Distribution, cellular localization, and colocalization of several peptide neurotransmitters in the central nervous system of Aplysia

Neuropeptides are widely used as neurotransmitters in vertebrates and invertebrates. In vertebrates, a detailed understanding of their functions as transmitters has been hampered by the complexity of the nervous system. The marine mollusk Aplysia, with a simpler nervous system and many large, identified neurons, presents several advantages for addressing this question and has been used to examine the roles of tens of peptides in behavior. To screen for other peptides that might also play roles in behavior, we observed immunoreactivity in individual neurons in the central nervous system of adult Aplysia with antisera raised against the Aplysia peptide FMRFamide and two mammalian peptides that are also found in Aplysia, cholecystokinin (CCK) and neuropeptide Y (NPY), as well as serotonin (5HT). In addition, we observed staining of individual neurons with antisera raised against mammalian somatostatin (SOM) and peptide histidine isoleucine (PHI). However, genomic analysis has shown that these two peptides are not expressed in the Aplysia nervous system, and we have therefore labeled the unknown peptides stained by these two antibodies as XSOM and XPHI There was an area at the anterior end of the cerebral ganglion that had staining by antisera raised against many different transmitters, suggesting that this may be a modulatory region of the nervous system. There was also staining for XSOM and, in some cases, FMRFamide in the bag cell cluster of the abdominal ganglion. In addition, these and other studies have revealed a fairly high degree of colocalization of different neuropeptides in individual neurons, suggesting that the peptides do not just act independently but can also interact in different combinations to produce complex functions. The simple nervous system of Aplysia is advantageous for further testing these ideas.

Cocaine increases quantal norepinephrine secretion through NET-dependent PKC activation in locus coeruleus neurons

The norepinephrine neurons in locus coeruleus (LC-NE neurons) are essential for sleep arousal, pain sensation, and cocaine addiction. According to previous studies, cocaine increases NE overflow (the profile of extracellular NE level in response to stimulation) by blocking the NE reuptake. NE overflow is determined by NE release via exocytosis and reuptake through NE transporter (NET). However, whether cocaine directly affects vesicular NE release has not been directly tested. By recording quantal NE release from LC-NE neurons, we report that cocaine directly increases the frequency of quantal NE release through regulation of NET and downstream protein kinase C (PKC) signaling, and this facilitation of NE release modulates the activity of LC-NE neurons and cocaine-induced stimulant behavior. Thus, these findings expand the repertoire of mechanisms underlying the effects of cocaine on NE (pro-release and anti-reuptake), demonstrate NET as a release enhancer in LC-NE neurons, and provide potential sites for treatment of cocaine addiction.

Expression of p-Akt in sensory neurons and spinal cord after peripheral nerve injury

Akt has been implicated in pro-survival and anti-apoptotic activities in many cell types, including dorsal root ganglion (DRG) and spinal motor neurons. In this immunohistochemical study we have monitored phosphorylated Akt (p-Akt) levels in adult mouse DRGs and spinal cord following unilateral peripheral sciatic nerve transection (axotomy) or carrageenan-induced inflammation. In control animals around half of the lumbar DRG neuron profiles (NPs), mainly small and medium-sized ones, were p-Akt immunoreactive (IR), and of these around 50% expressed calcitonin gene-related peptide and/or isolectin IB4. Two weeks after axotomy, the number of p-Akt-positive NPs was only slightly reduced, but p-Akt immunofluorescence intensity was strongly increased. One third of the ipsilateral p-Akt-IR NPs was galanin positive, but virtually without colocalization with neuropeptide Y. Furthermore, p-Akt-like immunoreactivity significantly increased in intensity in the ipsilateral spinal dorsal horn after axotomy and expanded into deeper layers. Carrageenan-induced peripheral inflammation increased the number of p-Akt-IR NPs after 1 h. Both axotomy and inflammation caused a clear increase in nuclear p-Akt-like immunoreactivity in DRG neurons. Our findings support a role for Akt as a key signaling molecule in sensory neurons and spinal cord after peripheral injury.

Regulation of neuropeptide Y Y1 receptors by testosterone in vascular smooth muscle cells in rat testis

It is well established that testosterone and neuropeptide Y (NPY), via its Y1 receptor (Y1R), are involved in the central control of the gonadotrope axis in male rats. Here we examined if a similar interaction also occurs in the male peripheral reproductive target organ, the testes. Expression of the Y1R transcript and protein and changes in testicular microcirculation were studied in normal rats and 12 days following hypophysectomy with and without testosterone substitution (1 or 25 mg s.c.). In situ hybridization and immunohistochemistry showed strong expression of, respectively, Y1R messenger RNA (Y1R mRNA) and Y1R-like immunoreactivity (Y1R-LI) in vascular smooth muscles in the testes of control and hypophysectomized rats treated with testosterone, but was not seen without testosterone substitution. In parallel, control animals and hypophysectomized, testosterone-supplemented rats showed a strong (approximately 40%) decrease in testicular blood flow following intratesticular (i.t.) injection of the Y1-R agonists, [Leu(31), Pro(34)]NPY, [D-Arg(25)]NPY or NPY, an effect which was completely blocked by prior intravenous administration of the Y1R antagonist, BIBP3226. No significant change in testicular blood flow following i.t. injection of NPY was seen in hypophysectomized rats without testosterone substitution. These findings suggest that the high levels of Y1R mRNA and Y1R-LI in the testes reflect expression of functional Y1Rs mediating vasoconstriction, and that testosterone regulates expression of functional Y1Rs.

Galanin expressed in the excitatory fibers attenuates synaptic strength and generalized seizures in the piriform cortex of mice

The neuropeptide galanin is considered to be an endogenous antiepileptic agent, presumably acting via inhibition of glutamate release. Previously, we have demonstrated that in mice ectopically overexpressing galanin in cortical and hippocampal neurons, particularly in granule cells and their axons, the mossy fibers, hippocampal kindling epileptogenesis is suppressed and is associated with attenuated frequency facilitation in mossy fiber-CA3 cell synapses. We hypothesized that changes in synaptic transmission might occur also in other excitatory synapses of the galanin overexpressing (GalOE) mouse, contributing to seizure suppression. Lateral olfactory tract (LOT) synapses, formed by axons of olfactory bulb (OB) mitral cells and targeting piriform cortex (PC) pyramidal cells, ectopically express galanin in GalOE mice. Using whole-cell patch-clamp recordings, we found that excitatory synaptic responses recorded in PC pyramidal cells during high frequency stimulation of the LOT were attenuated in GalOE mice as compared to wild-type controls. This effect was mimicked by bath application of galanin or its agonist galnon to wild-type slices, supporting the notion of ectopic galanin action. Since the high frequency activation induced in vitro resembles epileptic seizures in vivo, we asked whether the observed synaptic inhibition would result in altered epileptogenesis when animals were kindled via the same synapses. In male GalOE mice, we found that the latency to convulsions was prolonged, and once animals had experienced the first stage 5 seizure, generalized seizures were less sustainable. These data indicate that the PC is a possible target for epilepsy treatment by ectopically overexpressing galanin to modulate seizure activity.

Presence of delta opioid receptors on a subset of hypothalamic gonadotropin releasing hormone (GnRH) neurons

Opioid peptides exert an inhibitory effect on hypothalamic gonadotropin releasing hormone (GnRH) secretion mainly by interacting with mu-opioid receptors. Although a direct role for opioids via delta-opioid receptors (DORs) has been suggested, the presence of these receptors on GnRH neurons has never been demonstrated. In the present study, we determined the distribution of DORs in the basal hypothalamus of rat with special focus on their relation to GnRH neurons. Double-labelling immunofluorescence and confocal microscopy revealed that DORs are exclusively present in a subpopulation of GnRH nerve terminals, with the highest density in the external layer of the median eminence. We then studied the functional characteristics of DORs in an immortalized GnRH-secreting neuronal cell line (GT1-1) known to endogenously express this receptor. Here, pertussis toxin pretreatment abolished the delta-agonist (DPDPE) inhibitory effect on cAMP accumulation. We also analyzed the type of G proteins involved in the signal transduced by the DOR and showed that GT1-1 cells express the inhibitory Go and Gi2 alpha-subunits. However, only Go was down-regulated under chronic DPDPE exposure. Finally, since DOR is expressed postnatally in brain, we compared GnRH neuronal cells immortalized at different developmental stages (the more mature GT1-1 and GT1-7 cells, versus the more immature GN11 cells), evidencing that only mature neurons express DOR. In conclusion, our study indicates that a direct control of opioids via delta-receptors occurs on GnRH neurons and validates the use of GT1 cells to further investigate the nature of the DOR present on GnRH neurons.

Distinct roles of dopamine D2L and D2S receptor isoforms in the regulation of protein phosphorylation at presynaptic and postsynaptic sites

Dopamine D2 receptors are highly expressed in the dorsal striatum where they participate in the regulation of (i) tyrosine hydroxylase (TH), in nigrostriatal nerve terminals, and (ii) the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), in medium spiny neurons. Two isoforms of the D2 receptor are generated by differential splicing of the same gene and are referred to as short (D2S) and long (D2L) dopamine receptors. Here we have used wild-type mice, dopamine D2 receptor knockout mice (D2 KO mice; lacking both D2S and D2L receptors) and D2L receptor-selective knockout mice (D2L KO mice) to evaluate the involvement of each isoform in the regulation of the phosphorylation of TH and DARPP-32. Incubation of striatal slices from wild-type mice with quinpirole, a dopamine D2 receptor agonist, decreased the state of phosphorylation of TH at Ser-40 and its enzymatic activity. Both effects were abolished in D2 KO mice but were still present in D2L KO mice. In wild-type mice, quinpirole inhibits the increase in DARPP-32 phosphorylation at Thr-34 induced by SKF81297, a dopamine D1 receptor agonist. This effect is absent in D2 KO as well as D2L KO mice. The inability of quinpirole to regulate DARPP-32 phosphorylation in D2L KO mice cannot be attributed to decreased coupling of D2S receptors to G proteins, because quinpirole produces a similar stimulation of [(35)S]GTPgammaS binding in wild-type and D2L KO mice. These results demonstrate that D2S and D2L receptors participate in presynaptic and postsynaptic dopaminergic transmission, respectively.

Effects of peripheral axotomy on neuropeptides and nitric oxide synthase in dorsal root ganglia and spinal cord of the guinea pig: an immunohistochemical study

The effect of axotomy (3, 10 and 21 days) on the expression of some neuronal markers was analysed in dorsal root ganglia and spinal cord of guinea-pigs using immunohistochemistry. Three weeks following injury, substance P-like immunoreactivity (-LI) was slightly reduced in the DRGs of the ipsilateral side, whereas a marked increase in neuropeptide Y(NPY)-LI could be detected ipsilaterally and a smaller increase contralaterally. NPY-LI was mainly expressed in small, but also some medium-sized and large neuron profiles after axotomy. Galanin-LI showed a moderate bilateral increase. No significant changes could be observed in DRGs for calcitonin gene-related peptide (CGRP)-, vasoactive intestinal polypeptide-, peptide histidine isoleucine- or nitric oxide synthase-LIs. In the ventral horn CGRP-LI was slightly increased bilaterally in motoneurons, most pronounced on the injured side. Autotomy behaviour was seen in seven of the nine animals in the twenty-one day group. The present results demonstrate that also in guinea-pigs several peptides undergo distinct changes in their expression after peripheral nerve injury. However, in contrast to rats and monkeys, galanin-LI is only moderately increased in guinea-pigs. Neuropeptide Y showed a dramatic increase mainly in small neurons, in contrast to the upregulation in large neurons in the rat. Thus, distinct species differences exist with regard to the cellular response to nerve injury.

Antibodies to a segment of tyrosine hydroxylase phosphorylated at serine 40

A synthetic peptide corresponding to residues 32-47 of rat tyrosine hydroxylase (TH) was phosphorylated by protein kinase A at Ser40 and used to generate antibodies in rabbits. Reactivity of the anti-pTH32-47 antibodies with phospho- and dephospho-Ser40 forms of TH protein and peptide TH32-47 was compared with reactivity of antibodies to nonphosphorylated peptide and to native TH protein. In antibody-capture ELISAs, anti-pTH32-47 was more reactive with the phospho-TH than with the dephospho-TH forms. Conversely, antibodies against the nonphosphorylated peptide reacted preferentially with the dephospho-TH forms. In western blots, labeling of the approximately 60-kDa TH band by anti-pTH32-47 was readily detectable in lanes containing protein kinase A-phosphorylated native TH at 10-100 ng/lane. In blots of supernatants prepared from striatal synaptosomes, addition of a phosphatase inhibitor was necessary to discern labeling of the TH band with anti-pTH32-47. Similarly, anti-pTH32-47 failed to immunoprecipitate TH activity from supernatants prepared from untreated tissues, whereas prior treatment with either 8-bromoadenosine 3',5'-cyclic monophosphate or forskolin enabled removal of TH activity by anti-pTH32-47. Lastly, in immunohistochemical studies, anti-pTH32-47 selectively labeled catecholaminergic cells in tissue sections from perfusion-fixed rat brain.

Chemical Messengers and Their Coexistence in Individual Neurons

The original concept of chemical neural transmission was based on the assumed principle that individual neurons utilize only one type of neurotransmitter. There is now histological evidence that single nerve cells in both the central and the peripheral nervous system often contain multiple chemical messengers. This article describes the evidence for and the functional significance of multiple messengers in synaptic transmission.

Immunocytochemical studies of somatostatin neurons in brain

Immunohistochemical studies with antisera to somatostatin have, in many instances, led the way to our present understanding of the peptidergic nervous system. Somatostatin was among the first of the hypophysiotropic hormones shown to be contained in diverse neuronal circuits outside of the hypothalamus. For example, somatostatin is found within neurons ranging in location from the cerebral cortex to primary sensory neurons to enteric neurons within the gut wall. Somatostatin was also the first neuropeptide demonstrated to coexist within vertebrate neurons that also produce a classical neurotransmitter. Since this initial demonstration in sympathetic ganglionic neurons, somatostatin and numerous other neuropeptides have been demonstrated to coexist with a variety of classical neurotransmitters. The "rules" for coexistence are not clear, since somatostatin coexist in some instances with norepinephrine, in other cases with GABA, and probably with other classical transmitters as well. In some neurons, somatostatin also coexists with certain other neuropeptides. Finally, the specificity of immunohistochemical localizations of somatostatin has now been confirmed by virtue of the co-staining of somatostatin neurons with antisera to other portions of the biosynthetic precursor to somatostatin.

Enhancement of sexual behavior in the female rat by nicotine

The effects of nicotine alone or in combination with mecamylamine, and the effects of both in combination with DA agonists and antagonists and d-LSD (5-HT agonist) have been studied on the sexual behaviour of castrate, estrogen-treated female rats. The results show that nicotine (as low as 50 mug/kg) significantly increases sexual receptivity. The pharmacological analysis suggests this effect to be mediated by a central, nicotine-like cholinergic receptor whose relationship to DA and 5-HT pathways known to exert inhibitory influences on receptivity in the female rat is also discussed.