Habituation and sensitization of the rat startle response: effects of raphe lesions

Serotonergic Modulation of Walking in Drosophila

To navigate complex environments, animals must generate highly robust, yet flexible, locomotor behaviors. For example, walking speed must be tailored to the needs of a particular environment. Not only must animals choose the correct speed and gait, they must also adapt to changing conditions and quickly respond to sudden and surprising new stimuli. Neuromodulators, particularly the small biogenic amine neurotransmitters, have the ability to rapidly alter the functional outputs of motor circuits. Here, we show that the serotonergic system in the vinegar fly, Drosophila melanogaster, can modulate walking speed in a variety of contexts and also change how flies respond to sudden changes in the environment. These multifaceted roles of serotonin in locomotion are differentially mediated by a family of serotonergic receptors with distinct activities and expression patterns.

Subchronic use of rivastigmine increases procognitive flexibility across multimodal behavioral tasks in healthy male rats

Rivastigmine (RVT) is a reversible inhibitor of cholinesterase approved worldwide for the treatment of cognitive dysfunctions, especially in Alzheimer's disease. Most previous pre-clinical studies have examined the effects of RVT treatment in a wide variety of pathological research models. Nonetheless, the effects of this drug on sensorimotor gating, memory, and learning tasks in healthy subjects remains unclear. In this study, we investigate the procognitive effects of RVT treatment in healthy rats through sensorimotor gating evaluations (measured as prepulse inhibition of the acoustic startle reflex), active avoidance learning, and spatial memory learning in a radial maze. There is an increase in the amplitude of the startle reflex in RVT-treated rats compared to the control groups, whereas the latency remained constant. Sensorimotor gating values were also incremented compared to those values from controls. In active avoidance, rats treated with RVT learned faster to successfully perform the task compared to controls, but afterwards all groups exhibited virtually identical results. During the sessions in the radial maze, RVT-treated rats committed fewer errors in both the working and reference memory compared to controls. All in all, our results support the hypothesis that RVT treatment may entail procognitive effects in healthy subjects.

Transient inhibition and long-term facilitation of locomotion by phasic optogenetic activation of serotonin neurons

Serotonin (5-HT) is associated with mood and motivation but the function of endogenous 5-HT remains controversial. Here, we studied the impact of phasic optogenetic activation of 5-HT neurons in mice over time scales from seconds to weeks. We found that activating dorsal raphe nucleus (DRN) 5-HT neurons induced a strong suppression of spontaneous locomotor behavior in the open field with rapid kinetics (onset ≤1 s). Inhibition of locomotion was independent of measures of anxiety or motor impairment and could be overcome by strong motivational drive. Repetitive place-contingent pairing of activation caused neither place preference nor aversion. However, repeated 15 min daily stimulation caused a persistent increase in spontaneous locomotion to emerge over three weeks. These results show that 5-HT transients have strong and opposing short and long-term effects on motor behavior that appear to arise from effects on the underlying factors that motivate actions.

DOI:http://dx.doi.org/10.7554/eLife.20975.001

The brain controls sleep, movement and the other behaviors that an animal needs to survive. A chemical called serotonin plays an important role in controlling these behaviors as it regulates the activity of nerve cells (known as neurons) throughout the brain. Serotonin is produced by a specific group of neurons found in an area at the base of the brain called the raphe nuclei. From there, serotonin is released into other parts of the brain to influence different behaviors. Although drugs that target serotonin are widely used as antidepressants, how this chemical signal acts in the brain remains a mystery. This is due, in part, to it being technically challenging to carry out experiments on the serotonin-producing neurons.

A technique called optogenetics uses light to selectively activate or inhibit individual cells in live animals. Here, Correia, Lottem et al. use optogenetics to activate serotonin-producing neurons in the dorsal raphe nucleus of mice. The experiments show that triggering serotonin production for a few seconds causes the mice to move around more slowly as they explore their surroundings. This short-term release of serotonin only slows the mice down if they are not already occupied with other activities, such as finding water or balancing on a moving object. These experiments suggest that serotonin decreases an individual’s motivation to move but that this can be overcome by sufficiently powerful goals. In contrast, repeatedly activating the serotonin neurons over a period of several weeks led to long-term changes of the opposite kind – the mice begin to move around more quickly.

The findings of Correia, Lottem et al. have possible implications for the use of drugs that target serotonin to treat mental disorders as it suggests important links between serotonin, movement, and the ability of the brain to change how it responds to certain situations. The next steps will be to investigate how the two different effects of serotonin are connected, which areas in the brain are involved and how best to apply these findings to clinical studies.

DOI:http://dx.doi.org/10.7554/eLife.20975.002

Origin and function of short-latency inputs to the neural substrates underlying the acoustic startle reflex

The acoustic startle reflex (ASR) is a survival mechanism of alarm, which rapidly alerts the organism to a sudden loud auditory stimulus. In rats, the primary ASR circuit encompasses three serially connected structures: cochlear root neurons (CRNs), neurons in the caudal pontine reticular nucleus (PnC), and motoneurons in the medulla and spinal cord. It is well-established that both CRNs and PnC neurons receive short-latency auditory inputs to mediate the ASR. Here, we investigated the anatomical origin and functional role of these inputs using a multidisciplinary approach that combines morphological, electrophysiological and behavioral techniques. Anterograde tracer injections into the cochlea suggest that CRNs somata and dendrites receive inputs depending, respectively, on their basal or apical cochlear origin. Confocal colocalization experiments demonstrated that these cochlear inputs are immunopositive for the vesicular glutamate transporter 1 (VGLUT1). Using extracellular recordings in vivo followed by subsequent tracer injections, we investigated the response of PnC neurons after contra-, ipsi-, and bilateral acoustic stimulation and identified the source of their auditory afferents. Our results showed that the binaural firing rate of PnC neurons was higher than the monaural, exhibiting higher spike discharges with contralateral than ipsilateral acoustic stimulations. Our histological analysis confirmed the CRNs as the principal source of short-latency acoustic inputs, and indicated that other areas of the cochlear nucleus complex are not likely to innervate PnC. Behaviorally, we observed a strong reduction of ASR amplitude in monaural earplugged rats that corresponds with the binaural summation process shown in our electrophysiological findings. Our study contributes to understand better the role of neuronal mechanisms in auditory alerting behaviors and provides strong evidence that the CRNs-PnC pathway mediates fast neurotransmission and binaural summation of the ASR.

Sensation seeking: A comparative approach to a human trait

A comparative method of studying the biological bases of personality compares human trait dimensions with likely animal models in terms of genetic determination and common biological correlates. The approach is applied to the trait of sensation seeking, which is defined on the human level by a questionnaire, reports of experience, and observations of behavior, and on the animal level by general activity, behavior in novel situations, and certain types of naturalistic behavior in animal colonies. Moderately high genetic determination has been found for human sensation seeking, and marked strain differences in rodents have been found in open-field behavior that may be related to basic differences in brain neurochemistry. Agonistic and sociable behaviors in both animals and humans and the trait measure of sensation seeking in humans have been related to certain common biological correlates such as gonadal hormones, monoamine oxidase (MAO), and augmenting of the cortical evoked potential.

The monoamine systems in the rodent brain are involved in general activity, exploratory behavior, emotionality, socialization, dominance, sexual and consummately behaviors, and intracranial self-stimulation. Preliminary studies have related norepinephrine and enzymes involved in its production and degradation to human sensation seeking. A model is suggested that relates mood, behavioral activity, sociability, and clinical states to activity of the central catecholamine neurotransmitters and to neuroregulators and other transmitters that act in opposite ways on behavior or stabilize activity in the arousal systems. Stimulation and behavioral activity act on the catecholamine systems in a brain–behavior feedback loop. At optimal levels of catecholamine systems activity (CSA) mood is positive and activity and sociability are adaptive. At very low or very high levels of CSA mood is dysphoric, activity is restricted or stereotyped, and the organism is unsocial or aggressively antisocial. Novelty, in the absence of threat, may be rewarding through activation of noradrenergic neurons.

Effects of haloperidol and clozapine on sensorimotor gating deficits induced by 5-hydroxytryptamine depletion in the brain

Evidence is increasing for a role of brain 5-hydroxytryptamine (5-HT) systems in schizophrenia. We previously showed that brain 5-HT depletion causes disruption of prepulse inhibition, a measure of sensorimotor gating that is deficient in schizophrenia. Antipsychotic treatment has been reported to reverse these deficits in patients with schizophrenia. The present study was designed to investigate the ability of antipsychotic drugs to reverse prepulse inhibition deficits caused by lesions of the brain 5-HT system in rats. In male Sprague-Dawley rats, selected parts of the brain 5-HT systems were lesioned by micro-injection of the 5-HT neurotoxin 5,7-dihydroxytryptamine into the dorsal raphe nucleus (DRN) or median raphe nucleus (MRN). The effects of antipsychotic drugs on lesion-induced changes in prepulse inhibition were examined 2 weeks after the surgery. There was significant disruption of prepulse inhibition in the MRN-lesioned group compared to sham-operated controls. This deficiency in prepulse inhibition was restored by clozapine (1 and 5 mg kg(-1)) treatment, and by treatment with a relatively high dose of haloperidol (0.25 mg kg(-1)). There was no significant effect of the DRN lesions on prepulse inhibition compared with sham-operated controls. These results indicate that 5-HT depletion in MRN-innervated brain structures leads to disruption of prepulse inhibition. Treatment with both antipsychotic drugs, haloperidol and clozapine, significantly increased prepulse inhibition in these animals back to the level seen in sham-operated controls. The present findings highlight the importance of the 5-HT systems in cognitive models of schizophrenia.

A subpopulation of dorsal raphe nucleus neurons retrogradely labeled with cholera toxin-B injected into the inner ear

Previous studies have shown that: (1) raphe neurons respond to acoustic and vestibular stimuli, some with a latency of 10-15 ms; (2) alterations of the raphe nuclei alter the acoustic startle reflex; (3) the dorsal raphe nucleus (DRN) is the major source of serotonergic neurons; and (4) approximately 57% of the DRN neurons are nonserotonergic. In the present study, cholera toxin subunit-B (CTB) was injected into cat cochleas, and the brain tissue was examined after a survival period of 5-7 days. Aside from neurons which were known to project to the inner ear, i.e., olivocochlear and vestibular efferent neurons, a surprising new finding was made that somata of a subpopulation of DRN neurons were intensely labeled with CTB. These CTB-labeled neurons were densely distributed in a dorsomedian part of the DRN with some in a surrounding area outside the DRN. The present results suggest that a novel raphe-labyrinthine projection may exist. A future study of anterograde labeling with injections of a tracer in the DRN will be needed to establish the existence of a raphe-labyrinthine projection more thoroughly. A raphe-labyrinthine descending input, together with an ascending input from the inner ear to the DRN through intervening neurons, such as the juxta-acousticofloccular raphe neurons (JAFRNs) described by Ye and Kim, may mediate a brain stem reflex whereby a salient multisensory (including auditory and vestibular) stimulus may alter the sensitivity of the inner ear. As a mammal responds to a biologically important auditory-vestibular multisensory event, the raphe projections to the inner ear and other auditory and vestibular structures may enhance the mammal's ability to localize and recognize the sound and respond properly. The raphe-labyrinthine projection may also modulate the inner ear's sensitivity as a function of the sleep-wake arousal state of an organism on a slower time course.

Differential role of serotonergic projections arising from the dorsal and median raphe nuclei in locomotor hyperactivity and prepulse inhibition

While an involvement of brain serotonin systems in schizophrenia has been suggested by many studies, the relative role of different serotonergic projections in the brain remains unclear. We therefore examined the effects of selective brain serotonin depletion on psychotropic drug-induced locomotor hyperactivity and prepulse inhibition, two animal models of aspects of schizophrenia. Pentobarbital-anesthetized (60 mg/kg, i.p.) male Sprague-Dawley rats were stereotaxically microinjected with 1 microl of a 5 microg/microl solution of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into either the dorsal or median raphe nucleus. At 2 weeks after the surgery, rats with dorsal raphe lesions did not show changes in psychotropic drug-induced locomotor hyperactivity, but displayed partial disruption of prepulse inhibition. In contrast, rats with median raphe lesions showed significant enhancement of phencyclidine-induced, but not amphetamine-induced locomotor hyperactivity and a marked disruption of prepulse inhibition. These results provide evidence for differential involvement of serotonergic projections in locomotor hyperactivity and prepulse inhibition. This study may help to explain the role of different serotonin projections in the brain in the pathophysiology of schizophrenia.

Acoustic startle eyeblink response after acute exercise

The acoustic startle eyeblink response (ASER) is a useful probe for investigating central nervous system activity associated with emotional responses to aversive and appetitive stimuli. Though the ASER is sensitive to change in emotional arousal, the effect of acute physical exertion on ASER has not been reported. We examined changes in ASER amplitude and latency in 26 healthy young men (24+/-5 yr) after 20 min of cycling at light and hard intensities (40% and 75% VO2peak) and after 20 min of quiet rest. Mixed model ANCOVA, controlling precondition scores, indicated no effects for ASER amplitude or latency in either sedentary or active participants (p>.10). Our findings indicate that possible effects of acute exercise on potentiated startle or ASER responses elicited by positive or negative foreground stimuli should not be expected to be confounded by an altered baseline acoustic startle eyeblink response when measured in healthy young men.

Lesions of the amygdala do not affect the enhancement of the acoustic startle response by background noise

The acoustic startle response (ASR) is enhanced in the presence of loud background noise. We examined whether or not this increase of response strength is mediated by the amygdala, which is known to be involved in various phenomena of enhancement of the ASR. To achieve this aim, we tested whether or not amygdaloid lesions with the excitotoxin N-methyl-D-aspartate (NMDA) would abolish the enhancement of the ASR by background noise in 13 Wistar rats. Loss of foot-shock sensitization in these rats, as well as histological evaluation, proved the successful destruction of the amygdala. However, the enhancement by background noise of the ASR, which was observed in sham-operated controls, was not affected in amygdala-lesioned rats. Therefore, we conclude that the background noise facilitation does not involve emotional components that are mediated by the amygdala. On the basis of these findings, we differentiate between the startle-enhancing effect of background noise and the amygdala-mediated effect of foot shocks on the ASR.

Modification of acoustic startle reactivity by cocaine administration during the postnatal period: comparison with a specific serotonin reuptake inhibitor

This study investigated whether exposure to cocaine during postnatal period affects the acoustic startle response (ASR) following administration of the serotonin (5-HT) agonists, 8-OH-DPAT and mCPP, in adulthood. To test the hypothesis that alterations in reactivity may be due to cocaine's effects at the 5-HT carrier, another group of rats was given fluoxetine, a specific 5-HT uptake inhibitor, during the same postnatal period and tested along with the cocaine-treated rats. Male and female rats received 25 mg/kg/day cocaine HCl, fluoxetine HCl, or vehicle SC during postnatal days 11-20. At 75 days of age, subjects were ASR tested for 30 min on 2 consecutive days. On the first test day, there was a significant effect of treatment and gender with post hoc analysis indicating that, overall, the males were more reactive than the females and that the fluoxetine-treated males showed a pattern of reactivity resembling sensitization. On the second test day, subjects received a dose of the 5-HT1A agonist 8-OH-DPAT, the 5-HT1B/2C agonist, mCPP, or saline prior to being placed in the startle chamber. Cocaine-exposed males showed an enhanced response to 8-OH-DPAT and a reduction in the depression produced by mCPP administration compared to their response to saline. Fluoxetine exposed males showed a significant increase in startle response following saline administration compared to the rats receiving vehicle during the postnatal period and 8-OH-DPAT produced an insignificant enhancement of that startle response. mCPP reduced startle in fluoxetine-treated males as it did in the postnatal vehicle-treated controls. In females, the postnatal cocaine and fluoxetine treatments did not alter the response to 8-OH-DPAT or mCPP compared to females receiving vehicle during the postnatal period. Together these data indicate that, in males, whereas postnatal cocaine alters the development of the 5-HT system as evidenced by an altered startle response to 5-HT agonists, cocaine does not produce the same alteration as that produced by the administration of a specific 5-HT uptake inhibitor during the same period of development.

5-HT1 receptors and behavior

The activation of different subtypes of the 5-HT1 receptor can be associated with specific behavioral responses. The present review discusses different categories of behavioral studies that have examined functional distinctions among 5-HT1 receptors. These include: 1) behavioral responses elicited by selective 5-HT receptor agonists; 2) drug discrimination experiments; 3) studies of sensorimotor reactivity and motivated behavior; and 4) behavioral models of clinical psychotherapeutic effects.

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.

Serotonergic modulation of the acoustic startle response in rats during preweaning development

The involvement of serotonin (5-HT) in modulating the acoustic startle response (ASR) is well established in adult rats, but 5-HT involvement during the preweaning period, when 5-HT neurons undergo extensive development, has not previously been described. Three 5-HT receptor subtypes are reported to modulate the ASR in adult rats: 5-HT1A and 5-HT2 receptor agonists facilitate the ASR, whereas 5-HT1B agonists decrease the response. In the present study, the effects of 5-HT agonists and generalized 5-HT depletion on the ASR were studied in preweanling animals, using independent groups of Long-Evans rats tested on postnatal day (PND) 13, 17 and 21. 8-Hydroxy-2-(di-n-propylamino) tetralin (8OHDPAT, 62-1000 micrograms/kg), a 5-HT1A receptor agonist, and 5-methoxy-N,N-dimethyl tryptamine (MeODMT, 2-4 mg/kg), a nonselective 5-HT agonist, had no effect on PND 13 and then increased the ASR on PND 17 and 21. The 5-HT2 receptor antagonists cyproheptadine (5 mg/kg) and ketanserin (5 mg/kg) blocked the effect of MeODMT at both ages, providing some evidence that MeODMT increased the ASR through 5-HT2 receptors. 1-(m-Chlorophenyl) piperazine (mCPP, 1-5 mg/kg), a 5-HT1B agonist, had no effect on ASR amplitude on PND 13 or 17 and then produced a dose-related decrease in the response on PND 21. Generalized depletion of 5-HT by 80-90% in whole-brain and spinal cord, using p-chlorophenylalanine (PCPA, 300 mg/kg 24 hr prior to testing), did not alter ASR amplitude at any age.(ABSTRACT TRUNCATED AT 250 WORDS)

A fostering/crossfostering analysis of the effects of prenatal ethanol exposure in a liquid diet on offspring development and behavior in rats

Pregnant Long-Evans rats were fed a liquid diet containing either 35% ethanol-derived calories (group E) or were pair-fed to group E dams using the same diet with an equivalent amount of sucrose-derived calories (group PFC) on days 7-18 of gestation. At birth half the litters were fostered to other dams of the same treatment and half were crossfostered to dams of the opposite treatment. Half the dams in each group had blood samples taken on their last day of treatment. Mean (+/- SE) serum ethanol concentration in group E was 167.1 +/- 22.6 mg/dl. Main effects of prenatal ethanol exposure were obtained on offspring body weight, olfactory orientation, figure-8 activity, straight channel swimming time, and gap-induced startle inhibition. Group E offspring weight differences were small (less than 7%) and transient (occurring between days 21-69). Group E offspring oriented less to their home cage scent, were more active in the figure-8 test, spent more time swimming the straight channel, and startled less than controls. Fostering was a significant main effect only on olfactory orientation and day 50-51 open-field. The tests of prime interest were interactions involving group and fostering. No such interactions were found. The data support the interpretation that postnatal maternal influences induced by prenatal treatment with ethanol in a liquid diet are not sufficiently strong modifying variables on offspring development or behavior to be a source of experimental concern.

Early postnatal deprivation of active sleep with desipramine or zimeldine impairs later behavioural reactivity to auditory stimuli in rats

To examine the functional significance of early postnatal active sleep for the development of behavioural reactivity to auditory stimuli, rat pups were daily injected i.p. from the 7th to the 18th postnatal days with 5 mg kg-1 (6.6 mmol l-1) desipramine or 25 mg kg-1 (12.2 mmol l-1) zimeldine. Sleep-wake behaviour was recorded with a static-charge-sensitive bed (SCSB) method. Both desipramine and zimeldine suppressed the percentage of active sleep relative to the total recording time throughout the treatment period. In addition, these drugs increased the percentage of quiet state and waking. At the age of 38 days the zimeldine-treated rats showed more motor activity in the open field than the controls. At the age of 39 and 78 days all rat groups behaved similarly in the open field. Startle measures and motor activation, provoked by auditory stimulation, were determined by the SCSB method when the rats were 4 months of age. Auditory stimuli, consisting of a series of ten clicks, induced a greater number of startles as well as strong movement responses in the control rats than in the desipramine- or zimeldine-treated rats. The number of small movement responses did not differ between the rat groups. These findings indicate that early postnatal active sleep and the monoaminergic systems regulating it may be important for the normal development of neuronal circuitry associated with later reactivity to auditory stimuli.

Serotonergic mechanisms in the behavioral effects of buspirone and gepirone

The literature describing the role of serotonin (5-HT) in the mediation of anxiety is a controversial one. Serotonergic involvement in the mechanism of action of two nonbenzodiazepine anxiolytics, buspirone and gepirone, supports a role for serotonin in anxiety. The anticonflict effect of both drugs is blocked by serotonin lesions, and gepirone induces the serotonin syndrome. A shift in the gepirone dose-response curve to the left in serotonin lesioned rats suggests that this may be 5-HT-receptor mediated. Both buspirone and gepirone enhance the acoustic startle response and gepirone's effect is attenuated in serotonin lesioned animals. While other components of buspirone's mechanism of action may suppress the behavioral expression of its serotonergic interactions, results from these studies suggest that serotonin agonist-like activity may be an important mechanism in the actions of a clinically proven nonbenzodiazepine anxiolytic (buspirone), and anxiolytic candidate (gepirone).

Developmental alterations in maturing rats caused by chronic prenatal and postnatal diazepam treatments

The post treatment effects of early prenatal, late prenatal, early postnatal or combined prenatal and neonatal treatment with diazepam on the development of pain sensitivity, acoustic startle responsiveness, and benzodiazepine receptors in the cerebral cortex were investigated in rats between 14 and 90 days of age. Tail-flick latency was significantly decreased by combined prenatal and neonatal and by early prenatal diazepam treatment, but not by diazepam during the last half of gestation or during the neonatal period alone. Acoustic startle response was decreased by either late prenatal or neonatal diazepam treatment, but not by early prenatal treatment alone. Density of benzodiazepine receptors in the cortex was increased from postnatal day 1 to 21 by either early or late prenatal diazepam treatment. Neonatal diazepam treatment suppressed cortical benzodiazepine receptor or development until postnatal day 21; thereafter, receptor density increased to significantly higher values than in controls at 90 days of age. The results demonstrate that diazepam can alter development of pain sensitivity by actions during early gestation, startle responsiveness by actions late in pregnancy, and cortical benzodiazepine receptors by actions throughout gestation and the early postnatal period.

Parachlorophenylalanine-induced serotonin depletion increases offensive but not defensive aggression in male rats

Cerebral 5-HT depletion has been shown to facilitate elicitation of various kinds of aggressive behavior in rats. The question as to whether both offensive and defensive aggressive reactions are affected to the same extent was examined in a resident-intruder paradigm where an ethological analysis of the two animals allows an evaluation of non-social activities as well as agonistic interactions, including both offense and defense. PCPA (375 mg/kg IP) was administered either to the resident or the intruder and the interactions with an untreated conspecific were recorded in the resident's home cage for an 8 min period three days after injection when 5-HT was maximally reduced. PCPA treatment increased the occurrence of social approach and offensive postures in resident rats, whereas their untreated partners displayed more defensive reactions. When intruders were injected, only non-significant increases in approach and offense were observed. In no case did PCPA affect occurrence of defensive postures in the injected animals. These results confirm that serotonin plays a role in controlling offensive aggression but not defensive behavior.

Cardiovascular concomitants of tactile and acoustic startle responses in spontaneously hypertensive and normotensive rats

In the present study we have simultaneously investigated cardiovascular and behavioral responses to repeated tactile and acoustic stimulation in adult, male, spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Blood pressure and heart rate responses were increased in SHR vs. WKY rats following both types of stimuli. The pressor responses, but not the tachycardic responses, habituated with repeated stimulation in both strains and with both stimulus modalities. The rates of habituation were not significantly different between the two strains. Magnitudes of the behavioral startle responses were significantly elevated in SHR vs. WKY rats with tactile, but not with acoustic stimulation. No significant differences were found between the two strains with respect to the latency, with which the startle responses occurred. These data indicate that SHR as compared to WKY rats respond to repeated stimulation with two different stimulus modalities (tactile and acoustic) with significantly increased pressor and tachycardic responses. This cardiovascular hyperreactivity is not due to different degrees of habituation between the two strains and can be observed even in the absence of significant differences in standard behavioral startle response measures.

Separation of the associative and non-associative effects of brain serotonin released by p-chloroamphetamine: dissociable serotoninergic involvement in avoidance learning, pain and motor function

p-Chloramphetamine (PCA, 0.63-5 mg/kg IP) injected 30-60 min before testing produced a dose-related impairment of avoidance acquisition, prolonged reaction time in the hot-plate test and increased locomotor activity. Pretreatment with the selective serotonin (5-HT) uptake inhibitor zimeldine (10 mg/kg IP) blocked these behavioral effects. Degeneration of brain 5-HT neurons by a high neurotoxic dose of PCA (2 X 10 mg/kg IP) or inhibition of tryptophan hydroxylase by p-chlorophenylalanine (300 mg/kg IP) also blocked the behavioral effects of PCA. There was a complete blockade of the PCA-induced avoidance deficit following pretreatment with metergoline, a central 5-HT receptor blocking agent. On the other hand, metergoline failed to block the hot-plate analgesia and the increased locomotion caused by PCA. Depletion of brain NA and DA by the tyrosine hydroxylase inhibitor H44/68 did not counteract the PCA effect on avoidance or hot-plate performance, but reduced the locomotor stimulating effect. The selective NA neurotoxin DSP4 (50 mg/kg IP) or the opiate antagonist naloxone (1 mg/kg) failed to affect the PCA-induced modulations of the behaviours studied. In addition, PCA administration in doses that caused avoidance deficits, did not result in motor impairment as assessed by the tread mill test. The above results support the hypothesis that the PCA-induced impairment of active avoidance acquisition does not involve changes in nociception or altered locomotor activity. It is concluded that behavioural processes related to serotonergic neurotransmission can be independently modified, suggesting differences in the underlying 5-HT mechanisms.