Interaction of serotonin with somatosensory cortical neuronal responses to afferent synaptic inputs and putative neurotransmitters

Serotonin-, substance P- and glutamate/aspartate-like immunoreactivities in medullo-spinal pathways of rat and primate

Serotonergic neurons of the medulla oblongata have been proposed to play a role in the control of sensory, motor and autonomic cells in the spinal cord. Many of these raphe neurons have been shown to contain the undecapeptide substance P as well as the tripeptide thyrotropin-releasing hormone, but evidence for the presence of an excitatory amino acid in these pathways has not yet been documented. In colchicine-treated rats, we have used a combination of retrograde tracing and tri-color immunohistofluorescence techniques to study co-localization of serotonin- and substance P- with glutamate- or aspartate-like immunoreactivities in medullary neurons and the possible spinal projections of these cells. In addition, the distributions of serotonin-, substance P- and glutamate-immunoreactive terminal fields in the dorsal, ventral and lateral horns of the spinal cord were examined with tri-color immunofluorescence in the rat and the primate Macaca fasciculata. In colchicine-treated rats, glutamate- and aspartate-like immunoreactivity was found in practically all serotonin- and substance P-immunoreactive neurons of the B1, B2 and B3 cell groups. Some of these neurons also contained wheat-germ agglutinin conjugated to inactivated horseradish peroxidase and colloidal gold particles retrogradely transported from the spinal cord. In the spinal cords of non-colchicine-treated monkeys and rats, striking co-localization of serotonin, substance P- and glutamate-like immunoreactivities was seen in large boutons, surrounding the dendrites and cell bodies of large alpha motor neurons in the ventral horn. These observations suggest the existence of spinally projecting serotonin/substance P neurons containing excitatory amino acids such as glutamate or aspartate.(ABSTRACT TRUNCATED AT 250 WORDS)

Clozapine's mechanisms of action: non-dopaminergic activity rather than anatomical selectivity

The mechanisms of clozapine's unusual actions were investigated in anesthetized rats. Sensory field potentials were recorded in the nucleus accumbens, striatum and somatosensory cortex. Both haloperidol and clozapine suppressed responses in the striatum and accumbens. Clozapine's but not haloperidol's subcortical actions were unaffected by dopamine depletion. In addition, the cortical effects of the two drugs differed. These data indicate that clozapine has non-dopaminergic activity but no particular specificity for limbic basal ganglia structures. While it is known that clozapine acts as a serotonergic and cholinergic antagonist, several considerations suggest that action on still another transmitter system, possibly glutamate, needs to be investigated.

The 5,7-DHT-induced anticonflict effect is dependent on intact adrenocortical function

We have recently reported that the anxiolytic-like effect observed in rats severely depleted of brain serotonin (5-HT) by means of 5,7-DHT is indirect and probably involves the GABA(A)/benzodiazepine chloride ionophore receptor complex (GABAA/BDZ-RC). One tentative explanation for this effect considered the involvement of corticosteroids. In the present series of experiments we have therefore investigated the effect of adrenalectomy (ADX) on the 5,7-DHT-induced anxiolytic-like effect displayed by rats in Vogel's conflict test. ADX totally abolished the anticonflict effect of the 5,7-DHT lesion. Replacement treatment with corticosterone, but not with dexamethasone, reinstated the anticonflict effect. These results indicate that an intact adrenocortical function, possibly via brain steroid type I receptors, is required for the expression of the 5,7-DHT-induced anxiolytic-like effect. It is postulated that ADX lowers the concentration of endogenous positive modulators at the GABAA/BDZ-RC to a level no longer sufficient to produce anxiolytic-like effects in 5,7-DHT-lesioned animals. The finding that 5,7-DHT-lesioned animals were more sensitive than sham-lesioned controls to the anticonflict effect of the barbiturate-like corticosteroid THDOC provides further support for the contention that an increased endogenous activity at the GABAA/BDZ-RCes is involved in the anxiolytic-like effect observed in rats with a severe depletion of brain 5-HT.

Changes in motor performance and rubral single unit activity in cats after microinjections of serotonin into the red nucleus area

The serotonergic control exerted on the red nucleus (RN) was studied in unrestrained cats during the performance of a simple reaction time task which consisted of releasing a lever in response to an auditory go-signal. The effects of microinjections of serotonin-oxalate salt into the rubral area on the motor activity and on the firing of neurons recorded concomitantly in the red nucleus were investigated. Injections of serotonin (5-HT) (200-400 ng) into the red nucleus or its dorsal border induced subtle alterations in the conditioned motor performances but had no major effects on the spontaneous motor behavior. The changes in the conditioned motor output (an increase in the static force exerted on the lever and a speeding up of the lever release) are reminiscent of the facilitatory influence of serotonin on various motor reflexes previously reported. Changes in the neuronal activity were observed concomitantly with the effects on the motor output: 5-HT either enhanced or reduced the firing rate of the rubral neurons. These effects were apparently dependent on the discharge pattern of the neurons during the static motor activity. The results suggest that the serotonergic input to the red nucleus may participate in motor control by exerting a dual modulatory action on the activity of rubral neurons.

Serotoninergic innervation of the ferret cerebral cortex. I. Adult pattern

We have investigated the serotoninergic innervation of the adult ferret cerebral cortex with immunohistochemical techniques. Distribution pattern of serotoninergic fibers in the ferret neocortex is characterized by a decrease in the density of fibers as one moves from the pial surface towards the white matter. Throughout the entire cerebral cortex, the serotoninergic fibers are very dense within the supragranular layers, especially within layer 1. In contrast, granular and infragranular layers exhibit only a sparse innervation. Although this general pattern of innervation is roughly the same in all cortical areas, significant variations in the fiber density are apparent in different regions. Areas 17, 1, 6, and 8 (primary visual cortex, presumptive somatosensory cortex, presumptive motor cortex, and prefrontal cortex, respectively) are described in more detail to illustrate the diversity of the serotoninergic innervation patterns. The density of innervation is highest in areas 1 and 6, intermediate in area 8, and lowest in area 17. It is noteworthy that while areas 1, 6, and 8 show a marked decrease in fiber density at the boundary between layer 3 and 4, the less strongly innervated area 17 shows a change in density in the transition from layer 2 to layer 3. The types of fibers found within the ferret cortex are similar to those described in other mammalian species. The bulk of the innervation is made by very fine fusiform axons with small ovoid varicosities. In addition to this fiber type, axons with thick round varicosities and some smooth nonvaricose axons were found. The latter types occur in very small numbers within the supragranular layers and mostly in more anterior cortical regions. While the general innervation pattern and the fiber types are similar to those described in the cat cerebral cortex, the pericellular baskets found in the cat cortex (Mulligan and Törk, J Comp Neurol 270:86-110, 1988) are not seen in the ferret.

Effects of systemically and locally applied cocaine on cerebrocortical neuron responsiveness to afferent synaptic inputs and glutamate

The goal of the present study was to determine the effects of systemically or locally applied cocaine on rat somatosensory cortical neuron responsiveness to afferent synaptic inputs or putative transmitter application and to compare these results with previously observed actions of endogenous cortical monoamines on the same parameters of neuronal function. Individual cells in rat cortex were activated by stimulation of thalamocortical afferents or local iontophoretic application of glutamate. Extracellularly recorded responses to these stimuli were monitored before and after parenteral or microiontophoretic administration of cocaine. The results indicate that while high doses (greater than 2.0 mg/kg i.p.) of the drug can suppress both evoked and spontaneous activity of cortical neurons, low doses (0.5 mg/kg i.p.) can selectively enhance stimulus-evoked discharge. These facilitating effects can also be observed during iontophoretic application of cocaine directly onto recorded cells, thus suggesting that at least a component of the drug's influence on neuronal responsiveness is mediated by local actions at synapses within the cortex. Of the 3 major endogenous cortical monoamines whose synaptic reuptake is influenced by cocaine, the actions reported here mimic those described previously for norepinephrine but not those of dopamine or serotonin. As such these findings suggest that cocaine may enhance the responsiveness of sensory cortical neurons to afferent synaptic inputs via its ability to activate noradrenergic modulatory mechanisms within the cerebrocortical circuitry.

Second messenger-mediated actions of norepinephrine on target neurons in central circuits: a new perspective on intracellular mechanisms and functional consequences

Ever since the initial demonstration of a widespread distribution of noradrenergic fibers to functionally diverse regions of the mammalian forebrain, there has been considerable interest in determining the electrophysiological effects of norepinephrine (NE) on individual neurons within these target areas. While early studies showed that NE could directly inhibit cell firing via increased intracellular levels of cyclic AMP, more recent work has revealed a spectrum of noradrenergic actions, which are more accurately characterized as neuromodulatory. More specifically, numerous experimental conditions have been described where NE at levels subthreshold for producing direct depressant effects on spontaneous firing can facilitate neuronal responses to both excitatory and inhibitory synaptic stimuli. The goal of this report is to review recent evidence which suggests that the various modulatory actions of NE on central neurons result from the activation of different adrenoceptor-linked second messenger systems. In particular, we have focused on the candidate signal transduction mechanisms that may underlie NE's ability to augment cerebellar and cortical neuronal responsiveness to GABAergic synaptic inputs. The consequences of such NE-induced changes in synaptic efficacy are considered not only with respect to their influences on feature extraction properties of individual sensory cortical neurons but also with regard to the potential impact such actions would have on the signal processing capabilities of a network of noradrenergically innervated cortical cells.

Involvement of the GABAA/benzodiazepine chloride ionophore receptor complex in the 5,7-DHT Induced anticonflict effect

The effects of drugs interacting with the GABAA/benzodiazepine chloride ionophore receptor complex (GABAA/BDZ-RC) on the anticonflict and biochemical effects observed after intracerebroventricular (i.c.v.) administration of 5,7-dihydroxytryptamine (5,7-DHT; 450 micrograms -14 days) were investigated in the rat using a modified Vogel's drinking conflict test. The GABAergic antagonistic drugs bicuculline, picrotoxin and Ro 15-4513 all counteracted the 5,7-DHT induced anxiolytic-like action in doses that did not alter the behavior per se, whereas flumazenil was ineffective in this respect. Also i.c.v. administration of 5-HT antagonized the 5,7-DHT induced anticonflict effect. Furthermore, 5,7-DHT-lesioned animals appeared more sensitive to the anticonflict effects of diazepam than sham-lesioned controls. The 5,7-DHT treatment produced marked depletions of 5-HT in the limbic system (80-90%) and hippocampus (90-95%), and an increase in the 5-HIAA/5-HT quotient in hippocampus. The effects on the levels of noradrenaline were comparatively small. The doses of bicuculline and picrotoxin antagonizing the 5,7-DHT induced anticonflict effect did not uniformly influence 5-HT levels or 5-HIAA/5-HT quotients. It is suggested that the anxiolytic-like effect observed in 5,7-DHT-lesioned rats in Vogel's drinking conflict test involves enhanced transmission at the GABAA/BDZ-RC.

Inhibition of spontaneous and evoked unit activity in the rat medial prefrontal cortex by mesencephalic raphe nuclei

The rat medial prefrontal cortex (PFC) receives a serotoninergic (5-HT) innervation which originates from the mesencephalic raphe nuclei. In the present study we determined the influence of the 5-HT ascending systems on the spontaneous and evoked activity of PFC neurons in anesthetized rats. Stimulation of the dorsal (DRN) and of the median raphe (MRN) nuclei inhibited the spontaneous activity of 35.0% and 52.8% of the PFC cells tested (mean duration of the inhibition: 75.5 and 82.2 ms, respectively). These inhibitory responses are likely mediated by the 5-HT-containing neurons since they were decreased markedly following selective destruction of ascending 5-HT pathways induced by local injections of 5,7-dihydroxytryptamine. Moreover, the inhibitory effect of MRN stimulation could be blocked by systemic administration of the 5-HT2 receptor antagonists: ketanserin and ritanserin. The effects of MRN stimulation on two types of evoked responses were studied. The excitatory responses of PFC neurons induced by the stimulation of the mediodorsal nucleus of the thalamus (MD) were inhibited by MRN stimulation applied before that of MD. Similarly, the activation of PFC cells induced by a noxious tail pinch was suppressed by a concomitant stimulation of the MRN. These results indicate that 5-HT neurons exert an inhibitory control on spontaneous or evoked activity in the rat PFC.

Modulation of rat cortical area 17 neuronal responses to moving visual stimuli during norepinephrine and serotonin microiontophoresis

The present study was conducted to examine the actions of norepinephrine (NE) and serotonin (5-HT) on the multiphasic, visually evoked discharges of cells recorded from the visual cortex (area 17) of anesthetized Long-Evans pigmented rats. Visual responses of 51 cells, evoked by computer controlled presentation of moving visual stimuli, were examined before, during and after low level microiontophoretic application of NE (1-55 nA) or 5-HT (1-50 nA). Drug-induced changes in stimulus-evoked and spontaneous discharges were quantitatively assessed by computer analysis of peri-event histograms. In the majority of cases tested, NE produced a net enhancement of visually evoked responses by facilitating excitatory and inhibitory components of stimulus-bound discharges. By contrast, 5-HT tended to suppress stimulus-evoked excitation and inhibition in many cases to the extent that neurons were no longer responsive to appropriate visual stimuli. In selected cases we were able to demonstrate additional effects of NE and 5-HT on response threshold, direction selectivity and discrimination of receptive field borders. For example, in some cells NE was capable of revealing evoked responses to visual stimuli which were previously ineffective in eliciting stimulus-bound discharges. In other instances, changes in cell activity evoked by stimulus movement across the visual field were accentuated during NE application in such a way that unit discharges at receptive field borders were more sharply defined in comparison to control conditions. 5-HT, on the other hand, was capable of decreasing the contrast between spontaneous and visually evoked discharge at receptive field boundaries. In summary, these results suggest that endogenously released NE and 5-HT may modulate, by complimentary actions, the magnitude of responses of visual cortical neurons to afferent synaptic inputs. Moreover, these monoaminergic projection systems may also have the capacity to modify the threshold of detection of afferent signals within a neuronal network as well as alter feature extraction properties of the circuit.

Effects of iontophoretically applied monoamines on somatosensory cortical neurons of unanesthetized rats

The response of somatosensory cortical neurons to iontophoretic applications of monoamines was studied in unanesthetized rats. The animal's head was held in a stereotaxic apparatus by means of a painless head-restraining system implanted 8 days prior to the recording sessions. The electrodes consisted of a recording micropipette attached to a multibarreled iontophoresis micropipette. The electrode penetrations were reconstructed on camera lucida drawings of frontal brain sections. The percentage of cortical neurons responding to application of monoamines were 76% after noradrenaline, 58% after dopamine and 66% after serotonin. The differences observed among percentages of responses seemingly correlate with the relative abundance of terminal axons and receptors for each of the three monoamines in the somatosensory cortex. The vast majority of the responding neurons were inhibited by monoamines and this inhibitory effect was independent of the level of spontaneous activity. The depressant effect of the monoamines on glutamate and acetylcholine-evoked responses supports a modulatory role for these substances. Serotonin was the most potent, followed by noradrenaline and dopamine. The present study shows that when the influence of anesthesia is eliminated, the predominant effect of monoamines on cortical first somatosensory neurons is one of inhibition. These findings contrast with results obtained under some anesthetic conditions, as well as under in vitro conditions.

Chemoanatomical organization of the noradrenergic input from locus coeruleus to the olfactory bulb of the adult rat

The locus coeruleus contains noradrenergic neurons which project widely throughout the CNS. A major target of locus coeruleus projections in the rat is the olfactory bulb (Shipley et al.: Brain Res. 329:294-299, '85) but the organization of the projections within the bulb has not been systematically examined. In this study, the laminar distribution and densities of locus coeruleus-noradrenergic fibers in the main and accessory olfactory bulbs were determined with anterograde tracing and immunocytochemical techniques. Following iontophoretic injections of 1% wheat germ agglutinin-horseradish peroxidase into the locus coeruleus, the densest anterograde label in the accessory olfactory bulb was observed in the external plexiform layer, granule cell layer, and especially in the internal part of the mitral cell layer. Virtually no label was observed in the glomerular layer. In the main olfactory bulb, labelled axons were observed in the granule cell layer, in the internal and external plexiform layers, occasionally in the mitral cell layer, and least often in the glomerular layer. Noradrenergic fibers in the olfactory bulb were identified by using immunocytochemistry with an antibody to dopamine-beta-hydroxylase. Laminar patterns and densities of noradrenergic innervation were determined with quantitative image analysis. In the accessory olfactory bulb, the densest innervation was in the innermost portion of the mitral cell layer followed by the granule cell layer, the superficial part of the mitral cell layer, and the external plexiform layer. The density of fibers in the glomerular layer was least. The laminar pattern of noradrenergic fiber distribution in the main olfactory bulb was similar to that in accessory olfactory bulb. The present studies demonstrate that locus coeruleus-noradrenergic fibers terminate preferentially in the internal plexiform, granule cell, and external plexiform layers. This suggests that the major influence of the locus coeruleus input to both the main and accessory the olfactory bulbs is on the predominant neuronal element in those layers, the granule cells. Additional studies are needed to resolve how this input influences specific olfactory bulb circuits.

Does the PCPA induced anticonflict effect involve activation of the GABAA/benzodiazepine chloride ionophore receptor complex?

The effects of the benzodiazepine (BDZ) receptor antagonist flumazenil (Ro 15-1788) and the GABAA receptor antagonist bicuculline on the anticonflict effect observed after depletion of brain serotonin (5-HT), were examined in a modified Vogel's punished drinking conflict model. Pretreatment with para-chlorophenylalanine (PCPA; 300 mg/kg/day for three days, last injection - 24 h) markedly decreased brain 5-HT levels and produced clearcut anticonflict effects. The anticonflict effect, but not the biochemical effect, of PCPA pretreatment was completely counteracted by both flumazenil (10 mg/kg, - 30 min) and bicuculline (2.0 mg/kg, - 10 min), in doses not altering the behavior per se. The findings suggest a behavioral interaction between 5-HT systems and the GABAA/BDZ chloride ionophore receptor complex, possibly involving a direct neuronal interaction, neuromodulation or hormonal alterations.

Disruption of neocortical lamina V neuronal bursts by serotonin in urethane anaesthetized rats

Time-shared high speed cyclic voltametry using carbon fibre multibarrelled microelectrodes was used to monitor the concentration of 5-Hydroxytryptamine administered by iontophoresis to locations in lamina V of Sm l neocortex and to record spontaneous neuronal spike activity. In the absence of 5-Hydroxytryptamine at any one recording location the firing of two or more individual units was seen to be synchronized so that the pattern of multi-unit activity consisted of synchronized clusters of spike activity interspersed with period of neuronal silence. The repetition rate of such clusters of neuronal activity was seen to be between 0.5 and 4Hz. Maximum concentrations of 2.7 x 10(-7) M 5-Hydroxytryptamine produced by iontophoresis disrupted synchronized neuronal cluster activity. 5-Hydroxytryptamine at a concentration of 6.2 x 10(-8) M resulted in a greater than 50% inhibition of activity for 47 single units but a change in firing pattern from cluster restricted high frequency activity to a continuous mode of firing for a separate population of 11 units. Intraperitoneal administration of P-Chloroamphetamine produced similar changes of neuronal firing and hence loss of synchrony.

Lack of response of serotonergic neurons in the dorsal raphe nucleus of freely moving cats to stressful stimuli

Changes in brain serotonin (5-HT) neurotransmission have been implicated in the mammalian response to stressful stimuli. The purpose of this study was to examine the extracellular single-unit activity of 5-HT neurons in cats exposed to three stressors: loud (100 dB) white noise, restraint, and confrontation with a dog. Serotonergic neurons were recorded in the dorsal raphe nucleus (DRN) and were identified by (i) slow and regular spontaneous activity, (ii) long duration (approximately 2 ms) waveform, (iii) complete suppression of activity during REM sleep and after systemic administration of 5-methoxy-N-N-dimethyltryptamine (250 micrograms/kg i.m.), and (iv) histological localization in the DRN. Despite behavioral and physiological evidence that all three manipulations induced a stress response, the maximal firing rate of 5-HT neurons was not significantly different from that observed under unstressed conditions. These data are consistent with previous studies from our laboratory which have indicated that very few manipulations are able to perturb the slow and regular activity of these neurons. In contrast, previous work has shown that the firing rate of noradrenergic neurons in the locus ceruleus is dramatically increased by these stressors. The relative imbalance in the activity of these two neuronal groups observed during stress may affect postsynaptic neuronal processing patterns and have adaptive significance during stressful conditions.