Local Microinfusion Research Articles

Neonatal 6-hydroxydopamine lesions of the frontal cortex in rats: Persisting effects on locomotor activity, learning and nicotine self-administration

Dopaminergic innervation of the frontal cortex in adults is important for a variety of cognitive functions and behavioral control. However, the role of frontal cortical dopaminergic innervation for neurobehavioral development has received little attention. In the current study, rats were given dopaminergic lesions in the frontal cortex with local micro-infusions of 6-hydroxydopamine (6-OHDA) at 1 week of age. The long-term behavioral effects of neonatal frontal cortical 6-OHDA lesions were assessed in a series of tests of locomotor activity, spatial learning and memory, and i.v. nicotine self-administration. In addition, neurochemical indices were assessed with tissue homogenization and HPLC in the frontal cortex, striatum, and nucleus accumbens of neonatal and adult rats after neonatal 6-OHDA lesions. In neonatal rats, frontal 6-OHDA lesions as intended caused a significant reduction in frontal cortical dopamine without effects on frontal cortical 5-HT and norepinephrine. The frontal cortical dopamine depletion increased 5-HT and norepinephrine levels in the nucleus accumbens. Locomotor activity assessment during adulthood in the figure-8 maze showed that lesioned male rats were hyperactive relative to sham-lesioned males. Locomotor activity of female rats was not significantly affected by the neonatal frontal 6-OHDA lesion. Learning and memory in the radial-arm maze was also affected by neonatal frontal 6-OHDA lesions. There was a general trend toward impaired performance in early maze acquisition and a paradoxical improvement at the end of cognitive testing. Nicotine self-administration showed significant lesion×sex interactions. The sex difference in nicotine self-administration with females self-administering significantly more nicotine than males was reversed by neonatal 6-OHDA frontal cortical lesions. Neurochemical studies in adult rats showed that frontal cortical dopamine and DOPAC levels significantly correlated with nicotine self-administration in the 6-OHDA-lesioned animals but not in the controls. Frontal cortical 5-HT and 5HIAA showed inverse correlations with nicotine self-administration in the 6-OHDA-lesioned animals but not in the controls. These results show that interfering with normal dopamine innervation of the frontal cortex during early postnatal development has persisting behavioral effects, which are sex-specific.

Secretion of ghrelin from rat stomach ghrelin cells in response to local microinfusion of candidate messenger compounds: A microdialysis study

Ghrelin is produced by A-like cells (ghrelin cells) in the mucosa of the acid-producing part of the stomach. The mobilization of ghrelin is stimulated by nutritional deficiency and suppressed by nutritional abundance. In an attempt to identify neurotransmitters and regulatory peptides that may contribute to the physiological, nutrient-related regulation of ghrelin secretion, we challenged the ghrelin cells in situ with a wide variety of candidate messengers, including known neurotransmitters (e.g. acetylcholine, catecholamines), candidate neurotransmitters (e.g. neuropeptides), local tissue hormones (e.g. serotonin, histamine, bradykinin, endothelin), circulating gut hormones (e.g. gastrin, CCK, GIP, neurotensin, PYY, secretin) and other circulating hormones/regulatory peptides (e.g. calcitonin, glucagon, insulin, PTH). Microdialysis probes were placed in the submucosa of the acid-producing part of the rat stomach. Three days later, the putative messenger compounds were administered via the microdialysis probe (reverse microdialysis) at a screening dose of 0.1 mmol l − 1 for regulatory peptides and 0.1 and 1 mmol l − 1 for amines and amino acids. The rats were awake during the experiments. The resulting microdialysate ghrelin concentration was monitored continuously for 3 h (radioimmunoassay), thereby revealing stimulators or inhibitors of ghrelin secretion. Dose–response curves were constructed for each candidate messenger that significantly ( p < 0.05) affected ghrelin mobilization at the screening dose. Peptides that showed a (non-significant) tendency to affect ghrelin release at the screening dose were also given at a dose of 0.3 or 1 mmol l − 1 . Adrenaline, noradrenaline, endothelin and secretin stimulated ghrelin release, while somatostatin and GRP inhibited. Whether these agents act directly or indirectly on the ghrelin cells remains to be investigated. All other candidate messengers were without measurable effects, including acetylcholine, serotonin, histamine, GABA, aspartic acid, glutamic acid, glycine, VIP, PACAP, CGRP, substance P, NPY, PYY, PP, gastrin, CCK, GIP, insulin, glucagon, GLP and glucose.

Evidence for a critical role of entorhinal cortex at pre‐exposure for latent inhibition disruption in rats

Latent inhibition (LI), that is the decrease in conditioned response induced by the repeated nonreinforced pre-exposures to the to-be-conditioned stimulus, is disrupted by entorhinal cortex (EC) lesions. The mechanism involved in this disruption is unknown, and in particular the experimental stage (pre-exposure or conditioning) at which the integrity of EC is necessary has to be determined. The purpose of this study was to address this issue by using reversible inactivation of the EC by local micro-infusion of tetrodotoxin (TTX). TTX was infused either before the pre-exposure phase, before the conditioning phase, or before both phases. LI was unaffected in rats that received TTX before conditioning or before both pre-exposure and conditioning. In contrast, LI was disrupted in rats that received TTX before pre-exposure only. These results are discussed in the framework of LI models.

Effect of systemic and intracortical administration of phenytoin in two genetic models of absence epilepsy

1. Spontaneous 7-10 Hz spike-wave discharges (SWDs) are the electroencephalographic hallmark of absence seizures, as can be observed in WAG/Rij as well as in GAERS, two commonly used well-validated genetic rat models of absence epilepsy. A local upregulation of sodium channels within the perioral region of the primary somatosensory cortex indicated an initiation site for SWDs in WAG/Rij rats, in line with a new theory that assumes that SWDs have a cortical focal origin in the perioral region of the somatosensory cortex. We tested whether bilateral microinfusion at this focal site of the sodium channel blocker phenytoin, which is known to aggravate SWDs after systemic administration, reduces SWDs in both models. 2. WAG/Rij rats and GAERS, chronically provided with cortical EEG electrodes and bilateral cortical cannula's, were used. The EEGs were recorded before and after or systemic or bilateral infusion of phenytoin. 3. Microinfusion of phenytoin at the perioral region of the somatosensory cortex produced an immediate cessation of seizure activity in WAG/Rij rats, while systemic injection produced an increase in both genetic models. Microinfusion of the same and higher concentrations of phenytoin in GAERS at the same stereotactic coordinates showed no effect. Phenytoin was effective in GAERS 2 mm more posteriorly.4. The data suggest that both genetic models have a cortical area at which diametrically opposite effects of phenytoin can be found compared to systemic injections: a decrease after local microinfusion and aggravation after systemic administration, although the exact cortical location may be different. Moreover, a deficit in sodium channels might be an ethiological factor underlying an increased probability for the initiation of SWDs in the somatosensory cortex.

Behavioral effects of local microinfusion of pituitary adenylate cyclase activating polypeptide (PACAP) into the paraventricular nucleus of the hypothalamus (PVN)

Pituitary adenylate cyclase activating polypeptide (PACAP) has been implicated in the regulation of several autonomic and neuroendocrine functions. In the hypothalamic paraventricular nucleus (PVN), for example, PACAP-immunoreactive fibers densely innervate corticotropin-releasing hormone (CRH)-containing neurons in the medial parvocellular region, suggesting that PACAP acts to mediate stress responses. Therefore, we examined the behavioral effects of an intra-PVN PACAP injection (25 pmol) in combination with a mild stressor. PACAP or artificial cerebrospinal fluid (aCSF) was microinjected into the PVN (0.25l) and then animals were restrained or placed in their home cage for 5 min. Exploratory activity (total distance traveled) and scored behaviors (face washing, body grooming, wet dog shakes, and rearing) were observed in a familiar open field for 10 min. In animals receiving aCSF, there were no behavioral differences between restrained and unrestrained groups. For the entire 10-min observation period, animals receiving PACAP, whether restrained or not, displayed elevated face washing and body grooming with decreased locomotor activity and rearing. Among PACAP-injected animals, restrained animals displayed increased body grooming compared to unrestrained animals during the first 2 min in the open field suggesting a summation of the effects of peptide injection and stressor. The observed elevation in grooming is consistent with previous studies reporting similar increases following electrical-, NMDA-, CRH-, or stressor-induced activation of the PVN. Thus, at the level of the PVN, PACAP may act as an excitatory neuropeptide and augment behavioral responses to stressors.

Antiangiogenic therapy by local intracerebral microinfusion improves treatment efficiency and survival in an orthotopic human glioblastoma model.

Targeting active angiogenesis, which is a major hallmark of malignant gliomas, is a potential therapeutic approach. For effective inhibition of tumor-induced neovascularization, antiangiogenic compounds have to be delivered in sufficient quantities over a sustained period of time. The short biological half-life of many antiangiogenic inhibitors and the impaired intratumoral blood flow create logistical difficulties that make it necessary to optimize drug delivery for the treatment of malignant gliomas. In this study, we compared the effects of endostatin delivered by daily systemic administration or local intracerebral microinfusion on established intracranial U87 human glioblastoma xenografts in nude mice. Noninvasive magnetic resonance imaging methods were used to assess treatment effects and additional histopathological analysis of tumor volume, microvessel density, proliferation, and apoptosis rate were performed. Three weeks of local intracerebral microinfusion of endostatin (2 mg/kg/day) led to 74% (P < 0.05) reduction of tumor volumes with decreased microvessel densities (33.5%, P < 0.005) and a 3-fold increased tumor cell apoptosis (P < 0.002). Systemic administration of a 10-fold higher amount of endostatin (20 mg/kg/day) did not result in a reduction of tumor volume nor in an increase of tumor cell apoptosis despite a significant decrease of microvessel densities (26.9%, P < 0.005). Magnetic resonance imaging was used to successfully demonstrate treatment effects. The local microinfusion of human endostatin significantly increased survival when administered at 2 mg/kg/day and was prolonged further when the dose was increased to 12 mg/kg/day. Our results indicate that the local intracerebral microinfusion of antiangiogenic compounds is an effective way to overcome the logistical problems of inhibiting glioma-induced angiogenesis.

Significance of dopamine transmission in the rat medial prefrontal cortex for conditioned fear.

Previous studies have demonstrated activation of dopamine transmission in the medial prefrontal cortex (mPFC) by conditioned fear stimuli. Therefore, the present study investigated the functional significance of mPFC dopamine for a conditioned fear response to a tone. We examined the effects of inhibition or stimulation of mPFC dopamine transmission by local microinfusion of the D1/D2-receptor antagonist cis-flupenthixol or the indirect dopamine receptor agonist D-amphetamine, respectively, in a classical fear-conditioning paradigm in Wistar rats. Rats received tone-shock pairings and were later exposed to the tone alone. Freezing was used as measure of conditioned fear. Presence of the drugs in the mPFC during the tone-shock pairings did not affect freezing during subsequent presentation of the tone alone. However, when cis-flupenthixol and D-amphetamine were present in the mPFC during presentation of the tone alone, freezing to the tone was reduced. We demonstrated that the decreased freezing could be explained neither by state dependency nor infusion-induced alterations in activity. Our data indicate that mPFC dopamine transmission is important for the retrieval/expression, but not the formation, of conditioned fear. The reduction of conditioned fear by prefrontal infusion of both cis-flupenthixol and D-amphetamine may reflect normal expression of conditioned fear requires an optimal level of mPFC dopamine activity.

Importance of the paraventricular nucleus of the hypothalamus as a component of a neural pathway between the brain and the testes that modulates testosterone secretion independently of the pituitary.

We previously reported that in adult male rats, the intracerebroventricular (icv) injection of corticotropin-releasing factor (CRF) or the beta-adrenergic agonist isoproterenol (ISO) significantly inhibited the ability of human chorionic gonadotropin (hCG) to stimulate testosterone (T) secretion. The finding that this phenomenon also took place when LH release had been blocked with an LHRH antagonist suggested that icv CRF and ISO did not alter Leydig cell function by influencing the activity of pituitary gonadotrophs. We therefore proposed the existence of a neural pathway connecting the brain to the testes, whose activation by icv CRF or ISO interfered with T secretion. Based on the intratesticular injection of the transganglionic tracer pseudorabies virus, we recently identified the paraventricular nucleus (PVN) of the hypothalamus as a component of this neural link. The aim of the present work was to investigate the functional role of this brain area in mediating the ability of CRF and ISO to inhibit the ability of hCG to stimulate T secretion. We first demonstrated that local microinfusion of CRF or ISO directly into the PVN mimicked the effect of their icv injection, suggesting that the PVN does indeed represent a site of action of ISO and CRF in altering Leydig cell responsiveness to gonadotropin. In contrast, neither CRF nor ISO microinfusion into the central amygdala or the frontal cortex influenced hCG-stimulated T secretion. To further investigate the role of the PVN in ISO- and CRF-induced blunting of hCG stimulation of T, we determined the effect of icv CRF or ISO on testicular activity of rats with electrolytic lesions of the PVN. These lesions, which did not in themselves influence Leydig cell responsiveness to hCG, blocked the effect of both icv ISO and CRF on hCG-induced T release. Collectively, these results support the hypothesis that CRF- and ISO-induced activation of cells in the area of the PVN decreases the ability of gonadotropin to release T and suggests that this nucleus represents an important site of the proposed neural connection between the brain and the testes.

Modulation of taste-induced Elk-1 activation by identified neurotransmitter systems in the insular cortex of the behaving rat

Taste consumption activates the extracellular responsive kinases 1-2 (ERK1-2) and the transcription factor Elk-1 in the insular cortex (IC) of the behaving rat. ERKs activation, an obligatory step for the encoding of long- but not short-term memory, was shown to be regulated by multiple neurotransmitter systems in the IC. Here I show, by the use of local microinfusions of pharmacological agents into the IC of the behaving rat, that a set of similar systems is required for the taste-induced activation of Elk-1. N-Methyl- d-aspartate (NMDA), glutamate metabotropic (mGlu), ionotropic AMPA/kainate (AMPA), muscarinic, and dopaminergic receptors, which all contribute to the acquisition of taste memory, are also responsible for Elk-1 activation. However, blockade of the β-adrenergic transmission does not affect Elk-1 activation. I also show that the basal level of Elk-1 activation in cortex is mainly maintained by GABAergic transmission. Thus, the formation of taste memory triggers the activation of Elk-1 in the IC of the behaving rat via selected neurotransmitter and neuromodulatory systems.

Activation of Ventral Tegmental Area Cells by the Bed Nucleus of the Stria Terminalis: A Novel Excitatory Amino Acid Input to Midbrain Dopamine Neurons

We examined the role of excitatory amino acids (EAAs) in the activation of midbrain dopaminergic (DA) neurons evoked by stimulation of the ventromedial and ventrolateral (subcommissural) bed nucleus of the stria terminalis (vBNST). Using anesthetized rats and extracellular recording techniques, we found that 84.8% of ventral tegmental area (VTA) DA neurons were activated synaptically by single-pulse electrical stimulation of the vBNST. In contrast, similar stimulation did not affect the activity of presumed GABA neurons in the VTA. Three characteristic responses were observed in VTA DA neurons: short latency activation (<25 msec; 55.1% of cells), long latency activation (>65 msec; 56% of cells), and inhibition (61.8% of cells, usually followed by long latency excitation). Microinfusion of antagonists of EAA receptors (3 mm kynurenic acid, 100 microm AP-5, or 50 microm CNQX) from a micropipette adjacent to the recording electrode significantly reduced both short and long latency activations evoked in DA neurons by vBNST stimulation. Specific responses were attenuated similarly by AP-5 alone, CNQX alone, or a cocktail of AP-5+CNQX, indicating that joint activation of NMDA plus non-NMDA receptors was required. Stimulation of the vBNST by local microinfusion of glutamate increased the firing and bursting activity of VTA DA neurons. Similar microinfusion of GABA decreased bursting of VTA DA neurons without altering their firing rate. Retrograde and anterograde labeling and antidromic activation of vBNST neurons by VTA stimulation confirmed a direct projection from the vBNST to the VTA. These results reveal that inputs from the vBNST exert a strong excitatory influence on VTA DA neurons mediated by both NMDA and non-NMDA receptors.

Sexual dimorphism and developmental regulation of substantia nigra function.

The substantia nigra is an important brain nucleus involved in the expression of movement disorders and seizures. The two most common movement disorders affecting the substantia nigra, Parkinson's disease and Tourette syndrome, show gender differences and age-related onset. To assess the substrates for the gender and age specificity of substantia nigra-related disorders, we determined the functional properties of the substantia nigra gamma-aminobutyric acid (GABAA) system along its anterior-posterior axis, using localized microinfusions of muscimol (a GABAA agonist) and susceptibility to motor seizures in rats. In the substantia nigra, there are sex-specific differences in the topographic segregation and functionality of GABAA systems. In mature male rats there are two distinct regions mediating opposite effects on seizures; in female rats there is only one region that can affect seizures. In the neonatal period, the presence of circulating testosterone is essential for the development of a substantia nigra region that exerts proconvulsant effects throughout the rat's life, a unique feature of the male substantia nigra. The final maturation of the substantia nigra occurs in the peripubertal period, and is in part regulated by testosterone as well. The recognition of the existence of distinct sex- and age-specific substantia nigra features can be translated into new cures of disorders affecting the substantia nigra.

On brain lesions, the milkman and Sigmunda

Lesion studies have been of historical importance in establishing the brain systems involved in memory processes. Many of those studies, however, have been overinterpreted in terms of the actual role of each system and of connections between systems. The more recent molecular pharmacological approach has produced major advances in these two areas. The main biochemical steps of memory formation in the CA1 region of the hippocampus have been established by localized microinfusions of drugs acting on specific enzymes of receptors, by subcellular measurements of the activity or function of those enzymes and receptors at definite times, and by transgenic deletions or changes of those proteins. The biochemical steps of long-term memory formation in CA1 have been found to be quite similar to those of long-term potentiation in the same region, and of other forms of plasticity. Connections between the hippocampus and the entorhinal and parietal cortices in the formation and modulation of short- and long-term memory have also been elucidated using these techniques. Lesion studies, coupled with imaging studies, still have a role to play; with regard to human memory, this role is in many ways unique. But these methods by themselves are not informative as to the mechanisms of memory processing, storage or modulation. Trends Neurosci. (1998) 21, 423–426

Differential Behavioral Responses to Dopaminergic Stimulation of Nucleus Accumbens Subregions in the Rat

The following experiments investigated the behavioral response to local microinfusion of dopamine (DA) and selective DA agonists into the core and shell subregions of the nucleus accumbens. Rats were implanted with chronic indwelling cannulae aimed at these subregions. Two experiments were conducted. In experiment 1, the response to DA (0, 2, 5, 10 μg/0.5 μl/side), the D-1 agonist SKF-82598 (0, 0.1, 1.0 μg), the D-2/3 agonist quinpirole (0, 1, 5, 15 μg) and the D-3 preferring agonist pramipexole (0.1, 1.0, 10.0 μg) was examined in photocell activity cages. Locomotor (horizontal) and rearing (vertical) activities were measured. DA and SKF-82958 induced relatively greater increases in activity following stimulation of the shell as compared with the core. Quinpirole induced a dose-dependent suppression of activity after infusion into both sites, although the core was more sensitive to the suppressive effect than the shell. Pramipexole induced time-dependent, biphasic effects that were small in magnitude and did not differentiate between site. In experiment 2, an observation procedure was used to record behaviors (locomotion, rearing, feeding, drinking). Dopamine (0, 2, 10 μg) elicited greater increases in rearing and feeding behavior in the shell than in the core. SKF-82958 (0, 0.75 μg) enhanced locomotion and rearing to a similar extent in both subregions in this test, whereas a mixture of a low dose (0.25 μg) of the D-1 and D-2 agonists selectively induced behavioral activation in the shell. In contrast to the results in the activity cage test, quinpirole (0, 1, 5 μg) increased motor activity at the lower dose when infused into the shell but not into the core. No alterations in feeding were observed following infusion of selective agonists, and no changes in drinking were found with any of the treatments. In summary, the shell appears to be relatively more sensitive to the motor activating effects of DA agonists than the core. Moreover, circuits associated with shell may be preferentially involved in feeding.

Neurotransmitters regulating defensive rage behavior in the cat

This review of summarizes recent findings of our laboratory that have been directed at: (1) identifying the neural circuits underlying the expression and modulation of defensive rage behavior in the cat and the neurotransmitters associated with these pathways; and (2) determining which components of the circuitry are affected by alcohol administration and which significantly alter the rage mechanism. The experiments described herein incorporated a number of converging methods, which include brain stimulation, behavioral pharmacology, immunocytochemistry, retrograde tract tracing and receptor binding. For behavioral pharmacological studies, monopolar electrodes and cannula-electrodes were implanted into selected regions along the limbic-midbrain axis for electrical stimulation and local microinfusion of drugs. The findings demonstrated: (1) a direct pathway from the anterior medial hypothalamus to the dorsal periaqueductal gray (PAG) over which this response is mediated. This pathway utilizes excitatory amino acids that act upon NMDA receptors within the midbrain PAG; (2) that the region of the dorsal PAG, from which defensive rage could be elicited, receives other inputs from the basal amygdala that facilitate this response by acting upon NMDA receptors; (3) a pathway from the medial amygdala to the medial hypothalamus that also facilitates defensive rage and whose functions are mediated by substance P receptors within the medial hypothalamus; (4) that the PAG also receives enkephalinergic inputs from the central nucleus of amygdala, which act upon μ receptors, and which powerfully suppress defensive rage; and (5) that recent findings reveal that ethanol administration facilitates defensive rage by virtue of its interactions with the medial hypothalamus, its descending projection to the PAG, and possibly with NMDA receptors within this pathway.

Role of Dopamine in Behavioral Effects of Serotonin Microinjected into Rat Striatum

Bilateral local microinfusion of serotonin (5-hydroxytryptamine; 5-HT) into the ventrolateral striatum (VLS) of the rat forebrain induces quantifiable stereotyped orofacial behaviors. The role of presynaptic dopamine (DA) and structural requirements of indoles for expression of this behavioral effect and for inhibition of neuronal transport of [ 3H]DA were examined. Bilateral local injection of 6-OHDA (8 μg/side) into VLS depleted DA and markedly diminished the behavioral effects of 5-HT. Intracerebral pretreatment with the potent DA transport inhibitors GBR-12909 (6 μg/side) or nomifensine (4 μg/side) also markedly decreased behavioral responses to 5-HT. A series of indoles and tyramine were examined for ability to induce stereotypy following infusion into the VLS. Of compounds tested, only p-tyramine, 5-HT, tryptamine and L-5-hydroxytryptophan (5-HTP) elicited strong orofacial behaviors; indoles lacking a free amino group or containing other substituents were virtually inactive in vivo, and the effect of 5-HTP was prevented by systemic pretreatment with the decarboxylase inhibitor NSD-1015, indicating its required conversion to 5-HT. Uptake of [ 3H]DA (0.1 μM) into rat striatal synaptosomes was inhibited in a concentration-dependent manner in the following apparent rank-order: p-tyramine, N-methyl-5-HT, tryptamine, 5-HT, N-methyltryptamine (IC 50 = 44-718 nM), other indoles (IC 50 = 10-100 μM). These results support the conclusion that oral stereotypy induced by microinjection of 5-HT or other aromatic amines into rat VLS is mediated by local release of endogenous DA. These results extend previous findings indicating that this effect of 5-HT was not blocked by 5-HT receptor antagonists, and suggest mediation by a neuronal transport process involved in the uptake or storage of DA.

Astressin, a novel and potent CRF antagonist, is neuroprotective in the hippocampus when administered after a seizure

Corticotropin-releasing factor (CRF), the principle hypothalamic regulator of the adrenocortical axis, also functions as a neurotransmitter. In this latter role, CRF causes electrophysiological activation and epileptiform activity in various brain regions. That finding, coupled with the observation that CRF mRNA is induced in endangered brain regions following necrotic insults, suggests that the peptide might contribute to necrotic neuron loss. Supporting that, a number of studies have shown that CRF antagonists decrease ischemic or excitotoxic damage to neurons. In the present report, we demonstrate the considerable neuroprotective potential of a novel and potent CRF antagonist, astressin, against kainic acid-induced excitotoxic seizures. Intracerebroventricular infusion of the peptide both 30 min before and 10 min after seizures decreased damage in some hippocampal cell fields by as much as 84%, a magnitude of protection greater than reported for other CRF antagonists against other models of necrotic neuronal injury. Administration of astressin was done against both local microinfusion (0.035 μg) or systemic infusion (10 mg/kg body weight) of the excitotoxin; furthermore, the peptide protected even if administered only 10 min following excitotoxin exposure. This fulfills a critical prerequisite for any eventual therapeutic use of CRF antagonists, namely that they need not be administered in anticipation of a neurological insult.

Long-latency responses of brain noradrenergic neurons to noxious stimuli are preferentially attenuated by intravenous morphine

The nucleus locus coeruleus (LC) has been strongly implicated in the processing of noxious stimuli. Consistent with this, previous studies have shown that spontaneous LC discharge is depressed by morphine. However, effects of morphine on evoked responses of LC neurons to noxious stimuli have not been systematically examined. We reported recently that responses to footshock stimuli in rat locus coeruleus neurons consist of an early (A-fiber mediated) component and a previously undescribed late (C-fiber mediated) component. In the present study, we administered analgesic doses of morphine (0.1, 0.5, or 1.0 mg/kg, i.v.) to determine the effect on A- and C-fiber components of footshock responses in LC neurons. Doses of 0.5 and 1.0 mg/kg significantly attenuated the C-fiber mediated response of LC neurons without affecting the A-fiber response component. Spontaneous LC discharge was reduced by administration of all doses of morphine. Both depressive effects of morphine were abolished by intravenous administration of naloxone. In contrast, local microinfusion of naloxone into the LC abolished the morphine-induced decrease of spontaneous discharge but did not prevent the depression of the C-fiber mediated footshock response by morphine. This indicates that the site of action for morphine's attenuation of the late LC response to footshock stimulation is outside of the LC. The results are consistent with the hypothesis that the late (C-fiber-mediated) footshock responses in locus coeruleus are involved in the processing of noxious stimuli and may contribute to anti-nociceptive mechanisms.

Dopamine D1 receptors in the amygdala enhance the immune response in the rat.

Indirect evidence suggests that dopamine within the brain may participate in the regulation of immune responses in both man and rodents. The aim of the present study was to investigate the possible role played by the specific dopamine D1 receptor subtype within the central amygdala in the modulation of immunity. Mitogen responsiveness of splenocytes and NK cell activity were measured in rats following local microinfusion of SKF 38393, a specific dopamine D1 receptor agonist. Microinfusion of SKF 38393 (100 nmol) within the central amygdala increased the proliferative response of splenocytes to Con A, whereas it did not modify the proliferative response of splenocytes to LPS or NK cell activity. The effects of SKF 38393 were prevented by prior systemic administration of SCH 23390 (0.3 mg/kg ip), a specific D1 receptor antagonist. These results indicate that activation of dopamine D1 receptors within the central amygdala induced selective stimulation of mitogen responsiveness of splenocytes and suggest that specific activation of dopamine neurotransmission within selected areas of the limbic system may produce immunoenhancing effects. These findings further confirm the immunomodulatory role played by dopaminergic mechanisms in the brain.

NMDA-receptor-mediated sensory responses of brain noradrenergic neurons are suppressed by in vivo concentrations of extracellular magnesium.

Recent studies reveal at least three receptor subtypes for excitatory amino acid (EAA) neurotransmitters. Activation of one of these, the N-methyl-D-aspartate (NMDA) receptor-channel complex, has been strongly implicated in neuronal mechanisms of several important brain processes, including learning and memory. As NMDA receptors are highly sensitive to extracellular magnesium (Mg++), we tested whether in vivo concentrations of this ion are sufficient to suppress NMDA-receptor-mediated responses. We show that slow, local microinfusion of Mg(++)-free artificial cerebrospinal fluid onto noradrenergic locus coeruleus (LC) neurons reveals an NMDA-receptor-mediated component of their response to a sensory stimulus. This is the first demonstration that the in vivo concentration of extracellular Mg++ ions suppresses synaptically mediated NMDA receptor activation. We also present evidence that unmasking this NMDA receptor activity induces prolonged enhancement of the EAA-mediated sensory response of LC neurons.

Medial amygdala and aggressive behavior: Interaction between testosterone and vasopressin

This paper considers the functional significance of the testosterone-dependent vasopressinergic neurons of the medial amygdala (Ame) in intermale aggressive behavior of rats. Local microinfusion of vasopressin into the medial amygdala causes an increase in offensive behavior both in gonadally intact males and in animals that were castrated 1 week or 6 months prior to the experiment. The absolute level of offense after arginine-vasopressin (AVP) treatment declined with time after castration indicating that the behavioral consequences of AVP in the Ame are at least partially independent of testosterone. The discussion considers the possibility that AVP in the medial amygdala is involved in the modulatory role of this structure in social learning and memory processes.