Transient expression of opioid receptors in defined regions of developing brain: are embryonic receptors selective?

Expression of kappa opioid receptors in developing rat brain - Implications for perinatal buprenorphine exposure

Buprenorphine, a mu opioid receptor partial agonist and kappa opioid receptor (KOR) antagonist, is an emerging therapeutic agent for maternal opioid dependence in pregnancy and neonatal abstinence syndrome. However, the endogenous opioid system plays a critical role in modulating neurodevelopment and perinatal buprenorphine exposure may detrimentally influence this. To identify aspects of neurodevelopment vulnerable to perinatal buprenorphine exposure, we defined KOR protein expression and its cellular associations in normal rat brain from embryonic day 16 to postnatal day 23 with double-labelling immunohistochemistry. KOR was expressed on neural stem and progenitor cells (NSPCs), choroid plexus epithelium, subpopulations of cortical neurones and oligodendrocytes, and NSPCs and subpopulations of neurones in postnatal hippocampus. These distinct patterns of KOR expression suggest several pathways vulnerable to perinatal buprenorphine exposure, including proliferation, neurogenesis and neurotransmission. We thus suggest the cautious use of buprenorphine in both mothers and infants until its impact on neurodevelopment is better defined.

Afferent islands are larger than mu-opioid receptor patch in striatum of rat pups

Dopamine afferent islands were observed in rodent caudate-putamen only during development, whereas patches with intense mu-opioid receptor (MOR) immunoreactivity were seen throughout the life. We performed direct comparison between MOR patches and dopamine islands in the caudate-putamen of rat pups, by double immunofluorescence labeling for MOR and tyrosine hydroxylase. MOR patches were included in dopamine islands at postnatal day (P) 0 to P8, although the patches occupied the same region as the islands at P12-16. Furthermore, the regions of glutamatergic afferents with intense vesicular glutamate transporter 1 and vesicular glutamate transporter 2 immunoreactivities well corresponded to those of dopamine islands at P4. These results suggest that the striatal 'afferent islands' are larger than MOR patches in the early postnatal life.

The effects of maternally administered methadone, buprenorphine and naltrexone on offspring: review of human and animal data

Most women using heroin are of reproductive age with major risks for their infants. We review clinical and experimental data on fetal, neonatal and postnatal complications associated with methadone, the current "gold standard", and compare these with more recent, but limited, data on developmental effects of buprenorphine, and naltrexone. Methadone is a micro-opioid receptor agonist and is commonly recommended for treatment of opioid dependence during pregnancy. However, it has undesired outcomes including neonatal abstinence syndrome (NAS). Animal studies also indicate detrimental effects on growth, behaviour, neuroanatomy and biochemistry, and increased perinatal mortality. Buprenorphine is a partial micro-opioid receptor agonist and a kappa-opioid receptor antagonist. Clinical observations suggest that buprenorphine during pregnancy is similar to methadone on developmental measures but is potentially superior in reducing the incidence and prognosis of NAS. However, small animal studies demonstrate that low doses of buprenorphine during pregnancy and lactation lead to changes in offspring behaviour, neuroanatomy and biochemistry. Naltrexone is a non-selective opioid receptor antagonist. Although data are limited, humans treated with oral or sustained-release implantable naltrexone suggest outcomes potentially superior to those with methadone or buprenorphine. However, animal studies using oral or injectable naltrexone have shown developmental changes following exposure during pregnancy and lactation, raising concerns about its use in humans. Animal studies using chronic exposure, equivalent to clinical depot formulations, are required to evaluate safety. While each treatment is likely to have maternal advantages and disadvantages, studies are urgently required to determine which is optimal for offspring in the short and long term.

Ultrastructural localization of ?-opioid receptors in the rat caudate-putamen nucleus during postnatal development: Relation to synaptogenesis

During development, delta-opioid receptors (DORs) in the rat caudate-putamen nucleus (CPN) appear later than mu-opioid receptors (MORs), whose developmental pattern specifically relates to synaptogenesis. We used electron microscopic immunocytochemistry to determine whether there are also age-related changes in subcellular localization of DORs in the rat CPN. Sections from postnatal day (P) 0-P30 and adult dorsomedial CPN were immunogold-silver labeled to examine the plasmalemmal and cytoplasmic distribution of these receptors. In addition, immunoperoxidase labeling was used to determine the numerical density of synapses relative to DOR-labeled profiles. Immunolabeling for DOR was undetectable at P0, light at P5, and dense from P10 onward. The labeling during P5-P10 was mainly localized in somatodendritic profiles but also was readily seen in axon terminals, most of which formed asymmetric synapses with dendrites. From P15, a few immunogold particles were seen in contact with postsynaptic densities in spines, and the proportion of these particles significantly increased in P30 and adult CPN. Other particles were localized in the cytoplasm of dendrites and terminals without significant age-related changes. Stereological analysis showed that compared with labeled dendritic shafts and spines, labeled axon terminals have a closer correlation with synapse formation. These results are in marked contrast with MORs, which show an age-related increase in association with dendritic plasma membrane and a good correlation in the developmental pattern of MOR-labeled spines with synapse formation (Wang et al. [2003] Neuroscience 118:695-708). Together, our results suggest receptor-type specific roles for endogenous opioids acting at both pre- and postsynaptic sides in the developing CPN.

Postnatal development of mu-opioid receptors in the rat caudate-putamen nucleus parallels asymmetric synapse formation

The mu-opioid receptor (MOR) in the caudate-putamen nucleus (CPN) appears early during prenatal development, and shows a patch-like distribution throughout the postnatal period and adulthood. In the adult rat CPN, neurons in patch compartments receive glutamatergic excitatory input mainly from the cortex through synapses onto spines, many of which express MORs. Thus, MOR expression in spines may be related to corticostriatal synaptogenesis. We used electron microscopic immunocytochemistry to determine potential age-dependent changes in the distribution pattern of MOR during postnatal synaptogenesis in the rat CPN. Immunogold-silver labeling revealed that the dendritic plasmalemmal density of MOR at postnatal day (P) 0 was significantly lower than, but after P10 was similar to, that of adult. In contrast, such age-dependent changes were not observed in axon terminals. Stereological analysis of immunoperoxidase labeling for MOR showed a good correlation in the developmental numerical densities of synapses with MOR-labeled spines and those of total asymmetric axospinous synapses, linear correlation coefficient r=0.99. Synapses with MOR-labeled dendrites, however, had a low correlation with axodendritic synapses (r=0.61), and synapses with MOR-labeled terminals showed no correlation with axospinous and axodendritic synapses (r=0.19). These results provide ultrastructural evidence that the targeting of MOR on the plasma membrane of dendrites and spines parallels the peak period of synaptogenesis during the third postnatal week in the rat CPN. Thus, the postnatal spatiotemporal expression pattern of MOR appears to match the functional maturation of corticostriatal glutamate transmission.

Comparative developmental profile of the neuropeptide Y Y1 receptor gene and protein in the rat brain

Neuropeptide Y (NPY) is one of the most abundant peptides found in the mammalian central nervous system (CNS) and plays several important roles in regulating brain function. Physiological roles of NPY in the brain are mediated by at least six receptor subtypes (Y1 to Y6). In the present study, a rat Y1 receptor cRNA probe was used for in in situ hybridization experiments in order to determine the developmental profile of this receptor mRNA and to compare it with its translated protein using receptor autoradiography with the radiolabelled ligand [125I][Leu31,Pro34]PYY. The NPY Y1 receptor mRNA is expressed as early as by the 12th day of gestation while specific [125I][Leu31,Pro34]PYY binding is observed by day 14 of gestation. Thereafter, both signals steadily increased, with Y1 receptor mRNA increasing faster than its translated protein during fetal life. The in situ hybridization signals reached a plateau around birth and remained high during the first 2 post-natal weeks to display the adult distribution by the end of the 3rd post-natal week. Similarly, specific [125I][Leu31,Pro34]PYY binding constantly increased during brain maturation and reached a plateau by the end of the 3rd post-natal week. In some brain areas, such as the cerebral cortex, specific binding declined slightly before attaining its adulthood pattern. Throughout ontogenesis, the profile of both the Y1 receptor mRNA and protein was well-matched except in hypothalamic areas where relatively higher mRNA signals were observed. Taken together, these results along with previous reports describing NPY-like immunoreactivity in the early developmental rat brain, suggest that the NPY Y1 receptor may play an important role in early brain development and maturation on the basis of its very early pattern of embryonic expression.

Atypical location of cannabinoid receptors in white matter areas during rat brain development

Previous evidence suggests that the endogenous cannabinoid system could emerge and be operative early during brain development. In the present study, we have explored the distribution of specific binding for cannabinoid receptors in rat brain at gestational day 21 (GD21), postnatal days 5 (PND5) and 30 (PND30), and at adult age (> 70 days after birth) by using autoradiography with [3H]CP-55,940. Our results indicated that specific binding for cannabinoid receptors can be detected in the brain of rat fetuses at GD21 in the classic areas that contain these receptors in adulthood-in particular, in the cerebellum and the hippocampus and, to a lesser extent, in the basal ganglia, several limbic structures, and cerebral cortex. The density of cannabinoid receptors in all these structures increased progressively at all postnatal ages studied until reaching the classical adult values in 70-day-old animals. Interestingly, cannabinoid receptor binding can also be detected at GD21 in regions, in which they are scarcely distributed or not located in the adult brain and that have the particularity of all being enriched in neuronal fibers. Among these were the corpus callosum, anterior commissure, stria terminalis, fornix, white matter areas of brainstem, and others. This atypical location was quantitatively high at GD21, tended to wane at PND5, and practically disappeared at PND30 and in adulthood, with the only exception being the anterior commissure, which exhibited a moderate density for cannabinoid receptors. Moreover, the binding of [3H]CP-55,940 to cannabinoid receptors in the white matter regions at GD21 seems to be functional and involves a GTP-binding protein-mediated mechanism. Thus, the activation of these receptors with an agonist such as WIN-55,212-2 increased the binding of [35S]-guanylyl-5'-O-(gamma-thio)-triphosphate, measured by autoradiography, in the corpus callosum and white matter areas of brainstem of fetuses at GD21. This increase was reversed by coincubation of WIN-55,212-2 with SR141716, a cannabinoid receptor antagonist. As this antagonist is specific for the cerebral cannabinoid receptor subtype, called CB1, we can assert that the signal found for cannabinoid receptor binding in the fetal and early postnatal brain likely corresponds to this receptor subtype. Collectively, all these data suggest the existence of a transient period of the brain development in the rat, around the last days of the fetal period and the first days of postnatal life, in which CB1 receptors appear located in neuronal fiber-enriched areas. During this period, CB1 receptors would be already functional acting through a GTP-binding protein-mediated mechanism. After this transient period, they progressively acquire the pattern of adult distribution. All this accounts for a specific role of the endogenous cannabinoid system in brain development.

Vicia villosa agglutinin labels a subset of neurons coexpressing both the mu opioid receptor and parvalbumin in the developing rat subiculum

Vicia villosa agglutinin (VVA), anti-parvalbumin antiserum and an affinity-purified anti-mu opioid receptor antibody were used to triple-label neurons in the postnatal rat subiculum. VVA labeled a subset of mu opioid receptor-positive neurons that were also immunoreactive for parvalbumin. The morphology of the triple-labeled neurons was heterogeneous, and included multipolar, ovoid and pyramidal-shaped neurons. Neurons single-labeled for the mu opioid receptor, VVA or parvalbumin were also morphologically heterogeneous. The postnatal development of mu opioid receptor immunoreactivity (IR), parvalbumin-IR and VVA binding was investigated using triple-labeling immunocytochemistry. Mu opioid receptor-IR appeared first and was present at postnatal day 1 (P1). Parvalbumin-IR was first observed in somata at P10, followed by proximal and distal dendrites at P15 and P20 respectively. Faint VVA labeling was seen first at P10 and surrounded a limited number of neurons. The intensity of labeling and the number of neurons labeled with VVA increased between P10 and P20; however, both measures remained below adult levels at P20. This study further illustrates the neurochemical heterogeneity of interneurons in the hippocampal formation and shows the developmentally early appearance of mu opioid receptor-IR compared to the late appearance of VVA binding and parvalbumin-IR.

kappa-opioid receptor expression defines a phenotypically distinct subpopulation of astroglia: relationship to Ca2+ mobilization, development, and the antiproliferative effect of opioids

To assess the role of kappa-opioid receptors in astrocyte development, the effect of kappa-agonists on the growth of astroglia derived from 1-2-day-old mouse cerebra was examined in vitro. kappa-Opioid receptor expression was assessed immunocytochemically (using KA8 and KOR1 antibodies), as well as functionally by examining the effect of kappa-receptor activation on intracellular calcium ([Ca2+]i) homeostasis and DNA synthesis. On days 6-7, as many as 50% of the astrocytes displayed kappa-receptor (KA8) immunoreactivity or exhibited increases in [Ca2+]i in response to kappa-agonist treatment (U69,593 or U50,488H). Exposure to U69,593 (100 nM) for 72 h caused a significant reduction in number and proportion of glial fibrillary acidic protein-immunoreactive astrocytes incorporating bromodeoxyuridine (BrdU) that could be prevented by co-administering the kappa-antagonist, nor-binaltorphimine (300 nM). In contrast, on day 14, only 5 or 14%, respectively, of the astrocytes were kappa-opioid receptor (KA8) immunoreactive or displayed functional increases in [Ca2+]i. Furthermore, U69,593 (100 nM) treatment failed to inhibit BrdU incorporation at 9 days in vitro. Experimental manipulations showed that kappa-receptor activation increases astroglial [Ca2+]i both through influx via L-type channels and through mobilization of intracellular stores (which is an important Ca2+ signaling pathway in cell division). Collectively, these results indicate that a subpopulation of developing astrocytes express kappa-opioid receptors in vitro, and suggest that the activation of kappa-receptors mobilizes [Ca2+]i and inhibits cell proliferation. Moreover, the proportion of astrocytes expressing kappa-receptors was greatest during a period of rapid cell growth suggesting that they are preferentially expressed by proliferating astrocytes.

Prenatal morphine exposure differentially alters expression of opioid peptides in striatum of newborns

The biochemical and cellular mechanisms involved in the development and/or maintenance of morphine tolerance remain unclear. In the adult central nervous system (CNS) results are contradictory. For the neonate, a variety of drug induced deficits have been observed following prenatal addiction to opioids, although very little work on the biochemical and molecular level has been done. Therefore, the present study was carried out to investigate the effects of prenatal morphine treatment on the levels and expression of endogenous opioid peptides in brain regions of newborns. Dams were implanted with one morphine pellet (75 mg each) 1 week prior to the birth of pups. Changes in mRNA levels for the opioid peptides were determined by Northern blot analysis. Alterations in opioid peptide levels were determined by radioimmunoassays. Prenatal morphine treatment significantly increased proenkephalin mRNA levels and decreased met-enkephalin levels in striatum of newborns. These data are in contrast to what is observed in the adult CNS. These data indicate that prenatal morphine treatment may increase met-enkephalin release and/or cause inhibition at the level of translation. In addition, increased transcription may be necessary to maintain equilibrium in the system when there is an increase in met-enkephalin release.

Effects of adenosine on cAMP production during early development in the optic tectum of chicks

Accumulation of cyclic adenosine monophosphate (cAMP) elicited by adenosine was studied in slices and membrane preparations of optic tectum from chicks aged 1-13 days post-hatch. Accumulation of cAMP promoted by adenosine declined with age, the highest value being observed in three-day-old chicks and the lowest in 11-day-old chicks. However, when the slices were incubated with adenosine and the phosphodiesterase inhibitor-Ro 20-1724 the differences between the two ages were abolished, suggesting a higher phosphodiesterase activity in 11-day-old chicks. In membrane preparations, although basal adenylate cyclase activity was lower in three-day-old chicks, the guanylyl-imidodiphosphate (Gpp(NH)p) concentration curves for stimulation of adenylate cyclase activity indicated a higher sensitivity of G protein to Gpp(NH)p at this age. This hypothesis was reinforced by the observation that the binding of [3H]Gpp(NH)p to the membrane preparation was greater in three-day-old animals. In spite of these differences, the percentage of adenylate cyclase activity stimulation by 2-chloroadenosine (2CADO)+Gpp(NH)p was the same at both ages. These findings suggest that the decreased response evoked by adenosine during development is probably due to increased phosphodiesterase activity and a lower sensitivity of adenylate cyclase activity to Gpp(NH)p.

Postnatal development of functional dopamine, opioid and tachykinin receptors that regulate acetylcholine release from rat neostriatal slices. Effect of 6-hydroxydopamine lesion

In the present work we have studied the postnatal development of functional dopamine, opioid and tachykinin receptors, which regulate cholinergic activity in the neostriatum. The release of endogenous acetylcholine from rat striatal slices was measured using a chemiluminescent method. We have observed that the inhibition mediated by dopamine through D2 receptors was not detectable until postnatal day 10, whereas the inhibition mediated by opioid receptors was detectable at postnatal day 15 for delta-receptors ([D-Pen2,D-Pen5]-enkephalin) and at postnatal day 21 for mu-receptors ([D-Ala2,Gly(ol)5]-enkephalin). Excitatory effect mediated by tachykinins through NK1 ([Sar9,Met(O2)11]- Substance P), NK2 ([Nle10]-Neurokinin A4-10), or NK3 (senktide) receptors was already detectable at postnatal day 5. In order to examine the influence of dopamine in the development of tachykinin and opioid systems in the neostriatum, we induced dopamine deficiency by intraventricular injection of 6-hydroxydopamine at postnatal day 3. We observed an increase in senktide-evoked acetylcholine release at postnatal day 30. The effect produced by [Sar9,Met(O2)11]-Substance P and [Nle10]-Neurokinin A4-10 was not modified. Furthermore, at postnatal day 35, we could observed that the two opioid receptor agonists have no effect. Our results show that dopamine, tachykinins and opioids are already able to mediate the modulation of acetylcholine release in early stages of development with a different pattern of postnatal development. Furthermore, the integrity of a dopaminergic system plays an important role in the functional development of the neostriatal cholinergic neurons which are differentially modulated by opioids or tachykinins.

Developmental expression of cannabinoid receptor mRNA

Cloning of the cannabinoid receptor affords the opportunity to examine its developmental expression. Other G-protein-coupled receptor systems, those for the opioids for example, exhibit distinct ontogenies. For the initial study, therefore, cannabinoid receptor mRNA expression was assessed in rat pups postnatal days 3, 5, 8, 10, 12, 15, 18 and 21. The brains were grossly dissected into cerebellum/brainstem and forebrain, and total RNA was extracted by a modified acid-extraction method. Expression of the cannabinoid receptor was analyzed by two methods: polymerase chain reaction (PCR) and Northern blot analysis. Oligonucleotide primers based on bp 1-21 and bp 824-843 on the opposite strand were chosen for use in the PCR. The probe used in the Northern blot analysis was a full length cDNA corresponding to the rat cannabinoid receptor and was cloned in our lab based on published sequence information. Our results indicate that by postnatal day 3, cannabinoid receptor mRNA can be detected in the brain. Our results further indicate that cannabinoid mRNA expression steadily increases in the cerebellum/brainstem until postnatal days 18-21, while expression in the forebrain does not change. The findings from the present study indicate that cannabinoid receptor mRNA is present in very young rats. Our data also suggest, however, regional differences in the relative expression of message which may parallel cerebellar proliferation and organization.

Immunocytochemical visualization of kappa-opioid receptors on chick embryonic neurons differentiating in vitro

The present paper is the first immunocytochemical demonstration of kappa-opioid receptors in neurons isolated from seven-day-old chick embryonic forebrains and cultivated for one to seven days. The monoclonal antibody KA8 (IgG1-k) [Maderspach et al. (1991) J. Neurochem. 56, 1897-1904] was raised against the frog brain kappa-opioid receptor as an antigen and recognizes an epitope in or near the ligand binding site. The KA8 immunostaining of the neurons displayed individual variations and changed with the in vitro differentiation. Receptors often appeared at the pole of the primary outgrowing process, later on in the whole soma and finally on the branched processes. Specific radioligand binding and KA8 immunocytochemistry both presented an increase in the receptor concentration with development. The equilibrium binding values that were measured at 1 nM [3H]naloxone concentration were 2.9 and 6.1 fmol/10(6) cells on the first and sixth cultivation days, respectively. Neurons were treated with 10(-7) M bremazocine or dynorphine (agonists with relative specificity to kappa-opioid receptors) on the second and third cultivation days. The agonist promoted the morphological differentiation which was already visible within 24 h. It also promoted the expression of the 200,000 mol. wt neurofilament protein, this became pronounced after two to three days. The changes provoked by the agonist were reduced by the opioid antagonist norbinaltorphimine (10(-7) M) or naloxone (10(-5) M) indicating that the effect was receptor-mediated. The hypothesis that kappa-opioid agonists through their receptors may function as regulatory signals in the early neuronal differentiation is discussed.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.