Central serotonin involvement in the elaboration of the startle reaction in rats

Acoustic startle reduction in cocaine dependence persists for 1 year of abstinence

RATIONALE

Chronic cocaine use results in long-lasting neurochemical changes that persist beyond the acute withdrawal period. Previous work from our group reported a profound reduction in the acoustic startle response (ASR) in chronic cocaine-dependent subjects in early abstinence compared to healthy controls that may be related to long-lasting neuroadaptations following withdrawal from chronic cocaine use.

OBJECTIVES

This study aims to investigate the persistence and time course of the decrements in the ASR of cocaine-dependent subjects during prolonged abstinence.

METHODS

Seventy-six cocaine-dependent (COC) subjects and 30 controls (CONT) were tested, the former after a period of heavy cocaine dependence. COC subjects were retested sequentially for 1 year of abstinence or until relapse. ASR testing was conducted at 3-dB levels and the eye-blink component of the startle response was quantified with electromyographic recording of the orbicularis oculi muscle.

RESULTS

While there was no difference in startle magnitude between CONT and COC in early abstinence, by day 40 of abstinence COC subjects exhibited a statistically significant decline (p = 0.0057) in ASR magnitude as compared with CONT and this decrement persisted for up to 1 year of abstinence (p = 0.0165). In addition, startle latency was slower in COC subjects as compared with CONT at all stages of abstinence.

CONCLUSIONS

These results replicate and expand upon the earlier finding that chronic cocaine use impairs the ASR in a manner that persists beyond the acute withdrawal period. This phenomenon may represent a biological measure of long-term neural changes accompanying cocaine dependence and subsequent withdrawal.

5-Hydroxytryptamine-interacting drugs in animal models of anxiety disorders: more than 30 years of research

An overview of the behavioral data arising from the vast literature concerning the involvement of 5-hydroxytryptamine (5-HT) neurotransmission in the regulation of anxiety is presented. More than 1300 experiments were carried out in this area and they provide evidence that: (1) results obtained in ethologically based animal models of anxiety with drugs stimulating 5-HT transmission are most consistent with the classic 5-HT hypothesis of anxiety in that they show an increase in animals' emotional reactivity; (2) no category of anti-anxiety models are selectively sensitive to the anxiolytic-like effects of drugs targetting 5-HT1A, 5-HT2A or 5-HT2C receptor subtypes; (3) anxiolytic-like effects of 5-HT3 receptor antagonists, in the great part, are revealed by models based on spontaneous behaviors. Taken together, these observations lead to the conclusion that different 5-HT mechanisms, mediated by different receptor subtypes, are involved in the genesis of anxiety.

Facilitation of the acoustic startle reflex by ponto-geniculo-occipital waves: effects of PCPA

The relationship between ponto-geniculo-occipital (PGO) waves and motor activity during waking and non-rapid eye movement (non-REM) sleep stages was studied in cats treated with the serotonin synthesis inhibitor p-chlorophenylalanine (PCPA). PGO waves appeared in waking after daily treatment with PCPA. The magnitude of the acoustic startle elicited in the absence of prior PGO waves was increased (by a mean of 555%) by the PCPA treatment as compared to that of the pre-drug level. When startle-eliciting stimuli were presented shortly after the occurrence of the PGO wave, the response amplitude was further enhanced as compared to that of the baseline startle. The effect was maximal 50 ms following the peak of the PGO wave (average 192% of the baseline level), with return to the baseline startle level within 200 ms. A similar effect could also be seen with waking eye-movement potentials (EMPs) in drug-naive animals. Over half of the spontaneous PGO waves were found to be preceded or followed by discrete head-body movements. After PCPA, the amplitude of auditory-evoked LGN PGO waves increased during quiet waking (QW) while those in non-REM and REM sleep states did not change. It was concluded that serotonergic systems produce a tonic suppression of startle response and PGO amplitude in waking. PGO spikes in waking are associated with a phasic facilitation of the sensorimotor mechanisms involved in startle.

The role of 5HT1A receptors in the modulation of the acoustic startle reflex in rats

The modulatory role of serotonin (5-HT) on the acoustic startle reflex was studied using 5-HT receptor agonists and antagonists. 8-Hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) (1,2 and 4 mg/kg, SC) and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) (1,2 and 4 mg/kg, IP), putative 5-HT1a receptor agonists, increased the magnitude of the startle reflex, while quipazine (5, 10 and 20 mg/kg, SC), an agonist with mixed 5-HT2 and 5-HT1b receptor activity, decreased startle responsiveness. Pretreatment of rats with ketanserin (1, 2 and 4 mg/kg, SC), a 5-HT2 receptor antagonist, had no significant effect on the activity of 8-OHDPAT, 5-MeODMT, or quipazine. Metergoline (0.25, 0.5, 1 and 2 mg/kg, SC), a mixed 5-HT1/5-HT2 receptor antagonist attenuated the augmentation of the reflex by 8-OHDPAT and 5-MeODMT and the suppression produced by quipazine. At the doses used, metergoline produced a non-dose-dependent increase in startle, while ketanserin had no effect. None of the agents specifically affected the ability of a prepulse stimulus to inhibit the acoustic startle response. These data suggest that 5-HT1a and 5-HT1b receptors play opposite roles in the modulation of the acoustic startle response and that 5-HT plays little, if any, role in the prepulse inhibition of the acoustic startle response.

Effects of 8-OH-DPAT, lisuride and some ergot-related compounds on the acoustic startle response in the rat

Five ergot-related compounds were examined for their effects on the acoustic startle response in the rat. The startle amplitude and the startle latency were registered. 8-Hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT; 0.5-8 mg/kg) and lisuride (0.05-0.8 mg/kg) were found to enhance the startle amplitude, while the mainly DA receptor active ergot derivatives pergolide (0.2-0.8 mg/kg), bromocriptine (5-20 mg/kg) and LY 141865 (5-20 mg/kg) had no, or even the reverse, effect. All five compounds caused a prolongation of the startle latency. The increased startle amplitude caused by 8-OH-DPAT (2 mg/kg) and lisuride (0.2 mg/kg) was successfully antagonized by the 5-HT receptor antagonist methiothepin (0.1 mg/kg) but not by metergoline (1 mg/kg). 5-Hydroxy-L-tryptophan (L-5-HTP; 12.5-50 mg/kg), administered to pargyline- and benserazide-pretreated animals, was included for comparison. The serotonin precursor caused a marked increase in the startle amplitude and a shortening of the startle latency.

Discrimination of auditory from nonauditory toxicity by reflex modulation audiometry: effects of triethyltin

The evaluation of toxicity in sensory systems presents particular problems because of the need to distinguish specific sensory loss from other toxic effects. It has recently been shown that modulation of reflex behavior by low-intensity test stimuli can be used to provide a rapid assessment of sensory acuity and of ototoxic hearing loss. In this report the ability of the method to distinguish the known neuromuscular consequences of triethyltin (TET) from hearing loss is demonstrated. Rats treated with TET bromide (30 mg/liter) in their drinking water for 3 weeks showed a profound decrease in acoustic startle reflex amplitudes which was apparent prior to overt neuromuscular weakness and subsequent hindlimb paralysis. Following cessation of treatment, startle levels recovered to preexposure values. Despite these large changes in startle reflex amplitudes, TET treatment did not alter the ability of pure tone stimuli to modulate reflex behavior. These results indicate that TET as administered in these studies disrupted neuromuscular function but did not affect hearing.

The effects of footshock and handling on tyrosine hydroxylase activity in synaptosomes and solubilised preparations from rat brain

The effects of mild noxious stimulation on central noradrenergic activity were investigated in rats, using low-intensity electrical footshock as the noxious stimulus. Synaptosomes were prepared from brain regions of control and experimental rats and the rate of tyrosine hydroxylation was assayed: this is a measure of transmitter synthesis in the isolated noradrenergic terminals. Single footshocks were found to elevate the synthesis rate in the hippocampus, the extent of the elevation depending on the baseline rate. This elevation was not significant in hypothalamus or cerebellum. Repeated handling over a period of 14 days was also found to elevate synthesis rate in hippocampus; but rats subjected to daily footshock for a week, in addition to handling, had synthesis rates similar to those of controls. Maximal tyrosine hydroxylase activity was measured by solubilizing the enzyme and assaying at saturating concentrations of cofactor and substrate: this is a measure of the amount of enzyme in the nerve terminals. No significant changes in maximal tyrosine hydroxylase activity were found in the brains of handled or shocked animals. These results suggest that the assay of tyrosine hydroxylase activity in a suspension of synaptosomes provides a sensitive indicator of changes in noradrenergic activity related to mild noxious stimulation. The biochemical responses are more pronounced in the hippocampus than in other noradrenergic terminal areas; this suggests that the synthesis rate of noradrenaline in terminals of central noradrenergic neurones could be regulated by local mechanisms at the terminal as well as by impulses transmitted from the cell body.

Enhancement by the putative 5-HT receptor agonist 8-OH-2-(di-n-propylamino)tetralin of the acoustic startle response in the rat

Two 2-(di-n-propylamino)tetralin (DPAT) compounds, 8-OH-DPAT and 5-OH-DPAT, with reported effects on central 5-HT and DA receptors respectively, were tested for their effects on the acoustic startle response in rats. 8-OH-DPAT was given in doses of 0.25-2.0 mg/kg IP and 5-OH-DPAT in doses of 1.0-8.0 mg/kg IP. Both compounds increased the startle response significantly in a dose-dependent manner, but 8-OH-DPAT appeared to be about 30 times as potent and to have a higher efficacy than 5-OH-DPAT. In addition, the effects on the startle response of L-5-HTP, 25-100 mg/kg IP, and L-dopa, 25-100 mg/kg IP, administration to animals pretreated with the inhibitor of aromatic L-amino acid decarboxylase, benserazide (25 mg/kg IP) were included for comparison. A small, but significant increase in the startle amplitude was found after the highest dose of L-5-HTP, whereas no effects were observed after L-dopa administration.

Potentiation of d-amphetamine and L-dopa-induced acoustic startle activity after long-term exposure to amphetamine

The startle response to an auditory stimulus was potentiated by treatment with d-amphetamine sulfate. Administration of L-dopa after pretreatment with the extracerebral decarboxylase inhibitor MK-486 also increased startle activity. After long-term exposure to amphetamine the startle response to L-dopa and d-amphetamine was enhanced. These findings are consistent with the consequences of long-term amphetamine administration on other amphetamine-induced behaviors (e.g. stereotype), and are discussed in terms of the effects of long-term amphetamine treatment on pre- and postsynaptic dopamine receptors and serotonin.

In utero alcohol heightens juvenile reactivity

The behavioral teratogenicity of ethanol was studied in a laboratory model of the fetal alcohol syndrome. Pregnant rats were placed in one of three groups: Ethanol (4 g ethanol/kg intubated twice daily; Purina Chow ad lib.); Sucrose (7 g sucrose/kg intubated instead of ethanol; Untreated (no intubations; Purina Chow ad lib.). Ethanol offspring did not differ from either control group in neonatal body weight or developmental measures. On Day 35, 2 female offspring per litter were tested for reactivity to acoustic startle stimuli. Activity was measured during the pre-stimulus foreperiod and during inter-stimulus intervals. Ethanol pups displayed heightened startle reactivity in the absence of hyperactivity or disrupted habituation. These data indicate that ethanol in utero produces hyperreactivity in the absence of morphological, body weight or developmental abberations.

Excitatory and inhibitory effects of serotonin on sensorimotor reactivity measured with acoustic startle

Serotonin infused into the lateral ventricle in rats produced a dose-dependent depression of the acoustic startle reflex. When infused onto the spinal cord, serotonin produced a dose-dependent increase in startle. Thus the same neurotransmitter can modulate the same behavior in opposite ways, depending on which part of the central nervous system is involved.

Neurochemical modulation of sensory-motor reactivity: acoustic and tactile startle reflexes

The present review argues that the startle reflex is particularly well suited as a model system to analyze how drugs alter stimulus reactivity and reflex excitability. It then reviews all the literature to date on how drugs or lesions that are thought to alter neurochemical transmitter systems affect acoustic and/or tactile startle. Hypotheses are presented to account for how serotonin, dopamine, norepinephrine, acetylcholine, and opiates modulate startle. Effects on startle plasticity such as habituation, sensitization, and potentiation resulting from prior associative learning are also included.

Increased sensitivity to intracerebroventricular infusion of serotonin and deaminated indoles after lesioning rat with dihydroxytryptamine

Rats were prepared with a chronic intracerebroventricular cannula, and treated with intracisternal 5,7-dihydroxytryptamine (DHT) after i.p. desmethylimipramine or control vehicle. After recovery, they were tested behaviorally by direct observation and electronic monitoring of motor activity. Intraventricular infusion of a placebo or 5-hydroxyindoleacetic acid (5-HIAA) had little effect, but serotonin (5-HT) decreased, and norepinephrine increased locomotor activity in intact rats. Following pretreatment with 5,7-DHT, a small increase in locomotor activity was noted which was not altered by intracranial infusion of vehicle. In contrast, infusions of 5-HT produced a striking dose-dependent (ED50 = 5 micrograms/min) pattern of hyperactivity, 'myoclonic' jerking movements, postural changes, and autonomic responses. Norepinephrine increased locomotor activity in the DHT-lesioned rats (but not significantly more than in controls), but failed to produce the myoclonic syndrome. The deaminated indoles, indoleacetaldehyde and 5-HIAA were more potent than 5-HT in producing the myoclonic response; tryptamine when infused at an equimolar dose had no effect. The putative serotonin antagonists, cyproheptadine and methiothepin (i.p.), were more effective in blocking responses to infused 5-HT than to equipotent doses of deaminated indoles. These behavioral responses may represent exaggerated ex"itatory effects mediated by serotonin in the brain stem and spinal cord, possibly modified by altered forebrain mechanisms. A neurophysiologic or neuropharmacologic role for deminated indoles should be reconsidered as they may not merely be inactive metabolites.

5-Methoxy-N,N-dimethyltryptamine: spinal cord and brainstem mediation of excitatory effects on acoustic startle

The effects of different doses (0.03, 0.06, 0.12, 0.25, 1.0, 2.0, 4.0, and 8.0 mg/kg body weight) of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) were tested on the acoustic startle reflex in rats. Beginning at 0.12 mg/kg, 5-MeODMT increased startle monotonically up to the highest dose used. 5-MeODMT still increased startle in acutely decerebrate rats or when infused directly onto the spinal cord. The excitatory effects of a high systemic dose of 5-MeODMT were completely blocked by cinanserin, cyproheptadine, and propranolol, but not by parachlorophenylalanine, alpha-methyl-p-tyrosine, haloperidol, sotalol, or phenoxybenzamine. The results were discussed in terms of a new theory, which suggests that stimulation of serotonin receptors in the spinal cord enhance startle whereas serotonin receptors in the forebrain inhibit startle.

Ascending 5-HT pathways and behavioural habituation

Microinjections of 5,7 -dihydroxytryptamine into both the dorsal and median raphe nuclei resulted in 70-85% depletions in striatal and hippocampal 5-HT concentrations but did not affect habituation of orienting in the lick-distraction test, habituation of activity in an open-field or habituation of exploration in the holeboard test. Lesioned animals were hypoactive in the latter two tests and defaecated more than control rats in the open-field suggesting an increase in emotionality or fear. Rats with selective 5,7 -dihydroxtryptamine lesions of either the dorsal or the median raphe nucleus also showed no impairment of habituation of orienting or exploration. However, median raphe lesioned animals were hyperactive at some stages of the holeboard test. In contrast to previous studies, the results suggest intact ascending 5-HT pathways are not necessary for behavioural habituation of orienting, activity or exploration. Rather, 5-HT neurones may be involved in modulation of activity or responsiveness to aversive environments.

Development of the acoustic startle response in the rat: ontogenetic changes in the magnitude of inhibition by prepulse stimulation

Three experiments examined the development of the acoustic startle reflex and its modification by a preliminary stimulus in the infant rat during the 2nd and 3rd postnatal weeks. The 1st experiment employed a white noise S1 (20 msec, 70 dB), the 2nd a cutaneous S1 (.5 msec, .5 mA and 1.0 mA shock), and the 3rd identical S1-S2 pairs (20 msec, 10 kHz, 110 dB tones). The results demonstrate a similar maturation of the prepulse modification pattern over days in the 3 experiments, evidenced mainly in the growth of inhibition. The findings indicate peripheral and central mechanisms that are maturing during the period of life under observation and that contribute to the developmental patterns of modification.

Involvement of norepinephrine in startle arousal after acute and chronic d-amphetamine administration

Treatment with d-amphetamine produced a dose-dependent increase in startle amplitude in response to a buzzer. This increase appeared to be a reflection of a sensitization effect, i.e., enhanced responsivity as a function of repeated stimulus presentations. Treatment with alpha-methyl-p-tyrosine, which reduced whole brain concentrations of dopamine (DA) and norepinephrine (NE), or treatment with FLA-63, which reduced only NE, antagonized the effects of d-amphetamine on the startle reflex, suggesting a role of NE in this behavior. Startle amplitude was also reduced following chronic d-amphetamine treatment. The effect of d-amphetamine on startle was found to be independent of changes in drug-induced locomotor excitation. The data of the present investigation, together with earlier reports, suggests that tolerance occurs to those behaviors that involve a noradrenergic component.

Brain 5-hydroxytryptamine correlates of behavior: studies involving spontaneously hypertensive (SHR) and normotensive Wistar rats

Brain area 5-hydroxytrhptamine (5-HT) turnover and behavioral correlates were compared in spontaneously hypertensive rats (SHR) and normotensive Wistar rats. SHR appeared to have hyperfunctional 5-HT systems as evidenced by the finding that brain area 5-HT turnover times were 53% lower in SHR, with significant changes seen in the limbic forebrain. SHR, while less active upon initial testing of spontaneous motor activity than Wistar rats, did not habituate as readily to the testing procedures as evidenced by higher activity than Wistar rats in repeated testing stiuations. In the light of other data indicating that 5-ht systems are predominantly inhibitory to ongoing behavior, these data suggest that habituation, as determined in this study, is an active process that is retarded in the SHR because of hyperfunctional 5-HT systems. Spontaneous activity of SHR was inhibited less than that of Wistar rats by a conditioned stimulus which is consistent with the observations of others that 5-HT systems tend to suppress responsiveness to external stimuli. While spontaneous activity data suggested that SHR were more emothinal than Wistar rats, the SHR also exhibited a higher avoidance rate in an unsignaled shuttle avoidance procedure which is usually indicative of a lower emotional state.

The startle response in rats: effect of ethanol

The effects of acute and chronic ethanol intake on the startle response was examined in male rats. Ethanol given IP produced a dose-dependent decrease in the amplitude of the startle response measured 30 min later. With a dose of 1 g/kg, the effect was evident at 15 min and had recovered substantially by 60 min. The effect of ethanol on the startle response was potentiated by pretreatment of the animals with pimozide, haloperidol, and p-chlorophenylalanine but not by propranolol, phenoxybenzamine, alpha-methyltyrosine, or pargyline. After 3 weeks on an ethanol-containing diet, the startle response was greater than that shown by rats on the control iso-caloric, sucrose-containing diet. After ethanol withdrawal, the startle response was further increased, with a peak about 9 to 12 hr after discontinuation of ethanol; thereafter, the response declined. This time course of heightened startle response during ethanol withdrawal corresponds to the time course of the activation of noradrenergic neurons during withdrawal. It appears that dopaminergic and serotonergic neurons are involved in the mediation of the startle response in rats.

p-Chloroamphetamine (PCA): acute and chronic effects on habituation and sensitization of the acoustic startle response in rats

In a series of 6 experiments the effects of p-chloroamphetamine (PCA) on the acoustic startle response in rats were investigated. 15 min after 5 mg/kg PCA startle amplitude was inhibited, 2-15 hr after PCA startle was facilitated. Rate of habituation however was not altered. Both the inhibitory and excitatory effects of PCA were blocked by pretreatment with p-chlorophenylalanine but not by alpha-methyl-p-tyrosine. 24 hr, 1 week and 4 weeks after PCA, initial startle amplitude was unchanged but PCA increased rate of sensitization over successive tone blocks. Increased sensitization was most pronounced at 10 mg/kg and absent at 2.5 mg/kg. The early inhibitory effect of PCA but not the later facilitatory effect was eliminated by reducing the level of background noise. The results suggest that inhibition of startle sensitization is associated with enhanced release of serotonin (5-HT) whereas enhancement of startle sensitization is associated with 5-HT depletion.

Effects of parachlorophenylalanine and amphetamine on habituation of orienting

Parachlorophenylalanine significantly reduced the orienting response to the first presentation of a tone but did not alter the size of responses to subsequent tone presentations nor the rate of habituation. In contrast amphetamine did not alter the orienting response but significantly impaired habituation. It was concluded that there was little evidence for a serotonergic involvement in behavioral habituation, although a role for the catecholamine system could not be excluded.

Physiological and pharmacological evidence for a serotonergic projection to the hippocampus

Rat hippocampal pyramidal cells were studied for their response to serotonin applied iontophoretically and to stimulation of the midbrain raphe nuclei. Ninety-two percent of the cells studied were inhibited by serotonin. Fourty-eight percent of the cells responded by inhibition to dorsal and median raphe stimulation. The inhibitory response to raphe stimulation was absent when the rats were pretreated with p-chlorophenylalanine (PCPA), a serotonin synthesis inhibitor; PCPA effects were alleviated by 5-HTP or 5-HT administration. The response to raphe stimulation was blocked by emthysergide and cyproheptadine. The responses to raphe stimulation were potentiated by chlorimipramine, a serotonin reuptake blocker. These data satisfy several of the criteria required to identify serotonin as the inhibitory neurotransmitter for the raphe-hippocampal pathway