Coexistence of neuronal messengers--an overview

Desensitization of the nicotinic acetylcholine receptor: molecular mechanisms and effect of modulators

1. Loss of response after prolonged or repeated application of stimulus is generally termed desensitization. A wide variety of phenomena occurring in living organisms falls under this general definition of desensitization. There are two main types of desensitization processes: specific and non-specific. 2. Desensitization of the nicotinic acetylcholine receptor is triggered by prolonged or repeated exposure to agonists and results in inactivation of its ion channel. It is a case of specific desensitization and is an intrinsic molecular property of the receptor. 3. Desensitization of the nicotinic acetylcholine receptor at the neuromuscular junction was first reported by Katz and Thesleff in 1957. Desensitization of the receptor has been demonstrated by rapid kinetic techniques and also by the characteristic "burst kinetics" obtained from single-channel recordings of receptor activity in native as well as in reconstituted membranes. In spite of a number of studies, the detailed molecular mechanism of the nicotinic acetylcholine receptor desensitization is not known with certainty. The progress of desensitization is accompanied by an increase in affinity of the receptor for its agonist. This change in affinity is attributed to a conformational change of the receptor, as detected by spectroscopic and kinetic studies. A four-state general model is consistent with the major experimental observations. 4. Desensitization of the nicotinic acetylcholine receptor can be potentially modulated by exogenous and endogenous substances and by covalent modifications of the receptor structure. Modulators include the noncompetitive blockers, calcium, the thymic hormone peptides (thymopoietin and thymopentin), substance P, the calcitonin gene-related peptide, and receptor phosphorylation. Phosphorylation is an important posttranslational covalent modification that is correlated with the regulation and desensitization of the receptor through various protein kinases. 5. Although the physiological significance of desensitization of the nicotinic receptor is not yet fully understood, desensitization of receptors probably plays a significant role in the operation of the neuronal networks associated in memory and learning processes. Desensitization of the nicotinic receptor could also possibly be related to the neuromuscular disease, myasthenia gravis.

Immunoreactive Calcitonin Gene-Related Peptide, Vasoactive Intestinal Polypeptide, and Somatostatin in Developing Chicken Spinal Cord Motoneurons

The distribution of calcitonin gene-related peptide (CGRP)-, vasoactive intestinal polypeptide (VIP)-, and somatostatin (SOM)-like immunoreactivities (-LI) in neurons of the spinal cord of developing chickens was characterized by use of the indirect immunofluorescence technique, and the findings related to a possible role for these peptides in the development of muscles and motor endplates. CGRP-LI in presumptive motoneurons of the ventral horn was first observed at embryonic day 6. During the following days the number of CGRP-immunoreactive (IR) cells increased reaching high numbers between days 12 and 18 of incubation, and thereafter decreasing in numbers until hatching. SOM-LI was first observed on embryonic day 7, while VIP-LI appeared on days 12 - 13. The number of SOM- and VIP-IR cells was considerably lower than that observed for CGRP-LI, but they also exhibited an initial increase followed by a decrease towards hatching. Intrathecal administration of colchicine increased the number of CGRP-IR motoneurons at days 7 and 30 after hatching and of VIP-IR ones at day 7, while at day 30 no expression of VIP-LI was found. Colchicine treatment did not cause any significant change in the number of SOM-IR motoneurons after hatching. The effect of VIP, SOM, and CGRP on cAMP accumulation in primary cultures of chick muscle cells was determined after labelling of the cells by (2-3H) adenine and by radioimmunoassay. All three peptides stimulated the accumulation of cAMP. However, the development of the pharmacological response of each of the peptides followed a different time course during in vitro differentiation of the primary cultures. The response of CGRP was the latest to develop and did not significantly decrease after the maximal response had been reached around day 3. The data are discussed in terms of 'trophic' effects of these neuropeptides upon muscle and spinal cord differentiation and synaptogenesis.

Cranial motor neurons contain either galanin- or calcitonin gene-related peptidelike immunoreactivity

The demonstration of coexistence of a peptide or peptides in neurons that produce a small molecule neurotransmitter has become increasingly frequent. The calcitonin gene-related peptide (CGRP) is known to be colocalized in the cholinergic neurons of both cranial and spinal motor nuclei. The present study demonstrates that all somatic motor cranial nerve nuclei contain CGRP- and galaninlike immunoreactivity. The perikaryal content of both peptides is increased by colchicine pretreatment and by transecting axons arising from the perikarya, and both peptides are found in nerve fibers innervating striated musculature. CGRP- and galaninlike immunoreactivity appear to be present in different populations of neurons. In contrast to CGRP, galaninlike immunoreactivity was not detected in spinal motor neurons. These observations suggest that galanin and CGRP participate in the process of synaptic transmission at the neuromuscular junction of cranial motor neurons.

Neurotransmitter phenotype plasticity in cultured dissociated adult rat dorsal root ganglia: an immunocytochemical study

Culturing sympathetic ganglion neurons in vitro may modify phenotypic expression of some neurotransmitters. For dorsal root ganglia (DRG), contradictory results have been reported; most studies have used immature material. We have therefore performed a detailed immunocytochemical analysis of the transmitter content of cultured adult rat DRG neurons. To demonstrate possible modifications of neurotransmitter phenotypes, we have compared the results obtained with the same techniques on neurons cultured for 3 days and on freshly dissociated DRG cells. Also, the transmitter profile of cultured neurons was compared with that known from in situ studies. Out of 22 antigens studied, 20 were detected in cultured DRG neurons. All of them were expressed in small and/or intermediate-sized cells. Large neurons only contained CGRP, VIP, NPY, beta-END, ENK, and GABA. The percentage of immunostained neurons varied for the various antisera: less than 10% of cultured neurons were positive for ENK, beta-LPH, beta-END, DYN, VASO, and OXY; 10-30% for SOM, CCK, CAT, and SP; and greater than 30% for NPY, CRF, GLU, NT, VIP, GABA, GRP, CGRP, 5-HT, and TRH. In the latter two groups of transmitters (except CGRP), the proportion of immunoreactive neurons was by far larger in cultured than in freshly dissociated DRG. The most pronounced (greater than 25%) increase in the proportion of positively stained neurons after culturing was observed for the GRP, CRF, TRH, and 5-HT antisera. Serotonin was the only transmitter identified in cultured but not in freshly dissociated cells. These data indicate, on one hand, that various antigens, for example, CAT, GABA, NT, TRH, NPY, beta-LPH, and beta-END, which up to now have not been described in DRG in situ, can be detected immunocytochemically a few hours after dissociation of adult rat DRG. On the other hand, several transmitters, for example, VIP, NPY, SP, GABA, GLU, NT, GRP, CRF, TRH, and 5-HT, are expressed in a significantly higher proportion of cells in cultured than in freshly dissociated preparations. This might reflect a change in the phenotypic expression of transmitters due to the new environment generated by the culture conditions, a hypothesis that can be tested by measuring specific mRNA levels. Moreover, considering the plasticity and multipotentiality of their transmitter phenotype, cultured adult DRG neurons might represent an interesting material for autografts into the injured central nervous system.

Activity-dependent regulation of gene expression in muscle and neuronal cells

In both the central and the peripheral nervous systems, impulse activity regulates the expression of a vast number of genes that code for synaptic proteins, including neuropeptides, enzymes involved in neurotransmitter biosynthesis and degradation, and membrane receptors. In recent years, the mechanisms involved in these regulations became amenable to investigation by the methods of recombinant DNA technology. The first part of this review focuses on the activity-dependent control of nicotinic acetylcholine receptor biosynthesis in vertebrate muscle, a model case for the regulation of synaptic protein biosynthesis at the postsynaptic level. The second part summarizes some examples of neuronal proteins whose biosynthesis is under the control of transsynaptic impulse activity. The first, second, and third intracellular messengers involved in membrane-to-gene signaling are discussed, as are possible posttranscriptional control mechanisms. Finally, models are proposed for a role of neuronal activity in the genesis and stabilization of the synapse.

The acetylcholine receptor: its molecular biology and biotechnological prospects

The structure and behaviour of the acetylcholine receptor (AChR) is described, and the evidence that it is an allosteric protein is discussed. The genes for the AChR subunits are subject to a complex set of spatio-temporal transcriptional controls during development of the motor endplate, and these findings are reviewed here. Finally, the biotechnological prospects suggested by the new data are noted.

Pituitary gland: neuropeptides, neurotransmitters and growth factors

The hypothalamus receives neuronal afferents from numerous sources including inputs from limbic structures, such as the amygdala and hippocampus, and from brainstem regions involved in the regulation of the cardiovascular system and other autonomic functions. These afferents using a vast array of neurotransmitters and neuropeptides influence the activity of the hypothalamic neurons which synthesize and secrete the hypothalamic releasing and release-inhibiting factors into the hypophyseal portal circulatory system. The afferents can modulate the activity of the hypothalamic neurons by forming synapses on the neuronal cell body, on the nerve terminals in the median eminence or both. The chemicals most frequently used as neurotransmitters are the biogenic amines, including the catecholamines (norepinephrine, dopamine and epinephrine), serotonin, acetylcholine and gamma-aminobutyric acid (GABA). The stimulatory influence of norepinephrine, serotonin, and acetylcholine on the secretion of corticotropin (ACTH) in rodents and man will be discussed, whereas GABA exerts an inhibitory effect on the secretion of ACTH in both man and rodents. These effects appear to be mediated by changes in the secretion of the corticotropin-releasing hormone (CRH) and vasopressin into the hypophyseal portal circulation. Numerous neuropeptides appear to alter the secretion of ACTH in the rat. We will discuss the stimulatory actions of neuropeptide Y (NPY), angiotensin II, and peptides of immune cell origin on the secretion of ACTH and CRH. The opioid peptides inhibit the secretion of CRH into the portal blood, however, they exert a potent stimulatory effect on prolactin secretion in the rat and man.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventral mesencephalic neurons containing both cholecystokinin- and tyrosine hydroxylase-like immunoreactivities project to forebrain regions

The coexistence of cholecystokinin- and tyrosine hydroxylase-like immunoreactivities within neurons of the rat ventral mesencephalon was analyzed by using an indirect immunofluorescence technique for the simultaneous demonstration of two antigens in the same tissue section. A high degree of colocalization was observed in the substantia nigra pars compacta, in which 80-90% of all labeled neurons at rostral and up to 70% at intermediate levels contained both cholecystokinin and tyrosine hydroxylase. At caudal levels, the incidence of colocalization declined to approximately 30-50%. All of the immunoreactive perikarya in the substantia nigra pars lateralis were labeled with both substances. Other areas of the ventral midbrain that exhibited a moderate proportion of neurons immunoreactive for both cholecystokinin and tyrosine hydroxylase included the ventral tegmental area, interfascicular nucleus, and rostral and caudal linear nuclei. In addition, coexistence was occasionally observed within neurons of the central and ventral periaqueductal gray matter, supramammillary region, peripeduncular region, retrorubral field, and extremely rarely, within the substantia nigra pars reticulata. Cell bodies containing tyrosine hydroxylase-like immunoreactivity (indicative of dopamine) usually outnumbered those containing the peptide except in the supramammillary region and in the ventral periaqueductal gray matter, where the cholecystokinin perikarya were present in higher numbers. The double-labeling colocalization technique was combined with fluorescence retrograde tracing to determine some of the forebrain projections of these neurons. Ventral midbrain neurons containing both cholecystokinin and tyrosine hydroxylase were found to project to the caudate-putamen, nucleus-accumbens, prefrontal cortex, and amygdala. These projections originated from neurons located predominantly in the substantia nigra pars compacta and the ventral tegmental area. Thus, cholecystokinin occurs within the well-known dopaminergic nigrostriatal pathway in the rat. Overall, these results demonstrate that a significant proportion of the dopamine neurons giving rise to the ascending mesotelencephalic projections also contain the peptide cholecystokinin.

Antidepressant action of imipramine and iprindole in mice is enhanced by inhibitors of enkephalin-degrading peptidases

The implication that opioid peptides are involved in the action of the antidepressants imipramine and iprindole was investigated in mice by using the forced swimming test as an experimental model of depression. Both the drugs were found to shorten the immobility time in this test. This effect of imipramine and iprindole was reversed by the opiate antagonist naloxone. Moreover, when subeffective doses of either imipramine or iprindole were given together with an intracerebroventricular injection of an inhibitor of their degradation (thiorphan or bestatin), the immobility time was again decreased. Interestingly, the reduction of the time of immobility was found to be not related to the effect of the drugs on locomotor activity. These data might be taken as further evidence for the involvement of opioid peptides in the pharmacological action of antidepressant drugs.

Acetylcholine receptor expression in primary cultures of embryonic chick myotubes--I. Discoordinate regulation of alpha-, gamma- and delta-subunit gene expression by calcitonin gene-related peptide and by muscle electrical activity

The neuropeptide calcitonin gene-related peptide calcitonin gene-related peptide, and muscle electrical activity regulate in opposite directions the content of nicotinic acetylcholine receptor alpha-subunit mRNA in primary cultures of chick embryonic myotubes. Indeed, treating the cells with calcitonin gene-related peptide or blocking the spontaneous activity of muscle cells by tetrodotoxin (an inhibitor of sodium channels) increases, although to different levels, the content of acetylcholine receptor alpha-subunit mRNA [Fontaine B., Klarsfeld A. and Changeux J. P. (1987) J. Cell Biol. 105, 1337-1342; Klarsfeld A. and Changeux J. P. (1985) Proc. natn. Acad. Sci. U.S.A. 82, 4558-4562]. In this paper, we demonstrate that, under these in vitro culture conditions, calcitonin gene-related peptide (0.1 microM) and tetrodotoxin (0.5 microM) regulate to a smaller extent (no more than 2.5-fold above control) the levels of acetylcholine receptor gamma- and delta-subunit mRNAs. No effect of either compound on acetylcholine receptor biosynthesis was observed during the initial three days of culture. The response to calcitonin gene-related peptide was already maximal when the cells were treated between days three and four after plating (about 3-fold increase of the alpha-subunit mRNA level). The effect of tetrodotoxin resulted in a six-fold increase of the acetylcholine receptor alpha-subunit mRNA level in cells treated between days three and four, and still increased when the cells were exposed to tetrodotoxin through days six and eight (up to a maximum of 20-fold).(ABSTRACT TRUNCATED AT 250 WORDS)

Development of immunoreactivity to the invertebrate neuropeptide small cardiac peptide B in the tapeworm Diphyllobothrium dendriticum

Neurons immunoreactive to small cardiac peptide B (SCPb) occur in the scolex and neck region of adult Diphyllobothrium dendriticum. The localisation of the SCPb-IR neurons in the peripheral nervous system is very pronounced; they are closely associated to the bothridial musculature in the scolex. SCPb-IR neurons were not observed in plerocercoid larvae but appeared after cultivation in vitro at 37 degrees C for 30 h. Functional and developmental aspects of the SCPb-IR neurons are discussed.

Ontogeny of substance P-containing neurons in relation to serotonin-containing neurons in the central nervous system of the mouse

In order to begin to investigate the developmental factors influencing co-localized neurotransmitters in the central nervous system, we have studied the ontogenesis of coexisting transmitters, substance P and serotonin, in cells of the medullary raphe of the fetal mouse using double-label immunocytochemistry. The results indicate that initial detection of these neurotransmitters occurs at embryonic day 12 in non-overlapping cell populations. Substance P-like immunoreactivity is first co-localized with serotonin in cells of the caudal medullary raphe (raphe pallidus and raphe obscurus) at embryonic day 13. The percentage of serotonin cells containing substance P was estimated at embryonic day 13 and subsequent gestational ages both by double-label immunofluorescence using fluorescein- and rhodamine-conjugated secondary antibodies and also by the elution technique. Double-label immunofluorescence routinely and reliably yielded the highest proportion of co-localized cells. At embryonic day 13, 89% of serotonin cells contained substance P-like immunoreactivity. A consistently high degree of co-localization occurred throughout embryonic development (87% at postnatal day 3) and into adulthood (87%). Colchicine treatment was required at older ages to elicit these data. Unexpectedly, we found that neurons containing substance P-like immunoreactivity in the piriform cortex and in the hypothalamus transiently expressed serotonin immunoreactivity during normal ontogenesis.

Coexistence of catecholamine and methionine enkephalin-Arg6-Gly7-Leu8 in neurons of the rat ventrolateral medulla oblongata. Application of combined peptide immunocytochemistry and histofluorescence method in the same vibratome section

An overlapping distribution of catecholamine-containing cells and proenkephaline-A derived peptide-containing neurons have been identified in the rat medulla oblongata. However, it is not evident whether the coexistence of these bioactive substances occurs in the same neurons or not. Therefore, we examined the coexistence of catecholamine and methionine-enkephalin-Arg6-Gly7-Leu8 (MEAGL), a proenkephaline-A derived peptide, using a combination of histofluorescence and peroxidase-anti-peroxidase (PAP) immunohistochemical (modified formaldehyde-glutalaldehyde (Faglu)) methods on the same tissue sections. We found one third of A1/C1 catecholamine fluorescent cells show MEAGL-like immunoreactivity.

Transient coappearance of glucagon and insulin in the progenitor cells of the rat pancreatic islets

Ontogenetic appearances of glucagon, insulin and tyrosine hydroxylase (TH) were immunohistochemically investigated on developing pancreatic islets of rats. Glucagon immunoreactivity appeared first in some epithelial cells (g-cells) of the dorsal anlage of the pancreas on day 11.5 of gestation. On day 12.5, g-cells increased in number manufacturing the primitive islets, in which some cells appeared to be immunoreactive for insulin (i-cells) and about 40% of g-cells indicated also a slight immunoreactivity for insulin (g/i-cells). Afterwards, all the islet cells, especially g-cells, increased in number, and almost half of g-cells were g/i-cells. After day 16.5 of gestation, numerical increase of the cells with insulin immunoreactivity exceeded that of the cells with glucagon immunoreactivity, and about one fifth of g-cells were g/i-cells. After 20.5 days, however, no g/i cells were found. On day 16.5 of gestation, the immunoreactivity for TH appeared in occasional cells of the islets, but the cells did not show immunoreactivity for glucagon or insulin. It is concluded that the progenitor cells of the pancreatic islets appear to synthesize both glucagon and insulin by day 20.5 of gestation, but differentiate giving rise to mature A and B cells of adult islets afterward.

Calcitonin gene-related peptide enhances the rate of desensitization of the nicotinic acetylcholine receptor in cultured mouse muscle cells

Calcitonin gene-related peptide (CGRP) is a neuropeptide that coexists with acetylcholine in spinal cord motoneurons. The effects of CGRP on the functional properties of the nicotinic acetylcholine receptor (AcChoR) were examined by electrophysiological methods. Using the whole-cell patch-clamp technique and a mouse cell line derived from soleus muscle, we found that CGRP produces a progressive and reversible enhancement of the rapid-decay phase of AcChoR desensitization. Single-channel data further show that CGRP decreases acetylcholine-activated channel opening frequency. This decrease occurs when CGRP and acetylcholine are applied on different cell-surface areas and thus is likely mediated by a second-messenger system. CGRP is also shown to increase cAMP accumulation in this cell line. The effects of CGRP on macroscopic acetylcholine-activated currents are mimicked by external application of forskolin (10 microM) or by internal perfusion of the cell with cAMP (1 microM). In both these cases, further application of CGRP produces no additional enhancement of AcChoR desensitization. These results suggest that, on mouse muscle cells, CGRP regulates AcChoR desensitization by a mechanism that involves, at least in part, cAMP-dependent phosphorylation of the AcChoR.

Existence and coexistence of calcitonin gene-related peptide, vasoactive intestinal polypeptide- and somatostatin-like immunoreactivities in spinal cord motoneurons of developing embryos and post-hatch chicks

By use of immunocytochemical methods, it is shown that immunoreactive calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP) and somatostatin (SOM) are present in motoneurons in the chicken spinal cord. While CGRP-like immunoreactivity (LI) is present in numerous motoneurons both before and after hatching, SOM- and VIP-LI markedly decline at the end of the embryonic period. Evidence is also provided for coexistence of some of these peptides in certain spinal motoneurons.

Platelets as a model for neurones?

The multiple biochemical and pharmacological similarities existing between blood platelets and 5-hydroxytryptamine (5-HT)-containing neurones of the CNS point to the platelets as a reliable model for the biochemical characterization of 5-HT releasers and uptake blockers which interfere with the storage and the active carrier mechanism of 5-HT in the neurones, respectively. In addition, the affinity displayed by dopamine and by dopaminergic neurotoxin MPP+ for the platelet 5-HT transport and storage indicates also some similarities between platelets and the dopaminergic system of the CNS. Since human platelets contain almost exclusively monoamine oxidase type B (MAO-B), they can be used as a source for the purification and characterization of this human enzyme. Human platelets thus offer an excellent peripheral model to indirectly assess the degree and duration of MAO-B inhibition occurring in the CNS. To date, knowledge of the many biochemical mechanisms underlying platelet physiology is still fragmentary. In fact, the functional role of binding sites located on the platelet cytoplasmic membrane, i.e. their coupling to a specific transmembrane signalling mechanism, is still in need of a precise biochemical and physiological characterization.

Possible coexistence of amino acid (gamma-aminobutyric acid), amine (dopamine) and peptide (substance P); neurons containing immunoreactivities for glutamic acid decarboxylase, tyrosine hydroxylase and substance P in the hamster main olfactory bulb

The coexistence of immunoreactivities for glutamic acid decarboxylase (GAD), tyrosine hydroxylase (TH) and substance P (SP) was revealed in the hamster main olfactory bulb, using the peroxidase-antiperoxidase immunohistochemical method. Adjacent 40 micron thick Vibratome sections were incubated in different antisera and those cells which were bisected by the plane of sectioning were identified at the paired surfaces of two consecutive sections. The coexistence of the immunoreactivities for 1) TH and GAD, 2) TH and SP and 3) GAD and SP in the same cells could thus be determined by observing the immunoreactivity of the two halves of the cell incubated in two different antisera. About 70% of TH-like immunoreactive (TH-LI) neurons in the periglomerular region also contained GAD-like immunoreactivity, whereas about 45% of GAD-LI ones were also TH-like immunoreactive. Furthermore, almost all (more than 95%) of SP-LI neurons contained both GAD-like and TH-like immunoreactivities. These observations indicate that in the periglomerular region of the hamster main olfactory bulb, some neurons (about 9% of all neurons containing TH-like and/or GAD-like immunoreactivities) may contain three different categories of neuroactive substances, that is, amino acid (GABA), amine (dopamine) and peptide (SP).

Calcitonin gene-related peptide and muscle activity regulate acetylcholine receptor alpha-subunit mRNA levels by distinct intracellular pathways

In cultured chicken myotubes, calcitonin gene-related peptide (CGRP), a peptide present in spinal cord motoneurons, increased by 1.5-fold the number of surface acetylcholine receptors (AChRs) and by threefold AChR alpha-subunit mRNA level without affecting the level of muscular alpha-actin mRNA. Cholera toxin (CT), an activator of adenylate cyclase, produced a similar effect, which did not add up with that of CGRP. In contrast, tetrodotoxin, a blocker of voltage-sensitive Na+ channels, elevated the level of AChR alpha-subunit mRNA on top of the increase caused by either CGRP or CT. 12-O-Tetradecanoyl phorbol-13-acetate (TPA), an activator of protein kinase C, markedly decreased the cell surface and total content of [125I]alpha BGT-binding sites and reduced the rate of appearance of AChR at the surface of the myotubes without reducing the level of AChR alpha-subunit mRNA. Moreover, TPA inhibited the increase of AChR alpha-subunit mRNA caused by tetrodotoxin without affecting that produced by CGRP or CT. Under the same conditions, TPA decreased the level of muscular alpha-actin mRNA and increased that of nonmuscular beta- and gamma-actins mRNA. These data suggest that distinct second messengers are involved in the regulation of AChR biosynthesis by CGRP and muscle activity and that these two pathways may contribute to the development of different patterns of AChR gene expression in junctional and extrajunctional areas of the muscle fiber.

D-1 and D-2 agonists in Parkinson's disease

We have evaluated 5 DA agonists-bromocriptine, lergotrile, lisuride, pergolide, and mesulergine in studies encompassing 278 patients with advanced PD. In most of our patients the DA agonist was added to levodopa. Most of our patients were no longer satisfactorily responding to levodopa. Previous attempts at managing these patients by changing the dose of levodopa (increasing or decreasing it), the treatment schedule, or the ratio of levodopa to carbidopa or by temporarily discontinuing levodopa [drug holiday] were unsuccessful. The majority of our patients had diurnal fluctuations in performance, either "wearing off" or "on-off" phenomena. The addition of a DA agonist resulted in a decrease in parkinsonian disability in most patients and a decrease in the severity of the diurnal fluctuations in performance. Improvement in many patients was maintained for at least 2 years. Adverse effects included mental changes, dyskinesias, orthostatic hypotension, and nausea. All of the adverse effects were reversible when the agonist was decreased or discontinued. As a group the agonists behaved similarly but individual patients often responded better to one agonist than another. The main role of agonists is in combination with levodopa in the treatment of patients with early PD who have not yet developed dyskinesias or diurnal fluctuations in performance.

Coexistence of peptides with classical neurotransmitters

In the present article the fact is emphasized that neuropeptides often are located in the same neurons as classical transmitters such as acetylcholine, 5-hydroxy-tryptamine, catecholamines, gamma-aminobutyric acid (GABA) etc. This raises the possibility that neurons produce, store and release more than one messenger molecule. The exact functional role of such coexisting peptides is often difficult to evaluate, especially in the central nervous system. In the periphery some studies indicate apparently meaningful interactions of different types with the classical transmitter, but other types of actions including trophic effects have been observed. More recently it has been shown that some neurons contain more than one classical transmitter, e.g. 5-HT plus GABA, further underlining the view that transfer of information across synapses may be more complex than perhaps hitherto assumed.

Biochemistry of information storage in the nervous system

The use of molecular biological approaches has defined new mechanisms that store information in the mammalian nervous system. Environmental stimuli alter steady-state levels of messenger RNA species encoding neurotransmitters, thereby altering synaptic, neuronal, and network function over time. External or internal stimuli alter impulse activity, which alters membrane depolarization and selectively changes the expression of specific transmitter genes. These processes occur in diverse peripheral and central neurons, suggesting that information storage is widespread in the neuraxis. The temporal profile of any particular molecular mnemonic process is determined by specific kinetics of turnover and by the geometry of the neuron resulting in axonal transport of molecules to different synaptic arrays at different times. Generally, transmitters, the agents of millisecond-to-millisecond communication, are subject to relatively long-lasting changes in expression, ensuring that ongoing physiological function is translated into information storage.

Immunohistochemical evidence for colocalization of gamma-aminobutyric acid and serotonin in neurons of the ventral medulla oblongata projecting to the spinal cord

Fluorescence immunohistochemistry was used to analyze the medulla oblongata of colchicine-treated rats that had been incubated with guinea pig antibodies to serotonin (5-HT) and either rabbit or sheep antibodies to glutamic acid decarboxylase (GAD). Numerous cells in the rostral ventrolateral medulla in the region of nucleus raphe magnus were immunostained for either 5-HT or GAD. A substantial number of neurons showed positive immunoreactivity for both substances, and were most frequently observed in the lateral aspect of nucleus raphe magnus. In addition, a number of the 5-HT/GAD-containing neurons were retrogradely labelled with Fast blue dye that had been injected into the thoracic spinal cord. This work provides evidence for colocalization of the classical neurotransmitters 5-HT and GABA in single cells of the ventral medulla oblongata, some of which project to the spinal cord.