Medullary serotonergic neurons are insensitive to 5-MeoDMT and LSD

Activation of 5-HT1A receptors in medullary raphé disrupts sleep and decreases shivering during cooling in the conscious piglet

Activation of 5-HT1A receptors in the medullary raphé decreases sympathetically mediated brown adipose tissue (BAT) thermogenesis and peripheral vasoconstriction when previously activated with leptin, LPS, prostaglandins, or cooling. It is not known whether shivering is also modulated by medullary raphé 5-HT1A receptors. We previously showed in conscious piglets that activation of 5-HT1A receptors with (±)-8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT) in the paragigantocellularis lateralis (PGCL), a medullary region lateral to the raphé that contains substantial numbers of 5-HT neurons, eliminates rapid eye movement (REM) sleep and decreases shivering in a cold environment, but does not attenuate peripheral vasoconstriction. Hoffman JM, Brown JW, Sirlin EA, Benoit AM, Gill WH, Harris MB, Darnall RA. Am J Physiol Regul Integr Comp Physiol 293: R518–R527, 2007. We hypothesized that, during cooling, activation of 5-HT1A receptors in the medullary raphé would also eliminate REM sleep and, in contrast to activation of 5-HT1A receptors in the PGCL, would attenuate both shivering and peripheral vasoconstriction. In a continuously cool environment, dialysis of 8-OH-DPAT into the medullary raphé resulted in alternating brief periods of non-REM sleep and wakefulness and eliminated REM sleep, as observed when 8-OH-DPAT is dialyzed into the PGCL. Moreover, both shivering and peripheral vasoconstriction were significantly attenuated after 8-OH-DPAT dialysis into the medullary raphé. The effects of 8-OH-DPAT were prevented after dialysis of the selective 5-HT1A receptor antagonist WAY-100635. We conclude that, during cooling, exogenous activation of 5-HT1A receptors in the medullary raphé decreases both shivering and peripheral vasoconstriction. Our data are consistent with the hypothesis that neurons expressing 5-HT1A receptors in the medullary raphé facilitate spinal motor circuits involved in shivering, as well as sympathetic stimulation of other thermoregulatory effector mechanisms.

Activation of 5-HT1A receptors in the paragigantocellularis lateralis decreases shivering during cooling in the conscious piglet

Activation of 5-HT1A receptors in the medullary raphé decreases sympathetic outflow to thermoregulatory mechanisms, including brown adipose tissue (BAT), thermogenesis, and peripheral vasoconstriction when these mechanisms are previously activated with leptin, prostaglandins, or cooling. These same mechanisms are also inhibited during rapid eye movement (REM) sleep. It is not known whether shivering is also modulated by medullary raphé neurons. We previously showed in the conscious piglet that activation of 5-HT1A receptors with 8-OH-DPAT (DPAT) in the paragigantocellularis lateralis (PGCL), a medullary region lateral to the midline raphé that contains 5-HT neurons, decreases heart rate, body temperature and muscle activity during non-rapid eye movement (NREM) sleep. We therefore hypothesized that activation of 5-HT1A receptors in the PGCL would also attenuate shivering and peripheral vasoconstriction during cooling. During REM sleep in a cool environment, shivering, carbon dioxide production, and body temperature decreased, and ear capillary blood flow and ear skin temperature increased. Shivering associated with rapid cooling was attenuated after dialysis of DPAT into the PGCL. In animals maintained in a continuously cool environment, dialysis of DPAT into the PGCL attenuated shivering and decreased body temperature, but there were no significant increases in ear capillary blood flow or ear skin temperature. We conclude that both naturally occurring REM sleep and exogenous activation of 5-HT1A receptors in the PGCL are associated with a suspension of shivering during cooling. Our data are consistent with the hypothesis that 5-HT neurons in the PGCL facilitate oscillating spinal motor circuits involved in shivering but are less involved in modulating sympathetically mediated thermoregulatory mechanisms.

Inhibition of serotonergic neurons in the nucleus paragigantocellularis lateralis fragments sleep and decreases rapid eye movement sleep in the piglet: implications for sudden infant death syndrome

Serotonergic receptor binding is altered in the medullary serotonergic nuclei, including the paragigantocellularis lateralis (PGCL), in many infants who die of sudden infant death syndrome (SIDS). The PGCL receives inputs from many sites in the caudal brainstem and projects to the spinal cord and to more rostral areas important for arousal and vigilance. We have shown previously that local unilateral nonspecific neuronal inhibition in this region with GABA(A) agonists disrupts sleep architecture. We hypothesized that specifically inhibiting serotonergic activity in the PGCL would result in less sleep and heightened vigilance. We analyzed sleep before and after unilaterally dialyzing the 5-HT1A agonist (+/-)-8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT) into the juxtafacial PGCL in conscious newborn piglets. 8-OH-DPAT dialysis resulted in fragmented sleep with an increase in the number and a decrease in the duration of bouts of nonrapid eye movement (NREM) sleep and a marked decrease in amount of rapid eye movement (REM) sleep. After 8-OH-DPAT dialysis, there were decreases in body movements, including shivering, during NREM sleep; body temperature and heart rate also decreased. The effects of 8-OH-DPAT were blocked by local pretreatment with N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane-carboxamide, a selective 5-HT1A antagonist. Destruction of serotonergic neurons with 5,7-DHT resulted in fragmented sleep and eliminated the effects of subsequent 8-OH-DPAT dialysis on REM but not the effects on body temperature or heart rate. We conclude that neurons expressing 5-HT1A autoreceptors in the juxtafacial PGCL are involved in regulating or modulating sleep. Abnormalities in the function of these neurons may alter sleep homeostasis and contribute to the etiology of SIDS.

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.

Physiological identification of pontomedullary serotonergic neurons in the rat

Spinal serotonin is derived entirely from bulbar sources and plays an important role in spinal modulatory processes, including pain modulation. Establishing the electrophysiological properties of SEROTONERGIC bulbospinal neurons in the pontomedullary raphe and reticular formation is critical to understanding the physiological role of serotonin in the spinal cord. Neurons were characterized by their responses to noxious stimulation and their background discharge pattern in the lightly anesthetized rat. Characterized cells were intracellularly labeled with Neurobiotin, which was visualized with a Texas Red fluorophore. Sections containing the labeled cells were processed for serotonin immunocytochemistry with the use of a Bodipy fluorophore. Forty-seven intracellularly labeled cells were tested for serotonin immunoreactivity. The labeled neurons were located in raphe magnus, the nucleus reticularis magnocellularis, and the adjacent reticular and raphe nuclei at levels from the inferior olivary complex to the superior olivary complex. SEROTONERGIC cells were located in the raphe nuclei, in nucleus reticularis magnocellularis pars alpha, and in nucleus reticularis magnocellularis pars beta or nucleus reticularis gigantocellularis. Thirteen intracellularly labeled cells contained serotonin immunoreactivity. The background discharge rate of SEROTONERGIC cells average 1.8 Hz (range: 0.5-3.1 Hz). Discharge was steady and without sustained pauses or bursts in firing. Most serotonin-immunoreactive cells were unaffected or slightly excited by pinch and were unaffected by noxious heat. Three SEROTONERGIC cells were weakly excited by both noxious pinch and heat, whereas two SEROTONERGIC cells were briefly inhibited by these stimuli. Cells that lacked serotonin immunoreactivity were heterogeneous and included ON, OFF, and NEUTRAL cells. Nonserotonergic cells differed from SEROTONERGIC cells in having an irregular discharge pattern and/or a high mean discharge rate. A linear discriminant function, employing background discharge characteristics as independent variables, was calculated that successfully classified 13 of 13 SEROTONERGIC and 32 of 33 nonserotonergic neurons. The probability of misclassification with the use of this discriminant function was estimated to be < 10%. Employing the discriminant function on a test group of cells whose immunochemical content was unknown revealed a population of SEROTONERGIC-LIKE cells that resembled the labeled SEROTONERGIC cells in background discharge pattern, response to noxious stimulation, and nuclear location. The discharge of pontomedullary SEROTONERGIC neurons is slow and steady, suggesting that these neurons may have a role in the tonic, rather than phasic, modulation of spinal processes.

Pharmacological characterization of medullary serotonin neurons

In the present study we investigated the characteristics of medullary raphe serotonergic neurons. Specifically, we sought to examine further the similarities between medullospinal 5-HT neurons and the more extensively studied neurons of the dorsal raphe. Intravenous administration of 5-methoxy-dimethyltryptamine (5-MeODMT) produced a dose-related inhibition of the firing of midline medullary 5-HT neurons. Microiontophoretically applied 5-MeODMT also inhibited medullary 5-HT neurons. The inhibitory potency of 5-MeODMT was nearly identical to that observed for dorsal raphe 5-HT neurons. Microiontophoretic or intravenous administration of the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) did not alter the firing rate of medullary 5-HT neurons. Intravenous administration of the alpha 1-receptor antagonist prazosin resulted in an inhibition of the medullary 5-HT neuronal firing. The discharge of medullary 5-HT neurons increased during iontophoresis of norepinephrine. These data are discussed in relation to the identification and characterization of medullary 5-HT neurons. In addition, the data suggest that the firing rate of medullary 5-HT neurons is regulated in part by a tonic excitatory noradrenergic input.

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 evidence for a functional differentiation between subtypes of the 5-HT1 receptor

Serotonin (5-HT) neurons in the dorsal (DRN) and median (MRN) raphe nuclei, and dopamine (DA) neurons in the substantia nigra (SN) were recorded extracellularly in the anesthetized rat. Compounds which have a relatively high affinity for the 5-HT1A or 5-HT1B subtypes of the 5-HT1 receptor were administered and their effect on the firing rate of the monoamine cells was determined. 5-HT1A ligands were more potent in inhibiting impulse activity in the DRN than in the MRN, but had little effect in the SN. In contrast, 5-HT1B ligands increased the firing rate of MRN 5-HT units at low doses, and were also effective inhibitors of DA cell firing in the SN. These results could be correlated with recently described differences in the distribution of the 5-HT1A and 5-HT1B receptor subtypes, and were interpreted as indicating possible functional differentiation between these subtypes. In particular, agonist activity at the 5-HT1B autoreceptor site may decrease 5-HT release, suggesting a presynaptic locus for this receptor in the somatodendritic region. The site also appears to be implicated in 5-HT modulation of nigral DA impulse flow.

3-Acetylpyridine lesions and four serotonergic behavioral syndromes in the rat

We studied the effect of 3-acetylpyridine (3-AP) lesions on the serotonergic-myoclonic syndromes evoked by quipazine (QP), 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), fenfluramine (FF), and p-chloroamphetamine (PCA) in the adult rat. Eleven behaviors were scored from videotapes by an observer blind to drug status. In unlesioned rats, drugs could be differentiated by forelimb and axial myoclonus, pivoting and backing. All drugs significantly suppressed rearing. 3-AP produced a lasting action-enhanced body tremor which differed from axial myoclonus in its vertical direction and rhythmicity. 3-AP lesions modified the effect of drugs on several behaviors, increasing axial (QP, FF, PCA) and forelimb (5-MeO-DMT, FF, PCA) myoclonus and decreasing locomotor score. Prior lesions with 5,7-dihydroxytryptamine did not prevent the effect of 3-AP or any behaviors of the serotonin syndrome, but had a slight effect on the magnitude of forelimb myoclonus, head weaving, and hunching induced by some drugs. Neither lesion abolished or reduced myoclonus. These data suggest that intact 5-HT terminals are not requisite for the tremorogenic and cytotoxic effect of 3-AP. To the extent that chemical lesions with 3-AP are selective for the inferior olive (IO), the role of the IO in myoclonus in several 5-HT rodent myoclonic models appears to be regulatory rather than stimulatory.

Enteric receptors for 5-hydroxytryptamine

[3H]5-hydroxytryptamine [( 3H]5-HT) was used as a radioligand to study enteric 5-HT receptors. Membranes were derived from preparations of longitudinal muscle with adherent myenteric plexus and of mucosa-submucosa dissected from guinea pig and rabbit small intestines. Filtration and radioautographic analyses were used. Specific [3H]5-HT binding was found in both preparations. Binding was saturable and dissociable with equilibrium dissociation constants (Kd) of 2.7 and 1.4 nM, respectively. A kinetic estimate of Kd (1.5 nM) was similar to that determined by saturation analysis and the Hill coefficient approximated unity. Ring-hydroxylation of indoles was found to be a requirement for antagonism of [3H]5-HT binding. On the other hand, substitutions could be made in the aliphatic side chain of tryptamines without destroying the affinity of analogues for the binding sites. The inability of antagonists to displace [3H]5-HT indicated that the binding sites were not muscarinic or nicotinic receptors, alpha- or beta-adrenoceptors, H1 or H2 histamine receptors, dopamine receptors or either the S1 or S2 types of 5-HT receptor that have been found in the brain. Frozen section dry-mount radioautography revealed the [3H]5-HT binding sites to be located in ganglia of the myenteric plexus and at the boundary between the mucosa and submucosa. The similarity between the structure-activity requirements for affinity at the [3H]5-HT binding sites and activation of neural or M receptors for 5-HT in the gut, as well as the characteristics and location of the binding sites suggests that they are enteric neural receptors for 5-HT.

Effects of methiothepin and lysergic acid diethylamide on serotonin release in vitro and serotonin synthesis in vivo: possible relation to serotonin autoreceptor function

An in vitro system characterizing the presynaptic serotonin (5-HT) autoreceptor which controls the release of 5-HT from rat brain slices is described. Using this system, methiothepin (1-10 microM) demonstrated 5-HT autoreceptor antagonist activity by enhancing 5-HT release, while several recognized postsynaptic 5-HT receptor antagonists were inactive: mianserin, cinanserin, cyproheptadine, methysergide. The activity of methiothepin was highest in hypothalamic slices and lowest in striatal slices and was inhibited by the autoreceptor agonists lysergic acid diethylamide (LSD) and 5-methoxytryptamine (5-MT). The reversal of the methiothepin-enhanced 5-HT release from hypothalamic slices by LSD was not influenced by 0.3 microM tetrodotoxin. The peripheral administration of LSD to rats has been shown to reduce 5-HT synthesis and release by a mechanism thought to involve, in part, an autoreceptor-mediated reduction in impulse flow of 5-HT neurons. In the present experiments, intraperitoneal injection of methiothepin antagonized the LSD-induced reduction in hypothalamic 5-HT synthesis (5-hydroxytryptophan accumulation) while exerting no influence by itself. Conversely, compounds which were not active as 5-HT autoreceptor antagonists in vitro (i.e., cyproheptadine, methysergide, cinanserin) did not influence the effect of LSD on 5-HT synthesis. Further, the reduction in 5-hydroxytryptophan (5-HTP) accumulation by LSD showed regional differences in inhibition by methiothepin (hypothalamus greater than cortex greater than striatum) which paralleled the autoreceptor antagonist activity of methiothepin in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Raphe neurons: firing rate correlates with size of drug response

Significant negative correlations were obtained between the spontaneous discharge rate during waking and the neural response to systemic injections of either 5-MeODMT or LSD for serotonergic neurons in the dorsal raphe nucleus, nucleus centralis superior, and nucleus raphe pallidus of unanesthetized and unrestrained cats. These data are discussed in terms of an hypothesis which accounts for both the rate of spontaneous activity of serotonergic neurons and the magnitude of their response to serotonin agonist drugs in terms of autoreceptor density on individual neurons.