Rapid recovery of function after partial denervation of the rat pineal gland suggests a novel mechanism for neural plasticity

Limited recovery of pineal function after regeneration of preganglionic sympathetic axons: evidence for loss of ganglionic synaptic specificity

The cervical sympathetic trunks (CSTs) contain axons of preganglionic neurons that innervate the superior cervical ganglia (SCGs). Because regeneration of CST fibers can be extensive and can reestablish certain specific patterns of SCG connections, restoration of end organ function would be expected. This expectation was examined with respect to the pineal gland, an organ innervated by the two SCGs. The activity of pineal serotonin N-acetyltransferase (NAT) exhibits a large circadian rhythm that is dependent on the sympathetic input of the gland, with high activity at night. Thirty-six hours after the CSTs were crushed bilaterally, nocturnal NAT was decreased by 99%. Three months later, enzyme activity had recovered only to 15% of control values, a recovery dependent on regeneration of CST fibers. Nevertheless, a small day/night rhythm was present in lesioned animals. Neither the density of the adrenergic innervation of the gland nor the ability of an adrenergic agonist to stimulate NAT activity was reduced in rats with regenerated CSTs. In addition, stimulation of the regenerated CST at a variety of frequencies was at least as effective in increasing NAT activity as seen with control nerves. These data suggest that the failure of pineal function to recover is not attributable to a quantitative deficit in the extent of reinnervation or synaptic efficacy. Rather, we suggest that there is some loss of specificity in the synaptic connections made in the SCG during reinnervation, resulting in a loss of the central neuronal information necessary for directing a normal NAT rhythm and thus normal pineal function.

Removal of half the sympathetic innervation does not reduce vasoconstrictor responses in rat tail artery

Following reinnervation of denervated rat tail arteries, nerve-evoked contractions are at least as large as those evoked in normally innervated arteries despite a much lower nerve terminal density. Here nerve-evoked contractions have been investigated after transection of half the sympathetic innervation of normal tail arteries. After 1 week, the noradrenergic plexus 50-70 mm along the tail was about half as dense as control. Excitatory junction potentials recorded in smooth muscle cells of arterial segments isolated in vitro were half their normal amplitude. Surprisingly, nerve-evoked contractions of isometrically mounted segments were not reduced in amplitude, as was also the case after only 3 days. After 1 week, enhancement of nerve-evoked contractions by blocking either neuronal re-uptake of noradrenaline with desmethylimipramine or prejunctional α2-adrenoceptors with idazoxan was similar to control, suggesting that these mechanisms are matched to the number of innervating axons. The relative contribution of postjunctional α2-adrenoceptors to contractions evoked by long trains of stimuli was enhanced but that of α1-adrenoceptors was unchanged. Transiently, sensitivity to the α1-adrenoceptor agonist phenylephrine was slightly increased. After 7 weeks, amplitudes of nerve-evoked contractions remained similar to control, and sensitivity to phenylephrine had recovered but that to the α2-adrenoceptor agonist clonidine was slightly raised. The normal amplitude of nerve-evoked contractions after partial denervation is only partly explained by the greater contribution of α2-adrenoceptors. While the post-receptor mechanisms activated by nerve-released transmitter may be modified to amplify the contractions after partial denervation, our findings suggest that these mechanisms are normally saturated, at least in this artery.

Circadian regulation of pineal gland rhythmicity

The pineal gland is a neuroendocrine organ of the brain. Its main task is to synthesize and secrete melatonin, a nocturnal hormone with diverse physiological functions. This review will focus on the central and pineal mechanisms in generation of mammalian pineal rhythmicity including melatonin production. In particular, this review covers the following topics: (1) local control of serotonin and melatonin rhythms; (2) neurotransmitters involved in central control of melatonin; (3) plasticity of the neural circuit controlling melatonin production; (4) role of clock genes in melatonin formation; (5) phase control of pineal rhythmicity; (6) impact of light at night on pineal rhythms; and (7) physiological function of the pineal rhythmicity.

A standardized surgical technique for rat superior cervical ganglionectomy

Superior cervical ganglionectomy (SCGx) is a valuable microsurgical model to study the role of the sympathetic nervous system in a vast array of physiological and pathological processes, including homeostatic regulation, circadian biology and the dynamics of neuronal dysfunction and recovery after injury. Despite having several experimental applications in the rat, a thorough description of a standardized procedure has never been published. Here, we provide a brief review of the principal features and experimental uses of the SCGx, the surgical anatomy of the neck and sympathetic cervical chain, and a step-by-step description of how to consistently remove the superior cervical ganglia through the omohyoid muscle or the carotid triangle. Furthermore, we suggest procedures and precautions to be taken during and after surgery to optimize results and describe tools to validate surgical success. We expect that the following standardized and optimized protocol will allow researchers to organize knowledge into a cohesive framework in those areas where the SCGx is applied.

Evidence for a predominant intrinsic sympathetic control of cerebral blood flow alterations in an animal model of cerebral arteriovenous malformation

In terms of neurogenic cerebral blood flow (CBF) control, the activity of the sympathetic nervous system (SNS) has a regulating effect. The impact of a manipulation of both the peripheral (via the perivascular sympathetic net) and central components (via the intracortical noradrenergic terminals originating from the locus coeruleus) on CBF-and especially on hyperperfusion syndromes-is unclear. To test the specific patterns following such alterations, cortical oxygen saturation (rSO2), regional CBF (rCBF), and cortical interstitial norepinephrine (NE) concentrations were measured. Twelve weeks after either the creation of an extracranial AV fistula or sham operation, 80 male Sprague-Dawley rats underwent one of the following procedures: (1) no SNS manipulation, (2) peripheral SNS inhibition via bilateral sympathectomy, (3) central SNS inhibition via the neurotoxin DSP-4, or (4) complete SNS inhibition. Norepinephrine concentrations were lowest after complete inhibition (NE [nmol]: pre, 1.8 ± 1.2; post, 2.4 ± 1.8) and highest following peripheral inhibition (NE [nmol]: pre, 3.6 ± 1.9; post, 6.6 ± 4.4). Following fistula occlusion, rCBF (laser Doppler unit [LDU]) and rSO2 (%SO2) increases were highest after complete inhibition (pre: 204 ± 14 LDU, 34 ± 3%SO2; post: 228 ± 18 LDU, 39 ± 3%SO2) and lowest after peripheral inhibition (pre: 221 ± 18 LDU, 41 ± 2%SO2; post: 226 ± 14 LDU, 47 ± 2%SO2). Thus, a complete inhibition down-regulates SNS activity and provokes a cortical hyperperfusion condition. With this, the hitherto unknown predominant role of the intrinsic component could be demonstrated for the first time in vivo.

Mammalian pinealocytes: ultrastructural aspects and innervation

In the mammalian pineal gland it is notoriously difficult to relate structure to function. The pineal-specific cells, the pinealocytes, contain only inconspicuous numbers of secretory granules, and the variable amounts of smooth and rough endoplasmic reticulum also do not point to a particular function. In addition to these widely known cellular components, pinealocytes contain organelles, the so-called 'synaptic' ribbons, histophysiological studies of which provide important insights into the structural and functional complexity of the organ. As synaptic ribbons may be involved in neuronal functions of pinealocytes it is notable that these organelles are structurally heterogeneous. Ribbons fall into at least two categories: rod-like (RSR) and sphere-like (SS) structures. RSR and SS usually do not lie within the same pinealocyte profile and appear to be regulated by different mechanisms. It is conceivable that they are important components of the biological clock system. These findings are related to our knowledge of the innervation of the mammalian pineal gland and to electrophysiological characteristics of pinealocytes.

Nicotinic acetylcholine receptor subunit proteins alpha7 and beta4 decrease in the superior cervical ganglion after axotomy

Synaptic transmission in the superior cervical ganglion (SCG) is mediated by nicotinic acetylcholine receptors (nAChR). After transection of the postganglionic nerves of the SCG in the adult rat, the transcript levels of four of the five nAChR subunits present in the ganglion, alpha3, alpha5, alpha7, and beta4, decrease dramatically. In the present study, the effect of axotomy on nAChR subunit expression was examined at the protein level, focusing on the alpha7 and beta4 subunits. Immunohistochemistry with monoclonal antibody mAb306 (for the alpha7 subunit) and polyclonal antibody 4886 (for the beta4 subunit) showed that immunoreactivities for both alpha7 and beta4 subunits were concentrated in neurons in the intact ganglion. Results from double staining with antibodies to these subunits and to tyrosine hydroxylase, the enzyme that catalyzes the rate-limiting step in the biosynthesis of the sympathetic neurotransmitter norepinephrine, demonstrated that most neurons in the SCG express both the alpha7 and beta4 subunits. Three days after axotomy, the number of immunolabeled neurons and the intensity of the immunostaining per labeled neuron were decreased for both subunits. Decreases in subunit levels were also observed by Western blot analysis. Observing changes in these subunits over time after surgery revealed that, while the protein level of the alpha7 subunit recovered substantially within 2 weeks after the lesion, that of the beta4 subunit stayed low. These data demonstrate that decreases in nicotinic receptor subunits are among the changes in proteins that occur in axotomized sympathetic neurons, and suggest that these decreases may contribute to the depression in ganglionic synaptic transmission observed in axotomized ganglia.

p-Chlorophenylalanine treatment depresses the number of synaptic ribbon profiles in the rat pineal gland, but does not abolish their day-night rhythm

It is largely unknown as to how the day/night rhythm of pineal synaptic ribbon number is regulated. Transcriptional events related to the nocturnal formation of new synaptic ribbons occur early in the morning, when pineal serotonin levels begin to increase. The present study was carried out in order to elucidate as to how altered serotonin levels affect the day/night changes in the number of synaptic ribbon profiles. To this end, male Sprague-Dawley rats received a single dose of p-chlorophenylalanine (pCPA, 300 mg/kg body weight, 72 hours before sacrifice), which depresses tryptophan hydroxylase activity and hence pineal serotonin levels. Control animals received saline injections. Experimental and control animals were killed in the middle of the light period and in the middle of the dark period, respectively. The pineal glands were removed and bisected. One half was used for electron microscopic quantification of synaptic ribbon profile numbers, and the other half for determination of several biochemical parameters (see below). pCPA decreased pineal serotonin levels to c. 30% of the controls and synaptic ribbon profile numbers by c. 30%, both at day and night. The day/night rhythm of the two parameters did not appear to be affected. The rate-limiting enzyme of melatonin synthesis, serotonin N-acetyltransferase, which is only measurable at night, and pineal melatonin levels were not affected at night. The intermediary product of melatonin formation, N-acetylserotonin, was significantly depressed at night. It is concluded that serotonin plays a more important modulatory role on pineal synaptic ribbons than does melatonin.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in ovine pineal gland neuron-specific enolase immunoreactivity following bilateral, but not unilateral, superior cervical ganglionectomy

Pineal gland tissue from control and from unilaterally or bilaterally superior cervical ganglionectomized (SCGX) sheep was found to contain neuron-specific enolase immunoreactive cells and nerve fibers. Morphological characteristics of pineal cells exhibiting immunoreactivity indicated that they were predominantly pinealocytes, while other cell types were nonimmunoreactive. Whereas bilateral SCGX resulted in a reduction in the size, and possibly number, of immunoreactive cells in the pineal, unilateral denervation did not result in any significant effects when compared with control pineals. Concomitant with the reduction in immunoreactivity in bilaterally denervated pineals was a significant increase in the volume of interstitial space, but not the number of nonimmunoreactive cells. These results suggest that sympathetic nerve fibers innervating the pineal of unilaterally sympathectomized sheep exhibited a degree of neural plasticity that resulted in denervated pinealocytes being reinnervated by remaining intact nerve terminals, thus preventing the occurrence of degenerative changes normally associated with complete loss of neural input through bilateral denervation. The fact that in unilaterally denervated sheep neither left nor right SCGX produced any discernible effects in either half of the pineal indicates that nerve fibers from each of the ganglia cross over to innervate the contralateral as well as the ipsilateral pineal half. In the stalk of the pineal an extensive network of immunoreactive nerve fibers was found in both the caudal and habenular commissures, and occasionally these fibers were observed to enter the body of both intact and sympathetically denervated pineals. This latter result suggests that the sympathetic innervation enters the pineal over its surface and not via the stalk.

Time course of adaptations in dopamine biosynthesis, metabolism, and release following nigrostriatal lesions: implications for behavioral recovery from brain injury

Alterations in neostriatal dopamine metabolism, release, and biosynthesis were determined 3, 5, or 18 days following partial, unilateral destruction of the rat nigrostriatal dopamine projection. Concentrations of dopamine and each of its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 3-methoxytyramine (3-MT) were markedly decreased in the lesioned striata at 3, 5, or 18 days postoperation. The decline in striatal high-affinity [3H]dopamine uptake closely matched the depletion of dopamine at 3 and 18 days postoperation. However, neither DOPAC, HVA, nor 3-MT concentrations were decreased to as great an extent as dopamine at any time following lesions that depleted the dopamine innervation of the striatum by greater than 80%. In these more severely lesioned animals, dopamine metabolism, estimated from the ratio of DOPAC or HVA to dopamine, was increased two- to four-fold in the injured hemisphere compared with the intact hemisphere. Dopamine release, estimated by the ratio of 3-MT to dopamine, was more increased, by five- to sixfold. Importantly, the HVA/dopamine, DOPAC/dopamine, and 3-MT/dopamine ratios did not differ between 3 and 18 days postlesioning. The rate of in vivo dopamine biosynthesis, as estimated by striatal DOPA accumulation following 3,4-dihydroxyphenylalanine (DOPA) decarboxylase inhibition with NSD 1015, was increased by 2.6- to 2.7-fold in the surviving dopamine terminals but again equally at 3 and 18 days postoperation. Thus, maximal increases in dopamine metabolism, release, and biosynthesis occur rapidly within neostriatal terminals that survive a lesion. This mobilization of dopaminergic function could contribute to the recovery from the behavioral deficits of partial denervation by increasing the availability of dopamine to neostriatal dopamine receptors. However, these presynaptic compensations are not sufficient to account for the protracted (at least 3-week) time course of sensorimotor recovery that has been observed following partial nigrostriatal lesion.

Hamster and rat pineal gland beta-adrenoceptor characterization with iodocyanopindolol and the effect of decreased catecholamine synthesis on the receptor

Rat and hamster pineal glands were used in binding studies to characterize their beta-adrenoceptors with a new specific antagonist ligand, iodocyanopindolol. The receptors were saturable, and the ligand was selective and demonstrated stereospecificity for both species. The rat pineal had a 20-fold greater density of beta-adrenoceptors, while the affinity was one-third that of the hamster pineal. utilizing this radioligand, we examined the effects of decreased sympathetic input to the pineal on beta-adrenergic receptors in both species. Decreased noradrenergic input to the pineal gland of the hamster was accomplished by superior cervical ganglionectomy, or by exposing the animals to continuous light for 36 hours. Parallel studies were conducted with hamster pineal gland in which catecholamine synthesis was measured. The results indicate that a selective decrease in catecholamine synthesis in the hamster pineal does not change the beta-adrenoceptor density or affinity. In contrast, a concomitant increase in beta-adrenoceptor density but not affinity occurs in the rat pineal gland after similar decreased sympathetic input.

The influence of the frequency and pattern of sympathetic nerve activity on serotonin N-acetyltransferase in the rat pineal gland

The activity of the pineal enzyme arylamine: N-acetyltransferase (NAT) was determined following direct stimulation of the preganglionic or post-ganglionic nerves of the superior cervical ganglia. 1. Stimulation of the preganglionic trunks at 10 c/sec during the day or night was sufficient to increase NAT activity approximately 50-fold, to levels comparable to those observed at night in the intact animal. The time course of this effect of nerve stimulation differed between day and night. 2. The responses of pineal NAT to certain frequencies of stimulation were similar for preganglionic and post-ganglionic stimulation. In both cases the responses to stimulation at 5 c/sec appeared to be maximal, 10 c/sec causing no further increase. However, at 10 c/sec, stimulation was more effective post-ganglionically than preganglionically. 3. Various patterns of preganglionic stimulation, having the same average frequency, differed in their ability to increase the activity of NAT. Some, though not all, of these differences between patterns were observed during post-ganglionic stimulation. 4. Unilateral stimulation of the preganglionic nerves produced an increase in NAT activity that was less than half the increase produced by bilateral stimulation, suggesting that the innervation from the two ganglia interact within the pineal gland. 5. These data indicate that changes in the firing rates of sympathetic nerves innervating the pineal gland, within the range of frequencies typically observed for sympathetic neurones, would be sufficient to account for the circadian rhythm in NAT activity observed in the intact rat. Changes in the over-all pattern of sympathetic activity, in addition to changes in the total number of stimuli, could play a significant role in the pineal response.