Distribution of putative neurotransmitters in the neocortex

Fine structural studies on a type of somatostatin-immunoreactive neuron and its synaptic connections in the rat neostriatum: a correlated light and electron microscopic study

Somatostatin-immunoreactive neurons in the rat neostriatum were studied by correlated light and electron microscopy using the peroxidase-antiperoxidase immunocytochemical technique. Immunoreactivity was localized in neuronal perikarya and processes. The perikarya were of spindle or fusiform shape (average length 16.9 microns) and were found in all parts of the neostriatum. From each neuron there arose two to four straight immunoreactive dendritelike processes, which could frequently be traced as far as about 130 microns from their perikaryon. Immunoreactive varicose axonlike processes were occasionally found, some of which were proximal axons of identified immunoreactive cells. Nine of the light microscopically identified neurons showing somatostatin-immunoreactivity were studied in the electron microscope; two of them had proximal axons with varicosities. Each neuron had an oval or elongated nucleus, which was always indented. These morphological features correspond well to those of certain "medium-size aspiny" neurons classified by Golgi studies. Although the immunoreactive endproduct was diffusely located throughout the neuron, it was characteristically located in the saccules and large granules (diameter 133 nm) of the Golgi apparatus, and large immunoreactive vesicles of similar size to those in the Golgi apparatus frequently occurred in all parts of axon. Very little synaptic input was found on the perikarya and dendrites of somatostatin-immunoreactive neurons. The perikarya and proximal dendrites received both symmetrical and asymmetrical synaptic input, while the distal dendrites usually received boutons that formed asymmetrical contacts. The somatostatin-immunoreactive boutons contained pleomorphic electron-lucent vesicles (diameter 39.3 nm) and a few large immunoreactive granular vesicles; these boutons always formed symmetrical synapses. Their postsynaptic targets were dendritic shafts, spines, and unclassified dendritic profiles. On the other hand, the varicosities of identified proximal axons of somatostatin-positive neurons did not form typical synapses, since they lacked clusters of small vesicles, but some of them were in direct apposition (via membrane specializations) to unlabelled perikarya or dendrites. It is concluded that somatostatin is a useful marker for a particular type of neuron in the neostriatum. The presence of somatostatin immunoreactivity in synaptic boutons is consistent with the view that somatostatin could be a neurotransmitter in the neostriatum.

Immunohistochemical demonstration of serotonin nerve fibers in the neocortex of the monkey (Macaca fuscata)

Using a peroxidase-antiperoxidase immunohistochemical method, the distribution of serotonin nerve fibers was studied in the neocortex of the monkey (Macaca fuscata). All layers of the neocortex showed evidence of serotonin fibers, both fine and thick. Unlike the distribution in rodents, different patterns of distribution were noted in each neocortex of the primate. Among the cortical areas--area 4, 3-1-2, 17, 18, 41 and 42--the primary visual cortex (area 17) contained the highest density of immunoreactive fibers, while the primary motor cortex (area 4) possessed the lowest concentration. The most outstanding finding was a dense and laminar distribution of serotonin fibers in area 17, particularly within the upper portion of layer IVc. In area 3-1-2, 18, 41 and 42, a fairly uniform density of immunoreactive fibers was observed across the six cortical layers, apart from a relatively dense plexus of fine serotonin fibers in layer IV.

Pathological basis for neurotransmitter changes in Parkinson's disease

In eight patients with idiopathic Parkinson's disease, loss of nerve cells from the locus caeruleus, together with reductions in capacity for function in those of the dorsal motor vagus nucleus, the supraoptic and paraventricular nuclei of the hypothalamus and the nucleus basalis of Meynert in the substantia innominata are reported. These changes were more severe in three of these eight patients where mental impairment was present, though in none of these three was the number of senile plaques and neurofibrillary tangles or extent of granulovacuolar degeneration greater in the hippocampus and other cerebral cortical areas than would be expected for the age of the patient. Although these same three patients had received long term L-dopa, it seems that their pathology was related to the disease process and was not drug induced. It is possible therefore, that the dementia seen in some patients with Parkinson's disease is related to degeneration and failing function of noradrenergic and cholinergic neurotransmitter systems and not always associated with the coincidental acquisition of Alzheimer type lesions (Alzheimer's disease).

Neuron loss in the nucleus basalis of Meynert in parkinsonism-dementia complex of Guam

The nucleus basalis of Meynert, which supplies diffuse cholinergic fibers to the cerebral neocortex, was investigated in two cases of parkinsonism-dementia complex of Guam (PDG). The nucleus basalis of the two PDG patients showed extensive neuron loss when compared with age-matched non-Guamanian controls, suggesting the necessity of investigating activity of cholinergic enzymes in the brains of patients with PDG.

Acute subcortical lesions modify cortical muscarinic receptors in human brain

Post-mortem determinations of muscarinic cholinergic receptor parameters by means of binding procedures were carried out in human brains. In patients who died from internal capsule stroke a significant increase in cortical (Brodmann area 8) muscarinic receptor density was present when compared to non-neurological controls. No significant changes were detected in cortical choline acetyltransferase. Subcortical structures such as thalamus and caudate nucleus seemed to undergo opposite effects. It is suggested that acute interruption of fibers ascending to the cortex from subcortical areas can alter muscarinic receptor properties in the cerebral cortex.

A search for discrete cholinergic nuclei in the human ventral forebrain

Slices cut from five frozen human brains were dissected into 2-mm cubes and assayed for choline acetyltransferase (ChAT) activity and protein content. A pattern of enrichment of ChAT activity was found ventral to the anterior commissure; this finding is consistent with the location of the enzyme in the cells of the nucleus basalis of Meynert. The region beneath the anterior commissure was the only place a discrete enrichment of activity could be found, and the precise topography of the enrichment was somewhat variable from brain to brain. The results are discussed in the light of recent knowledge concerning the source of the cortical cholinergic innervation.

Lesions of cholinergic forebrain nuclei: changes in avoidance behavior and scopolamine actions

The acquisition of active (shuttle-box) and passive avoidance conditioned responses and the effects of scopolamine on acetylcholine (ACh) output in freely moving rats and on conditioned responses were investigated 20 days after placing a unilateral lesion in the magnocellular forebrain nuclei (MFN). In the lesioned rats spontaneous ACh output from the cerebral cortex ipsilateral to the lesion was slightly decreased, while on the other hand the increase in ACh output elicited by scopolamine was strongly reduced. Sham operated rats always performed more active avoidance responses than MFN lesioned rats in the daily training shuttle-box sessions, and the facilitating effect of scopolamine (1 mg/kg IP) on the shuttle-box performance was suppressed. However the lesion did not disrupt the shuttle-box performance whenever training had taken place before the lesion. In the lesioned rats retested 30 min after the training trial, an impairment of the passive avoidance response was found. The effect of the lesion was potentiated by scopolamine. The results show therefore that MFN lesions impair the cortical cholinergic mechanisms, whose activity seems to play an important role in cognitive functions.

Somatostatin is not co-localized in cholinergic neurons innervating the rat cerebral cortex-hippocampal formation

The somatostatin content and choline acetyltransferase activity of the rat cerebral cortex and hippocampus were examined after lesions to the nucleus basalis, the fornix and the dorsal hippocampus. Lesions of the nucleus basalis caused reductions in cortical choline acetyltransferase activity but had no effect on the concentration of somatostatin. Fornix transection caused a reduction in choline acetyltransferase activity in the dorsal hippocampus, but again was without effect on the somatostatin content. Excitotoxin lesions of the dorsal hippocampus caused a 69% reduction in somatostatin concentration in this structure, with no reduction in choline acetyltransferase activity. The results indicate that somatostatin and choline acetyltransferase are in separate populations of neurons in the rat cerebral cortex and hippocampus.

Cholinergic projections from the basal forebrain to frontal, parietal, temporal, occipital, and cingulate cortices: a combined fluorescent tracer and acetylcholinesterase analysis

The morphologies, intercellular organization, and cortical projection patterns of putative cholinergic neurons in the basal forebrain of the rat were examined by use of fluorescent tracer histology in combination with the pharmacohistochemical regimen for acetylcholinesterase (AChE). Intensity staining AChE-containing cells projecting to frontal sensorimotor (Area 10), parietal (Area 2), and temporal (Area 4) cortices were found ipsilaterally in nucleus preopticus magnocellularis, in nucleus basalis, and in association with the substantia innominata, the ansa lenticularis, and the lateral hypothalamic area; an essentially rostrocaudal topography was observed for these projections. AChE-containing pathways to cingulate (Area 29) and visual (Area 17) cortices derived from ipsilateral somata associated with the vertical and horizontal limbs of the diagonal band, nucleus preopticus magnocellularis, rostral portions of nucleus basalis, and the substantia innominata. Neurons innervating Area 29 were generally located more rostrally than those giving rise to AChE afferents to Area 17. The vast majority of cells appeared to innervate relatively discrete areas of the cortex. Evidence for collateralization was found only in neurons projecting to visual and cingulate cortices, and these represented only 3.2% of the cells providing AChE afferents to Areas 17 and 29. The basal forebrain AChE projection cells were typically large (greater than 25 micron in maximum cell body extent), and their somata were predominantly oval, with lesser proportions being fusiform or triangular. Many were organized in clusters, particularly in nucleus basalis.

Excitatory effect of acetylcholine on different types of neurons in the first somatosensory neocortex of the rat: laminar distribution and pharmacological characteristics

In rats anaesthetized with either urethane, pentobarbital or fluothane the effects of acetylcholine, cholinergic agonists and antagonists (applied by iontophoresis) were studied on single cortical neurons of first somatosensory region. The laminar distribution of the neurons excited by acetylcholine was determined by the reconstruction of each electrode track based on a dye-deposit made at the last recording site. Neurons were identified using antidromic stimulation of the pyramidal tract, the ventrobasal thalamus and the corpus callosum. Neurons excited by acetylcholine could be segregated into two groups: one encompassing layer Vb and the upper part of layer VI, the other more deeply located at the limit between the cerebral cortex and the subjacent white matter. Neuronal responses to glutamate and nicotine, unlike those to actylcholine were evenly distributed in the cortex. Pyramidal tract neurons had corticothalamic neurons were frequently excited by acetylcholine and were shown to be located with the first group of acetylcholine sensitive neurons. Commissural neurons were rarely excited by acetylcholine and were not restricted to either group. The analysis of neuronal responses to acetylcholine and various agonists (carbachol, nicotine, acetyl-beta-methylcholine, carbamyl-beta-methylcholine, butyrylcholine) and antagonists (atropine, mecamylamine) revealed a prominent but not exclusive muscarine character. It is included (i) that cortical neurons of first somatosensory cortex which are excited by acetylcholine belong to two populations, one consisting, at least in part, of projection neurons (upper group) and the other of interneurons (lower group); (ii) that cortical acetylcholine receptors are of a 'mixed' type strongly weighted toward the muscarinic side.

Biogenic monoamine turnover in discrete rat brain regions is correlated with conditioned emotional response and its conditioning history

The content and turnover of dopamine, norepinephrine and 5-hydroxytryptamine (serotonin), and the content of their respective major metabolites were evaluated in 19 discrete brain areas of rats exposed to conditioned emotional response (CER), and in control groups which received either equivalent yoked shock (shock only) or compound stimulus presentation (tone only). On test day, CER animals suppressed responding and exhibited forms of emotional behavior after presentation of the conditioned stimulus (CS); while shock only and tone only control groups, and CER animals which received an acute dose of diazepam prior to testing, did not suppress. Few changes were observed in content of the biogenic amines or their metabolites, suggesting that the behavioral manipulations were acting within normal physiological limits. On the other hand, numerous changes were observed in the utilization of the 3 biogenic monoamines, which were correlated with the conditioning-anxiety (comparisons of CER vs shock only) and the shock history (comparison of shock only vs tone only). These observations are consistent with putative neural pathways in the frontal cortex, septum, nucleus accumbens, amygdala, striatum, hippocampus and brain stem (which utilize specific monoamines), and with discrete brain areas which have been implicated in classical conditioning and CER-related phenomena. These observations suggest roles for biogenic monoamines in mediating or responding to the classical conditioning and emotional components of the paradigm.

Somatostatin: localization and distribution in the cortex and the subcortical white matter of human brain

Examination of the cortex and the subcortical white matter by use of an immunocytochemical technique--the per oxidase anti-per oxidase method--shows that somatostatin is located in a widespread neuron system with cell bodies localized in both the cortex and the subcortical white matter of the human brain. In the cortex, the somatostatin cell bodies and fibers are found in all layers, but the fibers are especially numerous in layer I located tangentially to the brain surface. The fibers are very long and subdivide into many branches which form a network of pathways in the deeper cortical layers. There are numerous varicosities along the fibers and they come into close contact with other non-immunoreactive neuronal cells. The somatostatin cells located in the white matter are larger than the somatostatin cells in gray matter. They are giant cells with a size ranging from 50 to 120 micrometers. The fibers from these cells are varicose and can be followed both rostrally into the cortical gray matter and caudally in the subcortical white matter. The localization and the morphology of the somatostatin neurons in the cortex and the subcortical white matter indicate that somatostatin may be able to exert sustained influence in various brain areas and thereby modulate integrative and/or specific functions, not only via connection in the gray matter but also by influencing the neuronal circuits passing through the subcortical white matter.

Cholinergic innervation in neuritic plaques

Although several studies of Alzheimer's disease suggest that the frequency of neuritic plaques in the cerebral cortex is correlated with the severity of dementia and with reduction in presynaptic cholinergic markers in the cortex, the relationship between cholinergic cortical innervation and the pathogenesis of plaques is unknown. The hypothesis was tested that the neurites in the plaque consist, in part, of presynaptic cholinergic axons, many of which arise from neurons in the basal forebrain. This hypothesis was tested by analyzing the character and distribution of plaques in monkeys, aged 4 to 31 years, with staining for acetylcholin-esterase and also with Congo red and silver stains. Immature and mature plaques were rich in acetylcholinesterase. As the plaques matured, the amount of amyloid increased, and the number of neurites and the activity of acetylcholinesterase decreased. End-stage amyloid-rich plaques lacked acetylcholinesterase. These observations indicate that changes in cortical cholinergic innervation are an important feature in the pathogenesis and evolution of the neuritic plaque.

Brain neurotransmitter turnover correlated with morphine-seeking behavior of rats

Neurochemical substrates of intravenous opiate self-administration were investigated in rats using littermate controls for vehicle and passive morphine infusion. The rates of turnover of the putative neurotransmitters, dopamine, norepinephrine, serotonin, gamma-aminobutyric acid, aspartate and glutamate were concurrently measured in eleven brain regions of rats intravenously self-administering morphine and yoked-morphine or yoked-vehicle infused littermates. The passive infusion of morphine resulted in significant changes in the rates of turnover of the biogenic monoamine and amino acid neurotransmitters in six brain regions with the caudate nucleus-putamen-globus pallidus showing the most changes. The contingent infusion of morphine resulted in changes in utilization rates that were generally greater in both magnitude and number than the effects of the drug itself. Twenty-nine significant changes were observed in the self-administering group with most changes occurring in limbic structures. The neurotransmitter turnover rate changes resulting from contingent administration suggest that the drug administration environment is an important factor that should be considered in studies of interactions between drugs and neuronal systems.

Neuropeptides and cerebral electric activity in rabbits

The effects of some neuropeptides infused into the cerebral ventricles on the spontaneous cerebral electric activity were studied in unanasthetized rabbits. The following peptides were investigated: physalaemin, caerulein, bombesin, litorin (supplied by Farmitalia). The rabbits were prepared according to Monnier and Gangloff's [10] method in order to record the spontaneous cortical activity. Each of these substances affects the electroencephalographic (EEG) records in a specific and dose-related way. Bombesin induces a biphasic pattern (synchronization followed by a partial activation), litorin is partially activating and physalaemin brings about a marked desynchronization. In spite of the marked structural analogy between bombesin and litorin, their EEG effects differ.

The substantia innominata in Alzheimer's disease: an histochemical and biochemical study of cholinergic marker enzymes

Acetylcholinesterase staining was examined in the substantia innominata of 3 normal human brains. Large intensely stained neurones were seen within the region of the basal nucleus of Meynert which is believed to be the origin of the cholinergic projection to the neocortex in animals. On the basis of the acetylcholinesterase staining pattern, the substantia innominata was dissected from post-mortem brain tissue of 19 cases of Alzheimer's disease (AD) and 16 controls so as to include the basal nucleus. Choline acetyltransferase (ChAT) activity was found to be reduced in the substantia innominata and amygdala in AD but not in the adjacent lentiform nucleus and hypothalamus.

Changes in high affinity choline uptake in rat cortex following lesions of the magnocellular forebrain nuclei

High affinity choline uptake (HACU) and choline acetyltransferase (CAT) were measured in the cerebral cortex of rats 4 and 20 days after placing electrolytic lesions in the magnocellular forebrain nuclei (MFN) or in the pallidum. Four days after MFN lesion a 40-50% decrease in ipsilateral cortical HACU was found and a slightly smaller decrease was found 4 days after the pallidum lesion. Twenty days after the lesion, HACU activity returned to control values in the ipsilateral parietal cortex, its decrease was smaller than 4 days postlesion in the ipsilateral frontal cortex and a significant increase was found in the contralateral cortex. CAT activity showed a 40% decrease in the frontal, parietal and occipital ipsilateral cortex 4 days after MFN lesion. The same decrease was found 20 days postlesion. However, at this time a significant increase in CAT activity was detected in the contralateral cortex. The ipsilateral recovery of HACU activity 20 days after the lesions and the contralateral increase in HACU and CAT activity demonstrate the remarkable and widespread functional adjustment associated with discrete brain lesions. The existence of a large cholinergic pathway projecting to the neocortex from the basal forebrain region is also confirmed.

Brainstem innervation of prefrontal and anterior cingulate cortex in the rhesus monkey revealed by retrograde transport of HRP

The cells of origin of projections from the brainstem to the dorsolateral and orbital prefrontal granular cortex and to the anterior cingulate cortex of the rhesus monkey were analyzed by means of retrograde axonal transport of the enzyme horseradish peroxidase (HRP). Following injections in various portions of the dorsolateral prefrontal and in the cingulate cortex, HRP-positive neurons were found in three main locations: (1) the ventral midbrain including the anterior ventral tegmental area, the medial one-third of the substantia nigra pars compacta, and the retrorubral nucleus; (2) the central superior nucleus and the dorsal raphe nucleus, primarily in its caudal subdivision; and (3) the locus coeruleus and adjacent medial parabrachial nucleus. Labeled neurons in the raphe nuclei and locus coeruleus were distributed bilaterally. A basically similar pattern of labeled somata was found in the brainstem with HRP injections in the orbital prefrontal cortex. Scattered HRP-positive cells were found throughout the ipsilateral ventral tegmental area and in ventromedial portions of the retrorubral nucleus, and a large number of HRP-positive cells were distributed bilaterally in the dorsal raphe and central superior nuclei as well as the dorsolateral pontine tegmentum. However, in contrast to the results obtained with injections on the dorsolateral and medial aspects of the hemisphere, labeled neurons were not found in any portion of the substantia nigra. The neurons labeled retrogradely after injection of HRP in these various regions of the frontal lobe in rhesus monkey correspond both in location and morphology to the monoamine-containing neurons of the brainstem and are thus very likely the source of dopamine, norepinephrine, and serotonin found in the frontal cortex of the same species.

Acetylcholinesterase in pontomedullary catecholamine neurons of the adult albino rat

This study investigated the cholinesterasic reactivity of catecholamine neurons in the rat hindbrain with the aid of a two-step histochemical procedure. First, catecholamine cells were visualized by their formaldehyde/glutaraldehyde induced specific histofluorescence and then poststained in the same tissue with a thiocholine technique for acetylcholinesterase (AChE). Processing the vibratome-sectioned tissue in phosphate buffer subsequent to initial aldehyde fixation permitted satisfactory preservation of both amine fluorophores and esterasic reactivity. Our results, in both randomly sampled and serially sectioned material, unequivocally establish the presence of AChE in all pontomedullary cell groups emitting catecholamine fluorescence, the majority of which are known to consist of noradrenaline perikarya. Hence in contrast to previous reports the occurrence of AChE in central noradrenaline neurons appears to be generalized. The intensity of histofluorescence and esterasic staining were uncorrelated in most regions. It remains for future study to determine whether AChE in brain catecholamine neurons indicates their cholinoceptivity or subserves the catabolism of other neuromediators such as substance P.

Amino acid neurotransmitter utilization in discrete rat brain regions is correlated with conditioned emotional response

The content and utilization of amino acid neurotransmitters were evaluated in discrete brain areas of rats exposed to a conditioned emotional response (CER) procedure and in control groups which received either equivalent yoked shock history (shock only) or compound stimulus presentation (tone only). On test day, CER animals suppressed responding and exhibited anxious behavior after presentation of the CS, while shock only and tone only control groups, or CER animals which received an acute dose of diazepam prior to testing, did not suppress. Few changes were observed in the content of amino acids, suggesting that the behavioral manipulations were acting within normal physiological limits. On the other hand, numerous changes were observed in the utilization (turnover, metabolism) of the amino acid neurotransmitters. The effects of a history of shock presentation (shock only versus tone only) were persistent long after the conditioning sessions were terminated, and resulted in decreased turnover of the amino acids in many areas. CER conditioning-emotion (CER versus shock only) produced an increase in the turnover of aspartate and glutamate in many structures, while changes in GABA turnover were generally limited to decreases in limbic areas. If CER represents an animal model of anxiety, these observations may suggest roles for neurons which utilize amino acids in mediating or responding to emotional components of the paradigm.

Altered neuronal responsiveness to biogenic amines in rat cerebral cortex after serotonin denervation or depletion

To further investigate monoaminergic mechanisms in cerebral cortex, responsiveness of cortical neurons to microiontophoretic applications of serotonin (5-HT), dopamine (DA) or noradrenaline (NA) was examined in the frontoparietal region of control, 5,7-dihydroxytryptamine (5,7-DHT)- and p-chlorophenylalanine (PCPA)-treated rats anesthetized with urethane. As a rule, 100 nA applications of either one of these biogenic amines induced marked slowing or total interruption of 'spontaneous' firing overlasting the 30 s period of ejection. Given the large amounts of monoamines ejected, it could be inferred that such microiontophoretic applications produced a maximal activation of receptors. In control rats, the responses to 5-HT, DA and NA were of approximately equal duration (approximately equal to 5 min). Two to 4 weeks after denervation with 5,7-DHT, most neurons (75%) exhibited greatly prolonged responses to 5-HT (approximately equal to 14 min), and marked depressions of firing could be induced by small ejection currents (approximately equal to 2 nA) having little or no effect in the controls. In addition, 85% of the units supersensitive to 5-HT showed considerably shortened responses to DA and NA (approximately equal to 1 min). After 2-14 days of depletion with PCPA, there was no change in the responsiveness to 5-HT in spite of a 91% lowering of cortical 5-HT content equivalent to that measured after denervation. Nevertheless, responsiveness to DA and NA was again diminished in a majority (80%) of the units tested. In control or PCPA-treated rats, acute administration of the 5-HT re-uptake blocker fluoxetine increased the duration of depressions induced by 100 nA applications of 5-HT but did not enhance responsiveness to low ejection currents. This suggested that, after 5-HT denervation, the suppression of re-uptake was mainly responsible for the prolongation of 5-HT responses ('presynaptic' component of supersensitivity), whereas a modification of 5-HT receptors accounted for the greater efficacy of small doses of 5-HT ('postsynaptic' component). Responsiveness to the microiontophoretic application of phenylephrine (PHE), a noradrenergic a-agonist, was comparable with that to NA in PCPA- and 5,7-DHT-treated as well as in control rats. Therefore, the hyposensitivity to DA and NA appeared indicative of a desensitization of catecholamine receptors caused by the absence of 5-HT. Such a desensitization may be viewed as an adaptive change resulting from an increased release of endogenous DA and NA. This interpretation would in turn imply that, normally, 5-HT regulates catecholamine release in the neocortex.

The structure of cerebral cortex in the rat following prenatal administration of 6-hydroxydopamine

The early, prenatal formation of noradrenergic projections to the forebrain has led to the proposition that these axons exert a trophic influence on cerebral cortex during ontogeny. To test this hypothesis, we have examined a number of different structural features of cortical development following prenatal lesions of the ascending noradrenergic axons. The parameters that were analyzed include cytoarchitecture, dendritic morphology, and the distribution of monoaminergic and nonmonoaminergic cortical afferents. Rat fetuses were administered the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) by transuterine, intraperitoneal injection on embryonic day 17. Vehicle-injected controls and fetuses treated with the catecholamine uptake inhibitor desmethylimipramine (DMI) prior to 6-OHDA were prepared. After reaching maturity (200-300 g), the brain of treated and control rats were examined using Nissl and Golgi preparations (for cytoarchitecture and dendritic morphology), histofluorescence (for monoaminergic afferents, especially dopaminergic axons), and serotonin and dopamine-beta-hydroxylase (DBH) immunocytochemistry. Effective lesioning of the ascending noradrenergic system was confirmed in each case, using DBH immunocytochemistry. Prenatal treatment with 6-OHDA resulted in complete and long-lasting destruction of the noradrenergic innervation of the cerebral cortex, along with hyperinnervation of the diencephalon and brain stem. Despite the widespread denervation of cerebral cortex, no significant alterations in cytoarchitecture, dendritic morphology, or spine counts were found in treated brains. In particular, no abnormalities were observed in the apical dendrites of layer VI pyramidal cells, based on qualitative criteria. The distribution, density and morphology of serotonergic and dopaminergic afferents were unaffected. Thalamocortical afferents had developed normally as reflected by the cortical barrels. In 33% of the 6-OHDA-treated fetuses foci of ectopic neurons were found at the cortical surface. The ectopias contain neuronal processes, somata, and synapses interspersed with collagen and other connective tissue elements. While the ectopias may result from selective damage to the noradrenergic neurons, the finding of similar (but smaller) malformations in DMI-protected animals is equally consistent with a non-specific effect of 6-OHDA upon non-adrenergic cells. The examination of intervening stages will be needed to resolve this question. Based on the parameters of cortical structure analyzed in this study we conclude that the neocortex develops normally even in the absence of the noradrenergic system.

Chemical circuitry and central cardiovascular regulation

In attempting to sort out the functional components responsible for central cardiovascular regulation, circuits which can be defined by their neurotransmitter provide a focus for an initial simplified scheme. Three classes of transmitter-specific circuits can be developed from this perspective. The most prominent class, amino acid-mediated circuits, are illustrated by the presumptive excitatory glutamatergic excitatory efferent projections from baroreceptors to the nucleus tractus solitarius (NTS) and by presumptive inhibitory interneurons intrinsic to reticular system which use gamma-aminobutyrate as a transmitter. Secondly, monoamine-mediated intrinsic and extrinsic projections to the NTS are generally subsumed neuroanatomically within the so-called "non-specific" projection systems but recent cytochemical studies indicate that some organizational principles exist within NTS regions. A third class of chemically defined transmission consists of those circuits attributed to peptidergic neurons, such as endorphins, enkephalins, Substance P, or vasoactive-intestinal peptide (VIP). Although no classes of transmitters or transmitter mechanisms wholly unique to cardiovascular regulation have yet been identified, these 3 classes of transmitter can each mediate a broad range of integrative actions, through unique molecular mechanisms.

Distribution of neuropeptides in the limbic system of the rat: the amygdaloid complex

The distribution of six neuropeptides (vasoactive intestinal polypeptide, cholecystokinin octapeptide, substance P, neurotensin, methionine-enkephalin and somatostatin) has been mapped in the amygdala using immunocytochemical methods. Cell bodies containing each peptide showed a differential distribution throughout the various subnuclei. Large numbers of vasoactive intestinal polypeptide and cholecystokinin-octapeptide-containing cell bodies were located in the lateral and cortical nuclei respectively, neurotensin-and methionine enkephalin-containing cell bodies in the central nucleus, and substance P-containing cell bodies primarily in the medial nucleus. Somatostatin-containing cell bodies were found in all nuclei. Neuropeptide-containing fibres were also differentially distributed. Substance P and cholecystokinin fibres formed dense plexuses in the medial nucleus whilst the greatest concentration of vasoactive intestinal polypeptide, neurotensin and methionine enkephalin fibres were seen in the central nucleus. Close observation of serial sections showed that all the neuropeptides studied had extensive intra-amygdaloid pathways and connections with other brain areas. The central nucleus and stria terminalis have particular importance in the organisation of peptides within the amygdala. The central nucleus acts as a focus for a number of converging/diverging peptide pathways and incoming catecholaminergic afferents. The stria terminalis contains all six peptides and represents the major efferent peptidergic system. The amygdala is thought to control a number of endocrine responses and to regulate complex behavioural functions. The abundance of neuropeptides within the amygdala and their complex pattern imply that they may act to regulate endocrine responses to external events (e.g. stress) or alter emotional tone functions thought to be controlled by the amygdala.

Parkinson's disease and Alzheimer's disease as disorders of the isodendritic core

Parkinson's disease and Alzheimer's disease may represent two parts of a spectrum of disease characterised by a primary loss of cells of the isodendritic core. Secondary cell loss from the striatum and cerebral cortex therefore occurs as a consequence of the loss of ascending projections from the isodendritic cells. The anatomy of this system should provide a unique opportunity for therapeutic intervention. Neurotransmitter replacement treatment may be provided either by enhancing transmitter release by any remaining neurones or by direct agonists. The wide dispersal of the isodendritic projection systems affected in Parkinson's and Alzheimer's disease and the possibility that they are tonically active create an opportunity for neurotransmitter replacement treatment. Animal studies should be able to show whether such treatment can delay secondary cell loss, and, together with human postmortem studies, whether the hypothesis that the primary lesion is a loss of isodendritic cells is correct.

Cholinergic and catecholaminergic afferents to the olfactory bulb in the hamster: a neuroanatomical, biochemical, and histochemical investigation

A series of neuroanatomical, biochemical, and histochemical studies have been conducted to determine the sources of cholinergic afferents to the main olfactory bulb (MOB) in the hamster. Following horseradish peroxidase (HRP) injections that are restricted to the MOB, retrograde neuronal labeling is observed bilaterally in the anterior olfactory nucleus, locus coeruleus, and raphe nuclei, and ipsilaterally in the ventral hippocampal rudiment, dorsal peduncular cortex, piriform cortex, nucleus of the lateral olfactory tract, anterior pole of the medial septal area and vertical limb of the diagonal band, nucleus of the horizontal limb of the diagonal band (HDB), and hypothalamus. Spread of HRP into the accessory olfactory bulb results in additional neuronal labeling ipsilaterally in the bed nucleus of the accessory olfactory tract, medial amygdaloid nucleus, and bed nucleus of the stria terminalis, and bilaterally in the posteromedial cortical amygdaloid nucleus. Retrograde tracing studies also have been conducted in cases with lesions in the basal forebrain or hypothalamus to assess the extent to which such lesions interrupt fibers of passage from other sources of centrifugal afferents, and the effects of such lesions on choline acetyltransferase (CAT) activity and catecholamine content in the MOB and on acetylcholinesterase (AChE) activity in the forebrain have been evaluated. Lesions in the basal forebrain reduce or eliminate CAT and AChE activity in the MOB in direct relationship to the extent of damage to the HDB. Norepinephrine (NE) content in the MOB also is reduced by basal forebrain lesions, but in relationship to damage of the medial forebrain bundle (MFB). The hypothalamic lesions have no effect on AChE activity in the forebrain or on CAT activity in the MOB, but they eliminate retrograde labeling in the locus coeruleus and raphe nuclei and reduce the NE content of the MOB to undetectable levels. The dopamine content of the MOB is not reduced by any of the lesions. Anterograde tracing studies have been conducted to compare the rostral projection patterns of the HDB with the distribution of AChE activity. Most of the rostrally directed axons travel in association with the MFB. A small component of axons travels in association with the lateral olfactory tract. Within the MOB, the axons terminate predominantly in the glomerular layer and in the vicinity of the internal plexiform layer. The projection and termination patterns of the HDB correspond well with the distribution of AChE activity. These various results indicate that the HDB is the major source of cholinergic afferents to the MOB.

A rapid method for preparing synaptosomes: comparison, with alternative procedures

A method for the rapid (1-1.5 h) preparation of nerve ending particles (synaptosomes) from rat cerebral cortex is described. The synaptosome fraction has been characterized by quantitative electron microscopy and enzyme distribution studies. By these criteria, the fraction showed a degree of enrichment in synaptic structures which was comparable to that of the standard (4-5 h) preparation, and substantially better than an alternative fast (2-2.5 h) method. On incubation, synaptosomes obtained by the new procedure accumulated a high tissue concentration of potassium and showed a high, linear rate of oxygen uptake. Depolarization by veratrine caused a significant increase in the rate of respiration and in the release of the physiologically active amino acids; glutamate, aspartate and GABA, as well as a significant reduction in tissue potassium. Thus, the new procedure compared favourably with alternative methods as judged by these indices of metabolic and functional performance. The new preparation method has been found to be of value in metabolic studies of synaptosomes prepared from human post-mortem brain.

Bipolar neurons in rat visual cortex: a combined Golgi-electron microscope study

Golgi-impregnated bipolar neurons in rat visual cortex have been examined by both light and electron microscopy. Bipolar neurons are encountered throughout layers II to V and are recognized by their spindle-shaped cell bodies and vertically elongate, narrow dendritic trees which may traverse the cortex from layer II to layer V. Although a single primary dendrite usually extends from each end of the cell body, two primary dendrites may extend from one pole, usually the lower one, and an additional short dendrite may emerge from one side. In the electron microscope gold-toned Golgi-impregnated neurons are seen to have folded nuclear envelopes and except at the poles of the cell body where the dendrites emerge, the nucleus is surrounded by only a thin rim of cytoplasm. Both the cell body and the dendrites form asymmetric and symmetric synapses. Usually the axon of a bipolar neuron arises from one of the primary dendrites and it soon assumes a vertical orientation, to either descend or ascend through the cortical neuropil. Some bipolar neurons have myelinated axons and only the initial portion is impregnated in Golgi preparations, but when they are unmyelinated the axons can be seen to form vertical plexuses and asymmetric synapses. Most commonly the terminals synapse with dendritic spines, some of which are derived from apical dendrites of pyramidal cells, but other terminals synapse with the shafts of apical dendrites, and with the cell bodies and dendrites of nonpyramidal cells. It is apparent that these bipolar neurons are the cells which others have shown to label specifically with antisera to vasoactive intestinal polypeptide (VIP), and it is suggested that the prime role of these cells in the cerebral cortex is to excite the clusters of pyramidal cells.

Glutamate binding in the rat cerebral cortex during ontogeny

Two binding sites for L-glutamate have been identified on adult rat brain cortical membranes. One of these sites is Na+-dependent with Kd of 1.3 micro M and a Bmax of 210 pmol/mg protein. The other is Na+-independent with a Kd of 0.37 micro M and Bmax of 6.2 pmol/mg protein. There is a sharp rise in total number of Na+-independent sites per cortex up to 20 days postnatally followed by a more gradual rise to adult levels at 50 days. Na+-dependent binding is also low at birth rising to a peak at 20 days followed by a drop in total levels of binding to 30 days and then a very sharp rise up to 50 days. The kinetics of binding at 20 days gives a Kd for the Na+-dependent site of 1.77 micro M and a Bmax of 82 pmol/mg protein. The Na+-independent site at 20 days has a Kd = 1.3 micro M and Bmax of 8.47 pmol/mg protein. The ability of several acidic amino acid analogues to displace specifically bound L-glutamate was investigated by estimating IC50 values at 20 and 50 days of age. The Na+-independent site is stereospecific for L-glutamate at both ages, but will also interact with L-aspartate at 20 days. The Na+-dependent site has a similar affinity for L- and D-glutamate and L-aspartate at 50 days. The L-glutamate analogue kainate will not displace any bound L-glutamate.

Vasoactive intestinal polypeptide induces glycogenolysis in mouse cortical slices: a possible regulatory mechanism for the local control of energy metabolism

Mouse cerebral cortex slices will synthesize [3H]glycogen in vitro. Vasoactive intestinal polypeptide (VIP) stimulates the enzymatic breakdown of this [3H]glycogen. The concentration giving 50% of maximum effectiveness (EC50) is 26 nM. Under the same experimental conditions norepinephrine also induces a concentration-dependent [3H]glycogen hydrolysis with an EC50 of 500 nM. The effect of VIP is not mediated by the release of norepinephrine because it is not blocked by the noradrenergic antagonist d-1-propranolol and is still present in mice in which an 85% depletion of norepinephrine was induced by intracisternal 6-hydroxydopamine injections. Other cortical putative neurotransmitters such as gamma-aminobutyric acid, aspartic acid, glutamic acid, somatostatin, and acetylcholine (tested with the agonist carbamylcholine) do not induce a breakdown of [3H]glycogen. This glycogenolytic effect of VIP and norepinephrine, presumed to be mediated by cyclic AMP formation, should result, at the cellular level, in an increased glucose availability for the generation of phosphate-bound energy. Given the narrow radial pattern of arborization of the intracortical VIP neuron and the tangential intracortical trajectory of the noradrenergic fibers, these two systems may function in a complementary fashion: VIP regulating energy metabolism locally, within individual columnar modules, and norepinephrine exerting a more global effect that spans adjacent columns.