Ontogenesis of enkephalinergic systems in rat brain: post-natal changes in enkephalin levels, receptors and degrading enzyme activities

Asymmetric intra and inter-hemispheric subcellular rat brain correlation of arginyl-aminopeptidase activity during development and aging

The functional significance of brain asymmetry is still largely unknown. Studying the level of correlation of neuropeptide-degrading activities between subcellular fractions such as synaptosomal, of the left and right hemispheres of male rats during development and aging could provide relevant data on their functional role during these periods. The present study analyzes the level of correlation of a enkephalin- or angiotensin III-degrading activity, such as membrane-bound arginyl-aminopeptidase activity (M-B ArgAP) between the left versus right homogenate and/or synaptosomal subcellular fractions obtained and processed independently from both brain hemispheres during development and aging. The tested ages were: fetuses, rats of one week, one month, five month and two year old. In homogenate, the results demonstrate high levels of positive correlations (left versus right homogenate) with high levels of significance, without differences among the ages analyzed. In synaptosomes the results demonstrate positive correlations with a level close to statistical significance (left versus right synaptosomes) in fetuses, significant correlations at one week, one month and five months, particularly at one week and five months, and a radical decrease in the level of left versus right correlation between synaptosomes of two-year-old animals, which could suggest a functional loss of the bilateral synaptic interaction that could be carried out at earlier ages by M-B ArgAP activity. The interaction between left or right synaptosomes versus left or right homogenates demonstrate decreasing levels of positive correlation from fetuses to five month old rats, without differences between correlations of the left synaptosomes with correlations of the right ones. However, in two year old rats the values of correlations of the left synaptosomes diverged significantly from the right ones. While left synaptosomal correlations exhibited positive values, the right correlations exhibited negative ones, showing. a clear asymmetry between both sides in aged rats suggesting a marked reduction with aging of the synaptic function in the right hemisphere.

The early origins of food preferences: targeting the critical windows of development

The nutritional environment to which an individual is exposed during the perinatal period plays a crucial role in determining his or her future metabolic health outcomes. Studies in rodent models have demonstrated that excess maternal intake of high-fat and/or high-sugar "junk foods" during pregnancy and lactation can alter the development of the central reward pathway, particularly the opioid and dopamine systems, and program an increased preference for junk foods in the offspring. More recently, there have been attempts to define the critical windows of development during which the opioid and dopamine systems within the reward pathway are most susceptible to alteration and to determine whether it is possible to reverse these effects through nutritional interventions applied later in development. This review discusses the progress made to date in these areas, highlights the apparent importance of sex in determining these effects, and considers the potential implications of the findings from rodent models in the human context.

Stimulation of GABAB receptors increases the expression of the proenkephalin gene in slice cultures of rat neocortex

In rat neocortex the proenkephalin gene is expressed in GABAergic interneurons. Immunocytochemistry and in situ hybridisation show only a small number of cells in layers II to VI which express the gene. In organotypic slices of rat neocortex, the GABAA receptor inhibitor bicuculline methiodide enhances the expression of the gene in numerous cells. In the present study, we have investigated how GABA regulates the expression of the proenkephalin gene. The GABAA receptor antagonist bicuculline methiodide and the inhibitor of ligand-gated Cl- channels picrotoxin strongly enhanced the expression of the gene in numerous cells which were arranged in neocortical layers II/III and V/VI. Since bicuculline methiodide can also block Ca(++)-activated K+ channels, the possible involvement of such channels was tested. However, apamin which blocks only Ca(++)-activated K+ channels had no effect on the expression of the proenkephalin gene indicating that the effect of bicuculline methiodide was due to inhibition of GABAA receptors. In addition, the GABAB receptor agonist baclofen increased the neocortical expression of the proenkephalin gene mainly in cells located in layers V/VI of the neocortex. The effect of baclofen was inhibited by the GABAB receptor antagonists CGP35348 and CGP52432. Also muscimol, an agonist at GABAA receptors, enhanced the expression of the proenkephalin gene. This effect was blocked by CGP52432 confirming previous observations that muscimol can also stimulate GABAB receptors. Our results indicate that GABA can regulate the expression of the opioid peptide in neocortical neurons in a bidirectional manner. The expression is suppressed via GABAA and enhanced via GABAB receptors.

Expression of angiotensin-converting enzyme (ACE) in the developing chicken retina

Angiotensin-converting enzyme (ACE) performs two contrasting enzymatic effects: as part of the renin-angiotensin system it converts angiotensin I into physiologically active angiotensin II, and it inactivates a number of peptides, e.g. substance P. These peptides are well known neurotransmitters in the retina and recently angiotensin II was described in retinal neurons. We therefore investigated a possible involvement of ACE in retinal metabolism by determining the mRNA and protein expression of ACE in the developing and mature chicken retina. ACE-mRNA expression was investigated by RT-PCR in the iris/ciliary body, the choroid, the optic nerve head, pecten, and the retina. Levels of ACE-mRNA were quantified by competitive PCR with heterologous competitor fragments in the retina at different developmental stages. To localize protein expression of ACE in the mature chicken retina an antibody directed against ACE was used. ACE-mRNA was present in all ocular tissues examined. Quantification of ACE-mRNA in avascular retinas of developing chickens revealed small amounts (0.13 attomol microl(-1)) at embryonic day 7 and values of about 0.6 attomol microl(-1)during embryonic days 7-17. ACE-mRNA expression transiently increased ten-fold (7.3 attomol microl(-1)) on postnatal day 1, decreased again to about 1.4 attomol microl(-1)on postnatal day 6, and remained constant thereafter. ACE-immunohistochemistry revealed labeling of photoreceptors, bipolar cells, amacrine cells, and cells in the ganglion cell layer as well as of Müller glia. Our data show that ACE-mRNA is an intrinsic component of the retina and that ACE itself has a widespread but distinct cellular distribution. The transient high expression of ACE-mRNA directly after hatching indicate, that ACE may be involved in fine tuning the neuropeptidergic equipment of the retinal network during the initial phase of visual experience.

Mu- and delta-opioid receptor densities in respiratory-related brainstem regions of neonatal swine

The piglet displays similar postnatal development in respiration and sleep-wake behavior to the human. To shed light on the possible influence of opioid systems on these functions, this study assessed the density of mu- and delta-opioid receptors in brainstems of 2-3 and 5-7 (young), 14-17 (intermediate) and 20-21 (older) day-old piglets, using quantitative autoradiography. Serial 10 microns sections from fresh-frozen brains were incubated with either mu-(125I-DAGO) or delta-(125I-DPDPE) opioid ligands. The binding characteristics of each receptor remained unchanged over the age-range studied. delta-opioid receptor density was minimal in the young piglets, and increased over the age-range studied in all brainstem regions. mu-opioid receptor density exceeded delta-opioid density in all brainstem regions in young and older piglets, and remained unchanged with age. We conclude that, as in other species, the development of delta-opioid receptors in swine lags behind that of mu-opioid receptors, and that the distribution of each in the piglet's brainstem is distinct. The present findings help explain the changing influence of the mu- and delta-opioid systems on breating and state during postnatal development.

Development of proenkephalin gene expression in rat neocortex: a non-radioactive in situ hybridization study

Products of the proenkephalin gene are not only neurotransmitters but may also influence brain development. The ontogeny of the expression of the proenkephalin gene in neocortex was studied in embryonic and postnatal rats with in situ hybridization. At embryonic day 14, the proliferating cells in the ventricular zone strongly expressed the gene. Thereafter, the expression decreased and was hardly detectable up to embryonic day 21. At the day of birth and during the subsequent week, proliferating cells in the subventricular zone were labelled. The expression of the proenkephalin gene in proliferating neuronal and glial progenitors indicates that gene products may affect proliferation and/or commitment. In the neocortex, cells which strongly expressed the gene were first seen at postnatal day 7 in the outer part of the neocortex. Seven days later, a second band of positive cells had appeared in the inner part of the cortex, i.e. the adult pattern of distribution had been established. Thus, in rat neocortex the expression of the proenkephalin gene developed in an outside-first, inside-last mode.

Distribution and changes in mu- and kappa-opiate receptors during the midlife neurodevelopmental period of Coho salmon, Oncorhynchus kisutch

Parr-smolt transformation (PST) in coho salmon is associated with a plasma thyroid hormone (PT4) surge and a critical period of neural development that includes axonal sprouting, neurogenesis, and surges of select neurotransmitters. Here we provide a description of the selectivity, distribution, and the changes in the density of mu- and kappa-opiate receptors during PST, as revealed by quantitative in vitro autoradiography of [3H]Tyr-D-Ala-Gly-NMe-Phe-Gly-ol ([3H]DAMGO) and [3H]ethylketocyclazocine ([3H]EKC), respectively. The concentration of mu-receptors increased significantly in select cell groups in the early stages of parr-smolt transformation, until a peak was reached at the time coinciding with the peak of the PT4 surge. In other cell groups, the peaks occurred 1 or 2 weeks later. With one exception, this increase was followed by a decrease in concentration. The brain areas showing the highest concentrations are the dorsal nucleus of the ventral telencephalic area, the glomerular region, the granular layer of the valvula cerebelli, the nucleus diffuses of the inferior lobe, and the nucleus diffuses of the torus lateralis. Other regions with distinctly elevated mu-receptor concentrations are the stratum griseum centrale of the optic tectum and the preoptic area. The distribution of kappa-receptors is more diffuse, and the densities are considerably lower. The overlap in distribution of mu- and kappa-receptors is considerable, but significant exceptions are noted. For example, the dorsomedial nucleus of the dorsal telencephalic area, the habenular nucleus, and the dorsomedial nucleus of the thalamus exhibit a surge in density of kappa-receptors at the time of the PT4 surge, while the density of mu-receptors in these nuclei remain very low throughout parr-smolt transformation. The kappa-receptor containing cell groups are not identifiable until 3 weeks before the PT4 surge because of low densities. The most prominently labeled kappa-receptor regions are the ventral and dorsal nuclei of the ventral telencephalic area, the medial and dorsal zones of the dorsal telencephalic area, the optic tectum (all layers), the dorsomedial nucleus of the thalamus, the torus lateralis of the ventral hypothalamus, and the preoptic area. An increase of mu- and kappa-opiate receptor densities in specific brain regions may reflect roles in the alteration of brain organization, olfactory imprinting, neuroendocrine activity or other physiological activities. The overall distribution of these receptors are relatively more extensive in salmon than in other vertebrates so far studied.

Expression of the proenkephalin A gene in organotypic cultures of neocortex from newborn rats

In rats, the proenkephalin A gene is expressed in proliferating cells of the neuroepithelial zone which later give rise to neocortical neurones and glial cells. Therefore, organotypic cultures of neocortex of newborn rats were used in the present study to examine whether neurones as well as glial cells expressed the gene. The slices were prepared at birth and kept in culture for 7-13 days. Proenkephalin mRNA was visualised by in situ hybridisation, while immunocytochemical staining for MAP-2 and GFAP was used to identify neurones and astroglial cells, respectively. In the analysed slices, only neurones contained proenkephalin mRNA. Activation of protein kinase C with tetradecanoylphorbol acetate (1 mumol/l) caused a strong increase in the number of neurones expressing proenkephalin mRNA. Our results indicate that a large number of neurones is able to express the proenkephalin gene under these conditions. However, only a few of them have a basal expression which is strong enough to be detected with in situ hybridisation.

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.

Gene-peptide relationships in the developing rat brain: the response of preproenkephalin mRNA and [Met5]-enkephalin to acute opioid antagonist (naltrexone) exposure

[Met5]-enkephalin, encoded by the preproenkephalin (PPE) gene, serves as a growth factor during brain development in addition to its role as a neurotransmitter. This study examined the relationship of gene and peptide expression in the developing (postnatal day 6) rat brain by disrupting peptide-receptor interaction with either a brief (4-6 h) or continuous opioid receptor blockade using a single injection of 1 or 50 mg/kg naltrexone (NTX), respectively; such perturbations result in growth inhibition or acceleration, respectively. In the caudate putamen, an area that has completed neurogenesis by postnatal day 6 and has an abundance of PPE mRNA and enkephalins in adulthood, NTX did not influence PPE mRNA in either NTX group, or the enkephalin levels in the 1 mg/kg NTX group. [Met5]-enkephalin values in the neostriatum, however, were 67-183% greater than controls in rats given 50 mg/kg NTX, beginning 5 min after drug injection. In the cerebellum, PPE mRNA expression was depressed from 5 min to 4 h in the 1 mg/kg NTX group, and was normal thereafter; mRNA levels in the 50 mg/kg NTX group were markedly subnormal for 24 h. Enkephalin levels were significantly depressed within 5 min of drug injection and remained so for 4 h in the 1 mg/kg NTX group, but were elevated to approximately 135% of control values at 8, 16, and 24 h. Enkephalin levels were not changed in the cerebellum of the 50 mg/kg NTX group, or in the plasma of either NTX group. These data suggest that a single exposure to NTX can affect transcriptional and translational mechanisms related to PPE mRNA and opioid peptide expression in a rapid and sustained manner, and that this treatment elicits a specific pattern of alterations dependent upon the brain region sampled, drug dosage, and/or the duration of opioid receptor blockade. Additionally, our results indicate that the decreased DNA synthesis in external germinal cells occurring after opioid receptor blockade as recorded earlier may be related to an increase in the potent opioid growth factor, [Met5]-enkephalin.

Preproenkephalin mRNA expression in the developing and adult rat brain

[Met5]-Enkephalin is derived from the protein precursor, proenkephalin A, which in turn is encoded by the preproenkephalin (PPE) gene. [Met5]-Enkephalin is not only a putative neuromodulatory substance, but also serves as a growth factor (= opioid growth factor, OGF). OGF exerts an inhibitory influence on the developing nervous system and is especially targeted to cell proliferative and differentiative events. This study examined the relationship of PPE mRNA expression to late prenatal and postnatal rat brain development. Northern blot analysis of the whole brain and cerebellum showed that message is present in the fetal nervous system on prenatal day 15 (the earliest timepoint examined), is expressed at relatively similar levels within each tissue during the first 2 postnatal weeks, and reaches adult levels by the beginning of the 3rd postnatal week. In situ hybridization methodology revealed that PPE mRNA was prominent in areas associated with cell generation. Message was found in sites of primary (i.e., ventricular region) and secondary (e.g., external germinal layer of the cerebellum) cellular replication, as well as in discrete foci of cell proliferation (e.g., medullary layer of the cerebellum). PPE mRNA was also present for varying periods of time in postmitotic cells. During development, a number of patterns (decrease, increase, and no perceptible change) of PPE mRNA could be detected in relationship to the fetal/neonatal period. Given the strong evidence (e.g., regulation of cell proliferation and differentiation, temporal and spatial patterns of peptide and zeta opioid receptor) that enkephalin immunoreactivity is associated with proliferating and differentiating neurons and glia, these results suggest that the source of [Met5]-enkephalin is both autocrine and paracrine in nature.

Conversion of dynorphin A(1-8) to [Leu5]-enkephalin in rat central nervous tissue during development

Rat central nervous tissue contains enzymic activity that is able to convert the kappa-receptor preferring opioid peptide dynorphin A(1-8) to the delta-nu-receptor preferring opioid peptide [Leu5]enkephalin. The ontogeny of this conversion process has been studied in vitro using cortex, striatum, cerebellum and spinal cord tissues of the developing rat brain. Evidence for the enzymic cleavage of the Leu5-Arg6 bond of dynorphin A(1-8) to afford [Leu5]enkephalin was observed as early as neonatal day 1. The degree of conversion increased up to day 7, at which time adult levels were attained. Results in all tissues studied were similar. The relationship between the increase in the conversion process with age and the ontogeny of opioid peptides and their receptors may indicate an important role for the process in the developing nervous system.

The development of morphine-induced antinociception in neonatal rats: a comparison of forepaw, hindpaw, and tail retraction from a thermal stimulus

Two parallel experiments in rats 2-21 days of age investigated the onset and characteristics of morphine-induced antinociception. One measure of reactivity to pain, limb retraction from a hotplate, was utilized for three different limbs (forepaw, hindpaw, and tail) to chart the development of opioid sensitivity. Morphine-induced antinociception, even in 2-day-old rats, was obtained for all limbs, in a dose-related fashion, and reached peak sensitivity at 6-7 days of age. Naltrexone did not affect limb retraction latencies in nonmorphine treated rats at any age. These studies demonstrate early antinociception to low doses of an opiate and establish that the pain system, like positive reinforcement systems, is opiate sensitive.

Ontogeny of zeta (zeta), the opioid growth factor receptor, in the rat brain

Opioid growth factor (OGF), [Met5]enkephalin, serves as an inhibitory influence on the developing nervous system and is especially targeted to cell proliferative events. OGF interacts with the zeta (zeta) opioid receptor to perform its function. Using [3H]-[Met5]enkephalin, the ontogeny of the zeta receptor in the whole brain and cerebellum of rats was explored. Specific and saturable binding was recorded at the earliest time sampled, prenatal day 15 (E15). In the whole brain, binding capacity (Bmax) was two-fold greater at E15 than at E18 and E20. The quantity of zeta receptor appeared to increase in the first postnatal week, reaching a maximum on postnatal day 8. Binding decreased the remainder of the 2nd week and between postnatal days 15 and 25 binding was no longer recorded. In the cerebellum, binding capacity increased from E20 to the 2nd postnatal week, reaching a maximum on postnatal days 8-10. The Bmax of the zeta receptor decreased precipitously on postnatal day 11, being 5.4-fold lower than on postnatal day 10. Between postnatal days 21 and 30, no binding was observed. The binding affinities of the whole brain and cerebellum were 2.3 and 2.7 nM, respectively, and no differences between ages could be detected. Continuous opioid receptor blockade from birth to postnatal day 6 increased body weight, the Bmax of the zeta receptor in the whole brain and cerebellum (but not the Kd), and increased the number of layers of germinal cells in the cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the subcutaneous administration of enkephalins on tail-flick response and righting reflex of developing rats

The s.c. administration of [Met5]-enkephalin to 10-day-old rats pretreated with the mixture of 3 peptidase inhibitors, amastatin, captopril and phosphoramidon, produced the inhibition of tail-flick response and loss of righting reflex. When infant rats were pretreated with the mixture of any combination of two peptidase inhibitors, however, the change in both the response and the reflex were not produced at all by enkephalin injection, indicating that 3 kinds of enzymes, amastatin-sensitive aminopeptidase(s), captopril-sensitive peptidyl dipeptidase A and phosphoramidon-sensitive endopeptidase 24.11, played an important role in the inactivation of enkephalin after its systemic administration. Additionally, the fact that the two enkephalin-induced effects were more effectively antagonized by naloxone, a relatively selective mu-opioid antagonist, than by naltrindole, a specific delta-antagonist, or by nor-binaltorphimine, a specific kappa-antagonist, showed that these two effects were produced by the interaction of enkephalin with mu receptors. Moreover the involvement of mu receptors in the production of these two effects was shown by the fact that the s.c. administration of [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin, a selective mu agonist, also produced these two effects which were more effectively antagonized by naloxone than by naltrindole or nor-binaltorphimine. Since the magnitude of the two effects induced by enkephalins in 15-day-old rats was significantly lower than that in 10-day-old rats, and the two enkephalin-induced effects were not produced at all in 20-day-old rats, a maturation-induced decrease in the permeability of the blood-brain barrier against opioid peptides was indicated.

The development of stimulation-produced analgesia (SPA) in the rat

The present study studied the development of stimulation produced analgesia (SPA) from the periaqueductal gray (PAG) in rats. A monopolar stimulating electrode was lowered into the dorsal or ventral PAG of animals aged 7, 14, 21, or 90-120 days. Constant current cathodal pulses (100 Hz, 100 microseconds) were delivered, starting 10 s before analgesia was tested by the tail-flick (TF) test and continuing throughout each TF trial or until cut-off (7 s). Current intensity was increased stepwise (3-200 microA). It was found that SPA can be elicited starting at 21 days, but not earlier. However, supraspinal modulation of nociception is still immature at 3 weeks after birth. First, stimulation intensities needed to produce SPA are higher in 21-day-old pups than in adult animals. Second, in 21-day-old pups, but not in adults effective current intensities in the dorsal PAG are higher than in the ventral PAG. Third, naltrexone decreases SPA from the ventral PAG in 21-day-old pups, but not in adult animals. These findings indicate that supraspinal modulation of nociception develops only 3 weeks after birth, with the ventral PAG maturing prior to the dorsal PAG, and that the contribution of endogenous opioids to SPA does not remain constant throughout the ontogeny of rats.

Differential ontogenetic expression and regulation of proenkephalin and preprosomatostatin mRNAs in rat caudate-putamen as studied by in situ hybridization histochemistry

Specific oligonucleotide probes and in situ hybridization histochemistry were used to study the ontogeny and regulation of the mRNAs for proenkephalin A and preprosomatostatin in rat brain. In adult brain the most intense hybridization signal for the proenkephalin A mRNA was in caudate putamen, nucleus accumbens and olfactory tubercle. By contrast, the hybridization signal for preprosomatostatin mRNA was more diffusely scattered throughout the brain, with high signals in the neocortex, olfactory bulb and hippocampal formation. Studies of the ontogeny of these mRNAs revealed a different pattern of ontogenetic expression and differential regulation by dopaminergic input. The mRNA for preposomatostatin reached the highest level within the first postnatal week, whereas proenkephalin A mRNA progressively increased throughout the entire period studied. In addition the proenkephalin A mRNA showed a medial to lateral gradient in 2-day-old rat striatum which disappeared with increasing age, whereas preprosomatostatin mRNA increased in most brain areas in fairly uniform fashion with increasing age. Treatment of newborn rats with 6-hydroxydopamine increased the expression of proenkephalin A mRNA by 1.6 fold but had no effect on the expression of preprosomatostatin mRNA. The 6-hydroxydopamine-induced change in proenkephalin A mRNA expression was not observed until postnatal day 32, indicating that enkephalin-containing neurons of the developing striatum are relatively insensitive to dopamine input and that they cannot compensate for the neonatal lesion, despite the fact that the insult was given in a period of high plasticity of the neural tissue.

Ontogeny and distribution of opioid receptors in the rat brainstem

The distribution and postnatal ontogeny of opioid receptors have been investigated using in vitro quantitative receptor autoradiography. Rats were studied at postnatal day 1 (P1), P5, P10, P21 and P120 (adult). Opioid receptor sites for (D-Ala2,N-MePhe4,Gly-ol5)-enkephalin (DAMGO) binding were labelled with 4 nM of 3H-DAMGO; (D-Ala2,D-Leu5)-enkephalin (DADLE) binding sites were labelled with 4 nM of 3H-DADLE in the presence of 1 microM unlabelled mu-agonist (N-MePhe3,D-Pro4)-morphiceptin (PL107). We found that both binding sites have strikingly different distributional patterns. [3H]DADLE binding sites were rather homogeneous, whereas the distribution of [3H]DAMGO binding was very heterogeneous with the highest density in the nucleus of the solitary tract (NTS), ambiguus nucleus, dorsal motor nucleus of the vagus and the parabrachial areas. [3H]DAMGO binding density was 2- to 40-fold higher than [3H]DADLE binding sites in most brainstem nuclei. [3H]DAMGO binding sites appeared in most brainstem nuclei at birth, with a high density in cardiorespiratory-related nuclei, whereas [3H]DADLE binding sites were too scarce to be quantitated at P1. Both binding sites increased with age, but the developing patterns depended on the nucleus and the type of binding site. In most areas, the densities of both binding sites reached a maximum between P10 and P21 and then decreased to an adult level, but in some nuclei (e.g. the caudal part of the NTS and dorsal raphe nucleus), [3H]DAMGO binding sites kept increasing until adulthood. In contrast with the brainstem, cortical areas had a lower binding density in the newborn and reached peak levels later than brainstem regions (post P21). We conclude that (1) since [3H]DAMGO binding sites mainly reflect mu-receptors and [3H]DADLE binding sites delta-receptors (in the presence of PL017), the brainstem is essentially a mu-receptor region through delta-receptors are present; (2) both opioid receptors are present at birth but delta-receptors are very scarce in the newborn; (3) both receptors increase with age, but the time course depended on various nuclei and receptor types; (4) cardiorespiratory-related nuclei have high density of mu-receptors at all ages; and (5) opioid receptors develop earlier in the brainstem than in the cortex.

Pro-opiomelanocortin mRNA and peptide co-expression in the developing rat pituitary

Pro-opiomelanocortin (POMC) is synthesized in both the pituitary gland and the brain. Various peptide products of this precursor, namely beta-endorphin, ACTH and alpha-MSH are co-localized in the anterior lobe corticotrophs, all intermediate lobe cells and in hypothalamic neurons. Messenger RNA (mRNA) for POMC has further been shown to exist in these tissues. In this study, we have shown that POMC mRNA, and peptide accumulation as detected by in situ hybridization and immunocytochemistry, respectively, occur simultaneously within the rat pituitary gland during ontogeny and that their maturation occurs in parallel during prenatal and early postnatal development.

Opioids influence neurotransmitter phenotypic expression in chick embryonic neuronal cultures

There is considerable interest in the role of endogenous opioid peptides in neural growth and differentiation. In this study we used neuron-enriched cultures derived from 3-day-old chick embryos to test the effects of endogenous enkephalins on neurotransmitter phenotypic expression. Cultures were grown in serum-free chemically defined medium and were treated with either Met-enkephalin antiserum (anti-Met) to immunoneutralize enkephalins, or with naloxone, a universal opioid receptor antagonist, to block receptor-mediated actions of released endogenous opioids. The enzyme activities of choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH) were used as markers for cholinergic and catecholaminergic phenotypic expression, respectively. We found that cultures treated with anti-Met or naloxone exhibited strikingly different neuronal growth patterns as compared to controls. In addition, ChAT activity was enhanced by anti-Met, and TH activity by both anti-Met and naloxone. These findings lend support to the possibility that neuropeptides may be co-localized with neurotransmitters and that peptides released into the microenvironment affect neuronal phenotypic expression by differential receptor subtypes.

Postnatal development of preproenkephalin mRNA containing neurons in the rat lower brainstem

Postnatal developmental changes of preproenkephalin (PPE) gene expression in rat brainstem neurons were studied by in situ hybridization histochemistry. On the basis of PPE mRNA expression, brainstem neurons were categorized into three types: 1) type I neurons were characterized by constant or increasing expression of PPE mRNA during postnatal development; 2) type II neurons started to express PPE mRNA several days after birth and continued to do so thereafter; and 3) type III neurons showed transient expression of PPE mRNA or stopped expressing the mRNA during early postnatal development. Type I PPE neurons were observed in diverse brainstem structures including the mesencephalic and pontine central gray matter, various reticular and raphe nuclei, the ventral tegmental area of Tsai, the interpeduncular nucleus, the nucleus of the brachium of the inferior colliculus, the ventral and dorsal tegmental nuclei of Gudden, the sphenoid nucleus, the laterodorsal tegmental nucleus, Barrington's nucleus, the parabrachial region, the lateral lemniscus and its related nuclei, the trapezoid nucleus, the rostral and ventromedial periolivary nuclei, the mesencephalic trigeminal and principal sensory trigeminal nuclei, the locus coeruleus, the subcoeruleus nucleus, the medial and spinal vestibular nuclei, the dorsal and ventral cochlear nuclei, the medial and lateral cerebellar nuclei, the Roller nucleus, and the intermedius nucleus of the medulla. Type II PPE neurons were found in the superior colliculus, the inferior colliculus, the central part of the dorsal tegmental nucleus, and as Golgi neurons in the granular layer of the cerebellum. Type III PPE neurons were located in the substantia nigra, the red nucleus, the superior olive, the motor trigeminal nucleus, the facial nucleus, the inferior olive, the dorsal motor nucleus of the vagus, and the hypoglossal nucleus. Such region-specific expression of the PPE gene during postnatal ontogeny suggests that rat brainstem PPE neurons may be involved in a variety of developmental events, such as cell proliferation, differentiation, and migration.

In ovo and in culture development of chick retinal angiotensin converting enzyme

The activity of angiotensin I-converting enzyme (ACE, EC 3.4.15.1), measured using Hip-His-Leu as substrate, was determined in the developing chick retina, and in monolayer and aggregate cultures of embryonic retinal cells. ACE specific activity in chick retinal homogenate increased 86-fold from embryonic day 13 until the 7th post-hatching day. The development of ACE activity occurred in parallel with that reported for synapse and photoreceptors. ACE activity expression in aggregates, but not in monolayer culture, was similar to that observed in the developing retina in ovo. At culture, day 13, ACE specific activity was 11.8-fold higher in the aggregate than in the dispersed cell culture, and was comparable to that in a 21-day-old embryonic intact retina. Our results suggest that histotypic association of retinal cells during development may be an important event controlling the expression of ACE activity in the CNS.

The effect of neonatal exposure to chronic footshock on pain-responsiveness and sensitivity to morphine after maturation in the rat

Rat pups in 3 groups respectively were given daily footshock, exposure to a footshock apparatus without shock, or no handling from birth to 21 days of age and reared with no manipulation afterwards. After maturation (90-100 days of age), they were assessed for hot-plate paw-lick latency, morphine-induced analgesia and opiate receptor binding assay. In footshocked animals, a significant increase was found in paw-lick latency and in antinociceptive effects of morphine (1.25, 2.5, and 5.0 mg/kg) in comparison with two control groups. The antinociceptive effect of morphine in all 3 groups was antagonized by pretreatment with naloxone (2.0 mg/kg). No significant difference was found in binding activities (Bmax and Kd) for both [3H]naloxone and [3H]Dala2, D-Leu5-enkephalin between the 3 groups. These results suggest that exposure to footshock stress in the preweanling period has a long-term effect on the sensitivity of rats to painful events, probably due to chronic functional changes in endogenous opiate systems at presynaptic level rather than in postsynaptic opiate receptor binding activity.

Effect of p-mercuribenzoate on the subestimation of angiotensin-converting enzyme measurement during chick retina development

The time course of dipeptidase activity and the effect of p-mercuribenzoate (PCMB) on the subestimation of the fluorometric determination of angiotensin-converting enzyme (ACE, EC 3.4.15.1) during development was studied. ACE and dipeptidase activities were measured fluorometrically in homogenates of the developing chick retina using Hip-His-Leu and His-Leu as substrates, respectively, both either in the presence or in the absence of 1 mM PCMB. ACE activity was inhibited by captopril (IC50 1.7 nM), MK 422 (IC50 4.8 nM), BPP9a (IC50 0.25 microM) and BPP5a (IC50 1.2 microM), thus suggesting that avian retinal ACE catalytically resembles the mammalian enzyme. Dipeptidase activity varied 3.4-fold throughout development, leading to a large and variable (28-83%) subestimation of ACE activity during chick retina ontogenesis. PCMB (1 mM) inhibited 67-94% dipeptidase activity during development, thus greatly reducing any subestimation of ACE activity determination during the development of the chick retina.

Three-dimensional distribution of 3H-naloxone binding to opiate receptors in the human fetal and infant brainstem

Despite the putative role of opioids in disorders of the developing human brainstem, little is known about the distribution and ontogeny of opioid-specific perikarya, fibers, terminals, and/or receptors in human fetuses and infants. This study provides baseline information about the quantitative distribution of opiate receptors in the human fetal and infant brainstem. Brainstem sections were analyzed from three fetuses, 19-21 weeks gestation, and seven infants, 45-68 postconceptional weeks, in whom the postmortem interval was less than or equal to 12 hours. Opiate receptors were localized by autoradiographic methods with the radiolabelled antagonist 3H-naloxone. Computer-based methods permitted quantitation of 3H-naloxone binding in specific nuclei, as well as three-dimensional reconstructions of binding patterns. High 3H-naloxone binding corresponds primarily to sensory and limbic nuclei, and to nuclei whose functions are known to be influenced by opioids, e.g., trigeminal nucleus (pain), nucleus tractus solitarii and nucleus parabrachialis medialis (cardio-respiration), and locus coeruleus (arousal). The regional distribution of opiate receptors as determined by 3H-naloxone binding is similar in human infants to that reported in human adults and animals and corresponds most closely to that of mu receptors. We found, however, that opiate receptor binding is high in the fetal and infant inferior olive, in comparison to low binding reported in this site in adult humans, primates, and rodents. In addition, opiate receptors are sparse in the fetal and infant substantia nigra, as in reports of the adult human substantia nigra, compared to moderate densities reported in primates and rodents. By midgestation, the regional distribution of 3H-naloxone binding in human fetuses is similar, but not identical, to that in infants. Highest 3H-naloxone binding occurs in the inferior olive in fetuses at midgestation, compared to the interpeduncular nucleus in infants. Tritiated naloxone binding quantitatively decreases in virtually all nuclei sampled over the last trimester, but not to the same degree. The most substantial binding decrease (two- to fourfold) occurs in the inferior olive and may reflect programmed regressive events, e.g., neuronal loss, during its development. Definitive developmental trends in 3H-naloxone binding are not observed in the postnatal period studied. The heterogeneous distribution of opiate binding in individual brainstem nuclei underscores the need for volumetric sampling in quantitative studies.

Localization of enkephalinase mRNA in rat brain by in situ hybridization: comparison with immunohistochemical localization of the protein

The messenger RNA (mRNA) encoding enkephalinase (EC. 3.4.24.11; neutral endopeptidase) has been localized in rat brain by in situ hybridization using 35S- or 32P-labelled cRNA probes. Hybridization was observed only in few brain areas, and was particularly strong in the striatum, olfactory bulb and pontine nuclei. The enkephalinase protein was also localized in brain sections using a radiolabelled monoclonal antibody. While some brain regions contained both the mRNA and its translation product, others, including in particular the substantia nigra, were rich in enkephalinase but did not contain any detectable amount of enkephalinase mRNA. Enkephalinase mRNA-containing cells could be identified in regions containing neurons known to project to the substantia nigra. The discrepancy between the mRNA and the protein labelling is likely to reflect the fact that the mRNA is exclusively located within the soma of the cells while the translated protein may be found anywhere along the axonal processes.

Postnatal expression of opioid genes in rat brain

Developmentally regulated expression of opioid genes in rat brain was studied by Northern blot hybridization, using cloned rat genes as probes. Total RNA was isolated from the rat medulla oblongata, hypothalamus and striatum at 11 stages of development from birth to postnatal day 28. The steady-state levels of opioid precursor mRNAs in the above tissues were determined by hybridization to 32P-labeled opioid DNA probes and densitometric scanning of the appropriate bands. Analysis of the relative steady-state concentrations of opioid mRNAs has shown that the tissue distribution and size of these mRNAs during postnatal development of the CNS are similar to the pattern described in the adult. However, up to 50-fold changes in the relative mRNA concentrations were observed. These changes are tissue-specific, characterized by a peak within the postnatal stage of development, and are followed by a build-up of mRNA levels characteristic of the adult. Moreover, comparison of the changes in mRNA levels of proenkephalin to those of prodynorphin at their main site of synthesis in the brain, namely the striatum, revealed striking similarities in pattern. The association of this phenomenon with a possible role of opioid genes in development is suggested.

Detailed immunoautoradiographic mapping of enkephalinase (EC 3.4.24.11) in rat central nervous system: comparison with enkephalins and substance P

The metallopeptidase enkephalinase known to participate in the inactivation of endogenous enkephalins and, possibly, other neuropeptides such as tachykinins, was visualized by autoradiography using a [125I]iodinated monoclonal antibody. A detailed mapping of the enzyme in rat brain and spinal cord was established on 10-micron serial sections prepared in a frontal plane as well as a few sections in a sagittal plane. On adjacent sections, and for the purpose of comparison, substance P-like and enkephalin-like immunoreactivities were also visualized by autoradiography using a 125I-monoclonal antibody and a polyclonal antibody detected by a secondary 125I-anti-rabbit antibody respectively. Histological structures were identified on adjacent Nissl-stained sections. Using the highly sensitive 125I-probe, enkephalinase immunoreactivity was found to be distributed in a markedly heterogeneous manner in all areas of the central nervous system. Immunoreactivity was undetectable in white matter areas, for example the corpus callosum or fornix, and had a laminar pattern in, for example, the cerebral cortex or hippocampal formation. Hence, although immunodetection was not performed at the cellular level, a major neuronal localization of the peptidase is suggested. The latter is consistent with the detection of a strong immunoreactivity in a pathway linking the striatum to the globus pallidum, the entopeduncular nucleus and the substantia nigra, as well as with a series of biochemical and lesion data. The strong immunoreactivity also present in choroid plexuses and ependymal cells as well as in the intermediate lobe and in scattered cells of the anterior lobe of the pituitary suggests that populations of glial and endocrine cells also express the peptidase. The highest density of enkephalinase immunoreactivity was observed in basal ganglia and limbic areas (caudate putamen, globus pallidus, nucleus accumbens, olfactory tubercles) as well as in areas involved in pain control mechanisms (superficial layers of the spinal nucleus of the trigeminal nerve or of the dorsal horn of the spinal cord) which also display the highest immunoreactivities for both enkephalins and substance P (except in globus pallidus for the latter). These localizations account for the opioid-like analgesic and motor effects of enkephalinase inhibitors inasmuch as a selective or predominant participation of the peptidase in enkephalin inactivation is assumed. A number of other areas appear richly endowed in both enkephalinase and enkephalins whereas substance P is hardly detectable. This is particularly the case for the olfactory bulb, bed nucleus of the accessory olfactory tract, the cerebellum (where enkephalinase mainly occurs in the molecular layer) and the hippocampal formation (namely in the molecular layer of the dentate gyrus).(ABSTRACT TRUNCATED AT 400 WORDS)

Developmental aspects of nociception

Research has documented the existence of multiple, endogenous systems that modulate nociception. Based on the effects of opioid antagonists and endocrine lesions, endogenous analgesia systems have been organized into four classes: neural-opioid, neural-nonopioid; hormonal-opioid; hormonal-nonopioid. Developmental research on the ontogeny of endogenous analgesic function has revealed differential rates of maturation. Front-paw shock, a stimulus that activates a neural-opioid analgesic response, has been shown to be functionally mature by 28 days of age in the rat. Similarly, hind-paw shock, a stimulus that elicits a neural-nonopioid analgesic response, reaches maturity after two months of age. However, the hormonal-opioid analgesic system activated by cold-water immersion reaches adult levels by 10 days of age. Food deprivation produces a hormonal-opioid analgesic response in adult rats, and food deprivation/isolation of rat pups has been found to elicit an analgesic response in 6-day-old rats. From these data it seems that the rate of development of the different endogenous analgesic systems is related to the activation of neural or hormonal components. Whether the differential rates of development and the neural-hormonal distinction are related to the ecological validity of the activating stimulus remains to be determined.

The ontogeny of opiate tolerance and withdrawal in infant rats

The acquisition of morphine analgesic tolerance was investigated in neonatal rats. Morphine was found to produce a potent analgesia, as measured by latency to retract a hindpaw from a 52 degree C hotplate, in rat pups as young as 1 day of age. Morphine analgesic tolerance, however, did not develop in rats until the third week of life. Rats given the same daily morphine regimen starting at 15 days of age or older showed rapid tolerance development. The data from four experiments indicate that experience with morphine prior to this age (Day 15) does not impact on the analgesic efficacy of the drug. Similarly, when morphine treatment was discontinued and the rats given a naloxone challenge, withdrawal symptoms were not observed in very young rats. Opiate withdrawal was first detected in rats that started their daily morphine treatment at 30 days of age and were then challenged with naloxone at 52 days of age. Therefore, two correlates of opiate addiction, tolerance and withdrawal, appear to be relatively late-developing phenomena in the rat.

Ontogeny of an endogenous, nonopioid and hormonally mediated analgesic system

Rats of different ages (10-day, 28-day, and 3-month-old) were exposed to cold-water stress in order to activate an endogenous analgesic system. The effects of naltrexone (7 mg/kg) and dexamethasone (.4 mg/kg) were also studied to examine the role of the opioid and hormonal systems in cold-water-induced analgesia. Following cold-water exposure, nociception was measured with the tail-flick procedure for 2 hr. Results revealed that cold water produced a significant level of analgesia in the 10-day, 28-day, and 3-month-old age groups with no differences between age groups. In addition, in each age group naltrexone did not block the analgesia while dexamethasone attenuated the analgesia produced by cold water. The effects of naltrexone and dexamethasone confirm that cold-water immersion activates a nonopioid, hormonally mediated analgesic system in each age group. Thus, this experiment found that the endogenous, nonopioid, and hormonally mediated analgesic system activated by cold water is functional early in the development of the rat. The early development of this hormonally mediated analgesic system is in contrast to the slower development of endogenous analgesia systems that are mediated by the central nervous system.

Differential maturation of mu and delta opioid receptors in the chick embryonic brain

The developmental profiles of the binding of mu and delta opiate receptors agonists was investigated using the chick embryo brain. Binding of opioids was performed at embryonic days 5, 6, 15, 18, and 20 in the developing chick embryo brain. [3H]dihyromorphine was used as a mu ligand and with 5 X 10(-7) M levorphanol for non-specific binding, and [3H](D-Ala2-D-Leu5)-enkephalin was used as a delta with 5 X 10(-7) M (D-Ser-Gly-Phe-Leu-Thr)-enkephalin for non-specific binding. Crude membranes were prepared from whole brain at days 5, 6 and cerebral hemispheres at days 15, 18, and 20 of embryonic age. Both mu and delta opiate receptors were present during early embryogenesis and as early as day 5. Analysis of binding sites revealed high and low affinity mu sites during early embryogenesis but only one delta site. By 18 days of embryonic age, only one mu site remained. This developmental change is interpreted as a transitory state of the receptor to the adult mu pattern. The presence of only one delta site is constant throughout embryonic age; it is high during early embryogenesis reaching a lower level by 18 days. The presence of a dual binding site pattern for the mu receptor in early embryogenesis is implicated to have a functional significance in the pluripotential role of the endogenous opioids in early development.

Differential ontogeny of rat brain peptidases: prenatal expression of enkephalin convertase and postnatal development of angiotensin-converting enzyme

We quantitated the levels of two peptidases in the developing rat brain as a means to determine their function. Enkephalin convertase (EC 3.4.17.10), a carboxypeptidase B-like enzyme detected by [3H]guanidinoethylmercaptosuccinic acid (GEMSA) autoradiography, is present in high concentration throughout the brains of rat fetuses 3 days prior to birth. During the first 3 postnatal weeks, the density of [3H]GEMSA-labeled enkephalin convertase drops to adult levels. The expression of enkephalin convertase prior to that of most neuropeptides supports a role for this enzyme in propeptide processing. The regional distribution of [3H]GEMSA binding is similar in fetal and adult rats except that the thalamus exhibits the highest levels of [3H]GEMSA binding prenatally, and among the lowest levels in adult rats. Thus, peptide(s) formed in high concentration in the prenatal thalamus may be substrates for enkephalin convertase. Angiotensin-converting enzyme (ACE, EC 3.14.15.1) was visualized in the perinatal period by [3H]captopril autoradiography. Striatonigral ACE is undetectable at birth and increases to adult levels by two weeks of age. The expression of ACE after the initial presence of known peptides in the basal ganglia implies that the enzyme is not essential for peptide synthesis, suggesting instead a degradative role. In contrast to the striatonigral system, the choroid plexus contains high concentrations of ACE prior to birth, consistent with previous proposals of different substrates for ACE in the choroid plexus and the basal ganglia.

Ontogeny of substance P receptors in rat spinal cord: quantitative changes in receptor number and differential expression in specific loci

Anatomic distribution and functional studies of substance P (SP) and its binding sites show a role for the peptide in sensory (nociception), autonomic and somatic motor control. These physiologic functions show postnatal developmental changes, which, if mediated by SP, suggest that the receptors for the peptide may also undergo postnatal changes. This hypothesis was tested by using light microscopic autoradiography and membrane homogenate binding of 125I-Bolton-Hunter-SP (125I-BH-SP) to study SP binding sites in the spinal cord of rats of different ages. In cervicothoracic segments of rat spinal cord, the autoradiographs showed that specific binding of 125I-BH-SP occurred predominantly in the grey matter and varied inversely to age. In pups, up to about 15 days old, binding sites were diffusely distributed over the grey matter, and became progressively more defined in specific nuclei as the rats aged. A novel nucleus which is located in the ventrolateral ventral horn of caudal cervical segments and contained a high density of SP binding sites has been identified. High densities of SP binding sites in this nucleus and the intermediolateral cell column were visualized from the first postnatal day; however, those in the phrenic motor nucleus and in the dorsal horn were not fully expressed until after the 8th postnatal day. The age-related binding was confirmed in a membrane homogenate binding study of whole spinal cord which showed that the ratio for the concentration (cpm/mg protein) of specific binding was 106:12:4:1, for rats 11 (26 g), 38 (145 g), 90 (329 g) and 260 (553 g) days old. The ratio for the specific binding to the spinal cord (uncorrected for tissue weight) for the same groups of rats was 6:3:2:1. These data suggest that SP receptors decreased as a function of age. Furthermore, the decrease in SP receptors was not entirely due to growth of the spinal cord.

Ontogeny of gamma-melanocyte-stimulating hormone in the brain and hypophysis of the rat: an immunohistochemical analysis

The ontogeny of gamma-melanocyte stimulating hormone (gamma-MSH)-like immunoreactive (gamma-MSHI) structures in the brain and hypophysis was investigated in the rat by means of indirect immunofluorescence. gamma-MSHI neurons in the arcuate nucleus appeared at Day 13 of gestation, in the anterior hypophysis at Day 16, in the intermediate lobe at Day 18, and in the nucleus commissuralis after birth. gamma-MSHI fibers first appeared at Day 15 of gestation in the hypothalamic area and extended dorsally to reach the surface of the diencephalon. At Day 16 of gestation, another gamma-MSHI fiber bundle was found that ran laterally along the ventral surface of the diencephalon. At Day 19, immunoreactive fibers first appeared in the forebrain, diencephalon, midbrain and upper pons, and thereafter they increased in number, reaching a maximum at postnatal Day 15. gamma-MSHI fibers in the lower pons and medulla oblongata first appeared after birth. They also increased in number with age and reached a maximum at postnatal Day 15. The present ontogenetical study has demonstrated the different times of first appearance of each of the 3 major gamma-MSHI structures-containing cell groups in the brain, i.e., the arcuate nucleus, hypophysis and nucleus commissuralis. These findings may reflect the different functions of 3 groups of cells.

In vivo and in vitro development of alpha-MSH and ACTH in the embryonic and postnatal rat brain

The appearance of immunoreactive alpha-melanotropin (alpha-MSH) and adrenocorticotropin (ACTH) during development was studied in 3 areas of the rat brain--cerebral hemispheres, midbrain and hindbrain--from embryonic day (ED) 13-14 until day 21 postnatally. The alpha-MSH content in vivo was always highest in the midbrain; a peak content at birth was followed by a transient decline and a later, higher plateau from postnatal day 7 onwards. The alpha-MSH content in the cerebral hemispheres rose progressively after birth reaching a peak at day 21. Values in the hindbrain rose at day 3 and changed relatively sue taken at ED 15-16 showed a gradual increase in alpha-MSH content over the 20 days. The alpha-MSH content of hindbrain cultures remained at constant low levels, while no alpha-MSH was detectable in cerebral hemisphere cultures. ACTH appeared in vivo earlier than alpha-MSH and was detectable in embryonic brains at ED 13-14. A transient rise was seen at ED 17-18 and major peaks at birth, day 2 and day 3, in the midbrain, hemispheres and hindbrain, respectively. In vitro, the ACTH content increased in all brain regions during the first 5 days in culture and showed no further change thereafter. Comparisons of the in vivo and in vitro development of alpha-MSH and ACTH demonstrate that (i) these two peptide systems are independent in respect to their localization and time of appearance; (ii) they undergo maturation both in vivo and in vitro; (iii) epigenetic factors, such as interactions with other neurotransmitter systems may modulate the developmental pattern of these two peptides.

Characterization of opioid receptor binding in adult and fetal sheep brain regions

Opioid receptor binding was studied in 3 brain regions from maternal and fetal sheep at various gestational ages. [3H]dihydromorphine [( 3H]DHM) and [3H]D-Ala2,D-Leu5-enkephalin ([3H]DADLE) were employed as radioligands to characterize mu- and delta-opioid receptor binding sites, respectively. [3H]DHM binding was found to be highest in maternal cerebellum, intermediate in frontal cortex, and lowest in hippocampus. [3H]DADLE binding was highest in frontal cortex, intermediate in hippocampus and lowest in cerebellum. Cerebellum was the only tissue studied which contained more [3H]DHM than [3H]DADLE binding sites. Dissociation constants for [3H]DHM binding were similar in all 3 brain regions from both maternal and fetal sheep, while the dissociation constant for [3H]DADLE binding was significantly higher in cerebellum than in frontal cortex or hippocampus. Binding of both mu- and delta-receptor-selective ligands was 70% of maternal values in fetal cerebellum at 97-101 days of gestation and gradually increased over the remainder of the gestational period studied. Levels of [3H]DHM binding in frontal cortex and hippocampus were also similar to maternal levels at all timepoints studied. In contrast, [3H]DADLE binding was only 40-45% of maternal levels in fetal frontal cortex and hippocampus prior to 110 days of gestation, followed by a rapid increase in binding in both brain regions.

Ontogeny of tolerance to haloperidol: behavioral and biochemical measures

Developing and adult Sprague-Dawley rats were tested after acute or repeated haloperidol administration. Although 8-day-old rat pups showed a form of immobility in response to a single injection of haloperidol (1 mg/kg), 14-day-old rats did not show any behavioral response to the neuroleptic. By 21 days of age, an acute dose of haloperidol induced a cataleptic response similar to that described for adult animals. Following 7 days of repeated haloperidol administration, the cataleptogenic effects of haloperidol were attenuated in animals aged 21 days and older, but not in 8- and 14-day-old rats. Subjects were sacrificed 70 min after the injection of the test dose of haloperidol or saline and the corpus striatum and olfactory tubercles were dissected for HPLC determination of dopamine (DA) and its metabolites, homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC). In the corpus striatum, an area believed to be important for DA-related catalepsy, acute and repeated haloperidol induced a slight increase in concentrations of HVA in 8-day-old rats, and an increase in both DOPAC and HVA concentrations in animals aged 14 days and older. Tolerance after repeated haloperidol administration, in the form of an attenuation of the haloperidol-induced increase in DA metabolites, was not apparent until 35 days of age. These data contrast with the behavioral data, which indicate that the ability to develop a tolerance to the cataleptogenic effects of haloperidol matures by 21 days of age. The pattern of responses in the olfactory tubercles differed from those observed in the striatum. Following acute haloperidol, subjects did not show any increase in HVA until 14 days of age, and in both HVA and DOPAC until 21 days of age. At no age, including adults, was one week of repeated administration of haloperidol sufficient to induce tolerance to the effects of haloperidol on DA metabolites in the olfactory tubercles. In addition to providing information about the development of certain aspects of DA systems in rats, these studies suggest that an attenuation of the haloperidol-induced increase in DA metabolites is not necessary for the development of tolerance to haloperidol-induced catalepsy.

Ontogenesis of proenkephalin products in rat striatum and the inhibitory effects of low-level lead exposure

Certain developmental abnormalities have been associated with environmental exposure to lead and our previous studies have indicated that the endogenous opioid system is disrupted by this metal. In connection with this we report the ontogeny of proenkephalin products in the rat striatum determined by combined HPLC and bioassay and the effects of low-level lead exposure on this ontogeny. The development of Met-enkephalin levels was dissimilar from that of the other proenkephalin products, Met-enkephalyl-Arg6-Phe7, Met-enkephalyl-Arg6-Gly7-Leu8 and Leu-enkephalin. The ratios of Met-enkephalin containing peptides to Leu-enkephalin was less than the 6:1 ratio predicted from the proenkephalin structure. Lead (administered in the maternal drinking water, from conception to weaning at 100, 300 and 1000 ppm) caused a dose-related depression of the levels of proenkephalin products in rat striatum at 10, 21 and 30 days after birth. The most pronounced effects were observed at 10 days and the most persistent effects were seen with Met-enkephalin. Peak blood lead levels were below 45 micrograms/100 ml in the 100 and 300 ppm lead-dosed groups and in all lead-dosed groups at 10 days after birth. It is suggested that lead may have inhibitory effects on proenkephalin-processing enzymes.

Ontogeny and regional distribution of proenkephalin- and prodynorphin-derived peptides and opioid receptors in rat hippocampus

Levels of prodynorphin- and proenkephalin-derived peptides were determined in whole hippocampus of prenatal and early postnatal rats and in five regions of the hippocampus of the adult rat. Using autoradiography, opioid receptor subtypes were localized in coronal sections of adult hippocampus. The opioid peptides are present in very low concentrations in prenatal hippocampus, with only dynorphin B and alpha-neo-endorphin being present in significant amounts. The main increase in concentrations of the opioid peptides occur between day 7 and 14 postnatally, when dynorphin A, dynorphin A-(1-8), dynorphin B and alpha-neo-endorphin reach their adult levels. beta-Neo-endorphin and [Met]enkephalyl-Arg-Gly-Leu do not reach their maximal level until later in development. There is a distinct differential distribution of the opioid peptides in the subregions of the hippocampus; the subiculum and CA1 are relatively poor in prodynorphin-derived peptides but do contain significant amounts of [Met]enkephalin and [Leu]enkephalin. Very high concentrations of dynorphin B and alpha-neo-endorphin are present in region CA4. Dynorphin A-(1-8) and [Met]enkephalin have their highest concentrations in the dentate gyrus. There is a 5-fold higher concentration of [Met]enkephalin in the ventral hippocampus compared to the dorsal hippocampus. A similar trend is seen with dynorphin A-(1-8) but not with the other opioid peptides. The most abundant opioid receptor population in the hippocampus is of the mu type and it is densest in and around stratum pyramidale of the region CA3. There are relatively few kappa opioid receptors in the rat hippocampus. These results indicate the presence of at least two independent opioid neuronal systems (enkephalin and dynorphin) in rat hippocampus and the presence of mu-, delta- and kappa-opioid receptor subtypes.

Time of origin of opioid peptide-containing neurons in the rat hypothalamus

By using a combined technique of immunocytochemistry and [3H]thymidine autoradiography, we have determined the "birth date" of opioid peptide-containing neurons in several hypothalamic nuclei and regions. These include proopiomelanocortin (POMC) neurons (represented by ACTH immunoreactivity) in the arcuate nucleus; dynorphin A neurons in the supraoptic and paraventricular nuclei and the lateral hypothalamic area; and leu-enkephalin neurons in the periventricular, ventromedial, and medial mammillary nuclei, as well as in preoptic and perifornical areas. Arcuate POMC neurons were born very early in embryonic development, with peak heavy [3H]thymidine nuclear labelling occurring on embryonic day E12. Supraoptic and paraventricular dynorphin A neurons were also labelled relatively early (peak at E13). The lateral hypothalamic dynorphin A neurons showed peak heavy labelling also on day E12. By contrast, leu-enkephalin neurons in the periventricular nucleus and medial preoptic area exhibited peak heavy nuclear labelling on day E14. Furthermore, perifornical and ventromedial leu-enkephalin neurons were also born relatively early (peak on days E12 and E13, respectively). However, the leu-enkephalin neurons in the medial mammillary nucleus were born the latest of all cell groups studied (i.e., peak at E15). The results indicate a differential genesis of these opioid peptide-containing neuronal groups in different hypothalamic nuclei and regions.

Development of methionine-enkephalin in microdissected areas of the rabbit brain

Microdissected areas of the rabbit brain were isolated at prenatal day E-29, postnatal days P-3, 7, 14, 21, 2 months and adults. Methionine-enkephalin (ME) was assayed by RIA and ME concentration [ME] was expressed relative to the protein content of the extracted brain tissues. In brain nuclei with important roles in respiratory control [ME] was higher in prenatal and early postnatal life than in adults. In contrast, the prenatal and early postnatal [ME] levels in other nuclei were lower than or equal to adult values. These data suggest an important and changing role for ME in respiratory control throughout development. Early high [ME] levels within brainstem respiratory control nuclei may contribute to the newborn's increased susceptibility to respiratory depression.

Opioid antagonist-induced regulation of organ development

Naltrexone, a potent opioid antagonist, was given to preweaning rats in order to explore the influence of endogenous opioid systems on organogenesis. Sprague-Dawley rats were injected (SC) daily with either 1 or 50 mg/kg naltrexone to invoke a temporary or complete blockade, respectively, of opioid receptors; animals injected with sterile water served as controls. At weaning (Day 21), wet and dry weights, relative organ weight, and tissue water content were determined in 10 organ systems. Naltrexone's effects on growth depended on dosage, sex, and the organ system examined. In general, dosages of 1 and 50 mg/kg naltrexone caused significant decreases and increases, respectively, in organ weight. These changes in wet weight were not due to the state of hydration, but rather to dry weight, indicating that the content of cellular matter was altered. The changes in wet weight were similar to those for body weight, suggesting that a proportional increase or decrease in animal growth took place. Although the same organs in males and females within a dosage group were influenced by naltrexone, and usually to a similar degree, a dosage of 1 mg/kg naltrexone often affected different organ systems than the 50 mg/kg dosage. These results serve as the foundation for subsequent investigations directed towards delineating the role of endogenous opioid systems in developmental biology.

Naltrexone's influence on neurobehavioral development

The ontogeny of spontaneous motor and sensorimotor behaviors of preweaning rats, as well as ambulation, emotionality, and nociception at weaning (day 21), were studied in rats given chronic administration of 1 or 50 mg/kg naltrexone from birth to day 21. The age at which a specific spontaneous motor behavior or performance initially appeared and the age at which 100% of the animals demonstrated a particular behavior were accelerated in animals given 50 mg/kg naltrexone, but delayed in rats injected with 1 mg/kg naltrexone. In general, ambulation, emotionality, and nociceptive responses were not affected by naltrexone treatment, although the frequency of face-washing in both naltrexone groups and activity cage performance in the 50 mg/kg naltrexone group deviated from control levels. Observations of head-shake and wet-dog shake behaviors in naltrexone-treated animals at 2 hr and 10 hr post-drug injection were similar to controls with the exception of an abnormal increase in the 1 mg/kg naltrexone group at 10 hr. Although these results may imply that endogenous opioid systems play a role in regulating neurobehavioral development, further study is needed to distinguish whether these changes are a consequence of the somatic and morphological alterations known to occur with naltrexone administration or if the timetable of behavioral ontogeny is governed by endorphin-opiate receptor interaction.

Maturation of kainic acid seizure-brain damage syndrome in the rat. III. Postnatal development of kainic acid binding sites in the limbic system

The progressive appearance of [3H]kainic acid binding sites with age has been studied in membrane suspensions prepared from various regions of the rat limbic system, and by autoradiography. Binding sites with fast dissociation rate appeared earlier than binding sites with slow dissociation rate. Scatchard analysis demonstrated apparent receptor heterogeneity for both subclasses. High affinity components were detected in the hippocampus as early as 10 days after birth, but in the amygdala + piriform lobe were found only towards the end of the third week, when animals also respond to parenteral kainic acid, for the first time, with limbic seizures accompanied by metabolic activation of the amygdala. Slice autoradiography revealed distinct labelling of the hippocampal CA3 region by postnatal day 10. A comparison with the ontogenesis of the kainic acid-induced seizure-brain damage syndrome suggests a role of high affinity receptors as mediators of metabolic nerve cell activation by kainic acid. However, this receptor interaction per se does not result in neuronal damage to the vulnerable region of the Ammon's horn, which will only occur at an age when also the amygdala is activated by the neurotoxin.

Naltrexone modulates body and brain development in rats: a role for endogenous opioid systems in growth

Preweaning rats receiving daily injections of 20, 50, or 100 mg/kg naltrexone, a potent opiate antagonist, had body and brain weights that were increased 16-22% and 6-13%, respectively, from control levels on day 21 (weaning). All of these dosages of naltrexone blocked the opiate receptor for 24 hr/day as measured in opiate challenge experiments. Dosages of 0.1, 1, and 10 mg/kg naltrexone, which blocked the opiate receptor for less than 12 hr/day, inhibited growth. Repetitive administration of low dosages (3 mg/kg naltrexone, 3 times daily), which blocked the receptor 24 hr/day, increased body and brain development by 31% and 10%, respectively, whereas a cumulative dosage of 9 mg/kg naltrexone given once daily retarded growth. These results show that developmental events are dictated by the duration of opiate receptor blockade and provide compelling evidence that endogenous opioid systems play a crucial role in growth.

Differential, postnatal ontogeny of opiate and benzodiazepine receptor subtypes in rat cerebral cortex: binding characteristics of tifluadom and brotizolam

The postnatal development of 3[H] ethylketocyclazocine (EKC) binding characteristics was examined using membranes from the cerebral cortex of rats at various ages. Binding site affinity did not vary significantly between postnatal days 1 and 90. However, the apparent density of cortical binding sites increased fivefold between birth and adulthood. These results were similar to another ontogenic study of brain opiate receptor binding. Whereas EKC was equally potent as a competitor for 3[H] EKC binding in cortex from neonatal and adult rats, tifluadom was three times more potent in neonatal cortex than in adult cortex as a displacer of specific EKC binding. Brotizolam, a new thienodiazepine and a potent sedative hypnotic, also was distinctly more potent as an inhibitor of 3H-diazepam binding in neonatal rat brain cortex than in adult rat brain cortex. These results suggest that subtypes of benzodiazepine receptors, as well as some opiate receptor subtypes, exhibit different rates of postnatal development.