Brain stem self-stimulation attenuated by lesions of medial forebrain bundle but not by lesions of locus coeruleus or the caudal ventral norepinephrine bundle

Medullary Norepinephrine Projections Release Norepinephrine into the Contralateral Bed Nucleus of the Stria Terminalis

Central norepinephrine signaling influences a wide range of behavioral and physiological processes, and the ventral bed nucleus of the stria terminalis (vBNST) receives some of the densest norepinephrine innervation in the brain. Previous work describes norepinephrine neurons as projecting primarily unilaterally; however, recent evidence for cross-hemispheric catecholamine signaling challenges this idea. Here, we use fast-scan cyclic voltammetry and retrograde tracing to characterize cross-hemispheric norepinephrine signaling in the vBNST. We delivered stimulations to noradrenergic pathways originating in the A1/A2 and locus coeruleus and found hemispherically equivalent norepinephrine release in the vBNST regardless of stimulated hemisphere. Unilateral retrograde tracing revealed that medullary, but not locus coeruleus norepinephrine neurons send cross-hemispheric projections to the vBNST. Further characterization with pharmacological lesions revealed that stimulations of the locus coeruleus and its axon bundles likely elicit vBNST norepinephrine release through indirect activation. These experiments are the first to demonstrate contralateral norepinephrine release and establish that medullary, but not coerulean neurons are responsible for norepinephrine release in the vBNST.

Noradrenaline is necessary for the hedonic properties of addictive drugs

To determine whether noradrenaline (NA) is an essential neurotransmitter for addictive and appetitive behaviors, we measured drug and food seeking in transgenic mice lacking dopamine beta-hydroxylase (Dbh), the enzyme responsible for synthesizing NA. Using the conditioned place preference test (CPP), we show that Dbh -/- mice do not exhibit rewarding behavior to morphine, cocaine, or the mixed reuptake inhibitor bupropion. In spite of their lack of preference for drugs, Dbh -/- mice had an unaltered preference for food. Drug seeking was induced when NA was restored to the central nervous system of Dbh -/- mice by administration of l-threo-3,4-dihydroxyphenylserine (DOPS) and carbidopa. When a NK1 receptor antagonist was co-administered with morphine or cocaine, it produced aversive behavior in Dbh -/- mice while it abolished place preference in the controls. NK1 antagonists alone did not have any rewarding or aversive effect in the CPP suggesting that substance P opposes some of the unpleasant effects of morphine and cocaine. Our results show that NAergic transmission is necessary for motivated behaviors, the dysregulation of which is a co-morbid factor of many depressive states. The reversibility of this phenomenon, by restoring NA, indicates that even when this behavioral deficit is genetically determined it can be reversed.

The central extended amygdala network as a proposed circuit underlying reward valuation

The phenomenon of medial forebrain bundle self-stimulation offers a powerful model of reward-based behavior. In particular, it appears to activate a neural system whose natural function is to compute the survival value or utility of present stimuli and to help orchestrate responses toward those inputs. Although the anatomical identity of this system is as yet unknown, recent descriptions of anatomical macrosystems within the basal forebrain lead to the proposal that it may be largely contained within the central extended amygdala network. This paper reviews decades' worth of behavioral and neurophysiological investigations of brain stimulation reward that support or are at least consistent with this idea. The proposed network circuitry underlying self-stimulation is also placed into the larger context of basal forebrain function, specifically, the role of the ventral striatopallidum in linking motivation to behavior, the role of the amygdala in detecting motivationally significant inputs, and the role of the magnocellular complex in communicating reward information to cortical and hippocampal targets.

Electrical self-stimulation in the central amygdaloid nucleus after ibotenic acid lesion of the lateral hypothalamus

This experiment was carried out in order to investigate the involvement of lateral hypothalamus (LH) in electrical self-stimulation of the central amygdaloid nucleus (CeA). Adult male Sprague-Dawley rats were bilaterally implanted with a guide cannula situated above each LH and with two electrodes in the CeA. Self-stimulation was subsequently obtained separately from both right and left electrodes. The LH was then lesioned unilaterally by ibotenic acid (IBO) injection. Eight days later, the effect of this unilateral lesion on self-stimulation of the ipsilateral and contralateral CeA was tested. Then the neurons of the remaining non-lesioned LH side were lesioned with IBO and self-stimulation was tested 15 days after the second lesion. Both unilateral as well as bilateral lesions of LH produced a significant decrease in CeA self-stimulation rates but had no significant effect on the reward effectiveness. The unilateral lesions did not produce any modification of the rate-intensity function in the contralateral CeA. This lesion-induced depression in performance was reversed by treatment with phenobarbital. These results provide clear evidence that the rewarding effects of CeA electrical stimulation do not result from the activation of the LH outputs and that the apparent decrease in CeA self-stimulation may result from the LH lesion-induced increase in the frequency of epileptiform manifestations that occur following amygdaloid stimulation.

The relationship between self-stimulation and sniffing in rats: does a common brain system mediate these behaviors?

The relationship between brain self-stimulation and brain-stimulation induced sniffing behavior was examined at three brain sites (frontal cortex, hypothalamus and lower brain stem). In the first experiment, sniffing was elicited in the prefrontal cortex and pontine reticular formation (PRF) of anesthetized rats. These sites corresponded to reported self-stimulation sites. In non-anesthetized animals (Expt. 2), all self-stimulation sites in the medial prefrontal cortex (MPC) and lateral hypothalamic-medial forebrain bundle (LH-MFB) also supported sniffing. In the PRF, this was also the case except for one subject which exhibited self-stimulation and jaw movements without sniffing. After unilateral lesions either in the MPC or PRF, stimulation-induced sniffing from the ipsilateral LH-MFB was not influenced. While MPC lesions did not affect self-stimulation either, medial PRF lesions disrupted ipsilateral self-stimulation. In summary, stimulation-induced sniffing and self-stimulation behavior appear to share strikingly similar anatomical loci, but the PRF appears to be differentially involved in these behaviors. The results were discussed from an appetitive motivational hypothesis of self-stimulation.

Excitatory amino acid pathways in brain-stimulation reward

A range of agonists and antagonists active at different glutamate/aspartate (Glu/Asp) receptor subtypes were injected into rat ventral tegmental (VTA) sites downstream from self-stimulation electrodes in the medial forebrain bundle. Control injections were made into the contralateral tegmentum. Variable-interval (VI 10 s) self-stimulation was not significantly affected by a specific antagonist of N-methyl-D-aspartate (NMDA)-type receptors (D,L-2-amino-5-phosphonovaleric acid (2-AP5), 10 and 50 nmol). Broad-spectrum excitatory amino acid (EAA) antagonists viz cis-2,3-piperidine dicarboxylate (cPDA) (10 and 50 nmol), gamma-D-glutamylaminomethyl sulphonic acid (GAMS) (10 nmol) and p-chlorobenzoyl-2,3-piperazine dicarboxylic acid (pCB PzDA) (2.0 and 10 nmol), active at kainate, quisqualate, as well as NMDA receptors, all produced significant depression of responding when injected into the ipsilateral, but not the contralateral, tegmentum. Compounds inhibiting Glu/Asp reuptake had variable effects: strong depression with dihydrokainic acid (7.5 nmol), or no significant effect (L-threo-3-hydroxyaspartic acid, 2.0 and 10 nmol). The receptor agonist, NMDA (10 nmol), depressed responding regardless of injection side; kainic and responding regardless of injection side; kainic and quisqualic acid elicited myoclonic and other non-specific responses in preliminary tests, and were not examined further; enhanced responding was not seen. The side-specific blockade of responding by non-NMDA antagonists indicates the existence of non-NMDA EAA terminals in the VTA, signalling the receipt of hypothalamic brain-stimulation reward. Caudally directed EAA projections terminating on A10 dopamine cell bodies may account for depression of self-stimulation by EAA antagonists.

Effects of 5-HT and alpha 1 adrenoceptor antagonists on kappa opioid-induced sedation

The kappa opioid agonists PD-117302 and U-50488 were found to produce dose-dependent reductions in spontaneous locomotor activity in mice. The magnitude of the response to a given dose of each kappa agonist was found to be clearly potentiated by pretreating the animals with either ketanserin (1 mg/kg) or prazosin (0.5 mg/kg). Pretreatment with the selective 5-HT2 receptor antagonist ritanserin given at a high dose (1 mg/kg), the nonselective 5-HT antagonist methysergide or the 5-HT synthesis inhibitor parachlorophenylalanine did not alter the magnitude of the response to the kappa agonist. These results suggest that 5-HT systems are not involved in the sedative effects of kappa opioid agonists and that the potentiating effect seen in animals pretreated with ketanserin is due to the alpha 1 blocking properties of this compound since the effect was mimicked by the alpha 1 antagonist prazosin.

Cholinergic antagonists in ventral tegmentum elevate thresholds for lateral hypothalamic and brainstem self-stimulation

Frequency thresholds for lateral hypothalamic self-stimulation are elevated following microinjections of atropine into ventral tegmentum (73). Many self-stimulation sites in brainstem are situated near cholinergic cell groups and axons, and ventral tegmentum receives cholinergic afferents terminals. To test the hypothesis that ventral tegmental muscarinic receptors are involved in lateral hypothalamic and brainstem self-stimulation, stimulating electrodes were placed in lateral hypothalamus and dorsal tegmentum near the midbrain-pons border, and cannulae were implanted in ventral tegmentum. Microgram injections of muscarinic antagonists, atropine or scopolamine, or a choline uptake blocker, hemicholinium-3, elevated frequency thresholds for both self-stimulation sites in a dose-dependent and time-dependent fashion. In addition, summation and collision between the two self-stimulation sites was tested using paired-pulse methods (53). Summation ranged from 31 to 87% (i.e., 24 to 47% reductions in frequency threshold were observed at long intrapair intervals), but no collision-like effects were observed at short intrapair intervals. The ventral tegmentum is a likely site for the convergence of dorsal tegmental and lateral hypothalamic self-stimulation pathways.

Neurotransmitters, pathways and circuits as the neural substrates of self-stimulation of the prefrontal cortex: facts and speculations

Through a multidisciplinary approach considerable progress has been made in understanding the neural substrates of self-stimulation (SS) of the medial prefrontal cortex (MPC). Thus, neuroanatomical studies have revealed that intrinsic neurones in the MPC seem to be the central elements responsible for initiating and maintaining this phenomenon in this area of the brain. Complementary to this central finding are the electrophysiological and neurohistological data reviewed here, showing that neurones in the MPC are directly activated and have monosynaptic feed-back connections with neurones located in areas which also support SS. These findings have given rise to the hypothesis that several single feed-back pathways or single circuits exist between points of SS in the MPC and points of SS in other areas of the brain. This hypothesis implies that SS in a particular area would depend not only on the intrinsic local activity induced by the electrical stimulation but on the functional and specific activity of other nuclei in the brain. The fact that lesions of single circuits, which are apparently involved in SS of the MPC such as the medial prefrontal cortex-ventrotegmental area-medial prefrontal cortex and medial prefrontal cortex-n. dorsomedialis of the thalamus-medial prefrontal cortex, do not produce a permanent decrease of SS, together with the finding that transynaptic connections seem to exist between MPC and other areas of the brain, suggests further that a complex rather than several single independent circuits could be at the neural basis of SS of the MPC. If that were the case, then SS of the MPC would not only depend upon local and single feed-back activity but upon specific functional feed-back activity among the nuclei, which in turn have single feed-back connections with the MPC (see the concept of 'complex circuit' outlined in the section of Behavioural studies). On the basis of this hypothesis no permanent changes should be expected after lesions of single pathways since physiological and even anatomical compensation could be reached through the rest of the undamaged circuit. That terminals containing specific neurotransmitters exist in layers of the PC where electrodes for SS are located has been reviewed in this paper. Some of these neurotransmitters have been suggested to be part of the local substrates activated by SS.(ABSTRACT TRUNCATED AT 400 WORDS)

Brain stimulation reward and dopamine terminal fields. II. Septal and cortical projections

The boundaries and relative sensitivities of the substrates of septal and cortical brain stimulation reward were mapped in relation to the dopamine terminal fields in these regions using a dorsal-ventral moveable electrode. Brain stimulation was rewarding at all levels of the posterior lateral septum and not just in the region of dopamine terminal innervation. Reward thresholds, ease of training, maximum response rates and stability of responding were all unrelated to the proximity of the stimulating electrode to the band of dopamine terminals revealed by glyoxylic acid-induced dopamine fluorescence. Stimulation was also rewarding in the anterior lateral septum; the best sites were in the ventral portions of this region although dopamine terminal fluorescence was uniform throughout. Thus the anatomy of the brain stimulation reward substrate of the lateral septal nucleus does not bear a special relation to the anatomy of dopamine terminals within this region. Stimulation was also rewarding in each of the dopamine terminal fields of the cerebral cortex. The best self-stimulation was obtained with electrodes in the medial frontal cortex; sulcal frontal cortex was next best, entorhinal cortex was next, and pyriform cortex, though reliably positive, supported the weakest self-stimulation. Variations in self-stimulation threshold were seen as electrodes were moved through homogeneous regions of dopamine terminal density in some regions, while stable thresholds were associated with movements through areas of varying dopamine terminal density in others; thus, again, there was no special relation between goodness of self-stimulation and density of dopaminergic innervation. These data suggest that rewarding brain stimulation in these regions is not due to direct activation of either the dopaminergic terminals or the cells that they innervate.

Intrinsic neurons are involved in lateral hypothalamic self-stimulation

The recent technique of using ibotenic acid to lesion selectively local neurons while sparing fibers of passage permitted us to answer a long-standing question: is lateral hypothalamic self-stimulation supported by fibers of passage or are the intrinsic hypothalamic neurons involved? Three groups of adult male Sprague-Dawley rats were used. In a normal group, electrodes were bilaterally implanted in the lateral hypothalamus and self-stimulation (ICSS) was obtained separately with the right and left electrodes, at various current intensities, using a nose-poke response. In the experimental group, the intrinsic neurons of the lateral hypothalamus were destroyed unilaterally by local injection of ibotenic acid (4 or 6 micrograms in 0.5 microliter); the other side served as the sham-lesion control. Ten days later ICSS electrodes were implanted bilaterally, one in the lesioned area, the other in the contralateral hypothalamus. As in the case of the normal animals, the rate of nose-poking (ICSS) was then determined separately for each electrode. In the normal rats, ICSS rates were the same with stimulation on either side and the increase in ICSS rate as a function of the increase in current intensity was the same on each side. In the experimental rats, ICSS of the lesioned side was decreased in all cases; moreover, after lesion with the 6 micrograms dose, ICSS was totally suppressed. Self-stimulation of the sham-lesioned side was not significantly different from that observed in the normal rats. In 6 rats sampled from the lesioned groups as well as in 3 additional unimplanted animals, biochemical assays compared dopamine and serotonin contents of the two striata and noradrenaline and serotonin contents of the two hippocampi. No difference was observed for these two structures between the side ipsilateral to the lesion and the contralateral side. Moreover, none of these monoamine levels differed from those seen in the unimplanted rats. These results, taken together, suggest that intrinsic lateral hypothalamic neurons are involved in ICSS.

Afferent pathways to points of self-stimulation in the medial prefrontal cortex of the rat as revealed by the horseradish peroxidase technique

Afferent projections to points of self-stimulation (SS) in the medial prefrontal cortex (MPC) of the rat were studied using the horseradish peroxidase (HRP) technique. Intracranial microinjections of HRP (30%) were delivered at the same stereotaxic points at which the electrodes eliciting SS were located. Retrogradely transported HRP labeled neurons in different thalamic, hypothalamic, mesencephalic and pontine areas. In the thalamus, labeled neurons were found in the dorsomedial, anteromedial, anteroventral, ventral, ventromedial, posteromedial, paratenial, parafascicular nuclei and n. reuniens. Labeled neurons in mesencephalic areas were found in the n. interpeduncularis, ventral tegmental area (AVT) and substantia nigra (SN). In the pons, labeled neurons were found in the locus coeruleus and in the periaqueductal gray. Other nuclei in which labeled neurons were also found were: lateral hypothalamus (LH), periventricular gray and zona incerta (ZI). Theoretically it is possible that all these afferent areas contribute to SS of MPC. This assumption is discussed and criticized in connection with previous literature on SS. It is suggested that only specific areas and their projections are good candidates for the neural mechanisms involved in the reward produced by electrical stimulation of the prefrontal cortex.

Time course analysis of para-chlorophenylalanine induced suppression of self-stimulation behavior

The hypothesis of a serotonergic basis of reward rests partly on data showing that serotonin (5-HT) depletion by para-chlorophenylalanine (p-CPA) causes depression of self-stimulation (SS) rates. These data do not clearly demonstrate a time course relationship between 5-HT depletion and SS suppression. The present study shows that SS behavior has fully recovered from p-CPA induced suppression when 5-HT levels are still maximally depleted. The data reveal that the effects of electrical brain SS on p-CPA induced reductions of 5-HT cannot explain the temporal dissociation between 5-HT depletion and SS suppression. As a whole the data suggest that midbrain 5-HT neurons are not critically involved in SS behavior.

Intracranial self-stimulation in the thalamus of the rat

Rats were tested for intracranial self-stimulation (ICSS) via bipolar electrodes situated throughout the thalamus. Of 112 animals in the study, 55 met the criteria for ICSS, with scores ranging from 55 to 921 bar-presses in a 15 minute session. A map of both positive and neutral placements is presented. Positive sites for ICSS were found in all aspects of the mediodorsal nucleus, except for the central segment. The ventromedial nuclear complex was also a positive area of ICSS, with the exception of the submedial nucleus (nucleus gelatinosus). Each of the intralaminar nuclei (central medial, parafascicular, paracentral, and central lateral) supported ICSS, as did each of the midline nuclei (rhomboid, paratenial and paraventricular). No placements were found in the nucleus reuniens. Both "major" relay nuclei, the ventrobasal and ventrolateral, supported ICSS; but neither the laterodorsal nor the lateral posterior nuclei had positive caudal thalamus. As a general rule, ICSS scores appeared to be higher as the electrode placements approached the midline. Sites in which no positive placements were seen included the reticular nucleus, as well as the stria medularis, the mammillothalamic tract, and the fasciculus retroflexus.

Intracranial self-stimulation in relation to the ascending noradrenergic fiber systems of the pontine tegmentum and caudal midbrain: a moveable electrode mapping study

Chronically implanted moveable electrodes were used to map the pontine tegmentum and caudal midbrain for intracranial self-stimulation in relation to the ascending noradrenergic systems as revealed by fluorescence histochemistry. In no area tested was there a consistent correlation between the quality or the presence of self-stimulation and the degree of noradrenergic fiber density or cellular aggregation. Of particular importance was the failure to obtain self-stimulation from the locus coeruleus, despite repeated testing and extensive attempts at behavioral shaping. Those areas supporting self-stimulation included the dorsal raphe nucleus, the superior cerebellar peduncle and the mesencephalic and motor nuclei of the trigeminal nerve. These data appear to rule out activation of the ascending noradrenergic systems as an explanation of the rewarding effects of stimulation in these areas. A gustatory-visceral fiber system is suggested as an alternative possible substrate.

Afferent projections to the self-stimulation regions of the dorsal pons, including the locus coeruleus, in the rat as demonstrated by the horseradish peroxidase technique

Afferent projections to the dorsal pons of the rat have been studied using the horseradish peroxidase (HRP) technique. HRP injections were made in each of the following regions: the vicinity of the locus coeruleus (LC); the periventricular gray, medial to the LC; the medial parabrachial region, lateral to the LC; the ventral cerebellum, dorsal to the LC; and the pontine reticular formation, ventral to the LC. Because intracranial self-stimulation (ICSS) has been obtained in these regions, the afferents have been discussed in terms of their possible contributions to the behavior. Previous ICSS studies of the dorsal pons have focussed on the LC as playing a central role. Presently identified inputs to the LC include: the dorsal raphe nucleus: the ventrolateral periaqueductal gray: the pontine reticular formation: the areas that contain the pontine and medullary noradrenergic and adrenergic cell groups: the lateral hypothalamic area: the contralateral LC: the deep cerebellar nuclei: the ventrolateral and parafascicular thalamic nuclei: and the parabrachial regions of the pons and midbrain.

Mesencephalic lesions in female rats resulting in normophagia and reduced body weight

The effects of bilateral mescencephalic lesions in the area of the ventral noradrenergic bundle (VNA) were studied in immature and mature female rats. The food intake consumption of the immature lesioned rats did not differ from sham operated controls whether fed a chow or high fat diet. However, after the surgery the body weights of the two groups began to diverge with the immature lesioned rats obtaining a significantly lower body weight. By the end of the study the immature lesioned rats were also significantly shorter than the controls. Both groups showed normal body composition throughout the measurement period. Upon refeeding after a one day fast the immature female rats defended their lower than normal pre-fast body weight. These data are in good agreement with our earlier findings using immature male rats. The mature lesioned animals also showed normal food consumption when fed a chow type diet. However the lesioned animals did show a brief hyperphagia when placed on a high fat diet. Unlike the immature lesioned rats the body weight of these lesioned animals did not differ from their sham operated controls. It is suggested that lesions in the area of the VNA may result in hyperphagia or, in no effects on food intake or body weight as reported by others or normophagia with reduced body weight. Exact lesion placement may be responsible for these divergent findings.

Intracranial self-stimulation site specificity: the myth of current spread

Locus-specificity of intracranial self-stimulation (ICSS) was determined for 78 electrode placements using monophasic stimulation. ICSS rate comparisons between each pole of a bipolar electrode when each served as cathode were made when the anodal source was either the other pole of the bipolar electrode or a skull screw. In hypothalamic areas, medial forebrain bundle electrode tips elicited significantly higher rates than electrode tips of the same bipolar electrodes located in perifornical or far-lateral diencephalic placements. In turn, perifornical electrode tips elicited higher rates than more dorsally or medially placed tips. In dorsal pontine areas, locus coeruleus electrode tips elicited significantly higher rates than more medially or laterally placed tips. In periaqueductal midbrain and substantia nigra placements, tips located along the midline or in the substantia nigra elicited significantly higher rates than tips located lateral to or ventral to those respective structures. Anodal locus did not change these results. These results suggest that ICSS behavior is delimited by and corresponds to neuroanatomically discrete entities and that cathodal, rather than anodal factors seem to most crucially determine ICSS integrity.

Self-stimulation at the lateral hypothalamus and locus coeruleus after specific unilateral lesions of the dopamine system

A group of rats was trained to press levers for electrical stimulation from bipolar electrodes aimed at the lateral hypothalamus (LH), and another group was trained to self-stimulate from electrodes in the locus coeruleus (LC). All rats in both groups were subjected to unilateral injections of 6-hydroxydopamine into the substantia nigra and midbrain ventral tegmentum. The lesions produced profound depletions of dopamine from the ipsilateral frontal cortex, nucleus accumbens/olfactory tubercle, and corpus striatum. Pretreatment with desmethylimipramine prevented loss of noradrenaline in excess of that caused by electrode implantation. The destruction of the dopamine projections produced a persistent and pronounced deficit in intracranial self-stimulation from ipsilateral electrodes in both the LH and LC groups, but only transient effects on self-stimulation from contralateral electrodes at these sites. These results suggest that an intact dopamine system is required for the expression of self-stimulation behaviour.

Cerebellar inhibition and ICSS from stimulation in the area of the nucleus locus coeruleus

The purpose of this study was to determine the relationship between intracranial self-stimulation (ICSS) and the long-lasting inhibition (LLI) of cerebellar Purkinje cells which are produced by stimulation around the dorsal pontine nucleus locus coeruleus (LC). No strong correlation was found between the dorsal pontine sites which produced LLI and those sites which yielded ICSS. Moreover, ICSS sites were no more effective than non-ICSS sites in producing LLI. LLI of Purkinje cells was produced most effectively by stimulation of an area dorsolateral to the LC where axons arising from the LC collect to ascend to the cerebellum. The LLI produced by stimulation of this dorsolateral region was less often associated with short latency excitations, compared to the LLI produced by stimulation of the cerebellar white matter. This characteristic may be useful as an indication of LC-produced LLI. Sites yielding ICSS were scattered around the LC but were most consistent ventrolateral to the LC. These results indicate that ICSS and LLI of Purkinje cells appear to be independent phenomena which depend on different mechanisms.

Monoamine involvement in hippocampal self-stimulation

The roles of noradrenergic and serotonergic projections to the hippocampus were investigated with respect to their involvement in the intracranial self-stimulation of this structure. In the first study, 6-hydroxydopamine-induced lesions of the dorsal tegmental noradrenergic bundle, which depleted hippocampal NE by 97%, had no effect on hippocampal self-stimulation in rats. In the second study, intragastric administration of para-chlorophenylalanine (PCPA) decreased hippocampal self-stimulation, suggesting the importance of a serotonin input in maintaining this behavior. Identical PCPA treatments resulted in temporary depletions of brain serotonin which paralleled the changes in hippocampal self-stimulation. The maximal decreases in both the biochemical and behavioral measures occured at 4 days post-drug. Interpretations of this deficit in hippocampal self-stimulation in terms of gross sensory and/or motor changes were ruled out as animals with lateral hypothalamic electrodes showed increases in self-stimulation paralleling the post-drug serotonin changes. An intra-sessional analysis of the PCPA-induced behavioral changes revealed that lateral hypothalamic self-stimulation was facilitated mainly during the first hour of the two-hour test sessions, whereas the depression in hippocampal self-stimulation occurred primarily in the last hour of the sessions. The differential effects of PCPA on lateral hypothalamic and hippocampal self-stimulation provide evidence against simple monoamine theories of reinforcement.

Dorsal noradrenergic bundle lesions fail to disrupt self-stimulation from the region of locus coeruleus

Intracranial self-stimulation (ICSS) from the region of the locus coeruleus (LC) was not attenuated by ipsilateral dorsal noradrenergic bundle (DNB) lesions. Bilateral DNB lesions also failed to affect LC ICSS, whereas the same lesions, in the same animals, resulted in a marked enhancement of lateral hypothalamic (LHA) ICSS responding. Behavioral, neuroanatomical and fluorescent histochemical data confirmed destruction of the DNB, thus suggesting that the dorsal noradrenergic system is not necessary for ICSS from the region of the LC.

Hypothalamic self-stimulation and operant activity in the mottled mutant mouse

Some behavioural effects of the mottled mutation were studied in the variegated heterozygote female. In mutants, compared to normals, operant activity was increased, whereas hypothalamic self-stimulation was always decreased. These results are discussed relative to the excepted central catecholaminergic disturbances resulting from the mutant gene.

Norepinephrine-dopamine interactions and behavior

The proposed hypothesis is directed toward explaining a number of disparate findings in terms of a stress-related interaction between the NE- and DA-containing systems in the brain. The deleterious behavioral effects of decreased DA activity, for example, may be counterbalanced by a similar decrease occurring in NE activity, such compensation being most likely to occur under conditions of stress. This hypothesis may have application to the understanding of neurological and mental disorders such as Parkinson's disease and schizophrenia.

Self-stimulation of the MFB or VTA after microinjection of haloperidol into the prefrontal cortex of the rat

Haloperidol, a dopamine receptor antagonist, was microinjected in doses of 12 or 24 microng into the prefrontal cortex of the rat. Its effects on self-stimulation of the ventral tegmental area (VTA) or the medial forebrain bundle (MFB) were examined. It was found that these injections failed to attenuate self-stimulation at either structure. However, when haloperidol was injected into the caudate-putamen complex, a decrease in self-stimulation occurred within these structures. These results suggest that dopamine in the medial prefrontal cortex is not necessarily a part of the neurochemical substrate underlying self-stimulation of the ventral tegmental area or medial forebrain bundle.

Differential effects of para-chlorophenylalanine on self-stimulation in caudate-putamen and lateral hypothalamus

Rats were prepared with chronic bipolar electrodes aimed at either the caudate-putamen or lateral hypothalamus and those displaying consistent self-stimulation were given additional training at half-maximal current intensities. All subjects received an intragastric injection of para-chlorophenylalanine (400 mg/kg) and self-stimulation tests continued until pre-injection rates were re-established. Responding in both brain areas was suppressed 24 h after drug treatment. The next day, self-stimulation rates in the hypothalamus increased to 115% of pre-drug levels reaching a level of 180% by the third day of post-drug testing. In contrast, self-stimulation of sites in the neostriatum continued to decline, with minimal levels reaching 48% of control on the fourth post-drug day. Self-stimulation rates in both groups had returned to control levels by post-drug day 6. These data indicate that the role of serotonergic mechanisms in brain stimulation is locus specific, and that the specific nature of this role may be determined by interaction with other neurochemical systems. The possible interaction between dopaminergic and serotinergic mechanisms in the neostriatum is discussed as a model of self-stimulation in this region of the brain.

Evidence that self-stimulation of the region of the locus coeruleus in rats does not depend upon noradrenergic projections to telencephalon

Rats with intracranial self-stimulation (ICSS) electrodes in the locus coeruleus and adjacent pontine tegmental structures received stereotaxically placed bilateral injections of 6-hydroxydopamine (4 mug/2 mul) into the mesencephalic trajectory of the dorsal tegmental noradrenergic bundle. The consequent depletions of norepinephrine in the cerebral cortices and hippocampi (96.7%) did not result in significant changes in ICSS. Thus, diencephalic and telencephalic noradrenergic projections of the locus coeruleus do not appear to be critical for the occurrence of ICSS from that nucleus or its surrounding region. Nor do these projections appear to be crucially involved in the enhancement of this ICSS by D-amphetamine. Rats in this study showed two-fold increases in responding following injections of D-amphetamine sulfate (0.5 mg/kg) both before and after the lesions of the dorsal tegmental bundle. These results suggest that the ascending projections of the locus coeruleus are not critically involved in ICSS of the dorsal pontine tegmentum.

Attenuation of self-stimulation from substantia nigra but not dorsal tegmental noradrenergic bundle by lesions of sulcal prefrontal cortex

Rats that self-stimulated from electrodes implanted in either the substantia nigra pars compacta (SNC) or the dorsal tegmental noradrenergic bundle (DTB) received bilateral electrolytic lesions of the prefrontal cortex dorsal to the rhinal sulcus. Immediately after the lesions, animals with SNC electrodes (n = 9) showed significant and permanent reductions in self-stimulation rates. The average reduction in the SNC group was 67% of prelesion bar-pressing scores. In contrast, rats with DTB electrodes (n = 11) were not significantly affected by similar sulcal lesions. Subsequent examination of the brains for prograde degeneration using procedure I of Fink and Heimer 11 revealed a descending system of sulcal efferents that was very dense in the region of the SNC but only scattered in the area of the DTB. The close relation between the effects of sulcal lesions on self-stimulation and the resultant density of degeneration as a function of stimulation site is discussed in terms of the contribution of prefrontal cortex to self-stimulation in general, and of the implications for the catecholamine hypothesis of self-stimulation in particular.

The role of nerve-growth factor (NGF) in the central nervous system

NGF is a protein that stimulates growth and differentiation of sympathetic and sensory components of the peripheral nervous system. The purpose of this review is to examine the evidence that NGF has similar activity in the central nervous system. First, the primary mode of interaction of NGF with the nerve cell will be discussed, and the possibility that such an interaction takes place in the brain will be examined. Recent studies have demonstrated that NGF promotes regenerative sprouting of damaged catecholamine-containing neurons in the brain. The next part of the paper reviews this literature, and other findings that indicate or contraindicate a role of NGF in brain maturation of maintenance. The final part of this paper suggests specific avenues for future research in this area, and presents conclusions regarding the literatureon brain activity of NGF to date.

Stimulation of rat medial or sulcal prefrontal cortex during passive avoidance learning selectively influences retention performance

Low-level unilateral electrical stimulation was delivered during passive avoidance learning through a bipolar electrode to the prefrontal cortex of the adult albino rat. No brain stimulation was applied during a retention test measured 24 h later. Ventromedial prefrontal cortex stimulation produced retention impairment over and above that observed with chronic electrode implantation. Sulcal cortex stimulation, in contrast, actually attenuated the retention deficit produced by chronic implantation in the sulcal cortex. Stimulation of an afferent common to both prefrontal regions, the dorsomedial thalamus, resulted in retention disruption, but stimulation of another common afferent, the locus coeruleus, did not. Acquisition of the inhibitory response was not affected by stimulation of any of the above brain regions. The present results demonstrate, again, that the functional role in memory of particular brain regions can be dissected by low-level electrical stimulation. The functional separation of rat sulcal and medial cortices revealed by the effects of stimulation suggests that these prefrontal subfields subserve different functions in the information storage process.

Mesencephalic lesions resulting in normophagia, reduced weight and altered metabolism

The effects of bilateral lesions of the ventral noradrenergic bundle (VNA) were studied in male rats. In contrast to data reported by others, hyperphagia and obesity were not observed following VNA lesions. Indeed, except for a depression during the first three postoperative days, food intake (FI) of the VNA lesioned animals (VNAL) was normal. Interestingly, the body weight (BW) of the VNAL was significantly reduced compared to the controls, and a pair feeding study indicated that this depression of BW was not due to their FI. Computation of FI per metabolic size showed that the VNAL actually had a significantly increased FI compared to the controls. After a two day fast the VNAL lost more metabolic size than controls and upon refeeding they defended their pre-fast BW. The VNAL rats showed normal body composition and circulating glucose, insulin and prolactin. They had reduced free fatty acids, triglycerides, growth hormone and body length. The data suggest that the mesencephalon influences BW set point, some metabolites and possibly overall metabolism.