Alterations of norepinephrine metabolism in rat locus coeruleus neurons in response to axonal injury

Chronic Treatment with Serotonin Selective Reuptake Inhibitors Does Not Affect Regrowth of Serotonin Axons Following Amphetamine Injury in the Mouse Forebrain

A current hypothesis to explain the limited recovery following brain and spinal cord trauma stems from the dogma that neurons in the mammalian central nervous system lack the ability to regenerate their axons after injury. Serotonin (5-HT) neurons in the adult brain are a notable exception in that they can slowly regrow their axons following chemical or mechanical lesions. This process of regrowth occurs without intervention over several months and results in anatomical recovery that approximates the preinjured state. During development, serotonin is a trophic factor, playing a role in both cell survival and axon growth. Additionally, some studies have shown that stroke patients treated after injury with serotonin selective reuptake inhibitors (SSRIs) appeared to have improved recovery. To test the hypothesis that serotonin can influence the regrowth of 5-HT axons, mice received a high dose of para-chloroamphetamine (PCA) to induce widespread retrograde degeneration of 5-HT axons. Then, after a short rest period to avoid any interaction with the acute injury phase, SSRIs were administered daily for 6 or 10 weeks. Using immunohistochemistry in 5-HT transporter-GFP BAC transgenic mice, we determined that while PCA led to a rapid initial decrease in total 5-HT axon length in the somatosensory cortex, visual cortex, or area CA1 of the hippocampus, treatment with either fluoxetine or sertraline (two different SSRIs) did not affect the recovery of axon length. These results suggest that chronic SSRI treatment does not affect the regrowth of 5-HT axons and argue against SSRIs as a potential therapy following brain injury.

Neuronal Redevelopment and the Regeneration of Neuromodulatory Axons in the Adult Mammalian Central Nervous System

One reason that many central nervous system injuries, including those arising from traumatic brain injury, spinal cord injury, and stroke, have limited recovery of function is that neurons within the adult mammalian CNS lack the ability to regenerate their axons following trauma. This stands in contrast to neurons of the adult mammalian peripheral nervous system (PNS). New evidence, provided by single-cell expression profiling, suggests that, following injury, both mammalian central and peripheral neurons can revert to an embryonic-like growth state which is permissive for axon regeneration. This “redevelopment” strategy could both facilitate a damage response necessary to isolate and repair the acute damage from injury and provide the intracellular machinery necessary for axon regrowth. Interestingly, serotonin neurons of the rostral group of raphe nuclei, which project their axons into the forebrain, display a robust ability to regenerate their axons unaided, counter to the widely held view that CNS axons cannot regenerate without experimental intervention after injury. Furthermore, initial evidence suggests that norepinephrine neurons within the locus coeruleus possess similar regenerative abilities. Several morphological characteristics of serotonin axon regeneration in adult mammals, observable using longitudinal in vivo imaging, are distinct from the known characteristics of unaided peripheral nerve regeneration, or of the regeneration seen in the spinal cord and optic nerve that occurs with experimental intervention. These results suggest that there is an alternative CNS program for axon regeneration that likely differs from that displayed by the PNS.

Readministration of adenoviral gene delivery to dopamine neurons

An approach currently being explored as treatment for Parkinson's disease is gene therapy. An important question concerns the duration of transgene expression in dopamine neurons and the issues of vector persistence, neuronal damage and the feasibility of readministering vector to the same neuronal population. We show, using an adenoviral vector expressing the LacZ reporter gene, that transgene expression declined over time but with minimal loss of dopamine neurons or vector DNA. Readministration of vector resulted in low levels of transgene delivery to the neurons. Moreover, the neurons to which vector had already been delivered were unable to transport the retrograde tracer fluorogold. Our findings indicate that transgene expression declined in dopamine neurons despite the persistence of virus, and the capacity to readminister vector to these neurons was limited.

Stimulation of the locus coeruleus elicits noradrenaline and dopamine release in the medial prefrontal and parietal cortex

Our previous studies have suggested that dopamine and noradrenaline may be coreleased from noradrenergic nerve terminals in the cerebral cortex. To further clarify this issue, the effect of electrical stimulation of the locus coeruleus on extracellular noradrenaline, dopamine and DOPAC in the medial prefrontal cortex, parietal cortex and caudate nucleus was analysed by microdialysis in freely moving rats. Stimulation of the locus coeruleus for 20 min with evenly spaced pulses at 1 Hz failed to modify cortical catecholamines and DOPAC levels. Stimulation with bursts of pulses at 12 and 24 Hz increased, in a frequency-related manner, not only noradrenaline but also dopamine and DOPAC in the two cortices. In both cortices noradrenaline returned to baseline within 20 min of stimulation, irrespective of the stimulation frequency, whereas dopamine returned to normal within 20 and 60 min in the medial prefrontal cortex and within 60 and 80 min in the parietal cortex after 12 and 24 Hz stimulation, respectively. DOPAC remained elevated throughout the experimental period. Phasic stimulation of the locus coeruleus at 12 Hz increased noradrenaline in the caudate nucleus as in the cerebral cortices but was totally ineffective on dopamine and DOPAC. Tetrodotoxin perfusion into the medial prefrontal cortex dramatically reduced noradrenaline and dopamine levels and suppressed the effect of electrical stimulation. These results indicate that electrical stimulation-induced increase of dopamine is a nerve impulse exocytotic process and suggest that cortical dopamine and noradrenaline may be coreleased from noradrenergic terminals.

The morphological and neurochemical effects of diffuse brain injury on rat central noradrenergic system

The central noradrenergic system is widely distributed throughout the brain and is closely related to spontaneous motility and level of consciousness. The study presented here evaluated the morphological as well as neurochemical effects of diffuse brain injury on the central noradrenergic system in rat. Adult male Sprague-Dawley rats were subjected to impact-acceleration brain injury produced with a weight-drop device. Morphological changes in locus coeruleus (LC) neurons were examined by using immunohistochemistry for dopamine-beta-hydroxylase, and norepinephrine (NE) turnover in the cerebral cortex was measured by high performance liquid chromatography with electrochemical detection. The size of LC neurons increased by 11% 24 h after injury but had decreased by 27% seven days after injury. Axons of noradrenergic neurons were swollen 24 h and 48 h after injury but the swelling had dwindled in seven days. NE turnover was significantly reduced seven days after injury and remained at a low level until eight weeks after injury. These results suggest that focal impairment of axonal transport due to diffuse brain injury causes cellular changes in LC and that the neurochemical effect of injury on the central noradrenargic system lasts over an extended period of time. Chronic suppression of NE turnover may explain the sustained behavioral and psychological abnormalities observed in a clinical situation.

Heightened transcription for enzymes involved in norepinephrine biosynthesis in the rat locus coeruleus by immobilization stress

BACKGROUND

The locus coeruleus (LC), a target for CRH neurons, is critically involved in responses to stress. Various physiological stresses increase norepinephrine turnover, tyrosine hydroxylase (TH) enzymatic activity, protein and mRNA levels in LC cell bodies and terminals; however, the effect of stress on other enzymes involved in norepinephrine biosynthesis in the LC is unknown.

METHODS

Rats were exposed to single (2 hour) or repeated (2 hour daily) immobilization stress (IMO). Recombinant rat dopamine b-hydroxylase (DBH) cDNA was expressed in E. coli and used to generate antisera for immunohistochemistry and immunoblots in LC. Northern blots were used to assess changes in mRNA levels for TH, DBH, and GTP cyclohydrolase I (GTPCH) in the LC in response to the stress. Conditions were found to isolate nuclei from LC and to use them for run-on assays of transcription.

RESULTS

Repeated stress elevated the DBH immunoreactive protein levels in LC. Parallel increases in TH, DBH and GTPCH mRNA levels of about 300% to 400% over control levels were observed with single IMO, and remained at similar levels after repeated IMO. This effect was transcriptionally mediated, and even 30 min of a single IMO significantly increased the relative rate of transcription.

CONCLUSIONS

This study is the first to reveal transcriptional activation of the genes encoding catecholamine biosynthetic enzymes in the LC by stress. In addition to TH, changes in DBH and GTPCH gene expression may also contribute to the development of stress-triggered affective disorders.

Norepinephrine and traumatic brain injury: a possible role in post-traumatic edema

Unilateral cerebral contusion is associated with an early (30 min) increase in norepinephrine (NE) turnover followed by a later (6-24 h) depression of turnover which is bilateral and widespread throughout the brain. Blockade of NE function during the first few hours after traumatic brain injury (TBI) impedes subsequent recovery of function without enlarging the size of the lesion. The current studies were carried out to characterize further the timing of the switch from increased to decreased NE turnover and to investigate the pathogenesis of the delayed recovery of function associated with blocking NE function. Adult male rats had unilateral somatosensory cortex contusions made with a 5 mm diameter impact piston. They were killed after 2 h and their brains analyzed for NE turnover by HPLC with electrochemical detection. In general, NE turnover (the ratio of 3-methoxy-4-hyroxyphenylglycol to NE levels) had returned to sham-lesion control levels in most brain regions by 2 h after either left or right sided contusions. The only exceptions were a persistent 87% increase at the lesion site after right-sided contusions and 22% and 32% increases in the contralateral cerebellum after right- and left-sided contusions, respectively. Blockade of alpha1-adrenoceptors by treatment with prazosin (3 mg/ kg, i.p.) 30 min prior to TBI produced edema in the striatum and hippocampus at 24 h which was not seen saline-treated rats nor in rats where NE reuptake was blocked with desmethylimipramine (DMI; 10 mg/kg, i.p.). DMI increased edema at the lesion site at 24 h, however. These data suggest that the early increase in NE release following unilateral cerebral contusion is protective and that this may act to stabilize the blood-brain barrier in areas adjacent to the injury site. Drugs that interfere with this enhanced noradrenergic function might enhance the damage caused by TBI.

Differential effects of the intraventricular administration of 6-hydroxydopamine on the induction of type II beta-tubulin and tyrosine hydroxylase mRNA in the locus coeruleus of the aging Fischer 344 rat

Noradrenergic neurons of the locus coeruleus have been shown to respond to injury by increasing the synthesis of neurotransmitter (via the activation and induction of tyrosine hydroxylase, the rate-limiting catalyst in the production of catecholamines) and initiating compensatory axonal sprouting. However, this laboratory has recently described a significant deficit in the activation of tyrosine hydroxylase in the aged Fischer 344 rat, in contrast to the young and mature rat, following partial damage to cortical and hippocampal noradrenergic terminals induced by the neurotoxin 6-hydroxydopamine. To extend these observations, we measured changes in the relative levels of neuron-specific type II beta-tubulin and tyrosine hydroxylase mRNA in locus coeruleus neurons of 2, 12, and 24-month-old Fischer 344 rats following intraventricular infusions of 6-hydroxydopamine by using in situ hybridization histochemistry. These measures were used as markers of the responsiveness of these neurons to injury. 6-Hydroxydopamine treatment induced a persistent increase (at least 10 days) in the expression of type II beta-tubulin mRNA only in 2-month-old animals; this marker decreased in the 12 and 24-month-old animals. Relative levels of tyrosine hydroxylase mRNA increased in 2 and 12-month-old lesioned animals both 3 and 10 days post-treatment. In contrast, the induction of tyrosine hydroxylase mRNA in 24-month-old animals, seen three days post-treatment, was attenuated by 10 days. These data indicate that the capacity of locus coeruleus neurons to compensate for injury by either initiating a potential sprouting response or increasing their capacity to synthesize neurotransmitter is reduced in older animals.

Widespread and lateralization effects of acute traumatic brain injury on norepinephrine turnover in the rat brain

Norepinephrine (NE) has been implicated in recovery of function following traumatic brain injury (TBI). While bilateral decrease in brain NE turnover occur at 6-24 h after TBI, it is unknown what effects unilateral TBI might have on brain NE turnover the first few minutes after injury. Her male Sprague-Dawley rats had unilateral confusions of either the right or left somatosensory cortex produced by an air between piston. At 30 min after TBI, brain NE turnover was assessed by measuring the ratio of 3-methoxy-4 hydroxyphenylglycol (MHPG) to NE levels in various brain regions. Both right and left TBI produced 32-103% increases in NE turnover at the injury site and in the ipsilateral cerebral cortex surrounding, rostral and caudal to the injury as compared to the contralateral, uninjured site or to the homologous sites in uninjured controls. NE turnover was also altered selectively in some brain areas not affected by right TBI. Left TBI decreased NE turnover by 29% in the frontal cortex contralateral to the injury and by 24% bilaterally in the hypothalamus while increasing locus coeruleus NE turnover by 72% compared to uninjured controls. Thus, unilateral cortical TBI produced predominantly ipsilateral increases in cortical NE turnover but variable, bilateral changes in NE turnover in subcortical areas which were dependent upon the side of injury. These subcortical differences may explain some of the lateralized effects of cortical injury on post-injury behavior.

Focal traumatic brain injury causes widespread reductions in rat brain norepinephrine turnover from 6 to 24 h

The effect of right sensorimotor traumatic brain injury (TBI) in male Sprague-Dawley rats on brain norepinephrine (NE) turnover was assessed by measuring the decline of endogenous NE levels following tyrosine hydroxylase inhibition produced with alpha-methyl-p-tyrosine. Right sensorimotor cortex contusions were produced by a pneumatically driven piston which depressed the dural surface by 2 mm at 3.2 m/s. TBI rats were compared to uninjured, anesthetized controls at 6 h and 24 h after surgery. While NE turnover was not affected at the lesion site at 6 h after TBI, it was either abolished or decreased by 33-75% bilaterally in the hypothalamus and in the cerebral cortex surrounding and rostral to the lesion site. In the cortex caudal to the lesion site, NE turnover was completely abolished. NE turnover in cerebral cortex opposite the lesion site and in the contralateral cerebellum was decreased by 51 and 43%, respectively, at 6 h. At 24 h, NE turnover was either abolished or decreased bilaterally by 45-92% in all cortical areas, in the hypothalamus, cerebellum, locus coeruleus and medulla. Thus, right sensorimotor cortex contusion causes a marked, early and widespread depression of brain NE turnover. Since amphetamine increases NE turnover, this may explain the dramatic improvement in behavioral deficits which occurs following amphetamine administration at 24 h after such lesions.

Alpha 1-adrenoceptors in the adult rat barrel field: effects of deafferentation and norepinephrine removal

Norepinephrine (NE), acting on brain adrenoceptors, plays an important role in barrel field neuronal activity and plasticity. For this reason, the distribution of alpha 1- and alpha 2-adrenoceptors in the somatosensory cortex barrel field was studied by autoradiographic techniques in rats undergoing plastic change or NE depletion. In layers IV and V of the cortex, the pattern of alpha 1-adrenoceptors (assessed by [3H]prazosin binding) varied across the barrel field. There was relatively low binding within the barrels themselves, with 21% higher binding in the surrounding septa. alpha 2-Adrenoceptor binding (assessed with [3H]paraminoclonidine) was almost homogeneous across the entire barrel field. Two weeks after noradrenergic deafferentation by unilateral lesioning of the locus coeruleus, there was a 16% upregulation of [3H]prazosin binding. This then returned to control levels of by 8 weeks. Peripheral deafferentation of sensory input to the barrel field produced the opposite effect on alpha 1-adrenoceptors. Unilateral removal of all but the central (C3) vibrissa (which induces plastic changes in the cortical representation of the spared virbrissa) caused a 12% decrease in [3H]prazosin binding in the whole barrel field at 2 weeks after surgery which returned to normal by 8 weeks. Therefore, alpha 1-adrenoceptors in the barrel field of the rat are affected in opposite ways by changes in NE content and afferent sensory input. We hypothesize that alpha 1-adrenoceptor levels are modulated after vibrissectomy through either an indirect reaction to reduced cortical gamma-aminobutyric acid levels, or by a reordering of metabolic priorities during plastic change of the cortical neuronal network.

Fetal neocortical transplants into the medial forebrain bundle attract ingrowth of catecholaminergic fibers in adult rat brain

The hypothesis that fetal tissue grafts may exert a trophic influence on damaged catecholaminergic fibers was examined. Ascending dopamine and norepinephrine axons normally innervate frontal cortex targets in the intact rat brain. These and other ascending catecholaminergic fibers were disrupted with stereotaxic injections of 6-hydroxydopamine into the medial forebrain bundle (mfb), followed after 1 or 14 days by grafts of fetal neocortical tissue placed into the injection site, or by sham grafts. Glyoxylic acid histofluorescence techniques were then used to examine catecholaminergic fiber distribution. When such lesions were made without subsequent grafting, virtually no growth of catecholaminergic fibers occurred beyond the injection site and frontal cortex norepinephrine levels were depleted to 15% of control levels. However, when grafts of fetal neocortical tissue were made into the lesion site and animals examined 3 months later, catecholaminergic fibers grew through the lesion site to ramify within the graft tissue. Catecholaminergic fibers were seen in all portions of most grafts, though they were most dense on the caudal and ventral edges of the graft, close to the path of the mfb. Similar densities of graft innervation were seen 3 months after animals received grafts placed into the same site without prior lesioning of catecholaminergic fibers. Fetal neocortical grafts thus induce collateral sprouting from intact host catecholaminergic axons and may also promote regenerative sprouting when such fibers are otherwise irreparably damaged.

Dose-dependent destruction of the coeruleus-cortical and nigral-striatal projections by MPTP

In order to determine whether 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces neuronal death or the loss of tyrosine hydroxylase (TH) immunoreactivity, 4 catecholaminergic nuclei in the mouse: substantia nigra compacta (SNc), locus coeruleus (LC), ventral tegmental area (VTA) and the A13 nucleus in the hypothalamus were quantitatively examined. Serial sections were taken through the rostrocaudal extent of each nucleus: alternate sections were incubated with TH antiserum and reacted with an immunoperoxidase technique while the alternate set was Nissl stained. Counts and 3 dimensional reconstructions of TH reactive somata were made for each nucleus for saline-treated controls and mice treated with different doses of MPTP (37.5, 75, 150 and 300 mg/kg). TH-positive neurons were counted along with their counterparts on the Nissl-stained alternative sections to both identify the catecholaminergic neurons and to measure their destruction. Concentrations of striatal dopamine and cortical norepinephrine were measured for all dosages of MPTP in order to determine the relationship between dosage, target tissue neurotransmitter concentration and neuronal destruction. By 20 days after MPTP injection there was a dose-dependent random loss of TH-immunoreactive neurons that was almost identical in all 4 nuclei examined. Analysis of the Nissl versus TH cell counts revealed that MPTP resulted in neuronal destruction in the SNc and the LC rather than just a loss of TH immunoreactivity. There was no difference in sensitivity to MPTP between the SNc and the LC. Decreases in cortical norepinephrine concentrations were about one third of the decreases of LC neuronal counts for all MPTP doses; while decreases in striatal dopamine and SNc cell loss was similar to the LC for the two lower doses of MPTP but for the higher doses, the relationship approached or exceeded a one to one ratio. Hence estimates of neuronal death based upon target tissue transmitter concentrations could not be made using the same relationship for SNc and the LC catecholaminergic neurons and use of the same relationship for higher MPTP dosages results in an underestimate of LC neuronal destruction relative to that in the SNc.

Activation of the noradrenergic system facilitates an attentional shift in the rat

The noradrenergic system was pharmacologically activated with the alpha 2 receptor antagonist, idazoxan (2 mg/kg i.p.), during the acquisition of a complex appetitive task requiring a shift in attention to stimulus dimension and in response strategy. Rats first learned a fixed path of 6 successive choices in a linear maze. The task was then changed to a visual discrimination task in which the spatial configuration of the correct path was indicated by visual cues and changed on each daily trial. During this part of the task, the rats were injected before each trial with idazoxan, a drug which increases the firing rate of neurons in the locus coeruleus and the release of noradrenaline in the cortex and hippocampus. Two control experiments showed that the drug treatment had no effect on the acquisition of either component of the task - the successive place learning or the visual discrimination. The drug was found to be effective only during the shift phase of the experiment, the idazoxan-treated rats taking fewer trials to reach criterion than the saline. A second experiment showed that idazoxan increased the amount of time spent investigating novel and unexpected objects in a familiar hole board. These results implicate the noradrenergic system in problem-solving which requires an attentional shift or a shift in responding from familiar to novel stimuli.

Noradrenergic innervation does not affect chronic regulation of [125I]pindolol receptors in fetal rat brain transplants or host neocortex

Fetal (E15-16) somatosensory cortex (n = 15) or cerebellum (n = 9) were placed into the somatosensory cortex (SmI) of adult rat hosts to study the relative importance of tissue origin versus host milieu on graft beta-adrenoceptor regulation. Autoradiographic studies of [125I]pindolol ([125I]pin) binding in the presence of 3 microM serotonin were performed as an index of beta-receptor binding in both intact hosts and those with ipsilateral locus coeruleus (LC) lesions and/or ipsilateral superior cervical ganglionectomy. [125I]pin binding within fetal grafts was highly variable with areas of highest specific binding in cortical grafts (Kd = 209 +/- 30 pM, Bmax = 106 +/- 7 (fmol/mg protein) being comparable to host cortex (Kd = 211 +/- 41 pM, Bmax = 111 +/- 9 fmol/mg protein). Average total binding in whole cortical grafts was 73% and in cerebellar grafts was 60% of that in comparable adult cortex. Host cortex had 66-73% and cerebellum had 4-8% beta 1-receptors while cortical grafts had 59% and cerebellar grafts had 43% beta 1-receptors as determined by competitive binding with ICI 89406 and 118551. Noradrenergic fibers derived from both the host LC and superior cervical ganglion grew into fetal cortical grafts. Binding to high affinity uptake sites ([3H]desmethylimipramine, [3H]DMI) on noradrenergic terminals in cerebellar grafts was 28% higher than that in cortical grafts; superior cervical ganglionectomy decreased [3H]DMI binding in cortical grafts by 37% but had no effect on cerebellar grafts. Neither ganglionectomy nor LC lesions affected total specific binding or binding to beta-receptor subtypes in the grafts or host cortex 3-6 months after removal. Therefore, anatomic site of origin appeared to be the predominant factor in determining the development of beta-adrenoceptors in fetal cortical tissue. In ectopically placed cerebellar grafts, beta-receptor subtypes did not develop comparably to host cerebellar receptors suggesting that host milieu may be of critical importance in receptor development in this tissue.

Axonal transport of beta-receptors during the response to axonal injury and repair in locus coeruleus neurons

Injections of the catecholamine neurotoxin, 6-hydroxydopamine, were placed in the ascending locus coeruleus (LC) pathway in the right cerebral cortex of rats partially destroying the noradrenergic projection to the somatosensory cortex. Norepinephrine (NE) levels fell to a nadir of 49% of control over the first 14 days, associated with a 40% increase in the number of beta-adrenoreceptor binding sites (labeled with [3H]dihydroalprenolol; [3H]DHA) in the denervated cortex. Both NE levels and cortical beta-receptor binding returned to control levels by 28 days. Similar changes, of lesser magnitude, also occurred in the unlesioned, left somatosensory cortex. Catecholamine histofluorescence studies supported these findings of denervation and reinnervation of the right cortex over a 3-month period. Anterograde axonal transport of beta-receptors was assessed by measuring the accumulation of beta-receptor binding sites ([3H]DHA) behind a second lesion placed in the more proximal portion of the ipsilateral LC pathway. Anterograde transport was completely blocked at 4 days, during the initial fall of NE levels, then was increased to 200% of control at 14-21 days, when recovery of cortical NE levels was beginning, and then returned towards control levels by 2-3 months when normal NE levels had been restored.(ABSTRACT TRUNCATED AT 250 WORDS)

Reserpine and the role of axonal transport in the independent regulation of pre- and postsynaptic beta-adrenoreceptors

The response of pre- and postsynaptic beta-adrenoreceptors to depletion of brain norepinephrine (NE) with reserpine in the rat was characterized by studying the anterograde and retrograde axonal transport of presynaptic receptors and the receptor binding changes induced in postsynaptic frontal cortex cells. Anterograde transport was shown to occur by the linear accumulation of [3H]dihydroalprenolol ([3H]DHA) binding sites (by in vitro binding assay) proximal to a 6-hydroxydopamine (6-OHDA) lesion placed in the ascending pathway of the locus coeruleus and was blocked by more proximal lesions in the pathway. Retrograde transport was demonstrated by the accumulation of [125I]iodocyanopindolol binding distal to similar lesions. Autoradiograms from sections of 6-OHDA injected brains were produced with [3H]DHA binding in the presence of the beta 2-agonist, zinterol, and suggested that the anterograde accumulation of binding sites was primarily of the beta 1-subtype. A single injection of reserpine (5 mg/kg, i.p.) produced a long lasting (6-8 weeks), biphasic decrease in cortical NE levels with nadirs and 4 and 28 days (10% and 45% of control, respectively). Frontal cortex binding of [3H]DHA increased to a maximum at 7-14 days and again at 28 days post-reserpine (230% and 167% of control, respectively). These increases were not prevented by the destruction of presynaptic noradrenergic nerve terminals with intraventricular administration of 6-OHDA 1 day prior to sacrifice and therefore appeared to take place solely in postsynaptic cells. Presynaptic, anterograde axonal transport of beta-receptors was completely blocked from 4-14 days post-reserpine, increased to 323% of control at 21 days, was blocked again at 6 weeks and returned to control by 8 weeks. Retrograde transport of beta-receptors followed a similar pattern suggesting that the presynaptic alterations in beta-receptors in noradrenergic neurons of the locus coeruleus take place independently from those in postsynaptic cortical beta-receptors as a response to NE depletion by reserpine.

Retrograde axonal transport of beta-adrenoreceptors in rat brain: effect of reserpine

Retrograde axonal transport of beta-adrenoreceptors was assessed by measuring the accumulation of binding sites for the beta-receptor ligand [125I]iodocyanopindolol [( 125I]ICP) distal to a unilateral 6-hydroxydopamine (6-OHDA) lesion placed in the ascending noradrenergic axons of the locus coeruleus. Accumulation of binding sites was linear over a 3 day period and was blocked by intracerebroventricular 6-OHDA given 1 day prior to sacrifice. A single dose of reserpine (5 mg/kg, i.p.) caused a long lasting (6-8 week) biphasic depletion of frontal cortex norepinephrine (NE) associated with increased frontal cortex binding of another beta-receptor ligand, [3H]dihydroalprenolol [( 3H]DHA), at 7-14 days, and again at 28 days post-reserpine. Unlike the changes in cortical beta-receptors, retrograde transport of [125I]ICP in presynaptic noradrenergic neurons was decreased or blocked completely at 7-14 days and at 6 weeks, and was increased to 470% and 240% of control at 21 days and 8 weeks after reserpine. Anterograde transport of [3H]DHA binding sites was measured by accumulation proximal to a 6-OHDA lesion in this pathway. This transport varied in a pattern similar to that seen for retrograde transport of [125I]ICP binding sites. These data and others suggest that presynaptic beta-receptors are regulated independently of frontal cortex beta-receptors, which appear to be located primarily on postsynaptic cells. On the other hand, the regulation of both anterograde and retrograde transport appears to be interrelated since both types of transport were altered in a similar way in the face of long-term NE depletion by reserpine.