Conditioned place-preference analysis in the goldfish with the H1 histamine antagonist chlorpheniramine

The Histamine System in Zebrafish Brain: Organization, Receptors, and Behavioral Roles

Three of the four histamine receptors have been identified in zebrafish. Whereas only one histamine receptor 1 gene (hrh1) is known, two copies of histamine receptor 2 (hrh2a and hrh2b) have been identified. Although initially only one gene encoding for histamine receptor 3 (hrh3) was recognized in zebrafish, the genome database contains information for two more hrh3-like genes, whereas no genes corresponding for histamine receptor 4 with expression mainly in the immune system have been identified. Hrh1 and hrh3 show prominent uneven expression in the zebrafish brain, with the strongest expression in the dorsal telencephalon. Quantitatively significant expression of hrh1, hrh2, and hrh3 can also be found in several peripheral organs. Whereas antagonists of hrh1, hrh2, and hrh3 all affect the locomotor activity of zebrafish larvae, interpretation of the data is hampered by a lack of information on receptor binding and signaling characteristics. Zebrafish mutants lacking any of the three histamine receptors have shown modest behavioral phenotypes, possibly due to genetic compensation. None of the receptor mutant fish have shown significant sleep phenotypes. Adult zebrafish lacking hrh3 display decreased locomotor activity. The zebrafish histamine system shows significant life-long plasticity: presenilin 1 mutant zebrafish develop an abnormally large number of histamine neurons and increased thigmotaxis and anxiety-related phenotype. Overexpression of histidine decarboxylase (hdc) in larval zebrafish is associated with an increased number of hypocretin neurons, whereas translation inhibition of hdc or exposure to α-fluoromethylhistidine leads to decreased numbers of hypocretin neurons. Current pharmacological evidence suggests that this may be mediated by hrh1. Further studies using acute, e.g., pharmacogenetic or optogenetic manipulation of selected components of brain circuits, are required to understand the full range of physiological functions of zebrafish histamine receptors.

Dominance behaviour in a non-aggressive flatfish, Senegalese sole (Solea senegalensis) and brain mRNA abundance of selected transcripts

Dominance is defined as the preferential access to limited resources. The present study aimed to characterise dominance in a non-aggressive flatfish species, the Senegalese sole (Solea senegalensis) by 1) identifying dominance categories and associated behaviours and 2) linking dominance categories (dominant and subordinate) with the abundance of selected mRNA transcripts in the brain. Early juveniles (n = 74, 37 pairs) were subjected to a dyadic dominance test, related to feeding, and once behavioural phenotypes had been described the abundance of ten selected mRNAs related to dominance and aggressiveness was measured in the brain. Late juveniles were subjected to two dyadic dominance tests (n = 34, 17 pairs), related to feeding and territoriality and one group test (n = 24, 4 groups of 6 fish). Sole feeding first were categorized as dominant and sole feeding second or not feeding as subordinate. Three social behaviours (i. "Resting the head" on another fish, ii. "Approaching" another fish, iii. "Swimming above another" fish) were associated with dominance of feeding. Two other variables (i. Total time occupying the preferred area during the last 2 hours of the 24 h test, ii. Organisms occupying the preferred area when the test ended) were representative of dominance in the place preference test. In all tests, dominant fish compared to subordinate fish displayed a significantly higher number of the behaviours "Rest the head" and "Approaches". Moreover, dominant sole dominated the sand at the end of the test, and in the group test dominated the area close to the feed delivery point before feed was delivered. The mRNA abundance of the selected mRNAs related to neurogenesis (nrd2) and neuroplasticity (c-fos) in dominant sole compared to subordinate were significantly different. This is the first study to characterise dominance categories with associated behaviours and mRNA abundance in Senegalese sole and provides tools to study dominance related problems in feeding and reproduction in aquaculture.

Cognitive appraisal of aversive stimulus differs between individuals with contrasting stress coping styles; evidences from selected rainbow trout (Oncorhynchus mykiss) strains

In animals, personality variations in response to stress and energy demands have been established. Cognitive processing of negative stimuli correlates with stress response patterns. Still, the relative contribution of cognitive appraisal or physiological demands to the behavioural output needs to be clarified. In this study we utilized reactive (high-responsive, HR) and proactive (low-responsive, LR) rainbow trout strains to investigate how contrasting reactions to hypoxia are related to individual variation in metabolism and/or cognition. The HR-LR strains did not differ in standard metabolic rate or hypoxia tolerance. HR trout displayed more pronounced avoidance to a signal cue after being conditioned with hypoxia, suggesting that they experienced this stimulus more aversive than LR trout. Together with differences in forebrain c-fos activation patterns in dorsomedial pallium, these results suggest cognitive differences between the strains. These results demonstrate that differences in personality/stress coping style can be related to contrasts in cognition, which are independent of metabolic differences.

Learning and memory in zebrafish larvae

Larval zebrafish possess several experimental advantages for investigating the molecular and neural bases of learning and memory. Despite this, neuroscientists have only recently begun to use these animals to study memory. However, in a relatively short period of time a number of forms of learning have been described in zebrafish larvae, and significant progress has been made toward their understanding. Here we provide a comprehensive review of this progress; we also describe several promising new experimental technologies currently being used in larval zebrafish that are likely to contribute major insights into the processes that underlie learning and memory.

Conditioned place preference behavior in zebrafish

This protocol describes conditioned place preference (CPP) in zebrafish following a single exposure to a substance. In the CPP paradigm, animals show a preference for an environment that has previously been associated with a substance (drug), thus indicating the positive-reinforcing qualities of that substance. The test tank consists of two visually distinct compartments separated by a central alley. The protocol involves three steps: the determination of initial preference, one conditioning session and the determination of final preference. This procedure is carried out in ∼2 d; other reported CPP protocols take up to 2 weeks. An increase in preference for the drug-associated compartment is observed after a single exposure. Establishment of this high-throughput protocol in zebrafish makes it possible to investigate the molecular and cellular basis of choice behavior, reward and associative learning. The protocol is also a tool for testing psychoactive compounds in the context of a vertebrate brain.

l-histidine induces state-dependent memory deficit in mice mediated by H(1) receptor

This study investigated the role of H(1) receptor in the state-dependent memory deficit induced by l-histidine (LH) in mice using Trial 1/2 protocol in the elevated plus-maze (EPM). The test was performed for two consecutive days: Trial 1 (T1) and Trial 2 (T2). Before both trials, mice received a combined injection i.p. of saline+saline (SAL/SAL), 500 mg/kg L-histidine+saline (LH/SAL), 500 mg/kg L-histidine+16 mg/kg chlorpheniramine (LH/CPA) or saline+16 mg/kg chlorpheniramine (SAL/CPA). The trials were performed in the EPM 10 min after the last injection. Each animal was placed in the center of the maze facing the open arm and had five minutes to explore it. On both days, test sessions were videotaped. The behavioral measures were scored from videotape. Data were analyzed based on Analysis of Variance (ANOVA) and the Fisher's LSD test. The data showed no effects on anxiety since there was no difference between the SAL/SAL and the other groups in Trial 1, respectively, open arm entries (OAE), open arm time (OAT) and their percentages (%OAE and %OAT). During Trial 2, OAE, OAT, %OAE and %OAT were reduced in mice treated with SAL/SAL, LH/CPA and SAL/CPA, while the group LH/SAL did not show any difference in these measures. No significant changes were observed in enclosed arm entries (EAE), an EPM index of general exploratory activity. Thus, it can be suggested that LH induces emotional memory deficit and the treatment with chlorpheniramine was able to revert this effect, suggesting this action of LH was mediated by the H(1) receptor.

Chlorpheniramine impairs functional recovery in Carassius auratus after telencephalic ablation

We determined the effect of an H1 receptor antagonist on the functional recovery of Carassius auratus submitted to telencephalic ablation. Five days after surgery the fish underwent a spatial-choice learning paradigm test. The fish, weighing 6-12 g, were divided into four groups: telencephalic ablation (A) or sham lesion (S) and saline (SAL) or chlorpheniramine (CPA, ip, 16 mg/kg). For eight consecutive days each animal was trained individually in sessions separated by 24 h (alternate days). Training trials (T1-T8) consisted of finding the food in one of the feeders, which were randomly blocked for each subject. Animals received an intraperitoneal injection of SAL or CPA 10 min after the training trials. The time spent by the animals in each group to find the food (latency) was analyzed separately at T1 and T8 by the Kruskal-Wallis test, followed by the Student Newman-Keuls test. At T1 the latencies (mean +/- SEM) of the A-SAL (586.3 +/- 13.6) and A-CPA (600 +/- 0) groups were significantly longer than those of the S-SAL (226.14 +/- 61.15) and S-CPA (356.33 +/- 68.8) groups. At T8, the latencies of the A-CPA group (510.11 +/- 62.2) remained higher than those of the other groups, all of which showed significantly shorter latencies (A-SAL = 301.91 +/- 78.32; S-CPA = 191.58 +/- 73.03; S-SAL = 90.28 +/- 41) compared with T1. These results support evidence that training can lead to functional recovery of spatial-choice learning in telencephalonless fish and also that the antagonist of the H1 receptor impairs it.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Involvement of the histaminergic system on appetitive learning and its interaction with haloperidol in goldfish

This study investigated the actions of the histaminergic system on appetitive learning and memory, and its interaction with the dopaminergic system in goldfish. It consisted of nine sessions, in which fish were tested in a four-arm tank. On day 1, the animals were habituated for 10 min. On day 2, they were placed in one arm and had to find food at the left or the right arm. Time to begin feeding was recorded, and the procedure repeated for more 3 days (training phase). On training day 4, seven groups were injected with saline, seven with haloperidol (2.0 mg/kg) and one with DMSO solution before training and after feeding, three groups received saline, six chlorpheniramine (CPA) (1.0, 4.0 and 8.0 mg/kg), and six l-histidine (LH) (25, 50 and 100 mg/kg). Saline groups were considered as control of CPA and LH treated groups and DMSO as control of haloperidol. A non-injected group was also included. Testing occurred after 24 h. A reversal procedure was conducted 24h after testing and repeated for 3 days. The groups receiving CPA at 1.0 and 8.0 mg/kg and LH at 25, 50 and 100 mg/kg differed between Test and Reversal day 1. Pre-treatment with haloperidol plus 8.0 mg/kg of CPA and 25 and 50 mg/kg of LH reverted the treatment effect. However, in the groups treated with 1.0 mg/kg of CPA and 100 mg/kg of LH, the difference remained. This study confirmed the interaction between the histaminergic and the dopaminergic systems on memory process in goldfish.

Anxiolytic-like effect of chlorpheniramine in inhibitory avoidance in goldfish submitted to telencephalic ablation

The aim of the present study was to verify the consequences of telencephalic ablation on the learning of inhibitory avoidance and anxiety in goldfish. The animals were submitted to telencephalic ablation or sham operations five days prior to the experimental procedure. The inhibitory avoidance procedure was performed in 3 days using a rectangular aquarium divided into two compartments (black and white) with a central door. On the first day, the animals were habituated for 10 min. On the second and third days, they were injected with saline (SAL), 16 mg/kg Chlorpheniramine (CPA), 40% Propylene glycol (PPG) or 1 mg/kg Diazepam (DZP) twenty minutes before training. Then the animals were placed in the white compartment, the central door was opened and the time spent for crossing between compartments was recorded. After the fish crossed the line between the compartments a 45-g weight was dropped. This procedure was performed three times in a row. The groups submitted or not to telencephalic ablation and treated with SAL presented a difference between training sessions; however, the groups treated with CPA, PPG or DZP did not show any differences between them. These results suggest that the treatment with CPA, PPG or DZP impaired the acquisition of inhibitory avoidance conditioning in animals regardless of telecenphalic ablation. In conclusion, telencephalic ablation does not disrupt the animals' capacity to learn the inhibitory avoidance task, and based on the fact that CPA showed similar effects to those of DZP on the animals submitted or not to telencephalic ablation, we suggest that the CPA presents an anxiolytic-like effect mediated by the diencephalon in goldfish.

Involvement of histamine receptors in the acquisition of inhibitory avoidance in Carassius auratus

This study investigated the involvement of H(1) and H(2) histaminegic receptors on the acquisition of a new task in Carassius auratus by using an inhibitory avoidance paradigm in which the animals had to learn to avoid an aversive stimulus. Before training, the fish received injections of H(2) antagonist zolantidine at a dose of 20 mg/kg, or H(1) antagonist chlorpheniramine at a dose of 4 or 16 mg/kg. Control animals were injected with distilled water. A facilitatory effect of chlorpheniramine was observed at the dose of 16 mg/kg. On the other hand, the administration of 20 mg/kg of zolantidine inhibited acquisition. Place preference conditioning was used to observe the aversive or reinforcing effects of the drugs, which could interfere with the inhibitory avoidance procedure; however, no effects were observed. Thus, it can be suggested that both receptors, H(1) and H(2), are involved in the acquisition of a new task in this species.

Effects of Chlorpheniramine and L-histidine on vestibular compensation in goldfish, Carassius auratus

Histamine is thought to be involved in the recovery of vestibular function after damage to the vestibular receptors of the inner ear. This study evaluated the effects of post-operative treatment using Chlorpheniramine (H1 histamine antagonist) and L-histidine, (a histaminergic precursor), after hemilabyrinthectomy in goldfish. In this lesion model, the unilateral removal of the labyrinth induces a transient postural imbalance in response to light. After the lesion, the animals were injected intraperitoneally, during 12 consecutive days, with Chlorpheniramine, L-histidine and saline. All the substances were administered in a volume of 1 ml/kg body weight. Another group, which served as a non-lesion control, did not receive hemilabyrinthectomy or systemic injections. Chlorpheniramine accelerated the functional recovery when compared with that of the saline group. These data suggest that the inhibition of the histaminergic system facilitates the functional recovery in goldfish.

Haloperidol and chlorpheniramine interaction in inhibitory avoidance in goldfish

The purpose of this study was to investigate a possible interaction between histaminergic and dopaminergic systems in learning and memory processes, in an inhibitory avoidance test in goldfish. Haloperidol, a dopaminergic antagonist, was administrated pre-training and the chlorpheniramine (CPA), a histaminergic antagonist, post-training. The inhibitory avoidance procedure was performed in 3 days, using a rectangular aquarium divided into two compartments (black and white), with a central door. On the first day, the animals were habituated for 10 min. On the second day, they were injected with 2 mg/kg of haloperidol or dimethyl sulfoxide (DMSO) 20 min before training. Then, the animals were placed in the white compartment, the central door was opened and the time spent for crossing between compartments was recorded. After the fish crossed the line between compartments a 45 g weight was dropped. This procedure was done five times in a row. Immediately after the fifth trial, the fish were injected intraperitoneally (i.p.) with either saline or CPA (0.4, 1.0, 4.0, 8.0 or 16 mg/kg). On the next day (test) the time to cross was recorded again. On the training trials, the animals treated with DMSO or haloperidol presented a significant increase in the latencies indicating learning (Friedman P = 0.0062 and 0.0001). The latencies in the test day showed that groups pre-treated with haloperidol and treated with CPA presented a dose-dependent increase in latencies, and those treated with the 16 mg/kg CPA group showed a significant increase (ANOVA two-way followed by Student-Newman-Keuls (SNK) P < 0.01). Thus, it can be suggested that the facilitatory action occurs due to an additive interaction between both systems, in a dose-dependent way.

Dimenhydrinate produces a conditioned place preference in rats

Dimenhydrinate (DMH; trade names Gravol and Dramamine) is a compound of diphenhydramine (DP) and 8-chlorotheophylline in equimolar ratios. DMH has been reported to be abused by humans for its euphoric and hallucinogenic properties but few studies have evaluated its reinforcing effects in animals. To evaluate the hypothesis that DMH and its constituents DP and 8-chlorotheophylline are rewarding in animals, rats were tested for conditioned place preference (CPP). The paradigm consisted of pre-exposure (three 15-min sessions of access to both sides of the chamber), conditioning [eight 30-min pairings of one side with drug (four sessions) and, on alternate days, the other side with vehicle (four sessions)] and test phases (three 15-min sessions of access to both sides of the chamber). Significant preferences for the drug-paired location were found on test session one after conditioning with 60.0, but not 25.0, 40.0 or 50.0 mg/kg of DMH, and after conditioning with 37.8 but not 27.0 or 32.4 mg/kg of DP. No preference was found after conditioning with 23.0, 27.6 or 32.2 mg/kg of 8-chlorotheophylline. All three drugs stimulated locomotor activity during conditioning sessions and DMH and DP showed sensitization over conditioning sessions. DMH doses that showed sensitization (25.0 and 40.0 mg/kg) were lower than the dose (60.0 mg/kg) that produced a CPP revealing a dissociation of locomotor stimulating versus rewarding effects. Results reveal that DMH and DP have rewarding properties, although the molar equivalent dose-response curve for DP appeared to be further to the right than that for DMH. Future investigations into the neurotransmitter systems modulating this effect are awaited.

Circadian variation of brain histamine in goldfish

Teleosts may make an excellent model to study brain histamine function. Fishes are phylogenetically closer to the basic vertebrate blueprint than higher vertebrates. They appear to have a simpler histaminergic system in terms of central nervous system distribution and, contrary to higher vertebrates, brain histamine appears to be strictly neuronal. In this preliminary study, we examined circadian variation of brain histamine in goldfish, Carassius auratus, as this neurotransmitter correlates with circadian behavior of some mammals. Two groups of juvenile goldfish were held in 24 60L aquaria, six fish per aquarium, on reversed photoperiods; L:D 12:12 with light onset either at 0700 or 1900h. Fish were sampled every 4h. At a sampling time, all the fish in a tank were taken; each sampling, for both groups, was done in replicate. Brain histamine was determined by immunoassay. There was a significant circadian variation in histamine on both photoperiod regimes with the highest levels during the photophase. These results support the hypothesis of an early phylogenic role for histamine in vertebrate circadian physiology.

Behavioral screening for cocaine sensitivity in mutagenized zebrafish

Understanding the molecular basis of addiction could be greatly aided by using forward genetic manipulation to lengthen the list of candidate genes involved in this complex process. Here, we report that zebrafish exhibit cocaine-induced conditioned place preference. In a pilot screen of 18 F(2) generation families of mutagenized fish, we found three with abnormally low responses to cocaine. This behavior was inherited by the F(3) generation in a manner that suggests the abnormalities were because of dominant mutations in single genes. Performance profiles in secondary behavioral screens measuring visual dark-adaptation and learning suggest that the defects were the result of mutations in distinct genes that affect dopaminergic signaling in the retina and brain.

Analysis of the role of histamine in inhibitory avoidance in goldfish

1. Teleost fish have histaminergic cell bodies on the posterior part of the basal hypothalamus. It was suggested that they are homologous to the tuberomammillary E group in rats. However, unlike in rats, fish have fewer ascending fibers. The main projection runs through the ventral telencephalic area reaching the dorsal telencephalon. This projection is considered homologous to the prosencephalic forebrain bundle. 2. The aim of this study was to verify if the histaminergic system has an inhibitory action on learning and memory in goldfish, as suggested previously for higher vertebrates. 3. A two-compartment aquarium with a central sliding door was used. The animals were placed in one of them, the central door was opened after 30 sec and the time spend for crossing between compartments was recorded. After the fish dorsal fin crossed the line between the compartments a 45 g weight was dropped into the compartment the fish entered. 4. On the training day this procedure was done 3 times. Immediately after the 3rd trial the fish was injected i.p. with either vehicle (2 ml/kg), chlorpheniramine (CPA; 1.0, 4.0 and 8.0 mg/kg) or histidine (500 mg/kg). On the next day, fishes were placed in the start compartment and the latency to cross between compartments was again recorded. 5. The group treated with CPA at the dose of 8 mg/kg, presented a significant increase in the latency to leave the start compartment (Wilcoxon rank sum test, p<0.0232). On the other hand, the vehicle and 1-histidine (500 mg/kg) treated groups, presented a decrease in test latency. 6. Thus, we suggest that also in fish, the histaminergic system has an inhibitory role on learning and memory.

Differential sensitivity of the caudal and rostral nucleus accumbens to the rewarding effects of a H1-histaminergic receptor blocker as measured with place-preference and self-stimulation behavior

A recent series of studies in rats has demonstrated positively reinforcing and memory enhancing effects following lesions of the nucleus tuberomammillaris, which is the only known source of neuronal histamine. The aim of the present experiments was to assess whether inhibition of histaminergic neurotransmission in the ventral striatum has positively reinforcing effects. In Experiment 1 rats with chronically-implanted cannulae were injected with the H1 receptor blocker d-( + )-chlorpheniramine at doses of 0.1, 1.0 and 10.0 microg into the rostral or caudal parts of the nucleus accumbens, a brain region known to be involved in reward-related processes. Immediately after the treatment the animals were placed into one of four restricted quadrants of a circular open field (closed corral) for a single conditioning trial. During the drug-free test for conditioned place preference, when a choice among the four quadrants was provided, those rats injected with 10.0 microg chlorpheniramine in the caudal nucleus accumbens spent more time in the treatment corral, indicative of a positively rewarding drug action. In Experiment 2 the question was posed whether injection of chlorpheniramine into the nucleus accumbens influences electrical self-stimulation of the lateral hypothalamus. For this purpose rats were chronically implanted with two bipolar electrodes aimed at the lateral-hypothalami and with two additional guide cannulae aimed either at the rostral or caudal nucleus accumbens. After having established reliable self-stimulation behavior at one of the two electrode sites the animals were allowed to self-stimulate for one hour (baseline). Then they were unilaterally injected with 10.0 microg chlorpheniramine or vehicle and allowed to self-stimulate for another hour (test). On the next day the same procedure took place, except for the difference that the animals received an injection aimed at the hemisphere not treated so far. Animals treated with chlorpheniramine in the caudal and in the rostral nucleus accumbens displayed higher rates of ipsihemispheric self-stimulation behavior. Moreover, the animals treated with the H1 receptor blocker in the caudal nucleus accumbens displayed higher rates of ipsihemispheric self-stimulation than those having received an injection in the rostral pole. Upon completion of this part of the experiment all animals received an additional intraperitoneal treatment with chlorpheniramine (20 mg/kg) or vehicle, respectively, and were tested in the same way described above. This treatment also resulted in an amplification of intracranial self-stimulation behavior. These results support the hypothesis that histaminergic neurotransmission is involved in the inhibitory control of a central system subserving reward-related processes. The present data also further highlight the nucleus accumbens as functionally heterogenous along its rostrocaudal axis, with the caudal-shell subregion being more sensitive to antihistaminic induced reward than the rostral entity.

Post-trial administration of H1 histamine receptor blocker improves appetitive reversal learning and memory in goldfish, Carassius auratus

Based on the hypothesis that neuronal histamine exerts an inhibitory influence on learning and reinforcement, goldfish were tested for post-trial effects of the H1 receptor blocker chlorpheniramine (CPA) on learning the location of a food source in one of two compartments, one black the other white, with a feeder located in each compartment. Testing was carried out over 6 days. On the training day a food pellet was placed into the feeder of one of the compartments. After consumption of the food the fish were injected i.p. with either vehicle or CPA either immediately after training or 3 h later. Twenty-four-hours later, food was placed in the same compartment and the time to begin feeding was recorded. On the next day the location of the food pellet was reversed, and testing was continued for 4 days. On the first test day the time to begin feeding was significantly longer for the vehicle injected fish as compared with those injected with CPA. The vehicle group also took longer to begin feeding than the CPA group on the first reversal test day. The results of the 3-h delay groups indicated no significant differences between vehicle and drug for any experimental session. These results suggest that post-trial blockade of the H1 histamine receptor can affect appetitive learning in goldfish either by improving long-term memory consolidation and/or by the additive reinforcing effects of CPA (known from previous studies) on behavior.

Memory improvement by post-trial injection of lidocaine into the tuberomammillary nucleus, the source of neuronal histamine

Brain histamine is exclusively contained within and released from neurons whose cell bodies are clustered in the tuberomammillary nucleus (TM) of the posterior hypothalamus. This experiment examined the effects of a transient inactivation of the TM on inhibitory avoidance learning. Rats with chronically implanted cannulae were tested on a 1-trial step-through avoidance task. Immediately following training, the rats received unilateral intra-TM infusions (0.5 microl) of lidocaine (5 or 20 microg). Control groups included vehicle-injected rats and a group given an injection of 20 microg lidocaine 5 h after training. When tested 24 h later, rats treated with 20 microg lidocaine exhibited longer step-through latencies than vehicle-treated controls, indicative of superior learning of the task. The failure of the delayed post-trial injection of lidocaine to significantly influence step-through latencies indicates that the compound influenced learning by modulating memory storage processes rather than by acting on performance variables during retrieval of the task. Thus, inactivation of the TM by lidocaine can exert facilitatory effects on mnemonic processing, which might be related to a temporary reduction of histaminergic activity during the early phase of memory consolidation.

Development of the histaminergic neurons and expression of histidine decarboxylase mRNA in the zebrafish brain in the absence of all peripheral histaminergic systems

The histamine-storing neural system in adult and developing zebrafish (Danio rerio) was studied with immunocytochemical and chromatographical methods. Furthermore, the gene for histidine decarboxylase was partially cloned and its expression mapped with in situ hybridization. The histamine-storing neurons were only seen in the caudal hypothalamus, around the posterior recess of the diencephalic ventricle. Almost all parts of the brain, except the cerebellum, contained at least some histamine-immunoreactive fibres. The ascending projections had the rostral part of the dorsal telencephalon as a major target. Descending projections terminated in the torus semicircularis, central grey and inferior olive. A prominent innervation of the optic tectum, which has not been reported in other fish, was seen. The in situ hybridization gave a strong signal in cells with the same anatomical position as the histamine-immunoreactive neurons. The first histamine-immunoreactive neurons appeared in the ventral hypothalamus at about 85 h post-fertilization, and at 90 h, immunoreactive fibres terminated in the dorsal telencephalon. The embryonic histamine production described in mammals was lacking in this species. Both immunocytochemical and chromatographical studies indicated that histamine is absent in all other parts of the zebrafish body, and no specific hybridization was seen in any other part of the fish than the hypothalamus. The zebrafish could therefore be a very useful model for pharmacological in vivo studies of the histaminergic system of the brain, since the powerful peripheral actions of histamine should be lacking in this species.

Measuring reward with the conditioned place preference paradigm: a comprehensive review of drug effects, recent progress and new issues

This review gives an overview of recent findings and developments in research on brain mechanisms of reward and reinforcement from studies using the place preference conditioning paradigm, with emphasis on those studies that have been published within the last decade. Methodological issues of the paradigm (such as design of the conditioning apparatus, biased vs unbiased conditioning, state dependency effects) are discussed. Results from studies using systemic and local (intracranial) drug administration, natural reinforcers, and non-drug treatments and from studies examining the effects of lesions are presented. Papers reporting on conditioned place aversion (CPA) experiments are also included. A special emphasis is put on the issue of tolerance and sensitization to the rewarding properties of drugs. Transmitter systems that have been investigated with respect to their involvement in brain reward mechanisms include dopamine, opioids, acetylcholine, GABA, serotonin, glutamate, substance P, and cholecystokinin, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs, or indirectly, by studying the effects of these drugs on the reward induced by other drugs. For a number of these transmitters, detailed studies have been conducted to delineate the receptor subtype(s) responsible for the mediation of the observed drug effects, particularly in the case of dopamine, the opioids, serotonin and glutamate. Brain sites that have been implicated in the mediation of drug-induced place conditioning include the 'traditional' brain reward sites, ventral tegmental area and nucleus accumbens, but the medial prefrontal cortex, ventral pallidum, amygdala and the pedunculopontine tegmental nucleus have also been shown to play important roles in the mediation of place conditioning induced by drugs or natural reinforcers. Thus, although the paradigm has also been criticized because of some inherent methodological problems, it is clear that during the past decade place preference conditioning has become a valuable and firmly established and very widely used tool in behavioural pharmacology and addiction research.

The H1- and H2-histamine blockers chlorpheniramine and ranitidine applied to the nucleus basalis magnocellularis region modulate anxiety and reinforcement related processes

This study examined the effects of the H1-antagonist chlorpheniramine and the H2-antagonist ranitidine on reinforcement and anxiety-parameters following unilateral injection into the vicinity of the nucleus basalis magnocellularis (NBM). In Experiment 1, rats with chronically implanted cannulae were injected with chlorpheniramine or ranitidine (each at doses of 0.1, 1, 10 and 20 microg) and were placed into one of four restricted quadrants of a circular open field (closed corral) for a single conditioning trial. During the test for conditioned corral preference, when provided a choice between the four quadrants, only those rats injected with 10 or 20 microg chlorpheniramine spent more time in the treatment corral, indicative of a positively reinforcing action. None of the other doses of chlorpheniramine or of the H2-antagonist influenced rats' preference behavior. In Experiment 2, the elevated plus-maze (EPM) was used to gauge possible anxiolytic or anxiogenic effects of intra-basalis injection of chlorpheniramine or ranitidine (each at doses of 0.1, 1, 10 and 20 microg). A single injection of chlorpheniramine at 0.1 or 20 microg as well as ranitidine at 20 microg was found to exert anxiolytic-like effects in the EPM. Both compounds elevated the time spent on the open arms and increased scanning over the edge of an open arm. None of the other doses of the H1- and H2-antagonist influenced rats' behavior in the EPM. In sum, these findings show that H1- and H2-receptor antagonists differentially modulate reinforcement and fear-related processes in the NBM and thus, provide the first evidence for a behavioral relevance for the histaminergic innervation of this brain site.