Effect of neurotoxic lesions in the mammillary bodies on the distribution of brain histamine

Effects of alpha-adrenoceptor agonists and antagonists on histamine-induced impairment of memory retention of passive avoidance learning in rats

The effect of alpha-adrenoceptor agents on the impairment induced by histamine was measured for memory retention of passive avoidance learning in rats. Post-training intracerebroventricular (i.c.v.) injection was carried out in all the experiments. Histamine (5, 10 and 20 microg/rat) reduced, while a histamine H(1) receptor antagonist, chlorpheniramine (0.1, 1 and 10 microg/rat), increased memory retention. The histamine H(2) receptor antagonist, ranitidine (0.1, 1, 10 and 20 microg/rat), did not elicit any response in this respect. Different doses of chlorpheniramine but not ranitidine reversed the histamine-induced impairment of memory. Clonidine and prazosin decreased, but yohimbine and phenylephrine increased, memory retention. Yohimbine decreased the inhibitory response to histamine. Phenylephrine, clonidine and prazosin did not alter the histamine effect. It is concluded that a histamine-induced impairment of memory retention through histamine H(1) receptors and an alpha(2)-adrenoceptor mechanism may be involved in the histamine response.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

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.

Enhanced learning by posttrial injection of H1-but not H2-histaminergic antagonists into the nucleus basalis magnocellularis region

The aim of this study was to examine the effects of histaminergic antagonists on memory upon injection into the region of the nucleus basalis magnocellularis (NBM). In experiment 1, rats with chronically implanted cannulae were trained on the uphill avoidance task, which involves a punishment of a high-probability turning response on a tilted platform (negative geotaxis). Immediately after the training trial, that is, after a tail shock was administered upon performing the response, rats received one microinjection (0.5 microliter) of H1-receptor blocker chlorpheniramine (dose range 0.1 to 20 microgram) or the H2-receptor blocker ranitidine (same dose range) or saline into the NBM region. When tested 24 h later, rats treated with chlorpheniramine (20 micrograms) had significantly longer uphill latencies than vehicle controls and ranitidine-treated animals, indicative of superior learning of the avoidance response. In experiment 2, a test for possible proactive effects of posttrial chlorpheniramine on performance during the retention trial was performed. Animals were injected with either 20 micrograms chlorpheniramine or saline immediately after the training trial of the uphill task. One chlorpheniramine control group was treated with a delay of 5 h. Additional groups which received chlorpheniramine or vehicle after the training trial but no trail shock were included. When tested 24 h later, rats injected with 20 micrograms chlorpheniramine again exhibited significantly longer uphill latencies than did vehicle-injected rats. Retention latencies for the rats of the chlorpheniramine 5-h delayed group did not differ from those of the vehicle-injected rats, ruling out proactive effects of chlorpheniramine on performance. In summary, the histaminergic H1-blocker chlorpheniramine can enhance mnemonic functioning in addition to its reinforcing effects upon NBM injection as reported previously.

Stress-induced release of anterior pituitary hormones: effect of H3 receptor-mediated inhibition of histaminergic activity or posterior hypothalamic lesion

The effect of stress- or lipopolysaccharide (LPS) endotoxin-induced release of ACTH, beta-endorphin (beta-END) and prolactin (PRL) was investigated in two groups of conscious male rats: (1) Rats pretreated with different H3 receptor agonists, which inhibit neuronal histamine (HA) synthesis and release, and (2) rats with bilateral posterior hypothalamic lesion, which destroys the histaminergic perikarya exclusively localized in the mammillary nuclei. The H3 receptor agonists R(alpha)methyl-HA, BP 2-94 or imetit injected intraperitoneally (ip) had no effect on basal secretion of ACTH or PRL but inhibited the ACTH and PRL responses to restraint stress and the ACTH response to LPS endotoxin. LPS had no effect on PRL secretion. The inhibitory effect of the agonists was prevented by prior ip administration of the H3 receptor antagonist thioperamide. Bilateral lesion of the posterior hypothalamus inhibited the ACTH, beta-END and PRL responses to restraint stress, ether stress and LPS endotoxin, whereas sham operation had no effect compared to nonoperated control rats. In addition, posterior hypothalamic lesion inhibited the PRL response but not the ACTH and beta-END responses to activation of serotonergic neurons induced by ip administration of the 5-HT precusor 5-hydroxytryptophan in combination with the 5-HT re-uptake inhibitor fluoxetine. Thus, serotonergic pathways were not damaged by the lesions. The present results support our previous findings that inhibition of neuronal HA synthesis by alpha-fluoromethylhistidine as well as blockade of H1 or H2 receptors inhibit the ACTH, beta-END and PRL responses to stress and LPS endotoxin and further substantiate an important role of histaminergic neurons in the mediation of the stress-induced release of pituitary stress hormones. Furthermore, in accordance with our previous findings, the lesion experiments indicated the existence of an interaction between the histaminergic and serotonergic system in regulation of the stress- and LPS-induced PRL release.

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.

Unilateral lesion of dorsal hippocampus enhances reinforcing lateral hypothalamic stimulation in the contralateral hemisphere

Whereas convincing evidence exists for an important role of the hippocampus in mechanisms underlying memory and encoding of location in space, the contribution of the hippocampus to the system underlying central processes of reinforcement is less well established. Scattered data suggesting that hippocampal ablation increases the effectiveness of positive reinforcers have alternatively been interpretated in terms of general and unspecific behavioral disinhibition, which results in higher levels of activity and rates of responding. In the present experiment, 22 Wistar rats were either given a neurotoxic or a sham lesion in the CA1 region of the hippocampus, and the effect on lateral hypothalamic self-stimulation behavior was assessed. To control for nonspecific performance effects rates of lever pressing were assessed ipsi- and contralateral to the lesioned hemisphere as well as under condition of extinction (current set to zero). Following the neurotoxic lesion the animals displayed significant higher rates of self-stimulation at the electrode sites in the hypothalamus situated contralateral but not ipsilateral to the hemisphere with the lesion compared with controls. The increase in self-stimulation commenced on the third day postlesion and was maintained over the 8 days of testing. The lesion did not change the animals' behavior under extinction. Thus, the hippocampal lesion led to an amplification of rewarding lateral hypothalamic self-stimulation behavior, indicative of a lesion induced disinhibition of the brain's reinforcement system.

Lateralized attenuation of hypothalamic self-stimulation after injecting histamine synthesis blocker alpha-FMH into the E2 tuberomammillary subnucleus

The tuberomammillary nucleus (TM), located in the posterior hypothalamic region, is the only known source of neuronal histamine. Unilateral lesions in the rostroventral part of this nucleus enhanced ipsihemispheric lateral hypothalamic self-stimulation behavior, suggesting that this region exerts inhibitory control over the neuronal systems related to reward or reinforcement processes. To examine whether the amplification of reinforcing stimulation following lesions of histamine synthesizing neurons is indeed histamine mediated, we blocked histamine synthesis unilaterally by injection of 200 microg alpha-fluoromethylhistidine into the E2 region of the TM, and assessed the effects on electrical self-stimulation behavior in the lateral hypothalamus (LH) of rats. Based on the finding that TM lesions facilitated such self-stimulation behavior, we hypothesized that this treatment would have similar effects. Unexpectedly, there was a sharp decrease in the rate of ipsihemispheric lateral hypothalamic self-stimulation following the injection of alpha-FMH compared to the contralateral hemisphere of treated animals as well as compared to the vehicle group. Response rates were most strikingly attenuated 1 h postinjection, but remained low over the whole 7 days of testing. Opposite behavioral effects of TM lesions and alpha-FMH application have been reported previously, and the effectiveness of alpha-FMH in reducing brain histamine levels is known to differ between brain regions. The fact that the alpha-FMH injection affected self-stimulation only in the ipsilateral hemisphere rules out an interpretation of the results in terms of unspecific effects of the treatment on arousal and other performance variables, and, instead, indicates a functional interaction with a subsystem linked to lateral hypothalamic reinforcement processes.

Effects of SDZ ENA 713, novel acetyl cholinesterase inhibitor, on learning of rats with basal forebrain lesions

1. The effects of SDZ ENA 713, a novel acetyl cholinesterase inhibitor, on rat learning was studied using a step-down avoidance paradigm. 2. Injection of ibotenic acid into the caudolateral part of the basal forebrain (BF) innervating cholinergic neurons to the cerebral cortex, resulted in an increase in the number of trials required to obtain 300-second-latency, and also a decrease in the latency period after attaining 300-second-latency. 3. It is shown that the BF-lesioned rats are impaired in both acquisition and retention of learning. 4. Intraperitoneal injection of 0.10-0.05 mg/kg/day SDZ ENA 713 to the BF-lesioned rats showed amelioration of the learning impairment, with a decreased number of trials required to obtain 300-second-latency as well as an increase in the latency time after repeated training. 5. These results indicate that SDZ ENA 713 improves acquisition and retention impairment in BF-lesioned rats, and that this drug may be useful for demented patients with cholinergic dysfunction, such as Alzheimer's disease.

Effects of neurotoxic lesions in the posterior hypothalamic region on psychomotor activity and learning

Histamine (HA) acts as a neurotransmitter and/or neuromodulator in mammalian brain. Central HA has been found to be involved in the regulation of behavioral, cognitive, neurovegetative, neuroendocrine and neuroimmune functions. In this study we have evaluated psychomotor activity (PMA) and passive avoidance behavior (PAB) in rats with bilateral neurotoxic lesions in the posterior hypothalamic region (PHR) (L), where histaminergic neurons are located, and in sham-operated rats (S), two weeks after neurosurgery. In an open-field paradigm, lesioned rats showed higher PMA scores than sham-operated animals. However, L rats exhibited a significant decrease in PMA on consecutive days (motor habituation) similar to that found in S rats. In a maze paradigm, in which the animals had to learn to stay on a neutral platform in order to avoid a 1.5 mA electric footshock during 10 trails, no significant differences were observed between L and S rats on the task performance. According to the present results, it seems that bilateral neurotoxic lesions in the PHR induced hyperactivity with no apparent effects on PAB, suggesting that neuronal HA might be involved, directly and/or by influencing arousal/alertness-mediated mechanisms, in the regulation of PMA processes.

Effects of neurotoxic lesions in histaminergic neurons on brain tumor necrosis factor levels

Histamine (HA) is a biogenic amine involved in the regulation of neurovegetative, cognitive, neuroendocrine and neuroimmune functions in the central nervous system (CNS). A bidirectional interaction between the CNS and the neuroimmune system has been demonstrated in recent years. However, data concerning brain HA-cytokine interactions are scarce. In this study we have evaluated tumor necrosis factor-alpha (TNF-alpha) levels in the posterior hypothalamic region (PHR) and hippocampus (HP) of rats with: (a) bilateral ibotenic acid neurotoxic lesions in histaminergic neurons located in the PHR (I); (b) saline injections in the PHR (S); and (c) sham operation (C), two weeks after neurosurgery. The bilateral disruption of PHR HA neurons with ibotenic acid decreased TNF-alpha levels in the PHR with respect to sham-operated (C), but not saline-injected (S), rats. In contrast, hippocampal TNF-alpha concentrations were higher in lesioned rats (I) than in C and S animals. Our results indicate that the neurotoxic destruction of HA neurons decreases TNF-alpha synthesis in the hypothalamus while enhancing TNF-alpha production in the hippocampus, suggesting that neuronal HA might be involved in the regulation of the brain TNF-alpha system.

The tuberomammillary nucleus region as a reinforcement inhibiting substrate: facilitation of ipsihypothalamic self-stimulation by unilateral ibotenic acid lesions

The tuberomammillary nucleus (TM), located in the posterior hypothalamic region, consists of five subgroups and is the only known source of brain histamine. Knowledge about the function of this nucleus is still scarce. In a previous study we found an increase in the rate of ipsihemispheric hypothalamic self-stimulation following a dc lesion in the rostroventral part of this nucleus, suggesting that this region has an inhibitory action on a neuronal reward system or on the brain's reinforcement mechanism. In the present study we examined whether this facilitating effect on reinforcement was due to the destruction of fibers passing through the lesion area or of intrinsic cells, by lesioning subgroups of the TM with ibotenic acid, an excitatory amino acid, that selectively destroys neural cell bodies, leaving fibers largely intact. Following such lesions in the rostroventral part of the TM the operant response rates increased over the six days of testing when the animals stimulated themselves in the lateral hypothalamus in the hemisphere located ipsilateral but not contralateral to the lesion. No significant changes in response rate occurred following the lesion in the caudal part of the ventral TM. The results indicate that the region influenced by the lesion exerts inhibitory control over lateral hypothalamic self-stimulation, and that it is possible that histamine-containing neurons are involved in this effect.

Amplification of rewarding hypothalamic stimulation following a unilateral lesion in the region of the tuberomammillary nucleus

The tuberomammillary nucleus, a cluster of cells in the posterior hypothalamus, is the only known source of brain histamine. Although this nucleus is well described in terms of anatomy and neurochemistry, only little is known about its function. In the present study, the effect of a lesion in the region of this nucleus on intracranial self-stimulation was examined. Rats were implanted bilaterally with stimulating electrodes in the lateral hypothalamus and unilaterally with one lesion electrode in the region of this nucleus. After three days of baseline testing, half of the animals were given an electrolytic lesion. The animals were retested for six consecutive days, and thereafter weekly for another seven weeks. From the second day postlesion on, we unexpectedly found a gradual increase in response rate, which peaked on day 13 in the ipsilateral hemisphere only. Although there was no further increase over subsequent days, response rates remained elevated during the following seven weekly tests. The observed increase in lateral hypothalamic self-stimulation after an electrolytic lesion of the tuberomammillary nucleus is discussed in terms of an inhibitory system, possibly located in the region of this nucleus which, when removed by the lesion, increased reinforcing effects of the electrical brain stimulation. The fact that the effects on self-stimulation were lateralized to one hemisphere rules out an interpretation in terms of unspecific "performance" variables that could influence rate of lever pressing.

The role of histamine in the anterior hypothalamus and its functional interaction with the hippocampus on exploratory behavior in adult male rats

The possible effects of histamine (HA) locally applied into the preoptic area (POA) on hippocampus-mediated behaviors were studied in adult male rats. Animals were double-implanted unilaterally with microinjection cannulae into POA and hippocampus (HPC). In experiment 1, HA was injected into POA and pyrilamine (H1-HA antagonist) or ranitidine (H2-HA antagonist) were microinjected into the ipsilateral HPC in two different doses. In Experiment 2, HA was injected into POA and the histamine antagonists were microinjected into the contralateral HPC. Ten min later the animals were tested in an automatic monitor activity. Horizontal, ambulatory and vertical movements were measured as general motor exploratory behaviors. Contact time (in seconds) to a circular metal rack positioned in the center of the animal activity monitor was also recorded as goal-directed exploratory activity. Results of Experiment 1 showed that HA in POA exerted an inhibitory influence on general motor behaviors and also on goal-directed activity. Ipsilateral administration of HA-antagonists into HPC blocked the HA effect on behavior. Results of Experiment 2 showed that the administration of the HA-antagonists in any of the two doses used were not able to block the depressive actions on behavior caused by HA into POA. In conclusion, data suggest that POA is linked to the ipsilateral HPC through histaminergic influence to control behavioral patterns induced by novelty.

The release of endogenous histamine in distinct brain areas is modified by electrical stimulation

In anaesthetized cats, mamillary bodies, hypothalamic areas and medial amygdaloid nuclei were bilaterally superfused through push-pull cannulae and the effects of the electrical stimulation on the release of endogenous histamine were investigated. Electrical stimulation of the mamillary body increased the release of histamine in the stimulated area, as well as in the contralateral mamillary body. Electrical stimulation of the lateral hypothalamic area enhanced the histamine release in the contralateral hypothalamic area. Stimulation of the posterior hypothalamic area led to a delayed increase in the histamine release in the stimulated area. Stimulation of the medial amygdaloid nucleus reduced the release if histamine in the ipsilateral posterior hypothalamic area, while the histamine release in the contralateral lateral hypothalamic area was enhanced. The results demonstrate that electrical stimulation of distinct brain areas rich in histaminergic neurons may either increase, or decrease the release rate of histamine in the stimulated area and/or in remote brain areas.