Role of medial forebrain bundle catecholaminergic fibers in the modulation of glucocorticoid negative feedback effects

The possible role of medial forebrain bundle (MFB) catecholaminergic fibers in the previously observed MFB-mediated modulation of hypothalamic sensitivity to glucocorticoid negative feedback effects has been investigated. MFB catecholaminergic fibers were destroyed by bilateral MFB injections of 6-hydroxydopamine (6-OHDA) to adult male rats. The inhibitory effect of dexamethasone (10 μg, i.p., 4h) upon the adrenocortical secretory response to ether stress, as estimated by serum corticosterone levels, was enhanced in the 6-OHDA-treated animals, as compared to vehicle-injected controls. This study suggests that MFB noradrenergic neurons are at least partially responsible for the MFB-mediated modulation of hypothalamic sensitivity to glucocorticoid hormones.     

Strength-duration analysis of the organization of reinforcement pathways in the medial forebrain bundle of rats

Strength-duration curves were determined for electrical self-stimulation of the brain in rats implanted with lateral hypothalamic electrodes. The rats self-stimulated in different behavioral situations which required them to make different responses, and the parameters of the strength-duration curves determined in each situation were compared. The comparisons suggested that two distinct groups of neurons were involved in the mediation of brain stimulation reinforcement of the bar pressing response, and that one of these groups was primarily involved in mediating the reinforcement of an alley running response, while the other group primarily mediated the reinforcement of a response in which a restrained rat moved only its tail. The data from other responses suggested that the nature of the response a rat is required to perform determines the exact combination of neurons from the two groups which participate in mediating the brain stimulation reinforcement of the task. The possible functional significance of these two neuron groups was discussed.     

Hypothalamic afferent connections mediating adrenocortical responses that follow hippocampal stimulation

This study identified some neural pathways which mediate the adrenocortical responses that follow hippocampal stimulation. The increase in plasma corticosterone following dorsal hippocampus stimulation, in rats with electrodes chronically implanted under pentobarbital anesthesia, was blocked by dorsal fornix and lateral septal lesions and by small posterior hypothalamic deafferentation. Fimbria transection, lateral septal lesions, and posterior hypothalamic deafferentation, but not midbrain reticular formation lesions, also blocked the adrenocortical responses to ventral hippocampus stimulation. Our present and previous studies indicate that the dorsal and ventral hippocampal effects on the hypothalamus, which increase plasma corticosterone concentrations, are mediated by the dorsal fornix and fimbria, respectively, as well as by the lateral septum. A posterior hypothalamic input, which does not involve the medial forebrain bundle or the midbrain reticular formation is also essential for the activation of this response.     

Tolerance to amphetamine''s facilitation of self-stimulation responding: Anatomical specificity

Further work on the phenomenon reported by Leith and Barrett, wherein tolerance was shown to develop to the well-known D-amphetamine-induced facilitation of self-stimulation, clearly indicates that the development of such tolerance is dependent on the location of the stimulating electrode. Thirty-seven Fisher or Harlan rats were trained to bar press for hypothalamic stimulation (60 Hz, AC). Following several sessions during which small doses of D-amphetamine were administered to demonstrate facilitation, the subjects were placed on a 4-day D-amphetamine regimen. During this time they were given three daily injections of continuously increasing doses of D-amphetamine (total 78 mg/kg). Subsequent tolerance was shown for electrodes stimulating dorsal or medial hypothalamic structures (H2 field of Forel, dorsal medial forebrain bundle, medial hypothalamic nuclei), but did not develop with ventral or lateral hypothalamic stimulation sites (fornix, ventral medial forebrain bundle).     

High frequency stimulation of the subthalamic nucleus improves treadmill locomotion in unilateral 6-hydroxydopamine lesioned rats

This study investigated the influence of electrical stimulation of the subthalamic nucleus (STN) on motor impairment induced by unilateral 6-hydroxydopamine (6-OHDA) lesions in the medial forebrain bundle. Rats were trained to walk on a treadmill and then implanted with microelectrode arrays in and near the STN. The neurotoxin 6-OHDA was injected into the medial forebrain bundle (MFB) unilaterally to produce a targeted lesion of the dopaminergic system. Successful lesions produced impaired treadmill walking behavior. High frequency stimulation (HFS) of the STN improved treadmill walking immediately and restored normal walking patterns. The same HFS failed to evoke visible side effects such as stepping, turning, raising of the head or facial muscle contraction in the absence of treadmill movement, or to change rotational behaviors elicited by the dopamine (DA) agonist apomorphine in unilateral lesioned rats. This suggests that the stimulation did not cause movement by an activation of brainstem locomotor regions or an increase attention leading to movement. Apomorphine-induced rotation may represent an imbalance of dopaminergic activation which remains during HFS. This work may provide a rodent model for deep brain stimulation (DBS) in patients with Parkinson's disease, and be suitable for further investigation of the neural mechanisms underlying the therapeutic effects of DBS.     

Adrenocortical responses to ether stress and neural stimuli in rats following the injection of 6-hydroxydopamine into the medial forebrain bundle

Adrenocortical responses, as expressed by changes in plasma corticosterone concentrations, after ether stress, or photic, acoustic, or sciatic nerve stimulation were studied in rats, with 6-hydroxydopamine or vehicle injected into the medial forebrain bundle (MFB). The neurotoxin partially inhibited the response to photic stimulation only, indicating the involvement of MFB catecholaminergic fibers in the transmission of this response which stimulates adrenocortical secretion.     

Neural pathways mediating the prolactin secretory response to acute neurogenic stress in the male rat

Adult male rats, intact (N) or bearing complete, anterior or posterior hypothalamic deafferentations (CHD, AHD or PHD, respectively), or bilateral medial forebrain bundle (MFB) lesions, were acutely exposed to visual or audiogenic stimulation. At 2, 4,10 or 30 min following stress onset the animals were decapitated and trunk blood was collected for prolactin (PRL) determinations. Basal serum PRL levels were found to be similar in all groups. In N animals, exposure to both modalities resulted in rapid and marked PRL secretory responses. These responses were totally abolished in the CDH group. In AHD rats, no significant elevation in serum PRL concentration was found upon stress exposure. In PHD animals, the PRL secretory responses were only slightly attenuated when compared with the N group. In MFB-lesioned rats, a marked elevation in serum PRL concentrations was recorded following visual stimulation; contrary thereto, the PRL secretory response following audiogenic stress was markedly attenuated. These data (1) describe the temporal aspects of the PRL secretory response to acute exposure to neurogenic stresses in the male rat, and (2) demonstrate that these PRL responses are elicited via a neural pathway impinging upon the medial basal hypothalamus from the rostral direction.     

Biochemical mapping of the noradrenergic ventral bundle projection sites: Evidence for a noradrenergic-dopaminergic interaction

Norepinephrine (NE) and dopamine (DA) concentration and dopamine turnover were measured 12 days after a unilateral or bilateral noradrenergic ventral bundle (VB) transection to determine the noradrenergic projection sites and possible interactions with dopaminergic systems.Both bilateral and unilateral VB transection resulted in a significant reduction of NE of the nucleus accumbens, lateral septal nucleus, medial forebrain bundle, ventromedial nucleus, dorsomedial nucleus and medial amygdaloid nucleus. Bilateral transection also decreased NE content of the median eminence and the periventricular and arcuate nuclei. In the medial preoptic nucleus, the nucleus interstitialis striae terminalis and the central gray catecholamine area, bilateral transection significantly decreased NE concentrations while unilateral lesions had no significant effect. The anterior hypothalamic, lateral preoptic, and paraventricular nuclei responded to bilateral VB transection with a decrease in NE concentration and to unilateral lesion with a bilateral increase in NE. In the dorsal hippocampus and the caudate nucleus, bilateral lesions had no effect on NE concentrations while unilateral transection significantly decreased NE concentrations. Regions in which neither bilateral nor unilateral VB transection produced a significant change in NE content are the olfactory tubercle, the nucleus tractus diagonalis, substantia nigra pars compacta and reticulata, ventral tegmental area, habenula, superior colliculus, and the cingulate and piriform cortices.Transection of the noradrenergic ventral bundle also produced changes in dopaminergic systems suggesting a noradrenergic-dopaminergic interaction. Bilateral VB transection decreased the dopamine concentration and turnover in the nucleus accumbens, increased steady-state levels and turnover in the nucleus tractus diagonalis and increased dopamine concentration in the lateral septum. Unilateral VB transection decreased DA concentration bilaterally in the caudate nucleus, olfactory tubercle, nucleus accumbens and the nucleus interstitialis striae terminalis but increased concentrations in the substantia nigra pars reticulata (ipsilateral) and in the ventral tegmental area (bilateral). These results indicate a broad projection field for the noradrenergic ventral bundle and suggest a noradrenergic-dopaminergic interaction.     

Selective Sprouting of Injured Dopaminergic Neurons Induced by Embryonic Brain Grafts

Following 6-hydroxydopamine lesions of the catecholaminergic fibers in the medial forebrain bundle (mfb), little fiber regrowth occurs through the lesion site. However, if grafts of embryonal neocortical tissue are placed into the lesion site 2 weeks after the initial lesions, host catecholaminergic fibers sprout through the lesion site into the graft over the ensuing 2 months. The present study examined the origin of these fibers. Both tyrosine hydroxylase (TH) and dopamine-β-hydroxylase (DβH) immunoreactive fibers grew into grafts placed in the intact mfb. However, when mfb lesions were made prior to grafting, TH-immunoreactive fiber ingrowth was similar to that seen without an mfb lesion. Conversely, there was only limited DβH immunoreactive fiber ingrowth. Similarly, such grafts contained patches of [³H]mazindol binding to dopamine transporter sites, while binding of [³H]desipramine to norepinephrine transporters was not demonstrable. Thus, embryonic brain grafts promote significant sprouting of injured dopaminergic axons and limited sprouting of injured noradrenergic/adrenergic fibers, neither of which would otherwise occur. This suggests the presence of a trophic interaction or permissive milieu provided by the graft which is relatively selective for dopamine neurons.     

Electrophysiological studies of olfactory projection to the mesencephalic reticular formation

Single unit responses and field potentials of the mesencephalic reticular formation following olfactory stimulation were analyzed in trigeminally de-afferented, unanesthetized and immobilized cats. Out of 167 units isolated, 97 units (58%) responded to odor stimulation, but 70 units (42%) were unresponsive. Two major types of response were identified in these responsive units; prolonged acceleration or reduction of the firing rate. These responses to odor were induced almost equally from any side of the nostril. Both responsive and unresponsive units were distributed widely in the mesencephalic reticular formation and also in the central gray matter. Reticular units could also be activiated by electrical stimulation of the olfactory bulb. By applying single shocks to the olfactory bulb field potentials were evoked from the prepyriform cortex, preoptic and posterior hypothalamic regions of the medial forebrain bundle ipsilateral to the stimulation, and bilaterally in the mesencephalic reticular formation. The latency of the response in these structures was increased in the order of recording points described above. The field potential in the mesencephalic reticular formation induced ipsilaterally or contralaterally was not affected by section of the anterior commissure, while following lesions of the medial forebrain bundle, the potentials induced from the olfactory bulb ipsilateral to the lesion were completely abolished. Thus, it was demonstrated that olfactory impulses project to the mesencephalic reticular formation through the medial forebrain bundle.     

Effects of periaqueductal gray stimulation on diencephalic neural activity

The effects of ventral tegmental area of Tsai (VTA) stimulation on lateral hypothalamic (LH), lateral preoptic area (LPA), and medial hypothalamic neuronal activity were determined in anesthetized rats. Recordings from 81 hypothalamic neurons indicate that stimulation produces predominantly decreases in hypothalamic neuron activity. Increases in activity due to VTA stimulation occurred less frequently. Following single rectangular pulse stimulation, 0.5 msec, 0–500 μA, short latency decreases in activity occurred. Longer latency increases in discharge frequency were also observed. Dose response relations were established for 56% of the LH neurons, 78% of the LPA neurons, and for 82% of the medial hypothalamic neurons following VTA stimulation. Decreases and in a few cases increases in activity seemed to involve only one or two synapses. Antidromic responses verified interconnections between the VTA and the hypothalamus and revealed relatively slow conduction velocities of 0.45 and 0.81 m/sec. The changes in discharge frequency which occurred following VTA stimulation were similar in direction to the effects of the direct microiontophoretic application of dopamine (DA) or norepinephrine (NE). Since DA increased or decreased while NE decreased discharge frequency, these microiontophoretic tests indicated that the shorter latency VTA stimulation induced increases and decreases in neural activity were associated with VTA dopaminergic neuron stimulation and that in some cases short and long latency decreases in neuronal activity were due to activation of VTA ventral bundle NE fibers of passage or to indirect polysynaptic mechanisms. Results demonstrate the interconnections between various regions of the hypothalamus and the VTA along the extent of the medial forebrain bundle (MFB). The cross-validation of neuroanatomical and various electrophysiological metods in establishing the nature of hypothalamic connections was discussed.     

Biochemical mapping of noradrenergic nerves arising from the rat locus coeruleus

Norepinephrine (NE) concentration was measured in discrete brain regions of the rat following unilateral electrolytic lesions of the locus coeruleus, to determine the distribution of its noradrenergic neurons. Discrete brain nuclei and subdivisions were dissected from frozen sections, and norepinephrine was measured by a sensitive radio-isotopic assay.A significant reduction by 29–63% of control values in norepinephrine content was observed ipsilateral to the lesion in the following areas: all portions of the cerebral cortex examined (entorhinal, hippocampal, cingulate, parietal, and occipital areas), anterior half of the cerebellar cortex, hypothalamic periventricular and paraventricular nuclei, anterior ventral thalamic nucleus, ventral thalamic nucleus, and habenula. It appears that these regions receive unilateral innervation by axons from the locus coeruleus. In 3 regions (the medial geniculate body, inferior colliculus, and posterior half of the cerebellum), NE was reduced bilaterally in a pattern suggesting bilateral innervation from the locus coeruleus. Since no reduction in NE concentration occurred in the medial preoptic nucleus, nucleus interstitialis stria terminalis (ventralis), dorsomedial hypothalamic nucleus, or medial forebrain bundle, axons from noradrenergic neurons in the locus coeruleus do not appear to innervate these regions.The biochemical mapping of noradrenergic nerves from the locus coeruleus is discussed in relation to distribution studies based on the histofluorescence method.     

Unit responses in the medial forebrain bundle to rewarding stimulation in the hypothalamus

The effects of different intensities of stimulation were studied on self-stimulation behavior and on the neural responses in the medial forebrain bundle correlated with it. Of the two neural responses observed, an excitatory type was more diffusely distributed throughout this pathway than an inhibitory type, and its properties varied more as a function of the rate of self-stimulation at the various intensities tested than the properties of the inhibitory responses did. The latter were more localized to the caudal portion of the medial forebrain bundle studied, and their threshold was generally lower than the threshold of self-stimulation behavior. Also, the inhibitory responses appeared to be more sensitive indicators of stimulation applied to the medial forebrain bundle, whereas the excitatory responses were more sensitive indicators of variations in the rate of responding for brain reward. The neural responses in the MFB to rewarding stimulation are evaluated in terms of the possibility that they are specific to the positive reinforcement mechanism in the diencephalon.     

Plasma catecholamine and cardiovascular responses following hypothalamic stimulation in the awake cat

Ninety-three hypothalamic sites were electrically stimulated, using constant parameters, in awake, restrained cats to determine those regions which maximally activated the sympatho-adrenal (SA) and cardiovascular (CV) systems. Plasma catecholamine levels were measured over time following hypothalamic stimulation; levels of norepinephrine (NE) and epinephrine (E) served as indices of adrenergic neural and adrenal medullary activities, respectively. CV parameters of heart rate (HR) and mean intra-arterial blood pressure (MAP) were continuously monitored. The greatest elevation in plasma catecholamines was elicited by stimulation of sites in the perifornical area, ventromedial nucleus, and medial forebrain bundle. Several sites were identified which preferentially elevated one of the sympatho-adrenal neurotransmitters. A differential increase in plasma E was most frequently obtained from sites around the border of the ventromedial nucleus and in the medial forebrain bundle. Differential elevation of plasma NE was observed following stimulation of sites in the anterior commissure, central preoptic area, and dorsal perifornical region posterior to the ventromedial nucleus. Sites which activated the CV and SA systems were not always coincident; those sites which activated the CV system alone tended to be located in the lateral hypothalamus.     

Fos expression following self-stimulation of the medial prefrontal cortex

Self-stimulation of the medial prefrontal cortex and medial forebrain bundle appears to be mediated by different directly activated fibers. However, reward signals from the medial prefrontal cortex do summate with signals from the medial forebrain bundle, suggesting some overlap in the underlying neural circuitry. We have previously used Fos immunohistochemistry to visualize neurons activated by rewarding stimulation of the medial forebrain bundle. In this study, we assessed Fos immunolabeling after self-stimulation of the medial prefrontal cortex. Among the structures showing a greater density of labeled neurons in the stimulated hemisphere were the prelimbic and cingulate cortex, nucleus accumbens, lateral preoptic area, substantia innominata, lateral hypothalamus, anterior ventral tegmental area, and pontine nuclei. Surprisingly, little or no labeling was seen in the mediodorsal thalamic nucleus or the locus coeruleus. Double immunohistochemistry for tyrosine hydroxylase and Fos showed that within the ventral tegmental area, a substantial proportion of dopaminergic neurons did not express Fos. Despite previous suggestions to the contrary, comparison of the present findings with those of our previous Fos studies reveals a number of structures activated by rewarding stimulation of both the medial prefrontal cortex and the medial forebrain bundle. Some subset of activated cells in the common regions showing Fos-like immunoreactivity may contribute to the rewarding effect produced by stimulating either site.     

Distribution of cytochrome oxidase in response to rewarding brain stimulation: Effect of different pulse durations

Cytochrome oxidase histochemistry was used to evaluate neuronal changes in oxidative metabolism in response to rewarding brain stimulation of the medial forebrain bundle. Rats with single lateral hypothalamic electrodes self-stimulated daily for ten days for trains of either 0.1 or 2.0 ms pulses that corresponded to about 75% of maximum responding. Quantitative comparison of stimulated-to-unstimulated sides revealed differences in relative optical density in few structures, notably in the lateral septal nucleus and the nucleus accumbens, when the brief pulse duration was used. In contrast, the longer pulse duration gave rise to metabolic increases in several dopaminergic projections, including the frontal cortex, olfactory tubercle, and lateral habenula, and also enhanced activity in the lateral septal nucleus. These data suggest that mesocorticolimbic structures may be implicated in medial forebrain bundle self-stimulation.     

Reinnervation of transplanted hypothalamic neurons by host aminergic fibers in rats

Fetal mediobasal hypothalamic tissue which receives an extensive noradrenergic innervation in the adult brain, was implanted into the vicinity of the medial forebrain bundle of adult rats to determine whether host noradrenergic fibers would innervate the ectopically placed tissue in an organotypic manner. Tissue from the site of implantation was prepared for light and ultrastructural immunocytochemistry at 6, 12, or 20 weeks postsurgery. Putative host catecholamine fibers formed dense plexuses in localized portions of the grafts and made synaptic contacts with dendrites and somata of transplanted neurons. This suggests that a mature host central nervous system is capable of a region-specific integration of transplanted neurons.     

The effect of rewarding hypothalamic stimulation on behavioral and neural hippocampal responses during trace eyeblink conditioning in rabbit (Oryctolagus cuniculus)

Rabbits were trace-conditioned with a tone as a conditioned stimulus and an airpuff as an unconditioned stimulus. Electrical stimulation to the medial forebrain bundle in the lateral hypothalamus was delivered either before or after the tone-airpuff pair. The purpose of the present study was to test whether the effect of post-trial hypothalamic stimulation differed from the effect of pre-trial hypothalamic stimulation on trace conditioning in the same subjects. Additionally, hippocampal responses were measured during sessions to see if hypothalamic stimulation activated dopaminergic fibres and affected hippocampal cell functioning and thus learning. The results showed that behavioral nictitating membrane conditioned responses were acquired quickly and hippocampal multiple unit activity increased with post-trial hypothalamic stimulation in comparison to almost non-existent conditioned responses and activity changes in the pre-trial hypothalamic stimulation sessions. The results suggest that hypothalamic stimulation affects trace conditioning differently depending on its time of delivery during conditioning.     

Forebrain origins and terminations of the medial forebrain bundle metabolically activated by rewarding stimulation or by reward-blocking doses of pimozide

Using [14C]-2-deoxyglucose autoradiography, we determined which forebrain and diencephalic areas showed metabolic alterations in response to unilateral electrical stimulation of the posterior medial forebrain bundle at parameters chosen to produce a just-submaximal rewarding effect. At these parameters, only a few areas were activated. There was no detectable activation anterior or dorsal to the genu of the corpus callosum. Just anterior to the anterior commissure, there was strong activation of the vertical limb of the diagonal band of Broca, with a focus in the nucleus of the diagonal band. Just posterior to the anterior commissure, there was strong activation of compartment "c" of the medial forebrain bundle (MFB), with weaker activation of the bed nucleus of the stria terminalis and the medial preoptic area. At midhypothalamic levels, the dorsolateral, dorsomedial, and ventral MFB all showed activation. There was bilateral suppression of activity in the lateral habenula. Activation appeared to end in the anterior ventral tegmental area of Tsai. Reward-blocking doses of the neuroleptic pimozide activated the caudate and the lateral habenula but did not alter any of the unilateral effects of stimulation. Using longer pulse durations and/or shifting the site of stimulation to the substantia nigra activated many of the systems not activated in the first experiment, including all of the major dopaminergic projection systems, proving the capacity of the technique to reveal activation of these systems. The results permit one to define a discrete projection system that merits electrophysiological investigation as a likely substrate for the rewarding effect of MFB stimulation. They also suggest that dopaminergic projection systems may not form part of the reward pathway itself.     

Lesions of pontomesencephalic cholinergic nuclei do not substantially disrupt the reward value of medial forebrain bundle stimulation

This study examines the effects of lesioning the pedunculopontine tegmentum (PPTg) and laterodorsal tegmentum (LDTg) on the reward effectiveness of medial forebrain bundle (MFB) stimulation. Although the focus is on the effects of unilateral lesions made ipsilateral to stimulation sites in the hypothalamic and ventral tegmental MFB, the effects of contralateral lesions of both targets are also investigated. Reward effectiveness was assessed using the rate–frequency curve shift paradigm. In nine rats with unilateral PPTg lesions and five rats with unilateral LDTg lesions, the frequency required to maintain half-maximal response rats was generally not changed by more than 0.1 log units relative to prelesion baseline mean. In three rats with contralateral PPTg lesions and four rats with contralateral LDTg lesions, required frequency was also not substantially changed. The results are interpreted in terms of a previously proposed hypothesis regarding the role in MFB self-stimulation of ascending cholinergic input from the pontomesencephalon to ventral tegmental dopaminergic neurons.