Behavioral correlates of the activity of serotonergic and non-serotonergic neurons in caudal raphe nuclei

Abstract

We investigated the behavioral correlates of the activity of serotonergic and non-serotonergic neurons in the nucleus raphe pallidus (NRP) and nucleus raphe obscurus (NRO) of unanesthetized and unrestrained cats. The animals were implanted with electrodes for recording single unit activity, parietal oscillographic activity, and splenius, digastric and masseter electromyographic activities. They were tested along the waking-sleep cycle, during sensory stimulation and during drinking behavior. The discharge of the serotonergic neurons decreased progressively from quiet waking to slow wave sleep and to fast wave sleep. Ten different patterns of relative discharge across the three states were observed for the non-serotonergic neurons. Several non-serotonergic neurons showed cyclic discharge fluctuations related to respiration during one, two or all three states. While serotonergic neurons were usually unresponsive to the sensory stimuli used, many non-serotonergic neurons responded to these stimuli. Several non-serotonergic neurons showed a phasic relationship with splenius muscle activity during auditory stimulation. One serotonergic neuron showed a tonic relationship with digastric muscle activity during drinking behavior. A few non-serotonergic neurons exhibited a tonic relationship with digastric and/or masseter muscle activity during this behavior. Many non-serotonergic neurons exhibited a phasic relationship with these muscle activities, also during this behavior. These results suggest that the serotonergic neurons in the NRP and NRO constitute a relatively homogeneous population from a functional point of view, while the non-serotonergic neurons form groups with considerable functional specificity. The data support the idea that the NRP and NRO are implicated in the control of somatic motor output.

Genioglossus muscle activity and serotonergic modulation of hypoglossal motor output in obese Zucker rats

“…Periods of quiet wakefulness were characterized by the lack of overt body movements, identified from the EMG signals and video recordings, in contrast to active wakefulness that included overt behaviors such as grooming and exploring. Such periods of wakefulness were differentiated because 5-HT caudal raphe neurons are thought to be activated more during motor tasks associated with such behaviors (18,39), and hence may be more responsive to the effects of 5-HT receptor antagonism at those times, whereas the 5-HT raphe inputs may have a lesser role in modulating the respiratory component of GG activity during basal breathing (44). Only those epochs comprising at least 30 s of uninterrupted sleep without arousal, or wakefulness without drowsiness, were included in the analysis.…”

Central control of cardiovascular function during sleep

“…3F) in the midline medulla evoked a differentiated sympathetic response very similar to that observed in REMS (i.e., increased muscle SNA and decreased renal SNA). The region from which this response was evoked contains nonserotonergic neurons that are active in REMS (70,100). Nonserotonergic neurons in the NRO include both putative sympathoinhibitory and putative sympathoexcitatory neurons (57,60).…”

Role of Endogenous Serotonin in Modulating Genioglossus Muscle Activity in Awake and Sleeping Rats

“…For example, dorsal raphe activity in cats is strongly modulated by changes in sleep-wake state, with large alterations in discharge frequency paralleling the changes in behavioral state (58), whereas caudal raphe neurons are only weakly modulated from wakefulness to non-REM sleep, although there remains a large suppression of activity in REM sleep (59). Serotonergic caudal raphe neurons are also believed to be more activated during motor tasks associated with rhythmic behaviors, such as chewing and licking (60,61), and may have a lesser role in modulating the respiratory component of GG activity during basal breathing (21). Nevertheless, although the results of the present study show only a weak serotonergic modulation of the HMN during unstimulated breathing in this intact, freely behaving rodent preparation, this does not contradict the concept that raphe neurons per se can significantly influence GG activity in a sleep-and wakefulness-dependent manner, as nonserotonergic raphe neurons also project to the HMN (62)(63)(64).…”