Norepinephrine depletion and responsiveness of norepinephrine-sensitive cyclic AMP generating systems in guinea pig brain.

The accumulation of radioactive cyclic AMP was examined in [¹⁴C]adeninelabeled slices of guinea pig neocortex following in vivo treatments designed to lower the levels of putative neurotransmitters such as norepinephrine, serotonin, and histamine. Lesions of the medial forebrain bundle and treatment with 6-hydroxydopamine or reserpine decreased cortical concentrations of norepinephrine in guinea pig. Based on studies in rats, enhanced responses of norepinephrine-sensitive cortical cyclic AMP-generating systems were to be expected after such treatments. Instead, no evidence for enhanced responses of cyclic AMP-generating systems to norepinephrine, serotonin, or histamine was obtained with cortical slices from the treated guinea pigs. Adenosine-sensitive cyclic AMP-generating systems in guinea pig cortical slices did become hyperresponsive after reserpine.     

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

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

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

Midbrain tegmental intracranial self-stimulation (ICSS) was not attenuated by ipsilateral or bilateral locus coeruleus lesions. Certain of these lesions were followed by histochemical confirmation that the majority of locus coeruleus neurons was destroyed, and biochemical evidence that over 80% of the cortical norepinephrine was depleted. To test the possibility that the surviving ICSS was due to stimulation of another norepinephrine system, histochemically verified ipsilateral or bilateral lesions of the ventral norepinephrine bundle were administered to a second group of midbrain tegmental ICSS animals. These lesions resulted in marked loss of body weight, but had no effect on ICSS. In a third experiment, lesions were made in the medial forebrain bundle (MFB) ipsilateral to midbrain tegmental ICSS electrodes. These lesions resulted in attenuation of ICSS which was directly proportional to the extent of MFB damage. On the basis of these data alone, however, it was not possible to identify the ciritical fibers supporting ICSS. It was oncluded that the locus coeruleus does not play a necessary role in midbrain tegmental ICSS.     

Effects of damage to the monoamine axonal constituents of the medical forebrain bundle on reactivity to foot shock and ingestive behavior in the rat.

Rats suffering damage to the medial forebrain bundle evidenced significant decreases in forebrain norepinephrine and 5-hydroxytryptamine as compared to controls; the threshold amount of shock required to elicit a jump response decreased significantly in rats with lesions in the medial forebrain bundle. Rats with lesions in which norepinephrine, but not 5-hydroxytryptamine, was reduced (norepinephrine group) showed a comparable significant decrease in jump threshold. The jump threshold of rats in which 5-hydroxytryptamine, but not norepinephrine, was reduced by midbrain raphe lesions did not change significantly. These results support the conclusion that damage to the ascending norepinephrine axon system is responsible for medial forebrain bundle lesion induced increases in pain sensitivity, as assessed by reactivity to noxious levels of foot shock. Rats in which forebrain dopamine was reduced showed a loss of body weight which was significantly different from the weight gains shown by the control animals. Four of the animals with lesions in the medial forebrain bundle and four of the norepinephrine group animals showed increases in body weight that were well above the weight gains observed in any control animals. Forebrain dopamine level of the norepinephrine group increased significantly over the control level; a number of reasons for this increase, and its possible functional implications, are discussed.     

Crossed unilateral lesions of the medial forebrain bundle and either inferior temporal or frontal cortex impair object-reward association learning in Rhesus monkeys

In an accompanying paper we showed that combined transection of the fornix, amygdala and temporal stem in monkeys produced dense amnesia, including an impairment in visual object-reward association learning. We proposed that this combined surgical section had its effect by isolating temporal cortex from the ascending projections of the basal forebrain and midbrain structures. To test this hypothesis, in the present experiment we disconnected the inferior temporal cortex from these basal forebrain and midbrain structures, while sparing cortical white matter, by crossed unilateral lesions of the medial forebrain bundle in one hemisphere and inferior temporal cortex in the opposite hemisphere. The aim of the medial forebrain bundle lesion was to section axons of cells, both those that project to the cortex via the medial forebrain bundle, and those which control the activity of these same structures. A single unilateral lesion alone had no effect on the ability to learn and remember visual object-reward associations, but the crossed unilateral lesions produced an impairment in this task which was equal in severity to the impairment seen earlier after bilateral section of the fornix, amygdala and temporal stem. The impairment was not an effect of interrupting fibres to the cortex from the ventromedial hypothalamus, or of unilateral sensory neglect. This supports the hypothesis that these midbrain and basal forebrain afferents to the inferior temporal cortex are important for new visual learning. Furthermore, an impairment of equal severity was demonstrated in a separate group of animals that received crossed unilateral lesions of the medial forebrain bundle in one hemisphere and of the frontal cortex in the opposite hemisphere. We propose that the frontal cortex acts to modulate basal forebrain activity which in turn reinforces object representations in the inferior temporal cortex during learning.     

Evidence concerning the anatomical location of the dopamine stimulated adenylate cyclase in the substantia nigra.

The dopamine (DA)-sensitive adenylate cyclase in the substantia nigra was assayed in rats which had been subjected to 3 different kinds of brain lesion: (1) unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle; (2) unilateral lesions of the descending strio-nigral and pallido-nigral projections; (3) total lesions of the serotoninergic raphe-nigral pathway. Lesions of the medial forebrain bundle causing 97% depletion of striatal DA, 72% depletion of nigral tyrosine hydroxylase, and no change in nigral glutamate decarboxylase (GAD), resulted in no change in basal or DA-stimulated cyclic AMP production ipsilateral to the injection. Lesions of the globus pallidus, causing 70% and 79% reductions in GAD and substance P respectively in the ipsilateral nigra, produced a reduction in basal cyclic AMP production and abolished the normal increase in cyclic AMP produced by DA on the side of the lesion. Lesions to the dorsal and median raphe nuclei did not affect the normal DA-sensitive adenylate cyclase response in the nigra. The results suggest that one of the neurotransmitter functions of DA in this brain region may be to modulate the release of psi-aminobutyric acid (GABA) or substance P from synaptic terminals afferent to the nigra.     

Crossed unilateral lesions of medial forebrain bundle and either inferior temporal or frontal cortex impair object recognition memory in Rhesus monkeys

In monkeys, section of the fornix, amygdala and anterior temporal stem results in a severe anterograde amnesia. Immunolesions of the cholinergic cells of the basal forebrain suggest that this amnesia is a result of isolating the inferior temporal cortex and medial temporal lobe from their cholinergic basal forebrain afferents. In this experiment, six monkeys were trained in a delayed match-to-sample task and then received a section of the medial forebrain bundle in one hemisphere and an ablation of either the frontal or inferior temporal cortex in the opposite hemisphere. All the animals were severely impaired in the performance of this task following this surgery, and the severity of the impairment was independent of the cortical area from which the medial forebrain bundle was disconnected. These results support a model of fronto-temporal interaction via the basal forebrain in new learning, in which midbrain sites related to reward modulate the cholinergic basal forebrain activity.     

Generation of cortical event-related slow potentials in the rat involves nucleus basalis cholinergic innervation

These experiments were conducted to gather information regarding the role of cholinergic innervation to the cortex in the generation of event-related slow potentials. The effects of unilateral drug treatments or lesions on ipsilateral and contralateral frontal cortex slow potential (SP) responses were examined in rats. The SP responses were recorded with silver-silver chloride electrodes and were generated by a 2 sec light cue which preceded rewarding medial forebrain bundle stimulation. The following approaches were used: microinjection of GABA, procaine or saline into the nucleus basalis magnocellularis; microinjection of atropine or saline subdurally in the SP recording area; electrolytic lesion of the nucleus basalis area; and kainic acid lesion of the nucleus basalis area. The following bilateral measurements were obtained lesion studies: choline acetyltransferase (ChAT) in cortex and hippocampus; serotonin in cortex, hippocampus, striatum and nucleus accumbens; norepinephrine in cortex and hippocampus; dopamine in striatum and nucleus accumbens; and metabolites of serotonin, norepinephrine and dopamine in these areas. The cortical SP responses were reduced on the side ipsilateral to the injections of GABA and procaine into the nucleus basalis, and on the side of the subdural atropine injection. With either type of lesion, the SP responses on the lesioned side were significantly reduced as compared to the non-lesioned side. Reductions in cortical ChAT and other measures were observed ipsilateral to the electrolytic lesion, but only cortical ChAT activity was reduced in the kainic acid-lesioned animals. Thus, pharmacological depression of nucleus basalis neurons, blockade of cholinergic muscarinic receptors in the cortex, and nucleus basalis lesions that reduce cortical choline acetyltransferase activity depress event-related slow potentials in the rat frontal cortex. These results provide evidence that cortical slow potential responses in the rat are dependent upon cholinergic innervation from the nucleus basalis.     

Lesions of the Cerebral Cortex and Caudate-Putamen Enhance GABA Function in the Rat Superior Colliculus

Unilateral lesions of the rat frontal cortex were made either alone or in combination with the caudate-putamen in order to examine (a) their morphological influence on the substantia nigra and (b) their neurochemical influence on GABA function in the superior colliculus. One to two months following the combined lesion, neuronal somata in the ipsilateral pars reticulata of the substantia nigra were clearly hypertrophied (+ 30%). Morphological changes in the substantia nigra were not evident contralaterally or in animals bearing only cortical lesions. One to two months following cortex-only lesions, no significant alterations in tectal GABA concentration were observed. However, the combined lesion induced elevations of GABA within both the medial and lateral sectors of the intermediate and deep layers of the superior colliculus. This effect was restricted to the ipsilateral side and was most pronounced in lateral sectors. The vast majority of GABA released from superfused control tectal slices by a depolarizing stimulus (35 mM KCl) was calcium-dependent. Such evoked GABA release from ipsilateral tectal slices was significantly reduced (- 25%) by unilateral lesions of the substantia nigra, a structure that is known to provide GABA-containing inputs to the tectum. In contrast, cortical lesions alone significantly enhanced the evoked tectal GABA release (+ 66%), although their influence was again confined to the ipsilateral side. Combined lesions of the cerebral cortex and caudate-putamen significantly enhanced the evoked GABA release from tectal slices in both hemispheres but the changes were most marked ipsilaterally (+ 147%). It is suggested that the hypertrophy of GABA-containing nigrotectal somata seen after removal of corticostriatal, corticotectal and in particular GABA-containing striatonigral fibres may reflect concomitant increases in GABA synthesis within and/or sprouting of nigrotectal terminals.     

Telencephalic efferents of the tiger salamander Ambystoma tigrinurn tigrinum (Green)

The efferent projections of the telencephalon in the tiger salamander were examined by the Nauta and Fink-Heimer methods following unilateral hemispherectomies, rostral hemispheric ablations and pallial lesions. The cerebral hemisphere connects with most areas of the contralateral hemisphere via the pallial, anterior and habenular commissures. The descending fibers travel in the medial and lateral forebrain bundles and in the tracts comprising the stria medullaris. Degenerating fibers and terminals were present throughout the diencephalon but were more abundant ipsilaterally. Fibers reach the pretectum and optic tectum via dorsal and ventral pathways. There is a heavy projection to the midbrain tegmentum and a sparse projection to the tectum via the ipsilateral lateral forebrain bundle. This tract continues into the medulla oblongata and the cervical spinal cord. Rostral and dorsal hemispheric ablations revealed that the majority of fibers forming the olfacto-peduncular tract originate in the ventral, rostral one-third of the hemisphere. It was also determined that the majority of the descending efferent fibers located in the lateral forebrain bundle originate from the caudal lateral hemispheric wall, and that these fibers form connections characteristic of mammalian corticofugal and striatofugal systems. The cytoarchitecture and connections of the caudal lateral hemispheric wall suggest that it is homologous to parts of motor isocortex and amygdala of amniotes.     

Prenatal and early postnatal beta-adrenergic receptor-mediated increase of cyclic AMP in slices of rat brain.

The levels of cyclic AMP in slices of cerebral cortex and cerebellum from newborn rats were significantly, but transiently, increased by exposure to the beta-adrenergic agonist, isoproterenol. Isobutylmethyxanthine, an inhibitor of phosphodiesterase, enhanced this effect and permitted its detection in cerebral cortex obtained from the prenatal rat. These results are consistent with the possibilities that functional noradrenergic synapses are formed early in the ontogeny of the CNS, and that norepinephrine may exert cyclic AMP-mediated influences on brain development.     

Effect of 6-hydroxydopamine on daily variations of 5-HT synthesis in the hypothalamus of the rat

In the cortical gray matter of human brains, which were obtained by permissible resection at the time of surgery, cyclic 3′, 5′-AMP was found in a range of concentrations between 42 and 143 pmole/mg protein. The subcortical white matter and various types of brain tumor contained much lower concentrations of the nucleotide. The level of the cyclic nucleotide in incubated slices of gray matter was elevated 20–50-fold by norepinephrine (0.5–1.0 mM) or veratridine (0.05 mM), 4–7-fold by histamine (1.0mM) or adenosine (0.2mM) and 1.5–2-fold by serotonin. With respect to the stimulatory effect on the cyclic AMP level in human brain slices, norepinephrine was inferior to epinephrine or isoproterenol, and was completely antagonized with a β-adrenergic blocking agent, propranolol, but only slightly by α-blocking agents, such as phentolamine and dibenamine.The particulate fraction of human brain homogenate was capable of synthesizing cyclic AMP from ATP. This enzymic activity was 0.19–0.24 nmole/min/mg protein. This activity was stimulated 2–3-fold by the addition of 10mM NaF but not by any of the biogenic amines tested. The activity of nucleotide phosphodiesterase of human brain was almost equal to that of rat brain: only a single K_m for the substrate cyclic AMP was observed around 0.9 × 10^?4M.     

A horseradish peroxidase study of the olfactory system of the frog, Rana esculenta

The olfactory system of the frog Rana esculenta was studied by using horseradish peroxidase (HRP) tracing of axonal pathways. Injections of HRP were made in the main olfactory bulb (MOB), accessory olfactory bulb (AOB), anterior olfactory nucleus (AON), the amygdala (AMY), and in a zone of the leteral wall of the telencephalic hemisphere immediately posterior to the AOB. Projections from these sites are described and are generally similar to those obtained by degeneration methods. However, HRP reveals more extensive olfactory connections than previously reported. Ipsilateral, contralateral, and bilateral projections are described. The MOB, AOB, and AON have ipsilateral connections to each other. The MOB and AOB have very different projections. The MOB and AON project via the habenular commissure (HC) to the contralateral medial wall of the telencephalon. Ipsilateral MOB fibers also terminate in this cell-free zone where the medial forebrain bundle (MFB) originates. The AOB projects to the lateral cortex of the contralateral telencephalic hemisphere via the HC and also to the ipsilateral AMY and lateral forebrain bundle (LFB) from where some fibers project contralaterally. HRP injections in the AMY retrogradely fill cells in the ipsilateral AOB, two nuclei of the ipsilateral hypothalamus and a nucleus of cells caudal to the ipsilateral nucleus isthmi. Fibers are also labeled that project to the contralateral AMY. Few fibers were observed to decussate in the interpeduncular nucleus or optic chiasma. No olfactory fibers were found to project to the habenular nuclei, and no labeled neurons were found to project to the olfactory bulbs. No morphological asymmetry was observed qualitatively in the distribution of olfactory fibers in the two halves of the brain.     

Altered responsiveness of adenosine 3′,5′-monophosphate-generating systems in brain slices from adult rats after neonatal treatment with 6-hydroxydopamine

Sprague-Dawley rat pups were administered 6-hydroxydopamine (100 mg/kg) subcutaneously on each of the first 4 days after birth. At 4–5 mo of age the rats and untreated littermates were killed, and the accumulation of [¹⁴C]cyclic AMP elicited by various agents in [¹⁴C]adenine-labeled brain slices was investigated. The accumulation of cyclic AMP elicited by norepinephrine was significantly greater in slices from neocortex and midbrain of 6-hydroxydopamine-treated rats than in corresponding slices from littermate controls. Accumulations in slices from medulla-pons and cerebellum were not significantly altered by the neonatal treatment with 6-hydroxydopamine. Thus, an enhanced response to norepinephrine pertains both in a region, the neocortex, where norepinephrine-levels were permanently reduced by neonatal treatment with 6-hydroxydopamine and in a region, the midbrain, where adult levels of norepinephrine were nearly doubled by the neonatal treatment with 6-hydroxydopamine. Enhanced responses of cyclic AMP-generating systems in neocortical slices from 6-hydroxydopamine-treated rats also pertained with isoproterenol, prostaglandin E₁ and an adenosine-norepinephrine combination, but not with adenosine alone. Depletion of norepinephrine throughout the brain by administration of either reserpine or intraventricular 6-hydroxydopamine to adult rats enhanced responses to norepinephrine in slices from neocortex, but not midbrain. Reserpine had no effect on responses in either neocortex or midbrain of rats treated neonatally with 6-hydroxydopamine.     

Sequential changes of cholinergic and dopaminergic receptors in brains after 6-hydroxydopamine lesions of the medial forebrain bundle in rats

Summary. We studied sequential changes in muscarinic cholinergic receptors, high-affinity choline uptake sites and dopamine D₂ receptors in the brain after 6-hydroxydopamine lesions of the medial forebrain bundle in rats. The animals were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks postlesion. [³H]Quinuclidinylbenzilate (QNB), [³H]hemicholinum-3 (HC-3) and [³H]raclopride were used to label muscarinic cholinergic receptors, high-affinity choline uptake sites and dopamine D₂ receptors, respectively. The degeneration of nigrostriatal pathway produced a transient decrease in [³H]QNB binding in the parietal cortex of both ipislateral and contralateral sides at 2 and 8 weeks postlesion. [³H] QNB binding also showed a mild but insignificant decrease in the ipsilateral striatum throughout the postlesion periods. No significant change was observed in the substantia nigra (SN) of both ipsilateral and contralateral sides throughout the postlesion periods. In contrast, [³H]HC-3 binding showed no significant change in the parietal cortex of both ipsilateral and contralateral sides during the postlesion. However, [³H]HC-3 binding was upregulated in the ipsilateral dorsolateral striatum throughout the postlesion periods. The ventromedial striatum also showed a significant increase in [³H]HC-3 binding at 1 week and 2 weeks postlesion. On the other hand, no significant change in [³H]raclopride binding was found in the parietal cortex of both ipsilateral and contralateral sides during the postlesion. [³H]Raclopride binding showed a conspicuous increase in the ipsilateral striatum (35–52% of the sham-operated values in the lateral part and 39–54% in the medial part) throughout the postlesion periods. In the contralateral side, a mild increase in [³H]raclopride binding was also found in the striatum (10–15% of the sham-operated values in the lateral part and 22% in the medial part) after lesioning. However, a significant decline in [³H]raclopride binding was observed in the ipsilateral SN and ventral tegmental area during the postlesion. The present study indicates that 6-hydroxydopamine injection of medial forebrain bundle in rats can cause functional changes in high-affinity choline uptake site in the striatum, as compared with muscarinic cholinergic receptors. Furthermore, our studies demonstrate an upregulation in dopamine D₂ receptors in the striatum and a decrease in the receptors in the SN and ventral tegmental area after the 6-hydroxydopamine injection. Thus, these findings provide further support for neurodegeneration of the nigrostriatal pathway that occurs in Parkinson's disease. Received April 26, 1999; accepted November 12, 1999     

The spontaneous firing pattern of forebrain neurons. I. The effects of dopamine and non-dopamine depleting lesions on caudate unit firing patterns

Unilateral lesions in and around the course of the nigrostriatal pathway (in the substantia nigra, in the supranigral region and more rostrally in the area of the medial forebrain bundle) in cats produced a marked slowing of single unit firing in the caudate nucleus contralateral to the lesion. In addition, the medial forebrain bundle lesion produced a tendency for an increase in unit firing rates in the caudate nucleus ipsilateral to the lesion. Lesions in the supranigral region in monkeys produced indications of a slowing in the firing rate of single units in the contralateral caudate and an increase in the firing rate of units in the caudate ipsilateral to the lesion. These alterations in spontaneous unit firing were independent of lesion-induced changes in concentrations of striatal dopamine or the activities of tyrosine hydroxylase and DOPA decarboxylase.     

Prenatal and early postnatal β-adrenergic receptor-mediated increase of cyclic AMP in slices of rat brain

The levels of cyclic AMP in slices of cerebral cortex and cerebellum from newborn rats were significantly, but transiently, increased by exposure to the β-adrenergic agonist, isoproterenol. Isobutylmethyxanthine, an inhibitor of phosphodiesterase, enhanced this effect and permitted its detection in cerebral cortex obtained from the prenatal rat. These results are consistent with the possibilities that functional noradrenergic synapses are formed early in the ontogeny of the CNS, and that norepinephrine may exert cyclic AMP-mediated influences on brain development.     

Prefrontal cortex lesions attenuate substantia nigra self-stimulation: a reward summation analysis

A curve-shift paradigm was used to assess the effects of lesions of the prefrontal cortex on self-stimulation from electrode sites in the substantia nigra. Combined lesions of the medial and sulcal cortical regions severely attenuated substantia nigra self-stimulation. These results are discussed in the context of the frontal cortex and the substantia nigra as belonging to a reinforcement system that is largely independent of the medial forebrain bundle system.     

Discriminative conditioning-related slow potential and single-unit responses in the frontal cortex of urethane-anesthetized rats

A model is described for obtaining long-term and stable discriminative conditioning-related slow-potential and single-unit responses from the frontal cortex of urethane-anesthetized rats. Responses were recorded and analyzed to reinforced (rewarding medial forebrain bundle stimulation) and non-reinforced tone cues. In the present study, cortical event-related slow potentials provided an adequate index of the level of discriminative conditioning. Single-unit response patterns are described for 57 neurons which demonstrated a discriminative response to either the reinforced or non-reinforced tone cue.     

The Corridor Task: a simple test of lateralised response selection sensitive to unilateral dopamine deafferentation and graft-derived dopamine replacement in the striatum

In this experiment, we report a novel drug-free behavioural test of lateralised neglect which is sensitive to unilateral dopamine-denervating lesions and subsequent graft-derived striatal dopamine replacement. For the task, white plastic lids containing sugar pellets were placed along the left and right sides of the floor of a long narrow corridor at regular intervals. Hungry female Sprague-Dawley rats were placed individually into the corridor where they were allowed to make up to 20 pellet retrievals. The number of retrievals each rat made from its left and right sides was counted. Complete mesencephalic or partial nigrostriatal lesions were induced by injection of 6-hydroxydopamine into the medial forebrain bundle or striatum, respectively. Both lesions induced a pronounced ipsilateral retrieval bias in the task. Five weeks after lesion surgery, half of the rats from each lesion group were given E14 ventral mesencephalic cell suspension transplants into the denervated striatum, and were then re-tested in the Corridor Task 5 and 10 weeks later. There was no amelioration of the side bias in rats with medial forebrain bundle lesions. In contrast, in nigrostriatal-lesioned rats, the graft significantly reduced the lesion-induced ipsilateral bias. We conclude that the Corridor Task is a sensitive test of lateralised sensorimotor response selection, and is suitable for assessing deficits and recovery associated with lesions and grafts within the nigrostriatal system.