Cerebral arterial innervation: II. Development of calcitonin-gene-related peptide and norepinephrine in the rat

Fetal Cerebral Circulation as Target of Maternal Alcohol Consumption

Alcohol (ethanol [EtOH]) is one of the most widely used psychoactive substances worldwide. Alcohol consumption during pregnancy may result in a wide range of morphological and neurodevelopmental abnormalities termed fetal alcohol spectrum disorders (FASD), with the most severe cases diagnosed as fetal alcohol syndrome (FAS). FAS and FASD are not readily curable and currently represent the leading preventable causes of birth defect and neurodevelopmental delay in the United States. The etiology of FAS/FASD remains poorly understood. This review focuses on the effects of prenatal alcohol exposure (PAE) on fetal cerebrovascular function. A brief introduction to the epidemiology of alcohol consumption and the developmental characteristics of fetal cerebral circulation is followed by several sections that discuss current evidence documenting alcohol-driven alterations of fetal cerebral blood flow, artery function, and microvessel networks. The material offers mechanistic insights at the vascular level itself into the pathophysiology of PAE.

Parasympathetic innervation of vertebrobasilar arteries: is this a potential clinical target?

This review aims to summarise the contemporary evidence for the presence and function of the parasympathetic innervation of the cerebral circulation with emphasis on the vertebral and basilar arteries (the posterior cerebral circulation). We consider whether the parasympathetic innervation of blood vessels could be used as a means to increase cerebral blood flow. This may have clinical implications for pathologies associated with cerebral hypoperfusion such as stroke, dementia and hypertension. Relative to the anterior cerebral circulation little is known of the origins and neurochemical phenotypes of the parasympathetic innervation of the vertebrobasilar arteries. These vessels normally provide blood flow to the brainstem and cerebellum but can, via the Circle of Willis upon stenosis of the internal carotid arteries, supply blood to the anterior cerebral circulation too. We review the multiple types of parasympathetic fibres and their distinct transmitter mechanisms and how these vary with age, disease and species. We highlight the importance of parasympathetic fibres for mediating the vasodilatory response to sympathetic activation. Current trials are investigating the possibility of electrically stimulating the postganglionic parasympathetic ganglia to improve cerebal blood flow to reduce the penumbra following stroke. We conclude that although there are substantial gaps in our understanding of the origins of parasympathetic innervation of the vertebrobasilar arteries, activation of this system under some conditions might bring therapeutic benefits.

Sympathetic reinnervation of peripheral targets following bilateral axotomy of the adult superior cervical ganglion

The ability of adult injured postganglionic axons to reinnervate cerebrovascular targets is unknown, yet these axons can influence cerebral blood flow, particularly during REM sleep. The objective of the present study was to assess quantitatively the sympathetic reinnervation of vascular as well as non-vascular targets following bilateral axotomy of the superior cervical ganglion (SCG) at short term (1 day, 7 day) and long term (8 weeks, 12 weeks) survival time points. The sympathetic innervation of representative extracerebral blood vessels [internal carotid artery (ICA), basilar artery (BA), middle cerebral artery (MCA)], the submandibular gland (SMG), and pineal gland was quantified following injury using an antibody to tyrosine hydroxylase (TH). Changes in TH innervation were related to TH protein content in the SCG. At 7 day following bilateral SCG axotomy, all targets were significantly depleted of TH innervation, and the exact site on the BA where SCG input was lost could be discerned. Complete sympathetic reinnervation of the ICA was observed at long term survival times, yet TH innervation of other vascular targets showed significant decreases even at 12 weeks following axotomy. The SMG was fully reinnervated by 12 weeks, yet TH innervation of the pineal gland remained significantly decreased. TH protein in the SCG was significantly decreased at both short term and long term time points and showed little evidence of recovery. Our data demonstrate a slow reinnervation of most vascular targets following axotomy of the SCG with only minimal recovery of TH protein in the SCG at 12 weeks following injury.

Age-dependent changes in Ca2+ homeostasis in peripheral neurones: implications for changes in function

Calcium ions represent universal second messengers within neuronal cells integrating multiple cellular functions, such as release of neurotransmitters, gene expression, proliferation, excitability, and regulation of cell death or apoptotic pathways. The magnitude, duration and shape of stimulation-evoked intracellular calcium ([Ca2+]i) transients are determined by a complex interplay of mechanisms that modulate stimulation-evoked rises in [Ca2+]i that occur with normal neuronal function. Disruption of any of these mechanisms may have implications for the function and health of peripheral neurones during the aging process. This review focuses on the impact of advancing age on the overall function of peripheral adrenergic neurones and how these changes in function may be linked to age-related changes in modulation of [Ca2+]i regulation. The data in this review suggest that normal aging in peripheral autonomic neurones is a subtle process and does not always result in dramatic deterioration in their function. We present studies that support the idea that in order to maintain cell viability peripheral neurones are able to compensate for an age-related decline in the function of at least one of the neuronal calcium-buffering systems, smooth endoplasmic reticulum calcium ATPases, by increased function of other calcium-buffering systems, namely, the mitochondria and plasmalemma calcium extrusion. Increased mitochondrial calcium uptake may represent a 'weak point' in cellular compensation as this over time may contribute to cell death. In addition, we present more recent studies on [Ca2+]i regulation in the form of the modulation of release of calcium from smooth endoplasmic reticulum calcium stores. These studies suggest that the contribution of the release of calcium from smooth endoplasmic reticulum calcium stores is altered with age through a combination of altered ryanodine receptor levels and modulation of these receptors by neuronal nitric oxide containing neurones.

Modulatory effect of interleukin-1β on rat isolated basilar artery contraction

An increased level of cytokine interleukin-1 (IL-1) has been detected around the site of stroke. However, the effect of IL-1beta on the basilar artery has received little attention. We evaluated the effects of IL-1beta on the contractile response of rat isolated basilar artery by measuring isometric tension change. IL-1beta (10 ng/ml) and phenylephrine (0.1 nM) markedly enhanced U46619 (30 and 100 nM)-induced basilar artery contraction. The IL-1beta-mediated potentiation was partly suppressed by zinc protoporphyrin (3 microM) and was abolished by tetrodotoxin (TTX, 100 nM), (-)-perillic acid (1 microM), PD98059 (0.3 microM), SB203580 (1 microM) and prazosin (1 microM). Our data suggest that IL-1beta (10 ng/ml) causes an enhancement of U46619-mediated basilar artery contraction that probably involves TTX-sensitive neuronal release of an alpha1-adrenoceptor agonist and activation of p42/p44 and p38 mitogen-activated protein kinases/p21(ras) pathways.

Development of nitric oxide synthase-immunoreactive nerves in the cerebral arteries of the rat

Development of cerebrovascular nitrergic nerves was investigated in the rat, using immunohistochemistry for nitric oxide synthase (NOS) and quantitative analysis. Cerebral perivascular NOS nerves usually appeared on the walls of both the intracranial part of the internal carotid artery (ICA) and the internal ethmoidal arteries (IEA) at birth. NOS nerves via the IEA grew more rapidly than those via the ICA. They extended over all the major arteries located more rostral than the middle part of the basilar arteries during the third postnatal week, while those from the ICA remained limited to the caudal segment of the anterior circulation and to the rostral segment of the posterior circulation throughout development. The appearance of NOS nerves on the vertebrate artery (VA) was not demonstrated before the third postnatal week, being apparently far late in development as compared to that of the same nerve type on the ICA and IEA.

The arterial circle of Willis of the mouse helps to decipher secrets of cerebral vascular accidents in the human

The human brain represents an elaborate product of hominizing evolution. Likewise, its supporting vasculature may also embody evolutionary consequences. Thus, it is conceivable that the human tendency to develop cerebral vascular accidents (CVAs) might represent a disease of hominization. In a search for hominizing changes on the arterial circle of Willis (hWAC), we attempted an anatomical comparison of the hWAC with that of the mouse (mWAC) by injecting aliquots of resin into the vasculature of the mouse and then creating vascular endocasts of the mWAC. The internal carotid artery of the mouse (mICA) unites with the mWAC midway between the middle cerebral artery (mMCA) and posterior cerebral artery (mPCA). The mWAC does not complete a circle: the mWAC nourishes the anterior portion of the circle which branches out to the olfactory artery (OlfA) and mPCA, along with the mMCA, and the basilar artery (mBA) does not connect to the mPCA. The OlfA is thicker than the mMCA. The relative brain weight of the mouse was 74 g on average for a 60 kg male and 86 g for a 60 kg female, respectively, as compared with 1424 g for a 60 kg man. These findings are consistent with the mouse being a nocturnal carnivore that lives on olfactory information in contrast to the human that lives diurnally and depends on visual and auditory information. In man, the human ICA (hICA) unites with the hWAC at a point where the human middle cerebral artery (hMCA) branches out, and thus, blood from the hICA does not flow through the hWAC but drains into the hMCA directly. The hMCA is thicker than the anterior cerebral artery. The hPCA receives blood from the hBA rather than from the hICA, and thus, the entire hWAC forms a closed circuit. Since the hICA drains directly into the hMCA without flowing a distance through the hWAC, the capacitor and equalizer functions of the WAC will be mitigated so much that the resultant hemodynamic changes would render the hMCA more likely to contribute to CVAs. Thus, anatomical findings and possibly functions of the arterial circle of Willi may vary from one species to another, depending on one's specific cerebral evolution.

Nerve fibers innervating the cranial and spinal meninges: morphology of nerve fiber terminals and their structural integration

Pachymeninx and leptomeninx of cranial cavity and spine are considerably different in their collagenous fiber texture, cellular composition, vascularization, and innervation. The majority of meningeal nerve fibers terminate as free nerve endings whereas encapsulated and lamellated nerve terminals additionally occur in higher vertebrates including man. With respect to nerve fiber classification, arborization pattern, topography, and organization of the microenvironment at the termination site afferent and efferent nerve terminals are differentiated. Only the dura mater and the pial subcompartment of the leptomeninx possess the morphological prerequisites for neurogenic inflammation. In the current review, the results of morphological studies regarding the meningeal innervation including the sites of CSF (cerebrospinal fluid) production and absorption are discussed with emphasis on their structure-function relationships.

Effects of maturation on adrenergic neurotransmission in ovine cerebral arteries

The present studies examine the hypothesis that multiple adrenergic neuroeffector mechanisms are not fully developed in fetal, compared with adult, ovine middle cerebral arteries. In arteries denuded of endothelium and pretreated with 1 microM atropine to block involvement of muscarinic receptors, 10 microM capsaicin to deplete sensory peptidergic neurons, and 10 microM nitro-L-arginine methyl ester (L-NAME) to block possible influences from nitric oxidergic innervation, transmural stimulation at 16 Hz increased contractile tensions to 9.5 +/- 3.7% (n = 6) of the potassium maximum in adult arteries. Corresponding values in fetal arteries, however, were significantly less and averaged only 1.1 +/- 0.6% (n =10). However, postsynaptic sensitivity to norepinephrine (NE) was similar in the two age groups; NE pD(2) values (-log EC(50)) averaged 6.11 +/- 0.12 (n = 6) and 6.33 +/- 0.09 M (n = 9) in fetal and adult arteries, respectively. Similarly, NE content measured via HPLC was also similar in the two age groups and averaged 32.4 +/- 5.0 (n = 17) and 32.5 +/- 3.9 ng/ng wet wt (n = 13) in fetal and adult middle cerebral arteries, respectively. In contrast, stimulation-induced NE release was greater in fetal than in adult arteries, whether calculated as total mass released [883 +/- 184 (n = 17) vs. 416 +/- 106 pg NE/mg wet wt (n = 13)] or as fractional release [51.1 +/- 5.3 (n = 17) vs. 22.8 +/- 3.8 pg/pg NE content per pulse x 10(-6)]. Measured as an index of synaptic density, neuronal cocaine-sensitive NE uptake was similar in fetal and adult arteries [1.55 +/- 0.40 (n = 10) and 1.84 +/- 0.51 pmol/mg wet wt (n = 7), respectively]. Overall, age-related differences in postsynaptic sensitivity to NE, NE release, and NE uptake capacity cannot explain the corresponding age-related differences in response to stimulation. The data thus suggest that total synaptic volume and cleft width, in particular, are probably greater and/or that adrenergic corelease of vasoactive substances other than NE is altered in fetal compared with adult middle cerebral arteries.

Neural and behavioral effects of intracranial 192 IgG-saporin in neonatal rats: sexually dimorphic effects?

The consequences of neonatal cholinergic lesions were examined in male and female rats. Rats were injected intraventricularly with 600 ng of 192 IgG-saporin at 7 days of age and examined behaviorally and histologically at 21, 45 and 90 days of age. 192 IgG-saporin profoundly reduced low affinity neurotrophin receptor (p75NTR)-immunoreactive (IR) and, to a lesser extent, choline acetyltransferase-IR cells in the basal forebrain. Presumptive sympathetic ingrowths (p75NTR- and dopamine beta-hydroxylase-IR) into the hippocampus were first apparent at 45 days of age and were not significantly greater at 90 days. Behaviorally, 192 IgG-saporin increased the time females, but not males, spent on the open arms of the elevated plus maze. Lesioned rats had longer platform location latencies in the Morris water maze only at the first hidden platform training session and did not differ on the rate of learning the platform location or on the no-platform probe trial. Generally, the effects of neonatal cholinergic lesions were not sex dependent and are unlikely to model Rett syndrome, a disorder characterized by forebrain cholinergic deficit which is seen almost exclusively in females.

Innervation of cerebral arteries by nerves containing 5-hydroxytryptamine and noradrenaline

Noradrenaline (NA)-containing nerves, mainly originating in the sympathetic superior cervical ganglia, supply large and small cerebral arteries. In large cerebral arteries, nerves containing serotonin (5-hydroxytryptamine, 5-HT) may represent neuronal uptake of circulating 5-HT by sympathetic nerves. 5-HT-containing nerves supplying small pial vessels probably have a central origin in the dorsal raphe nucleus. In most species, NA is a weak vasoconstrictor (alpha 1- or alpha 2-adrenoceptors), while 5-HT is a potent vasoconstrictor (5-HT2 or 5-HT1-like receptors) of large cerebral arteries. In contrast, both NA and 5-HT tend to cause vasodilatation in small pial vessels and arterioles. Adrenergic and serotonergic transmission can be modulated by pH, a range of putative neurotransmitters and neuromodulators, and by the endothelium. Sumatriptan, a 5-HT1-like receptor agonist, has been shown to be effective in the treatment of migraine. Changes in NA- or 5-HT-containing nerves and/or in the responses of cerebral vessels to NA and 5-HT have been observed in a variety of vascular disorders, including cerebral vasospasm following subarachnoid haemorrhage, hypertension, and atherosclerosis.

Calcitonin gene-related peptide innervation and binding sites in rat aorta during development

Indirect immunohistochemistry performed on whole mounts of arch and thoracic part of the rat aorta at six developmental stages (from embryonic day 17 to 6 months, in males and females) revealed that calcitonin gene-related peptide (CGRP) innervation is highest in the arch. The highest density of innervation is found at the three first postnatal ages investigated (day 1, day 3 and 5 weeks; 2.6 +/- 0.6 intercepts/mm in the arch at 1 day); however, all values are low compared to other arteries. The innervation grows from a few short isolated fibres in the embryo to a more complex meshwork in older animals. No striking differences were noticed between males and females. Autoradiographic studies were performed on serial sections at several levels of the aorta but did not reveal binding sites for CGRP in the vascular wall. This might be due to the technique which does not allow visualization of low density of binding sites, or to binding sites of weak affinity. We discuss the possible importance of CGRP in rat aortic smooth muscle development.

Cerebrovascular nerve fibers immunoreactive for tryptophan-5-hydroxylase in the rat: distribution, putative origin and comparison with sympathetic noradrenergic nerves

The distribution of serotonergic nerves in major basal and isolated small pial arteries (diameter > or = 50 microns) was investigated immunohistochemically using an antibody directed against tryptophan-5-hydroxylase (TPOH), the rate-limiting enzyme in the synthesis of 5-hydroxytryptamine (5-HT or serotonin), and compared to that of the noradrenergic system labeled for the selective noradrenaline (NA) synthesizing enzyme, dopamine-beta-hydroxylase (DBH). In addition, the possible peripheral and/or central origins of the cerebrovascular serotonergic (TPOH-positive) nerve fibers were examined. Strongly labeled TPOH-immunoreactive (TPOH-I) fiber bundles were observed in major basal arteries and gave rise to small varicose fibers organized in a meshwork pattern. The highest density of TPOH-I fibers was found in the middle cerebral artery followed by the anterior cerebral and the anterior communicating arteries, with a moderate to low density in the internal carotid and the vertebro-basilar trunk. Of the isolated pial arteries, only the larger ones (diameter > 75 microns) were significantly endowed with TPOH-I varicose fibers. However, free floating TPOH-I nerves were observed coursing through the pia-arachnoid membranes and reaching small pial vessels. In contrast, DBH-I nerve fibers were fine and were visualized primarily as numerous varicosities distributed in a circumferential manner around the vessel wall. A very high density of DBH-I varicosities was seen in the rostral part of the circle of Willis, with the internal carotid being the most richly supplied followed by the anterior cerebral and the anterior communicating arteries; comparatively, the middle cerebral artery was moderately innervated. The differences in distribution pattern and density between TPOH-I and DBH-I cerebrovascular fibers clearly suggest that these two innervation systems are not exactly superimposable. Superior cervical ganglionectomy caused an almost complete disappearance of TPOH-I nerves in all vascular segments, with some residual fibers in selected vessels. Lesion of the central serotonergic component with the neurotoxin 5,7-dihydroxytryptamine had virtually no effect on the TPOH-I fibers in the major basal and isolated pial arteries. These results strongly suggest that the serotonergic innervation of major cerebral as well as pial arteries has a prominent peripheral origin closely related to the sympathetic system. Processing of superior cervical ganglion slices for TPOH immunocytochemistry, however, failed to unequivocally detect TPOH-I neurons.

Development of cerebral arterial innervation: synchronous development of neuropeptide Y (NPY)- and vasoactive intestinal polypeptide (VIP)-containing fibers and some observations on growth cones

The pre- and postnatal development of sympathetic fibers containing neuropeptide Y (NPY) and parasympathetic fibers containing vasoactive intestinal polypeptide (VIP) supplying the cerebral arteries were studied with immunohistochemistry in rats. The innervation patterns and densities of NPY and VIP fibers were similar at all stages of development and similar to that previously reported for norepinephrine (NE). There was a striking reorganization of the innervation pattern of all three fiber systems between the first and second postnatal weeks. At all stages of development prior to the first postnatal week, growth cones were present on individual fibers at the distal part of major cerebral arteries and the middle segment of the basilar artery. The growth cones had a range of shapes from blunt to stellate, lanceolate or filiform. NPY and VIP immunoreactive granules were commonly present. The present results taken with our earlier developmental study of NE fibers (J. Comp. Neurol., 271 (1988) 435-444), demonstrate that: (1) both sympathetic and parasympathetic perivascular nerves on immature cerebral vessels develop with similar sequences: first longitudinal fibers and fiber bundles are present; these transform to a meshwork pattern and finally transform again into the mature, predominantly circumferential pattern; (2) both the classical (NE) and peptidergic transmitters (NPY) within the sympathetic system appear to develop identically in terms of time of appearance, innervation patterns, densities and reorganization.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcitonin gene-related peptide and corneal innervation: a developmental study in the rat

The development of calcitonin gene-related peptide-like immunoreactive (CGRP-LI) nerves was studied in neonatal and adult rat corneas stained immunohistochemically according to an avidin biotin peroxidase procedure. At birth, rat corneas already contained dense plexuses of CGRP-LI nerve fibers. Most of the nerves entered the cornea in 12-15 prominent stromal nerve bundles located at regular intervals around the circumference of the cornea. Fibers in these bundles entered the epithelium approximately midway between the limbus and the center of the cornea and supplied extensive central and pericentral areas of the tissue. In addition, smaller numbers of axons entered the cornea individually and in small fascicles located in between the larger bundles and supplied mainly peripheral territory. In the epithelium, the CGRP-LI nerves formed a complex, highly anastomotic meshwork that ramified uniformly throughout central and peripheral areas of the tissues. Fibers in the plexus gave origin to numerous short, stout terminal axons that extended into the adjacent epithelium in all directions with no preferred orientation. During the first week of neonatal life, several changes in CGRP-LI innervation occurred: 1) the innervation density of the central and pericentral cornea increased relative to the peripheral cornea; 2) intraepithelial axons became progressively longer, increased in branching complexity, and oriented preferentially towards the center of the cornea; and 3) a dense innervation of the corneoscleral limbus and, in particular, the branches of the marginal artery, developed. Midway through the second week of life, immature versions of corneal epithelial "leashes," the dominant feature of the adult corneal innervation, were first observed. Over the next 10 days, the leash formations in the central and pericentral cornea gradually became more complex and gave rise to greater numbers of terminal axons, compared to developing leashes in the peripheral cornea. The mature pattern of corneal CGRP-LI innervation was reached on day 21 and remained constant (except for compensatory growth-related elongation of axons) for at least the first 6 months of life. Transection of the ophthalmomaxillary nerve or neonatal administration of the sensory neurotoxin capsaicin resulted in the total loss of CGRP-LI staining from the cornea. In contrast, removal of the superior cervical ganglion had no effect on corneal CGRP-LI staining. The extraordinary density and complexity of the CGRP-LI innervation of the rat cornea demonstrated at all stages of development in this study suggests that these nerves may play important roles in corneal sensory, reflex, and trophic functions.

Anatomic distribution of autonomic neural tissue in the developing dog heart: II. Nonadrenergic noncholinergic innervation by calcitonin gene-related peptide-immunoreactive tissue

We used immunocytochemical localization of calcitonin gene-related peptide (CGRP) to trace the ontogenesis and anatomic distribution of this component of nonadrenergic noncholinergic (NANC) innervation in fetal, neonatal, and mature canine hearts and autonomic ganglia which control cardiac function. Rare varicose CGRP-immunoreactive nerve processes were present in the heart during late gestation. Abundant CGRP-immunoreactive neural tissue in the neonate suggested a burst of NANC innervation around birth. Neonatal, 1-, and 2-month-old animals all had many varicose individual nerve processes in addition to processes within bundles; however, the density of all CGRP-immunoreactive tissue appeared to decrease during this stage of development. Similarly, there were relatively more varicose stained nerve processes in the epicardial ganglia and numerous CGRP-immunoreactive cells and smooth nerve processes in the stellate ganglia of the neonate, as compared with older animals. In the mature animal CGRP-immunoreactive neural tissue in the heart was more sparse and largely confined to heterogeneous nerve bundles in the epicardium. The extramural coronary arteries were virtually the only site of innervation by individual nerve processes; CGRP-immunoreactive neural tissue was not found adjacent to working cardiac muscle fibers. At all developmental stages, the area of the sinoatrial node was the primary focus of CGRP innervation, although the atrioventricular nodal region was also preferentially innervated. In general, the atria contained more CGRP-immunoreactive tissue than the ventricles, which were only sparsely innervated. The perinatal peak in density of CGRP-immunoreactive neural tissue with subsequent decline to reach the adult pattern suggests a developmental role for NANC innervation in the dog heart.

Development of AChE-positive, NA-containing and VIP- and NPY-immunoreactive nerves in the major cerebral arteries of the rat

The development of cerebrovascular nerves containing noradrenaline (NA), acetylcholinesterase (AChE), neuropeptide Y (NPY), and vasoactive intestinal polypeptide (VIP) was studied in rats from before birth to adulthood. All these nerves entered the cranial cavity along the cerebral carotid, internal ethmoidal, and vertebral arteries during the early stages of development, but the subsequent growth and distribution of NA-containing and NPY-immunoreactive (IR) nerves differed greatly from that of AChE-positive and VIP-IR nerves. NA-containing and NPY-IR nerves extended rapidly from the cerebral carotid artery and spread over all the major arteries of the internal carotid system by postnatal day 3, as well as descending the posterior ramus of the cerebral carotid to mingle with nerves from the vertebral artery around the mid-basilar artery by day 5. AChE-positive and VIP-IR nerves from the internal ethmoidal artery covered the whole internal carotid system during the first postnatal week, and projected to the upper basilar artery after the second week, while those from the cerebral carotid artery remained limited to the middle cerebral artery throughout development. By day 21, all major arteries of the internal carotid system had dense plexuses of the four nerve types that were similar to those observed in adult rats. The vertebrobasilar system also had a well-organized network of NA-containing and NPY-IR nerves, but only a poor supply of AChE-positive and VIP-IR nerves. Even on day 30, the latter two nerve types were sometimes absent from the middle to caudal basilar artery, owing to a lack of interdigitation by nerves from the internal ethmoidal and vertebral arteries.

Reorganization of cranial sympathetic pathways following neonatal ganglionectomy in the rat

Postganglionic sympathetic innervation normally is distributed ipsilaterally to lateral cranial targets. However, contralateral outgrowth occurs following unilateral ganglionectomy in neonatal rats. This study was conducted to determine the prevalence, morphological features, ganglionic derivations, and temporal sequence of sympathetic reinnervation of denervated cranial targets. Unilateral superior cervical ganglionectomy of mature rats revealed exclusively ipsilateral distributions of catecholaminergic histofluorescent fibers to orbital targets (Meibomian gland, tarsal muscle, orbital muscle, iris, ciliary body, vasculature) and the circle of Willis, with the exception of the anterior cerebral artery. In mature rats following neonatal unilateral ganglionectomy, all targets were reinnervated by fibers displaying morphologies and target relationships similar to normal innervation, but with lower densities. Acute excision of the remaining superior cervical ganglion eliminated all fibers in 7 of 8 preparations. In adult rats receiving neonatal bilateral superior cervical ganglionectomies, cerebral vasculature was reinnervated consistently, and orbital targets contained fluorescent fibers in 6 of 16 cases, indicating that reinnervation can derive from other sources when superior ganglion outgrowth is prevented. Observations in developing rats revealed fibers along the cranial portion of the contralateral optic nerve sheath at 2-3 days postganglionectomy, and within the orbit at later ages, reaching the most distal targets by 14 days. It is concluded that widespread sympathetic reinnervation of orbital and cerebrovascular targets derives primarily from the contralateral superior ganglion. Orbital ingrowth apparently originates intracranially and enters the orbit by an atypical pathway within the optic foramen.