Unilaterally activated systems in rats self-stimulating at sites in the medial forebrain bundle, medial prefrontal cortex, or locus coeruleus

Rats with electrodes in either the posterior medial forebrain bundle (MFB), the anterior MFB, the medial prefrontal cortex, or the locus coeruleus self-stimulated during a 45 min period following the injection of [14C]2-deoxyglucose. They were then sacrificed and their brains prepared for autoradiography. The autoradiographs were analyzed for unilaterally activated neural systems, using a computerized image analyzing system to compare the darkness of neural structures on the stimulated side with the darkness of the same structures on the unstimulated side. There was extensive overlap in the neural structures unilaterally activated by stimulation in the anterior and posterior MFB; but there was no overlap between the structures activated by MFB stimulation and the structures activated by stimulation at either of the extradiencephalic sites; nor did the forebrain, diencephalic, and midbrain sites have any readily apparent bilateral effects in common. If there is a substrate common to MFB self-stimulation and extradiencephalic self-stimulation, its activation is not revealed by 2-deoxyglucose autoradiography.     

Conditioning-related single unit activity in the frontal cortex of urethane anesthetized rats

Responses of frontal cortex single units to a tone preceding medial forebrain bundle (MFB) stimulation were recorded in urethane anesthetized rats. The animals were implanted with monopolar electrodes for MFB stimulation and, following recovery, stimulation parameters which supported self-stimulation were determined for each rat. Prior to the unit recording experiment, the animals were trained to associate a 2-sec tone with MFB stimulation. Trials were presented at variable intervals. Under urethane anesthesia, single units were isolated and the responses of units to paired and unpaired tones were determined. The results indicate that conditioning-related responses of frontal cortex single units can be recorded in urethane anesthetized rats.     

Diazepam modulates lateral hypothalamic self-stimulation but not stimulation-escape in rats

Rats electrically stimulated via chronically implanted lateral hypothalamic (LH) electrodes were assessed with and without diazepam (DZ), for thresholds of stimulation-bound feeding (SBF) and for barpressing rates to administer and to escape from the same current, Six pure-reward rats, who self-stimulated but did not escape LH stimulation, exhibited SBF. Their electrode tips lay in medical forebrain bundle (MFB) and zona inserta along the entire rostral-caudal extent of the ventromedial nucleus of the hypothalamus (VMH). Six reward-escape rats, who self-stimulated and escaped from LH stimulation, did not (with one histologically deviant exception) show SBF. Reward-escape electrode tips were anterior to all the pure-reward placements. They lay in MFB rostral to the VMH up to the level of the bed nucleus of the stria terminalis (with the deviant electrode tip located on the zona inserta/ventral thalamic border). After i.p. injections of DZ, self-stimulation (SS) rates increased for both groups of animals and SBF thresholds decreased. Stimulation-escape (SE) rates, however, remained unchanged by the drug. The results are consistent with the existence of dual substrates: a DZ-sensitive reward system, present in both groups of animals, and a simultaneously stimulated, drug-resistant aversion system which is powerfully engaged in reward-escape animals only.     

Cocaine enhances the reward value of medial prefrontal cortex self-stimulation.

Intracranial self-stimulation (ICSS) of at least some brain sites is thought to be mediated by mesolimbic dopamine (DA) neurons. However other ICSS sites, especially those in the medial prefrontal cortex (MFC), have been shown to be relatively insensitive to drugs (e.g. amphetamine, neuroleptics) that alter DA synaptic transmission. In the present study, rats with ICSS electrodes implanted in both the medial forebrain bundle (MFB) and the MFC were treated once per day for 10 days with cocaine (15.0 mg kg-1). Cocaine decreased the thresholds for both MFB (-51.4%) and MFC (-23.0%) ICSS. Cocaine also increased rates of responding for MFC but not MFB ICSS. These data provide additional support for the view that the MFC contributes to the rewarding effects of cocaine.     

Lesions of midline midbrain structures leave medial forebrain bundle self-stimulation intact.

Previous work with psychophysically-based collision methods and pharmacological manipulation suggests a role in medial forebrain bundle (MFB) self-stimulation for neurons lying along the midline between the cerebral hemispheres, in the mid- and/or hindbrain. Also, recently-proposed models of the anatomical substrate for medial forebrain bundle stimulation reward suggest that at least part of the directly-activated axons of this substrate arise from mid- and/or hindbrain somata, bifurcate, and send bilateral projections to the MFB of each hemisphere. Branches of these axons are thought to cross the midline at some point near the ventral tegmental area. This study examines the effects on MFB stimulation reward of lesioning midbrain structures that lie along the midline between hemispheres. In 13 rats, lesions of the median raphe, the decussation of the superior cerebellar peduncle, or the interpeduncular nucleus were all ineffective in altering the stimulation frequency required to maintain half-maximal levels of operant responding for stimulation reward. These results are discussed in terms of implications for recent models of the anatomical substrate for brain stimulation reward.     

Increased dopamine and serotonin metabolism in rat nucleus accumbens produced by intracranial self-stimulation of medial forebrain bundle as measured by in vivo microdialysis

In the present study, we have used a newly developed microdialysis system to perfuse the nucleus accumbens (NAC) of conscious rats during spontaneous intracranial self-stimulation of the medial forebrain bundle (MFB). Chromatographic (HPLC-ECD) analysis of the perfusates showed that dopamine (DA) release increased, but with an unstable pattern during the actual period of self-stimulation. On the other hand, the main DA metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, and a serotonin metabolite 5-hydroxyindoleacetic acid, were all markedly enhanced by self-stimulation, but with different time courses. These findings indicate that self-stimulation of the MFB in rats induces increases in both DA and serotonin activities in the NAC. Such changes may be involved in mediating self-stimulation of the MFB.     

Behavioral measurement of axonal thresholds

A behavioral method for measuring the electrical sensitivity of directly stimulated elements in the brain is described and applied to the medial forebrain bundle (MFB) reward path and the tectospinal circling path. Equations are derived from which threshold current densities may be calculated from knowledge of the electrode tip dimensions and the current required to produce criterion behavior, which is a function of electrode size. Four different sizes of electrode were implanted in the MFB of rats and self-stimulation rates plotted against stimulating current. The mean currents for criterion bar-pressing rates of 25% and 55% of maximum rate were determined for each electrode size and the values used to calculate average threshold current densities. Two sizes of electrode were implanted in the tectospinal tract of rats and the average currents to produce circling at 0.2 and 0.4 turns/s were measured. The threshold current densities for self-stimulation axons were about 5 times as large as those for circling, in accordance with other evidence that tectal circling path axons are larger than those of the MFB reward path.     

Intra-raphe neurokinin-induced hyperactivity: effects of 5,7-dihydroxytryptamine lesions

Rats were implanted with cannulae in the median raphe nucleus (MR). 5,7-Dihydroxytryptamine (5,7-DHT) or vehicle was infused either directly through the MR cannula, or bilaterally into the medial forebrain bundle (MFB). The MR 5,7-DHT lesions completely blocked the hyperactivity elicited by injections into the MR of the neurokinin (NK) 3 agonists, DiMe-C7 and senktide, and the NK-2 agonist, neurokinin A. In contrast, the MFB 5,7-DHT lesions did not affect the locomotor hyperactivity produced by intra-MR administration of DiMe-C7 and senktide, but appeared to attenuate the effects of NKA. The data indicate that intra-raphe neurokinin-induced hyperactivity is mediated by 5-HT neurons, and that 5-HT projections to the forebrain may be involved in the behavioral activation induced by intra-raphe neurokinin A administration, but not that induced by intra-MR NK-3 agonists.     

Cocaine enhances resistance to extinction of responding for brain-stimulation reward in adult prenatally stressed rats

The present experiment assessed whether prenatal stress (PS) can alter the ability of acute and chronic cocaine administration to increase and decrease the rewarding effectiveness of the medial forebrain bundle (MFB) using intracranial self-stimulation (ICSS), and also whether PS can affect the extinction of the MFB stimulation response. Adult male offspring of female rats that received PS or no PS (nPS) were implanted with MFB stimulating electrodes, and were then tested in ICSS paradigms. In both nPS and PS offspring, acute cocaine injection decreased ICSS thresholds dose-dependently. However, the threshold-lowering effects at any dose were not significantly different between groups. There was also no group-difference in the threshold-elevating effects of chronic cocaine administration. Nevertheless, chronically drug-administered PS rats exhibited a resistance to the extinguishing of the response for brain-stimulation reward when acutely treated with cocaine, as compared to extinction without cocaine treatment. The results suggest that PS may weaken the ability for response inhibition under cocaine loading in male adult offspring.     

Cholinergic involvement in lateral hypothalamic rewarding brain stimulation

Rats were implanted with stimulating electrodes in the lateral hypothalamus, and cannulae for chemical injections in the ventral tegmentum. Injections of atropine, a muscarinic antagonist, increased thresholds for self-stimulation in a dose-dependent fashion, without slowing bar pressing rates. Thresholds increased less for a self-stimulation site contralateral to the atropine injection. In a conditioned place preference test, the rats preferred compartments in which they received carbachol, a cholinergic agonist. Muscarinic receptors in ventral tegmentum therefore seem critical for medial forebrain bundle (MFB) reward. The possible cholinergic cells of origin are discussed.     

Conditioned cortical slow potential responses in urethane anesthetized rats

Cortical slow potential (SP) responses to tone or light stimuli preceding medial forebrain bundle (MFB) stimulation were recorded in urethane anesthetized rats. In the first study, rats were implanted with Ag-AgCl electrodes for recording frontal cortex SPs as well as monopolar electrodes for MFB stimulation. Following recovery, optimum stimulation parameters for SP conditioning were determined for each rat during self-stimulation sessions. These animals were then subjected to extensive associative conditioning in the unanesthetized state. Trials were presented at variable intervals and a 2-sec tone preceded a single 0.5 sec train of MFB stimulation. Negative SP responses developed with training and responses of similar waveform and amplitude were observed in the same animals under urethane anesthesia. Other rats were implanted with MFB stimulating electrodes and, after recovery, stimulation parameters were determined as above but the animals were not subjected to the conditioning procedure prior to urethane administration. Under urethane anesthesia, Ag-AgCl electrodes were placed on the dura over frontal cortex for recording SP responses during pseudoconditioning, conditioning, extinction and retraining trials, using either light or tone stimuli. Negative bilateral SP responses to the tone or light were minimal or nonexistent during pseudoconditioning, developed gradually with pairing, diminished markedly during extinction and returned to maximum amplitude with retraining. The SP responses also reflected discrimination between reinforced and nonreinforced tone and light stimuli as well as reversal conditioning. Furthermore, turning off a light could also serve as the conditioned stimulus for SP response generation. Cortical slow potential responses can be conditioned in urethane anesthetized rats. Therefore, it may be possible to apply additional neurophysiological techniques in these animals to investigate event-related slow potential mechanisms.     

Intracranial self-stimulation in the parafascicular nucleus of the rat

A behavioral analysis of intracranial self-stimulation was provided for parafascicular nucleus. To evaluate whether intracranial self-stimulation in this nucleus could be site-specific and to determine if the positive sites are the same parafascicular areas that facilitate learning when stimulated, rats were tested via monopolar electrodes situated throughout the parafascicular nucleus. Animals were trained to self-stimulate by pressing a lever in a conventional Skinner box (1-5 sessions). Twenty-two of the 42 animals included in the study, had the electrode at the parafascicular nucleus. Only two of them showed intracranial self-stimulation. Histological analyses indicated that the latter rats had the electrode implanted at the anterior area of the medial parafascicular. Other two animals also showed intracranial self-stimulation but they had the electrode in a more posterior brain region, between the Dark-schewitsch nucleus and the red nucleus. The animals implanted at the parafascicular showed higher response rates than the other two rats. These results confirm that: (a) the anterior region of the medial parafascicular is a positive site for stable and regular intracranial self-stimulation behavior, and (b) these positive sites do not coincide with the parafascicular regions related to learning improvement.     

Ascending monoamine-containing fiber pathways related to intracranial self-stimulation: histochemical fluorescence study

Lesions made via self-stimulation electrodes in the medial forebrain bundle (MFB) of rats always resulted in monoamine accumulation, as studied with the histochemical fluorescence method. Build-up was often in the monoamine systems of the MFB itself, those systems being the ventral norepinephrine bundle, the nigro-neostriate bundle, and the mesolimbic fibers. In other cases, build-up was restricted to a bundle of fibers dorsomedial to the MFB, probably originating in the locus coeruleus or the subcoeruleus area. In still other instances, build-up was seen in both this dorsal bundle of fibers and the catecholamine systems of the MFB. Electrodes in rats with sites of implantation in the region of the A1, A2 and A5 cell groups of origin of the ventral norepinephrine system, as identified with combined Nissl and histochemical examination, were never positive with respect to self-stimulation; nor were placements in this ventral system caudal to the origin of the dorsal bundle positive with respect to that behavior. These results argued against the association of the ventral system with self-stimulation. By implication, the catecholamine fibers of the MFB which are capable of supporting self-stimulation might be the two ascending dopamine systems of the ventral mesencephalic tegmentum. Lesions made via positive electrodes in the vicinity of the brachium conjuctivum always resulted in monoamine build-up in the dorsal norepinephrine fiber system. In sum, our results implicate the A9 and A10 dopamine ventral midbrain systems and the A6 norepinephrine dorsal bundle in intracranial self-stimulation.     

Competence of nerve tissue as distal insert promoting nerve regeneration in a silicone chamber

In some medial forebrain bundle (MFB) sites, self-stimulation is often modulated by hunger or satiety. With electrodes in the nucleus accumbens (NAC) such modulation rarely occurs. The influence of food deprivation on MFB self-stimulation is the main basis for the hypothesis that electrical stimulation of the MFB can mimic the rewarding effect of food for hungry animals. To investigate this hypothesis, unit activity was recorded from the lateral hypothalamic area (LHA) of freely moving rats during rewarding stimulation at loci in both MFB and NAC, and during food ingestion. Of 63 neurons tested during MFB stimulation, 41 were inhibited, 19 were activated, and 3 were not influenced. NAC stimulation suppressed 8 of the 31 neurons tested, excited 16, and elicited no response in the remaining 7. During ingestion, 29 of the 63 neurons tested were inhibited and one was facilitated. Of 29 neurons suppressed by food, 20 were also inhibited by rewarding MFB stimulation, but 10 of 13 neurons inhibited by food were excited by rewarding NAC stimulation. Thus, most LHA neurons inhibited during feeding were also inhibited by rewarding MFB stimulation. Rewarding NAC stimulation, however, does not inhibit most LHA neurons that are inhibited by food. This result suggests that LHA neurons which are inhibited by food might be involved in mediation of the rewarding effect of electrical stimulation at some sites in the MFB. Nevertheless, self-stimulation may occur by activating reward processes other than those related to food, because rewarding NAC stimulation does not inhibit LHA neurons which are suppressed by food.     

Immunohistochemical characterisation of Fos-positive cells in brainstem monoaminergic nuclei following intracranial self-stimulation of the medial forebrain bundle in the rat

Fos immunostaining was used as a marker of neuronal activity following intracranial self-stimulation (ICSS) of the medial forebrain bundle (MFB) in the rat, and was combined with immunostaining for tyrosine hydroxylase (TH), serotonin (5-HT), gamma-aminobutyric acid (GABA), or NR1 (one of the glutamate N-methyl- D-aspartate receptor subunits) for purposes of neurochemical identification. ICSS induced a significant but different degree of increase in the number of Fos-immunopositive (Fos+) cells in the six brainstem monoaminergic nuclei examined, which included the ventral tegmental area (VTA), substantia nigra pars compacta (SNc), dorsal raphe nucleus (DR), median raphe nucleus (MR), locus coeruleus (LC), and A7 noradrenaline cells. Densely labelled Fos+ cells were observed in the LC following ICSS, and many of these Fos+ cells were colocalized with TH. Similarly, many of Fos+ cells in the A7 and DR/MR were colocalized with TH and 5-HT, respectively. By contrast, a smaller number of Fos+ cells was detected in the VTA and SNc following the ICSS, and in these regions the majority of Fos+ cells were not colocalized with TH. Although results among regions quantitatively differed, the ICSS induced a significant increase in the number of double-labelled cells (GABA+/Fos+ or NR1+/Fos+) in all of the VTA, DR, and LC, in which the ICSS produced an ipsilaterally weighted increase in Fos-like immunoreactivity. These results suggest that ICSS of the MFB induces differential Fos expression within monoaminergic and GABAergic neurons in brainstem monoaminergic nuclei under modulation by glutamatergic afferents.     

Differential effect of self-stimulation on dopamine release and metabolism in the rat medial frontal cortex, nucleus accumbens and striatum studied by in vivo microdialysis.

Changes in the extracellular levels of dopamine (DA) and its metabolites in the dopaminergic terminal regions, the medial frontal cortex (MFC), nucleus accumbens (NAC), and striatum (STR), were measured by microdialysis during self-stimulation of the medial forebrain bundle (MFB) in rats pretreated with the DA uptake inhibitor, nomifensine (1 mg/kg, i.p.). Self-stimulation of the MFB in nomifensine-pretreated rats caused an increase in the extracellular DA level in the MFC and NAC but not in the STR. Self-stimulation also increased the extracellular concentrations of the main DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) to a similar extent in the MFC and NAC and to a lesser extent in the STR. Thus, there was a regional difference in the neurochemical changes following self-stimulation with either the MFC or the NAC showing larger extracellular levels of DA, DOPAC, and HVA than the STR. Furthermore, these changes were observed on both hemispheres ipsilateral and contralateral to the stimulation. The results indicate that self-stimulation of the MFB preferentially activates the mesocorticolimbic DA systems, thereby bilateral increases in the release of DA and its metabolism being produced in their terminal regions, the MFC and NAC.     

Non-invasive in-vivo autoradiographic method to measure axonal transport in serotoninergic neurons in the rat brain.

We studied axonal transport of serotoninergic neurons by autoradiography following intravenous administration of alpha-[14C]methyl-L-tryptophan (alpha-[14C]MTrp). Autoradiograms obtained 24 h after intravenous injection of the tracer demonstrated clearly all raphe nuclei and the major ascending pathway, the medial forebrain bundle (MFB). From these autoradiograms it was clear that radioactivity traveling along the MFB had already reached the substantia nigra and ventrolateral geniculate body nuclei, terminal field. The whole route of the MFB was well visualized from an axial cross-section of a three-dimensional display of data. Autoradiograms obtained at 6 h after injection revealed only the caudal part of the MFB but all raphe nuclei were labelled, indicating that the tracer was in the process of being transported, probably as an alpha-methyl-5-hydroxytryptamine, via the MFB. The axonal transport rate was estimated from the brain autoradiograms of 4 rats killed 6 h after injection of the tracer. The mean distance of the tracer transported via the medial forebrain bundle in 4 rats was 3.8 +/- 0.4 (S.D.) mm, which corresponded to the level of the posterior to mid-hypothalamus. The axonal transport rate calculated from this distance from the medial raphe was 0.63 +/- 0.07 mm/h (14 mm/day). There was no significant difference in the axonal transport rate between the right and left side of the MFB.     

Opposite medial thalamic unit responses to rewarding and aversive brain stimulation

Medial thalamus receives fibers from both medial forebrain bundle (MFB) and hindbrain and midbrain reticular formation (RET). The MFB and RET stimulations are rewarding and aversive respectively. In 32 unanesthetized cerveau isolé rats, 158 units were recorded. The MFB and RET effects converge on two thirds of the units recorded in the dorsal medial and paracentral nuclei of thalamus and are opposite in the following ways: post-stimulus pattern of unit discharge during 7 Hz stimulation; slow-wave recruiting with MFB, but not RET, 7 Hz stimulation; and at a “desynchronization” stimulus frequency (20 Hz), MFB elicits decreased unit discharge and RET elicits increased unit discharge, compared to the 7 Hz rates. In the intralaminar and parafasicular nuclei, post-train (60 Hz, 0.2 sec) decreases and increases in unit firing lasting seconds are often elicited with MFB and RET trains respectively. Stimulation of hypothalamic sites outside MFB did not elicit these MFB effects; parafasicular stimulation did not elicit the RET effects. The MFB effects were not seen in habenula or ventral basal thalamus. Hippocampal, habenular, and ventral medial thalamic units did not show opposite MFB and RET effects. Threshold currents for the opposite MFB and RET effects are similar to those eliciting self-stimulation and escape respectively in several operated rats tested both behaviorally and neurophysiologically.     

Early developmental exposure to methylphenidate reduces cocaine-induced potentiation of brain stimulation reward in rats.

BACKGROUND: Methylphenidate (MPH) is prescribed for the treatment of attention and hyperactivity disorders. We showed previously that early developmental exposure to MPH in rats causes behavioral alterations during adulthood, including reduced cocaine reward in place conditioning studies. Here we examined if early MPH exposure alters the ability of cocaine to potentiate the rewarding effects of electrical stimulation of the medial forebrain bundle (MFB) using intracranial self-stimulation (ICSS). METHODS: Rats received MPH or saline during pre-adolescence (P20-35) and were implanted with MFB stimulating electrodes at adulthood (P60). Rats then were tested with cocaine in the ICSS paradigm. RESULTS: Cocaine dose-dependently decreased ICSS thresholds in all rats, but the threshold-lowering effects of cocaine were smaller in rats exposed to MPH during pre-adolescence. There were no differences between groups in sensitivity to the rewarding effects of MFB stimulation itself. CONCLUSIONS: Early developmental exposure to MPH reduces the reward-related effects of cocaine in the ICSS paradigm. These results are consistent with previous studies in which early exposure to MPH reduced the ability of cocaine to establish conditioned place preferences, as well as the rewarding effects of sucrose and sexual behavior. Reduced sensitivity to these various types of reward may reflect general dysfunctions of brain reward systems.     

Lidocaine inactivation demonstrates a stronger role for central versus medial extended amygdala in medial forebrain bundle self-stimulation

Given recent attention to the role of the extended amygdala (EA) in brain reward processes, this study examines the relative contributions of the medial versus central aspects of that forebrain macrostructure to the rewarding effects of medial forebrain bundle (MFB) stimulation. Thirty-one rats were self-stimulated at either the rostral or caudal MFB before and after lidocaine-induced inactivation of an EA target. Relative to non-injection baseline tests, the injection of 0.5 or 1.0 microl of 4% lidocaine into the central EA structures of the lateral bed nucleus of the stria terminalis, the central sublenticular EA, and the interstitial nucleus of the posterior limb of the anterior commissure frequently and substantially disrupted the rewarding effect of MFB stimulation, whereas comparable saline infusions did not. The effects were most pronounced when the central EA was inactivated either bilaterally or ipsilateral to the stimulation site. Contralateral inactivation was less effective but did impair the stimulation's reward effects in several cases. Inactivation of medial EA structures did not have as great or as consistent effects on stimulation reward value except when the lidocaine infusion encroached on the MFB itself. These results support prior demonstrations of the EA's role in brain reward and motivational processes and further show that the central rather than medial aspects of the EA are particularly relevant. The results are discussed in the context of possible anatomical substrates supporting MFB self-stimulation.