Quantitative autoradiography of muscarinic cholinergic receptors in the rat brain

The evolution of the pilocarpine animal model of status epilepticus

The pilocarpine animal model of status epilepticus is a well-established, clinically translatable model that satisfies all of the criteria essential for an animal model of status epilepticus: a latency period followed by spontaneous recurrent seizures, replication of behavioural, electrographic, metabolic, and neuropathological changes, as well as, pharmacoresistance to anti-epileptic drugs similar to that observed in human status epilepticus. However, this model is also characterized by high mortality rates and studies in recent years have also seen difficulties in seizure induction due to pilocarpine resistant animals. This can be attributed to differences in rodent strains, species, gender, and the presence of the multi-transporter, P-glycoprotein at the blood brain barrier. The current paper highlights the various alterations made to the original pilocarpine model over the years to combat both the high mortality and low induction rates. These range from the initial lithium-pilocarpine model to the more recent Reduced Intensity Status Epilepticus (RISE) model, which finally brought the mortality rates down to 1%. These modifications are essential to improve animal welfare and future experimental outcomes.

Cholinergic modulation of slow cortical rhythm in urethane-anesthetized rats

Slow cortical rhythm (SCR) is characterized by rhythmic cycling of active (UP) and silent (DOWN) states in cortical cells. In urethane anesthesia, SCR appears as alternation of almost isoelectrical EEG periods and low-frequency, high-amplitude large shifts with superimposed high-frequency activity in the local field potentials (LFPs). Dense cholinergic projection reaches the cortex from the basal forebrain (BF), and acetylcholine (ACh) has been demonstrated to play a crucial role in the regulation of cortical activity. In the present experiments, cholinergic drugs were administered topically to the cortical surface of urethane-anesthetized rats to examine the direct involvement of ACh and the BF cholinergic system in the SCR. SCR was recorded by a 16-pole vertical electrode array from the hindlimb area of the somatosensory cortex. Multiple unit activity (MUA) was recorded from layer V to VI in close proximity of the recording array. Neither a low dose (10 mM solution) of the muscarinic antagonist atropine or the nicotinic agonist nicotine (1 mM solution) had any effect on SCR. In contrast, the higher dose (100 mM solution) of atropine, the cholinergic agonist carbachol (32 mM solution), and the cholinesterase inhibitor physostigmine (13 mM solution) all decreased the number of UP states, delta power (0-3 Hz) and MUA. These results suggest that cholinergic system may influence SCR through muscarinic mechanisms during urethane anesthesia. Cholinergic activation obstructs the mechanisms responsible for local or global synchronization seen during SCR as this rhythm was disrupted or aborted. Muscarinic antagonism can evoke similar changes when high dose of atropine is applied.

Behavioral effects of morphine and cocaine in M1 muscarinic acetylcholine receptor-deficient mice

RATIONALE

Muscarinic acetylcholine receptors (M1-M5) modulate the activity of the central nervous system and an array of physiological functions. Recent evidence has also implicated muscarinic receptors in behavioral effects of drugs of abuse such as morphine and cocaine. However, the genetic similarity between muscarinic receptors and the coexpression of multiple subtypes in most cells has impeded the development of selective antagonists and the determination of the role of each muscarinic receptor subtype in morphine's and cocaine's behavioral effects.

OBJECTIVE

The present studies employ mice deficient in the M1 receptor subtype (M1 KO) to assess morphine antinociception (2.5, 5.0, 10, or 20 mg/kg) and the conditioned rewarding effects of morphine and cocaine (2.5, 5.0, or 10 mg/kg).

METHODS

M1 KO and their wild-type (WT) littermates were tested using a 56 degrees C hotplate assay and a conditioned place preference procedure. Parallel studies using the M1 receptor antagonist, pirenzepine, were also conducted in the background strain C57BL6 mice.

RESULTS

The results demonstrate that M1 KO mice display a greater antinociceptive effect of morphine in the hotplate assay; however, the effects of morphine as well as cocaine were attenuated in the conditioned place preference procedure. Comparable results were obtained with the pharmacological antagonism of the M1 receptor by pirenzepine.

CONCLUSIONS

These results suggest a modulatory role of the M1 muscarinic receptor in opioid antinociception and conditioned drug reward, and demonstrate the utility of M1 receptor knockout models for the determination of the role of the M1 subtype in the behavioral effects of morphine and cocaine.

The lesional and epileptogenic consequences of lithium-pilocarpine-induced status epilepticus are affected by previous exposure to isolated seizures: effects of amygdala kindling and maximal electroshocks

In temporal lobe epilepsy, the occurrence of seizures seems to correlate with the presence of lesions underlying the establishment of a hyperexcitable circuit. However, in the lithium-pilocarpine model of epilepsy, neuronal damage occurs both in the structures belonging to the circuit of initiation and maintenance of the seizures (forebrain limbic system) as in the propagation areas (cortex and thalamus) and in the circuit of remote control of seizures (substantia nigra pars reticulata). To determine whether or not we could protect the brain from lesions and epileptogenesis induced by status epilepticus and identify cerebral structures involved in the genesis of epilepsy, we studied the effects of the chronic exposure to non-deleterious seizures, either focalized with secondary generalization (amygdala kindling, kindled-pilocarpine rats), or primary generalized (ear-clip electroshocks, electroshock-pilocarpine rats) on neuronal damage and epileptogenesis induced by lithium-pilocarpine status epilepticus. These animals were compared to rats subjected to status epilepticus but not pretreated with seizures (sham-kindled-pilocarpine or sham-electroshock-pilocarpine rats). Compared to sham-pilocarpine rats, neuronal damage was prevented in the limbic system of the kindled-pilocarpine rats, except in the hilus of the dentate gyrus and the entorhinal cortex, while it was enhanced in rats pretreated with electroshocks, mainly in the entorhinal and perirhinal cortices. Most sham-kindled- and sham-electroshock-pilocarpine rats (92-100%) developed recurrent seizures after a silent period of 40-54days. Likewise, all kindled-pilocarpine rats developed spontaneous seizures after the same latency as their sham controls, while only two of 10 electroshock-pilocarpine rats became epileptic after a delay of 106-151days. The present data show that the apparent antiepileptic properties of electroshocks correlate with extensive damage in midbrain cortical regions, which may prevent the propagation of seizures from the hippocampus and inhibit their motor expression. Conversely, the extensive neuroprotection of the limbic system but not the hilus and entorhinal cortex provided by amygdala kindling does not prevent epileptogenesis. Thus, the hilus, the entorhinal and/or perirhinal cortex may be key structure(s) for the establishment of epilepsy.

Modulation of carbachol-induced antinociception from the rat periaqueductal gray

The tail-flick latency (TFL) and the vocalisation test (VT) thresholds were all increased by microinjecting CCh into the dorsal periaqueductal gray (dPAG) of rats. The effects on the TFL were mimicked by dimethyl-phenylpiperazinium, and inhibited by local mecamylamine or intraperitoneal (i.p.) phenoxybenzamine. The effects on the VT were mimicked by bethanechol and inhibited by local mecamylamine, atropine or naloxone. The effects on the thresholds for motor defence reaction were inhibited by i.p. methysergide or naloxone, and prolonged by i.p. phenoxybenzamine. The effects on the threshold for vocalisation during the stimulation were blocked by i. p. methysergide and shortened by i.p. phenoxybenzamine or naloxone. No significant effect of CCh was found on open arm exploration of rats in the elevated plus maze paradigm. We conclude that the effects of CCh from the dPAG is not due to an anxiolytic effect, and depends on the activation of local cholinergic and opioid sites for the supraspinal modulation of "affective" component of pain response, and nicotinic sites for the activation of descending pain pathways.

Differential effects of nucleus basalis lesions in young adult and aging rats

To characterize age-related changes in frontal cortical plasticity, we assessed maze learning and frontal cortical pharmacology in young adult, middle-aged, and aged rats. Rats received either ibotenic acid or sham lesions of the nucleus basalis magnocellularis (NBM) and were then trained on a radial maze task. After training, we assessed [3H]desmethylimipramine (DMI), [3H]muscimol, [3H]AMPA, and [3H]QNB binding using quantitative autoradiography. Both middle-aged and aged rats were impaired on the radial maze task. DMI binding was increased in both middle-aged and aged rats, while QNB binding was decreased in aged rats. While lesions impaired maze performance at all ages, middle-aged and aged rats showed more profound lesion-induced deficits. Lesions increased GABA, and AMPA receptor binding in young adult rats only. These lesion-induced changes may reflect a compensatory response that is lost with advancing age.

Actions of NMDA and cholinergic receptor antagonists in the rostral ventromedial medulla upon beta-endorphin analgesia elicited from the ventrolateral periaqueductal gray

Analgesia elicited by morphine in the ventrolateral periaqueductal gray is mediated in part by NMDA and cholinergic receptors in the rostral ventromedial medulla because selective receptor antagonists applied to the latter structure reduced morphine analgesia elicited from the former structure. Previous studies have demonstrated that morphine and beta-endorphin employ different anatomical and neurochemical pathways in exerting their supraspinal analgesic effects. The present study evaluated whether pretreatment with either competitive (AP7, 3-10 microg) or non-competitive (MK-801, 3-10 microg) NMDA antagonists, or muscarinic (scopolamine, 5 microg) or nicotinic (mecamylamine, 1 microg) cholinergic antagonists administered into the rostral ventromedial medulla altered beta-endorphin (15 microg) analgesia elicited from the ventrolateral periaqueductal gray as measured by the tail-flick and jump tests in rats. Whereas AP7 produced minimal (11%) and transient (30 min) reductions in beta-endorphin analgesia on the jump test, MK-801 produced minimal (9%) and transient (30 min) reductions in beta-endorphin analgesia on the tail-flick test. Whereas mecamylamine failed to reduce beta-endorphin analgesia on either measure, scopolamine produced small (23%) and transient (30 min) reductions in beta-endorphin analgesia on the tail-flick test. Each of these antagonists administered into the rostral ventromedial medulla at comparable or lower doses virtually eliminated morphine analgesia elicited from the ventrolateral periaqueductal gray. The opioid mediation of beta-endorphin analgesia in the ventrolateral periaqueductal gray was confirmed by its sensitivity to naltrexone (1-20 microg) pretreatment into the same structure. These data provide further evidence for dissociations between the descending neuroanatomical and neurochemical circuitry mediating the supraspinal analgesic responses induced by morphine and beta-endorphin, and indicate that the latter response is mediated by either non-cholinergic and non-NMDA synapses within the rostral ventromedial medulla, and/or by brainstem sites outside of the rostral ventromedial medulla.

Correlation between hypermetabolism and neuronal damage during status epilepticus induced by lithium and pilocarpine in immature and adult rats

The correlation between seizure-induced hypermetabolism and subsequent neuronal damage was studied in 10-day-old (P10), 21-day-old (P21), and adult rats subjected to lithium-pilocarpine status epilepticus (SE). Local CMRglc (LCMRglc) values were measured by the [14C]2-deoxyglucose method for a duration of 45 minutes starting at 60 minutes after the onset of SE, and neuronal damage was assessed by cresyl violet staining at 6 days after SE. In P21 and adult rats, LCMRglc values were increased by 275 to 875% in all thalamic, cortical, forebrain, and hypothalamic regions plus the substantia nigra. In addition, at P21 there were also large increases in LCMRglc in brainstem regions. In P10 rats, metabolic increases were mostly located in cortical and forebrain regions plus the substantia nigra but did not affect hypothalamic, thalamic, or brainstem areas. In adult rats, there was an anatomical correlation between hypermetabolism and neuronal damage. At P21, although hypermetabolism occurred in regions with damage, the extent of damage varied considerably with the animals and ranged from an almost negligible to a very extended degree. Finally, in P10 rats, although quite pronounced hypermetabolism occurred, there was no neuronal damage induced by the seizures. Thus, in the present model of epilepsy, the correlation between marked hypermetabolism and neuronal damage can be shown in adult rats. Conversely, immature rats can sustain major metabolic activations that lead either to a variable extent of damage, as seen at P21, or no damage, as recorded at P10.

Spatial and temporal evolution of neuronal activation, stress and injury in lithium-pilocarpine seizures in adult rats

In order to follow the spatial and temporal evolution of neuronal damage, cellular activation and stress responses subsequent to lithium-pilocarpine seizures of various durations in the adult rat, we analyzed the expression of Fos protein and local cerebral glucose utilization as markers of cellular activation, HSP72 immunoreactivity and acid fuchsin staining as indicators of cellular stress and injury, and Cresyl violet staining for the assessment of neuronal damage. The expression of Fos appeared very early, 2-30 min after the onset of polyspikes and intensified during the following 4 h. Fos immunoreactivity was especially high in the hippocampus, cerebral cortex, amygdala and anterior olfactory nuclei. Local cerebral glucose utilization measured during the second hour of seizures was largely increased (350-580%) over control levels in cortical areas, amygdala, dentate gyrus, caudate nucleus and mediodorsal thalamus. HSP72 immunoreactivity never appeared earlier than 40-50 min after the onset of polyspikes, and was most prominent in hippocampal CA3 area, cerebral cortex (except the piriform cortex) and anterior olfactory nuclei. Acid fuchsin staining was maximal in the piriform cortex and the polymorphic layer of the dentate gyrus. Staining was moderate in the sensorimotor cortex and the amygdala. Neuronal damage was extensive in the piriform and entorhinal cortices, the hippocampal CA3 area and the polymorphic layer of the dentate gyrus, basal amygdala, mediodorsal thalamus and anterior olfactory nuclei. In conclusion, the present study shows that brain regions with the highest expression of Fos and the largest metabolic activation were also highly stained with acid fuchsin and most heavily damaged. Conversely, there is no clear relationship between HSP72 expression, cellular activation and neuronal damage.

Developmental changes in heterogeneous patterns of neurotransmitter receptor binding in the human interpeduncular nucleus

The interpeduncular nucleus (IPN) exhibits many complex features, including multiple subnuclei, widespread projections with the forebrain and brainstem, and neurotransmitter heterogeneity. Despite the putative importance of this nucleus, very little is known about its neurochemical development in the human. The human IPN is cytoarchitectonically simple, unlike the rat IPN, which displays considerable heterogeneity. In the following study, we hypothesized that the developing human IPN is neurochemically heterogeneous despite its cytological simplicity. The chemoarchitecture in this study was defined by neurotransmitter receptor binding patterns by using quantitative tissue autoradiography for the muscarinic, nicotinic, serotoninergic, opioid, and kainate receptors. We examined neurotransmitter receptor binding in the developing human IPN in a total of 15 cases. The midbrains of five midgestational fetuses (19-26 gestational weeks) and six infants (38-74 postconceptional weeks) were examined. The midbrain of one child (4 years) and three adults (20-68 years) were analyzed as indices of maturity. At all ages examined, high muscarinic binding was localized to the lateral subdivision of the IPN, high serotoninergic binding was localized to the dorsal IPN, and high opioid receptor binding was localized to the medial IPN. The developmental profile was unique for each radioligand. We report a heterogenous distribution of neurotransmitter receptor binding in the developing human IPN, which supports a complex role for it in human brain function.

Orally active NGF synthesis stimulators: potential therapeutic agents in Alzheimer's disease

The degeneration of cholinergic neurons may be responsible for cognitive impairment in patients with Alzheimer's disease (AD). Since nerve growth factor (NGF) plays an important role in the survival and maintenance of cholinergic neurons in the central nervous system, this factor may have some beneficial effects on the cognitive impairment observed in patients with AD. However, since NGF does not cross the blood-brain barrier and is easily metabolized when administered peripherally, it can only be used when directly injected into the brain. In this review, we show that repeated oral administration of the NGF synthesis stimulators, idebenone and propentofylline, partially restored the age-associated decrease of NGF in the frontal and parietal cortices. Furthermore, this treatment attenuated the impairment of performance in the water maze, passive avoidance, and habituation tasks in rats with bilateral forebrain lesions, and in rats which had received continuous infusion of anti-NGF antibody into the septum. The behavioral improvement induced by idebenone and propentofylline was accompanied by recovery of both the reduced activity of choline acetyltransferase and the changes in [3H]QNB binding. These results suggest that the use of NGF synthesis stimulators may provide a novel therapeutic approach to cholinergic dysfunction.

The temporal evolution of neuronal damage from pilocarpine-induced status epilepticus

The temporal evolution of irreversible neuronal damage from pilocarpine-induced seizures was studied by light microscopy. Neuronal cell death was judged on a 0-3 scale by estimating the percentage of acidophilic neurons in each of 23 brain regions. In addition, in the dorsal dentate hilus (CA4), quantitative cell counts of normal and acidophilic neurons were also performed. A few dead neurons (grade 0.5 damage) appeared in ventral hippocampal CA1 and CA3 regions after 20-min status epilepticus (SE). Slight-to-mild damage (grades 0.5-1.5) occurred in 14 and 12 brain regions after 40-min and 1-h SE respectively, and slight-to-moderate damage (grades 0.5-2.0) was found in 15 regions after 3-h SE. Twenty-four h and 72 h after 3-h SE, there was slight-to-severe damage (grade 0.5-3.0) in 22 and 21 regions respectively. Three-h SE produced more severe damage to 7 brain regions compared to 1-h SE, and 16 regions had more pronounced neuronal injury 24 h after rather than 0-4 h after 3-h SE. Eight brain regions had less damage 72 h compared to 24 h after SE, probably because of progressive neuronal lysis and dropout, but in mediodorsal and lateroposterior thalamic nuclei damage worsened from 24 to 72 h after SE. Neuronal cell counting revealed 20% acidophilic neurons in dorsal dentate hilus after 40-min SE and no difference between the 1-h and 3-h seizure groups (31% vs. 43% acidophilic neurons respectively). Among the 3 groups of rats with 3-h SE and varying recovery periods, the 24-h and 72-h recovery groups had higher percentages of acidophilic neurons (65% and 54% respectively) than the 0-4-h group (43%). Finally, the hippocampal CA2 region and dentate granule cell layer and the caudate-putamen, considered resistant to seizure-induced cell injury, were all damaged from SE lasting 40 min or more.

An autoradiographic study of [3H]flunitrazepam binding sites in the brain of rat made tolerant to and dependent on pentobarbital

The effects of continuous administration of pentobarbital on the benzodiazepine receptor labeled by [3H]flunitrazepam were investigated. Animals were made tolerant to pentobarbital by intracerebroventricular (i.c.v.) infusion with pentobarbital (300 micrograms/10 microliters/h) for 6 days through pre-implanted canulae connected to osmotic mini-pumps. The dependent rats were assessed 24 h after cessation of pentobarbital infusion. Changes in [3H]flunitrazepam binding were examined in 37 brain regions at a concentration of [3H]flunitrazepam of 1 nM. In subsequent saturation studies, the binding parameters Bmax and KD were also investigated in 17 brain regions, most of which showed significant changes in [3H]flunitrazepam binding in experiments using a fixed concentration of radioligand. The pentobarbital-tolerant rats showed a significant increase in Bmax with an increase in KD for [3H]flunitrazepam in the ventroposterior nucleus of thalamus. In the dependent rats, a significant increase in Bmax for [3H]flunitrazepam binding, without a change in KD, was observed in all layers of the frontal cortex, the caudate-putamen, olfactory tubercle, and some nuclei in thalamus, compared to those in the control. Increased [3H]flunitrazepam binding in the molecular layer of the olfactory bulb, the ventral pallidum, and the cerebellum of the pentobarbital dependent rats at a fixed concentration of [3H]flunitrazepam was also observed. There was no significant change in [3H]flunitrazepam binding in the hippocampus and several nuclei of the brain stem. These findings suggest that benzodiazepine receptors are closely involved in the development of tolerance to and dependence on pentobarbital. Further studies on changes in gamma-aminobutyric acid (GABA)A receptor subunit mRNA or the effects of pentobarbital on GABAA receptor phosphorylation would be necessary for an explanation of the precise mechanisms underlying the development of tolerance to and dependence on pentobarbital.

Three-dimensional distribution of [3H]quinuclidinyl benzilate binding to muscarinic cholinergic receptors in the developing human brainstem

Acetylcholine has been implicated in brainstem mechanisms of cardiac and ventilatory control, arousal, rapid eye movement (REM) sleep, and cranial nerve motor activity. Virtually nothing is known about the developmental profiles of cholinergic perikarya, fibers, terminals, and/or receptors in the brainstems of human fetuses and infants. This study provides baseline information about the quantitative distribution of muscarinic cholinergic receptors in fetal and infant brainstems. Brainstem sections were analyzed from 6 fetuses (median age: 21.5 postconceptional weeks), 4 premature infants (median age: 26 postconceptional weeks), and 11 infants (median age: 53 postconceptional weeks). One child and three adult brainstems were examined as indices of maturity for comparison. The postmortem interval in all cases was less than or equal to 24 hours (median: 10 hours). Muscarinic receptors were localized by autoradiographic methods with the radiolabeled antagonist [3H]quinuclidinyl benzilate ([3H]QNB). Computer-based methods permitted quantitation of [3H]QNB binding in specific nuclei and three-dimensional reconstructions of binding patterns. By midgestation, muscarinic cholinergic receptor binding is already present and regionally distributed, with the highest binding levels in the interpeduncular nucleus, inferior colliculus, griseum pontis, nucleus of the solitary tract, motor cranial nerve nuclei, and reticular formation. During the last half of gestation, [3H]QNB binding decreases in most, but not all of the nuclei sampled. The most substantial decline occurs in the reticular formation of the medulla and pons, a change that is not fully explained by progressive myelination and lipid quenching. Binding levels remain essentially constant in the inferior olive and griseum pontis. Around the time of birth or shortly thereafter, the relative distribution of binding becomes similar to that in the adult, with the highest levels in the interpeduncular nucleus and griseum pontis, although binding levels are higher overall in the infant. In the rostral pontine reticular formation, paramedian bands of high muscarinic binding are present which do not correspond to a cytoarchitectonically defined nucleus. By analogy to animal studies, these bands may comprise a major cholinoreceptive region of the human rostral pontine reticular formation involved in REM sleep. In the human interpeduncular nucleus in all age periods examined, muscarinic binding localizes to the lateral portions bilaterally, indicative of a heterogeneous chemoarchitecture. Muscarinic binding is high in the arcuate nucleus, a component of the putative respiratory chemosensitive fields along the ventral surface of the infant medulla. This observation is consistent with the known effects of muscarinic agents on chemosensitivity and ventilatory responses applied to the ventral medullary surface in animal models. The nonuniform distribution of muscarinic binding in the caudorostral plane in individual brainstem nuclei, as illustrated by three-dimensional reconstructions, underscores the need for rigorous sampling at precisely matched levels in quantitative studies. This study provides basic information toward understanding the neurochemical basis of brainstem disorders involving dysfunction of autonomic and ventilatory control, arousal, and REM sleep in preterm and full-term newborns and infants and for developing cholinergic drugs for such disorders in the pediatric population.

Decreased muscarinic receptor binding in rat brain after paradoxical sleep deprivation: an autoradiographic study

Previous work demonstrated that paradoxical sleep deprivation (PSD) leads to a decrease in yawning behavior elicited by cholinergic agonists, suggesting that a downregulation of cholinergic muscarinic receptors may occur after PSD. More recent work using intracerebral injections of muscarinic agonists has suggested a critical role for M2 receptors in paradoxical sleep. In this study [3H]AF-DX 384 was used to investigate the effects of PSD on M2-type cholinergic receptors throughout the brain using quantitative autoradiography. After 96 h of paradoxical sleep deprivation, [3H]AF-DX 384 binding was generally reduced throughout the brain, and significantly so in the olfactory tubercle (-20%), n. accumbens (-23%), frontal caudate-putamen (-16%), islands of Callejas (-20%), piriform cortex (-24%), lateral (-26%) and medial (-24%) septum, anteromedial (-19%), ventrolateral (-22%), and lateral geniculate (-15%) nuclei of thalamus, deep layers of the superior colliculus (-15%), entorhinal cortex (-12%) and subiculum (-23%). [3H]AF-DX 384 binding was reduced in pontine structures, but not to a higher degree than in other brain areas. The observed downregulation of M2-type muscarinic receptors after PSD may be causally related to the previously reported decrease in cholinergically induced behaviors after PSD.

Autoradiographic localization of CCK-8 binding sites in the rat brain: effects of chronic methamphetamine administration on these sites

The effects of chronic methamphetamine (MAP) administration (at a dose of 4 mg/kg for 14 days) on [3H]pCCK-8 binding sites in the rat brain were investigated by an in vitro quantitative receptor autoradiographic technique. The number of [3H]pCCK-8 binding sites was significantly reduced in layers III and IV of the medial frontal, anterior, and posterior cingulate cortices, in layers II-IV of the retrosplenial cortex, in layers III-VI of the dorsal insular cortex, and in the reticular nucleus of the thalamus, compared to these numbers in a control group of rats that received physiologic saline. Further, chronic methamphetamine administration led to a significant increase in the number of these binding sites in layer I of the entorhinal cortex. These findings indicate the CCK peptides in the limbic lobe may be closely related to the development of the behavioral changes associated with methamphetamine sensitization. In addition, these results provide supporting evidence for the involvement of the limbic system in the pathophysiology of schizophrenia.

Effects of physostigmine on local cerebral glucose utilization in the central components of the rat visual system

The effects of intravenous administration of physostigmine at doses of 0.03, 0.095, or 0.3 mg/kg on local cerebral glucose utilization (LCGU) were determined in 3 structures of the visual system of the rat brain by means of the quantitative 2-[14C]deoxyglucose method. LCGU was increased in the superior colliculus (superficial gray layer), but unchanged in the visual cortex and the lateral geniculate body. To determine whether the observed effect of physostigmine on the superior colliculus depended on input from the retina, the highest dose of physostigmine was administered to rats which had previously been enucleated bilaterally. Enucleation decreased LCGU in the superior colliculus of the animals not treated with physostigmine and blocked the effect of physostigmine on LCGU. The effect of physostigmine in the superior colliculus appears, therefore, to depend on input from the retina.

Effects of propentofylline, a NGF synthesis stimulator, on alterations in muscarinic cholinergic receptors induced by basal forebrain lesion in rats

Basal forebrain (BF) lesions induced by ibotenic acid produced increases in the Bmax and Kd values of [3H]QNB binding sites in the frontal cortex, parietal cortex, and hippocampus. Twenty-eight-day successive administration of propentofylline (10 and 25 mg/kg, p.o.) significantly reduced the Kd values of [3H]QNB binding sites, to the levels of those in a sham group, in a dose-dependent manner. Moreover, propentofylline (25 mg/kg, p.o.) significantly reduced the Bmax value of [3H]QNB binding sites compared with that in a vehicle-treated BF-lesioned group. These results suggest that successive administration of propentofylline ameliorates changes in muscarinic cholinergic receptors through improving presynaptic cholinergic dysfunction.

Differential visualization of dopamine D2 and D3 receptor sites in rat brain. A comparative study using in situ hybridization histochemistry and ligand binding autoradiography

At least five members of the dopamine receptor family have been characterized at the gene level. D2, D3 and D4 dopamine receptors are related pharmacologically. In order to visualize the differential expression of D1, D2 and D3 receptors in rat brain we have combined in situ hybridization histochemistry with receptor autoradiography. Regions enriched with D3 messenger RNA (mRNA) included the islands of Calleja (ioC) and nucleus accumbens. Very low or undetectable levels were present in the caudate-putamen. In contrast, no D2 transcripts were observed in the islands of Calleja, but there were high levels in the nucleus accumbens, caudate-putamen (CP) and pyramidal layer of the olfactory tubercle. A comparison of the binding pattern of six dopamine receptor radioligands hitherto regarded as D2 receptor-selective showed that the islands of Calleja were intensely labelled by [125I]iodosulpride, [3H]CV 205 502 and [3H]SDZ 205 501, while the binding of [3H]spiperone, [3H]raclopride and [3H]YM 09151-2 was much lower or undetectable. Pharmacological analysis of the binding of D2/D3 ligands to the islands of Calleja and caudate-putamen suggests that binding sites in these two regions are of different pharmacology, consistent with the presence of D3 sites in the islands of Calleja and the predominance of D2 sites in the caudate. These results demonstrate the expression of D3 binding sites in the rat brain and provide a procedure to differentiate D2 and D3 receptor populations in binding studies.

Ontogenic distribution of muscarinic receptors and acetylcholinesterase in the rabbit hippocampus

Ontogenic development of muscarinic receptors was examined in the hippocampus of rabbits (from P2 to P60) using radioautographic method. Muscarinic sites were labelled with (3H)-quinuclinidyl-benzilate and pharmacologically defined M1 and M2 receptor subtypes with (3H)-pirenzepine and (3H)-oxotremorine, respectively. The distribution of binding sites was compared to acetylcholinesterase (AChE) staining in adjacent hippocampal sections. The two cholinergic components are progressively set up in the hippocampus during the first three postnatal weeks. The AChE staining was very low in all hippocampal fields in P2 rabbits. At P8 and after, the AChE staining was more pronounced in CA3 and CA4 than in CA1 and CA2. On the contrary, the M1 muscarinic binding sites were more abundant in CA1 and CA2 hippocampal fields than in CA3 and CA4 at all ages studied. M2 muscarinic binding sites were only distinguishable at P45 and have a relatively homogeneous distribution. This study shows a differential developmental evolution in the distribution of AChE and muscarinic M1 receptors, and no obvious correspondence between these two cholinergic markers was observed.

Early changes in second messenger but not receptor binding sites after acute subdural hematoma: an in vitro autoradiographic study

Neurotransmitter receptor-coupled mechanisms have been recently recognized as important determinants of cell damage after central nervous system (CNS) trauma and ischemia. Many of these receptors exert their intracellular effects via second messenger systems. This study used in vitro autoradiographic radioligand binding to measure beta-adrenergic and muscarinic cholinergic receptors and adenylate cyclase and protein kinase C (PKC) binding sites two h after acute subdural hematoma in rats. Both beta-adrenergic and cholinergic receptor binding sites were unchanged in comparison to controls, while adenylate cyclase binding significantly decreased in the ischemic cortex under the hematoma. These changes may constitute a major limiting factor on receptor-linked therapeutic strategies in trauma and ischemia. Protein kinase C activation significantly increased in the ischemic area under the hematoma in these studies. This appears to be a response to calcium flux, which may be in part glutamate mediated.

Effect of muscarinic cholinergic drugs on ischemia-induced decreases in glucose uptake and CA1 field potentials in rat hippocampus slices

To clarify the role of muscarinic acetylcholine receptors in the hypoxia/hypoglycemia (ischemia)-induced functional deficit in hippocampal neurons, we examined the effect of cholinergic drugs on ischemia-induced impairments of glucose uptake and CA1 field potentials in hippocampus slices. Muscarinic receptors were subdivided into M1 (high affinity for pirenzepine) and M2 (low affinity for pirenzepine) subtypes. The M1 receptor subtype is coupled to an increase in phosphoinositide hydrolysis and the M2 receptor subtype is associated with inhibition of adenylate cyclase. The greater potency of carbachol in stimulating phosphoinositide hydrolysis resulted in exacerbated ischemia-induced deficits. Treatment with the muscarinic receptor antagonists scopolamine and pirenzepine (M1 receptor-selective antagonist) had a strong dose-dependent protective effect against ischemia-induced deficits. Oxotremorine and McN-A-343, weak stimulators of phosphoinositide hydrolysis and strong inhibitors of adenylate cyclase, had a weak neuroprotective action against ischemia-induced deficits. These results suggest that stimulation of M1 muscarinic receptors coupled with an increase in phosphoinositide hydrolysis may play a facilitatory role in ischemia-induced deficits. Stimulation of M2 muscarinic receptors may play an inhibitory role in ischemia-induced neuronal deficits.

The distribution of cholinergic muscarinic receptors in the dog frontal lobe

The topographical distribution of cholinergic muscarinic receptor (MChR) sites was studied by means of quantitative receptor autoradiography using [3H]quinuclidinyl benzilate ([3H]QNB) in the frontal (prefrontal, premotor and motor) cortex of the dog. The mean binding value in the frontal cortex was 408 +/- 5.0 fmol/mg tissue and the only area that differed significantly from the mean was the primary motor cortex, where the binding value was significantly lower. In the dorsal part of the prefrontal and premotor cortical subregions studied, a tri-laminar pattern of [3H]QNB labelling was observed, with a superficial dense band of label corresponding to cortical layers I, II and III. The deep high density band overlaid layer V and the upper part of the layer VI. In the ventral part of the prefrontal cortex this pattern gradually disappeared and in the most ventral part no laminar differences were seen. In contrast, in primary motor areas, the deep band of labelling corresponding to layer V was much less pronounced than in the frontal association cortex. Variations in the distribution of MChR sites seem to reflect to some extent the greater cytoarchitectonic differentiation of the dorsal zone and also the similarity between the ventral zone and the limbic cortex described by us previously.

Characteristics of muscarinic cholinergic, gamma-aminobutyric acid(A) and phencyclidine receptors in spontaneously epileptic rats; in vitro quantitative autoradiographic analysis

Characteristics of muscarinic cholinergic (mACh), gamma-aminobutyric acid(A) (GABAA) and phencyclidine (PCP) receptors in the spontaneously epileptic rats (SER), which exhibit both absence-like seizures and tonic convulsion, were examined using in vitro quantitative autoradiography. Computer analysis using autoradiographic technique revealed that the amount of the specific binding of [3H]quinuclidinyl benzilate (QNB) to mACh receptors in the striatum of SER was more than that of zitter rats, not exhibiting both seizures and convulsion. However, the specific bindings of [3H]muscimol and [3H]N-(1-[2-thienyl]cyclohexyl)3,4-piperidine (TCP) to GABAA and PCP receptors, respectively, of SER were not different from those of zitter rats in various regions tested. These results suggest that hyperfunction of mACh receptors in the striatum is involved in the appearance of absence-like seizures and tonic convulsion of SER.

Characterization of muscarinic receptor subtype of rat eccrine sweat gland by autoradiography

The muscarinic cholinergic receptor of rat eccrine sweat gland was characterized using quantitative autoradiography and [3H]QNB as radioligand. The distribution of radioligand was maximal in the secretory coil. Autoradiographic competition binding studies were performed using selective antagonists to M1 (pirenzepine), M2 (AF-DX 116), and M3 (4-DAMP) and the classical nonselective antagonist atropine. pKi for pirenzepine, AF-DX 116, 4-DAMP, and atropine was 6.58, 5.47, 8.50, and 8.66 respectively indicating that the eccrine sweat gland muscarinic receptor was predominantly M3.

Antinociception and behavioral changes induced by carbachol microinjected into identified sites of the rat brain

The sites of the rat brain in which intracerebral administration of carbachol (0.4 microgram/0.5 microliter) elevates the nociceptive threshold to thermic (tail-flick test) and mechanical (calibrated-pinch test) noxious stimuli were examined. An extensive mapping (510 sites) ranging from AP + 10.5 to AP-0.1 mm revealed that antinociception was obtained from 119 sites (23%) widely scattered in the brain, and reached structures distant from, or within the immediate vicinity of the ventricular system. The effects from most placement were demonstrated using the tail-flick test, whereas a smaller proportion (approximately 13%) of sites was effective in reducing the response to mechanical stimuli only. Structures containing sensitive sites include the dorsal raphe nucleus, lateral border of the superior cerebellar peduncle, caudal portion of the superior colliculus, medial geniculate body, habenular complex, amygdala, temporal pole of the ventral hippocampus, rostral aspect of the dorsal hippocampus, lateral septal area, and triangular nucleus of the septum. Analysis of the distribution of responsive sites indicated that they are poorly superposed to the known distribution of opiate-sensitive areas. Most of the structures found to be responsive to carbachol are also known to possess cholinergic receptors and to evoke antinociception following focal electrical stimulation. In various placements, particularly in limbic structures, microinjection of carbachol evoked jumping to mechanical noxious stimulation, hyperexcitability to non-noxious stimuli, convulsive reactions, and other less frequent reactions. On few occasions, however, these changes were accompanied by antinociception.

Regional distribution and properties of [3H]MK-801 binding sites determined by quantitative autoradiography in rat brain

[3H]MK-801 binding in rat brain was characterized using a quantitative autoradiographic binding assay. [3H]MK-801 binding (5 nM) reached equilibrium by 120 min at 23 degrees C. [3H]MK-801 appeared to label a single high affinity site with an affinity constant of approximately 11 nM. [3H]MK-801 binding was heterogeneously distributed throughout the brain with the following order of binding densities: hippocampal formation greater than cortical areas greater than striatum greater than thalamus. Competitive N-methyl-D-aspartate antagonists, DL-2-amino-5-phosphonopentanoic acid, DL-2-amino-7-phosphonoheptanoic acid, 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid, and cis-4-phosphonomethyl-2-piperidine carboxylic acid, inhibited [3H]MK-801 binding. Glycine antagonists, 7-chlorokynurenic acid and kynurenic acid, also inhibited [3H]MK-801 binding. Furthermore, the inhibition of [3H]MK-801 binding by the quinoxalinedione compounds 6-cyano-7-nitroquinoxaline-2,3-dione and 6,7-dinitroquinoxaline-2,3-dione was reversed by glycine. [3H]MK-801 binding was also inhibited by zinc ions [3H]MK-801 binding was enhanced by glycine or N-methyl-D-aspartate. These results demonstrate that [3H]MK-801 can be used in a quantitative autoradiographic assay as a functional probe for the N-methyl-D-aspartate receptor complex.

Antinociception elicited by electrical or chemical stimulation of the rat habenular complex and its sensitivity to systemic antagonists

The effects of intraperitoneal administration of antagonists to morphine, norepinephrine, acetylcholine, dopamine and 5-hydroxytryptamine (5-HT) have been studied on the antinociceptive effect of electrical stimulation of the rat habenular complex (HbC). The antinociceptive effect of agonists microinjected into the HbC was also examined. A 15-s period of 53 microA rms sine-wave stimulation of the HbC significantly increased the latency of the tail-flick reflex to noxious heat for periods of up to 15 min. This effect was significantly attenuated by pretreating rats with naloxone (1 mg/kg) or phenoxybenzamine (5 mg/kg). Methysergide (5 mg/kg), haloperidol (5 mg/kg), atropine (1 mg/kg), and mecamylamine (1 mg/kg) had little effect on the antinociceptive effect of HbC stimulation. L-Glutamate (3.5 and 7.0 micrograms), morphine (1.0 and 5.0 micrograms), and carbachol (0.4 and 0.8 micrograms), but not 5-HT (5 micrograms), dopamine (5 micrograms) or norepinephrine (5 micrograms), induced a dose-dependent increase in the tail-flick latency when microinjected into the HbC. The effect of carbachol was significantly attenuated in rats previously treated with intraperitoneal administration of atropine or mecamylamine and fully depressed in rats previously treated with a combination of these two cholinergic antagonists. It is concluded that antagonists of opiate receptors and alpha-adrenoceptors, but not dopamine or cholinergic receptors, reduce the antinociceptive effects of HbC stimulation. These observations differ from the reported effects of these antagonists on the antinociception caused by stimulating the periaqueductal gray, but resemble the antinociception caused by stimulating the ventrolateral medulla and locus coeruleus.(ABSTRACT TRUNCATED AT 250 WORDS)

Organization of serotonin 1A and 1B receptors in the nucleus of the solitary tract

We utilized 3H-8-hydroxy-N,N-dipropyl-2-aminotetralin (3H-DPAT) and 125I-iodocyanopindolol (125I-CYP) to label serotonin (5HT) 1A and 5HT1B receptors, respectively, in sections of the rat brain after characterizing the pharmacologic specificity of these agents. We then used quantitative autoradiography to measure the concentrations of 5HT1A and 5HT1B receptors in individual subnuclei of the nucleus of the solitary tract (NTS) and adjacent structures of the dorsal vagal complex. The highest 5HT1A receptor concentrations were observed within the central and intermediate subnuclei of the NTS, with low quantities of 3H-DPAT binding sites observed in the hypoglossal nucleus and dorsal motor nucleus of the vagus. In contrast, the density of 5HT1B receptors was relatively homogeneous through all NTS subnuclei, with the highest concentrations localized within the ventrolateral subnucleus. The hypoglossal and dorsal motor nuclei had slightly higher 5HT1B receptor densities than the NTS subnuclei, whereas the area postrema had a very low density. These data suggest that 5HT1A receptors are organized in a manner consistent with the cytoarchitectural and hodological parcellation of the NTS into individual subnuclei. The high concentrations of 5HT1A receptors in the central and intermediate subnuclei suggest a role for these receptors in medullary reflex pathways subserving deglutition. The relatively high density of 5HT1B receptors in the ventrolateral subnucleus suggests that these receptors modulate respiratory neurons, whereas the diffuse organization of 5HT1B receptors in the remaining subnuclei suggests that they are associated with central 5HT afferent pathways to the NTS. Further studies will be required to understand the physiologic role of 5HT1 receptors within the NTS.

Posterior midbrain-induced locomotion

The purpose of this study was to determine the nature of the neurochemical signals which impinge on the mesencephalic locomotor region (MLR) to produce locomotion in the rat. Injections of GABA antagonists into NADPH diaphorase-positive regions (PPN) were found to induce locomotion for short episodes (5-30 sec) which were repeated for several minutes (1-40 min). Such activity was blocked by injections of GABA and the GABA agonist, muscimol. Locomotion was induced by injection of substance P (SP), which also produced short, repeated episodes of locomotion. The more potent excitatory amino acid agonist, n-methyl-d-aspartic acid (NMDA), however, did produce dose-dependent, long-lasting (20 sec-5 min) locomotor episodes which were repeated over prolonged periods at the higher concentrations used (2-24 min). Additional injections of NMDA could drive stepping from a walk to a trot to a gallop. The effects of NMDA were blocked by injections of the excitatory amino acid antagonist, aminophosphonovalerionic acid (APV) (1-10 mM). Preliminary evidence suggests that carbachol (10-50 mM), a cholinergic agonist, inhibits NMDA-induced increases in muscle tone and episodes of stepping. The effect of carbachol was blocked by the cholinergic antagonist, atropine.

Central cardiovascular effects of dihydropyridines in spontaneously hypertensive rats

Intracerebroventricular (i.c.v.) injections of dihydropyridine derivatives calcium channel agonist (BAY K8644) and antagonist (nifedipine, nicardipine, PN 200-110) induced opposite long-lasting changes in blood pressure (BP) in pentobarbital anesthetized spontaneously hypertensive rats (SMR). I.c.v. nifedipine (NIF), nicardipine (NIC), and PN 200-110 decreased mean blood pressure dose-dependently and stereoselectively, (+) NIC and (+) PN being 8 and 3 times more potent than their (-) isomers, respectively. The decrease in BP was due to a withdrawal of the sympathetic tone, since NIF- and NIC-induced falls in BP were suppressed after either hexamethonium (HXM), 6 OHDA or bilateral adrenalectomy. I.c.v. BAY K8644 increased BP dose-dependently. The i.c.v. BAY K8644-induced hypertensive effect was inhibited: a), by NIF and (+) PN but not by (-) PN, therefore probably occurring at central DHP sites; b), by HXM and reserpine, thus probably mediated by an increase in sympathetic tone; c) by i.c.v. methylatropine (MA) while i.v. MA and i.c.v. HXM had no inhibitory effect, thus probably involving central muscarinic sites. In SHR, NIC did not after the K(+)-evoked ACh release but suppressed the BAY K8644-induced increase in ACh release. In anesthetized normotensive control rats (WKY), neither i.c.v. NIF, NIC or BAY increased BP and HR while, in conscious SHR it decreased BP without any change in HR. These data increased BP and HR while, in conscious SHR it decreased BP without any change in HR. These data suggest that central DHP sites may be involved in the cholinergic transmission and may participate in genetic hypertension via sympathetic tone.

Regional differences in the distribution of muscarinic cholinergic receptors in the macaque cerebral cortex

The in vitro autoradiographic technique was used to characterize the density and laminar distribution of muscarinic cholinergic receptors in 12 cytoarchitectonic areas in the frontal, parietal, and occipital lobes of the rhesus monkey. The entire population of muscarinic receptors was labeled with [3H]quinuclidinyl-benzilate; the M1 receptor subtype was labeled with [3H]pirenzepine; and the density of the M2 receptor subtype was estimated by subtracting the density of M1 receptors from the total population. The overall density of M1 and M2 receptor subtypes was similar throughout the cerebral cortex. However, their laminar distribution varied regionally. In cortical regions of the parietal and occipital lobes and in the primary motor cortex of the frontal lobe, both M1 and M2 receptor subtypes were concentrated in the supragranular layers. By contrast, in prefrontal cortical areas, the combined population of M1 and M2 receptors was evenly distributed across the cortical layers, though M1 receptors were most dense and M2 receptors least dense in layer IV. The difference in the distribution of cholinergic receptors in the prefrontal cortex compared to other neocortical areas reveals a degree of chemoarchitectural specificity of this region with respect to cholinergic markers that has escaped immunohistochemical and other anatomical and functional techniques.

A direct nicotinic receptor-mediated inhibition recorded intracellularly in vitro

Acetylcholine activates both nicotinic and muscarinic receptors in the central nervous system. Although the action of acetylcholine at muscarinic receptor has been well characterized, relatively little is known at the cellular level concerning nicotinic receptor stimulation in brain. Central nicotinic receptors have been implicated in Alzheimer's disease, seizure activity, the generation of slow-wave theta rhythm in the hippocampus and the potential abuse liability of nicotine. At the neuronal level, nicotinic agonists have been most often associated with postsynaptically mediated excitation and membrane depolarization at various sites, including Renshaw spinal motoneurons, locus coeruleus and the medial habenular nucleus. Nicotine acting presynaptically can produce either excitation or inhibition indirectly through the release of endogeneous transmitters or modulators. Whereas a direct inhibitory effect of nicotine has been suggested by one in vivo extracellular recording study in rat cerebellar Purkinje neurons, the mechanism(s) underlying this action is not yet known. We now report our findings obtained using in vitro intracellular methods in a submerged brain slice preparation in which application of nicotinic agonists to rat dorsolateral septal neurons reveal a direct membrane hyperpolarization mediated by an increase in potassium conductance.

Striatal muscarinic receptors are associated with substance P and somatostatin containing neurons

The cellular localization of muscarinic acetylcholine binding sites (mAChr) in relation to immunohistochemically characterized cell populations within the rat caudate nucleus has been determined using in vitro autoradiography of the reversible antagonist ligand, quinuclidinyl benzilate [( 3H]QNB). The pattern of autoradiographic silver grain deposition in the striatum was contrasted with the localization of two peptide-containing neuronal populations in the striatum. Substance P-immunoreactive somata demonstrated prevalent association of mAChr binding sites, as did somatostatin-immunoreactive cells. Substantially more striatal muscarinic binding sites were aggregated over the somatostatin interneuron population of the caudate nucleus than were associated with the substance P somata in concurrently performed experiments. This data further substantiates the heterogeneity in organization of the caudate nucleus, and the results are discussed in relation to the processing of information within this basal ganglia region.

Cholecystokinin binding sites in the rat forebrain: effects of acute and chronic methamphetamine administration

Using the in vitro quantitative receptor autoradiographical technique, changes in the binding parameters of [propionyl-3H] propionylated CCK-8 [( 3H]pCCK-8) binding sites in the rat forebrain were investigated following acute and chronic administration of methamphetamine (MAP). The (Kd)app values of [3H]pCCK-8 binding sites in the frontal medial cortex and anterior cingulate cortex were significantly reduced after a single injection of 4mg/kg MAP. On the other hand, chronic treatment (14 days) with MAP at this dose significantly decreased the Bmax value of [3H]pCCK-8 binding sites in the anterior cingulate cortex accompanied by supersensitivity of locomotor effects to MAP. These findings suggest that dopamine (DA) neurons in these two regions are functionally related to intrinsic CCK-containing cortical neurons, and that CCK subsensitivity, perhaps due to an alteration in DA transmission, is involved in MAP sensitization. These findings may be relevant to the DA hypothesis of schizophrenia.

Effect of in vivo exposure to hypoxia on muscarinic cholinergic receptor-coupled phosphoinositide turnover in the rat brain

We examined the effects of a transient exposure of rats to severe (5% O2, 95% N2 for 30 min) or mild (10% O2, 90% N2 for 30 min) hypoxia and a chronic exposure to mild hypoxia (10% O2, 90% N2 up to 48 h) on carbachol-stimulated phosphoinositide (PI) turnover and [3H]quinuclidinyl benzilate (QNB) binding in 4 regions of the brain (cerebral cortex, striatum, hippocampus and cerebellum). When the rats were exposed to hypoxia transiently, significant changes were caused only by severe hypoxia. In all 4 regions examined, basal incorporation of [3H]inositol into inositol phosphates (IPs) in the absence of carbachol was significantly increased following transient severe hypoxia and remained high for 48 h. Carbachol stimulation of PI turnover (% of basal) was significantly enhanced only in the hippocampus. The increase in carbachol stimulation in the hippocampus was observed only after a 6-h exposure to room air following the severe hypoxia and lasted for at least 48 h. In Ca2+-free incubation medium, the increase in basal incorporation of [3H]inositol into IPs elicited by severe hypoxia was not observed but carbachol still had an enhanced stimulatory effect. Binding studies showed that maximum binding capacity (Bmax values) of [3H]QNB binding was significantly increased only in the hippocampus and only after a 6-h exposure to room air following the severe hypoxic exposure and remained increased for 48 h. Kd values showed no significant change. While a transient exposure of rats to mild hypoxia caused no significant change either in carbachol-stimulated PI turnover or in [3H]QNB binding parameters, a chronic exposure to mild hypoxia for more than 6 h caused a significant increase in [3H]QNB binding capacity localized in the hippocampus, which was accompanied by an enhanced stimulatory effect of carbachol on PI turnover. These changes observed with mild hypoxia, however, were transient and tended to return to control levels following a 48-h exposure. These results suggest an up-regulation of muscarinic cholinergic receptor-coupled PI turnover in the rat brain caused either by transient severe hypoxia or by chronic mild hypoxia.

Autoradiographic analyses of agonist binding to muscarinic receptor subtypes

The binding of four muscarinic receptor agonists to regions of rat brain was examined through quantitative autoradiographic techniques. Oxotremorine, arecoline, pilocarpine and bethanechol were chosen based on their different potencies and efficacies in muscarinic second messenger systems. Overall, the order of potency for inhibition of [3H]-l-quinuclidinyl benzilate ([3H]-l-QNB) binding to rat brain slices was oxotremorine greater than pilocarpine = arecoline much greater than bethanechol. Regional assays of agonist potency indicated that all agonists were more selective for brainstem and thalamic regions than for hippocampal and cortical regions. The high selectivity of agonists for areas such as the paraventricular thalamus and the superior colliculus, which also display low affinity for pirenzepine, suggests that muscarinic agonists bind with higher affinity to M2 receptors. Of the four agonists examined, pilocarpine displayed the lowest selectivity for M2 receptors in that IC50 values for pilocarpine were only 3-fold higher in the hippocampal and striatal regions (e.g. CA3: 40.6 +/- 9.4 microM) than in thalamic and brainstem regions (e.g. paraventricular thalamus: 14.9 +/- 6.2 microM). Oxotremorine was 8-fold more potent in the brainstem and thalamus, while arecoline and bethanechol were, respectively, 19- and 100-fold more selective for brainstem and thalamic receptors. Scatchard analyses revealed heterogeneous binding profiles for some agonists within single brain regions, suggesting that multiple agonist sites exist even within regions of predominantly M1 or M2 receptors. For example, arecoline displayed curved Scatchard plots within the external layers of the cerebral cortex, layer CA1 of the hippocampus (predominantly M1 subtype), and the paraventricular thalamus (predominantly M2 subtype). The ability of agonists to recognize multiple sites within a single region may reflect the ability to recognize receptors coupled or uncoupled to second messenger systems through G-proteins.

Muscarinic receptors on cultured cells of rat hippocampus: cholinergic regulation and presence of subtypes

Muscarinic acetylcholine receptors in intact, cultured explants of rat hippocampus were investigated in binding experiments with tritiated quinuclidinyl benzilate ([3H]QNB) as ligand. Dissociation constants (Kd) were determined to 320-575 pM and maximal binding capacity (Bmax) to 67-87 fmol/explant. The KdS obtained in kinetic experiments were very similar. Hippocampal explants cultured alone contained more muscarinic receptors than hippocampal explants reinnervated by cholinergic fibers from co-cultured septal explants. Pretreatment of hippocampal explants with carbachol resulted in a down-regulation of receptor number which was counteracted by the simultaneous addition of atropine. Atropine added alone had no effect on receptor number in hippocampal explants cultured alone whereas it occasionally caused an up-regulation in co-cultured hippocampus. Displacement experiments with scopolamine and oxotremorine as competitors, showed that hippocampal explants cultured alone contain multiple types of muscarinic receptors. With atropine, pirenzepine and AF-DX 116, only one class of receptors could be detected.

The cholinergic innervation of normal and transplanted superior colliculus in the rat: an immunohistochemical study

The distribution of choline acetyltransferase was determined in normal and transplanted rat superior colliculus with choline acetyltransferase immunohistochemistry. This distribution was compared to the pattern of histochemically detected acetylcholinesterase activity. To determine cholinergic input to the superficial superior colliculus, double labelling experiments combining retrograde tracing methods and choline acetyltransferase immunohistochemistry were carried out. No choline acetyltransferase-containing neurons were observed in the rat superior colliculus. A dense network of choline acetyltransferase-immunoreactive fibres and terminals was seen in the intermediate layers of the normal superior colliculus. The distribution was patchy and very similar to the pattern of acetylcholinesterase activity. Occasional fibres and terminals were seen in the deep tectal laminae. The superficial layers contained a low number of choline acetyltransferase-stained fibres and terminals but a comparatively high level of acetylcholinesterase activity. Following a unilateral injection of a tracer into the superficial superior colliculus, retrogradely labelled choline acetyltransferase-immunoreactive neurons were found in the dorsal and ventral subnuclei of the ipsilateral parabigeminal nucleus. As in the normal superior colliculus, choline acetyltransferase-positive neurons were not found in tectal transplants. However, choline acetyltransferase-immunoreactive fibres and terminals were seen in grafts but only in those which had extensive connections with the host midbrain. The pattern of staining most closely resembled that seen in the intermediate layers of the normal superior colliculus. The similar arrangement of choline acetyltransferase and acetylcholinesterase activity in the intermediate layers of normal rat superior colliculus provides further evidence for cholinergic innervation to these layers, probably originating in the dorsal and pedunculopontine tegmental nuclei. The data from the double labelling experiments indicate that the choline acetyltransferase-immunoreactive terminals observed in the superficial layers represent the terminal field of an ipsilateral cholinergic projection from the parabigeminal nucleus. Tectal grafts receive cholinergic innervation from the host. The evidence suggests that much of this input derives from the cholinergic nuclei in the brainstem tegmentum which normally project to the intermediate tectal layers.

Distribution of major neurotransmitter receptors in the motor and somatosensory cortex of the rhesus monkey

The in vitro quantitative autoradiographic technique was used to characterize the distributions of alpha 1, alpha 2, beta 1 and beta 2 adrenergic, D1 and D2 dopaminergic, 5-HT1 and 5-HT2 serotonergic, M1 and M2 cholinergic, GABAA and benzodiazepine receptors in the motor (Brodmann's area 4) and somatosensory (Brodmann's areas 3, 1 and 2) cortex of the adult rhesus monkey. All receptor subtypes studied were present throughout all layers of both areas. In the somatosensory cortex, each receptor had its own laminar distribution. Some subtypes of the same receptor (5-HT1 and 5-HT2; alpha 1 and alpha 2) had complementary distributions while others (beta 1 and beta 2; D1 and D2; M1 and M2) had largely overlapping distributions. In contrast, different receptors had remarkably coincidental distributions in the motor cortex. In this area, they all tended to concentrate in layers I, II and the upper part of layer III. However, such coextensive distribution of many types of neurotransmitter receptors is not observed in motor cortex of rats and humans and therefore may be a distinctive feature of motor cortex in the rhesus monkey. The findings described in this paper indicate that somatosensory and motor areas are distinct in their receptor architecture and that receptor autoradiography provides a useful complement to classical histological techniques in elucidating areal differences in the cortex.

Characterization and quantitative autoradiographic distribution of [3H]acetylcholine muscarinic receptors in mammalian brain. Apparent labelling of an M2-like receptor sub-type

[3H]Acetylcholine receptor binding characteristics (under muscarinic conditions) have been investigated using membrane binding assays and in vitro receptor autoradiography. In rat, guinea-pig and monkey brain membrane preparations, [3H]acetylcholine binds with high affinity (25-50 nM) to an apparently single class of sites which is differentially distributed across brain regions. The ligand selectivity pattern reveals that the potency of (-)quinuclidinyl benzylate is greater than (greater than) atropine greater than scopolamine greater than oxotremorine greater than carbamylcholine greater than pirenzepine greater than methylcarbamyl-choline = nicotine in competing for [3H]acetylcholine binding sites, indicating that [3H]acetylcholine selectively binds to muscarinic sites under these incubation conditions. Moreover, the low potency of pirenzepine suggests that [3H]acetylcholine does not label a significant proportion of the M1 receptor sub-type but most likely binds to putative M2-like receptor sites. This hypothesis is also supported by the autoradiographic distribution of [3H]acetylcholine binding sites in all species studied here. High densities of [3H]acetylcholine binding sites are seen in various nuclei of the medulla and pons, certain thalamic nuclei, medial septum, laminae III, V and VI of the cortex and just above the pyramidal cell layer of the hippocampus. Such localization is much different from that seen with the non-selective antagonist [3H]quinuclidinyl benzylate and the selective M1 receptor ligand [3H]pirenzepine, although it resembles that of the selective M2 receptor antagonist [3H]AF-DX 116. Thus, [3H]acetylcholine apparently mostly binds with high affinity mainly to non-M1 muscarinic receptor types in mammalian brain tissues. Moreover, the ligand selectivity pattern and in vitro receptor autoradiographic data suggest that at low concentrations (10-20 nM) most of [3H]actylcholine labelled sites are of the M2-like receptor class.

Modulation of locomotor activity induced by injections of carbachol into the tegmental pedunculopontine nucleus and adjacent areas in the rat

The pedunculopontine nucleus (PPN) is a major component of the mesencephalic locomotor region. There is little known, however, about neurotransmitters in the PPN associated with locomotor activity. The purpose of the present study was to investigate a possible modulatory effect of the cholinergic system on locomotion. The effects of application of carbachol (CCh) into the PPN on locomotor activity of freely moving rats were studied. Unilateral injections of CCh into the PPN decreased spontaneous locomotor activity of rats. On the other hand, an increase in locomotor activity resulted from CCh injections into sites surrounding the PPN. These CCh-induced changes in locomotion were no longer observed after pretreatment of the PPN with atropine. Locomotor activity induced by injections of amphetamine into the nucleus accumbens was also reduced to control levels by ipsilateral injections of CCh into the PPn, whereas contralateral injections of CCh were ineffective. The results suggest that the muscarinic cholinergic system has a modulatory influence on locomotor activity presumably by affecting PPN cells involved in relaying locomotion-associated signals. The PPN receives signals from higher structures involved in initiation of locomotion while the muscarinic system seems to play a role in attenuation or inhibition of locomotor behaviour.

Sensory deafferentation fails to modify muscarinic receptor binding in raccoon somatosensory cortex

The characteristics and distribution of muscarinic acetylcholine (mACh) receptor binding in primary somatosensory (SI) cortex and the caudate nucleus of raccoons were studied using [3H]-QNB, a muscarinic antagonist. The binding characteristics were similar to reported values in rat and cat. Autoradiographs produced from tissue sections labeled with [3H]-QNB showed the distribution of mACh receptors in the forebrain of the raccoon. [3H]-QNB binding was highest in cerebral cortex, neostriatum and hippocampus. Within SI cortex, binding was high in layers I-III and VI and relatively low in layers IV and V. Autoradiographs obtained from animals that had undergone peripheral deafferentation of part of the forepaw revealed no changes in [3H]-QNB binding in the affected cortical region during the time that physiological reorganization is known to occur.

Transient concordant distributions of nicotinic receptors and acetylcholinesterase activity in infant rat visual cortex

For a short period during the second week after birth, layer IV of rat visual cortex displays intense acetylcholinesterase (AChE) activity which demarcates area 17. We now report that this transient AChE pattern is paralleled by a marked increase in [3H]nicotine binding sites in layer IV of area 17. Muscarinic receptors show a different pattern. Neonatal bilateral enucleation results in a reduction of AChE reaction product and [3H]nicotine labelling in area 17, while leaving the pattern of muscarinic labelling virtually unaffected. The close association of [3H]nicotine labelling and AChE activity suggests that presynaptic nicotinic cholinergic receptors may modulate transmitter release while functional geniculocortical synapses are being formed.

Autoradiographic analysis of muscarinic receptors in rat nasal glands

An in vitro method was developed for the biochemical and autoradiographic demonstration of low muscarinic receptor densities in peripheral tissue. Histological criteria point clearly to the necessity for fixation to preserve tissue quality. [3H]l-Quinuclidinylbenzilate bound specifically to a homogeneous class of binding sites in 0.5% glutardialdehyde-fixed cryostat sections (10 microns) of rat nasal glands with high affinity (Kd = 0.47 +/- 0.06 nM) and with a receptor density (Bmax) of 41 +/- 1 fmol/mg protein. This binding was linearly dependent on the thickness of the sections. Kinetic experiments resulted in a Kd value of 0.19 nM. Binding was stereoselectively inhibited by benzetimide enantiomers. Autoradiograms, generated after incubation with 0.6 nM [3H]l-quinuclidinylbenzilate and dipping in nuclear K2 emulsion, showed specific labelling of the glandular acini and excretory ducts. These in vitro observations provide conclusive evidence for the presence of acetylcholine receptors in the nasal glands of the rat.

Central interactions between dihydropyridines and cholinergic systems in the control of blood pressure in rat

Intracerebroventricular (i.c.v.) injection of the 1,4-dihydropyridine (DHP) calcium channel agonist, Bay K8644 (30 micrograms/kg) increased mean blood pressure and the K+-evoked release of [3H]acetylcholine ([3H]ACh) from hippocampal slices in spontaneously hypertensive rats (SHR). The Bay K8644-induced hypertension was inhibited by a pretreatment with methylatropine (80 micrograms/kg i.c.v.). In SHR, nicardipine, a DHP calcium channel antagonist, reduced mean blood pressure when i.c.v. injected (10 micrograms/kg). The nicardipine-induced hypotension was reduced by a pretreatment with hemicholinium-3 (20 micrograms, i.c.v.). Nicardipine (1 microM) did not modify, in SHR, the K+-evoked release of [3H]ACh, but inhibited the Bay K8644-induced increase in the ACh release. In normotensive rats, neither Bay K8644 nor nicardipine modify blood pressure, when centrally injected, or the stimulated release of [3H]ACh from hippocampal slices. The participation of central DHP sites in the cholinergic transmission in genetic hypertension is discussed.

The functional anatomy and pathology of lithium-pilocarpine and high-dose pilocarpine seizures

Subcutaneous treatment of rats with low doses of lithium and pilocarpine or a high dose of pilocarpine results in a severe seizure--brain damage syndrome. Rats thus treated were studied with multiple-depth electrodes, quantitative [14C]2-deoxyglucose autoradiography, and light and electron microscopy. Rats receiving lithium-pilocarpine did not differ from high-dose pilocarpine rats in behavioral, electrographic, metabolic or histopathological findings, but lithium-pilocarpine reproduced the syndrome more reliably and with a lower acute mortality rate. Organized electrographic seizure activity developed just prior to the onset of behavioral forelimb clonus and appeared to originate from ventral forebrain in the vicinity of the ventral pallidum and/or nucleus accumbens. From these sites activity spread rapidly to involve other regions. Once initiated, electrographic seizures persisted for hours. Increased glucose utilization was found in most brain regions during the period of continuous seizure activity. The greatest increases were found in the ventral pallidum, globus pallidus, hippocampus, entorhinal cortex, amygdala, lateral septum, substantia nigra, ventrobasal and mediodorsal thalamus and frontal motor cortex. Animals sustaining seizures displayed a disseminated pattern of neural degeneration not involving globus pallidus or ventral pallidum but otherwise coinciding with the above pattern of enhanced glucose utilization. No consistent correlation was observed between the pattern of brain damage and known regions of high muscarinic cholinergic receptor density. Ultrastructurally, the cytopathological changes, like those associated with various other sustained seizure syndromes, resemble the excitotoxic type of damage glutamate is known to cause. This seizure-brain damage syndrome and that induced by systemic kainic acid appear to be similar in behavioral but not in electrophysiological or metabolic manifestations. During kainic acid seizures, electrographic changes are first recorded in the hippocampus while they are first detected in the ventral forebrain region in pilocarpine seizures. Pilocarpine also induced metabolic activation of ventral forebrain sites not activated by kainic acid. The cytopathology associated with the two syndromes is identical in type but not in pattern, the cholinergic model being characterized by much greater neocortical and slightly less hippocampal damage. Further study of these cholinergic models may provide new insights into the roles of the major excitatory neurotransmitter systems (cholinergic and glutamergic) in limbic epilepsy.

Quantitative autoradiography of muscarinic and benzodiazepine receptors in the forebrain of the turtle, Pseudemys scripta

The distribution of muscarinic and benzodiazepine receptors was investigated in the turtle forebrain by the technique of in vitro receptor autoradiography. Muscarinic binding sites were labeled with 1 nM 3H-quinuclidinyl benzilate (3H-QNB), and benzodiazepine sites were demonstrated with the aid of 1 nM 3H-flunitrazepam (3H-FLU). Autoradiograms generated on 3H-Ultrofilm apposed to tissue slices revealed regionally specific distributions of muscarinic and benzodiazepine binding sites that are comparable with those for mammalian brain. Dense benzodiazepine binding was found in the anterior olfactory nucleus, the lateral and dorsal cortices, and the dorsal ventricular ridge (DVR), a structure with no clear mammalian homologue. Muscarinic binding sites were most dense in the striatum, accumbens, DVR, lateral geniculate, and the anterior olfactory nucleus. Cortical binding sites were studied in greater detail by quantitative analysis of autoradiograms generated by using emulsion-coated coverslips. Laminar gradients of binding were observed that were specific for each radioligand; 3H-QNB sites were most dense in the inner molecular layer in all cortical regions, whereas 3H-FLU binding was generally most concentrated in the outer molecular layer and was least dense through all layers in the dorsomedial cortex. Because pyramidal cells are arranged in register in turtle cortex, the laminar patterns of receptor binding may reflect different receptor density gradients along pyramidal cell dendrites.

Reversal of scopolamine-induced amnesia and alterations in energy metabolism by the nootropic piracetam: implications regarding identification of brain structures involved in consolidation of memory traces

Pretreatment with scopolamine, 3 mg/kg, prevented the acquisition of a passive avoidance task in rats. These amnesic effects of scopolamine could largely be overcome by treatment with 100 mg/kg of the nootropic drug piracetam. In order to identify the brain structures involved, the effects of these drugs on regional energy metabolism were measured throughout the brain, utilizing Sokoloff's 2-deoxyglucose autoradiographic procedures. Scopolamine, 3 mg/kg, reduced glucose utilization in several areas of the cerebral cortex. These effects were largest in the parietal and temporal cortices. Other areas affected included the sensorimotor and cingulate cortices, the ventral and lateral thalamus, and the dendritic neuropil of the CA1, CA2, and CA3 regions of the hippocampus. The regional depressions in glucose metabolism observed following scopolamine treatment in the rat had some resemblance to depressions in glucose metabolism reported for Alzheimer's disease patients in positron emission tomography studies. Piracetam, 100 mg/kg, did not alter the energy metabolism of any of the 41 brain regions examined. However, this dose of piracetam completely reversed the scopolamine-induced depressions in the hippocampus. Piracetam partially but significantly reversed the scopolamine effects in the cingulate cortex. It is concluded that the data provide support for the hippocampal-cholinergic theory of memory as originally formulated by Meyers and Domino in 1964 and give insight into the mechanisms by which nootropics work.