Muscarinic cholinergic receptors mediate the cerebrovasodilation elicited by stimulation of the cerebellar fastigial nucleus in rat

Dorsal extensions of the fastigium cerebelli: an anatomical study using magnetic resonance imaging

BACKGROUND AND PURPOSE

The fastigium cerebelli is an important topographical landmark for neurosurgeons and radiologists. However, few studies have characterized the morphology of the fastigium cerebelli. We aimed to investigate the fastigium cerebelli using postmortem specimens and magnetic resonance imaging (MRI) in vivo.

MATERIALS AND METHODS

Three cadaveric brains were midsagittally sectioned for observing the fastigium cerebelli. Additionally, 66 outpatients underwent MRI, including sagittal T1-weighted imaging, axial T2-weighted imaging, and coronal constructive interference in steady-state (CISS) sequence.

RESULTS

In the cadaveric specimens, the fastigium cerebelli was observed as a beak-like dorsal protrusion of the fourth ventricle. Its inner surface was observed as a small fovea. On serial CISS images, the fastigium cerebelli consistently possessed a pair of triangular-shaped, dorsal extensions lying parasagittally along the nodule. These extensions were classified as symmetrical, right-side dominant, or left-side dominant. The symmetrical type was the most predominant and comprised 60.6% of the extensions, while the right-side dominant and left-side dominant types comprised 13.6 and 25.8%, respectively. In 91% of the 66 patients, the number of slices covering the entirety of the dorsal extensions were the same on both sides. The fastigial angle (θ) formed by lines tangent to the superior and inferior medullary velums varied widely.

CONCLUSIONS

The fastigium cerebelli has a pair of dorsal extensions lying parasagittally along the nodule. Coronal CISS sequence is useful in delineating the fastigium cerebelli in vivo.

Cardiovascular modules in the cerebellum

Mapping with local lesions, electrical or chemical stimulation, or recording evoked field potentials or unit spikes revealed localized representations of cardiovascular functions in the cerebellum. In this review, which is based on literatures in the field (including our own publications), I propose that the cerebellum contains five distinct modules (cerebellar corticonuclear microcomplexes) dedicated to cardiovascular control. First, a discrete rostral portion of the fastigial nucleus and the overlying medial portion of the anterior vermis (lobules I, II and III) conjointly form a module that controls the baroreflex. Second, anterior vermis also forms a microcomplex with the parabrachial nucleus. Third, a discrete caudal portion of the fastigial nucleus and the overlying medial portion of the posterior vermis (lobules VII and VIII) form another module controlling the vestibulosympathetic reflex. Fourth, the medial portion of the uvula may form a module with the nucleus tractus solitarius and parabrachial nucleus. Fifth, the lateral edge of the nodulus and the uvula, together with the parabrachial nucleus and vestibular nuclei, forms a cardiovascular microcomplex that controls the magnitude and/or timing of sympathetic nerve responses and stability of the mean arterial blood pressure during changes of head position and body posture. The lateral nodulus-uvula appears to be an integrative cardiovascular control center involving both the baroreflex and the vestibulosympathetic reflex.

Neurogenic neuroprotection

1. Stimulation of the rostral-ventromedial pole of the cerebellar fastigial nucleus exerts powerful effects on systemic and cerebral circulation. 2. Excitation of fibers passing through the fastigial nucleus evokes sympathoactivation and increases in arterial pressure. 3. Increase in cerebral blood flow evoked by excitation of fibers passing through the FN is mediated by intrinsic brain mechanisms independently of metabolism. 4. Excitation of the fastigial nucleus neurons in contrast decreases arterial pressure and cerebral blood flow. The latter probably is secondary to the suppression of brain metabolism. 5. Excitation of the fastigial nucleus neurons significantly decreases damaging effects of focal and global ischemia on the brain. 6. The fastigial nucleus-evoked neuroprotection can be conditioned: 1-h stimulation protects the brain for up to 3 weeks. 7. Other brain structures such as subthalamic cerebrovasodilator area and dorsal periaqueductal gray matter also produce long-lasting brain salvage when stimulated. 8. More than one mechanism may account for neurogenic neuroprotection. 9. Early neuroprotection, which develops immediately after the stimulation, involves opening of potassium channels. 10. Delayed long-lasting neuroprotection may involve changes in genes expression resulting in suppression of inflammatory reaction and apoptotic cascade. 11. It is conceivable that intrinsic neuroprotective system exists within the brain, which renders the brain more tolerant to adverse stimuli when activated. 12. Knowledge of the mechanisms of neurogenic neuroprotection will allow developing new neuroprotective approaches.

The pedunculopontine tegmental nucleus issues collaterals to the fastigial nucleus and rostral ventrolateral reticular nucleus in the rat

The pedunculopontine-laterodorsal tegmental nuclear complex was identified as a major source of brainstem afferents terminating in the fastigial cerebellar nucleus and/or ventrolateral reticular nucleus (n.Rvl). Collaterals from the pedunculopontine nucleus (Ch5 area) to rostral [vasopressor] regions of the fastigial nucleus and ventral reticular formation were revealed with a combined retrograde tracing technique. The data implicate acetylcholine as a transmitter and raise the hypothesis that the identified afferents may contribute to the autonomic and behavioral responses to midline cerebellar stimulation.

Cholinergic and vasoactive intestinal polypeptidergic innervation of the cerebral arteries

Acetylcholine and vasoactive intestinal polypeptide are not only two vasoactive agonists that predominantly induce a vasodilatation of the cerebral arteries, but also correspond to neurotransmitters that innervate the various anatomical segments of the cerebral vasculature. The distinct patterns of the cerebrovascular cholinergic and vasoactive intestinal polypeptidergic innervation, their neurochemistry, in vitro and in vivo pharmacology, as well as the putative pathophysiological implications of these neurotransmission systems are critically summarized on the basis of the most recently published literature.

Role of the cerebellar fastigial nucleus in the physiological regulation of cerebral blood flow

Local cerebral blood flow (ICBF) was measured with [14C]iodoantipyrine in conscious, unrestrained rats during electrical stimulation of the fastigial nucleus (FN). Electrode position in the FN was determined by blood pressure (MABP) responses to stimulation under anesthesia. In nine rats in which MABP responses had been variable under anesthesia, bipolar stimulation (50 Hz, 0.5 ms, 1 s on/1 s off) with currents of 30-100 microA after recovery from anesthesia produced stereotypic behavior but little effect on MABP and ICBF. In seven other conscious rats currents could be raised to 75-200 microA without inducing seizures, resulting in sustained MABP elevations during the ICBF measurement and significantly increased ICBF in the sensory-motor (+45%), parietal (+31%), and frontal cortices (+56%) and the caudate-putamen (+27%) above control values (n = 9). Glucose utilization, measured with [14C]deoxyglucose, in rats similarly stimulated was significantly increased in six structures, including some of the above, indicating increases in ICBF due to metabolic activation. Unilateral or bilateral electrolytic lesions of the FN, placed 6-7 days before ICBF measurement, had negligible effects on resting ICBF and on autoregulation in conscious rats. These results fail to support a specific role for the FN in physiological regulation of cerebral blood flow in unanesthetized rats.

Light and electron microscopic immunocytochemical analysis of the neurovascular relationships of choline acetyltransferase and vasoactive intestinal polypeptide nerve terminals in the rat cerebral cortex

Acetylcholine or vasoactive intestinal peptide (VIP) nerve terminals closely related to intracortical blood vessels have previously been reported. Recent physiological evidence indicates that these central neuronal systems are involved in the fine control of local cerebral blood flow. In the present study, the intimate associations between choline acetyltransferase (ChAT) and VIP axon terminals and intracortical microvessels were characterized by light (LM) and electron microscopic (EM) immunocytochemistry. In semithin sections, LM analysis of the distribution of ChAT- and VIP-immunostained puncta juxtaposed to small intraparenchymal blood vessels demonstrated that neither type of terminal was enriched or impoverished around microvessels within the cerebral cortex. At the EM level, most ChAT- or VIP-immunolabelled elements located within a 3 microns perimeter around vessel walls were axon terminals. These perivascular terminals were associated primarily with capillaries but also, to a lesser extent, with microarterioles. Even though ChAT and VIP terminals were frequently found in the immediate vicinity (< or = 0.25 microns) of microvessels, they almost never contacted the outer basal lamina, usually abutting onto perivascular astroglial leaflets. There were no membrane specializations at the site of contact between ChAT or VIP terminals and perivascular astroglia. In all cortical areas examined, the average size of VIP-immunolabelled varicosities (0.56 +/- 0.04 microns 2) was significantly larger than that of their ChAT counterparts (0.32 +/- 0.02 microns 2; P < 0.001). Perivascular VIP terminals were more frequently engaged in synaptic contact than those immunostained for ChAT, which rarely exhibited a synaptic junction even in serial thin sections. Neither VIP nor ChAT immunostaining was ever observed in endothelial cells. These results suggest that both acetylcholine and VIP exert their effects on intracortical microvessels through indirect, paracrine mechanisms. The marked difference in synaptic incidence and average size between both types of perivascular terminals indicates that these two vasoactive agents are primarily located in distinct neuronal populations. Further, our results show that the astrocytic glia is the major direct target for both ChAT and VIP perivascular terminals and suggest that neuronal/glial/vascular interactions are a key element in the neurogenic control of the intracortical microcirculation.

Effect of acute and chronic arecoline treatment on cerebral metabolism and blood flow in the conscious rat

Treatment with the muscarinic agonist arecoline improves memory retention in patients with Alzheimer's disease (AD). In animal models, arecoline selectively increases local cerebral glucose utilization (LCGU). We examined (1) whether these focal increases in metabolism were coupled to local cerebral blood flow (LCBF) and (2) whether the effect of arecoline on LCGU and LCBF was dependent upon duration of drug administration. In groups of young Fischer-344 rats, LCGU and LCBF were determined in 59 brain regions by the [14C]2-deoxyglucose and the [14C]iodoantipyrine autoradiographic methods following either the acute administration of arecoline (2 mg/kg and 15 mg/kg) or the chronic three week administration of arecoline (50 mg/kg/day). In general, LCBF correlated closely with LCGU following arecoline 2 mg/kg administration, but heterogeneous regions were present. Following treatment with arecoline 15 mg/kg, the two parameters became uncoupled with LCBF increasing disproportionately in relation to LCGU. Coupling between LCBF and LCGU was preserved during chronic arecoline treatment (50 mg/kg/day) but some regions, such as the hippocampus, were uncoupled with LCGU increasing to a greater extent than LCBF. Thus, we demonstrate that acute and chronic administration of arecoline can differentially modulate LCBF and LCGU. Since clinical administration of arecoline can improve cognitive function in patients with AD, understanding the ability of arecoline to selectively alter LCBF and LCGU in regions such as the hippocampus may offer insight into the pathophysiology of AD and provide direction for the development of definitive therapy for neurodegenerative disorders.

Stimulation of the fastigial nucleus enhances EEG recovery and reduces tissue damage after focal cerebral ischemia

Stimulation of the cerebellar fastigial nucleus (FN) increases CBF but not metabolism and reduces the tissue damage resulting from focal cerebral ischemia. This effect may result from enhancing CBF in the ischemic tissue without increasing local metabolic demands. To test this hypothesis, we studied whether the reduction in tissue damage is restricted to the neocortex, a region in which the CBF increase is independent of metabolism, and whether stimulation of the dorsal medullary reticular formation (DMRF), a treatment that increases both cerebral metabolism and CBF, also protects the brain from ischemia. In halothane-anesthetized Sprague-Dawley rats, the middle cerebral artery (MCA) was occluded either proximally or distally to the lenticulostriate branches. The FN or DMRF were then stimulated for 1 h (50-100 microA; 50 Hz; 1 s on/l s off). Twenty-four hours later, the infarct volume was determined. FN stimulation substantially reduced the size of the infarct, an effect that was greater with distal (-69 +/- 8%; n = 6; p < 0.001; mean +/- SD) than with proximal (-38 +/- 8%; n = 8; p < 0.001) MCA occlusion. The reduction occurred only in neocortex (-43 +/- 9%; p < 0.001) and not in striatum (-16 +/- 21%; p > 0.05). Stimulation of the FN also enhanced recovery of EEG amplitude in the ischemic cortex (+48%; p < 0.003). DMRF stimulation (n = 7) did not affect the stroke size or EEG recovery (p > 0.05). Thus, stimulation of the FN, but not the DMRF, attenuates the damage resulting from focal ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Focal elevations in neocortical interstitial K+ produced by stimulation of the fastigial nucleus in rat

We studied whether K+, a potent cerebrovasodilator released by active neurons, participates in the increase in cortical cerebral blood flow (CBF) elicited by stimulation of the cerebellar fastigial nucleus (FN). Rats were anesthetized by continuous administration of halothane (1-3%), paralyzed and artificially ventilated. FN was stimulated electrically (8 s trains, 50 Hz, 5-10 V) through microelectrodes positioned stereotaxically. K+o (mM) was measured in sensory cortex by K(+)-sensitive micropipettes. In some experiments neocortical CBF was monitored continuously by laser-doppler flowmetry. Stimulation of the FN produced significant increases in K+o that averaged 0.91 +/- 0.16 mM (range 0.5-2.9 mM; n = 19) and were confined to sites corresponding to the intermediate cortical laminae (P less than 0.05, ANOVA). To determine whether such K+o elevations were able to produce increases in CBF comparable to those elicited by FN stimulation, cortical K+o was increased by superfusing the sensory cortex with 20-30 mM K+ in Ringer. K+o elevations of 2.8 +/- 0.6 mM increased CBF by 17 +/- 2% (n = 5), an increase considerably smaller than that elicited by FN stimulation in cerebral cortex. We conclude that K+ is unlikely to mediate the cortical cerebrovasodilation. Furthermore, the restricted spatial distribution of the K+o increase indicates that the cortical neural activity evoked by FN stimulation is highly focal. Thus the findings support the hypothesis that, in cortex, the vasodilation is mediated by activation of a restricted group of neural elements, perhaps neurons in laminae III-IV.

Choline acetyltransferase activity associated with cerebral cortical microvessels does not originate in basal forebrain neurons

Cerebral cortical microvessels are innervated by cholinergic fibers that are probably involved in the regulation of local cerebral blood flow and blood-brain barrier permeability. The possibility exists that the cholinergic terminals associated with the cortical microvasculature belong to neurons from the nucleus basalis magnocellularis (NBM), where 70% of the cortical cholinergic projections originate. To test this hypothesis, ibotenic acid (25 nmol) was injected unilaterally in the NBM in rats, and 14 days later, choline acetyltransferase (ChAT) activity was measured in the frontoparietal cortex and in a blood vessel fraction isolated from this region. Lesions of the NBM resulted in a 50% decrease of cortical ChAT as compared with control or sham-operated hemispheres; however, no changes were observed in the ChAT activity associated with cortical microvessels. These results indicate that, in rat cerebral cortex, the perivascular cholinergic terminals do not originate in the basal forebrain.

Electrical stimulation of cerebellar fastigial nucleus reduces ischemic infarction elicited by middle cerebral artery occlusion in rat

Electrical stimulation of the cerebellar fastigial nucleus (FN) globally and profoundly increases cerebral blood flow via a cholinergic mechanism. In cerebral cortex, the vasodilation is unassociated with alterations in cerebral glucose utilization, a condition favoring protection against cerebral ischemia. We sought to determine whether FN stimulation would modify the size of the focal ischemic infarction resulting from occlusion of the middle cerebral artery (MCA). The MCA was occluded in anesthetized rats of the spontaneously hypertensive (SHR) or Sprague-Dawley (SD) strains with or without 1 h of electrical stimulation of the FN. Twenty-four hours later, rats were killed and the volume of the infarction established in thionin-stained sections. in SHRs, FN stimulation reduced by 40% the well-established cortical and partially subcortical infarctions elicited by occlusion of the MCA (from 186 +/- 35.2 to 113 +/- 47.1 mm3, mean +/- SD, n = 15; p less than 0.001). The zone of retrieval was anatomically constant, consisting of a rim of cortex dorsal and ventral to the infarction and medially within the thalamus and striatum corresponding to the penumbral zone described by others. The effect was comparable in rats of the SD strain having smaller infarctions. The effect of FN stimulation appears to be selective for the FN system in that it is not evoked by stimulation of the dentate nucleus and is blocked by systemic administration of atropine (1.0 mg/kg). We conclude that excitation of an intrinsic system in brain represented in the rostral FN has the capacity to reduce substantially an ischemic infarction.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypercapnia and stimulation of the substantia innominata increase rat frontal cortical blood flow by different cholinergic mechanisms

This study was designed to further investigate in the rat how the cerebrovascular response to excitation of the projections from the substantia innominata (SI) to the frontal cerebral cortex was mediated. Local cortical blood flow (CoBF) (by helium clearance) and tissue gas partial pressures (pO2, pCO2) (indices of energy metabolic activity) were measured in the frontal cortex in unanesthetized adult Fischer rats in response to electrical stimulation of the SI and, for comparison, in hypercapnic conditions. SI stimulation and hypercapnia increased CoBF to a similar extent (+92% and +106%, respectively). Differences between the changes in tissue gas partial pressures under hypercapnia and SI stimulation suggest that different patterns of flow-metabolism coupling prevail in the mechanisms underlying the two cerebrovascular responses. Cortical pCO2 increased under hypercapnia, but decreased during SI stimulation, indicating that a 'vascular' mechanism (i.e. independent of energy metabolism activation) is at least partly responsible for the flow increase in the latter condition. However, cortical pO2 rose more under hypercapnia than during SI stimulation, suggesting that oxygen consumption, and hence energy metabolism, was increased in the latter case. The ability of the acetylcholine esterase inhibitor physostigmine and the muscarinic receptor antagonist scopolamine to modulate the responses was quantified. In both experimental conditions, CoBF changes were potentiated by 0.15 mg/kg/h physostigmine (by factors of about 2). In contrast, 1 mg/kg scopolamine reduced by 65% the frontal CoBF response elicited by SI stimulation but was without effect on the response to hypercapnia. Thus, although a cholinergic mechanism may be implicated in both responses, activation of muscarinic receptors appears to occur when the stimulation originates from the SI but not from the hypercapnia.

Regional cerebral blood flow during spreading cortical depression in conscious rats

Spreading cortical depression (SCD) of EEG activity was induced in one cerebral hemisphere of conscious restrained rats by direct current stimulation of the lateral frontal cortex. Regional CBF was measured using [14C]iodoantipyrine and brain dissection. An early phase of increased CBF was not measured in conscious rats, but an early relative hyperperfusion was measured if the resting CBF was first reduced by treatment with pentobarbital or indomethacin. A long-lasting reduction in CBF was measured in conscious rats following the passage of SCD. This flow reduction resolved after 3 h. In conscious rats, CBF decreased in the striatum and thalamus ipsilateral to the SCD, paralleling the CBF changes occurring in the cortex. The CBF change in these deep structures was abolished by pentobarbital. An early transient increase in regional CBF was measured in the cerebral cortex contralateral to the hemisphere involved with SCD in conscious rats. This early contralateral hyperperfusion was also abolished by pentobarbital or indomethacin but not by atropine or propranolol. The vascular response to SCD in conscious rats differs from that which occurs in anesthetized rats.

Neuronal connections between the cerebellar nuclei and hypothalamus in Macaca fascicularis: cerebello-visceral circuits

The purpose of this study was to identify the basic pattern of interconnections between the cerebellar nuclei and hypothalamus in Macaca fascicularis. The distribution of retrogradely labeled cells and anterogradely filled cerebellofugal axons in the hypothalamus of M. fascicularis was investigated after pressure injections of a horseradish peroxidase mixture (HRP + WGA-HRP) in the cerebellar nuclei. Following injections in the lateral, anterior, and posterior interposed cerebellar nuclei retrogradely labeled cells were present in the following areas (greatest to least concentration): lateral and dorsal hypothalamic areas, dorsomedial nucleus, griseum periventriculare hypothalami, supramammillary and tuberomammillary nuclei, posterior hypothalamic area, ventromedial nucleus and periventricular hypothalamus, around the medial mammillary nucleus, lateral mammillary nucleus, and infundibular nucleus. Cell labeling was bilateral with an ipsilateral preponderance. In these same experiments anterogradely labeled cerebellar efferent fibers terminated in the contralateral posterior, dorsal and lateral hypothalamic areas, and the dorsomedial nucleus. In these regions retrogradely labeled hypothalamic cells were occasionally found in areas that also contained anterogradely filled cerebellar axons. This suggests a partial reciprocity in this system. In addition, sparse numbers of labeled cerebellar fibers recross in the hypothalamus to distribute to homologous areas ipsilateral to the injection site. Subsequent to an injection in the medial cerebellar nucleus (NM), cell labeling was present in more rostral hypothalamic levels including the lateral and dorsal hypothalamic areas, the dorsomedial nucleus, around or in fascicles of the column of the fornix, and in the periventricular hypothalamic area. Although no fastigiohypothalamic fibers were seen in this study, on the basis of information available from the literature it is likely that such a connection exists in primates. In summary, hypothalamic projections to NM originated mainly from rostral to midhypothalamic levels, whereas those projections to the lateral three cerebellar nuclei came from mid and more caudal levels. The existence of direct hypothalamic projections to cerebellar nuclei in M. fascicularis and of cerebellofugal projection to some hypothalamic centers indicates that circuitry is present through which the cerebellum may influence visceral functions. Furthermore, the fact that projections to NM versus the other cerebellar nuclei originate from somewhat different regions of the hypothalamus would suggest that the visceral functions modulated by each pathway is not the same.

Continuous monitoring of cerebrocortical blood flow during stimulation of the cerebellar fastigial nucleus: a study by laser-Doppler flowmetry

Laser-Doppler flowmetry was used to continuously monitor cortical CBF during electrical stimulation of the fastigial nucleus (FN). Rats were anesthetized with isoflurane (0.75-5%), paralyzed, and artificially ventilated. The LDF probe was placed over a target region of the parietal cortex through a burr hole. The hypertension associated with FN stimulation was prevented by spinal cord transection at C1 with arterial pressure maintained by i.v. infusion of phenylephrine. After cord transection, CBF changed linearly with changes in arterial pco2 (r = 0.93; n = 23). FN stimulation (50-100 microA, 50 Hz, 1 s on/1 s off) produced sustained increases in CBF that developed slowly, reaching 50% of maximum within 24 +/- 1 s of stimulation (n = 17). After stimulation, CBF returned to baseline gradually within a time period (84-540 s) proportional to the duration of the stimulation (r = 0.93; n = 15). The CBF response was stimulus frequency and intensity dependent, was elicited only from restricted sites in FN, and was abolished by atropine (1 mg/kg, i.v.) or pentobarbital (30 mg/kg, i.v.). The slow temporal profile of the cerebrovasodilation is compatible with the hypothesis that in cerebral cortex local neurons mediate the vasodilation by interstitial release of vasoactive agents rather than by a direct action of neural processes on blood vessels. LDF is an effective technique for monitoring phasic change in CBF and may be useful in studies of the intrinsic neurogenic control of the cerebral circulation.

Small pial vessels, but not choroid plexus, exhibit specific biochemical correlates of functional cholinergic innervation

In an attempt to provide the biochemical foundations for a putative cholinergic innervation of small pial vessels and choroid plexus, we have assessed their ability to specifically accumulate choline, synthesize and release acetylcholine (ACh) in response to depolarization. Our results show that both small pial vessels and choroid plexus avidly accumulate choline via a sodium-dependent mechanism which could be inhibited by hemicholinium-3 (IC50 in pial vessels = 47.8 microM). Light microscopic examination of radioautographs from vessels incubated with [3H]choline revealed two distinct sites of accumulation in the vessel wall. One site probably corresponded to nerve terminals and the other was closely associated with the endothelial cells. In small pial vessels, a major proportion (60%-70%) of the choline acetyltransferase (ChAT) activity could be inhibited by 4-naphthylvinylpyridine (4-NVP), a potent inhibitor of neuronal ChAT; and, following either K+ or veratridine depolarization, a Ca2(+)-dependent release of authentic [3H]ACh could be measured. In contrast, the choroid plexus exhibited a rather low ChAT activity which was not inhibited by 4-NVP and no release of ACh could be detected in this tissue following depolarization. Altogether, the results of the present study show that (1) small pial vessels exhibit all the most selective biochemical markers that are characteristic of cholinergic nerves; (2) [3H]choline in pial vessels can be accumulated in non-neuronal elements which probably correspond to the endothelial cells; and (3) the choroid plexus failed to exhibit convincing biochemical markers that would attest in favor of a functional cholinergic innervation.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissociation by chloralose of the cardiovascular and cerebrovascular responses evoked from the cerebellar fastigial nucleus

We studied the effects of chloralose anesthesia on the elevation in arterial pressure (AP), heart rate (HR), and regional CBF (rCBF) elicited by stimulation of the cerebellar fastigial nucleus (FN). Rats were anesthetized with an initial dose of chloralose (40 mg/kg s.c.), paralyzed, and artificially ventilated. The FN was stimulated (50-100 microA, 50 Hz, 1 s on/1 s off) with microelectrodes stereotaxically implanted. During the stimulation AP was carefully maintained within cerebrovascular autoregulation. CBF was measured by the [14C]iodoantipyrine technique with regional dissection. In rats that received only the initial dose of chloralose, FN stimulation elevated rCBF in brain and spinal cord, up to 209 +/- 13% of control in frontal cortex (n = 5; p less than 0.01, analysis of variance). Administration of additional chloralose (10 mg/kg i.v., 30 min prior to measurement of CBF) did not affect resting rCBF (n = 5), the EEG, or the elevation in AP and HR elicited by FN stimulation (n = 4). However, the additional chloralose abolished the elevations in rCBF (n = 5; p greater than 0.05). Thus, the cerebrovasodilation elicited from the FN is more susceptible to the effects of additional anesthesia than the elevation in AP and HR. These results indicate that the cerebrovascular and cardiovascular responses elicited from the FN are functionally distinct and provide additional evidence for the notion that these responses are mediated by different neural pathways and transmitters.

Electrical stimulation of the fastigial nucleus increases total cerebral blood flow in the monkey

Internal and external carotid blood flows were monitored continuously in the barbiturate-anesthetized monkey with appropriately-sized electromagnetic flow probes. Electrical stimulation of the fastigial nucleus increased blood flow in both the internal and external carotid circulations in a frequency-dependent manner. This response was entirely intracerebral and independent of infra-bulbar mechanisms since it was present following high cervical spinal cord section. These data re-affirm the presence of the fastigial nucleus cerebral vasodilator response and add the observations that the response is present in the primate and markedly frequency-dependent.

Electrically stimulated increases in cochlear blood flow: II. Evidence of neural mediation

In a companion paper, we reported that electrical stimulation increased cochlear blood flow (CBF). This response was found to be an increasing function of current intensity and was frequency-selective, with the best response at approximately 500 Hz continuous sinusoidal current. The present investigation seeks to discover the mechanism of this effect. Direct measurement of cochlear temperature during electrical stimulation revealed no evidence of local heating. Autonomic neuronal activation is not likely, as neither atropine, hexamethonium, nor propranolol abolished the evoked CBF response. Strial activity could be suppressed by the use of furosemide, but the evoked CBF response persisted. Inactivation of auditory afferent neurons with kainic acid also did not change the evoked CBF response. Dimethyl sulfoxide, a potent oxygen-free radical scavenger, did suppress the evoked CBF response to a small but significant degree. This suggests that oxygen-free radicals may be produced within the cochlea during electrical stimulation. Finally, the evoked CBF response was completely suppressed by procaine and tetrodotoxin, with recovery of evoked CBF response accompanying recovery of cochlear action potentials. These data indicate that stimulation of neural fibers, distinct from autonomic and auditory afferent neurons, may modulate CBF.

Cortical blood flow increases induced by stimulation of the substantia innominata in the unanesthetized rat

The possible implication of projections from the substantia innominata (SI) to the cerebral cortex in the control of local cortical blood flow (CoBF) was studied in adult Fischer rats. Local blood flow (by helium clearance) and tissue gas partial pressures (pO2, pCO2) as metabolic indices, were measured in the frontal and parietal cortices in unanesthetized animals via chronically implanted probes connected to a mass spectrometer. Stimulating electrodes, also implanted chronically, were placed in the region of the SI. Out of 37 correctly located sites, 28 gave rise to cerebrovascular responses without significant hypertension or agitation. Both frontal (+114%) and parietal CoBF (+28%) increased significantly during ipsilateral 50 microA stimulation, but did not further significantly increase at 100 microA. Contralateral stimulation induced only small, non-significant effects. SI stimulation simultaneously increased cortical pO2 and decreased cortical pCO2, significantly more so in the frontal compared to the parietal cortex, and ipsilaterally compared to contralaterally. Both the CoBF and the tissue gas changes induced by SI stimulation were strongly potentiated by infusion of 0.15 mg/kg/h of the cholinomimetic agent physostigmine. The electrocorticogram (ECoG) was not systematically activated during the SI stimulation. The evidence presented favors a role for the cholinergic projections of the SI in control of CoBF (particularly frontal cortex), especially since the flow changes observed showed no obvious dependence on changes in local pCO2 or on paCO2, and could not be attributed to hypertension or behavioral changes.

Neuronal and endothelial sites of acetylcholine synthesis and release associated with microvessels in rat cerebral cortex: ultrastructural and neurochemical studies

We sought to establish what proportion of the cholinergic innervation of the cerebral cortex (CX) is associated with intraparenchymal blood vessels by using immunocytochemical and neurochemical techniques, and whether [3H]acetylcholine ([3H]ACh) is synthesized and released by elements associated with cortical microvessels (MV). MVs and, for comparison, tissue homogenates were prepared using sucrose gradient/differential ultracentrifugation methods. Efficacy of the separation technique was indicated by the activity of gamma-glutamyltranspeptidase (up to 29.2-fold enrichment), an endothelial cell marker enzyme, in the MV fraction and microscopy. The size of isolated microvessels ranged from 5 to 40 micron (o.d.) with 67.7% of the vessels less than 10 micron and 32.2% between 11 and 40 micron (690 vessels measured from 4 animals). By electron microscopy immunoreactive choline acetyltransferase (ChAT), the biosynthetic enzyme for ACh, was localized to: (a) axons and axon terminals opposed to the basal laminae of capillaries and small arterioles, and (b) capillary endothelial cells. ChAT-labeled elements associated with MVs were most prominent in layers I, III and V of the CX consistent with the local pattern of cholinergic innervation. The absolute amount of ACh synthesized (pmol Ach/100 mg wet wt.) by elements associated with cortical MVs was relatively small (2.3% total cortical homogenate activity). Inhibition of MV ChAT activity to 5% of control by the specific ChAT inhibitor, 4-naphthylvinylpyridine, and HPLC analysis of the product, indicated that authentic ACh was measured. Other tissues similarly synthesized small amounts of ACh relative to the CX, caudate nucleus (CN, 2.4%), cerebellum (CRB, 1.4%) and liver (LIV, 3.9%). Consistent with the known extent of the cholinergic innervation of the tissues examined, the rank order of ChAT associated for both MVs and homogenate were: CN greater than CX much greater than CRB greater than LIV. However, based on the specific activities of ChAT, cortical MVs have the remarkable capacity to synthesize ACh at rates 95% greater than cortical (S1 fraction) homogenate (59.0 +/- 3.5 nmol/mg protein/40 min; n = 7), which is enriched in nerve terminals. Except for LV (+11%), other tissues also had remarkably high ChAT activity in MV (% above corresponding homogenate; P less than 0.05, n = 5): CN (+269) and CRB (+313). Release of [3H]ACh from MVs and, for comparison, nerve terminals were graded to K+ depolarization stimulus (5-55 mM), maximal with 55 mM K+ and Ca2+ dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of basal forebrain in the primary cholinergic vasodilation in rat neocortex produced by systemic administration of cismethrin

The pyrethroid insecticide cismethrin (9 mumol/kg) causes a large blood flow increase in cerebral cortex, without a parallel increase in metabolism. A unilateral lesion of the basal forebrain attenuated the blood flow increase in the cortex ipsilateral to the lesion but augmented that in the contralateral cortex. Cortical choline acetyltransferase was similarly affected. Atropine sulphate substantially reduced the flow increase and was additive to the lesion effects. Systemic cismethrin is thus capable of activating a cholinergic vasodilation in the cortex and, in the parietal cortex at least, a substantial proportion of the flow increase is mediated by extrinsic projections from the basal forebrain.

Contribution of cholinergic vasodilators on the increase in cerebral cortical blood flow responses to the intravenous administration of thyrotropin releasing hormone in anesthetized rats

The effect of an intravenous administration of thyrotropin-releasing hormone (TRH) on the regional cerebral blood flow in the sensory cortex was studied in halothane-anesthetized adult Wistar rats. The regional cerebral blood flow was continuously monitored with the laser Doppler flowmetry. The cerebral blood flow increased dose-dependently following the administration of 3 x 10(2) and 3 x 10(3) microns/kg TRH. The systemic blood pressure also increased simultaneously. After maintaining the systemic blood pressure at a constant level via a pressure reservoir system, the TRH-induced increases in the cerebral blood flow continued to be observed. In atropinized animals, the blood pressure increased as high as that indicated in non-atropinized animals following TRH administration, but the responses of the cerebral blood flow in the latter were much attenuated. It was suggested that the cholinergic vasodilative system contributed to the TRH-induced increase in cerebral blood flow.

Cortical projection patterns of magnocellular basal nucleus subdivisions as revealed by anterogradely transported Phaseolus vulgaris leucoagglutinin

The present paper deals with a detailed analysis of cortical projections from the magnocellular basal nucleus (MBN) and horizontal limb of the diagonal band of Broca (HDB) in the rat. The MBN and HDB were injected iontophoretically with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L). After immunocytochemical visualization of labeled efferents, the distribution of projections over the cortical mantle, olfactory regions and amygdala were studied by light microscopy. Based on differences in cortical projection patterns, the MBN was subdivided in anterior, intermediate and posterior portions (MBNa, MBNi and MBNp). All subdivisions maintain neocortical projections and are subject to an anterior to posterior topographic arrangement. In the overall pattern, however, the frontal cortex is the chief target. Furthermore, all MBN parts project to various regions of meso- and allocortex, which are progressively more dense when the tracer injection is more anteriorly placed. The most conspicuous finding, however, was a ventrolateral to dorsomedial cortical projection pattern as the PHA-L injection site moved from posterior to anterior. Thus, the posterior MBN projects predominantly to lateral neo- and mesocortex while the anterior MBN sends more fibers to the medial cortical regions. Furthermore, the MBNa is a source of considerable afferent input to the olfactory nuclei and as such should be regarded as a transition to the HDB. The HDB, apart from projecting densely to olfactory bulb and related nuclei, maintains a substantial output to the medial prefrontal cortical regions and entorhinal cortex, as well. Comparison of young vs aged cases indicate that aging does not appear to have a profound influence on cortical innervation patterns, at least as studied with the PHA-L method.

Local cholinergic mechanisms participate in the increase in cortical cerebral blood flow elicited by electrical stimulation of the fastigial nucleus in rat

We sought to determine whether the increase in regional cerebral blood flow (rCBF) elicited within the cerebral cortex (CX) by electrical stimulation of the fastigial nucleus (FN) of the cerebellum is: prevented by local application of the muscarinic cholinergic receptor antagonist, atropine and temporally correlated with a stimulus-locked release of acetylcholine (ACh) from the cortical surface. Rats were anesthetized, paralyzed, ventilated, with arterial blood gases controlled and arterial pressure maintained within the autoregulated range. Bilateral craniotomies were performed over a standardized region of the sensory motor CX and superfusion devices stereotaxically positioned on the cortical surface. Cortical surface temperature, as well as pH, pCO2 and pO2 of the solutions applied to the cortex were also carefully controlled. rCBF was measured in dissected regions of frontal (FCX), parietal (PCX), and occipital cortices (OCX), caudate nucleus (CN), and hippocampus (HIPP) by the Kety principle using [14C]iodoantipyrine as indicator. Resting rCBF (ml/100 g/min) in unoperated control animals ranged from 70 +/- 5 in HIPP to 95 +/- 7 in PCX and was unaffected by bilateral craniotomies and placement of superfusion devices containing Kreb's bicarbonate buffer (vehicle) on the cortical surface. Local application of atropine (ATR, 100 microM) to the right PCX via the superfusion device did not affect resting rCBF. With FN stimulation rCBF increased bilaterally and symmetrically in all areas up to 227% in PCX. ATR application attenuated by 59% the FN-elicited increase in rCBF on the ipsilateral frontoparietal CX, without affecting blood flow in adjacent structures. ATR did not affect cortical cerebrovasodilation produced by hypercarbia (arterial pCO2 = 59.0 +/- 1.4 mm Hg). FN-stimulation resulted in a small (22%) but significant (P less than 0.05, n = 9) reduction in the release of [3H]ACh from the cortical surface, while supramaximal depolarization with 55 mM K+ increased [3H]ACh release by 251%. These studies indicate that: increases in cortical rCBF elicited by FN stimulation, but not hypercarbia, are in large part mediated by local muscarinic cholinergic receptors; resting rCBF is not tonically affected by muscarinic receptor activation; and the release of ACh from the cortical surface is, in general, reduced during FN-stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)