A1 noradrenergic action on medial preoptic-medial septal neurons: a neuropharmacological study

In an attempt to determine whether the excitatory and inhibitory orthodromic responses of single medial preoptic-medial septal (MPO-S) neurons to discrete electrical stimulation of the A1 noradrenergic region were mediated specifically by norepinephrine (NE) and involved different types of adrenoreceptors, a series of electrophysiological and neuropharmacological experiments was conducted. Extracellular single unit recording and local drug application techniques were used in female rats under urethane anesthesia. Chemical lesion of the catecholaminergic nerve terminal plexus in the medial preoptic area with 6-hydroxydopamine abolished both excitatory and inhibitory orthodromic effects of A1 region stimulation on MPO-S neurons, suggesting the noradrenergic nature of the effects. This conclusion was corroborated by the observation that the orthodromic effects were mimicked by locally applied exogenous NE. The excitatory effects were reliably mimicked by a low concentration of NE (0.5 mM; in-barrel concentration) and methoxamine (1.0 mM, an alpha-1 agonist), but not by either low or high concentrations (1 and 100 mM) of clonidine (an alpha-2 agonist) and isoproterenol (a beta agonist). The inhibitory orthodromic effects of A1 region stimulation were reliably mimicked by a high concentration of NE (50 mM), clonidine (100 mM) and isoproterenol (100 mM), but not by a low concentration of NE (0.5 mM), methoxamine (1 mM), clonidine (1 mM) or isoproterenol (1 mM). A high concentration (100 mM) of methoxamine mimicked the inhibitory effects less than 40% of the time. The low concentration (0.5 mM) NE-induced excitation that matched the excitatory orthodromic effect of A1 region stimulation was blocked by phentolamine (100 mM), an alpha blocker, but not by timolol (100 mM), a beta blocker. On the other hand, the high concentration (50 mM) NE-induced inhibition that matched the inhibitory orthodromic effect of A1 region stimulation was blocked by timolol, but not by phentolamine. Taken together, the present results are consistent with the hypotheses that the ascending noradrenergic projections from the A1 region affect the excitability of MPO-S neurons directly through NE and that the excitatory and inhibitory orthodromic effects involve different types of adrenoreceptors, i.e., alpha-1 and beta receptors, respectively.     

Identification of a GBR12935 homolog,LR1111, which is over 4,000-fold selective for the dopamine transporter,relative to serotonin and norepinephrine transporters

The di-substituted piperazines, GBR12909 (1-[2-[bis(4-fluorophenyl)-methoxy]ethyl]-4-[3-phenylpropyl]piperazine) and GBR12935 (1-[2-(diphenyl-methoxy)-ethyl]-4-(3-phenylpropyl)piperazine), are potent and selective (20-to 100-fold) inhibitors of [³H]dopamine reuptake, relative to [³H]5-HT and [³H]norepinephrine uptake. The GBR12935 analog, 1-(2-(diphenylmethoxy)ethyl)-4-(3-phenylpropyl)homopiperazine (LR1111), was synthesized as part of a systematic structure-activity study of analogs of GBR12935 and GBR12909. LR1111 differs from GBR12935 by the addition of a methylene group into the piperazine ring to yield a compound with a seven-member homopiperazine ring. The IC50 values for LR1111 at the dopamine, norepinephrine, and serotonin transporters were 7.2 nM, 34, 072 nM, and greater than 20,000 nM, respectively, whereas the IC50 values of GBR12935 were 3.7 nM, 289 nM, and 1261 nM for these same transporters. This demonstrates that the addition of a single methylene group in the piperazine ring results in a compound with similar affinity but significantly higher selectivity for the dopamine transporter. LR1111 increased motoric activity in rats after intravenous administration. These indicate that LR1111 is a potent and highly selective inhibitor of the dopamine transporter. Published 1993 Wiley-Liss, Inc.     

BDNF increases monoaminergic activity in rat brain following intracerebroventricular or intraparenchymal administration

We have previously demonstrated alterations in serotonin metabolism within descending pathways following infusion of brain-derived neurotrophic factor (BDNF) into the midbrain, near the periaqueductal gray and dorsal and median raphe nuclei. The aim of the present study was to extend these studies to include a comprehensive regional examination of monoamine (serotonin, dopamine and norepinephrine) and metabolite levels in discrete areas of the intact, adult rat forebrain following direct intraparenchymal midbrain BDNF infusion. We have compared neurochemical changes following midbrain infusion of BDNF to those obtained following intracerebroventricular (i.c.v.) infusion. Significant increases in levels of 5-HIAA and/or the 5-HIAA/5-HT ratio were found in all areas examined including the hippocampus, cortex, striatum, n. accumbens, substantia nigra and hypothalamus following both midbrain and i.c.v. infusion. Changes in dopaminergic activity were also observed, but displayed more regional specificity, i.e. changes were found primarily within the striatum and cortex. The two infusion sites produced similar patterns of neurochemical effects although the magnitude of the changes did vary in some areas. These results suggest that BDNF increased synthesis and/or turnover of serotonin, and to a lesser extent dopamine, in the mature rat forebrain. Furthermore, these data point to possible functional roles for BDNF in neuropsychiatric and neurodegenerative conditions which involve a dysregulation of these monoamine systems.     

The role of adrenoceptors in norepinephrine-stimulated VO2 in muscle.

The purpose of this study was to characterize the norepinephrine (NE)-evoked calorigenic response of an isolated perfused rat muscle preparation into alpha-and/or beta-adrenergic components. Epinephrine was as effective as NE in evoking the increase in oxygen consumption. The alpha-agonist, phenylephrine, was also as effective as NE in eliciting an increase in oxygen consumption by the muscle. Isoproterenol (beta-agonist) had no effect on the VO2 of the preparation. Phentolamine, an alpha-blocker, completely blocked the NE-stimulated VO2. Propranolol (beta-antagonist) could also completely block the NE induced VO2 but this drug was found to be less potent. It was concluded that the NE-induced calorigenic response of muscle cannot be classified either strictly as an alpha-or beta-response, but unlike brown adipose tissue, the alpha-response seems to predominate in skeletal muscle. Also, it appears as though different mechanisms of action may be involved in the NE-stimulated VO2 in skeletal muscle and brown fat.     

Norepinephrine and serotonin: Specificity of release with rewarding electrical stimulation of the brain

Using a push-pull cannula method the amygdala of rats was perfused to examine the release of labeled norepinephrine (NE) and labeled serotonin (5-HT) during electrical stimulation of the brain (ESB) determined by prior behavioral testing to be rewarding or non-rewarding. Simple sensory stimulation was used during perfusion to examine further the degree of specificity of release of these amines. Highly rewarding ESB, but not the sensory stimulation, was accompanied by release of both NE and 5-HT. Varying current intensity had significant effects on the amount of these amines released. Furthermore, non-rewarding ESB was accompanied by inhibition of release of NE and 5-HT and a control substance, urea, was not significantly released during rewarding ESB. The results were discussed as implicating both noradrenergic and serotonergic mechanisms in the mediation of reinforcement.     

Effects of reserpine on sleep and brain biogenic amine levels in the cat

Cats receiving 0.15 mg/kg of reserpine, i.p., had rapid eye movement (REM) sleep suppressed for two days and showed increased ponto-geniculo-occipital (PGO) spikes in waking and slow-wave sleep; these effects were not reversed by administration of dopa. Reserpine (0.125 mg) given intraventricularly, however, significantly increased REM sleep and did not change the frequency of PGO spikes in waking and slow-wave sleep. The differences in sleep characteristics seen between i.p. and central administration of reserpine are probably due to (a) the marked peripheral autonomic side effects accompanying reserpine, i.p., and (b) the much smaller central depletion of brain monoamines following intraventricular reserpine. Also, the correspondance between the regional levels of brain biogenic amines and the occurrence of the sleep states is poor since REM and slow-wave were normal on the third day after reserpine, i.p., while norepinephrine and serotonin levels were depressed by about 50% at that time.This study was supported by grants MH-02211, MH-10625 and RR-05528.     

Locus coeruleus and labile memory

Memory lability is defined as the period of time recently-formed memory remains susceptible to experimental modification. Electrolytic lesions delivered through chronic indwelling electrodes to the locus coeruleus (LC) complex of mice, made shortly after learning, resulted in an extension of memory lability. Mice with unilateral, but not bilateral LC damage became amnesic following electroconvulsive shock (ECS) administered 7 days (168 hr) after memory formation. ECS had no effect on memory in LC-lesioned mice when administered 14 days following training. In a second experiment, the temporal relationship between time of memory formation and time of LC damage was found to be critical to the occurrence of this extended period of lability. In a third experiment, we tested the possibility that prolonged trace lability was the result of weaker memory formation as reflected by decreased persistence (i.e. faster forgetting) of the memory. The results indicated equal rates of forgetting for normal and LC-lesioned mice. Present results support the hypothesis that the locus coeruleus complex normally plays an important role in delimiting the time-course of initially labile stages of memory. By inference, these data suggest further that such a delimiting function of the locus coeruleus is mediated through its noradrenergic modulation of other brain regions.     

A probable role for norepinephrine in feeding after hypothalamic injection of morphine

When morphine is instilled directly into the ventromedial hypothalamus of rats there is a latent period followed by a prolonged bout of feeding. This enhanced activity may be mediated by the release of norepinephrine; for morphine-induced feeding was depressed by the α-adrenergic receptor blocker phentolamine. Several neurotransmitter agonists and antagonists failed to duplicate this action: propranolol, serotinin, methysergide, apomorphine and haloperidol were ineffective in modifying ingestion elicited after morphine. Unlike apomorphine, dopamine augmented morphine's a feeding effect. This difference may exist because dopamine acts as a precursor for norepinephrine formation in local ventromedial hypothalamic neurons.     

Thermoregulation in the rat: deficits following 6-OHDA injections in the hypothalamus

Bilateral microinjections of 6-hydroxydopamine (6-OHDA) were made in a volume of 0.5--0.75 microliter through chronically implanted cannulae into anterior hypothalamic, preoptic loci. Sites were selected at which 1.0 to 12.5 microgram of norepinephrine (NE) had previously elicited a fall in the rat's body temperature. After 2.0 to 6.0 microgram of 6-OHDA were injected in the same volume at the same loci, a comparable hypothermia ensued. When the rats were exposed repeatedly for one-hour intervals to an environmental temperature of either 35.0 degrees C or 8.0 degrees C, they were unable to thermoregulate against the heat and their colonic temperature rose. In some experiments, the rats also failed to defend adequately against the cold ambient temperature, but mainly following the microinjection of the higher doses of 6-OHDA . The intakes of food and water were generally suppressed; this was accompanied by a transient decline in body weight. Overall, the severity, duration and direction of the thermoregulatory impairment depended upon the anatomical site of injection and the dose regimen of the neurotoxin employed. These results offer further evidence that an intact catecholaminergic pathway within the anterior hypothalamus is required for the rat's physiological control of heat loss in a warm environmental temperature.     

Temporary increase in forebrain norepinephrine turnover in mouse-killing rats

Increased forebrain norepinephrine levels and turnover occur in the aggressive mouse-killing rat 2 hr after a killing episode. These neurochemical effects persist for 24 hr and return to normal by 48 hr. Such changes appear to be related to the killing episode (i.e. stress) and unrelated to aggressiveness. Conceivably, activation or induction of tyrosine hydroxylase might be responsible for these effects.