The responses of thalamic neurons to iontophoretically applied monoamines

Monoaminergic Neuromodulation of Sensory Processing

All neuronal circuits are subject to neuromodulation. Modulatory effects on neuronal processing and resulting behavioral changes are most commonly reported for higher order cognitive brain functions. Comparatively little is known about how neuromodulators shape processing in sensory brain areas that provide the signals for downstream regions to operate on. In this article, we review the current knowledge about how the monoamine neuromodulators serotonin, dopamine and noradrenaline influence the representation of sensory stimuli in the mammalian sensory system. We review the functional organization of the monoaminergic brainstem neuromodulatory systems in relation to their role for sensory processing and summarize recent neurophysiological evidence showing that monoamines have diverse effects on early sensory processing, including changes in gain and in the precision of neuronal responses to sensory inputs. We also highlight the substantial evidence for complementarity between these neuromodulatory systems with different patterns of innervation across brain areas and cortical layers as well as distinct neuromodulatory actions. Studying the effects of neuromodulators at various target sites is a crucial step in the development of a mechanistic understanding of neuronal information processing in the healthy brain and in the generation and maintenance of mental diseases.

The effects of Cirazoline, an alpha-1 adrenoreceptor agonist, on the firing rates of thermally classified anterior hypothalamic neurons in rat brain slices

Peripheral exposure to LPS induces a biphasic fever thought to be initiated via vagal afferents to the preoptic area of the anterior hypothalamus (POAH), an important thermoregulatory control center in the brain. Previous studies have shown that norepinephrine synaptically mediates this Prostaglandin E(2) (PGE(2))-dependent change in temperature through the selective activation of alpha-2 adrenoreceptors (AR). However, there is clear evidence that alpha-1 AR activation of thermoregulatory hypothalamic neurons will result in a rapid hyperthermia that is not dependent on PGE(2). This direct action of norepinephrine in the POAH was tested in the present study by recording the single-unit activity of POAH neurons in a tissue slice preparation from the adult male rat, in response to temperature and the selective alpha-1 AR agonist Cirazoline (1-100 microM). Neurons were classified as either warm sensitive or temperature insensitive. Warm sensitive neurons responded to Cirazoline with a decrease in firing rate, while temperature insensitive neurons showed a firing rate increase. These responses are similar to those reported for PGE(2) and suggest that both warm sensitive and temperature insensitive neurons in the POAH are important in mediating this alpha-1 AR-dependent hyperthermic shift in body temperature.

From cAMP to adenosine: an illuminating shift of focus

In a remarkable career, straddling five decades, John Phillis pursued with fierce determination and exceptional energy the main goal of his scientific life, to throw light on the chemical agents that control brain function. Starting in Australia, he settled in North America, first in Canada, then in the USA, where his long tenure at Wayne State brought his career to its culmination.

The lateral hypothalamic area revisited: neuroanatomy, body weight regulation, neuroendocrinology and metabolism

This article reviews findings that have accumulated since the original description of the syndrome that follows destruction of the lateral hypothalamic area (LHA). These data comprise the areas of neuroanatomy, body weight regulation, neuroendocrinology, neurochemistry, and intermediary metabolism. Neurons in the LHA are the largest in the hypothalamus, and are topographically well organized. The LHA belongs to the parasympathetic area of the hypothalamus, and connects with all major parts of the brain and the major hypothalamic nuclei. Rats with LHA lesions regulate their body weight set point in a primary manner and not because of destruction of a "feeding center". The lower body weight is not due to finickiness. In the early stages of the syndrome, catabolism and running activity are enhanced, and so is the activity of the sympathetic nervous system (SNS) as shown by increased norepinephrine excretion that normalizes one mo later. The LHA plays a role in the feedback control of body weight regulation different from ventromedial (VMN) and dorsomedial (DMN). Tissue preparations from the LHA promote glucose utilization and insulin release. Although it does not belong to the classical hypothysiotropic area of the hypothalamus, the LHA does affect neuroendocrine secretions. No plasma data on growth hormone are available following electrolytic lesions LHA but electrical stimulation fails to elicit GH secretion. Nevertheless, antiserum raised against the 1-37 fragment of human GHRF stains numerous perikarya in the dorsolateral LHA. The plasma circadian corticosterone rhythm is disrupted in LHA lesioned rats, but this is unlikely due to destruction of intrinsic oscillators. Stimulation studies show a profound role of the LHA in glucose metabolism (glycolysis, glycogenesis, gluconeogenesis), this mechanism being cholinergic. Its role in lipolysis appears not to be critical. In general, stimulation of the VMN elicits opposite effects. Lesion studies in rats show altered in vitro glucose carbon incorporation into several tissue fractions both a few days, and one mo after lesion production. Several of these changes may be due to the reduced food intake, others appear to be due to a "true" lesion effect.

Electrophysiological interactions of ethanol with GABAergic mechanisms in the rat cerebellum in vivo

Biochemical studies indicate that ethanol (EtOH) will facilitate the activation of the GABAA/Cl- channel, and behavioral studies demonstrate that EtOH-induced sedative and incoordinating effects can be potentiated by GABA mimetics and blocked by GABA antagonists. It has been difficult, however, to demonstrate an EtOH-induced potentiation of the depressant electrophysiological effects of locally applied GABA in mammalian brain in vivo. Similarly, in this study, local EtOH applications only infrequently caused potentiations of the depressant effects of microiontophoretically applied GABA on cerebellar Purkinje neurons, and this interaction was modest when present. The predominant interaction of locally applied EtOH was an antagonism of GABA-induced depressions of neuronal activity. However, the GABAA receptor antagonist bicuculline reversibly and apparently competitively blocked the depressant effects of locally applied EtOH on single cerebellar Purkinje neurons. Our data suggest that EtOH potentiation of GABA responses alone is insufficient to account for EtOH-induced depressions of cerebellar Purkinje neurons. However, these data clearly imply that activation of a GABAA receptor is required for the expression of EtOH-induced depressions of neuronal activity in this brain area. It is less clear how lower, nondepressant doses of EtOH interact with GABA mechanisms. We hypothesize that either the GABAA receptor mechanism must be sensitized to the potentiative effects of EtOH through the influences of neuromodulatory and/or hormonal regulation, or that EtOH interacts directly with these regulatory processes.

Serotoninergic and noradrenergic projections to the ventral posterolateral nucleus of the monkey thalamus

In the present study, serotoninergic and noradrenergic varicosities were identified in the ventral posterolateral nucleus of the macaque monkey. Monoaminergic neurons projecting to the ventral posterolateral nucleus of the thalamus were identified by using retrograde labeling with horseradish peroxidase combined with immunocytochemical staining for serotonin or dopamine-beta-hydroxylase. The midbrain nucleus raphe dorsalis was the major site of origin for neurons providing a serotoninergic projection to the ventral posterolateral nucleus. A few retrogradely labeled serotonin-containing neurons were also observed in the central superior and the raphe pontis nuclei. Noradrenergic cells with projections to the thalamus were primarily located in the nucleus locus coeruleus with some projection neurons in the nucleus subcoeruleus, and the A5 catecholamine cell group of the pons.

GABAergic mechanisms in the electrophysiological actions of ethanol on cerebellar neurons

We have found that the partial inverse benzodiazepine agonists Ro 15-4513 and FG 7142 antagonize the depressant electrophysiological effects of locally applied ethanol in the cerebellum. Although absolute tissue concentrations are not known, dose-response curves constructed using pressure-ejection doses as previously described we found that FG 7142 was more efficacious, but less potent than Ro 15-4513. Our observation that ethanol and inverse benzodiazepine agonists have interactions which are not competitive might suggest that these two drugs act through separate, but interactive mechanisms in order to produce the observed ethanol antagonism. If such independent interactions were mediated at different sites on a given macromolecular complex, such as the GABAa/Cl- channel, then one might expect to find allosteric interactions between those sites as well as with the functional response of the complex to GABA activation. Indeed, this hypothesis is consistent with the recent finding of Harris and collaborators that ethanol potentiates the inverse agonist actions of Ro 15-4513 and FG 7142. On the other hand, we were unable to find large ethanol-induced potentiations of GABA effects on all neurons which showed depressant responses to ethanol administration in rat cerebellum. However we did find that the GABAa antagonist, bicuculline, blocks the depressant effects of ethanol on the same neurons. We conclude that the interaction between ethanol and GABA probably does not occur directly at the GABAa receptor site, but that the GABAa mechanism does play a permissive role in the ethanol-induced depressions of cerebellar Purkinje neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulatory role of serotonin on GABA-elicited inhibition of cerebellar Purkinje cells

The present study was designed to directly examine the postsynaptic actions of serotonin on GABA-mediated inhibition of cerebellar Purkinje cells. The findings indicate that serotonin at currents that produced minimal effects on the spontaneous firing rates of Purkinje cells modified GABA effects in a biphasic manner. Serotonin initially decreased GABA-mediated inhibitions followed secondarily by either continued inhibition or, in the majority of cases, augmentation of GABA responses. When a comparison was made of the secondary effects of serotonin on GABA-mediated inhibition with the initial spontaneous firing rates of the Purkinje cells, the group in which serotonin augmented GABA actions had a significantly higher initial firing frequency than the group in which serotonin attenuated GABA-mediated inhibition. Furthermore, with increasing firing rates, the proportion of cells showing augmentation of GABA inhibition increased, and the proportion of cells displaying attenuation of GABA effects decreased. Serotonin affected beta-alanine-mediated inhibitions in a manner similar to that seen with GABA, whereas glycine was differentially altered. This study identifies another neuromodulatory role of serotonin on Purkinje cells in the cerebellum. Furthermore, the effects of serotonin on GABA inhibition seem to be governed by some intrinsic property of the Purkinje cell, which is apparently related to the firing rate of the cell.

Noradrenaline excites neurons in the guinea pig cerebellar vermis in vitro

Noradrenaline (NA) was applied to the solution bathing the cavy cerebellar vermis in vitro and the responses of 98 neurons were recorded extracellularly. Two thirds (23/35) of the responses were excitations and the remaining third were inhibitions. The lowest concentration of NA with which responses could be obtained was 10(-11) M NA. Responses were generally transient and occurred with a mean latency of 61 +/- 8 sec. The excitation was generally direct as most responses (9/11) survived synaptic blockade. The excitations were thought to be mediated by alpha 1 receptors because they could be mimicked by phenylephrine and antagonised by prazozin.

Electrophysiological actions of norepinephrine in rat lateral hypothalamus. I. Norepinephrine-induced modulation of LH neuronal responsiveness to afferent synaptic inputs and putative neurotransmitters

The present studies were conducted as part of an ongoing investigation of the effects of norepinephrine (NE) in neuronal circuits of the mammalian brain. In this report, we describe noradrenergic actions in the lateral hypothalamus (LH), an area which has been implicated in the central integration of cardiovascular regulatory mechanisms, fluid balance and ingestive behaviors. Microiontophoretically applied NE was interacted with extracellularly recorded responses of LH neurons to iontophoretically applied putative neurotransmitters gamma-aminobutyric acid (GABA), acetylcholine (ACh) and glutamate (Glu); and activation of known input pathways from the reticular thalamus (RT) and the lateral preoptic area (LPO). Peri-event histograms of cell responses were computed before, during and after NE microiontophoresis (5-50 nA) and used to quantitatively evaluate monoamine-induced effects on spontaneous and stimulus evoked activity of LH neurons. In 16 of 23 LH neurons, RT-stimulus-induced inhibition was markedly prolonged from a mean of 28.3 +/- 4.8 ms to 44.7 +/- 5.2 ms, during iontophoretic application of NE. In 22 of 38 LH cells, LPO-stimulus-induced excitatory responses were enhanced above control levels during NE administration. In further tests, inhibitory responses of LH cells to iontophoretic pulses of GABA were potentiated during NE administration in 69% (24 of 35) of the cases tested. ACh-induced excitation was potentiated in 9 of 21 cells. In 4 of these cases, otherwise subthreshold doses of ACh caused marked increases in cell firing during the period of NE administration. By contrast, Glu-evoked excitation was antagonized by NE iontophoresis in 65.5% (17 of 26) of LH cells tested. These findings indicate that, as in other noradrenergic target regions of the CNS, NE can facilitate synaptically mediated responses of LH neurons. Taken together these observations suggest that NE may play an important regulatory role in the synaptic transfer of information within LH circuits, and consequently exert considerable influence over the homeostatic functions mediated by this structure.

Differential effects of serotonin on the spontaneous discharge and on the excitatory amino acid-induced responses of deep cerebellar nuclei neurons in rat cerebellar slices

The effects of steady iontophoretic applications of serotonin on the spontaneous discharge and on the excitatory responses induced in deep cerebellar nuclei neurons by iontophoretic pulse applications of L-glutamate, L-aspartate, N-methyl-D,L-aspartate and quisqualate were studied in rat cerebellar slices maintained in vitro. Serotonin increased the spontaneous firing rate of deep cerebellar nuclei neurons in 91% of the tested cells by 109% on the average and had no effect on the remaining recorded neurons. Conversely, the monoamine induced a depression of the excitatory responses induced by four agonists tested and the depressant potency of serotonin was in the order quisqualate, glutamate, aspartate, N-methyl-D,L-aspartate. These effects persisted in low calcium high magnesium solution, suggesting that the serotonin receptors involved in these phenomena were, at least partially, postsynaptically located. The serotonin-induced increase in the cell firing rate appeared to be methysergide-resistant whereas the serotonin-induced decrease in the responses elicited by excitatory amino acids was depressed by this antagonist, which could indicate that these differential effects of serotonin are mediated via different mechanisms and/or serotonin receptor subtypes.

Modulatory action of serotonin on glutamate-induced excitation of cerebellar Purkinje cells

The effects of serotonin (5-hydroxytryptamine, 5-HT) on glutamate-induced excitation of Purkinje cells were examined. Pulsatile iontophoretic applications of glutamate (1-25 nA) induced consistent increases in the spontaneous activity of Purkinje cells. When serotonin was applied continuously with currents that elicited minimum changes in the spontaneous rate, it inhibited or blocked glutamate-induced excitations significantly in most Purkinje cells. We also examined the effects of high currents of serotonin on spontaneous activity of Purkinje cells. High currents of serotonin induced 3 different effects: inhibitions, biphasic effects comprising transient inhibition followed by excitation, and excitations. Nonetheless, whether it inhibited or excited the activity of Purkinje cells, serotonin inhibited glutamate-induced excitations consistently. The effect of a putative 5-HT antagonist methysergide (UML) was also examined. Methysergide consistently attenuated or antagonized the inhibitory effects of serotonin on glutamate-induced excitations. This finding suggests strongly that inhibitory effects of 5-HT on glutamate excitations observed in the present study is the specific action of serotonin.

An ascending serotonergic pain modulation pathway from the dorsal raphe nucleus to the parafascicularis nucleus of the thalamus

Three types of spontaneously active neurons were found in the parafascicularis (PF) nucleus of the thalamus of the rat: slow firing units (0.5-10 spikes/s), bursting units (2-5 spikes/burst in 10-20 ms, one burst every 1-2 s) and fast firing units (15-40 spikes/s). A similar population of neurons was found in the PF of rats treated with 5,7-dihydroxytryptamine (5,7-DHT), a serotonin neurotoxin. Noxious tail pinch (TP) caused 68% of the PF neurons to increase their firing rates to 242% of their initial baseline activity, while non-noxious touch stimulation failed to induce a response. In the 5,7-DHT-treated rats, TP caused 85% of the neurons in the PF to increase their firing rates to 581% of their initial baseline activity and 22% of the neurons increased their firing in response to touching the tail. Both the number of cells responding (P less than 0.05) and the percentage increase (P less than 0.001) were statistically greater in serotonin-depleted rats than in controls. This indicates that serotonin (5-HT) has a tonic inhibitory influence on responses to both noxious and non-noxious sensory stimuli. In control rats, electrical stimulation of the dorsal raphe nucleus (DR) decreased the firing rates of PF neurons. In contrast, the same DR stimulation induced an increase in PF firing rates during stimulation in serotonin-depleted rats and this increase in firing rates remained several seconds after cessation of stimulation. This indicates that the DR may use at least two different neurotransmitters in its projections to forebrain structures. In control rats, the TP stimulation induced an increase in firing rates of rates of PF neurons while DR stimulation attenuated the excitation induced by TP stimulation. In serotonin-depleted rats, DR stimulation and TP both caused an increase in firing rates. This effect was not additive indicating that there may be a serotonergic projection from the DR to the PF which modifies responses to somatosensory stimuli. The inhibitory effects elicited by electrical stimulation were limited to the immediate area of the DR. Stimulation of the adjacent reticular formation 1 mm lateral to the DR produced the opposite effect, an increase in firing rate often accompanied by driven spike activity in the PF.

Transmitter sensitivities among various types of cultured brain stem neurons

We prepared cultures of dissociated cells from the lower brain stem of 14- to 15- day-old rat fetuses, and studied how neuronal activities of various types of neurons were influenced by application of various transmitters or transmitter-like substances. Unit discharges of the cultured neurons were recorded extracellularly, using suction micropipette, while the test substances were applied iontophoretically to the surface membrane of the cell body. We selected for testing 4 types of neurons with the typical appearance, i.e. multipolar, pyramidal, polygonal and fusiform one. Each type of neuron had specific sensitivities to acetylcholine and leucine-enkephalin. Amino acids and substance P acted to 4 types of neurons as either unanimous excitant or inhibitor. Monoamines and leucine-enkephalin had both inhibitory and facilitatory effect on unit discharges, although the inhibitory effect was dominant. These findings suggest that there is a specific relation between morphological properties of the brain stem neurons and their sensitivities to transmitters.

Activation of lateral geniculate neurons by norepinephrine: mediation by an alpha-adrenergic receptor

Adrenergic receptors in the vicinity of neurons in the lateral geniculate nucleus (LGN) of the rat were pharmacologically characterized using extracellular single-cell recording and microiontophoretic techniques. Application of norepinephrine (NE) at low iontophoretic currents (1-15 nA) produced a delayed activation of most LGN neurons. This activation was mimicked by various sympathomimetic amines. The relative potency series of agonists was typical of postsynaptic alpha-adrenergic receptors: epinephrine greater than NE greater than phenylephrine greater than or equal to alpha-methylnorepinephrine greater than dopamine greater than isoproterenol. The alpha-antagonists phentolamine, piperoxane and WB-4101, when applied at low iontophoretic currents (less than 10 nA), produced a selective, dose-dependent and reversible blockade of the response to NE. The beta-antagonist sotalol had weak and variable effects at these currents. At low currents, the presynaptic alpha-agonist clonidine was also able to block the response to NE but, at higher currents, produced a partial activation of some units, suggesting that it is a weak agonist. The ability of sympathomimetic amines to activate LGN neurons correlates well with their reported affinities for brain alpha1-adrenoceptors labeled with [3H]WB-4101. It is concluded that NE activates neurons in the LGN via a postsynaptic or alpha1-adrenergic receptor.

The differential effects of 5-hydroxytryptamine, noradrenaline and raphe stimulation on nociceptive and non-nociceptive dorsal horn interneurones in the cat

The effects of 5-hydroxytryptamine (5-HT), noradrenaline (NA) and stimulation of the inferior central nucleus of the raphe (RN) were examined on nociceptive and non-nociceptive spinal neurones in anaesthetized cats. 5-HT reduced excitation evoked by noxious stimulation, but increased spontaneous firing and firing evoked by DL-homocysteic acid (DLH) on both nociceptive and non-nociceptive cells. NA reduced spontaneous activity, DLH-evoked excitation and excitation evoked by a noxious stimulus on nociceptive neurones, but had little action on non-nociceptive units. RN inhibited spontaneous, stimulus-evoked and DLH-evoked firing of nociceptive cells and caused briefer inhibitions of non-nociceptive cells. Excitatory effects were also observed. Strychnine antagonized short-duration inhibitions from RN of non-nociceptive cells responding to hair movement, but failed to antagonize any of the other effects of RN. No antagonism of the inhibitory effect of RN was observed with phenoxybenzamine, phentolamine, sotalol, bicuculline or methysergide. However, methysergide antagonized some excitatory effects of 5-HT and RN, but also produced non-specific actions on some cells. It was concluded that, although glycine may mediate some of the brief duration inhibitions evoked by RN, the longer duration inhibitions were unlikely to have been mediated by either glycine or GABA. 5-HT may be a mediator of raphe-spinal actions but may have presynaptic inhibitory actions coupled with postsynaptic excitatory effects. NA could mediate some descending inhibition of nociceptive neurones.

Responses of identified spinal neurones to acetylcholine applied by micro-electrophoresis

1. The responses of identified cells in the cat Clarke's column and dorsal horn to micro-electrophoretically applied cholinomimetics and anti-cholinergic substances have been investigated. 2. Both antidromically identified (DSCT neurones) and synaptically activated neurones from the region of the Clarke's column of the spinal cord were excited by ACh. However, the proportion of ACh excited cells was greater in units synaptically activated by ipsilateral dorsolateral funiculus stimulation (78%) than in DSCT neurones (50%). In addition, about 55% of neurones activated either antidromically or synaptically by ipsilateral dorsal column stimulation were excited by ACh. 3. In contrast to a relatively weak excitatory potency on the DSCT neurones (maximum firing frequency did not exceed 130% of the control activated by ipsilateral dorsolateral funiculus stimulation (maximum firing frequency reached 430% of the control level). 4. ACh has a relatively quick and rapidly reversible excitatory effect on Clarke's column neurones and some types of dorsal horn interneurones, which can be obtained also with nicotine. However, the action of nicotine is frequently delayed in onset and recovery. This excitatory action of ACh can be blocked or markedly depressed by dihydro-beta-erythroidine. These results and those obtained with acetyl-beta-methylcholine and atropine seem to suggest that the receptors mediating excitation of the cholinoceptive spinal cells activated either antidromically or synaptically by ipsilateral dorsolateral funiculus stimulation besides predominantly nicotinic have also weak muscarinic properties. 5. Desensitization with repeated applications of ACh and nicotine has been observed in both DSCT neurones and units antidromically activated by ipsilateral dorsal column stimulation. 6. About 11% of units antidromically activated by ipsilateral dorsolateral funiculus stimulation were depressed by ACh. In addition, the depressant effect of ACh was more frequently encountered in the cells unresponsive either to the dorsolateral funiculus or dorsal column stimulation. ACh depression was also seen in units activated either antidromically or synaptically by ipsilateral dorsal column stimulation. In contrast, none of the units synaptically activated by the ipsilateral dorsolateral funiculus stimulation were depressed by ACh. The same was true for spinal neurones receiving convergent peripheral inputs activated either antidromically or synaptically by ipsilateral dorsolateral or dorsal column stimulation. 7. The findings that ACh depression of all tested DSCT neurones is blocked by atropine and readily evoked by acetyl-beta-methylcholine indicates that receptors mediating the effect are of muscarinic type.

The effects of beta-carbolines on responses to acetylcholine, noradrenaline, 5-hydroxytryptamine and amino acids in the rat spinal cord

Various drugs have been applied electrophoretically to Renshaw cells and to unidentified spinal neurones in pentabarbitone anaesthetized or decerebrated rats. Responses to noradrenaline (NA) and 5-hydroxytryptamine (5-HT) have not previously been described at this site and were of two types; either monophasic depression or biphasic depression-excitation. The effect of harmine on these responses was examined. Harmine and harmaline were also tested on the excitant responses to acetylcholine (ACh) and DL-homocysteate (DLH), and on the depressant responses to glycine and gamma-aminobutyric acid (GABA). On some cells harmaline antagonized ACh, but not DLH, and glycine, but not GABA, responses. Harmine caused only non-specific depression and spike configuration changes. The effects of harmine on NA and 5-HT responses were usually non-specific, and any anatagonism was usually accompanied by, or soon followed by spike changes. LSD was also tested on the amine responses. LSD itself had a clear depressant effect on neuronal firing rates. It could either antagonize or potentiate NA and 5-HT depressant responses, but the antagonism in particular was closely followed by spike changes. Somewhat more specific antagonism of the late excitation was seen.

Serotonin in the lateral geniculate

Serotonin was introduced, by means of a fine cannula, into the lateral geniculate body of cats immobilized with Flaxedil and artificially ventilated, while the electrical activity at the point of injection was monitored by means of microelectrodes. Doses of 1.25 to 30 mug dissolved in 0.5 to 2.0 mul of saline produced, in 2-30 min, changes in electrical activity characteristic of synchronization: increase in the rhythmicity and in the amplitude of the spontaneous gross waves and increase in the clustering of the spontaneous neuronal action potentials. At the same time the activity of neurons which produced action potentials of high amplitudes was decreased, the activity of neurons which produced action potentials of low amplitudes was increased. Action potentials of different amplitudes were produced, in this case, by neurons of different types. Thus, in the lateral geniculate as in other thalamic nuclei studied in previous investigations, the synchronization of spontaneous activity seems to require the simultaneous excitation and inhibition of two different types of neurons. The action of serotonin on activity evoked by stimulation with brief flashes of light was limited to the decrease in the amplitude of the average gross response and the inhibition of only one type of neuron. This suggests that, in the lateral geniculate body, serotonin may be implicated in different ways in the different network structures responsible for the development of spontaneous as contrasted with evoked activity.

Further studies on the mode of action of psychotomimetic drugs: antagonism of the excitatory actions of 5-hydroxytryptamine by methylated derivatives of tryptamine

1 The actions of 5-methoxytryptamine (5-MeOT), N,N-dimethyltryptamine (DMT), 5-hydroxy-N,N-dimethyltryptamine (bufotenine, 5-HODMT) and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), and their interactions with 5-hydroxytryptamine (5-HT), acetylcholine, (-)-noradrenaline, and glutamate were studied by microiontophoresis on single neurones in the brain stem of rats anaesthetized with urethane or decerebrate cats.2 Like D-lysergic acid diethylamide (LSD 25) the three psychotomimetic derivatives (DMT, 5-HODMT, 5-MeODMT) specifically antagonized 5-HT excitations of single neurones, but the non-psychotomimetic 5-MeOT had no antagonistic effects.3 In contrast to LSD 25, the psychotomimetic tryptamines only rarely antagonized glutamate effects, indicating that the excitatory 5-HT receptors and the glutamate receptors on the same neurones may be closely related spatially, but are separate.4 The methylated tryptamine derivatives were able to mimic the actions of 5-HT on neurones. The non-psychotomimetic 5-MeOT was most potent in this respect, while the other three derivatives which are psychotomimetic, were less active.5 The 5-HT mimicking actions of 5-MeOT were the same in rats pretreated with p-chlorophenylalanine or reserpine as in untreated rats. It therefore seems that the 5-HT mimicking actions are unlikely to be due to release of 5-HT, but are due to direct actions on 5-HT receptors.6 The evidence presented supports the hypothesis that LSD-like psychotomimetics act by an antagonism of 5-HT in the lower brain stem, and is not compatible with the suggestion that the psychotomimetic action of these drugs is related to 5-HT receptor stimulation.

A comparative study of some convulsant substances as gamma-aminobutyric acid antagonists in the feline cerebral cortex

1. By the use of microiontophoretic techniques, quantitative estimates were obtained of the depressant effects of gamma-aminobutyric acid (GABA) on single feline cortical neurones.2. Picrotoxin, bicuculline, strychnine, (+)-tubocurarine, penicillin and leptazol were also applied microiontophoretically to single neurones. Sequential GABA applications were made before, during and after the microiontophoresis of these substances and any effects on the time course of the GABA depression were measured as an estimate of antagonism or potentiation of GABA.3. (+)-Tubocurarine was found to be a potent GABA antagonist. Picrotoxin and bicuculline were rather less potent and strychnine and penicillin only weakly active as GABA antagonists. Leptazol appeared to be inactive against GABA depressions.4. In addition, bicuculline and strychnine were found to be capable of potentiating the depressant action of GABA. This property was not shared by the other substances studied.5. All the substances studied produced changes in neuronal firing rate that did not correlate with GABA antagonism.6. In conclusion, several potent convulsants have been shown to be capable of GABA antagonism. It is not yet clear that this effect, rather than a direct effect on neuronal excitability, is the prime mechanism behind their convulsant properties.

The excitation of neurons by noradrenaline

Responses of single cortical neurons to microelectrophoretically applied noradrenaline at pH 3.1 and 5.0 and to hydrogen ions were compared in the halothane-anaesthetized cat. Of 16 neurons sensitive to noradrenaline, 13 were excited and 3 were depressed by noradrenaline at both pH values, whereas hydrogen ions released from an HCl solution did not affect the firing rate. Dose-response studies showed that noradrenaline at pH 3.1 was more potent than at pH 5.0. It is concluded that excitatory responses to noradrenaline are not artifacts and that the adjustment of the pH of noradrenaline solutions with NaOH should be avoided.

Cholinergic mechanisms in the rat somatosensory cerebral cortex

1. The responses of identified cells in the rat cerebral cortex to cholinomimetic and anticholinergic substances has been investigated.2. Acetylcholine and muscarinic agonists have an excitatory action on 80% of pyramidal tract cells. This response is found especially on cells responding to specific thalamic stimulation and the burst of spikes evoked from this site can sometimes be blocked by the iontophoresis of atropine. This strongly suggests an excitatory transmitter function for acetylcholine in a specific thalamocortical pathway.3. Experiments on non-pyramidal tract cells have detected a muscarinic depression of some cells, and a nicotinic excitation of some cells above a depth of 600 mu in the cortex.4. It is suggested that the increased release of acetylcholine from the cortex produced by atropine administration may be due to an excess of muscarinic inhibitory over excitatory synapses in the cortex.

Actions of noradrenaline, other sympathomimetic amines and antagonists on neurones in the brain stem of the cat

1. The effects of (-)-noradrenaline ((-)-NA) and related compounds on brain stem neurones in decerebrate unanaesthetized cats have been investigated using the technique of iontophoretic application from micropipettes.2. Four types of response to (-)-NA have been described. These were short lasting inhibition, long lasting inhibition, excitation, and a biphasic response consisting of short lasting inhibition followed by excitation. A variable amount of desensitization of the excitatory response, but not of inhibitory responses, was observed.3. Experiments in which small currents were used to pass (-)-NA from pipettes with smaller tips did not lead to any appreciable change in the proportions of neurones excited or inhibited.4. A variety of sympathomimetic agonists was tested. Short lasting inhibition was less sensitive than excitation to changes in molecular structure. Long lasting inhibition was more sensitive to molecular change and was not mimicked by some of the agonists which mimicked short lasting inhibition.5. Although agonists without one ring hydroxyl had weaker effects than those with both, compounds in which both ring hydroxyl groups were absent (beta-hydroxyphenylethylamine, ephedrine and amphetamine) mimicked excitation strongly. It is possible that the compounds without both ring hydroxyl groups had some effect other than simple agonistic activity.6. A dissociation was observed between responses to dopamine and (-)-NA. p-Tyramine mimicked dopamine, rather than (-)-NA.7. Neither the alpha-agonist, phenylephrine nor the beta-agonist, isoprenaline mimicked neuronal responses to (-)-NA. The alpha-antagonists phentolamine and phenoxybenzamine and the beta-antagonists dichloroisoprenaline, propranolol and D(-)-INPEA and combinations of propranolol with phentolamine or phenoxybenzamine were ineffective in blocking either excitation or inhibition. Thus, the central receptors appear to be different from peripheral alpha- and beta-receptors.8. The most effective antagonist of excitation was (-)-alpha-methylnoradrenaline. Metaraminol and dihydroergotamine also had some antagonistic activity. None of the compounds tested blocked inhibition. The effects of (-)-alpha-methylnoradrenaline have been discussed in relation to the hypotensive action of alpha-methyldopa.

Antagonism of 5-hydroxytryptamine by LSD 25 in the central nervous system: a possible neuronal basis for the actions of LSD 25

1. 5-Hydroxytryptamine (5-HT), acetylcholine (ACh), noradrenaline (NA), glutamate, D,L-homocysteic acid (DLH), glycine and gamma-aminobutyric acid (GABA) were applied to single neurones in the brain stem of decerebrate cats by microiontophoresis. The abilities of D-lysergic acid diethylamide tartrate (LSD 25), methysergide maleate (UML 491) and 2-bromo-lysergic acid diethylamide (BOL 148) to antagonize the actions of these compounds were studied.2. LSD 25 antagonized 5-HT excitation of single neurones when applied iontophoretically or administered intravenously. LSD 25 also antagonized glutamate excitation of neurones which could be excited by 5-HT. Inhibitory effects of 5-HT, the action of glutamate on neurones which could be inhibited by 5-HT and the actions of all the other compounds tested were unaffected by LSD 25.3. Iontophoretically applied UML 491 was also a specific antagonist to 5-HT and glutamate excitation but was less potent than LSD 25, and BOL 148 rarely exhibited antagonism.4. It is suggested that antagonism to 5-HT and glutamate excitation of brain stem neurones may be the basis of the psychotomimetic action of LSD 25. It is also suggested that there may be similarities in the mechanisms by which 5-HT and glutamate produce excitation where they act on the same neurone.

The responses of cortical neurones to monoamines under differing anaesthetic conditions

1. Noradrenaline, isoprenaline, 5-hydroxytryptamine and acetylcholine have been applied into the environment of single neurones in the cat cerebral cortex by micro-iontophoresis. The influence of anaesthesia on the neuronal responses to these drugs was studied.2. In general, excitation of neurones by noradrenaline occurred commonly in both unanaesthetized encéphale isolé (48%) and N(2)O-halothane anaesthetized preparations (57%) while depressions were less frequent (24 and 20% respectively). The picture differed markedly in barbiturate anaesthetized animals where excitation was uncommon (12%) and the majority of cells (59%) was depressed by noradrenaline. Although fewer cells were studied, similar differences were obtained with isoprenaline and 5-hydroxytryptamine for the three types of preparation. In sharp contrast, the vast majority of cells was excited by acetylcholine in each of the preparations: encéphale isolé 84%; N(2)O-halothane 92%; barbiturate anaesthetized preparations 85%.3. The differing neuronal responses observed in these experimental situations were not the result of variations in the depth of anaesthesia.4. Although the depths at which neurones were encountered within the cortex did not differ in N(2)O-halothane and encéphale isolé preparations, significantly more cells were found in deeper layers of cortex in barbiturate preparations. The proportion of cells excited or depressed by noradrenaline was generally similar at each depth in the three preparations used.5. The distribution of rates of cell firing was strikingly similar in each preparation, and most of the cells studied had frequencies below 10/sec.6. The direction in which a cell responds to noradrenaline is mainly determined by the type of anaesthetic used, and not by the depth of anaesthesia, the rate of cell firing, or cell depth within the cortex. This suggests important differences in the central pharmacology of halothane and barbiturates.