Raphe neurons: firing rate correlates with size of drug response

A narrative synthesis of research with 5-MeO-DMT

BACKGROUND

5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT) is a naturally occurring, short-acting psychedelic tryptamine, produced by a variety of plant and animal species. Plants containing 5-MeO-DMT have been used throughout history for ritual and spiritual purposes. The aim of this article is to review the available literature about 5-MeO-DMT and inform subsequent clinical development.

METHODS

We searched PubMed database for articles about 5-MeO-DMT. Search results were cross-checked against earlier reviews and reference lists were hand searched. Findings were synthesised using a narrative synthesis approach. This review covers the pharmacology, chemistry and metabolism of 5-MeO-DMT, as well epidemiological studies, and reported adverse and beneficial effects.

RESULTS

5-MeO-DMT is serotonergic agonist, with highest affinity for 5-HT_1A receptors. It was studied in a variety of animal models, but clinical studies with humans are lacking. Epidemiological studies indicate that, like other psychedelics, 5-MeO-DMT induces profound alterations in consciousness (including mystical experiences), with potential beneficial long-term effects on mental health and well-being.

CONCLUSION

5-MeO-DMT is a potentially useful addition to the psychedelic pharmacopoeia because of its short duration of action, relative lack of visual effects and putatively higher rates of ego-dissolution and mystical experiences. We conclude that further clinical exploration is warranted, using similar precautions as with other classic psychedelics.

Involvement of 5-HT1A and 5-HT2A Receptors but Not α 2-Adrenoceptors in the Acute Electrophysiological Effects of Cariprazine in the Rat Brain In Vivo

Cariprazine, an orally active and potent dopamine D₃-preferring D₃/D₂ receptor partial agonist, is approved to treat adults with schizophrenia (in the United States and Europe) and manic or mixed episodes associated with bipolar I disorder (in the United States). Cariprazine also displays partial agonism at serotonin [5-hydroxytryptamine (5-HT)] 5-HT_1A receptors and antagonism at 5-HT_2A and 5-HT_2B receptors in vitro. The study objective was to determine whether cariprazine leads to functional alterations of monoamine systems in vivo via electrophysiological recordings from anesthetized rats. Dorsal raphe nucleus (DRN), locus coeruleus (LC), and hippocampus pyramidal neurons were recorded, and cariprazine was administered systemically or locally through iontophoresis. In the DRN, cariprazine completely inhibited the firing activity of 5-HT neurons, which was fully reversed by the 5-HT_1A receptor antagonist, WAY100635. In the LC, cariprazine reversed the inhibitory effect of the preferential 5-HT_2A receptor agonist, 2,5-dimethoxy-4-iodoamphetamine, on norepinephrine (NE) neurons (ED₅₀ = 66 µg/kg) but did not block the inhibitory effect of the α ₂-adrenergic receptor agonist, clonidine. Cariprazine, iontophorized into the hippocampus, diminished pyramidal neuronal firing through activation of 5-HT_1A receptors, while its concomitant administration did not dampen the suppressant effect of 5-HT. These results indicate that, in vivo, cariprazine acted as a 5-HT_1A autoreceptor agonist in the DRN, a 5-HT_2A receptor antagonist in modulating the firing activity of LC NE neurons, and a full agonist at 5-HT_1A receptors mediating the electrophysiological effect of 5-HT on pyramidal neurons. The modulatory actions of cariprazine on these monoaminergic systems may contribute to its therapeutic effectiveness in patients with depressive episodes.

High-frequency stimulation of the subthalamic nucleus inhibits the firing of juxtacellular labelled 5-HT-containing neurones

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) is an established neurosurgical therapy for movement disability in advanced Parkinson's disease (PD), but some patients experience psychiatric side-effects like depression. In a previous electrophysiological study, we observed that HFS of the STN inhibited a population of neurones in the rat dorsal raphe nucleus (DRN), with firing properties characteristic of 5-HT neurones. The present study extended these findings to a second population of neurones, and combined extracellular recording with juxtacellular-labelling to investigate the chemical identity of the neurones affected by HFS. Bilateral HFS (130 Hz, 100-200 μA, 5 min) of the STN inhibited (26.0±2.9%) the firing of 37/74 DRN neurones displaying a slow, regular firing pattern. Slower firing neurones were more strongly inhibited than those firing faster. Importantly, 10 inhibited DRN neurones were juxtacellular-labelled with neurobiotin, and all neurones contained 5-HT as shown by post-mortem 5-HT immunocytochemistry. A minority of slow firing DRN neurones (18/74) were activated by STN HFS (37.9±8.3%) which was not observed previously. Of these neurones, three were juxtacellular-labelled and one was 5-HT immunopositive. Also a small number of DRN neurones (19/74) did not respond to HFS, four of which were juxtacellular-labelled and all contained 5-HT. These data show that individual chemically-identified 5-HT-containing neurones in the DRN were modulated by STN HFS, and that the majority were inhibited but some were activated and some failed to respond. These data extend previous findings of modulation of the 5-HT system by STN HFS but suggest a destabilisation of the 5-HT system rather than simple inhibition as indicated previously. Although the mechanism is not yet known, such changes may contribute to the psychiatric side-effects of STN stimulation in some PD patients.

Inhibition of serotonergic medullary raphe obscurus neurons suppresses genioglossus and diaphragm activities in anesthetized but not conscious rats

Although exogenous serotonin at the hypoglossal motor nucleus (HMN) activates the genioglossus muscle, endogenous serotonin plays a minimal role in modulating genioglossus activity in awake and sleeping rats (Sood S, Morrison JL, Liu H, and Horner RL. Am J Respir Crit Care Med 172: 1338–1347, 2005). This result therefore implies that medullary raphe neurons also play a minimal role in the normal physiological control of the HMN, but this has not yet been established because raphe neurons release other excitatory neurotransmitters onto respiratory motoneurons in addition to serotonin. This study tests the hypothesis that inhibition of medullary raphe serotonergic neurons with 8-hydroxy-2-(di- n-propylamino)tetralin (8-OH-DPAT) suppresses genioglossus and diaphragm activities in awake and sleeping rats. Ten rats were implanted with electrodes to record sleep-wake states and genioglossus and diaphragm activities. Microdialysis probes were also implanted into the nucleus raphe obscurus (NRO). Experiments in 10 anesthetized and vagotomized rats were also performed using the same methodology. In anesthetized rats, microdialysis perfusion of 0.1 mM 8-OH-DPAT into the NRO decreased genioglossus activity by 60.7 ± 9.0% and diaphragm activity by 13.3 ± 3.4%. Diaphragm responses to 7.5% CO2 were also significantly reduced by 8-OH-DPAT. However, despite the robust effects observed in anesthetized and vagotomized rats, there was no effect of 0.1 mM 8-OH-DPAT on genioglossus or diaphragm activities in conscious rats awake or asleep. The results support the concept that endogenously active serotonergic medullary raphe neurons play a minimal role in modulating respiratory motor activity across natural sleep-wake states in freely behaving rodents. This result has implications for pharmacological strategies aiming to manipulate raphe neurons and endogenous serotonin in obstructive sleep apnea.

"Fatigue" of medullary but not mesencephalic raphe serotonergic neurons during locomotion in cats

Single unit activity of presumed serotonergic neurons in the medulla [n. raphe obscurus (NRO) and pallidus (NRP)] or the mesencephalon [n. raphe dorsalis (DRN)] was recorded in adult male cats during prolonged treadmill locomotion. Treadmill speed was set at a moderate level (0.4 m/s) in order to induce long-duration locomotion. The typical time to "fatigue" (failure to keep pace, falling behind and reluctance to continue) was approximately 40 min in both groups, at which point cats typically displayed marked panting and vocalization. The activity of DRN neurons was unchanged from baseline during the locomotion trial and during the recovery phase. By contrast, the activity of NRO/NRP neurons decreased steadily across the locomotion trial, reaching a mean decrease of approximately 50% (during the first min after the treadmill was turned off). Full recovery of single unit activity to a level approximating the baseline discharge rate required 30-45 min. Possible mechanisms underlying these changes are discussed as is the role of serotonin and fatigue in human pathology.

Inhibition of medullary raphé serotonergic neurons has age-dependent effects on the CO2 response in newborn piglets

Medullary raphé serotonergic neurons are chemosensitive in culture and are situated adjacent to blood vessels in the brain stem. Selective lesioning of serotonergic raphé neurons decreases the ventilatory response to systemic CO2 in awake and sleeping adult rats. Abnormalities in the medullary serotonergic system, including the raphé, have been implicated in the sudden infant death syndrome ( 48 ). In this study, we ask whether serotonergic neurons in the medullary raphé and extra-raphé regions are involved in the CO2 response in unanesthetized newborn piglets, 3-16 days old. Whole body plethysmography was used to examine the ventilatory response to 5% CO2 before and during focal inhibition of serotonergic neurons by 8-hydroxy-2-di- n-propylaminotetralin (8-OH-DPAT), a 5-HT1A receptor agonist. 8-OH-DPAT (10 or 30 mM in artificial cerebrospinal fluid) decreased the CO2 response in wakefulness in an age-dependent manner, as revealed by a linear regression analysis that showed a significant negative correlation ( P < 0.001) between the percent change in the CO2 response and piglet age. Younger piglets showed an exaggerated CO2 response. Control dialysis with artificial cerebrospinal fluid had no significant effect on the CO2 response. Additionally, 8-OH-DPAT increased blood pressure and decreased heart rate independent of age ( P < 0.05). Finally, sleep cycling was disrupted by 8-OH-DPAT, such that piglets were awake more and asleep less ( P < 0.05). Because of the fragmentary sleep data, it was not possible to examine the CO2 response in sleep. Inhibition of serotonergic medullary raphé and extra-raphé neurons decreases ventilatory CO2 sensitivity and alters cardiovascular variables and sleep cycling, which may contribute to the sudden infant death syndrome.

The serotonergic system and mysticism: could LSD and the nondrug-induced mystical experience share common neural mechanisms?

This article aims to explore, through established scientific research and documented accounts of personal experience, the similarities between religious mystical experiences and some effects of D-lysergic diethylamide or LSD. LSD predominantly works upon the serotonergic (serotonin-using neurons) diffuse neuromodulatory system, which projects its axons to virtually all areas of the brain including the neocortex. By its normal action it modulates awareness of the environmental surroundings and filters a high proportion of this information before it can be processed, thereby only allowing the amount of information that is necessary for survival. LSD works to open this filter, and so an increased amount of somatosensory data is processed with a corresponding increase in what is deemed important. This article describes the effects and actions of LSD, and due to the similarities with the nondrug-induced mystical experience the author proposes that the two could have common modes of action upon the brain. This could lead to avenues of research into mysticism and a wealth of knowledge on consciousness and how we perceive the universe.

Electrophysiological and neurochemical evidence that pindolol has agonist properties at the 5-HT1A autoreceptor in vivo

1. It has been hypothesized that 5-HT1A autoreceptor antagonists may enhance the therapeutic efficacy of SSRIs and other antidepressants. Although early clinical trials with the beta-adrenoceptor/5-HT1 ligand, pindolol, were promising, the results of recent more extensive trials have been contradictory. Here we investigated the actions of pindolol at the 5-HT1A autoreceptor by measuring its effect on 5-HT neuronal activity and release in the anaesthetized rat. 2. Pindolol inhibited the electrical activity of 5-HT neurones in the dorsal raphe nucleus (DRN). This effect was observed in the majority of neurones tested (10/16), was dose-related (0.2-1.0 mg kg(-1), i.v.), and was reversed by the 5-HT1A receptor antagonist, WAY 100635 (0.1 mg kg(-1), i.v.), in 6/7 cases tested. 3. Pindolol also inhibited 5-HT neuronal activity when applied microiontophoretically into the DRN in 9/10 neurones tested. This effect of pindolol was current-dependent and blocked by co-application of WAY 100635 (3/3 neurones tested). 4. In microdialysis experiments. pindolol caused a dose-related (0.8 and 4 mg kg(-1), i.v.) fall in 5-HT levels in dialysates from the frontal cortex (under conditions where the perfusion medium contained 1 microM citalopram). In rats pretreated with WAY 100635 (0.1 mg kg(-1), i.v.), pindolol (4 mg kg(-1), i.v.) did not decrease, but rather increased 5-HT levels. 5. We conclude that, under the experimental conditions used in this study, pindolol displays agonist effects at the 5-HT1A autoreceptor. These data are relevant to previous and ongoing clinical trials of pindolol in depression which are based on the rationale that the drug is an effective 5-HT1A autoreceptor antagonist.

Identification of serotonergic and sympathetic neurons in medullary raphe nuclei

The purpose of the present study was to identify midline medullary serotonin (5-HT) neurons and to determine if these neurons were distinct from previously identified sympathoinhibitory and sympathoexcitatory neurons. Identification of medullary 5-HT neurons was based on electrophysiological and pharmacological similarities to dorsal raphe 5-HT neurons. Sympathoinhibitory and sympathoexcitatory neurons were characterized by an irregular discharge pattern which was temporally related to inferior cardiac sympathetic nerve discharge (SND) and to the cardiac cycle. Sympathoinhibitory neurons increased their discharge rate and the discharge of sympathoexcitatory neurons decreased during baroreceptor reflex activation. A third type of neuron fired in an extremely regular fashion (as judged by interspike interval analysis), fired at a rate of 1.1 spikes/s and had spike durations of approximately 2 ms. The discharges of regularly firing neurons were not temporally related to SND and were not affected during baroreceptor reflex activation. Regularly firing neurons and sympathoinhibitory neurons could be antidromically activated by electrical stimulation of the intermediolateral cell column of the spinal cord. Axonal conduction velocity of sympathoinhibitory neurons (2.4 m/s) was significantly greater than that for regularly firing neurons (1.3 m/s). Regularly firing neurons were completely inhibited by low doses of the 5-HT1A agonist 8-hydroxy-dipropylamino-tetralin (8-OH-DPAT) (i.e. 2 micrograms/kg, i.v.) while much higher doses of the drug failed to affect the discharges of sympathoinhibitory and sympathoexcitatory neurons. Microiontophoretic application of 5-HT and 8-OH-DPAT profoundly depressed the firing of regularly discharging neurons. Based on the striking similarities between regularly firing medullary neurons and dorsal raphe 5-HT neurons it is concluded that the regularly firing neurons were 5-HT-containing neurons. Furthermore, these medullary 5-HT neurons are distinct from sympathoinhibitory and sympathoexcitatory neurons.

Electrophysiological and pharmacological evidence for the existence of distinct subpopulations of nigrostriatal dopaminergic neuron in the rat

The electrophysiological and pharmacological properties of dopaminergic neurons were systematically examined throughout the anterior-posterior extent of the substantia nigra zona compacta in the rat. Cells were characterized in terms of their (1) firing pattern, (2) firing rate, (3) antidromic response properties, and (4) inhibition in firing rate following dopaminergic agonist administration. These properties were then related to the cell's position within one of four anterior-posterior segments of the nucleus. There were three types of neuronal discharge pattern encountered; irregular, burst and regular. Cells which exhibited different firing patterns exhibited different firing rates and anatomical locations within the substantia nigra zona compacta. All neurons were antidromically activated from the striatum, however, the burst- and regular-firing cells exhibited significantly faster estimated conduction velocities than irregular-firing cells. The irregular-firing cells were most sensitive to dopaminergic autoreceptor agonists whereas the burst-firing cells were most sensitive to an indirect-acting dopaminergic agonist. These experiments provide both electrophysiological and pharmacological evidence to indicate that nigrostriatal dopaminergic neurons are composed of distinct subpopulations which are characterized by their firing pattern.

SKF 38393 alters the rate-dependent D2-mediated inhibition of nigrostriatal but not mesoaccumbens dopamine neurons

Previous electrophysiological studies have failed to identify significant effects of the D1 dopamine (DA) agonist SKF 38393, either alone or in combination with the D2 agonist quinpirole (LY 171555), on the spontaneous firing rate of midbrain DA neurons. We have utilized extracellular single-unit recording techniques to examine whether SKF 38393 can alter D2-mediated inhibition of DA cell activity. Quinpirole-induced inhibition of the spontaneous activity of midbrain DA neurons was observed to be positively correlated with the basal firing rate of the neuron being examined (i.e., faster cells required higher doses to achieve 50% and maximal inhibition). Pretreatment with SKF 38393 (1.0 mg/kg, i.v.; 4 minutes) eliminated the rate dependency of quinpirole-induced inhibition of nigrostriatal but not mesoaccumbens DA neurons. This effect of SKF 38393 was blocked both by the D1 antagonist SCH 23390 and by hemitransections of the forebrain. In summary, SKF 38393 appears to exert Dl-specific, feedback pathway-dependent effects on the profile of responsiveness of nigrostriatal DA neurons to D2-mediated inhibition of cell firing rate.

gamma-Aminobutyric acid and 5-hydroxytryptamine interrelationship in the rat nucleus raphe dorsalis: combination of radioautographic and immunocytochemical techniques at light and electron microscopy levels

Serotonin and gamma-aminobutyric acid (GABA) neurons in the nucleus raphe dorsalis were identified by immunocytochemistry using antibodies to 5-hydroxytryptamine or GABA. The pattern of the 5-hydroxytryptamine and GABA immunostaining presented similar features: 5-hydroxytryptamine or GABA immunoreactive somata were fusiform or ovoid (15-20 micron) and positive dendritic profiles were found either without any connection with other nerve elements or in contact with one or several terminals. In addition, some 5-hydroxytryptamine nerve endings were apposed to 5-hydroxytryptamine immunoreactive cell bodies or dendrites; also some GABA-immunopositive terminals were in contact with GABA-immunopositive nerve cell bodies. On the other hand, GABA and 5-hydroxytryptamine patterns may be differentiated in several respects: the 5-hydroxytryptamine-reactive nerve cell bodies were more numerous than the GABA ones. Some small, round (8-10 micron) nerve cell bodies were reactive with GABA antiserum, but no neurons of this type were reactive with a 5-hydroxytryptamine antiserum; finally, GABA nerve terminals were more numerous than 5-hydroxytryptamine ones. In order to understand the relationship between GABA and 5-hydroxytryptamine neurons, radioautographic and immunocytochemical procedures were combined: 5-hydroxytryptamine and GABA immunocytochemistry was combined with radioautography of [3H]GABA and [3H]5-hydroxytryptamine uptake, respectively. Some nerve cell bodies, dendrites or terminals, which were 5-hydroxytryptamine-immunopositive, were also capable of accumulating [3H]GABA and, conversely, some GABA-immunopositive elements were capable of accumulating [3H]5-hydroxytryptamine. Moreover, several nerve elements were reactive with both glutamate decarboxylase and 5-hydroxytryptamine antisera. These data confirm in electron microscopy previous studies suggesting the coexistence of both GABA and 5-hydroxytryptamine in the same neurons. The presence of uptake mechanisms for GABA and 5-hydroxytryptamine may indicate the action of both neurotransmitters in the same neuron. On the other hand, the [3H]GABA-labelled nerve endings in contact with 5-hydroxytryptamine-positive dendrites or nerve cell bodies indicate the possibility of a GABAergic control of the activity of some 5-hydroxytryptamine neurons; this corroborates biochemical and electrophysiological studies whereby a trans-synaptic control of the 5-hydroxytryptamine neurons by GABA may be envisaged.

Responsiveness to 'autoreceptor' doses of apomorphine inversely correlated with the firing rate of dopaminergic A9 neurons: action of baclofen

Supersensitivity of dopamine (DA) autoreceptors develops soon after acute treatments with baclofen or gamma-butyrolactone, two drugs which suppress DA neuronal firing. We have hypothesized that this effect might reflect a condition associated with the low firing rate of DA cells induced by such treatments rather than a long-term adaptive modification of DA receptor sensitivity. In this study we show that the degree of the intravenous apomorphine-induced inhibition of A9-DA neurons is inversely correlated to the basal firing rate of these neurons. When administered after baclofen, apomorphine further reduced DA cell activities in a manner proportional to the predrug firing rate, as in control rats. Since the minute doses of apomorphine which were used are thought to selectively activate DA autoreceptors, our finding may indicate that DA autoreceptor stimulation is more effective when the activity of the DA neurons is low, or it is decreased by drugs such as baclofen.

Ontogenesis of unit activity in the raphe dorsalis of the behaving kitten: its relationship with the states of vigilance

The relationship between the spontaneous unit activity in the raphe dorsalis (RD), and the sleep-wakefulness cycles, was analyzed in the cat from birth to 40 days of age. Electrodes for polygraphic sleep monitoring were implanted under anesthesia, and unit recordings were obtained from bundles of microwires positioned in the RD area in kittens of different ages. Attention was paid only to units with slow firing in wakefulness (W) (1-6 spikes/s), and two types of discharge patterns during this state were obtained: a 'regular' type, whose discharge in W had the same characteristics of regularity as those described for the adult under the same conditions, was always found inside the RD. An 'irregular' type was always found in sites outside the RD. Injections of different doses of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT i.m.) induced a transient decrease in the firing rate of the regular type of cells, and no change for the units of the irregular type, suggesting that the regular neurons were of serotoninergic nature. Whereas the cells of the irregular type exhibited an increase of discharge frequency in active or paradoxical sleep (AS-PS), those which fired in a clock-like manner during W exhibited a rate of discharge which progressively decreased in quiet or slow wave sleep (QS-SWS) and even more in AS-PS. Such a pattern was qualitatively close to the adult one at all ages, but the discharge rate in AS was significantly higher during the first and second weeks of life than later on. The observation that these serotoninergic neurons exhibited at birth an adult-like pattern of discharge during W, indicates that during ontogenesis there was no direct relationship between the RD activity and the behavioral output. It is proposed that the RD neuronal discharge would be largely under genetic influences, and that the maturation of sleep regulations at the brainstem and mesencephalic levels is achieved only after the second week of postnatal age.

Efflux of 3H-5-hydroxytryptamine from rat hypothalamic slices by continuous electrical stimulation: frequency-dependent responses to serotonergic antagonists and 5-hydroxytryptamine

Rat hypothalamic slices were incubated with 3H-5-hydroxytryptamine and superfused in the presence of paroxetine to inhibit 5-hydroxytryptamine (5-HT) reuptake. The slices were continuously stimulated electrically with rectangular pulses at varying frequencies. Continuous stimulation for up to 42 min at 1 Hz or at 3 Hz evoked a steady efflux of tritium that slowly decayed with time. The efflux produced by continuous stimulation at 5 Hz declined more rapidly with time. Continuous stimulation at 1 Hz in the presence of increasing concentrations of unlabelled 5-HT produced a concentration-dependent decrease in tritium efflux. The presence of methiothepin (0.5 mumol/l), quipazine (10 mumol/l) and (-)- but not (+)-propranolol (1 mumol/l) attenuated this response to 5-HT. From these data, the apparent pA2 values were calculated and found to be in agreement with published values. Frequency-dependent responses were determined using a "cumulative stimulation" protocol whereby the slices were subjected to three consecutive 14 min periods of stimulation at increasing frequencies (1, 3 and then 5 Hz). Unlabelled 5-HT (1 mumol/l) inhibited electrically-evoked tritium efflux more at 1 than at 5 Hz. Methiothepin (0.5 mumol/l) and quipazine (10 mumol/l) enhanced the stimulated efflux in a manner inversely related to the frequency of stimulation. Neither (+)- nor (-)-propranolol enhanced stimulated tritium efflux at any of the three frequencies tested. It is concluded that continuous electrical stimulation of rat hypothalamic slices at a low frequency provides a rapid means of obtaining apparent affinities and intrinsic activities of drugs that modify the serotonergic autoreceptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Activity of serotonin-containing neurons in nucleus raphe magnus in freely moving cats

Serotonergic neurons were recorded in the nucleus raphe magnus in freely moving cats and were initially identified on-line by their characteristic slow and regular spontaneous activity during quiet waking (3.42 +/- 0.33 spikes/s; mean +/- SE). Discharge rates of these serotonergic neurons were highest during active waking (4.49 +/- 0.40 spikes/s), intermediate during slow-wave sleep (middle: 2.14 +/- 0.23 spikes/s), and lowest during REM sleep (0.20 +/- 0.03 spikes/s). Although these cells fired at a rate 31.3% higher during active waking than during quiet waking, their activity displayed no correlation with phasic elevations of the nuchal EMG or overt body movements. In addition, no relationship was observed between the activity of these neurons during slow-wave sleep and the occurrence of sleep spindles in the cortical EEG or pontogeniculooccipital waves recorded from the lateral geniculate nucleus. Serotonergic neurons of nucleus raphe magnus were also relatively unresponsive to phasic auditory and visual stimuli, with about half of the cells examined showing weak excitatory responses. These neurons did respond, however, to the administration of a small dose of the serotonin specific agonist, 5-methoxy-N,N-dimethyltryptamine (250 micrograms/kg, i.m.) with a mean decrease in unit activity of 73.6 +/- 4.5%. The results of this study are compared with those previously reported for serotonergic neurons in the dorsal raphe nucleus, nucleus centralis superior, and nucleus raphe pallidus of freely moving cats.

A10 dopamine neurons: role of autoreceptors in determining firing rate and sensitivity to dopamine agonists

The present experiments investigated the relationship between the spontaneous basal firing rate of A10 dopamine (DA) neurons and their sensitivity to the rate-suppressant effects of intravenously administered apomorphine (APO) and d-amphetamine (AMP) as well as microiontophoretically ejected DA. The results indicated highly significant inverse relationships between basal neuronal activity and sensitivity to DA and DA agonists, i.e. the faster the spontaneous rate of an A10 DA neuron, the less sensitive that cell was to agonist-induced suppression. This relationship was not found for the rate suppressant effects of iontophoretic gamma-aminobutyric acid. There were no significant differences between the effects of iontophoretic DA on pre-glutamate and glutamate-driven activity of the same A10 DA neurons indicating that faster firing rates, per se, did not determine the sensitivity of these cells to DA agonists. Rather, these results suggest that both spontaneous activity and sensitivity to DA agonists may be determined by the density (or sensitivity) of DA autoreceptors on A10 DA neurons. This hypothesis was supported by the finding that antidromically identified mesocortical DA neurons, which were significantly less responsive to DA, APO and AMP exhibited significantly faster firing rates than other A10 DA neurons. Thus, this subpopulation of A10 DA neurons is primarily made up of cells with low autoreceptor density (or sensitivity).