Michael E. Trulson

Activity of substantia nigra units across the sleep-waking cycle in freely moving cats

The activity of dopamine-containing substantia nigra units was recorded by means of movable 32 or 62 micrometers diameter insulated nichrome wires in freely moving cats. Nigral units displayed a slow, somewhat irregular activity during quiet waking (mean = 4.6 spikes per sec), and showed no significant change in activity during sleep. While nigral unit activity was somewhat higher during active waking, there was no relationship between unit discharge and phasic movement.

Raphe unit activity in freely moving cats: Effects of benzodiazepines

Benzodiazepines (chlordiazepoxide and diazepam) produced a dose-dependent decrease in the discharge rate of serotonin-containing neurons in the dorsal raphe nucleus of freely moving cats. This ranged from no significant change at doses of 0.5 and 1.0 mg/kg (i.p.), to greater than 90% reductions in unit activity at 10 mg/kg. The effects of benzodiazepines on raphe units occurred within 15-30 min of injection and the duration of action was dose-dependent and lasted from 1 to more than 6 hr. Doses of benzodiazepines that significantly decreased raphe unit activity (i.e. 2.5-10 mg/kg) also produced ataxia and decreased EMG activity. These data suggest that benzodiazepine-induced suppression of raphe unit activity is closely related to general motor behavior. Raphe unit activity remained suppressed during phasic increases in EMG activity during eating, grooming, or predatory behavior, suggesting that benzodiazepines also have a direct inhibitory action on raphe cells. The present results are discussed in the context of the serotonergic hypothesis of anxiety.

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 microns diameter insulated nichrome wires in freely moving cats. RPA units displayed a slow, rhythmic discharge rate during waking (mean = 5.3 spikes/s) and showed no significant change in activity during slow-wave sleep. However, these neurons showed a large decrease in activity during REM sleep (mean = 1.2 spikes/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.

Chloral hydrate anesthesia alters the responsiveness of dorsal raphe neurons to psychoactive drugs.

The effects of several psychoactive drugs on raphe unit activity in freely moving cats was compared with drug-induced effects in chloral hydrate anesthetized cats. The anesthesia greatly potentiated the depressant effects of LSD, phenoxybenzamine, clonidine, methiothepin, clozapine, and chlorimipramine on raphe units, but partially antagonized the depressant effects of diazepam. These results demonstrate that apparently discrepant reports of the affects of these drugs on raphe neurons in anesthetized rats versus freely moving cats are attributable to the use of anesthesia in rat studies. These data underscore the importance of conducting such drug studies in awake, freely moving animals, for which the results would be far more relevant to the issue of human drug use.

Behavioral and neurochemical effects of apomorphine in the cat

Administration of apomorphine (2-10 mg/kg i.p.) elicited a number of behaviors, such as limb flicking, abortive grooming, investigatory and hallucinatory-like responses, head and body shakes, and excessive grooming, which we have previously proposed as an animal model for studying the actions of LSD and related hallucinogens. Repeated administration of apomorphine resulted in a significant tolerance, which occurred within 2 h of the initial injection, and completely dissipated within 24 h. A pronounced LSD-apomorphine cross tolerance was observed; however, there was no significant apomorphine-LSD tolerance. Apomorphine-induced behavioral changes were blocked by prior treatment with haloperidol, but were unchanged by pretreatment with L-DOP[A. Administration of L-DOPA, in combination with a peripheral decarboxylase inhibitor, did not elicit these characteristic behavioral changes. Increasing synaptic serotonin levels by monoamine oxidase inhibition, precursor administration, or reuptake blockade in general did not alter the behavioral response to apomorphine. Similarly, pretreatment with serotonin receptor blockers produced no large changes in apomorphine-induced behaviors. Prior serotonin depletion with chronic p-chlorophenylalanine administration, however, potentiated certain apomorphine-induced behaviors. Neurochemical studies revealed that apomorphine administration increased striatal dopamine, and decreased dopamine metabolites. Norepinephrine levels were generally decreased throughout the CNS by apomorphine treatment. Administration of apomorphine increased CNS serotonin and 5-hydroxyindoleacetic acid levels, while tryptophan levels were unchanged. The biological bases of the limb flick model is discussed in the context of these pharmacological and neurochemical studies.

In vitro recording of raphe unit activity: Evidence for endogenous rhythms in presumed serotonergic neurons

The spontaneous activity of single neurons in the nucleus raphe dorsalis was recorded in vitro in mouse brain slices. The neurons displayed the slow and regular discharge pattern characteristic of raphe neurons recorded in vivo. When magnesium ion was added to increase the medium concentration to 20-30 mM for the purpose of inhibiting all synaptic transmission, raphe neurons continued to display the same discharge pattern and rate. The data suggest that the steady rhythmic firing of nucleus raphe dorsalis neurons is generated by an intracellular pacemaker mechanism.

Differential effects of phasic auditory and visual stimuli on serotonergic neurons in the nucleus raphe dorsalis and nucleus raphe pallidus in freely moving cats.

The effects of phasic auditory and visual stimuli upon single unit activity of serotonergic neurons within the nucleus raphe dorsalis (RD) and nucleus raphe pallidus (RPA) were studied in freely moving cats. RD units were driven by both auditory and visual stimuli, while RPA neurons were totally unresponsive to auditory and visual stimulation. These data support the hypothesis that ascending and descending serotonergic neurons subserve different functions.

Behavioral effects of quipazine in the cat

Administration of quipazine to cats elicits a number of behaviors, such as limb flicking abortive grooming, investigatory behavior and hallucinatory-like behavior, which we have previously proposed as an animal behavioral model for studying the actions of LSD and related hallucinogens. While recent studies have indicated that these model behaviors may not be totally specific for hallucinogenic drugs, the model can still be useful for studying drug action. Quipazine (0.5-5.0 mg/kg i.p.) produced significant increases in limb flicking, abortive grooming, investigatory behavior, hallucinatory-like behavior grooming, head and body shakes, staring and yawning. These behavioral changes persisted for 1-6 h, depending on the dose of quipazine employed. Administration of quipazine (5.0 mg/kg per day) for 5 consecutive days produced no significant tolerance effect on any of these model behaviors. These quipazine induced behavioral changes were potentiated by pretreatment with apomorphine, and partially blocked by pretreatment with haloperidol. Quipazine-induced behavioral changes were potentiated by prior serotonin depletion with p-chlorophenylalanine, and completely blocked by pretreatment with a monoamine oxidase inhibitor or the serotonin precursor, L-5-hydroxytryptophan. These quipazine-induced behavioral changes were also blocked by pretreatment with the serotonin receptor blockers, cinnanserin, methysergide or cyproheptadine. The mechanism of action of quipazine, as well as the neuropharmacology of the limb flick model, is discussed in the content of these studies with serotonergic and dopaminergic drugs.

Effects of chronic methamphetamine administration on tryptophan hydroxylase activity, [3H]serotonin synaptosomal uptake, and serotonin metabolism in rat brain following systemic tryptophan loading

Chronic administration of methamphetamine (20 mg/kg, i.p., every 12 hr for 6 days) produced significant decreases in the Vmax of brainstem (-32.8%) and forebrain (-31.5%) tryptophan hydroxylase when measured 12 hr after the final injection. Serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), and [3H]5-HT synaptosomal uptake were decreased by a similar magnitude following chronic drug treatment. Administration of fluoxetine prior to each methamphetamine injection prevented these neurochemical changes. Neither acute nor chronic methamphetamine treatment produced any significant changes in the Km of tryptophan hydroxylase for either substrate or cofactor. Systemic tryptophan loading (50 mg/kg, i.p.) one hour prior to sacrifice in chronic methamphetamine treated rats restored brain 5-HT and 5-HIAA levels to normal. These data suggest that chronic methamphetamine treatment decreases central serotonergic neurotransmission by reducing the activity of the rate-limiting enzyme in 5-HT biosynthesis, possibly by disrupting serotonergic nerve terminals.

Reduction in brain serotonin synthesis rate following chronic methamphetamine administration in rats

Chronic administration of methamphetamine (20 mg/kg i.p., every 12 h for 6 days) produced significant decreased in brain 5-hydroxytryptophan accumulation following decarboxylase inhibition and 5-hydroxyindoleacetic acid accumulation following probenecid treatment in rats. Administration of fluoxetine prior to each methamphetamine injection prevented these neurochemical changes. Acute methamphetamine treatment produced no changes in these neurochemical measures. These data demonstrate that chronic, but not acute, methamphetamine treatment reduces brain serotonin synthesis rate.