall, such fibers were shown to originate in a subgroup of ChAT-positive cells in the sphenopalatine, otic, and internal carotid ganglia, which, in addition, contained VIP. The ChAT-positive pial nerve fibers were few in relation to the VIP-immunoreactive fibers, as was also illustrated by the few TB-positive cells in the ganglia that were ChAT positive as compared with the number of cells that were VIP positive. Only a small population of ChAT-containing neurons in these ganglia appeared to project to the pial vessels. The pathway from the sphenopalatine ganglion is via a membranous structure on the medial orbital wall, through the ethmoidal foramen, and along the internal ethmoidal artery to reach the circle of Willis. The fibers from the internal carotid and otic ganglia probably bridge to the internal carotid artery in the carotid canal, those from the otic ganglion after an initial course in the lesser superficial petrosal nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hemoglobin on neurogenic responses and cholinergic parameters in porcine cerebral arteries

Electrically stimulated neurogenic vasodilation and endothelial-dependent cholinergic vasodilation in cerebral arteries are both blocked by hemoglobin. To determine if neurogenic vasodilation has a cholinergic component, we examined the effect of hemoglobin on neurogenic responses and perivascular cholinergic parameters in isolated porcine cerebral arteries. The perfused circle of Willis has a mixed response to transmural nerve stimulation (TNS) that is predominantly vasodilation. Exposure to hemoglobin (5 microM) causes constriction of this preparation while simultaneously blocking TNS-induced vasodilation. At similar concentrations, however, hemoglobin did not alter electrically stimulated, tetrodotoxin-sensitive release of acetylcholine. Hemoglobin also had no effect on neuronal choline uptake or esteratic inactivation of acetylcholine. These results demonstrate the ability of low concentrations of hemoglobin to alter cerebral neurogenic vasodilation. The failure of hemoglobin to affect any aspect of cholinergic transmission, however, provides further evidence against a direct vasodilatory role for acetylcholine as a terminal transmitter in isolated cerebral blood vessels.

Heterogeneous vasomotor responses of anatomically distinct feline cerebral arteries

1. The vasomotor reactivity to a number of neurotransmitters and blood-borne substances was evaluated in several anatomically distinct arteries of the cat cerebral circulation. Few regional differences were observed in their vasoconstrictor responses to noradrenaline, dopamine, 5-hydroxytryptamine and prostaglandin F2 alpha. Only the anterior cerebral artery reacted strongly to all vasoconstrictor agents. 2. Adenosine, acetylcholine and histamine induced pronounced relaxation in the vast majority of the major cerebral arteries. The relaxation elicited by adenosine showed a slight degree of heterogeneity between the arteries and the overall response accounted for 81 +/- 6% of the pharmacologically-induced tone. On the other hand, the dilatation induced by acetylcholine and histamine varied as a function of the anatomical localization of the cerebral arteries. The acetylcholine-induced vasodilatation was significantly more pronounced in the middle cerebral, anterior communicating and anterior cerebellar arteries, with respective responses of 72, 66 and 83% of the induced tone as compared to 43% in the other vessels. However, all arteries were equally sensitive to acetylcholine with an overall mean pD2 value of 7.47 +/- 0.06. The most heterogeneous results were obtained with histamine and applied both to the magnitude of the maximal response and the sensitivity of the various arteries to this amine. The intensity of the relaxation varied from 20% (anterior communicating artery) to 118% (posterior cerebellar artery). 3. Among the neuropeptides studied, substance P and bradykinin were considerably less potent than vasoactive intestinal peptide on all the cerebral arteries. The least responsive vessel to bradykinin was the anterior cerebral artery with a maximal response of 22 +/- 5% of the induced-tone and a pD2 value of 7.56 +/- 0.24. All vessels responded weakly to substance P and those from the vertebrobasilar circulation were significantly less sensitive to this neuropeptide with pD2 values around 8.07 as compared to 9.82 in the more rostral arteries. Although all vessels were equally sensitive to vasoactive intestinal peptide, the dilator responses were significantly less pronounced in the middle cerebral and basilar arteries (maximal response of 86 +/- 5% and 69 +/- 6% of the induced-tone, respectively, as compared to 110 +/- 9% in the other vessels). 4. The vertebrobasilar arteries were as reactive, if not more reactive, to vasoconstrictors than the vessels originating from the carotid circulation. In contrast, the dilator responses were less marked in most caudal arteries. Such dichotomies may be important in the regulation of local cerebral blood flow. 5. The results emphasize the considerable heterogeneity in the vasomotor responses to a given substance among the various cerebral arteries. Further, they suggest the presence of multiple receptor populations which mediate opposite effects and which are distributed in different proportions among the cephalic arteries.

Parasympathetic innervation of cutaneous blood vessels by vasoactive intestinal polypeptide-immunoreactive and acetylcholinesterase-positive nerves: histochemical and experimental study on rat lower lip

The distribution and origin of perivascular acetylcholinesterase-active and vasoactive intestinal polypeptide-immunoreactive nerve fibers were studied in the rat lower lip by means of acetylcholinesterase histochemistry and vasoactive intestinal polypeptide immunohistochemistry. The perivascular nerve fibers stained intensely with both histochemical techniques and were widely distributed on small arteries and arterioles of the lower lip, especially in the transitional zone between the hairy skin and the mucous membrane. The distributions of the two types of fibers were very similar and most of them showed overlapping coloration, on consecutive staining for vasoactive intestinal polypeptide and acetylcholinesterase. Both acetylcholinesterase-positive and vasoactive intestinal polypeptide-immunoreactive fibers were completely lost on removal of the otic ganglion, while they were not affected by sympathetic ganglion removal or sensory nerve sectioning. In the otic ganglion, most cells exhibited acetylcholinesterase activity, and about 60% of the cells showed light to heavy vasoactive intestinal polypeptide immunoreactivity. These findings indicate that vessels in the rat lip are innervated by parasympathetic fibers originating from the otic ganglion and support the view that vasoactive intestinal polypeptide is present in cholinergic neurons. This may suggest the possible control by the parasympathetic nervous system of cutaneous blood vessels through vasoactive intestinal polypeptide-containing cholinergic neurons, in general or at least in the facial area.

Neuronal versus endothelial origin of vasoactive acetylcholine in pial vessels

Functional pial vessels denuded in situ of the endothelial cell layer exhibit a markedly decreased choline uptake capacity (-53%) but integrally preserved choline acetyltransferase (ChAT) activity and acetylcholine (ACh) release mechanisms. These studies demonstrate that endothelial cells possess a specific choline uptake system. However, the unimpaired ChAT activity in denuded pial vessels implies that the endothelial pool of choline is not significantly metabolized into ACh. In spite of possible differences in the mechanisms that govern release processes in endothelial and neuronal elements, taken together the findings of the present study suggest that the ACh released following depolarization of pial blood vessels originates predominantly from cholinergic perivascular nerve terminals.

Reduction of common carotid resistance upon stimulation of an area dorsal to the facial nucleus of cats

In chloralose-urethane-anesthetized cats, electrical stimulation and glutamate injection on a small reticular area just dorsal to the facial nucleus (DFA) elicited an ipsilateral reduction in the common carotid resistance (CCR-reduction) with no or minimal change in other cardiovascular parameters. CCR-reduction was mediated via facial and glossopharyngeal nerves, involving partially muscarinic and partially non-muscarinic mechanisms.

Sympathetic, muscarinic vasodilation in cranial vessels of the cat

Pressor responses evoked by stimulation of the preganglionic sympathetic trunk of the feline superior cervical ganglion have been recorded in vivo from the vascular bed perfused by one external carotid and the vertebral artery. When vasoconstrictor activity is blocked and potential vasodilator activity enhanced by close, intracarotid injection of guanethidine and prostaglandin F2 alpha respectively, stimulation evokes a weak pressor response followed, on cessation of stimulation, by a prolonged vasodilation lasting for 6-8 min. The magnitude and duration of the poststimulation vasodilation was reduced significantly by atropine. Due to the prolonged nature of the vasodilation, it is unlikely that a sympathetic cholinergic vasodilation in the classical sense is involved.

Alpha-adrenoreceptors and muscarine receptors in human pial arteries and microvessels: a receptor binding study

Human pial arteries and intraparenchymal microvessels were isolated for enzyme assays and radioligand binding studies of receptors. Special attention was paid to contamination with brain tissue, which was assessed by luxol staining and cerebroside assays for myelin and by scanning electron microscopy. The amount of contamination was approximately 1% for pial vessels and 14% for microvessel preparations. Significant levels of alpha 1-adrenoreceptors (binding sites for [3H]prazosin) and alpha 2-adrenoreceptors (sites labeled by [3H]azidoclonidine) were found in both types of vessels, suggesting that each receptor can modify contractility in these human vessels. Levels of muscarine receptors (sites labeled with [3H]quinuclidinyl benzilate) and choline acetyltransferase activity were considered significant only in pial vessels.

Nimodipine has an inhibitory action on neurotransmitter release from human cerebral arteries

The effect of the dihydropyridine nimodipine was studied on the resting and K+-evoked release of [3H]acetylcholine (ACh) and [3H]5-hydroxytryptamine (5HT) from postmortem human cerebral arteries. Nimodipine, at a concentration of 30 microM, significantly reduced the K+-evoked release of [3H]ACh from anterior and middle cerebral arteries by 36 and 70%, respectively, and the K+-evoked release of [3H]5HT from basilar and middle cerebral arteries by 55 and 66%, respectively. The mode of action of nimodipine is interpreted in terms of a specific effect on the depolarisation-induced calcium current occurring in neuronal elements present in these preparations but absent from brain.

Vascular headaches and cerebral circulation: an overview

A complex network of neurotransmission systems underlies the control of the cerebral circulation. Classical neurotransmitters, vasoactive peptides and receptors have been found in cerebral arteries. Central and peripheral structures are also probably involved in the neurogenic control of the cerebral circulation. Vascular and neurotransmission changes reported in vascular headaches suggest that an alteration of the neurogenic control of the brain circulation may be implicated in vascular headaches. In particular, locus coeruleus, which may control the intracerebral adrenergic pathway, can induce vascular changes similar to those of migraine. Moreover, the trigeminal ganglion, which may induce the release of substance P, can change the extracranial and intracranial vasodilator activity. The vascular theory of migraine, proposed by Wolff, is re-evaluated on the grounds of a possible mediation of the vascular responses by neurotransmitters. It is hypothesized that a deficient modulation by enkephalins may cause alterations of locus coeruleus and/or trigeminal ganglion. The problem of pain in vascular headaches is also considered: whether it is of vascular origin or whether it is due to a dysfunction of the central nociceptive pathway. Knowledge of the neurogenic control of the cerebral circulation may be useful in understanding some pathogenetic mechanisms of vascular headaches.

Perivascular nerves with immunoreactivity to vasoactive intestinal polypeptide in cephalic arteries of the cat: distribution, possible origins and functional implications

The distribution of nerves containing vasoactive intestinal polypeptide(VIP)-immunoreactive material was examined in the cephalic arteries and cranial nerves of cats using an indirect immunofluorescence procedure on whole mounts. Perivascular VIP-immunoreactive nerves were widely distributed in arteries and arterioles supplying glands, muscles and mucous membranes of the face. Within the cerebral circulation, perivascular VIP-immunoreactive nerves were most abundant in the circle of Willis and the proximal portions of the major cerebral arteries and their proximal branches supplying the rostral brainstem and ventral areas of the cerebral cortex. Nerves containing VIP-immunoreactive material were absent from distal portions of arteries supplying the posterior brainstem, cerebellum and dorsal cerebral cortex. Cerebral perivascular VIP-immunoreactive nerves had extracerebral origins probably from VIP-immunoreactive perikarya within microganglia in the cavernous plexus and external rete. Extracerebral perivascular VIP-immunoreactive nerves probably arose from VIP-immunoreactive perikarya in microganglia associated with the tympanic plexus, chorda tympani, lingual nerve and Vidian nerve as well as from cells in the otic, sphenopalatine, submandibular and sublingual ganglia. Therefore, it seems likely that each major segment of the cephalic circulation is supplied by local VIP-immunoreactive neurons. If the VIP-immunoreactive nerves cause vasodilation, they are well placed to allow redistribution of arterial blood flow within the head. During heat stress, neurogenic vasodilation of the appropriate beds would permit efficient cooling of cerebral blood, particularly that supplying the rostral brainstem and surrounding areas of the cerebral cortex.

Evidence that vasoactive intestinal polypeptide is a dilator transmitter to some cerebral and extracerebral cranial arteries

Many arteries of the head upon electrical stimulation of the non-adrenergic nerve terminals present in their wall exhibit dilation that is only partially reduced by atropine. The results of three types of experiments are presented that tend to implicate vasoactive intestinal polypeptide (VIP) a known vasodilator, as an atropine-resistant dilator transmitter. VIP activity is high in vessels that exhibit neurogenic dilation and low in those that do not. It is released from two arteries that show such dilation upon neurogenic field stimulation and VIP antiserum reduces neurogenic dilation. It is proposed that VIP as well as acetylcholine are released from the innervation of some cranial arteries and together in a number of animals of least are responsible for part of the complex neurogenic dilation witnessed in these vessels. Although substance P is present in nerves found within the wall of cerebral and other cranial blood vessels, it is either ineffective as a dilator in vessels that show a sizeable dilation to electrical stimulation, or else exhibits marked, rapid, persistent tachyphylaxis.