Raphe unit activity in freely moving cats: Correlation with level of behavioral arousal

Dorsal raphe unit activity in freely moving cats showed a slow, rhythmic discharge rate during quiet waking (X=2.82 +/- 0.17 spikes/sec), and displayed a strong positive correlation with level of behavioral arousal. Presentation of an auditory stimulus during quiet waking resulted in significant increases in unit activity of 112% and 39% during the first sec and first 10 sec after the stimulus, respectively. This effect rapidly habituated with repeated stimulus presentations. During active waking, unit activity was significantly increased by 22% as compared to quiet waking, but there was no correlation between unit activity and gross body movements. Raphe unit activity showed a significant decrease of 17% during drowsiness (first appearance of EEG synchronization) as compared to quiet waking, and then progressive decreases during the early (--34%), middle (--52%) and late (--68%) phases of slow wave sleep. During all phases of slow wave sleep, the occurrence of sleep spindles was frequently associated with a transitory decrease in unit activity. The discharge rate would typically decrease during the few seconds immediately preceding the spindle, remains at this low level during the occurrence of the spindle, and then increase immediately after the spindle. Raphe unit activity showed decreases of 81% during pre-REM (the 60 sec immediately before REM onset) and 98% during REM, as compared to quiet waking. Unit activity reappeared 3.2 sec before the end of REM, with significant increases in unit activity of 83% and 17% during the first sec and first 10 sec of unit activity, respectively, as compared to quiet waking. The results of these studies are discussed in relation to the hypothesis that serotonin may play a modulatory, rather than mediative, role in behavioral and physiological processes.     

Comparison of the reflex responses to unexpected stimulation (startle reflexes) in freely moving and chloralose-anesthetized cats

Comparative study of startle reflexes recorded electromyographically in the same cat was performed under two different experimental conditions: in freely moving animal and after chloralose-anesthesia. A remarkable similarity in the component composition and temporal parameters (latency, duration) of the EMG responses in these two cases was found. The data obtained suggest the functional similarity of startle reflex discharges in freely-moving and anesthetized cats as well as the identity of their corresponding neural mechanisms.     

Effect of spontaneous exercise on reflex slowing of the heart in decerebrate cats

Exercise has been shown to reduce the ability of the baroreflex to slow the heart, and signals arising from cerebral cortex may cause this reduction. To test whether signals arising from the cerebral cortex are required to cause this inhibition, reflex slowing of the heart was assessed in decerebrate cats during rest and spontaneous walking. This reflex was quantified by the relation between systolic blood pressure and the subsequent heart beat interval or its inverse, beat to beat heart rate, during transient rises in pressure caused by injections of phenylephrine. Reflex slowing of the heart was reduced during spontaneous exercise compared to rest. Exercise may inhibit reflex cardiac slowing by activating beta-adrenoceptors that inhibit vagal effects on the heart. To test whether activation of beta-adrenoceptors caused the inhibition of reflex cardiac slowing produced by spontaneous walking in these decerebrate cats, the ability of the baroreflex to slow the heart during blockade of beta-receptors by propranolol was tested in 3 cats. Propranolol did not abolish the inhibitory effect of spontaneous walking on this reflex. These data indicate that the cerebral cortex and beta-adrenoceptors are not required for exercise to inhibit reflex cardiac slowing.     

Cholinergically-induced theta-like oscillations in the hippocampal formation of freely moving cats.

The present investigation was undertaken to assess the role of the cholinergics in the production of theta (theta) rhythm in the cat hippocampal formation. Intrahippocampal injections of carbachol, muscarine or eserine produced a well synchronized high frequency cholinergic theta activity (HFC) in a range of 5-12 Hz. Subsequent intrahippocampal injection of muscarinic antagonist, atropine sulphate, completely blocked this cholinergic-induced EEG pattern. The nicotinic antagonist, hexamethonium, was without any effect on the cholinergic-induced rhythmical waves. We suggest that HFC may result from the activation of the oscillatory mechanism intrinsic to the cat hippocampal formation.     

A critical role of the posterior hypothalamus in the mechanisms of wakefulness determined by microinjection of muscimol in freely moving cats

In order to determine critical sites within the hypothalamus responsible for the induction and maintenance of wakefulness (W), we performed microinjections of muscimol, a potent gamma-aminobutyric acid (GABA) agonist, in various lateral hypothalamic regions of freely moving cats. We found that bilateral injections of a small amount of muscimol (0.1-1.0 micrograms/0.5 microliters) in the preoptic and anterior hypothalamus and rostral mesencephalic tegmentum resulted in increased vigilance and insomnia. In contrast, microinjections of muscimol in the middle and anterior parts of the posterior hypothalamus induced long-lasting behavioral and electroencephalographic signs of sleep with short latency. The hypersomnia was characterized by a significant increase in both light and deep slow wave sleep (SWS), and a nearly complete suppression of paradoxical sleep (PS). Animals with muscimol microinjections in the ventrolateral part of the posterior hypothalamus, however, exhibited increased SWS followed by a significant increase in PS. When injected into the posterior hypothalamus of insomniac cats pretreated with p-chlorophenylalanine (PCPA), muscimol induced not only SWS but also PS with short latency. The present data thus support the hypotheses that the posterior hypothalamus plays a critical role in the mechanisms of W and that sleep might result from functional blockade of the hypothalamic waking center.     

Chemical and electrical stimulation of the caudate nucleus in freely moving cats: the role of dopamine

Behavioral effects of electrical stimulation of the medial part of the caput caudati were studied in freely moving cats in chronic experiments. These electrically evoked effects were also studied in animals locally pretreated with dexamphetamine, apomorphine, dopamine, 3-methoxytyramine, noradrenaline, serotonin, haloperidol,α-methyl-p-tyrosine, and procaine. Our data show that electrical stimulation of the medial part of the caput caudati results in behavioral responses similar to those evoked by dopamine application into the area, and that these responses are inhibited by blockade of the striatal dopamine system on the one hand, and facilitated by activation of the striatal dopamine system on the other hand. The importance of these data is discussed in view of the hypothesis that dopamine released from the ascending nigro-striatal pathways activates small inhibitory interneurons connecting the medial part with the anteroventral part of the caput caudati.     

Dopaminergic unit activity in freely moving cats: Lack of relationship to feeding, satiety, and glucose injections

The activity of dopaminergic neurons in the substantia nigra was recorded from freely moving cats during feeding and satiety, and following injections of glucose. At no time during feeding or in the postprandial satiety period was there a significant increase or decrease in firing rate of these neurons relative to baseline. Additionally, no change in firing rate was observed following injections of glucose (300, 500 and 1000 mg/kg) or glucose in combination with insulin (300 mg/kg glucose and 0.8 units/kg insulin).     

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.     

Caudate unit activity in freely moving cats: effects of phasic auditory and visual stimuli

Quantitative analyses of the single unit responses to sensory stimuli were made in caudate nucleus neurons in the unanesthetized freely moving cat. Over half of the cells were responsive to repetitive presentation of simple sensory stimuli. Typically, neurons gave similar responses to both auditory (click) and visual (flash) stimuli. This study confirms previous reports of the polysensory response characteristics of caudate nucleus neurons, and extends these observations to the freely moving animal.     

Raphe unit activity in freely moving cats: Effects of phasic auditory and visual stimuli

The effects of phasic auditory or visual stimuli upon the single unit activity of serotonergic neurons within the dorsal raphe nucleus (DRN) were studied in freely moving cats. The predominant response to auditory stimulation (86% of the cells) was excitation, with a mean latency of 40 ± 3 ms (S.E.M.) and a mean duration of 64 ± 4 ms. This was typically followed by a longer period (206 ± 32 ms) with unit activity below the baseline level. This did not appear to be a stimulus-induced inhibition of unit activity, however, since its duration closely corresponded to the normal interspike interval for that particular neuron. The response to repetitive auditory stimulation showed no evidence of habituation and was even present during sleep. A similar response, although generally of lesser magnitude, was evoked by a phasic visual stimulation in 64% of the cells tested. The mean latency for the response to visual stimulation was 53 ± 4 ms, the mean duration of excitation was 76 ± 7 ms, and the mean duration of the subsequent suppressed period was 239 ± 37 ms. The response to the visual stimulus also showed no evidence of habituation. These data indicate that serotonergic neurons of the DRN are driven, with similar temporal characteristics, by stimuli in two different sensory modalities. We hypothesize that these similar effects are attributable to a common excitatory input.     

Activity of nucleus raphe pallidus neurons across the sleep-waking cycle in freely moving cats

The activity of serotonin-containing nucleus raphe pallidus (RPA) units was recorded by means of movable 32 or 62 μm diameter insulated nichrome wires in freely moving cats. RPA units displayed a slow, rhythmic discharge rate during waking (X¯= 5.3spikes/s) and showed no significant change in activity during slow-wave sleep. However, these neurons showed a large decrease in activity during REM sleep (X¯= 1.2spikes/s). In contrast with results from studies on serotonin-containing nucleus raphe dorsalis units, RPA neurons showed no relationship to the occurrence of sleep spindles, and were not inhibited by low doses of LSD.     

Cortical evoked potentials and behavioral reactivity to photic stimuli in freely moving rats

Averaged evoked potentials at the visual cortex (VER) were studied in relation to behavioral reactivity to photic stimuli during operant behavior for food reinforcement. The VERs showed highly systematic increases in late components with repetition of the sensory stimuli. Amplitude of late components related inversely to behavioral indices of reactivity, but the correlations between these measures were low to moderate. Control experiments involving other freely moving and curarized animals served to evaluate various behavioral and stimulus factors in the VER changes. The results were interpreted in terms of habituation of ‘arousal reactions’, for which additional supporting data were provided. Electrographic arousal, however, was viewed as a varying substrate of all behavior, which complicates relationships between VERs and any single index of behavioral reactivity to stimuli in freely moving subjects.     

Waking selective neurons in the posterior hypothalamus and their response to histamine H3-receptor ligands: an electrophysiological study in freely moving cats

Neurons which discharge selectively during waking (waking selective) have been found in the tuberomamillary nucleus (TM) and adjacent areas of the posterior hypothalamus. Although they share some electrophysiological properties with aminergic neurons, there is no direct evidence that they are histaminergic. We have recorded from posterior hypothalamic neurons during the sleep-wake cycle in freely moving cats, and investigated the effects on waking selective neurons of specific ligands of histaminergic H3-receptors, which autoregulate the activity of histaminergic neurons. Two types of neurons were seen. Waking selective neurons, termed "waking-on (W-on)," were located exclusively within the TM and adjacent areas, and discharged at a low regular rate during waking (1.71-2.97 Hz), decreased firing during light slow wave sleep (SWS), became silent during deep SWS and paradoxical sleep (PS) and resumed their activity on, or a few seconds before, awakening. "Waking-related" neurons, located in an area dorsal to the TM, displayed a similar, although less regular, low rate of firing (1.74-5.41 Hz) and a similar discharge profile during the sleep-wake cycle; however, unlike "W-on" neurons, they did not completely stop firing during deep SWS and PS. Intramuscular (i.m.) injection of ciproxifan (an H3-receptor antagonist, 1mg/kg), significantly increased the discharge rate of W-on neurons and induced c-fos expression in histamine-immunoreactive neurons, whereas i.m. injection of imetit (an H3-receptor agonist, 1mg/kg) or microinjection of alpha-methylhistamine (another H3-receptor agonist, 0.025-0.1 microg/0.2 microl) in the vicinity of these cells significantly decreased their discharge rate. Moreover, the effect of the antagonist was reversed by the agonists and vice versa. In contrast, "waking-related" neurons were unaffected by these H3-receptor ligands. These data provide evidence for the histaminergic nature of "W-on" neurons and their role in cortical desynchronization during waking, and highlight the heterogeneity of posterior hypothalamic neuronal populations, which might serve different functions during the wakefulness.     

Serotonin neuron transplants: electrophysiological unit activity of intrahippocampal raphe grafts in freely moving cats

Fetal serotonin (5HT)-containing neurons from the cat midbrain raphe were transplanted to the hippocampus of adult host cat brain. These neurons displayed spontaneous electrophysiological activity in freely moving cats, and showed the pacemaker-like discharge pattern characteristic of these cells. However, these neurons showed no significant change in activity across the sleep-waking cycle, nor were they responsive to sensory stimulation in the auditory and visual modalities. Grafted neurons were inhibited by 5HT agonists, but required higher doses than those in intact cats. These data are the first electrophysiological recordings of grafted 5HT neurons in freely moving animals, and question whether such tissue grafts can restore functional activity in the central 5HT system.     

Dopamine-containing ventral tegmental area neurons in freely moving cats: activity during the sleep-waking cycle and effects of stress

The activity of dopamine-containing ventral tegmental area (VTA) units was recorded by means of movable 32- and 64-microns-diameter insulated Nichrome wires in freely moving cats. The VTA units displayed a slow, somewhat irregular activity during quiet waking (mean 3.63 +/- 0.41 spikes/s) and showed no significant change in activity during slow-wave sleep or REM sleep. Although VTA unit activity was somewhat higher and more erratic during active waking, there was no relationship between unit discharge and phasic movement. These neurons were inhibited (-87%) by small doses of apomorphine (1.0 mg/kg, i.p.) and excited (+43%) by small doses of haloperidol (0.5 mg/kg, i.p.). The stress of a conditioned emotional reaction (CER) paradigm resulted in a significant increase in the discharge rate of VTA neurons (+39%), compared with the quiet-waking baseline. The CER paradigm increased plasma glucocorticoids by 74%. Neurochemical studies revealed that the CER paradigm resulted in a significant decrease of dopamine in the limbic forebrain (-31%), whereas both homovanillic acid (+47%) and dihydroxyphenylacetic acid (+43%) concentrations were increased. No significant changes in dopamine metabolism were observed in the striatum under the CER situation. These data have implications in relation to the role of stress and dopamine in mediating certain psychiatric disorders.