Na+ dependence of tyrosine transport across the synaptosomal membrane reflects changes in the morphology of synaptosomes

The Na+ dependence of tyrosine uptake into rat brain synaptosomes and synaptosomal plasma membrane vesicles (SPMV) was examined in the present study. At low tyrosine concentrations, the isoosmotic substitution of Na+ by sucrose in the incubation medium led to an increase of tyrosine uptake in synaptosomes and to a decrease in SPMV. The removal of extracellular Ca2+ and Mg2+ and addition of isoosmotic sucrose completely prevented the augmented tyrosine uptake in Na+-free incubated synaptosomes. Morphological differences were found at the electron-microscopic level when synaptosomes were incubated in Na+-free and Na+-containing media. The internal volume measured for synaptosomes incubated in a Na+-free medium was almost half of that obtained in a Na+-containing medium, in good agreement with the observations made with the electron microscope. Also, the omission of Ca2+ and Mg2+ resulted in a specific swelling of only the synaptosomes incubated in Na+-free medium. When synaptosomes and SPMV were preloaded with several neutral amino acids, the tyrosine uptake rate was greatly increased, indicating fully operational exchange mechanisms for these amino acids. We propose that the enhancement of high-affinity synaptosomal tyrosine uptake observed in Na+-free medium is a consequence of a specific shrinkage of the synaptosomes and a parallel increase of the exchange rate with endogenous neutral amino acids.

Ontogeny of the novel tachykinins neurokinin A, neurokinin B and neuropeptide K in the rat central nervous system

Neurokinin A, neurokinin B and neuropeptide K content has been measured in several regions of the rat central nervous system at different stages of postnatal development. For this, we have employed a combination of HPLC separation and radioimmunoassay detection using a neurokinin A antiserum which also recognizes neurokinin B and neuropeptide K. All 3 tachykinins were detectable during postnatal development in the various regions studied (hypothalamus, striatum, substantia nigra, cerebral cortex and spinal cord). Interestingly, a general increase in the tachykinin concentrations was observed during the second week of life. Some of these concentrations reached values on postnatal day 15 which far exceeded those observed in the adult. After day 15 most areas showed a slow decline in their tachykinin content until adult values were finally achieved. The developmental profiles obtained for these tachykinins are in good agreement with previous studies on the ontogeny of substance P and its receptors and support the view that tachykinins may play an important role in the organization and maturation of the developing central nervous system.

In vitro and in vivo release of neurokinin A-like immunoreactivity from rat substantia nigra

In vivo and in vitro perfusion techniques have been used to study the release of neurokinin A-like immunoreactivity from the rat substantia nigra. Potassium depolarization and electrical field stimulation evoked calcium-dependent release from nigral slices. Potassium depolarization was also effective in vivo. Tetrodotoxin (1 microM) completely blocked electrically stimulated release but only diminished release in response to depolarizing potassium. Neurokinin A-like immunoreactivity release showed frequency dependence and a clear facilitation phenomenon between 5 and 25 Hz. High-performance liquid chromatography analysis of the immunoreactivity released in vitro revealed the presence of neurokinin A, neuropeptide K and neurokinin B, along with their sulphoxide forms. A marked depletion of neuropeptide K and neurokinin B content was observed when the tachykinin content of the nigral slices was examined before and after stimulation. However, the neurokinin A content of the slices was unchanged or even increased, suggesting an accelerated processing of neurokinin A precursors during the stimulation. The tachykinin peptides were degraded at different rates by substantia nigra homogenates; degradation was fastest for neuropeptide K and slowest for neurokinin A. The addition of a mixture of peptidases inhibitors (thiorphan, phosphoramidon, bestatin and captopril) substantially reduced the degradation of all three tachykinins, but did not completely block degradation. GABA-A receptor antagonists such as bicuculline and, particularly, picrotoxin potentiated the stimulated neurokinin A-like immunoreactivity release in vitro, but the GABA-agonist muscimol had no effect. Picrotoxin was even more potent in vivo. The results presented in this study demonstrate that neurokinin A, neuropeptide K and neurokinin B can be released by depolarizing stimuli from rat substantia nigra. Furthermore, the features exhibited by this release suggest that these peptides may have a neurotransmitter/neuromodulator role in the rat substantia nigra.

Opioid peptides inhibit noradrenergic transmission in the preoptic area to block LH secretion: evidence from neonatally androgenised rats

A series of experiments are described which show (a) that the release of noradrenaline in the preoptic area of rats is regulated by a presynaptic opioid receptor and, (b) that changes in the opioid control of preoptic noradrenaline release are associated with the anovulatory condition found in adult female rats which have been exposed to androgen neonatally. The experiments support the hypothesis that opioid-noradrenergic interaction in the preoptic area is an important component in the neural mechanism regulating gonadotrophin secretion.

Subcellular distribution of mammalian tachykinins in rat basal ganglia

A combined differential and density gradient centrifugation procedure was used to study the subcellular localisation of the mammalian tachykinins in rat caudateputamen and substantia nigra. Substance P, neurokinin A, neuropeptide K, and neurokinin B were found to be concentrated in the synaptosomal fractions and in fractions containing heavy synaptic vesicles in both regions studied. In contrast, the catecholamines dopamine and noradrenaline had a more widespread distribution throughout the gradient. HPLC analysis of the immunoreactivity recovered showed that the tachykinin immunoreactivity coeluted with the relevant synthetic tachykinins, except in the soluble gradient fraction where neurokinin A immunoreactivity eluted in position consistent with neurokinin A3-10. These results suggest that, in the basal ganglia, the mammalian tachykinins are localised in fractions containing large dense cored synaptic vesicles. This vesicular localisation would be consistent with the proposed role of the tachykinins as neurotransmitters and neuromodulators.

Effect of neonatal testosterone upon opioid receptors and the content of beta-endorphin, neuropeptide Y and neurotensin in the medial preoptic and the mediobasal hypothalamic areas of the rat brain

The content of beta-endorphin, neuropeptide Y and neurotensin-like immunoreactivity (beta-End, NPY and NT), and the total number of opioid binding sites, were measured in the medial preoptic area (MPOA) and mediobasal hypothalamus (MBH) of ovariectomized adult rats which were oestrogen-primed. The rats had been injected neonatally with either testosterone propionate (TP) or vehicle (oil). NPY content was found to be higher in the MPOA of animals which received TP, whereas no significant difference was observed in the MBH NPY content. However, the NT concentration in the MBH of TP-treated rats was almost twice the amount detected in oil-treated rats and with this peptide no significant changes were detected in the MPOA. Finally, beta-End and the total number of opioid binding sites were reduced in both the MPOA and MBH of the rats which were exposed to TP neonatally. Since exposure to testosterone neonatally masculinises the rat hypothalamus, to the extent that female rats cannot generate oestrogen-stimulated prolactin and luteinizing hormone surges, we suggest that the neurochemical changes reported in this paper reflect some aspects of this sexual differentiation of the rat hypothalamus.

Neonatal testosterone modifies LH secretion in the adult female rat by altering the opioid-noradrenergic interaction in the medial preoptic area

Noradrenergic-opioid interaction in the medial preoptic area (MPOA) was examined in ovariectomised adult rats which were oestrogen-treated and had been injected neonatally with either testosterone propionate (TP) or vehicle (oil). The first experiment involved electrical stimulation of the ventral noradrenergic tract (VNAT) in anaesthetised rats. Blood samples were collected before and after the stimulation to determine plasma levels of luteinising hormone (LH). Approximately half of the animals received naloxone i.v. 15 min before the onset of stimulation. In all groups, stimulation of VNAT elicited a significant increase in plasma LH concentration. However, pretreatment with naloxone in androgenised rats, but not in oil-treated animals, almost doubled the LH increment due to stimulation. Naloxone had no effect on plasma LH concentrations in unstimulated control rats. In the second experiment hypothalamic slices containing the MPOA were preincubated with [3H]noradrenaline [( 3H]NA) and then subjected to electrical field stimulation under the conditions of (a) no drug added and (b, c) morphine superfusion without and with naloxone. The opioid agonist morphine significantly reduced the net release of [3H]NA in normal and TP-treated female rats. Addition of equimolar naloxone reversed this effect in normal females, whereas in the androgenised group it not only reversed this effect but elicited a significant increase in [3H]NA release. From these data we conclude that (1) neonatal testosterone treatment alters noradrenergic-opioid interaction regulating LH secretion in adult females and (2) the site of this change may be the presynaptic opioid input to the noradrenergic terminals in the MPOA.

Neonatal administration of a specific neuropeptide Y antiserum alters the vasopressin response to haemorrhage and the hypothalamic content of noradrenaline in rats

The possibility that neuropeptide Y (NPY) exerts organizational effects on central noradrenergic systems was investigated by treating newborn rats with subcutaneous injections of a specific NPY-antiserum. Three months later, neuroendocrine function was determined by measuring plasma vasopressin following haemorrhage, since this response is known to be regulated by ascending noradrenergic pathways. Basal mean arterial pressure, heart rate and plasma vasopressin were similar in both control (normal rabbit serum-treated) and NPY-antiserum-treated rats. Treatment with this antiserum resulted in an impaired vasopressin response to haemorrhage, although the haemodynamic changes observed after this hypovolaemic challenge were similar to control rats. NPY and noradrenaline content in the hypothalamus and brainstem were examined at the end of these experiments. NPY-like immunoreactivity was similar in both groups of animals. However, electrochemical detection of noradrenaline after HPLC revealed significantly higher levels in the hypothalamus, but not brainstem, of NPY-antiserum-treated rats. The presence of enduring changes in noradrenaline levels and neurohypophyseal function following neonatal treatment with NPY-antiserum suggests a role for NPY in postnatal organization of the rat hypothalamus.

Opioid peptides inhibit the release of noradrenaline from slices of rat medial preoptic area

Previous circumstantial evidence suggested that endogenous opioid peptides inhibit an excitatory noradrenergic projection to the medial preoptic area (MPOA), and thereby suppress the activity of neurones containing luteinising hormone-releasing hormone and thus systemic concentrations of luteinising hormone (LH) itself. In this paper, we report that electrically stimulated release of 3H-Noradrenaline (3H-NA) from perifused slices of rat MPOA is diminished when opioid agonists are added to the incubation medium. Thus, morphine (10 microM), beta-Endorphin (1 microM) and met-Enkephalin (1 microM), but not Dynorphin A (1-8) (1 microM), caused a significant decrease in electrically stimulated 3H-NA release. The inhibition was reversed by addition of naloxone (10 microM) to the perifusion medium but 3H-NA release was unaffected by dopamine or acetylcholine (or their antagonists sulpiride and atropine, respectively), or serotonin, neurotensin, muscimol or bicuculline (the latter two being agonist and antagonist respectively for the GABA A receptor). Therefore, the experiments provide direct evidence that brain opioids modulate the noradrenergic input to MPOA neurones and support the hypothesis that this may be one mechanism for the regulation of LH secretion.

Tyrosine transport into isolated rat brain synaptosomes. Ionic dependence and kinetic studies

The ionic dependence and kinetics of the uptake of L-tyrosine into isolated rat brain synaptosomes has been investigated. L-Tyrosine has been found to enter the synaptosomes through three different transport systems showing distinct ionic requirements and kinetic characteristics. The one with the lowest affinity for tyrosine (Km 0.6 mM) showed a strong Na+ dependence. This system seems to provide the nerve cell with a safety mechanism that ensures the supply of tyrosine even in the presence of high levels of competing amino acids. The second one (Km 50 microM) does not appear to exhibit any strong ionic requirements and features most of the characteristics of the L-system for large neutral amino acids. Finally, the third shows the most interesting ionic dependence. Its activity increases at very low Na+ external concentrations, but this increase is prevented by the removal of divalent cations, Ca2+ and Mg2+. This ionic behaviour, along with the affinity constant of this system (Km 6 microM) (within the range of tyrosine extraneural concentrations), suggests that it is an initial regulatory step in the synthesis of catecholamines.

Morphine inhibits electrically stimulated noradrenaline release from slices of rat medial preoptic area

Experiments were undertaken to investigate whether the release of noradrenaline (NA) in the preoptic area of female rats is modulated by opiate mechanisms. Slices (300 microns) of preoptic area were incubated individually after loading with 3H-NA and then subjected to electric field stimulation. Morphine (10 microM) significantly reduced the amount of NA released in response to electrical stimulation, an effect that was completely prevented by simultaneous exposure to equimolar naloxone. This inhibitory action of morphine was not dependent upon prior exposure to oestrogen.