I. Walaas

Biochemical evidence for γ-aminobutyrate containing fibres from the nucleus accumbens to the substantia nigra and ventral tegmental area in the rat

Abstract Glutamate decarboxylase activity, a specific marker for γ-aminobutyrate-containing neurons, has been analysed in microdissected samples from rat mesencephalon following unilateral electrocoagulations of the nucleus accumbens. This lesion resulted in a consistent decrease of 50% in the enzyme activity in the rostromedial substantia nigra, and a slight, but insignificant decrease (−15%) in the medial parts of the caudal pars compacta of the substantia nigra. No change was found in the lateral pars compacta or the central pars reticulata. In the ventral tegmental area, the highest activity was found in the rostromedial part, adjacent to the mammillary body. At this level, a significant decrease of 20% was found in the ventral tegmental area on the lesioned side. In contrast, the activities in the medial accessory optic nucleus and the caudal ventral tegmental area adjacent to the interpenduncular nucleus were unchanged. The results indicate that the nucleus accumbens sends γ-aminobutyrate-containing fibres to the rostromedial substantia nigra and to the rostral ventral tegmental area. The caudal ventral tegmental area, the lateral pars compacta and the central pars reticulata do not receive measurable amounts of such fibres.

Biochemical evidence for glutamate as a transmitter in hippocampal efferents to the basal forebrain and hypothalamus in the rat brain.

Abstract The effects of bilateral transection of the fornix bundle on the high affinity uptake of glutamate and on the amino acid content in several nuclei of rat forebrain and hypothalamus were studied in order to investigate the possible role of glutamate as a transmitter of these fibres. This lesion decreased the high affinity uptake of l -glutamate by 60–70% in the mammillary body and lateral septum, and by 40–50% in the anterior diagonal band nucleus, the bed nucleus of the stria terminalis, the mediobasal hypothalamus and the nucleus accumbens. The content of endogenous glutamate in samples dissected from freeze-dried tissue also decreased significantly in these regions. Endogenous aspartate was slightly decreased in the anterior diagonal band nucleus and the mammillary body, but unchanged in the other regions. No significant changes were seen in the levels of serine, γ-aminobutyric acid, glutamine and taurine, except for an increase in glutamine and taurine in the bed nucleus of the stria terminalis. The high affinity uptake of γ-aminobutyric acid, tested in the bed nucleus of the stria terminalis, the mediobasal hypothalamus and the mammillary body, was unchanged after the lesion. The results indicate that allocortical efferents innervating subcortical nuclei through the fornix might use glutamate as a transmitter. The study further supports the concept that glutamate plays an important role as transmitter of several different corticofugal fibre systems in mammalian brain.

Biochemical evidence for overlapping neocortical and allocortical glutamate projections to the nucleus accumbens and rostral caudatoputamen in the rat brain

Abstract The high affinity uptake of l -glutamate has been used to investigate the origin and distribution of putative glutamate fibers in restricted parts of the rostral caudatoputamen and the nucleus accumbens of the rat brain. Ablation of the frontal cortex reduced the glutamate uptake heavily (−77%) in the dorsal part of the ipsilateral caudatoputamen, but also led to significant decreases in the ventral parts of the ipsilateral caudatoputamen (−62% and −53%), in the ipsilateral nucleus accumbens (−25% and −18%) and in the contralateral dorsal part of the caudatoputamen (−21%). Lesion of the caudal neocortex reduced the glutamate uptake in the dorsal part of the ipsilateral caudatoputamen only (−23%). Lesions of the fimbria/fornix reduced the glutamate uptake in both parts of the ipsilateral nucleus accumbens (−46% and −34%) and by approximately 20% in the whole dorsoventral extent of the anterior caudatoputamen. The results indicate that the frontal neocortex distributes fibers which may use glutamate as neurotransmitter both to the whole ipsilateral caudatoputamen and to the nucleus accumbens, and also to the dorsal parts of the contralateral caudatoputamen. The caudal neocortex probably sends such fibers to the dorsal ipsilateral caudatoputamen, and the caudal allocortex sends such fibers through the fimbria/ fornix to the nucleus accumbens and the ventral part of the ipsilateral caudatoputamen. The results thus corroborate previous suggestions of close similarities between the nucleus accumbens and the ventral caudatoputamen.

The effects of surgical and chemical lesions on neurotransmitter candidates in the nucleus accumbens of the rat

Abstract The origin of fibers containing different neurotransmitter candidates in the nucleus accumbens of rat brain has been studied with surgical and chemical lesion techniques. Destruction of the medial forebrain bundle decreased the activity of aromatic amino acid decar☐ylase by 80% in the nucleus. Cutting of the fornix or a hemitransection decreased the high affinity uptake of glutamate by 45% and the endogenous level of glutamate by 33%. The high affinity uptake of glutamate was concentrated in the synaptosomal fraction and the decrease after the lesion was most pronounced in this fraction. Restricted lesions indicated that fibers in the fimbria/fornix coming from the subiculum were responsible for this part of the glutamate uptake in the nucleus. Local injection of kainic acid into the nucleus was accompanied by a 75% decrease in choline acetyltransferase and a 35% decrease in acetylcholinesterase activities, a 70% decrease in glutamate decar☐ylase activity and a 60% decrease in the high affinity uptake of γ-aminobutyrate, a 45% decrease in high affinity glutamate uptake, and no change in aromatic amino acid decar☐ylase activity. Performing a lesion of the fornix after kainic acid injection led to an 85% decrease in high affinity glutamate uptake, without further affecting the other neuronal markers. The results indicate that all aminergic fibers to the nucleus accumbens are ascending in the medial forebrain bundle, that the subiculum—accumbens fibers are ‘glutamergic’, and that the nucleus also contains intrinsic glutamergic or aspartergic cells. Cholinergic and γ-aminobutyrate-containing cells are wholly intrinsic to the nucleus.

LOCALIZATION OF GABAERGIC, CHOLINERGIC AND AMINERGIC STRUCTURES IN THE MESOLIMBIC SYSTEM

Abstract— The localization of cholinergic, GABAergic and aminergic structures in the ‘mesolimbic’ system has been discussed from studies on the topographical distribution of choline acetyltransferase, glutamate decarboxylase and aromatic amino acid decarboxylase in normal rat brain and in brains hemitransected at the level of globus pallidus. The structures analysed included nucleus accumbens, olfactory tubercle, septum, medial forebrain bundle, striatum, substantia nigra, ventral tegmental area and nucleus interpeduncularis.

The distribution and origin of glutamate decar☐ylase and choline acetyltransferase in ventral pallidum and other basal forebrain regions

The distribution of choline acetyltransferase (ChAT) and glutamate decarboxylase (GAD), and the histochemical reaction for acetylcholinesterase have been studied in the basal forebrain and globus pallidus of unoperated rats and in rats with an electrolytic lesion of the nucleus accumbens. ChAT was highly concentrated in the substriatal region, the neostriatum and the lateral part of the rostral substantia innominata. The strongest intensity of staining for acetylcholinesterase was found in the substriatal grey and the neostriatum. Very high GAD activity was found in the substantia innominata, being even slightly higher than that in the pars reticulata of the substantia nigra. The lateral preoptic area, the bed nucleus of the stria terminalis and the globus pallidus also showed high activity of GAD. After lesions of the nucleus accumbens the activity of GAD decreased significantly in the substantia innominata and in a restricted part of the rostroventral globus pallidus, but not in the other regions studied. ChAT activity and acetylcholinesterase staining were unaffected in all regions. The results indicate that a dense GABAergic projection originates in the nucleus accumbens and terminates in the rostral substantia innominata and rostroventral part of the globus pallidus. The study gives neurochemical support to the suggestion that nucleus accumbens may be regarded as a ventral part of the neostriatum and that the rostral substantia innominata may be regarded as a ventral part of the globus pallidus.

THE EFFECT OF INTRAHIPPOCAMPAL KAINIC ACID INJECTIONS AND SURGICAL LESIONS ON NEUROTRANSMITTERS IN HIPPOCAMPUS AND SEPTUM

Local injection of kainic acid (2 μg) was accompanied by destruction of intrinsic neurons in the dorsal part of hippocampus. The lesion was accompanied by a 75% reduction in glutamate decarboxylase activity, a 60% reduction in the high affinity uptake of l‐glutamate, a 40‐60% reduction in the endogeneous levels of aspartate, glutamate and GABA and no changes in the activities of choline acetyltransferase or aromatic amino acid decarboxylase in the dorsal hippocampus. Unilateral destruction of neurons in the dorsal hippocampus was followed by a 20‐40% reduction in the high affinity uptake of glutamate in lateral, but not in medial septum, on both sides. There was no reduction in choline acetyltransferase, glutamate decarboxylase or aromatic amino acid decarboxylase activities in the lateral or medial part of the septum. Transection of fimbria and superior fornix was accompanied by a severe reduction in choline acetyltransferase and aromatic amino acid decarboxylase activity in hippocampus, in the high affinity uptake of glutamate and in the endogenous level of glutamate in the lateral septum. The results are consistent with the concept that in the hippocampus kainic acid destroys intrinsic neurons and not afferent fibres. It seems therefore that all GABAergic fibres in the hippocampus belong to intrinsic neurons whereas glutamergic and aspartergic neurons belong partly to local neurons. The connection from the hippocampus to the lateral septum probably uses glutamate as a transmitter.

Localization of Neurotransmitters, Particularly Glutamate, in Hippocampus, Septum, Nucleus Accumbens and Superior Colliculus

Publisher Summary This chapter presents a survey to identify the neurotransmitter in certain well-defined fibers in the hippocampal–septal system including the nucleus accumbens septi. The transmitters present in the neurons are identified by correlating the distribution of the transmitter marker to the distribution of axon terminals as defined by previous anatomical investigation; by studying the changes in the marker after interruption of specific afferent fibers and after destruction of intrinsic neurons by local injection of kainic acid; and also by studying the release of transmitter marker after stimulating specific nerve fiber bundles. The chapter discusses the structure as a ventral part of striatum, and its efferent projection as a ventral striato-pallidal pathway. Superior colliculus is, like the hippocampus, a laminar structure where many of the afferent fibers are well known. Hippocampus is a laminated structure where the pyramidal and granular cells are distributed in single layers that extend throughout the hippocampus proper and area dentata, respectively. The hippocampal neurons are excited by a series of known fibers such as commissural, perforant path, mossy fibers, and Schaffer collaterals, and inhibited by for example basket cells. The fibers are organized in a layered manner parallel to the cell bodies. In addition, all major neuronal excitatory members as well as the arteries to the hippocampal formation are arranged in lamellae parallel to each other in a plane nearly transverse to the longitudinal axis of the hippocampus.

The effect of parenteral glutamate treatment on the localization of neurotransmitters in the mediobasal hypothalamus

The localization of cholinergic, aminergic and amino acid-ergic neurones in the mediobasal hypothalamus has been studied in normal rat brain and in brains where neurones in nucleus arcuatus were destroyed by repeated administration of 2 mg/g body weight monosodium glutamate to newborn animals. In normal animals acetylcholinesterase staining, choline acetyltransferase and aromatic L-amino acid decarboxylase were concentrated in the median eminence and the arcuate nucleus. Glutamate decarboxylase was concentrated at the boundary between the ventromedial and the arcuate nuclei, with lower activity in the arcuate nucleus and very low activity in the median eminence. Nucleus arcuatus contained an intermediate level of high affinity glutamate uptake. In the lesioned animals, there were significant decreases in choline acetyltransferase, acetylcholinesterase staining and glutamate decarboxylase in the median eminence, whereas choline acetyltransferase activity and acetylcholinesterase staining, but not glutamate decarboxylase activity, were decreased in nucleus arcuatus. Aromatic L-amino acid decarboxylase was unchanged in all regions studied. The high affinity uptakes of glutamate, dopamine and noradrenaline, and the endogenous amino acid levels were also unchanged in the treated animals. The results indicate the existence of acetylcholine- and GABA-containing elements in the tuberoinfundibular tract. They further indicate that the dopamine cells in the arcuate nucleus are less sensitive to the toxic effect of glutamate than other cell types, possibly because they contain less glutamate receptors.

Localization of Neurotransmitter Candidates in Neostriatum

Publisher Summary This chapter discusses thelocalization of neurotransmitter candidates in neostriatum. It reviews the distribution of neurotransmitter candidates and receptors in the neostriatum, n. accumbens, and pallidum—both the external and internal segment—which is called nucleus entopeduncularis in rodents. The distribution of cholinergic parameters demonstrates that the activity of choline acetyltransferase and the endogenous level of ACh are high in neostriatum and n. accumbens, and low in globus pallidus. Apart from n. interpeduncularis, they are among the highest levels in brain. The ACh turnover in neostriatum and n. accumbens is rapid, which further indicates a considerable cholinergic activity in these regions. A high density of ACh receptors is also present in neostriatum. They are mostly muscarinic in nature, as measured by quinuclidinyl benzilate binding, but a sizeable population of proteins binding to bungarotoxin is also present. Several peptide transmitter candidates have been found in the basal ganglia. Methionine–enkephalin, whose Ca++ dependent release from globus pallidus has been demonstrated, is concentrated in globus pallidus, but it is also present in neostriatum and n. accumbens.