Internal Carotid Nerve Research Articles

Galanin and vasoactive intestinal peptide messenger RNAs increase following axotomy of adult sympathetic neurons.

The adult rat superior cervical ganglion (SCG) contains low levels of galanin- and vasoactive intestinal peptide-(VIP) like immunoreactivity, with very few immunostained principal neurons. Immunoreactivity for both neuropeptides increases in these neurons after explantation or postganglionic axotomy in vivo. Northern blot analysis has demonstrated concomitant increases in mRNAs encoding these peptides. To localize cells in axotomized ganglia which increase their expression of these mRNAs, we performed in situ hybridization studies. In control SCG, only a few principal neurons contained mRNA for either galanin or VIP. After 48 h in organ culture, galanin mRNA was expressed in the majority of principal neurons. At 48 h after in vivo axotomy of both postganglionic trunks of the SCG, the internal and external carotid nerves, the distribution and number of neurons, expressing galanin mRNA increased similarly to that seen in culture. Lesioning either trunk alone produced increases in galanin mRNA localized to those regions of the ganglion containing neurons that project into the lesioned trunk. Transection of the predominantly preganglionic cervical sympathetic trunk increased galanin mRNA expression in a small population of neurons near that nerve trunk. The distributions of these labeled neurons, together with previous neuroanatomical studies, suggests that they had been axotomized by the lesions. Similar studies examining VIP mRNA expression demonstrated that although considerably fewer VIP mRNA expressing neurons than galanin mRNA expressing neurons were present after axotomy, the distribution of neuropeptide mRNA-positive cells were similar in both cases. These observations suggest that increases in the peptides galanin and VIP after nerve transection result from changes in the levels of their mRNAs in those neurons that have been axotomized.

Induction of Tα1 α-Tubulin mRNA during Neuronal Regeneration Is a Function of the Amount of Axon Lost

Axotomy of mature peripheral neurons leads to upregulation of Tα1 α-tubulin mRNA. To distinguish whether this increase is due to interruption of target contact or to loss of the axon itself, we developed a model system that allowed us to axotomize mature sympathetic neurons proximal or distal to their cell bodies, severing contact with target tissue in all cases. Sympathetic neurons within the same superior cervical ganglion that project via the internal and external carotid nerves to the eye and to the ear, respectively, were differentially labeled with the retrograde tracers fast blue and fluorogold, and the labeled neurons were then transected close to or far from their cell bodies. In situ hybridization analysis 5 days postaxotomy (the time of peak Tα1 mRNA expression) indicated that when eye and ear neurons were both axotomized close to their cell bodies, Tα1 mRNA increased to a similar degree in both populations. In contrast, when ear neurons were again cut close, but eye neurons of the same ganglion were transected far from their cell bodies by eye removal, peak Tα1 mRNA levels were two- to threefold lower in the eye neurons. Thus, the increase in Tα1 mRNA was much lower when only a small amount of axon was lost, even though contact with target tissue was completely interrupted in all groups. These results therefore suggest that neurons normally monitor the status of their axon via a mechanism that allows for graded responses and that the axotomy-induced increase in Tα1 mRNA is, to a great extent, due to loss of the nonterminal axon.

Phenotypic plasticity in adult sympathetic ganglia in vivo: effects of deafferentation and axotomy on the expression of vasoactive intestinal peptide

The expression of neurotransmitters/neuromodulators in sympathetic neurons is regulated by anterograde and retrograde mechanisms. We have examined the role of such mechanisms in the regulation of the neuropeptide vasoactive intestinal peptide (VIP). The adult rat superior cervical ganglion (SCG) contains low levels of peptide-like immunoreactivity (IR) and mRNA for VIP. Some VIP-IR nerve processes, but only a few VIP-IR cell bodies, are detectable. Previous evidence demonstrates, however, that after the SCG is placed in organ culture for 48 hr, the level of VIP-IR and VIP mRNA and the number of VIP-IR cell bodies and fibers increase considerably. Two of the possible causes for these changes in peptide expression in sympathetic neurons are deafferentation and axotomy, both of which occur when the SCG is placed in culture. To determine the importance of deafferentation, the preganglionic cervical sympathetic trunk was cut and the ganglion left in situ. Forty-eight hours later, VIP-IR increased twofold. A corresponding increase in the number of VIP-IR nerve processes was seen, but there was no detectable change in the number of VIP-IR cell bodies. The content of VIP/PHI mRNA also increased by 1.8-fold. The effect of axotomy on VIP-IR was examined by cutting the postganglionic internal and external carotid nerves and leaving the ganglion in situ. Forty-eight hours later, the level of VIP-IR increased 22-fold, many immunostained neurons were found, and the content of VIP mRNA increased over fivefold. After either deafferentation or axotomy, changes in VIP-IR were accompanied by comparable changes in the related molecule peptide histidine isoleucine amide (PHI)-IR. Neuropeptide Y-IR, on the other hand, decreased after deafferentation and increased only twofold after axotomy. The results indicate plasticity in the expression of VIP- and PHI-IR in adult sympathetic neurons in vivo, and suggest that the changes previously seen in organ culture were primarily a response to axotomy.

Transynaptic regulation of low-affinity p75 nerve growth factor receptor mRNA precedes and accompanies lesion-induced collateral neuronal sprouting

The bilateral sympathetic innervation of the rat pineal gland from the two superior cervical ganglia (SCG) is a useful model system to investigate the mechanisms by which intact neurons compensate for neuronal losses. Cutting of the internal carotid nerve (ICN) on one side has been shown to result in the removal of approximately one-half of the innervation to the pineal gland within 2 days. This denervation is followed by the development of collateral neuronal sprouting from the contralateral “intact” SCG, most of which takes place during the next 2 days. Using a solution hybridization protection assay, levels of low-affinity NGF receptor p75 NGFR mRNA (pg/μg total RNA) were found to be increased 25%, with no change in cyclophilin mRNA, in the SCG contralateral to the lesion performed 1 or 3 days earlier. In situ hybridization with a 35S riboprobe complementary to p75 NGFR mRNA demonstrated a large increase in this mRNA in some cells of this intact SCG at both 1 and 3 days after a contralateral ICN cut lesion. The clustering of these cells toward the rostral portion of the SCG suggests that they may overlap with the population of sympathetic neurons which provides innervation to bilaterally innervated structures such as the pineal gland. The nature of the signals involved in the regulation of NGF receptor mRNA levels and their role in initiating and maintaining collateral sprouting remain to be fully established. Nevertheless, the time course of the changes in mRNA levels suggests that regulation of the low-affinity NGF receptor gene may be involved in the sequence of events associated with the collateral sprouting response by intact sympathetic nerve cells following partial denervation.

A role for protein kinase C in long term potentiation of nicotinic transmission in the superior cervical ganglion of the rat

The superior cervical ganglion of rats was perfused with Ringer solution containing hexamethonium to produce a steady, partial, nicotinic block. The compound action potential (CAP) evoked by supramaximal single shock stimulation of the cervical sympathetic trunk (CST) was recorded from the internal carotid nerve. Bolus injection of the protein kinase C (PKC) activators 4β-phorbol-12,13-dibutyrate (PDBu) or 4β-phorbol-12, 13-diacetate (PDAc) produced a marked, prolonged, dose-dependent potentiation of the CAP amplitude (e.g. 90% decay 2 h). A non-PKC activating phorbol ester (PE), 4α-phorbol-12,13-didecanoate, produced no potentiation. The PE-induced potentiation was antagonized by the PKC inhibitor H-7. In addition, after 1 h exposure to PDBu (3 μM) and recovery from the potentiation (e.g. 2–4 h), a second exposure to PDBu or PDAc produced no potentiation. A 5 s 40 Hz supramaximal train to the CST produced a long lasting potentiation of the CAP (long-term potentiation, LTP) as described previously. However, a similar train did not evoke LTP after perfusion for 2 h with PDBu. The train-evoked LTP was depressed by the PKC inhibitor H-7 at a concentration which antagonized the PE-evoked potentiation. These data suggest that (i) PKC activation potentiates nicotinic transmission, and (ii) a component of the train-evoked LTP is mediated by PKC.

Are there cholinergic through-fibers in the superior cervical ganglion of the mouse?

Choline acetyltransferase immunocytochemistry was used to detect the presence and distribution of cholinergic through-fibers in the superior cervical ganglion of adult mice. The results revealed a great number of choline acetyltransferase-positive axons in the cervical sympathetic trunk and the varicose terminal axons involved in the innervation of the principal ganglionic cells within the ganglion. Immunostained axons were scarce or absent in the external or internal carotid nerves. The immunocytochemical results argue against the physiological importance of cholinergic through-fibers in the postganglionic nerves.

On the histamine‐induced depolarization of the isolated superior cervical ganglion of the rat

1. Using a grease-gap technique, we studied the action of histamine on the d.c. potential recorded between the internal carotid nerve and the main body of the isolated superior cervical ganglion of the rat. 2. A small, slow depolarization was evoked by 10-300 microM histamine. This response was not reduced by lowering the calcium concentration in the superfusing medium (from 2.5 to 0.1 mM), or by superfusing tetrodotoxin, N-methylatropine, or propranolol (all at 1 microM). 3. Mepyramine (10 nM) antagonized this depolarization, but cimetidine (10 microM), metiamide (30 microM), burimamide (10 microM) and impromidine (1 microM) did not. Two other agonists also evoked a mepyramine-sensitive slow depolarization. The rank order of potencies was histamine greater than N alpha-methyl-histamine greater than 2-methyl-histamine. 4. At concentrations greater than 1 mM, histamine also evoked a larger, faster depolarization. This response was undiminished by reducing the calcium concentration of the medium to 0.1 mM or by adding 1 microM tetrodotoxin. The rank order of potency for the agonists was N alpha-methyl-histamine greater than histamine approximately 2-methyl-histamine. The histamine-induced fast response was not antagonized by any of the above-mentioned antagonists. It was slightly reduced by (+)-tubocurarine (100 microM) and N-methylbicuculline (100 microM) but such effects were not consistent with the blockade of nicotinic or GABAA receptor-mediated responses. 5. It was concluded that histamine depolarized the isolated superior cervical ganglion of the rat by activating H1 receptors. Relatively high concentrations of histamine also evoked a fast depolarization of this preparation, but this did not appear to be mediated by H1, H2 or H3 receptors.

Functional recovery and collateral neuronal sprouting examined in young and aged rats following a partial neural lesion

The rat pineal gland was chosen as a model system to study how aging affects the capacity of surviving neurons to compensate for partial destruction of a neural pathway. The pineal gland receives bilateral overlapping sympathetic innervation from the two internal carotid nerves, whose activity regulates several aspects of pineal metabolism in a circadian fashion. The most dramatic of these is the marked nighttime increase in the activity of N-acetyltransferase, the rate-limiting enzyme in melatonin synthesis. These features allow for the pineal gland to be used as a model system for studies on neuronal plasticity, since it is possible to create specific partial neural lesions and to evaluate functional recovery subsequently at the cellular level. We examined the activity of N-acetyltransferase and the content of melatonin in the pineal gland as indices of pineal function at various time points after unilateral surgical denervation (lesion of one of the two internal carotid nerves) in 4-month-(young) and 25-month-old (aged) rats. At both ages, the nighttime levels of the two parameters were significantly lower 8 h after this lesion than in sham-operated animals of the same age, indicating impaired function. When examined at later time points (i.e., 1.5 and 10 days after this lesion), both young and aged animals exhibited full recovery in these two parameters. Measurement of specific neuronal uptake of [ 3H]norepinephrine was utilized as an index of the number of sympathetic varicosities innervating the pineal gland. One and a half days after unilateral denervation. [ 3H]norepinephrine uptake in 4-, 10-, 16-, and 25-month-old animals was approximately 50% of that obtained in sham-operated animals of the same age, as expected if half of the innervation of the gland was lesioned. Ten days after surgery, [ 3H]norepinephrine uptake in 4- and 10-month-old animals was 78% of the value found in sham-operated animals of the same age, suggesting collateral sprouting of the remaining sympathetic nerve fibers. However, in 16- and 25-month-old animals, uptake remained at 50%. These data indicate that recovery of pineal function after unilateral denervation occurs to a similar extent and with a similar time course in young and aged animals. In contrast, the data suggest that collateral sprouting is impaired in aged animals. The fact that functional recovery occurs in young animals sooner after the lesions than does collateral sprouting and that recovery occurs in the aged animals in the apparent absence of collateral sprouting suggests that the relationship between this anatomical form of plasticity and functional recovery may be complex. Among other possibilities, collateral sprouting may play a role in the maintenance of recovered function rather than in the initial recovery process or sprouting may be functionally important after nerve losses greater than those represented by a 50% lesion.

Appearance of retrogradely labeled neurons in the rat superior cervical ganglion after injection of wheat-germ agglutinin-horseradish peroxidase conjugate into the contralateral ganglion

Injection of wheat-germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the superior cervical ganglion (SCG) of the rat results in accumulation of WGA-HRP in sympathetic postganglionic neurons in the contralateral SCG. The sympathetic pathways involved and the mechanism underlying the labeling were investigated. The labeling in neurons in the contralateral SCG was apparent 6 h after injection and increased in intensity with longer survival times. The number of labeled neurons reached 1300 at 72 h after the injection. Transection of the external (ECN) or internal carotid nerves (ICN) resulted in considerable reduction in the number of labeled neurons. Combined transection of both ECN and ICN virtually eliminated labeling in the contralateral SCG. This provides strong evidence that these two nerves are the major pathways for WGA-HRP transport out of the SCG. No labeling was observed in the contralateral SCG following injection of horseradish peroxidase (HRP). Therefore, it seems unlikely that a direct nerve connection exists between the bilateral ganglia. Instead, the labeling of contralateral SCG neurons appears to depend on the transneuronal transport capacity of WGA-HRP, which conveys the marker in an anterograde direction along the postganglionic fibers to terminals in sympathetic target organs, and then delivers it transneuronally to contralateral SCG neurons. We suggest that the sympathetic nerve fibers originating in the bilateral SCGs run intermingled and are in close contact in their peripheral target organs.

A pharmacological study of the responses induced by muscarinic agonists on the isolated superior cervical ganglion of the guinea-pig

We have studied the muscarinic agonist induced responses on the guinea-pig superior cervical ganglion in vitro, as recorded from the internal carotid nerve using a grease-gap. The principal response was a depolarization, but a small hyperpolarizing response could be revealed under certain conditions. We determined the pA 2 of a number of muscarinic antagonists against the muscarine induced depolarization. Four selective antagonists and atropine appeared to act competitively. The rank order of their pA 2s was 4-DAMP (8.5), atropine (8.4), pirenzepine (8.0), methoctramine (7.2) and AF-DX 116 (6.3). In addition to muscarine, we assessed the potency and relative maximum response of nine other muscarinic compounds to depolarize this preparation: carbachol, 5-methylfurmethide, oxotremorine, oxotremorine-M, pilocarpine, RS 86, AF102B and two novel compounds L-670548 and L-679512. L-670548 was the most potent and AF102B was the least potent agonist tested. Only AF102B evoked a maximum depolarization that was significantly smaller than muscarine. A hyperpolarizing response to carbachol (1 μM) could be recorded when the superfusing medium contained 0.3 μM pirenzepine and only 0.1 mM CaCl 2 (cf. usual 2.5 mM). This response was relatively small compared to that evoked on the superior cervical ganglion of the rat. It was blocked by the cardioselective antagonists methoctramine (0.1–0.3 μM) and AF-DX 116 (0.3–1.0 μM). Of the 10 agonists tested, only carbachol, oxotremorine and oxotremorine-M reproducibly evoked a hyperpolarizing response. It was concluded that muscarinic agonists can induce a depolarization of the guinea-pig superior cervical ganglion mediated by M 1 receptors. The activation of cardiac-like M 2 receptors resulted in a hyperpolarizing response that was relatively small.

The impact of sexual dimorphism on neuron numbers in the superior cervical ganglion of the rat

Other workers have reported a sexual dimorphism in the number of neurons that project from the rat superior cervical ganglion (SCG) via the internal carotid nerve (ICN). We have re-examined this situation by comparing ganglionic neuron numbers as well as the number of neurons labelled retrogradely from the iris and the pineal gland, in age-matched adult male and female rats. No significant dimorphism was seen in total cell numbers or in the cell populations supplying iris or pineal gland, although there was a trend towards more neurons in male animals. From evaluation of our results and the published data, we suggest that the variation in absolute numbers of neurons counted in the SCG is so wide, within genders, that any impact of gender differences on cell numbers is of little significance.

Recovery of function following unilateral denervation, but not unilateral decentralization, of the pineal gland as indicated by measurements of pineal melatonin content and urinary melatonin metabolites

The rat pineal gland is an attractive system for studies on the capacity of neural systems to recover following partial injury, allowing both for the creation of precise subtotal lesions and for the measurement of recovery of function at the cellular level. The pineal gland receives overlapping sympathetic innervation from the right and left internal carotid nerves from neurons whose cell bodies are located in the two superior cervical ganglia. This innervation regulates several aspects of pineal metabolism in a circadian fashion, with the most dramatic being a marked increase in the night-time activity of N-acetyltransferase, a key enzyme regulating the rate of melatonin synthesis. We have previously shown that a highly divergent pattern takes place in the night-time activity of this enzyme following two different unilateral lesions of the sympathetic innervation to the gland. Thus, following a unilateral lesion of the internal carotid nerve (unilateral denervation), there is an initial decline in N-acetyltransferase activity; however, normal activity is again seen during the second and subsequent nights. In contrast, a unilateral lesion of the cervical sympathetic trunk, the nerve that innervates the superior cervical ganglion (unilateral decentralization), results in “permanent” impairment of N-acetyltransferase activity. In the present study, we report that the functional capacity of the entire pathway for melatonin synthesis is similarly affected following these lesions, as reflected by the levels of melatonin and of its precursor N-acetylserotonin in the pineal gland, as well as the levels of the main melatonin metabolite 6-hydroxy-melatonin in the urine. N-Acetyltransferase activity was determined 1.5, 14 and 21 days after unilateral denervation and was found to be 97%, 122% and 98% of the night-time activity in sham-operated animals. At all three points, the pineal contents of N-acetylserotonin (84%, 129% and 105%) and of melatonin (90%, 110% and 115%) were also similar to the levels obtained in sham-operated animals. In contrast, after unilateral decentralization the activity of N-acetyltransferase (18%, 32% and 21%) and the content of both N-acetylserotonin (12%, 43% and 41%) and melatonin (35%, 55% and 47%) were lower than in sham-operated animals. Urinary measurements of 6-hydroxymelatonin, the main melatonin metabolite, showed a robust diurnal rhythm in control animals, with increased excretion at night. Measurements of this metabolite in the urine allowed assessment of pineal function in individual animals, before and after specific lesions. Following unilateral denervation, there was a return of normal 6-hydroxymelatonin levels by the second night after surgery. However, following unilateral decentralization, night-time 6-hydroxymelatonin measurements remained lower than prelesion values for at least 13 nights after the lesion. This study provides additional evidence for our hypothesis that, after certain lesions, intact, but electrophysiologically silent, nerve terminals of decentralized nerve cells may adversely affect the function of terminals from unoperated neurons and, thereby, prevent functional recovery.

Origins of substance P- and calcitonin gene-related peptide-containing nerves in the internal carotid artery of rat

An aggregation of substance P (SP)- and calcitonin gene-related peptide (CGRP)-containing nerve cells (internal carotid mini-ganglion) is described at the junction between the greater superficial petrosal nerve and the internal carotid nerve close to the internal carotid artery. A retrograde tracer dye technique demonstrates that this ganglion and the trigeminal and superior vagal ganglia supply the internal carotid artery with SP/CGRP fibers at, above and below this level, respectively. Implications of this finding for cranial painful syndromes in man are discussed.

Methoctramine and hexahydrodifenidol antagonise two muscarinic responses on the rat superior cervical ganglion with opposite selectivity

Two novel muscarinic antagonists, methoctramine and hexahydrodifenidol, have been assessed for their action against two muscarinic agonist-induced responses on the rat superior cervical ganglion in vitro. DC recordings were made between the desheathed ganglion and its internal carotid nerve using the grease-gap technique. Hexahydrodifenidol and methoctramine antagonised the muscarine-induced M 1-mediated depolarisation of this preparation with estimated pA 2 values of 7.5 and 6.5, respectively. In 0.3 μM pirenzepine and 0.1 mM CaCl 2, 1 μM muscarine evoked a hyperpolarisation mediated by cardiac-like M 2 receptors. Hexahydrodifenidol and methoctramine antagonised this response with pIC 50 values (−log 10IC 50) of 5.7 and 7.4, respectively. The selectivity of methoctramine for cardiac-like M 2 receptors over M 1 receptors is therefore confirmed and extended to these two neuronal responses. The selectivity of hexahydrodifenidol was opposite to, and greater than, that seen with methoctramine.

Quantitative analysis of the number and distribution of neurons richly innervated by GABA-immunoreactive axons in the rat superior cervical ganglion.

The superior cervical ganglion of rats contains a considerable number of nerve fibers with GABA-like immunoreactivity which show a nonuniform distribution within the ganglion. The topography of these fibers has been analyzed by using antibodies raised against GABA-BSA-glutaraldehyde complexes. GABA-positive axons and axon varicosities accumulated around a subpopulation of principal ganglion cells forming basketlike patterns. These neurons richly innervated by GABA-positive axons (RIG-neurons) in turn were aggregated in patches with strong immunoreactivity. The size and packing density of the patches containing RIG-neurons and GABA-positive axons approaching them had rostral-to-caudal and medial-to-lateral gradients. Similar patterns were found in right and left ganglia. In five ganglia, a quantitative analysis revealed on average 1,344 RIG-neurons per ganglion representing about 5% of the total neuron population, with small variations (standard deviation 122) despite the highly variable shape of the ganglia. The distribution of RIG-neurons resembles that of neurons sending their axons into the internal carotid nerve. To check this possible correlation, HRP was injected into the eye and applied to the transected external carotid nerve. Double staining for the retrogradely transported peroxidase and GABA immunohistochemistry revealed that RIG-neurons formed a small subpopulation of retrogradely labelled neurons in both experiments. This suggests that RIG-neurons innervate various target organs. This conclusion is in agreement with the observation that RIG-neurons also exist in other sympathetic ganglia. Data presented suggest that sympathetic ganglion cells can be classified on the basis of non-uniform innervation patterns formed by axons that use different neurotransmitters.

Evidence for GABAergic fibers entering the superior cervical ganglion of rat from the preganglionic nerve trunk

The origin of gamma-aminobutyric acid immunoreactive (GABA-IR) nerve fibers present in the superior cervical ganglion (SCG) of rat was investigated. With immunocytochemical techniques many nerve fibers showed GABA-like positivity in the cervical sympathetic trunk, whereas similar staining could not be revealed in the internal carotid nerve or in the external carotid nerve. Ligation of the cervical sympathetic trunk for 24 h resulted a dramatic reduction in the staining density in the ganglion and in the cervical sympathetic trunk distal to the ligature. After transection of the preganglionic nerve fibers for eleven days or more, very few fibers staining for GABA were seen in the ganglion. The immunohistochemical results suggest that a major source of GABA within the SCG is a population of GABAergic axons entering from the preganglionic trunk.

Trigeminal fibre collaterals storing substance P and calcitonin gene-related peptide associate with ganglion cells containing choline acetyltransferase and vasoactive intestinal polypeptide in the sphenopalatine ganglion of the rat. An axon reflex modulating parasympathetic ganglionic activity?

In immunohistochemical studies on rat two types of nerve fibres, both showing substance P and calcitonin gene-related peptide-like immunoreactivity, have been localized in the sphenopalatine ganglion, the principal cells of which contain both vasoactive intestinal polypeptide and choline acetyltransferase. One fine-calibre fibre type forms basket-like arrangements around approximately 3–5% of the principal neurons, whereas another, more coarse type traverses the ganglion without making contacts with the ganglion cells. By transection of nerves connecting with the ganglion, in combination with retrograde tracing experiments, it was concluded that the fine-calibre fibres exclusively come from the trigeminal ganglion, whereas the second type in addition, and mainly, originate in the internal carotid ganglion which is situated along the greater superficial petrosal nerve and the pterygoid nerve at their junction with the internal carotid nerve. The brain vasculature was shown to be one target structure for the innervated principal cells in the sphenopalatine ganglion. The arrangement provides the functional possibility for a modulatory interaction between the autonomie and sensory systems, thus resembling an axon reflex mechanism in the peripheral nervous system.

Neuroneuronal interconnections in the rat superior cervical ganglion; Possible anatomical bases for modulatory interactions revealed by intracellular horseradish peroxidase labelling

Electrophysiologically identified neurons of rat superior cervical ganglion were intracellularly injected with horseradish peroxidase and processed for light and electron microscopic observation. At light microscope level, neurons could be classified according to their dendritic arborization pattern in the vicinity of the soma into radiate, tufted and intermediate types. Upon electrical stimulation of the internal and external carotid nerves it was observed that radiate and intermediate neurons sent their axons into one or the other of these nerve trunks, whereas a majority of tufted neurons gave no response to stimulation of either of these postganglionic nerves. Electron microscopic exploration of horseradish peroxidase-labelled neurons revealed a surprisingly high prevalence of interconnectivity between ganglionic neurons. These contacts were both dendrosomatic and dendrodendritic, and were a universal feature of the labelled neurons explored. Twenty-two of the 23 labelled cells were found to receive direct dendritic appositions on their somata, and 13 of these 23 cells were seen each to send their dendrites into contact with at least one unlabelled neuronal soma. Dendrodendritic contacts were observed for 87% of the labelled neurons, and most of the cells (80%) were seen to form triadic contacts which included two dendrites and a preganglionic nerve ending. All these figures represent minimum incidences. None of the dendrosomatic or dendrodendritic appositions observed was overtly synaptic although several morphological features indicated the possibility of somatic and or dendritic release and uptake at sites of apposition. It is suggested that the observed appositions provide anatomical substrates for modulatory interactions between the ganglionic neurons, possibly involving slow potentials or the switching of metabolic pathways.

Origins and pathways of cerebrovascular vasoactive intestinal polypeptide-positive nerves in rat.

In order to clarify the origins and pathways of vasoactive intestinal polypeptide (VIP)-containing nerve fibers in cerebral blood vessels of rat, denervation experiments and retrograde axonal tracing methods (true blue) were used. Numerous VIP-positive nerve cells were recognized in the sphenopalatine ganglion and in a mini-ganglion (internal carotid mini-ganglion) located on the internal carotid artery in the carotid canal, where the parasympathetic greater superficial petrosal nerve is joined by the sympathetic fibers from the internal carotid nerve, to form the Vidian nerve. VIP fiber bridges in the greater deep petrosal nerve and the internal carotid nerve reached the wall of the internal carotid artery. Two weeks after bilateral removal of the sphenopalatine ganglion or sectioning of the structures in the ethmoidal foramen, VIP fibers in the anterior part of the circle of Willis completely disappeared. Very few remained in the middle cerebral artery, the posterior cerebral artery, and rostral two-thirds of the basilar artery, whereas they remained in the caudal one-third of the basilar artery, the vertebral artery, and intracranial and carotid canal segments of the internal carotid artery. One week after application of true blue to the middle cerebral artery, dye accumulated in the ganglion cells in the sphenopalatine, otic and internal carotid mini-ganglion; some of the cells were positive for VIP. The results show that the VIP nerves in rat cerebral blood vessels originate: (a) in the sphenopalatine, and otic ganglion to innervate the circle of Willis and its branches from anterior and caudally and (b) from the internal carotid mini-ganglion to innervate the internal carotid artery at the level of the carotid canal and to some extent its intracranial extensions.

Nerve pathways between the pterygopalatine ganglion and eye in cats.

By dissection of thiocholine-stained orbital preparations, it has been determined that three different nerve pathways link the pterygopalatine ganglion and the eye in cats. 1) Nerves from the proximal half of the ganglion join a plexus of nerves and ganglion cells in the rete mirabile of the maxillary artery. Branches of the internal carotid nerve also supply this plexus. Fine nerves from the plexus travel to the optic nerve and then to the eye, accompanying both the nasociliary nerve that passes through the rete and the ciliary arteries that arise from the rete. 2) One or more nerves from the nerve of the pterygoid canal and from a prominent accessory ganglion near the orbital apex course to the inferior optic nerve surface at the optic foramen; these then run distally along the optic nerve to fuse with ciliary nerves or to accompany ciliary arteries entering the eye. 3) Other nerves from the pterygopalatine ganglion travel medially around the extraocular muscle cone to join the ethmoidal and infratrochlear branches of the nasociliary nerve; some nerves from the ganglion then take a retrograde course to the optic nerve, where they join ciliary nerves or arteries to the eye. All three pathways may transmit sympathetic, parasympathetic and somatic sensory nerve fibers.