Activity Of Serotonin-containing Neurons Research Articles

Unit activity in the dorsal raphe in freely-moving cats: Effects of monoamine oxidase inhibitors

Two commonly used monoamine oxidase inhibitors, tranylcypromine and pargyline, produced dose-dependent decreases in the activity of serotonin-containing neurons in the dorsal raphe in awake, freely-moving cats. The onset of the suppression of unit activity occurred within 15–20 min after administration of drug and persisted for 6–16 hr, depending upon dose. Parallel neurochemical studies revealed that serotonin in the brain was significantly increased following inhibition of monoamine oxidase, and that concentrations of serotonin were still significantly elevated after unit activity in the raphe had returned to baseline levels. These data suggest that autoreceptors on neurons of the dorsal raphe may become tolerant following prolonged exposure to large concentrations of serotonin.

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 μg/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.

Activity of serotonin-containing neurons in nucleus centralis superior of freely moving cats

Abstract Serotonergic neurons were recorded in the nucleus centralis superior (NCS) in freely moving cats and were initially identified on-line by their slow and regular spontaneous activity (mean 2.55 ± 0.21 spikes/s). Discharge rates of NCS serotonergic neurons were highest during active waking (AW) (mean 2.94 ± 0.28 spikes/s), decreased during slow-wave sleep (middle of SWS: mean 1.38 ± 0.18 spikes/s), and were lowest during REM sleep (mean 0.46 ± 0.13 spikes/s). The activity of serotonergic NCS neurons did not significantly increase during transient elevations of the EMG during AW but did significantly decrease immediately preceding, and during the occurrence of, SWS spindles. These neurons were responsive to phasic auditory and visual stimuli, with most neurons showing excitatory responses. In response to a small dose of the serotoninspecific agonist 5-methoxy- N,N-dimethyltryptamine (50 μg/kg, i.m.), NCS serotonergic neurons responded with a mean decrease in unit activity of 43.9 ± 6.1%. Among the NCS serotonergic neurons a subpopulation differed from the remaining serotonergic neurons in that they showed a much smaller decrease in unit activity across the sleep-wake cycle and responded with an inhibition of activity to phasic auditory and visual stimuli. The results of this study are compared with those previously reported for serotonergic neurons in the dorsal raphe nucleus, nucleus raphe pallidus, and nucleus raphe magnus of freely moving cats.

Activity of serotonin-containing neurons in nucleus centralis superior of freely moving cats

Activity of serotonin-containing neurons in nucleus centralis superior of freely moving cats

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

Serotonergic neurons within nucleus raphe pallidus (NRP) of freely moving cats initially were distinguished by their slow (< 8 Hz), regular discharge and long duration (mean = 2.3 ms) action potentials. The activity of serotonergic NRP neurons was highest during active waking (mean = 4.85 ± 0.37 spikes/s) and gradually slowed, with little change in firing pattern, during the transition from waking through slow wave sleep (middle of SWS: mean = 3.76 ± 0.36 spikes/s). In REM sleep there was a precipitous decrease in firing rate (mean = 0.92 ± 0.23 spikes/s) and loss of discharge regularity. Although there was no significant difference in firing rate between active and quiet waking, discharge rates were significantly increased during transient elevations of the EMG, but these rate increases usually were associated with specific motor behaviors only. The activity of serotonergic NRP neurons during SWS was not related to the occurrence of either sleep spindles in the cortical EEG or PGO waves recorded from the lateral geniculate nucleus. These neurons also were relatively unresponsive to phasic auditory or visual stimuli, with most of the neurons examined showing weak excitatory responses. Activity of all serotonergic NRP neurons tested was suppressed (mean = −81.3 ± 4.3%) by the serotonergic agonist 5-methoxy-N,N-dimethyltryptamine (250 μg/kg, i.m.). The results of this study are compared with those previously reported for serotonergic neurons in the dorsal raphe nucleus of freely moving cats and the issue of homogeneity in central serotonergic systems is discussed.

Dreams and hallucinations: A common neurochemical mechanism mediating their phenomenological similarities

Dreams and drug-induced hallucinations have several phenomenological similarities, especially with respect to their visual and emotive components. This similarity is hypothesized to be due to a neurochemical mechanism which is common to both states: the inactivation of the brain serotonin system. This is supported by electrophysiological data indicating that the activity of serotonin-containing neurons is depressed during both dreaming (in REM and non-REM sleep) and in response to hallucinogenic drugs. Further support for the hypothesis derives from neuropharmacological data demonstrating that decreases in synaptic serotonin are associated with increased hallucinatory-like behavior or hallucinatory experience during waking, and increased duration of REM periods during sleep. Reciprocally, increases in synaptic serotonin are associated with decreased hallucinatory-like behavior or hallucinatory experience, and with decreased REM sleep time and dream reports. Neuroanatomical evidence that serotonin is heavily concentrated in brain areas which mediate visual perception and emotive experience is consonant with the strong visual and emotive components of dreams and hallucinations. When these data are considered in conjunction with the exclusively inhibitory synaptic action of serotonin in the forebrain, an explicit hypothesis can be formulated: A cessation, or decrease, in the discharge rate of serotonin-containing neurons, either spontaneously during REM and non-REM sleep, or in response to drugs such as LSD, precipitates, through disinhibition, a dramatic increase in activity of their target neurons in brain areas mediating visual sensation and emotional experience. These latter neural events are a primary physiological substrate for the emergence of strong sensory and emotive processes during dreams and drug-induced hallucinations.