The action of norepinephrine in the rat hippocampus. I. Iontophoretic studies

Brain chemistry and behaviour

The functional organization of chemically transmitting synapses in the brain are described with special emphasis on recent studies demonstrating the localization of different transmitters to specific anatomical circuitries. The use of pharmacological tools for manipulating levels of chemical transmitters is referred to briefly, but particular attention is given to the problems of studying the function of these pathways with lesion techniques. Noradrenaline (NA) and dopamine (DA) are selected for detailed consideration and experimental evidence reviewed, suggesting that these two catecholamines in the forebrain serve different functions: NA with processes of attention essential for learning, and DA with the execution of appropriate responses. Hypotheses suggesting dysfunction of forebrain DA and NA systems in schizophrenia are discussed.

Beta-adrenergic-receptor localization by light microscopic autoradiography

beta-Receptors were identified in rat brain by a light microscopic autoradiographic technique. The procedure involved binding 3H-labeled dihydroalprenolol to beta-receptors in intact slide-mounted tissue sections and generating autoradiograms by the apposition of emulsion-coated cover slips, Biochemical analysis of the binding indicated that these conditions provided a high degree of selective labeling of beta-receptors. High densities of receptors were found in superficial layers of the cerebral cortex, throughout the caudate-putamen, in the periventricular nucleus of the thalamus, in the molecular layer of the cerebellum, and in other areas. These results are in agreement with other electrophysiological and histochemical data. This radiohistochemical approach should be an important addition to other methods for mapping functional catecholamine neuronal pathways and sites of hormonal action.

Origin of dopaminergic innervation of the rat hippocampal formation

The origin of the dopaminergic afferents to the rat hippocampal formation has been investigated by measuring dopamine and DOPAC contents in this area after electrolytic or chemical lesion of the ventral tegmental area (A10) or substantia nigra (A9). The present study indicates that dopaminergic afferents to the hippocampal formation originate from the A10 and A9 dopaminergic cell groups. The 'anterior' hippocampal formation receives a major input from the A10 area whereas the 'posterior' hippocampal region receives dopaminergic afferents from both A9 and A10 cell groups. The dopaminergic afferents are entering the hippocampal region mainly through the dorsal route.

Intracellular studies on transmitter effects on neurones in isolated brain slices

Brain slice preparations seem to offer an opportunity to study the mechanism of action of some drugs. They allow long-lasting intracellular recording of good quality. Measurement of membrane parameters may be made under conditions in which drugs may be applied to the bath, by the microdrop method or by one or several iontophoretic electrode assembles to various positions on the same cell. Combined with microsurgery and various pre-treatments of the animal, the slice technique may extent our knowledge on the detailed mechanisms of drug interaction with particular neurons. A useful development of this technique probably requires a continuous comparison between the results obtained from slices with those acquired from experiments on the same cell types in intact preparations.

Activation of lateral geniculate neurons by norepinephrine: mediation by an alpha-adrenergic receptor

Adrenergic receptors in the vicinity of neurons in the lateral geniculate nucleus (LGN) of the rat were pharmacologically characterized using extracellular single-cell recording and microiontophoretic techniques. Application of norepinephrine (NE) at low iontophoretic currents (1-15 nA) produced a delayed activation of most LGN neurons. This activation was mimicked by various sympathomimetic amines. The relative potency series of agonists was typical of postsynaptic alpha-adrenergic receptors: epinephrine greater than NE greater than phenylephrine greater than or equal to alpha-methylnorepinephrine greater than dopamine greater than isoproterenol. The alpha-antagonists phentolamine, piperoxane and WB-4101, when applied at low iontophoretic currents (less than 10 nA), produced a selective, dose-dependent and reversible blockade of the response to NE. The beta-antagonist sotalol had weak and variable effects at these currents. At low currents, the presynaptic alpha-agonist clonidine was also able to block the response to NE but, at higher currents, produced a partial activation of some units, suggesting that it is a weak agonist. The ability of sympathomimetic amines to activate LGN neurons correlates well with their reported affinities for brain alpha1-adrenoceptors labeled with [3H]WB-4101. It is concluded that NE activates neurons in the LGN via a postsynaptic or alpha1-adrenergic receptor.

Hippocampal alpha- and beta-adrenergic receptors: comparison of [3H]dihydroalprenolol and [3H]WB 4101 binding with noradrenergic innervation in the rat

The relationship of alpha- and beta-adrenergic receptors to the noradrenergic innervation of the rat hippocampal formation was studied with histofluorescent, biochemical, and radioligand binding methods. The dentate gyrus received a major portion of the innervation and contained twice the norepinephrine content of the hippocampal gyrus. However, the density of beta-adrenergic receptors, determined by [3H]dihydroalprenolol (DHA) binding, was approximately equal in both gyri. By contrast, a presumed alpha-adrenergic binding site was relatively concentrated in the dentate gyrus, roughly correlating with the distribution of the noradrenergic innervation. Although a significant concentration of beta-adrenergic receptors in the pyramidal cell layer had been suggested in previous studies with fluorescent analogs of propranolol, direct microchemical measurements of [3H]DHA binding in stratum pyramidalis and stratum radiatum showed that beta-adrenergic receptors were uniformly distributed in the hippocampal gyrus. These data, demonstrating a ubiquitous distribution of beta-adrenergic binding in a brain region with a well-defined noradrenergic input, suggest that a portion, perhaps a majority, of beta-adrenergic receptors are not associated with noradrenergic nerve terminals.

Seizures and related epileptiform activity in hippocampus transplanted to the anterior chamber of the eye: modulation by cholinergic and adrenergic input

Transplants of rat hippocampus into the anterior chamber of the eye of a host animal were used to assess the effects of cholinergic and adrenergic neuronal inputs on the generation and duration of seizure activity. Cholinomimetics initiated both seizures and hypersynchronous neuronal activity in the transplants. Cyclic guanosine monophosphate (GMP) derivatives and isobutyl methylxanthine elicited similar changes. Reflex activation of the cholinergic parasympathetic input to the iris and transplant by illumination of the ipsilateral retina also induced seizures or increased the rate of penicillin-induced interictal spike discharge. Application of beta-adrenergic agonists inhibited interictal spikes and paroxysmal depolarizing shifts induced by penicillin. Fluorescence histochemical studies showed that host sympathetic adrenergic fibers derived from the ground plexus of the iris invaded the transplant to form fine varicose nerve terminals. Activation of these adrenergic afferents to the transplant diminished both the amplitude and frequency of penicillin-induced epileptiform activity. Epileptiform activity in hippocampal occular transplants is strongly modulated by cholinergic and adrenergic neuronal inputs, with the former exerting a facilatory influence and the latter, an inhibitory effect.

Catecholamines and convulsions

Severe depletion of brain noradrenaline and separately of brain dopamine was induced in rats by intracerebral injection of the selective neurotoxin 6-hydroxydopamine, and the susceptibility of the treated animals to various seizure-inducing manipulations was examined. A significant potentiation of the seizures induced both by Metrazol and by electroconvulsive shock was found in animals depleted of brain noradrenaline, but no alteration was seen after depletion of brain dopamine on either measure. The catecholaminergic drug cocaine also induced seizures, but these were found not to depend on either brain noradrenaline or dopamine as they continued to occur in the virtual absence of either catecholamine. It is concluded that cocaine induces seizures by a non-specific toxic mechanism and that noradrenaline, but not dopamine, is involved in reducing the suceptibility of the central nervous system to the several distinct forms of seizure induction examined.

Functional reinnervation of rat hippocampus by locus coeruleus implants

Transplants of the embryonic locus coeruleus (LC) region were implanted into the circuity of the hippocampal formation in adult rats in which the normal adrenergic afferents to the hippocampus had been removed. The growth of new adrenergic axons from the implant in the denervated hippocampus was followed for 1-14 months after surgery by means of fluorescence histochemistry, and the function of the implant-hippocampal connections was tested electrophysiologically after 2-3 months survival. In the successful cases the entire hippocampal formation was reinnervated from the LC implant within 3-6 months after operation, and the newly formed innervation still persisted unchanged by 14 months. The reinnervation was equally effective irrespective of the route by which the axons entered the hippocampus, i.e. along the lesioned fornix-fimbria or along a retrosplenial route. The pattern formed by the ingrowing LC axons mimicked to a large extent that of the normal LC afferents. Little growth was seen into denervated terminal fields of the commissural, septal or entorhinal afferents, pointing to a preference of the ingrowing LC fibers for the areas normally innervated by adrenergic afferents. In the electrophysiological experiments, stimulation of the LC implants caused (in 20 out of 29 cells monitored) an inhibition of the spontaneous activity of neurons in the host hippocampus. This inhibition had a relatively long latency and a long duration, similar to that observed after stimulation of the innate LC in the intact rat. As in the normal rat, the inhibitory responses were blocked by systemic or local application of the beta-adrenergic receptor blockers propranolol or sotalol. It is concluded that the adult rat brain is capable of carrying out all steps involved in correct functional reinnervation of a denervated region. Moreover, the implant-hippocampal preparation should be a highly suitable model system for functional studies of a central noradrenergic connection.

Theories of the dorsal bundle extinction effect

Selective destruction of the noradrenaline systems in the rat brain using the neurotoxin 6-hydroxydopamine has been found to cause resistance to extinction in a number of behavioural situations. Several theories concerning the behavioural mechanism altered by the lesion, and hence about the role of noradrenaline in normal brain functioning, are proposed and evaluated. Theories suggesting a role for noradrenaline in activity, perseveration, internal inhibition, frustrative non-reward, motivation, or secondary reinforcement, fail to explain all the available evidence and direct tests of each theory fails to support its predictions. A model which suggests that noreadrenaline is involved in attentional behaviour, specifically in filtering out or learning to ignore irrelevant environmental stimuli, is successful in explaining all available data and direct tests of the lesioned rats' attentional capacity serve to confirm many of the predictions of an attentional theory of the dorsal bundle extinction effect.

Failure of chronic lithium treatment to block tricyclic antidepressant-induced 5-HT supersensitivity

The ability of chronic lithium administration to modify tricylic antidepressant-induced supersensitivity development in cells receiving 5-hydroxytryptamine (5-HT) input was investigated using microiontophoretic techniques. In these experiments, chronic chlorimipramine (or imipramine) administration for a period of 14 days resulted in a 5-fold increase in the sensitivity of hippocampal pyramidal cells to iontophoretically applied 5-HT. This supersensitivity was not blocked by the concurrent administration of lithium. The data suggests that blockade of supersensitivity development by lithium as previously demonstrated in the dopamine system may not be generalized to all central amine systems.

Prostaglandin E1 raises the cAMP content of peripheral nerve tissue

Prostaglandins of the E type have been found to increase the cAMP content in peripheral nerve tissue of several species. In rabbit vagus nerve, for example, the effect is rapid (1--5 min), it can be elicited by 10(-6) M PGE1 and it is maximal at 10(-5) M. In this preparation the increase of the cAMP content caused by PGE1 is about 4-fold, whereas it is about 2-fold in the vagus of calf. PGE1 also causes an increase in cAMP level in the superior cervical ganglion of calf, rat and rabbit. In these tissues, however, the presence of a phosphodiesterase inhibitor is required for cAMP accumulation.

A pharmacologic study of analgesia produced by stimulation of the nucleus locus coeruleus

Pharmacologic studies of analgesia produced by stimulation of the nucleus locus coeruleus (LC) were conducted using the rat hot-plate test. A correlation between self-stimulation and analgesia produced by stimulation of LC was found. Analgesia produced by LC stimulation was attenuated by naloxone, a morphine antagonist, cyproheptidine, a serotonin antagonist, and WB-4101, an alpha-adrenergic antagonist. The analgesia was absent in 6-OHDA-treated rats. Catecholamine synthesis inhibition by a combination of reserpine and AMT or more specific inhibition of noradrenaline synthesis by DDC elevated latency to paw lick and yet did not affect stimulation-produced analgesia. It is suggested that morphinergic, serotonergic, and alpha-adrenergic mechanisms mediate LC stimulation produced analgesia.

Inhibition of neuronal firing by opiates: evidence against the involvement of cyclic nucleotides

1. Extracellular recordings were made in vitro from single neurones of the myenteric plexus of the guinea-pig ileum. 2. Neuronal firing was inhibited by morphine and normorphine (10 nM to 1 micrometer). Cyclic adenosine 3',5'-monophosphate (cyclic AMP) (100 micrometer to 1 mM) also inhibited the firing of the majority of the neurones. Prostaglandin E2 usually caused a short-lasting excitation of myenteric neurones and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine was usually without effect on firing rate. 3. The inhibition of neuronal firing by normorphine was unaffected by prior and/or concurrent administration of cyclic AMP, dibutyryl cyclic adenosine 3',5'-monophosphate, prostaglandin E2 or 3-isobutyl-1-methylxanthine. As these four treatments might be expected to elevate intracellular levels of cyclic AMP, the results lend no support to the notion that a reduction in intracellular cyclic AMP is essential to the inhibition of firing produced by morphine.

Depletion of brain noradrenaline, but not dopamine, by intracerebral 6-hydroxydopamine potentiates convulsions induced by electroshock

Intracerebral injection in rats of 4 microgram of the catecholamine neurotoxin 6-hydroxydopamine was used to deplete forebrain noradrenaline to less than 10% of control values and separately to deplete brain dopamine to less than 15% of control. The susceptibility of these animals to electroconvulsive shock-induced convulsions was examined, and a significant potentiation of the response was seen in the rats depleted of noradrenaline but not in those depleted of dopamine. The duration of the convulsion was significantly increased as a result of loss of forebrain noradrenaline.

Development of the noradrenergic innervation of neocortex

The development of the noradrenaline (NA)-neuron innervation of rat neocortex was studied by fluorescence histochemistry, high affinity uptake of [3H-]NA, and biochemical assay of regional NA content. Fluorescence histochemistry indicates that NA axons enter areas of developing neocortex prenatally and the innervation matures rapidly during early postnatal life. Frontal and lateral neocortical areas are the first to be innervated followed by occipital and parietal areas. All cortical layers receive innervation. The distribution and density of neocortical NA innervation achieves the adult pattern by the end of the first postnatal week. High affinity uptake studies confirm the observations from fluorescence histochemistry and show a very rapid maturation of the NA axon innervation with adult levels of uptake occurring by postnatal day 9. Following birth, there is a brief rise in NA content from PO to P2 in all neocortical areas. NA content then drops to low levels in all areas by P4. This is followed by a gradual increase in NA content in all areas occuring over several months. This pattern of development of NA axon innervation of neocortex demonstrates that the density and distribution of NA axons in developing neocortex matures much earlier than shown in previous studies whereas the NA content of the developing axonal plexus achieves adult levels later in postnatal life.

Modulatory role for biogenic amines in the cerebral cortex. Microiontophoretic studies

In order to investigate the mode of action of biogenic amines in rat cerebral cortex, the unitary activity of spontaneously firing neurons and their excitatory response to acetylcholine (ACh) were examined using microiontophoretic administration of dopamine (DA), noradrenaline (NA) and serotonin (5-HT). The predominant effect of these biogenic amines on the spontaneous activity was a profound and prolonged inhibition of firing (2-4 min), which attained its maximum within 15-120 sec. This response was generally more abrupt in onset and of greater magnitude with NA and 5-HT than with DA. Most units inhibited by DA, NA and 5-HT also showed marked depression of their excitatory response to ACh when pretreated with these biogenic amines. With repetitive administration of ACh, it could be shown that the total duration of inhibition of ACh responses by DA and NA was not as prolonged as the inhibition of the spontaneous firing of the same cells. With 5-HT, the initial ACh responses of many neurons could be completely blocked, and this inhibitory effect lasted as long as the inhibition of spontaneous firing. In view of the anatomical data demonstrating a relative sparsity of monoamine nerve terminals in cerebral cortex, the strong inhibition induced by DA, NA or 5-HT may have reflected slow inactivation of the biogenic amines. However, it could also be indicative of underlying mechanisms of action dependent on metabolic changes. Indeed, the interaction between biogenic amines and ACh might imply a balance between the intracellular pools of cAMP and cGMP is directly or indirectly influenced by the biogenic amines and ACh, respectively. This hypothesis would not exclude other modes of local interaction between DA, NA, 5-HT and ACh, and appears compatible with the modulatory role of biogenic amines in cerebral cortex.

Hypersensitivity to histamine in the guinea-pig brain: microiontophoretic and biochemical studies

Electrolytic lesions of the medial forebrain bundle induce a fall in histidine decarboxylase activity (the specific synthetic enzyme of brain histamine) in the ipsilateral cerebral cortex and hippocampus of the guinea pig brain; these results suggest the presence of an ascending histaminergic pathway in the guinea pig brain similar to that described in the rat. Possible alterations in the sensitivity of histaminergic receptors present in the target areas were studied following this type of lesion by combining electrophysiological and biochemical approaches. Microiontophoretic applications of histamine or noradrenaline reveal a hypersensitivity (lower ejecting currents for threshold and maximal responses) in cortical neurons ipsilateral but not contralateral to the lesion, whereas responses to iontophoretically applied GABA are not modified. In contrast the responsiveness of histamine-sensitive cyclic AMP generating systems is not modified, neither in the cerebral cortex nor in the hippocampus after this type of lesion. Similar conclusions are reached from the data obtained with specific agonists of the two classes of histaminergic receptors and measurements in the presence of a phosphodiesterase inhibitor. Several hypotheses are discussed in order to reconcile the finding of a denervation hypersensitivity revealed by iontophoresis contrasting with an unaltered responsiveness of the histaminergic receptors linked to the adenylate cyclase.

The in vivo binding of [3H]-desipramine and [3H]-chlorpromazine to areas in the rat brain

The distribution of [3H]-desipramine (DMI) and of [3H]-chlorpromazine (CPZ) in rat brain was determined by the incorporation of radioactivity into various regions of the brain and by autoradiography of transverse cryostat sections. The label from [3H]-DMI was rapidly distributed in all brain regions, reaching peak levels within 30 min and considerably decreasing 1--4 h after injection. Following the selective destruction of catecholaminergic nerve terminals by intracerebral administration of 6-hydroxydopamine, a marked reduction in the incorporation of DMI, but not of CPZ, was evident in all brain areas investigated. The autoradiographed sections clearly demonstrated a preferential uptake of both drugs by the caudate nucleus. These findings suggest that DMI might be largely bound to presynaptic dopamine and norepinephrine terminals, while the CPZ binding involves postsynaptic sites.

Comparison of the electrocortical changes induced by (+)-amphetamine and chlorpromazine when perfused directly into the dorsal raphé nucleus of the cat

1 (+)-Amphetamine mimicked the intermittent and sustained electrocortical desynchronization produced by (-)-noradrenaline (NA) when perfused directly into the dorsal raphé nucleus of cat encéphale isolé preparations. 2 The effects of amphetamine or NA were abolished or significantly attenuated by prior application of (-)-propranolol. 3 The effect of amphetamine, but not that of NA, was blocked by prior applications of guanethidine or chlorpromazine (CPZ). 4 Desmethylimipramine (DMI) produced dose-related changes in electrocortical activity which were similar to those induced by NA when applied to the same sites within the dorsal raphé nucleus. 5 DMI potentiated the effects of both amphetamine and NA, but guanethidine only abolished the DMI-induced potentiation of the amphetamine response. 6 (-)-Propranolol, guanethidine and CPA produced a short period of electrocortical desynchronization at the beginning of the perfusion period before antagonism of the amphetamine response was apparent. 7 The results suggest that CPZ and amphetamine have an action within the dorsal raphé nucleus possibly related to noradrenergic terminals.

Direct mapping of beta-adrenergic receptors in the rat central nervous system by a novel fluorescent beta-blocker

DL-N-(2-Hydroxy-3-napthyloxypropyl)-N'-dansylethylenediamine, dansyl analogue of propranolol (DAPN) is a novel fluorescent beta-adrenergic antagonist with high affinity to beta-receptors. The distribution pattern of DAPN fluorescence was studied in the rat central nervous system subsequent to its intravenous administration to living rats. DAPN distinctly labels specific regions and cells in the central nervous system (CNS). Highly dense DAPN fluorescence was observed in the pyramidal cell layer of the hippocampus, the granule cell layer of the dentate gyrus, the basal layers of the piriform cortex and the neocortex, the cerebellar Purkinje cell layer, and the spinal a-motoneurons. Pretreatment of control rats with DL-and L-propranolol markedly decreased the intensity and density of DAPN fluorescence in the tissue sections, whereas prior administration of D-propranolol had almost no effect. Pretreatment with large doses of reserpine did not alter the pattern of DAPN fluorescence. These findings were identical to those observed with another fluorescent beta-blocker, 9-aminoacridino-propranolol (9-AAP). Our data suggest that fluorescent beta-adrenergic antagonists may be used in vivo for the direct probing of the beta-receptors within the mammalian CNS.

Neuronal responses to extracellularly applied cyclic AMP:Role of the adenosine receptor

At low doses, theophylline blocks the neuronal depressant effects of 5'-AMP, but not cyclic AMP. Higher doses (100 mg/kg) block cyclic AMP responses and reduce the effects of noradrenaline and GABA. It is concluded that cyclic AMP does not depress neurones via the adenosine receptor.

Noradrenaline inhibition of acetylcholine release from guinea-pig brain

The effect of noradrenaline (NA) on acetylcholine (ACh) release from guinea-pig brain was investigated in superfused cerebral cortex slices and in unrestrained unanaesthetized animals provided with epidural cups. The amine reduced the ACh release from electrically stimulated tissue and its effect was antagonized by phentolamine and phenoxybenzamine, but not by propranolol and spiroperidol. The injection of NA (150 microgram) into the cerebral ventricles caused sedation, E.Co.G. synchronization and reduced ACh outflow from the parietal cortex. This inhibition was counteracted by alpha-blocking agents. A lower dose of NA (50 microgram) did not change the behaviour, but produced a late increase in ACh outflow, prevented by spiroperidol. These results fit well with the hypothesis that NA restrains, via alpha-receptors, the ACh secretion from the nerve endings and indirectly support the view that the amine reduces the firing rate of the corticopetal cholinergic neurones. The late increase in ACh outflow, observed in vivo, may be referred to secondary activation of the dopaminergic neurones, known to enhance the cortical ACh release in this animal species.

Long-term potentiation and depression of synaptic responses in the rat hippocampus: localization and frequency dependency

1. The consequences of repetitive activation of excitatory synaptic inputs to the CA1 pyramidal cells of rat hippocampus have been studied using in vitro techniques. 2. Single stimulation trains of 100 pulses at rates of 5-100/sec resulted in potentiation of population spike amplitudes lasting the duration of a 5 min test period in thirty-four out of thirty-five cases. Trains of 100 pulses delivered at 1/sec resulted in depression of the stimulated pathway in ten out of twelve experiments. 3. Responses to test stimulation of other excitatory inputs to the same cell population were depressed following conditioning trains at frequencies in the range 1-100/sec. Depression was seen both in the population spike amplitude (reflecting synchronous cell discharge) as well as the extracellularly recorded population e.p.s.p., and appeared to be maximal at lower frequencies. 4. Trains of antidromic stimulation of the CA1 cell population produced subsequent decreases in synaptically evoked responses, indicating that repetitive firing of pyramidal neurones or interneurones do not cause potentiation, but may be involved in heterosynaptic depression. 5. The results suggest that potentiation and heterosynaptic depression arise from different mechanisms, and that potentiation is confined to the set of terminals activated by a conditioning train, whereas the depression is generalized to the whole neurone.

Proposed effects of brain noradrenaline on neuronal activity and cerebral blood flow during REM sleep

We propose that the observed increases of both neuronal activity and cerebral blood flow seen throughout the brain during REM sleep may be effects of decreased central noradrenaline release.

A presynaptic site of action within the mesencephalic reticular formation for (+)-amphetamine-induced electrocortical desynchronization

1. Changes induced in the electrocorticogram by the bilateral perfusion of (+)-amphetamine into the mesencephalic reticular formation (MRF) have been studied in cat encéphale isolé preparations. 2. (+)-Amphetamine, applied for 5 min in the MRF, mimicked the electrocortical desynchronization induced by the perfusion of (-)-noradrenaline (NA) or (-)-alpha-methylnoradrenaline (AMNA) into the same sites. 3. Perfusion of 6-hydroxydopamine (6-OHDA) also induced desynchronization but, over the 1 h perfusion period, slow wave activity gradually returned to the electrical record. 4. Following the application of 6-OHDA the effect of (+)-amphetamine was abolished or significantly attenuated, whereas the effect of NA or AMNA was not affected. 5. The electrocortical desynchronization induced by (+)-amphetamine could be restored if its application was preceded by perfusion with NA or AMNA. 6. Fluorescence studies using AMNA indicated that 6-OHDA depleted noradrenergic nerve terminals near the cannulae tips. However, the terminals were still capable of taking up exogenously applied AMNA. 7. These results suggest that (+)-amphetamine has a presynaptic action on noradrenergic nerve terminals within the MRF.