Catecholamines released from cerebral cortex in the cat; decrease during sensory stimulation

New concepts of molecular communication among neurons

Recently a number of complex electrophysiological responses to neurotransmitters have been observed that cannot be described as simple excitation or inhibition. These responses are often characterized as modulatory, although there is no consensus on what defines modulation. Morphological studies reveal certain neurotransmitters stored in what might be release sites without synaptic contact. There is no direct evidence for nonsynaptic release from CNS sites, although such release does occur in the periphery and in invertebrates. Nonsynaptic release might provide a basis for diffuse one-cell-to-many communication, but it might also simply be a means of sending the transmitter to a broader area of a single neuron than occurs in typical synapses. Several kinds of macromolecules have been found to be transported in a retrograde direction – and in some cases transsynaptically. There have been suggestions that some neurons may release more than one type of transmitter. Particularly intriguing is the possibility of release of substances that modulate actions of a primary transmitter. Taken together this range of evidence suggests that neurons may use a variety of forms of molecular communication in addition to traditionally described synaptic transmission.

Several authors have suggested modes of communication distinct from classical synaptic transmission and have classified released substances using terms such as neurohumor, neurohormone, neuroregulator, and modulator. These suggestions have the heuristic value of drawing together diverse kinds of data, but it remains to be established that the pieces fit together in that fashion – for example, that complex electrophysiological effects are associated with substances released nonsynaptically. In order to reduce confusion, a flexible, generic approach to nomenclature for substances released from neurons and for hypothetical modes of communication is recommended. Some behavioral implications of nonconventional transmission are considered.

Effects of alpha-erabutoxin, alpha-bungarotoxin, alpha-cobratoxin and fasciculin on the nicotine-evoked release of dopamine in the rat striatum in vivo

Snake neurotoxins (NTX) have proven to be valuable tools for the characterisation of muscular nicotinic acetylcholine receptor structure and function. It is very likely that they could also be utilised to identify subtypes of neuronal nicotinic receptors controlling specific functions within the central nervous system. In this study we examined the effects of long alpha NTX (alpha-bungarotoxin, alpha-Bgt, and alpha-cobratoxin, alpha-Cbt) and short alpha NTX (alpha-erabutoxin a, alpha-Ebt) as well as the anticholinesterase toxin fasciculin-2 (FAS), on the nicotine-evoked release of dopamine (DA) in the striatum, using the in vivo push-pull technique. The short toxins alpha-Ebt and FAS blocked the extracellular increase of DA evoked by nicotine at 4.2 microM concentrations and alpha-Ebt was more potent, as reflected by the blockade at the lower dose of 0.42 microM. In contrast, the long toxins showed a different profile of action. Alpha-Cbt did not show any blockade of the nicotine-evoked release of DA at the doses studied while alpha-Bgt did block it only at the higher dose (4.2 microM) These results indicate that short neurotoxins show a stronger interaction with striatal nicotinic receptors subtypes controlling DA release when compared to the long ones. This interaction of short neurotoxin polypeptides and presynaptic receptors may permit the further elucidation of the particular nicotinic receptor populations responsible for the modulation of striatal DA release.

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

Ageing and monoamine turnover in the lateral geniculate nucleus and visual cortex of the rat

The effects of ageing on the turnover of dopamine, noradrenaline and serotonin in the lateral geniculate nucleus and the visual cortex were evaluated, using high performance liquid chromatography (HPLC) with electrochemical detection. Compared to adult animals, aged rats showed more changes in the visual cortex than in the lateral geniculate nucleus, with dopamine turnover decreased in both structures and noradrenaline turnover unaltered. Changes in serotonin turnover were witnessed only in the visual cortex. A decrease in the monoamine oxidase-A to -B ratio was also observed with increased age for both the lateral geniculate nucleus and visual cortex.

Disordered dopamine neurotransmission in the striatum of rats undergoing pilocarpine-induced generalized seizures, as revealed by microdialysis

In this study rats were fitted with a concentric dialysis probe in one striatum and extracellular concentrations of dopamine and HVA measured by reverse phase high performance liquid chromatography. Injections of saline or the D1 agonist SKF 38393 (30 mg/kg) did not affect the releases of these compounds. On the other hand, the D2 agonist LY 171555 (0.5 mg/kg) inhibited the release of both dopamine and HVA, whilst amphetamine (1 mg/kg) increased the output of dopamine but not HVA. Treatment with 200 mg/kg pilocarpine caused minimal signs of epileptic activity and did not affect striatal dopamine neurotransmission. Concomitant administration of SKF 38393 (30 mg/kg) to this dose of pilocarpine greatly facilitated the incidence and severity of motor seizures, which were accompanied by an irregular pattern of dopamine release and a significant rise in HVA overflow. Similar results were obtained with rats made to convulse with 400 mg/mg pilocarpine, and to a lesser extent if these animals were first pretreated with a protective dose of LY 171555 (0.5 mg/kg). It is concluded that dopamine neurotransmission in the striatum is disrupted in rats undergoing a pilocarpine-induced motor seizure, and that the extent of this disruption increases as the seizure becomes more severe. An irregular release of dopamine could signify a loss of sensorimotor control by the striatum, which might conceivably contribute to the intractability of the seizure. An increase in the dialysate concentrations of metabolite and not dopamine, is consistent with a heightened glutamate-stimulated release of dopamine from a discrete striatal pool, caused by the seizure spreading through the cortex and activating the cortico-striatal system.

Seizure threshold in juvenile myoclonic epilepsy with Graves disease

Thyroxine lowers the seizure threshold in experimental animals and humans. We report juvenile myoclonic epilepsy (JME) in two female patients with Graves' disease who had exophthalmos at age 11 (patient 1) and age 12 years (patient 2) but remained untreated until onset of seizures at ages 15 and 13 years, respectively. Seizures were not controlled well despite administration of antiepileptic drugs (AEDs) during the periods of excess serum thyroid hormones in Graves disease. When the serum levels of T3 were reduced to < 220 ng/dl with antithyroid drug treatment, both clinical seizures and paroxysmal EEG abnormalities disappeared despite discontinuation of AEDs and sleep deprivation. JME was noted only during periods of excess thyroid hormone and low compliance with antithyroid drug treatment. The excessively high level of thyroid hormones may have been a factor in precipitating the onset of JME.

Distribution and synaptic organization of dopaminergic axons in the lateral geniculate nucleus of the rat

In the present study, immunocytochemistry with an antiserum against dopamine (DA) revealed hitherto unknown terminal fields of DA axons in the lateral geniculate nucleus (LGN) of the rat. The innervation of all subdivisions of the LGN is achieved by a common set of afferent fibers that branch to form terminal fields of uneven density. The ventral lateral geniculate nucleus (LGv) receives slightly more DA axons than the dorsal lateral geniculate nucleus (LGd), whereas within the latter, DA afferents innervate the lateral part of the nucleus slightly more densely. Labeled axon terminals and varicosities, examined in single and serial ultrathin sections, were found in the extraglomerular neuropil in the LGd and in the neuropil of the LGv characterized by relatively simple synaptic relationships. They formed predominantly asymmetrical synaptic contacts with dendritic profiles. Occasionally, the postsynaptic elements were found to be presynaptic dendrites of presumptive interneurons. Some of the possible roles of this newly demonstrated DA afferent system in the physiology of the LGN and in the pathophysiology of diseases associated with impairment of dopaminergic activity are discussed.

Distribution of monoamines and metabolites in rabbit neostriatum, hippocampus and cortex

The monoamines noradrenaline (NA), dopamine (DA), adrenaline (AD) and 5-hydroxytryptamine (5-HT) were assayed in the putamen (PUT), the lateral (lCAU) and medial (mCAU) portions of the caudate, the dorsal (dHIP) and ventral (vHIP) hippocampus, as well as in four cortical areas, i.e., anterior cingulate (CIN), entorhinal-piriform (EnPi), sensorimotor (SSC; somatosensory) and primary visual (VIS). The use of an HPLC procedure enabled us to perform these measurements in microdissected samples and to assay as well monoamine metabolites. The DA levels were highest in the neostriatum, moderate in the EnPi and CIN and very low in the SSC, VIS and hippocampus. The distribution of NA was more uniform, although higher concentrations were measured in the neostriatum, hippocampus and EnPi. The largest amounts of 5-HT were in the EnPi, while moderate concentrations were found in the other regions. The ratios between the neurotransmitters and their metabolites were used as an index of turnover and indicate that the terminal fields of the monoamine systems are heterogenous within the neostriatal, hippocampal and cortical subdivisions.

Interactions between noradrenergic and cholinergic mechanisms involved in spinal nociceptive processing

Antinociceptive effects have been demonstrated after systemic and spinal administration of the adrenoceptor agonist clonidine and cholinomimetic drugs in animals and human. The present investigation was undertaken in rats to study the possible interactions between spinal noradrenergic and cholinergic mechanisms in modulating the reaction to nociceptive stimuli. Using the tail immersion test, an additive antinociceptive effect was found between intrathecal (IT) clonidine (10 micrograms) and physostigmine (15 micrograms, IT). The effect of clonidine was attenuated by atropine (15 micrograms, IT). Physostigmine (15 micrograms, IT) antinociception, which was of short duration was abolished by atropine (15 micrograms, IT) and attenuated by phentolamine (20 micrograms, IT). Neostigmine (5 micrograms, IT) produced a prolonged antinociceptive response. In animals pretreated with 6-hydroxydopamine IT, leading to a selective depletion of spinal cord noradrenaline, physostigmine (15 micrograms, IT) was ineffective in altering the nociceptive test response. Neither clonidine, nor physostigmine produced changes in latency times in the hot plate test (58 degrees C) in the doses employed. In conclusion, a clear-cut interaction exists between spinal noradrenergic and cholinergic systems for antinociception. To explain the interactions, several possible mechanisms may be considered, including cholinomimetic effects produced by clonidine, and the presence of muscarinic receptors in the dorsal horn of the spinal cord.

Acute effects of lithium on dopaminergic responses: iontophoretic studies in the rat visual cortex

The interactions between lithium and cortical dopaminergic receptors were investigated using the iontophoretic technique to record and apply dopaminergic compounds, GABA, acetylcholine and LiCl on neurons in the primary visual cortex of the rat. The main responses to dopamine (DA) or to the D1 agonist (+/- )SKF38393 on spontaneously-active (SA) or visually-driven (VD) units was a prolonged decrease in firing and a reduction in the responsiveness to pulses of acetylcholine. The D1 antagonist SCH23390, applied iontophoretically or intravenously, blocked or attenuated the inhibitory responses to both DA and (+/- )SKF38393. The D2 agonist quinpirole (LY171555) either produced only slight excitations or had no effects on both VD and SA units. The concomitant application of lithium blocked the inhibitory responses to DA and to (+/- )SKF38393 but did not modify the responsiveness to LY171555. In addition, the DA- and (+/- )SKF38393-induced decreases in responsiveness to acetylcholine were also suppressed by lithium. These effects were on dopaminergic mechanisms, since the excitatory responses to acetylcholine alone as well as the inhibitions caused by GABA were unchanged by the application of lithium. These results imply that the modifications in sensitivity to dopaminergic agents induced by lithium are mediated by dopamine D1 receptors and are discussed in relation to adenylate-cyclase.

Apomorphine in the evaluation of dopaminergic function in man

1. Apomorphine (Apo), a short acting dopamine (DA) receptor agonist, stimulates growth hormone (GH) secretion, decreases prolactin secretion, induces yawning, penile erections and other physiological effects in man. An effect on behavior, movement disorders and alcoholism has also been described. 2. Apo-mediated responses are used to evaluate DA function in psychiatric and neurological disorders. Many of the studies in schizophrenia using the GH response to Apo as an index of central DA function are difficult to interpret because of failure to control for key variables. 3. The GH response to Apo is a useful system to evaluate the effects of various drugs including peptides which may not cross the blood brain barrier on DA function in man. 4. Apo is a potent sedative. Specific antimanic, antischizophrenic, and anticraving effects in alcoholics have not been convincingly demonstrated. Side effects of Apo and failure to use active placebo make double-blind studies difficult. 5. Apo improves parkinsonian symptoms and certain forms of reflex epilepsy but beneficial effects in other involuntary movement disorders requires further documentation. 6. Apo may be a useful agent to evaluate DA function in impotent patients and predict a therapeutic response to long-acting dopaminergic agents. 7. Impairment of DA function may play a role in diabetic impotence. 8. The development of a simple polygraphic method to monitor the yawning response to Apo may facilitate clinical studies on the basic physiology of yawning in man and the use of the yawning response as a measure of central DA function in schizophrenia and other clinical disorders. 9. The use of Apo with 18F-fluorodeoxyglucose positron emission tomography to examine regional DA function in man opens up a promising area of research. 10. Though long-acting orally active aporphine DA agonists and antagonists have been developed the problem of tolerance may limit their therapeutic potential.

Dopaminergic innervation of the primary visual cortex in the rat, and some correlations with human cortex

Dopaminergic terminals have been identified in the primary visual cortex with three techniques; immunocytochemistry with an anti-dopamine antiserum, retrograde axonal transport techniques using unconjugated wheat germ agglutinin and HPLC determination of catecholamines and metabolites in microdissected sub-regions of occipital cortex in the rat. The results demonstrate a specific dopaminergic innervation, arising from the ventral tegmental area, which is found mainly in laminae VI and V, but with minor innervation also in lamina I. Dopaminergic innervation to adjacent cortical regions is also described. Neurochemical data from post-mortem human material suggests that a similar innervation exists in man. An analysis of the distribution of dopaminergic fibres in relation to the known connections and possible functions of the deep laminae of visual cortex suggests that dopaminergic axons may participate in the corticofugal control of visual afferent pathways.

Dopamine agonists suppress visual-cortical reflex myoclonus

Two patients with a diagnosis of olivo-ponto-cerebellar atrophy developed cortical reflex myoclonus to visual (flash) and somaesthetic stimuli. Oral treatment with levodopacarbidopa (1000/100 mg) or subcutaneous administration of apomorphine (1 mg) abolished the visually-triggered myoclonus, without modifying reflex myoclonus to electrical or tactile stimulation. Intravenous administration of lisuride (0.1 mg) produced a marked reduction in both types of reflex myoclonus. These results indicate a selective inhibitory effect of dopamine agonist drugs on visual reflex myoclonus of cortical origin.

Catecholamines and serotonin in the rat central nervous system after 6-OHDA, 5-7-DHT and p-CPA

The norepinephrine (NE), epinephrine (EPI), dopamine (DA) and serotonin (5-HT) contents were measured radioenzymatically in seven anatomically defined regions (frontal cerebral cortex, hippocampus, hypothalamus, midbrain, pons-medulla oblongata, cerebellum and spinal cord) in adult normal animals, after treatment with the tryptophan hydroxylase inhibitor p-chlorophenylalanine (p-CPA), and after the intraventricular administration of either 6-hydroxydopamine (6-OHDA) or 5,7-dihydroxytryptamine (5,7-DHT). The effects of p-CPA seemed not restricted to 5-HT, since reductions in catecholamine (CA) content were detected in several regions. After 5,7-DHT given under desimipramine (DMI) protection, comparable reductions in 5-HT levels were obtained but the changes in CA were less severe than after p-CPA. The neurotoxin 6-OHDA decreased the CA in all regions but also 5-HT content in hippocampus, hypothalamus and pons-medulla. The significance and the interpretation of these changes are discussed in relation to the specificity of the drugs employed, together with an assessment of the local monoamine turnover and the possible functional effects of monoamine interactions in the CNS.

Pathophysiology of generalized photosensitive epilepsy in the cat

Feline generalized penicillin-induced epilepsy is a reliable experimental model of epileptic photosensitivity. Binocular photic stimulation at 4-8 flashes/s consistently triggers generalized, bilaterally synchronous spike-and-wave discharges in cats submitted to long-term intramuscular penicillin administration at low dosage (50,000-150,000 IU/kg). The photically induced epileptic activity is first and mainly recorded from the cerebral cortex as compared with the lateral geniculate (GL) body, therefore supporting a cortical onset of epileptic photosensitivity. At this low penicillin dosage, only minimal spontaneous spike-and-wave activity is recorded. Bilateral GL stimulation is more effective than binocular photic stimulation in triggering spike-and-wave discharges, whereas unilateral GL stimulation is grossly ineffective. This observation implies a role of bilateral specific visual thalamocortical volleys in the genesis of epileptic photosensitivity. Five of 12 randomly selected cats receiving long-term penicillin exhibited spontaneous epileptic photosensitivity, a finding that suggests an underlying genetic predisposition of the feline species to this condition.

Visual evoked potentials in rats selected for high or low self-stimulation

Visual evoked potentials (VEPs) were analyzed in order to distinguish between rats from genetically high (HI) and low (LO) self-stimulation lines (LC2-HI and LC2-LO). Secondary VEP components - slow secondary negative wave (SNW) and sensory afterdischarge (SAD) - which are considered to be most sensitive indices of normal and pharmacologically-induced behavioral changes, were used for the comparison. Small, albeit statistically significant enhancement of SNW and SAD was obtained in LO rats. Unlike LO animals, HI rats gained in SNW amplitude and SAD area during repeated photic stimulation. The difference being highly significant. D,L-Amphetamine (1 mg/kg, i.p.) suppressed SAD and reduced the SNW amplitude in both HI and LO animals, although the predrug difference in their values remained practically unaltered. Apomorphine (0.25, 2.75, 5.25 mg/kg i.p.) had no measurable effect on VEP parameters even though it caused a regular picture of dose-related enhancement of locomotion and stereotypy. The effect of amphetamine can, therefore, be attributed to the activation of the norepinephrinergic system. Correspondingly, VEP variance in the two lines of rats is interpreted as related to the peculiarities of norepinephrine modulation of neocortical activity.

Familial photosensitive epilepsy: effectiveness of clonazepam

We describe the clinical features of a family with photosensitive epilepsy, followed for 13 years. Generalized paroxysmal discharges induced by photic stimulation appeared in all 9 siblings. Generalized seizures, myoclonic jerks, and absences appeared in variable combination in 7 of them who on clonazepam remained free from seizures for a seven-year follow-up period. On EEG the paroxysmal abnormalities induced by photic stimulation abated during clonazepam medication.

Effects of chlorpromazine and promazine on the visual aftereffects of tilt and movement

The effects of chlorpromazine (CPZ) and promazine on the visual aftereffects of tilt and motion were measured. CPZ markedly reduced the strength of both aftereffects, while promazine produced a smaller and not always significant reduction. Control experiments suggested that the effects were produced in the central visual system rather than by several possible peripheral artefacts or by drowsiness. The effects are discussed with reference to the pharmacological activity of the drugs and their influence on the strength of inhibition in the visual cortex, both in normal subjects and in schizophrenic illness.

Catecholamines and serotonin in rat frontal cortex after PCPA and 6-OHDA: absolute amounts and ratios

The catecholamines dopamine, epinephrine and norepinephrine, as well as the indoleamine serotonin were measured using radioenzymatic assays, in the frontal cortex of normal rats and of rats in which serotonin synthesis had been inhibited with p-chlorophenylalanine. Besides serotonin, norepinephrine levels were significantly reduced, thus changing the monoamine ratios. To further evaluate these changes, two models of catecholaminergic deafferentation were analyzed, i.e., the massive catecholamine depletion induced by intraventricular 6-OHDA and the more selective norepinephrine deafferentation, obtained by microinjecting 6-OHDA in the dorsal noradrenergic bundle. The results are interpreted in relation to the functional interactions between the catecholamines and between the catecholamines and serotonin.

Cholinergic neurons in the caudate-putamen complex proper are intrinsically organized: a combined Evans blue and acetylcholinesterase analysis

In an attempt to determine whether or not acetylcholinesterase (AChE)-containing neurons of the caudate-putamen proper were the source of striatal efferent fibers, we infused Evans Blue, a retrogradely transported fluorescent label, into the globus pallidus, entopeduncular nucleus, substantia nigra, or retrorubral area. Following microscopic analysis of the striatum for Evans Blue-labelled somata, the same brain sections were processed for AChE according to the pharmacohistochemical regimen and, after additional microscopic evaluation, were counterstained with cresyl violet. Histology for Nissl substance revealed that the areal density of cell bodies in the caudate-putamen complex proper was about 1510 somata/mm2. Striatal neurons labelled with Evans Blue, those considered to be projection cells, were medium-sized (approximate minor and major dimensions: 11 X 14 microns), had a density of roughly 833 cells/mm2, and were predominantly oval with lesser proportions being fusiform, triangular, or round. Each of the target structures received input from approximately 55% (range = 26-78%) of the total population of striatal neurons in regions where the projection cellsions: 11 X 14 microns), had a density of roughly 833 cells/mm2, and were predominantly oval with lesser proportions being fusiform, triangular, or round. Each of the target structures received input from approximately 55% (range = 26-78%) of the total population of striatal neurons in regions where the projection cellsions: 11 X 14 microns), had a density of roughly 833 cells/mm2, and were predominantly oval with lesser proportions being fusiform, triangular, or round. Each of the target structures received input from approximately 55% (range = 26-78%) of the total population of striatal neurons in regions where the projection cells were located. The two types of AChE-containing somata in the caudate-putamen complex proper--the medium-sized, lightly staining Type A and the large, intensely staining Type B cell--had densities of 14 and 15 somata/mm2, respectively. None of the AChE neurons contained Evans Blue, indicating that they were not the source of striatal efferent fibers but rather interneurons that could be categorized best as the aspiny or sparsely spined cells described in Golgi studies.

Histochemical evidence for a catecholaminergic (presumably dopaminergic) projection from the ventral mesencephalic tegmentum to visual cortex in the cat

An histochemical technique combining the retrograde transport of horseradish peroxidase (HRP) and catecholamine histofluorescence was used to identify those neurons of the cat's mesencephalic ventromedial tegmentum (VMT) which project to visual cortex. In a region corresponding to the ventral tegmental area-A10 nucleus we identified a group of fluorescent cells which were retrogradely labeled by HRP injections into visual cortex. It is suggested that these cells belong to a hitherto unknown, posterior part of the mesocortical dopaminergic system.

Distribution of catecholamines and serotonin in the rat cerebral cortex: absolute levels and relative proportions

The rat cerebral cortex was dissected in five regions and analyzed for the catecholamines noradrenaline, adrenaline and dopamine, and for the indoleamine serotonin using sensitive radioenzymatic assay methods with thin-layer chromatography. The noradrenaline concentration was highest in the ventral cortex, lateral to the hypothalamus, had intermediate values for the prefrontal, frontal and parietal cortical areas and was lowest in the occipital cortex. Dopamine levels were also highest in the cortex lateral to the hypothalamus, and moderate in the prefrontal and frontal cortical areas, with the lowest values measured for the occipital cortex. The ratios dopamine/noradrenaline further supports the hypothesis that they are independent transmitters. Traces of adrenaline were measured in all regions examined. The serotonin distribution was found to be non-homogeneous, with the highest values for the prefrontal cortex and ventral cortex lateral to the hypothalamus. The functional significance of these amines and their ratios are discussed in relation to their role as putative modulators of cortical neuronal excitability.

Brain noradrenergic responses to training and to amnestic frontal cortex stimulation

Rats were trained in a one-trial inhibitory (passive) avoidance task prior to receiving supraseizure electrical stimulation of frontal cortex, a treatment that results in amnesia. Forebrain and brain stem norepinephrine (NE) concentrations decreased by 23% 10 min after footshock training. Posttraining frontal cortex stimulation resulted in a potentiation of the forebrain NE response (to 31-33% below control values) and in attenuation of the brainstem response (0-5% lower than control values). These results are consistent with previous findings that indicate that good retention performance is predicted by training and treatment conditions that result in approximately a 20% decrease in brain NE content as measured 10 min after training; deviations from this optimal level, presumably reflecting more or less NE release, predict poor retention in comparably trained and treated rats. Thus, memory storage processing appears to be sensitive to many manipulations that alter the endogenous posttraining brain NE response to footshock.

The intracortical distribution of norepinephrine, dopamine and serotonin in the cerebral cortex of the cat

The intracortical distribution of the biogenic amines (BA), norepinephrine (NE), dopamine (DA) and serotonin (5-HT), was determined for the parietal and occipital cortical areas of the cat. Under microscopic control, slices of each region were dissected out into 4 fractions: (1) an upper gray fraction (G1); (2) a medium gray fraction (G2); (3) a lower gray fraction (G3) and (4) a white-matter fraction (W). The BA were assayed by means of specific and sensitive radioenzymatic techniques; using catechol-O-methyltransferase (COMT) for the catecholamines (CA), and N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) for the 5-HT. In the two cortical regions studied, NE and DA were found to be distributed in an orderly laminar fashion. The highest levels of endogenous CA were measured in the superficial layers (fractions G1 and G2) while the white matter (fraction W) exhibited the lowest content of both NE and DA. DA represented 32--38% of the total CA content of the parietal cortex and 32--41% of the total CA content in the occipital cortex. In the parietal cortex, the distribution of 5-HT was similar in pattern to that of the CA, i.e., a decreasing concentration gradient with the highest endogenous levels in the G1 fraction and the lowest content in the W fraction. In the occipital cortex, the distribution was also found to be laminar for the gray matter but the W fraction showed a higher endogenous content than the lower gray fraction (G3). The release of NE, DA and 5-HT was studied by means of cortical superfusion in an in vivo preparation. The concentrations of BA measured in 30 min interval superfusates was fairly constant in a given experiment under basal conditions but varied widely from experiment to experiment. Variations in the basal overflow of NE were often independent of those found for DA and the basal overflow of 5-HT seemed independent of those of CA. The laminar intracortical distribution of endogenous NE, DA and 5-HT in cerebral cortex demonstrated by microdissection and biochemical assay techniques suggests a definite and structured pattern of aminergic innervation. Furthermore, the fact that these BA are released and a basal overflow can be detected and measured appears compatible with significant roles of NE, DA and 5-HT in synaptic transmission in the cerebral cortex.