Hippocampal histamine receptors and conflictive exploration in the rat: studies using the elevated asymmetric plus-maze

Involvement of the dorsal hippocampal dopamine D2 receptors in histamine-induced anxiogenic-like effects in mice

Anxiety-related behaviors increase histamine and dopamine release in the brain. On the other hand, central histamine counteracts reward and reinforcement processes mediated by the mesolimbic dopamine system. We investigated the effects of the histaminergic system and dopamine D2 receptors agents and their interactions on anxiety-related behaviors using the elevated plus-maze (EPM). The intra-hippocampal (Intra-CA1) microinjection of histamine (10 μg/mouse) decreased the percentage of open arm time (%OAT) and open arm entries (%OAE) but not the locomotor activity, indicating an anxiogenic-like response. Quinpirole (0.5 and 2 μg/mouse) or sulpiride (0.3 and 1 μg/mouse) when injected into the dorsal hippocampus also induced anxiety-like behavior, however, the drugs reversed the anxiogenic response induced by the effective dose of histamine (10 μg/mouse). Taken together and under the present experimental design, our results indicate that activation of the dorsal hippocampal histaminergic receptors causes anxiety-like behaviors altered by dopamine D2 receptor agonist and antagonist. Histamine can decrease dopaminergic tone in the dorsal hippocampus through decreasing the endogenous dopamine release, whereas quinpirole does the same via the postsynaptic DA receptors' activation. Sulpiride however renders the same effect through autoreceptors' blockade and potentiated dopamine transmission. Thus, quinpirole and sulpiride seem to compensate the effects of the intra-CA1 injection of exogenous histamine, and tend to exert anxiolytic effects in the presence of histamine.

Histamine and motivation

Brain histamine may affect a variety of different behavioral and physiological functions; however, its role in promoting wakefulness has overshadowed its other important functions. Here, we review evidence indicating that brain histamine plays a central role in motivation and emphasize its differential involvement in the appetitive and consummatory phases of motivated behaviors. We discuss the inputs that control histaminergic neurons of the tuberomamillary nucleus (TMN) of the hypothalamus, which determine the distinct role of these neurons in appetitive behavior, sleep/wake cycles, and food anticipatory responses. Moreover, we review evidence supporting the dysfunction of histaminergic neurons and the cortical input of histamine in regulating specific forms of decreased motivation (apathy). In addition, we discuss the relationship between the histamine system and drug addiction in the context of motivation.

Withdrawal from repeated administration of morphine alters histamine-induced anxiogenic effects produced by intra-ventral hippocampal microinjection

In the present study, the influence of withdrawal from repeated administration of morphine on intra-ventral hippocampal microinjection of histamine-induced anxiety-like behavior was investigated in male Wistar rats. Three days subcutaneous administration of morphine (5-10 mg/kg) followed by five days free of the drug decreased the percentage open arm time and the percentage open arm entries. Intra-ventral hippocampal administration of histamine (2.5-7.5 microg/rat) decreased percentage open arm time and percentage open arm entries. Intra-ventral hippocampal histamine-induced anxiogenic effect was reversed in animals that had previously received the three days morphine (7.5 mg/kg) followed by five days free of the drug. Intra-ventral hippocampal administration of pyrilamine (5-20 microg/rat) or ranitidine (10-40 microg/rat) decreased percentage open arm time and percentage open arm entries. Pyrilamine- or ranitidine-induced anxiogenic effect was not changed in animals that had previously received the three days morphine (7.5 mg/kg) followed by five days free of the drug. Intra-ventral hippocampal injections of clobenpropit increased percentage open arm time. The percentage open arm time and percentage open arm entries were decreased in the morphine-treated animals compared with non-morphine-treated controls. Percentage open arm entries and locomotor activity was reduced with some doses of clobenpropit. It can be concluded that the histamine system is involved in anxiety-like behavior, and repeated injections of morphine followed by five days free of the drugs interact with histamine receptor mechanism.

The role of histamine on cognition

Histamine was intensively studied at the beginning of the 20th century because of its important role in allergic and inflammation processes. In those days it was very difficult that researchers could envisage another impacting function for the imidazolamine in the living systems. Once the imidazolamine was found located in neuron compartment in the brain, increasing evidence supported many regulatory functions including its possible role in memory and learning. The specific participation of histamine in cognitive functions followed a slow and unclear pathway because the many different experimental learning models, pharmacologic approaches, systemic and localized applications of the histamine active compounds into the brain used by researchers showed facilitating or inhibitory effects on learning, generating an active issue that has extended up to present time. In this review, all these aspects are analyzed and discussed considering the many intracellular different mechanisms discovered for histamine, the specific histamine receptors and the compartmentalizing proprieties of the brain that might explain the apparent inconsistent effects of the imidazolamine in learning. In addition, a hypothetical physiologic role for histamine in memory is proposed under the standard theories of learning in experimental animals and humans.

Motivated exploratory behaviour in the rat: The role of hippocampus and the histaminergic neurotransmission

Exploration is one of most basic adaptive behavioural responses, giving the animal an important evolutionary advantage to survive in a changing environment. Inspection of novel environments might be come with motivated exploratory behaviour. In spite that this type of exploration in the rat is known for many years, little attention has been given to the intrinsic mechanisms or the brain structures that are involved in. In the present work the hippocampus, the neurotransmitter histamine, and the geometrical features of novel objects were examined in a model of conflictive and non-conflictive exploration in the rat which evaluates incentive-motivated exploration. Young adult intact rats were tested in a neutral non-conflictive behavioural activity detector (OVM), with (eOVM) or without (sOVM) novel objects. Three different objects were used: a box, a toy duck, and a tower. Results show that animals decrease its general motor activity (horizontal, ambulatory and non-ambulatory activity) in favor to exploration of the objects. Motivated exploration was not the same for all three objects. Rats explored significantly more the Tower and the Box objects than the Duck item. Behavioral patterns of hippocampus-implanted rats showed decreased scores in motor activity but maintained the difference in the relation of "without/with objects" exploration. When hippocampus-implanted rats were tested in a conflictive exploration device (the elevated asymmetric plus-maze), exploration of the No Wall arm, considered the most fear-inducing environment, was significantly more explored by the animal when the tower object was positioned at its end than when it was absent. Microinjection into the ventral hippocampus of histamine abolished this motivated exploratory response. Pre-treatment with pyrilamine, and not with ranitidine, was effective to block the inhibitory effect of histamine on the object motivated exploration. Results confirm that the hippocampus is involved on incentive motivated exploration, and suggest that histamine is part of an analyzing neuronal circuit of novelty incentivating behavioural responses in rats.

Histaminergic H1 receptors mediate L-histidine-induced anxiety in elevated plus-maze test in mice

The central histaminergic system is reported to mediate behavioural, hormonal and physiological homeostasis of living organisms. Recent reports indicate its prominent role in various neurobehavioural disorders such as depression and psychosis. This study evaluated the effect of activation of the central histaminergic system in anxiety-like conditions, using the elevated plus-maze test in mice, and elucidated the role of different histaminergic receptors mediating such effects. Peripheral administration of L-histidine (L-His), in a dose-dependent manner, significantly decreased the exploration time in open arms and number of entries into open arms without modifying the number of entries into closed arms of the elevated plus-maze, indicating anxiogenesis. Further, such effects of central histamine were significantly attenuated, in a dose-dependent manner, by pretreatment with pyrilamine (H1 receptor antagonist). Pretreatment with either zolantidine (H2 receptor antagonist) or thioperamide (H3 receptor antagonist), however, failed to attenuate the L-His-induced anxiogenesis. Our results indicate that anxiogenic effects of central histaminergic system appear to be mediated prominently by activation of H1 receptors.

Behavioral neuroscience, exploration, and K.C. Montgomery's legacy

Exploration is a key animal and human behavior. Kay C. Montgomery (1921-1956) has made an important contribution to behavioral neuroscience of exploration, as well as motivation and learning. His works have many important applications to current experimental models of stress, fear and memory, continuing to influence research in this field. This paper, dedicated to the 85th anniversary of Montgomery's birth, and 50 years since his tragic death, summarizes Montgomery's contribution to behavioral neuroscience, and discusses its current importance for further progress in this field. It is aimed at neuroscientists with strong interests in both theory of animal exploration and motivation, and the history of behavioral neuroscience.

The effects of histaminergic agents in the ventral hippocampus of rats in the plus-maze test of anxiety-like behaviours

It has been suggested that histamine have modulatory influence on anxiety-related behaviours both in animals and humans. Ventral hippocampus (VHC) may also be an important brain site in the modulation of fear or anxiety. In the present study, the effects of histaminergic agents on anxiety-related behaviours in the rats, using plus-maze test has been investigated. Intra-VHC administration of histamine (2.5, 5 and 7.5 microg/rat) decreased %OAT and %OAE but not locomotor activity, showing an anxiogenic response. Pretreatment of animals with either pyrilamine, a H1 receptor antagonist (10 microg/rat), or ranitidine, a H2 receptor antagonist (10 microg/rat) reverse anxiogenic response of histamine (2.5, 5 and 7.5 microg/rat). However, intra-VHC microinjection of higher doses of pyrilamine (40 microg/rat) or ranitidine (20 and 40 microg/rat) alone increased anxiety-like behaviours in rats. Our results showed that histamine may modulate anxiety-like behaviours via H1 and H2 receptors in the ventral hippocampus of the rats.

The effects of histaminergic agents in the central amygdala of rats in the elevated plus-maze test of anxiety

Reports indicate that histamine and histaminergic agents can change anxiety-related behaviours in both animals and humans. The amygdala is an important brain site in the modulation of fear or anxiety. In the present study, we investigated the effects of intracentral amygdala microinjection of histaminergic agents on anxiety-related behaviours in rats, using the elevated plus-maze test of anxiety. Intracentral amygdala administration of histamine (0.01-0.5 microg/0.5 microl bilateral) decreased %open armtime and % open arm entries, but not locomotor activity, showing an anxiogenic response. Intracentral amygdala microinjection of pyrilamine (H1 receptor antagonist) and ranitidine (H2 receptor antagonist) (both at 1-20 microg/0.5 microl bilateral) did not change anxiety-related parameters in our experiments. In another series of experiments, histamine (0.5 microg/0.5 microl bilateral) was coadministrated with pyrilamine and ranitidine (both at 1-20 mg/0.5 microl bilateral). The results showed that pyrilamine but not ranitidine could significantly reverse the anxiogenic effect of histamine at doses of 10 and 20 microg/0.5 microl bilateral. The results suggest that histamine may modulate anxiety via H1 but not H2 receptors in the rat central amygdala.

Glutamic acid and histamine-sensitive neurons in the ventral hippocampus and the basolateral amygdala of the rat: functional interaction on memory and learning processes

The possibility of a functional interaction between the amygdala and the ventral hippocampus during learning of a conditioned avoidance response when both brain structures are chemically stimulated with glutamic acid and/or histamine receptor antagonists (pyrilamine, H1-histamine antagonist and ranitidine, H2-histamine receptor antagonist) was studied in rats. Adult male rats were stereotaxically implanted with guide cannulae into the basolateral amygdala (A) and the ventral hippocampus (H). Seventy-two hours after the implant, rats were microinjected with 1 microl of saline solution, 10 nmol glutamic acid or 45 nmol of histamine receptor antagonists in several brain structures combinations. These combinations were: HsalAsal; HmsgAmsg; HmsgAsal; HsalAmsg; HpyrAmsg; HmsgApyr; HranAmsg and HmsgAran. Five minutes after the injection, rats were subjected to a learning task which consisted to avoid an electric shock applied to the animal's feet when an ultrasonic tone of 40 kHz is on for 30 s. Results showed that the simultaneous application of glutamic acid into hippocampus and amygdala interfered with the latency to escape and memory consolidation process. Stimulation with glutamic acid alone into the hippocampus or into the amygdala (HsalAmsg and HmsgAsal groups) interfered slightly with latency but impaired the consolidation process. Blocking the H1-histamine receptors of the amygdala affected slightly latency and efficiency of learning, meanwhile the blocking of H2-histamine receptors interfered with both parameters. Blocking H1- and H2-histamine receptors of the hippocampus significantly impaired latency and efficiency of learning of rats stimulated with glutamic acid into the amygdala. In conclusion, the experimental evidence suggests that hippocampal glutamic acid-neurons functionally interact with histamine-neurons in the basolateral amygdala to modulate memory and learning process.

Blockade of histamine H2 receptors of the periaqueductal gray and inferior colliculus induces fear-like behaviors

Electrical and chemical stimulation of the dorsal periaqueductal gray matter (dPAG) and the inferior colliculus (IC) induces escape behavior, usually accompanied by autonomic responses and antinociception. Recently, we presented evidence for a tonic inhibitory control exerted by H(2) histamine receptors on defensive behaviors generated in these midbrain tectum sites. Since treatments of these areas that elicit the defensive behavior repertoire frequently also have anxiogenic effects, we here used the elevated plus-maze (EPM) test for assessing the effects of microinjections of histamine (5-40 nmol), dimaprit (5-10 nmol) and ranitidine (10-30 nmol) into either dPAG or IC, which have a relative abundance of histamine-containing cells and histaminergic receptors. Dimaprit is an agonist and ranitidine is an antagonist of H(2) histamine receptors. Immediately after the injections, the animals were submitted to the EPM test. Whereas dPAG injections of dimaprit had no behavioral effects, histamine (40 nmol) caused a significant reduction in exploratory activity. On the other hand, ranitidine alone or following saline had aversive-like effects in both structures, i.e. reduced open arm, but not closed arm, entries. This pattern is usually interpreted as representing an anxiogenic effect. These effects were more pronounced after injection into dPAG than into IC. Freezing, the most prominent effect produced by ranitidine, was significantly inhibited by histamine as well as dimaprit. Thus, H(2) receptor blockade has fear-like action in the midbrain tectum with predominance in the dPAG. Such an action can be understood as a concomitant of defensive behavior, which has been shown to be a consequence of H(2) receptor antagonism in both dPAG and IC. The functional significance of the different effects of H(2) receptor blockade in dPAG and IC is discussed in the light of the probable distinct roles of these structures in the organization of defensive behavior.

Further evidence for the involvement of histamine H2 receptors in the control of defensive behaviour generated in the midbrain tectum

The dorsal periaqueductal grey matter (dPAG) and inferior colliculus (IC) have been implicated in the control of defence reactions. Electrical and chemical stimulation of these structures induces escape behaviour, usually accompanied by autonomic responses and decreased pain sensitivity. Recently, we presented evidence for an involvement of histamine in the generation and organization of such defensive reactions in the midbrain tectum. In this study we have used the open field test to assess the effects of microinjections of histamine (40 nmol), dimaprit (10 nmol) and ranitidine (50 nmol) into the midbrain tectum of rats. Dimaprit is an agonist and ranitidine an antagonist of H2 histamine receptors. Immediately after the injections the animals were tested in an open field for 60 min. In an additional groups of rats, dimaprit was injected 15 min before the microinjections of ranitidine into either the dPAG or the IC. The results show that whereas histamine preceded by saline did not cause any apparent behavioural changes, ranitidine led to a behavioural reaction, with clear signs of fear, which was blocked by previous injection of dimaprit. Injections of only dimaprit had no apparent effects. The present results suggest that H2 receptors may be involved in the control of defensive behaviour following activation of the neural substrates of fear in the dPAG and IC.

Role of glutamate receptors in the nucleus accumbens on behavioural responses to novel conflictive and non-conflictive environments in the rat

The possible role of glutamic acid locally applied into the nucleus accumbens on exploratory behaviours measured in 'conflictive' and 'non-conflictive' environments was studied in adult male rats. As a model of conflictive environment, the elevated asymmetric-plus maze (APM) was used. As a model of a non-conflictive environment, a modified holeboard enriched with an object (OVM) was used. In order to characterize the possible glutamic acid receptors involved, the following antagonists were also used: AP3 (antagonist of the metabotropic glutamic acid receptor), AP7 (antagonist of NMDA glutamic acid receptor, and CNQX (antagonists of kainate/AMPA glutamic acid receptor). Results showed that injection of glutamic acid into the nucleus accumbens induced in the APM a decrease of exploration and an increase of the permanency score (non-exploratory behaviours) of the 'High and Low wall' arm. However, in the 'Two High Walls' arm, glutamic acid decreased permanency. In the OVM, no major changes in the motor activity were observed with glutamic acid. Nevertheless, the vertical activity (an index of rearing) and head-dipping were inhibited by the amino-acid treatment. In the APM, the decrease of exploration induced by glutamic acid was blocked by all three receptor antagonists. In the non-exploratory behaviours, the facilitatory effect observed in the 'High and Low walls' arm was blocked only by AP7 and CNQX. The inhibitory action of glutamic acid on the permanency score in the 'Two High Walls' arm was not blocked by the receptors antagonists. In the OVM, AP7 and CNQX were effective in blocking the inhibition of glutamic acid on the vertical activity, but in head-dipping, only AP3 and CNQX were able to block the effect of the amino acid on this behaviour. In conclusion, the present results are compatible with the concept that glutamatergic input fibres to the nucleus accumbens modulate the expression of exploratory behaviour induced by novelty in conflictive and non-conflictive conditions.

Exploratory behaviour after intra-accumbens histamine and/or histamine antagonists injection in the rat

The possible role of histamine locally applied into the nucleus accumbens on exploratory behaviours measured in 'conflictive' and 'non-conflictive' environments was studied in adult male rats. It was assumed that in conflictive environments the brain mechanisms involved in processing incentive environmental clues (novelty) were interacting with mechanisms involved in the processing of fearful or 'anxiogenic' environmental clues. As a model of conflictive environment, the elevated asymmetric-plus maze (APM) was used. As a model of a non-conflictive environment, a modified holebroad enriched with an object (OVM) was used. The exploration score in any of the arms of the APM was considered an approximate index of exploratory motivation. The permanency score (non-exploratory behaviours) was considered an inverse approximate index of emotionality. Other variables such as the frequency of entries into any arm, the latency time and central activity were also measured. In the OVM, the general motor activity and head-dipping, vertical rearing and focalized exploration were measured. Results show that histamine in the APM had a dual effect. On the one hand, an increase of exploration was observed in those arms considered more 'anxiogenic'. On the other hand, a decrease in exploration occurred in one of the arms considered less 'anxiogenic'. No changes of permanency was observed in the 'anxiogenic' arms, and a decrease of permanency took place in the arms considered less 'anxiogenic'. In the OVM, histamine did not change the overall motor activity, but head-dipping was inhibited by the imidazolamine treatment. Histamine effects on exploration parameters were counteracted by pre-treatment with H1- and H2-histamine antagonists. Nevertheless, some behaviours were not blocked by the histamine receptor antagonists. The present results give support to the role of the nucleus accumbens in the exploratory motivation mechanisms and suggest that histamine might be an endogenous regulator.

Behavioral profiles displayed by rats in an elevated asymmetric plus-maze: effects of diazepam

When rats are exposed to unknown environments where novelty and fear-inducing characteristics are present (conflictive environments), some specific behaviors are induced and exploration is apparently modulated by fear. In our laboratory, a new type of plus-maze was designed as a model of conflictive exploration. The maze is composed of four arms with different geometrical characteristics, differing from each other by the presence or absence of walls. The degree of asymmetry was as follows: NW, no wall arm; SW, a single high wall present; HL, a low and a high wall present, and HH, two high walls present. The four arms were arranged at 90 degrees angles and the apparatus was called the elevated asymmetric plus-maze (APM). The purpose of the present study was to assess the behavioral profile of rats exposed for a single time to the APM with or without treatment with benzodiazepine. Increasing doses of diazepam were injected intraperitoneally in several groups of male, 90-day-old Holtzman rats. Distilled water was injected in control animals. Thirty minutes after treatment all rats were exposed singly to a 5-min test in the APM. Diazepam induced a biphasic modification of exploration in the NW and SW arms. The increase in the exploration score was evident at low doses of diazepam (0.25-1.0 mg/kg body weight) and the decrease in exploration was found with the higher doses of diazepam (2.0-3.0 mg/kg body weight). Non-exploratory behaviors (permanency) were not affected by benzodiazepine treatment. In the HL arm, exploration was not modified but permanency was increased in a dose-dependent manner. In the HH arm, exploration and permanency were not affected. Results are compatible with the idea that exploration-processing mechanisms in conflictive environments are modulated by fear-processing mechanisms of the brain.