Cholinergic and Nadph-δ neurons in the pedunculopontine and laterodorsal tegmental nuclei of human and nonhuman primates

The brainstem pedunculopontine (PPN) and laterodorsal tegmental (LDTg) nuclei are involved in multifarious activities, including motor control. Yet, their exact cytoarchitectural boundaries are still uncertain. We therefore initiated a comparative study of the topographical and neurochemical organization of the PPN and LDTg in cynomolgus monkeys (Macaca fascicularis) and humans. The distribution and morphological characteristics of neurons expressing choline acetyltransferase (ChAT) and/or nicotinamide adenine dinucleotide phosphate diaphorase (Nadph-δ) were documented. The number and density of the labeled neurons were obtained by stringent stereological methods, whereas their topographical distribution was reported upon corresponding magnetic resonance imaging (MRI) planes. In both human and nonhuman primates, the PPN and LDTg are populated by three neurochemically distinct types of neurons (ChAT-/Nadph-δ+, ChAT+/Nadph-δ-, and ChAT+/Nadph-δ+), which are distributed according to a complex spatial interplay. Three-dimensional reconstructions reveal that ChAT+ neurons in the PPN and LDTg form a continuum with some overlaps with pigmented neurons of the locus coeruleus, dorsally, and of the substantia nigra (SN) complex, ventrally. The ChAT+ neurons in the PPN and LDTg are -two to three times more numerous in humans than in monkeys but their density is -three to five times higher in monkeys than in humans. Neurons expressing both ChAT and Nadph-δ have a larger cell body and a longer primary dendritic arbor than singly labeled neurons. Stereological quantification reveals that 25.6% of ChAT+ neurons in the monkey PPN are devoid of Nadph-δ staining, a finding that questions the reliability of Nadph-δ as a marker for cholinergic neurons in primate brainstem.

The ultrastructure of macaque mesencephalic trigeminal nucleus neurons

Primary afferents originating from the mesencephalic trigeminal nucleus provide the main source of proprioceptive information guiding mastication, and thus represent an important component of this critical function. Unlike those of other primary afferents, their cell bodies lie within the central nervous system. It is believed that this unusual central location allows them to be regulated by synaptic input. In this study, we explored the ultrastructure of macaque mesencephalic trigeminal nucleus neurons to determine the presence and nature of this synaptic input in a primate. We first confirmed the location of macaque mesencephalic trigeminal neurons by retrograde labeling from the masticatory muscles. Since the labeled neurons were by far the largest cells located at the edge of the periaqueductal gray, we could undertake sampling for electron microscopy based on soma size. Ultrastructurally, mesencephalic trigeminal neurons had very large somata with euchromatic nuclei that sometimes displayed deeply indented nuclear membranes. Terminal profiles with varied vesicle characteristics and synaptic density thicknesses were found in contact with either their somatic plasma membranes or somatic spines. However, in contradistinction to other, much smaller, somata in the region, the plasma membranes of the mesencephalic trigeminal somata had only a few synaptic contacts. They did extend numerous somatic spines of various lengths into the neuropil, but most of these also lacked synaptic contact. The observed ultrastructural organization indicates that macaque trigeminal mesencephalic neurons do receive synaptic contacts, but despite their central location, they only avail themselves of very limited input.

Activation of RMTg projections to the VTA reverse cocaine-induced molecular adaptation in the reward system

The rostromedial tegmental nucleus (RMTg) plays a crucial role in regulating reward-related behavior by exerting inhibitory control over the ventral tegmental area (VTA). This modulation of dopamine neuron activity within the VTA is essential for maintaining homeostasis in the reward system. Recently we have shown that activation of RMTg projections to the VTA during the acquisition of cocaine-conditioned place preference (CPP) reduces the rewarding properties of cocaine and decreases VTA dopamine neuron activity. By inhibiting dopamine neurons in the VTA, we hypothesized that RMTg projections hold the potential to restore reward system homeostasis disrupted by repeated cocaine use, and attenuate molecular adaptations in the reward system, including alterations in signaling pathways. Our study demonstrates that enhancing the GABAergic inputs from the RMTg to the VTA can mitigate cocaine-induced molecular changes in key regions, namely the VTA, nucleus accumbens (NAc), and prefrontal cortex (PFC). Specifically, we found that cocaine-induced alteration in the phosphorylation state of ERK (pERK) and GluA1 on serine 845 (S845) and serine 831 (S831), that play a major role in plasticity by controlling the activity and trafficking of AMPA receptors, were significantly reversed following optic stimulation of RMTg afferents to the VTA. These findings highlight the therapeutic potential of targeting the RMTg-VTA circuitry for mitigating cocaine reward. Ultimately, this research may pave the way for novel therapeutic interventions that restore balance in the reward system and alleviate the detrimental effects of cocaine.

Recruitment of inhibitory neuronal pathways regulating dopaminergic activity for the control of cocaine seeking

Drug seeking is associated with the ventral tegmental area (VTA) dopaminergic (DA) activity. Previously, we have shown that brief optogenetic inhibition of VTA DA neurons with 1 s pulses delivered every 9 s attenuates cocaine seeking under extinction conditions in rats without producing overt signs of dysphoria or locomotor sedation. Whether recruitment of neuronal pathways inhibiting VTA neuronal activity would suppress drug seeking remains unknown. Here, we asked if optogenetic stimulation of the lateral habenula (LHb) efferents in the rostromedial tegmental nucleus (RMTg) as well as RMTg efferents in VTA would reduce drug seeking. To investigate this, we measured how recruitment of elements of this inhibitory pathway affects cocaine seeking in male rats under extinction conditions. The effectiveness of brief optogenetic manipulations was confirmed electrophysiologically at the level of electrical activity of VTA DA neurons. Real-time conditioned place aversion (RT-CPA) and open field tests were performed to control for potential dysphoric/sedating effects of brief optogenetic stimulation of LHb-RMTg-VTA circuitry. Optogenetic stimulation of either RMTg or LHb inhibited VTA DAergic neuron firing, whereas similar stimulation of RMTg efferents in VTA or LHb efferents in RMTg reduced cocaine seeking under extinction conditions. Moreover, stimulation of LHb-RMTg efferents produced an effect that was maintained 24 h later, during cocaine seeking test without stimulation. This effect was specific, as brief optogenetic stimulation did not affect locomotor activity and was not aversive. Our results indicate that defined inhibitory pathways can be recruited to inhibit cocaine seeking, providing potential new targets for non-pharmacological treatment of drug craving.

Mesencephalic trigeminal nucleus neurons with collaterals to both eyelid and masseter muscles shown by fluorescent double-labeling, revealing a potential mechanism for Marcus Gunn Syndrome

Poking palpebral conjunctiva evoked upper-eyelid retraction during ophthalmic surgery. Iatrogenic eyelid ptosis occurred if eyelid branch of lachrymal nerve was sectioned. Mesencephalic trigeminal nucleus (Vme) neurons were labeled when tracer injected into lachrymal nerve innervating eyelid Mueller's muscle. Masseter afferent Vme neurons projecting to oculomotor nucleus (III) was observed in toad and rat, which helps amphibians to stare prey when they open mouth widely to prey. We hypothesized single Vme neurons may have peripheral collaterals to both eyelid and masseter muscles. WGA-594 was injected into upper eyelid, and WGA-488 was simultaneously delivered into ipsilateral masseter muscle in the same rat. Then, double labeled Vme neurons were found under both conventional and confocal microscope. Meanwhile, contact of WGA-594 positive eyelid afferent Vme neurons with WGA-488 labeled masseter afferent ones were observed sometimes. Combined with our previous observation of oculomotor projection Vme neurons, we thought WGA-594/488 double labeled Vme cells, at least some of them, are oculomotor projecting ones. Contact between eyelid and masseter afferent Vme neurons are supposed to be electrotonically coupled, based on a line of previous studies. If exogenous or genetic factors make these Vme neurons misinterpret masseter input as eyelid afferent signals, these Vme neurons might feedforward massages to eyelid retractor motoneurons in the III. Besides, oculomotor projecting Vme neurons might be co-fired by adjacent masseter afferent Vme neurons through electrotonic coupling once the masseter muscle is activated. In these cases, Marcus Gunn Syndrome might occur. This finding leads to a new hypothesis for the Syndrome.

Characterization of three cholinergic inputs to the cochlear nucleus

Acetylcholine modulates responses throughout the auditory system, including at the earliest brain level, the cochlear nucleus (CN). Previous studies have shown multiple sources of cholinergic input to the CN but information about their relative contributions and the distribution of inputs from each source is lacking. Here, we used staining for cholinergic axons and boutons, retrograde tract tracing, and acetylcholine-selective anterograde tracing to characterize three sources of acetylcholine input to the CN in mice. Staining for cholinergic axons showed heavy cholinergic inputs to granule cell areas and the dorsal CN with lighter input to the ventral CN. Retrograde tract tracing revealed that cholinergic cells from the superior olivary complex, pontomesencephalic tegmentum, and lateral paragigantocellular nucleus send projections to the CN. When we selectively labeled cholinergic axons from each source to the CN, we found surprising similarities in their terminal distributions, with patterns that were overlapping rather than complementary. Each source heavily targeted granule cell areas and the dorsal CN (especially the deep dorsal CN) and sent light input into the ventral CN. Our results demonstrate convergence of cholinergic inputs from multiple sources in most regions of the CN and raise the possibility of convergence onto single CN cells. Linking sources of acetylcholine and their patterns of activity to modulation of specific cell types in the CN will be an important next step in understanding cholinergic modulation of early auditory processing.

Increased Gray Matter Density in the Right Mesencephalic Tegmentum Is Associated With Better Engel Classes I and II After Radiosurgery for Hypothalamic Hamartomas

BACKGROUND

Hypothalamic hamartomas (HHs) are disabling congenital lesions, responsible for gelastic seizures frequently associated with catastrophic epilepsies, epileptogenic encephalopathy, and cognitive and psychiatric severe comorbidities. Stereotactic radiosurgery (SRS) is a well-established minimally invasive therapeutic approach.

OBJECTIVE

To assess whether pretherapeutic gray matter density (GMD) correlates with seizure outcome.

METHODS

We used voxel-based morphometry at whole-brain level, as depicted on pretherapeutic standard structural magnetic resonance neuroimaging. We examined 24 patients (10 male patients, 14 female patients; mean age, 12.7 yr; median, 9; range, 5.9-50) treated in Marseille University Hospital, France, between May 2001 and August 2018.

RESULTS

Most relevant anatomic area predicting postoperative Engel classes I and II vs III and IV after SRS for HHs was mesencephalic tegmentum. Higher pretherapeutic GMD in this area was associated with better outcomes for seizure cessation. The only other statistically significant clusters were right cerebellar lobule VIIIb and VIIIa. Lower pretherapeutic GMD in both clusters correlated with better Engel class outcomes. GMD decreased with age in the left mediodorsal thalamus.

CONCLUSION

Seizure cessation after SRS for HHs was associated with higher GMD in mesencephalic tegmental area, acknowledged to be involved in the neural control of explosive vocal behavior in animals. This area is connected by the mamillotegmental bundle to the lateral tuberal nucleus area of the hypothalamus, where HHs are known to rise. In the future, the detection of more gray matter in this "laugh" tegmental area based on pretherapeutic routine structural neuroimaging might help in patient selection for minimally invasive radiosurgery for HH.

Uncovering the Connectivity Logic of the Ventral Tegmental Area

Decades of research have revealed the remarkable complexity of the midbrain dopamine (DA) system, which comprises cells principally located in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). Neither homogenous nor serving a singular function, the midbrain DA system is instead composed of distinct cell populations that (1) receive different sets of inputs, (2) project to separate forebrain sites, and (3) are characterized by unique transcriptional and physiological signatures. To appreciate how these differences relate to circuit function, we first need to understand the anatomical connectivity of unique DA pathways and how this connectivity relates to DA-dependent motivated behavior. We and others have provided detailed maps of the input-output relationships of several subpopulations of midbrain DA cells and explored the roles of these different cell populations in directing behavioral output. In this study, we analyze VTA inputs and outputs as a high dimensional dataset (10 outputs, 22 inputs), deploying computational techniques well-suited to finding interpretable patterns in such data. In addition to reinforcing our previous conclusion that the connectivity in the VTA is dependent on spatial organization, our analysis also uncovered a set of inputs elevated onto each projection-defined VTADA cell type. For example, VTADA→NAcLat cells receive preferential innervation from inputs in the basal ganglia, while VTADA→Amygdala cells preferentially receive inputs from populations sending a distributed input across the VTA, which happen to be regions associated with the brain's stress circuitry. In addition, VTADA→NAcMed cells receive ventromedially biased inputs including from the preoptic area, ventral pallidum, and laterodorsal tegmentum, while VTADA→mPFC cells are defined by dominant inputs from the habenula and dorsal raphe. We also go on to show that the biased input logic to the VTADA cells can be recapitulated using projection architecture in the ventral midbrain, reinforcing our finding that most input differences identified using rabies-based (RABV) circuit mapping reflect projection archetypes within the VTA.

Stereological estimations and neurochemical characterization of neurons expressing GABAA and GABAB receptors in the rat pedunculopontine and laterodorsal tegmental nuclei

To better understand GABAergic transmission at two targets of basal ganglia downstream projections, the pedunculopontine (PPN) and laterodorsal (LDT) tegmental nuclei, the anatomical localization of GABAA and GABAB receptors was investigated in both nuclei. Specifically, the total number of neurons expressing the GABAA receptor γ2 subunit (GABAAR γ2) and the GABAB receptor R2 subunit (GABAB R2) in PPN and LDT was estimated using stereological methods, and the neurochemical phenotype of cells expressing each subunit was also determined. The mean number of non-cholinergic cells expressing GABAAR γ2 was 9850 ± 1856 in the PPN and 8285 ± 962 in the LDT, whereas those expressing GABAB R2 were 7310 ± 1970 and 9170 ± 1900 in the PPN and LDT, respectively. In addition, all cholinergic neurons in both nuclei co-expressed GABAAR γ2 and 95-98% of them co-expressed GABAB R2. Triple labeling using in situ hybridization revealed that 77% of GAD67 mRNA-positive cells in the PPT and 49% in the LDT expressed GABAAR γ2, while 90% (PPN) and 65% (LDT) of Vglut2 mRNA-positive cells also expressed GABAAR γ2. In contrast, a similar proportion (~2/3) of glutamatergic and GABAergic cells co-expressed GABAB R2 in both nuclei. The heterogeneous distribution of GABAAR and GABABR among non-cholinergic cells in PPN and LDT may give rise to physiological differences within each neurochemical subpopulation. In addition, the dissimilar proportion of GABAAR γ2-expressing glutamatergic and GABAergic neurons in the PPN and LDT may contribute to some of the functional differences found between the two nuclei.

[Comparison of diffusion tensor imaging-derived fractional anisotropy in multiple centers for identical human subjects]

The fractional anisotropy (FA) is calculated by using diffusion tensor imaging (DTI) with multiple motion probing gradients (MPG). While FA has become a widely used tool to detect moderate changes in water diffusion in brain tissue, the measured value is sensitive to scan parameters (e.g. MPG-direction, signal to noise ratio, etc.). Therefore, it is paramount to address the reproducibility of DTI measurements among multiple centers. The purpose of this study was to assess the inter-center variability of FA. We studied five healthy volunteers who underwent DTI brain scanning three times at three different centers (I-III), each with a 1.5 T scanner having a different MPG-schema. Then, we compared the FA and eigenvalue from the three centers measured in seven brain regions: splenium of corpus callosum (CCs), genu of corpus callosum (CCg), putamen, posterior limb of internal capsule, cerebral peduncle, optic radiation, and middle cerebellar peduncle. At the CCs and CCg, there was a statistical difference (p<0.05) between center Iand center IIfor the same MPG-directions. Furthermore, at CCs and CCg, there was a statistical difference (p<0.05) between center II and center III for different MPG-directions. Conversely, no statistical differences were found between center I and center III for the different MPG-directions for all regions. These results indicate that the FA value was affected by the MPG-schema as well as by the MPG-directions.

An unusual cause of fixed dilated pupils after cardiac surgery

BACKGROUND

The presence of fixed, dilated pupils after cardiac surgery is an ominous sign, typically indicating severe diffuse hypoxic-ischemic brain injury. Rarely, however, this finding can be seen as a result of focal midbrain ischemia. It is important to differentiate between these syndromes, as the latter might be amenable to acute stroke treatments, and because they affect consciousness very differently.

CASE

A 46-year-old man with diffuse atherosclerosis underwent coronary artery bypass grafting and closure of an incidentally discovered patent foramen ovale. He underwent neurologic evaluation on post-operative day 6 because he was not speaking and appeared to have a new right hemiparesis. Eye movements, pupillary, and lid function were all normal at this time. MRI showed multiple posterior circulation infarcts, involving both cerebral peduncles. On post-operative day 12, he became unresponsive, quadriplegic, and had bilaterally fixed, dilated pupils. CT showed low attenuation in nearly the entirety of both cerebral peduncles.

DISCUSSION

Midbrain infarction is rare. Bilateral midbrain infarction is even rarer and typically presents as locked-in syndrome with preservation of pupillary function and at least vertical eye movements. In our review of the literature, we found only three other cases of midbrain infarction associated with bilateral mydriasis. As in our case, these characteristically involved the anteromedial midbrain bilaterally.

CONCLUSIONS

Fixed, dilated pupils after cardiac surgery can rarely be caused by bilateral anteromedial midbrain infarctions. It is important to differentiate this from the much more common diffuse hypoxic-ischemic injury.

Gasperini syndrome, a report of two cases

We report 2 cases of Gasperini syndrome and consider them with the 11 previously reported cases to describe the clinical characteristics of this rare syndrome: Core neurological signs are peripheral facial nerve palsy and abducens nerve palsy of the affected side: Among all cases, imaging demonstrated a small lesion in the mediolateral tegmental pons (10/13 cases of microinfarction; 2/13 cases of microbleeding). We found that the responsible artery in ischemic Gasperini syndrome is mainly the long circumferential branch of the anterior inferior cerebellar artery; Case 1 is the first case thought to be caused by infarction of the basilar artery's paramedian branch.

The role of intraoperative monitoring of oculomotor and trochlear nuclei -safe entry zone to tegmental lesions

OBJECTIVE

A safe entry zone to tegmental lesions was identified based on intraoperative electrophysiological findings, the compound muscle action potentials (CMAP) from the extraocular muscles, and anatomic considerations. This entry zone is bordered caudally by the intramesencephalic path of the trochlear, laterally by the spinothalamic tract, and rostrally by the caudal margin of the brachium of the superior colliculus.

METHODS

Four intrinsic midbrain lesions were operated upon via the safe entry zone using the infratentorial paramedian supracerebellar approach. All lesions involved the tegmentum and included an anaplastic astrocytoma, a metastatic brain tumor, a radiation necrosis, and a cavernous angioma. CMAP were bilaterally monitored from the inferior recti (for oculomotor function) and superior oblique (for trochlear nerve function) muscles.

RESULTS

In three of four cases, CMAP related to the oculomotor nerve were obtained upon stimulation at the cavity wall after removal of the tumor. Stimulation at the surface of the quadrigeminal plate, however, did not cause any CMAP response. Using this monitoring as an indicator, the lesions were totally removed.

CONCLUSIONS

In the surgery of tegmental lesions, CMAP monitoring from extraocular muscles is particularly helpful to prevent damage to crucial neural structures during removal of intrinsic lesions, but less so to select the site of the medullary incision. The approach via the lateral part of the colliculi is considered to be a safe route to approach the tegmental lesions.

GDNF as a candidate striatal target-derived neurotrophic factor for the development of substantia nigra dopamine neurons

Glial cell line-derived neurotrophic factor (GDNF) has been known for many years to protect and restore dopamine neurons of the substantia nigra (SN) in lesion models of parkinsonism, but much less has been known of its normal physiologic role. We have found that GDNF injected into the striatum postnatally suppresses naturally-occurring cell death in SN dopamine neurons, and neutralizing antibodies augments it. Neutralizing antibodies augment cell death during the first phase, which occurs during the first postnatal week, but not during the second phase in the second week. To further explore the possible neurotrophic role of GDNF, we created double transgenic mice which overexpress GDNF exclusively in the target regions of mesencephalic neurons, particularly the striatum. As anticipated for a limiting, target-derived factor, this resulted in an increased surviving number of SN dopamine neurons after the first phase of cell death. However, this increase did not persist into adulthood. We conclude that GDNF is the leading candidate for a target-derived neurotrophic factor for SN dopamine neurons during the first phase of cell death, but that other factors must play an essential role in later development.

Differential effects of the 5-HT1A receptor agonist flesinoxan given locally or systemically on REM sleep in the rat

The effects of flesinoxan, a selective 5-HT1A receptor agonist on spontaneous sleep, were studied in adult rats implanted for chronic sleep recordings. Flesinoxan was administered systemically or infused directly into the dorsal raphe nucleus, the left laterodorsal tegmental nucleus or the medial pontine reticular formation. Systemic administration of flesinoxan (0.03 and/or 0.06 micromol/kg) significantly increased wakefulness and sleep latencies, and reduced rapid eye movement (REM) sleep and the number of REM periods, during the first and/or second 2-h period after treatment. Direct infusion of the 5-HT1A receptor agonist (0.06 and/or 0.12 nmol) into the dorsal raphe nucleus induced a significant increment of REM sleep and augmented the number of REM periods during the second and/or third 2-h period of recording. Microinjection of flesinoxan (0.03, 0.06 and/or 0.12 nmol) into the laterodorsal tegmental nucleus reduced REM sleep and the number of REM periods, and augmented REM sleep latency during the first, second and/or third 2-h recording period. Finally, direct infusion of flesinoxan (0.48 nmol) into the medial pontine reticular formation decreased REM sleep and the number of REM periods, and increased REM sleep latency during the first and second 2 h of recording. Our results indicate that the 5-HT1A receptor is involved in the inhibitory effect of serotonin on brainstem structures that act to promote and to induce REM sleep.

[Gustatory disturbance associated with putaminal hemorrhage]

48-year-old left-handed man experienced a sudden loss of taste, followed by dysarthria and dysphagia. Taste threshold examined by a small filter-paper disc method was severely elevated on both sides of the tongue. Brain CT revealed right putaminal hemorrhage that measured 9.3 ml in volume. In addition, brain MRI showed multiple cerebral infarcts bilaterally in the basal ganglia, and the corona radiata, and on the right side of the middle pontine tegmentum. His dysarthria and dysphagia seemed to be caused by interruption of bilateral cortico-bulbar tracts, and resolved within two weeks when edema around the hemorrhage regressed. The gustatory disturbance that persisted may be caused by the interruption of central gustatory pathways at the pons on one side, and subcortically close to the insular cortex on the other side.

[Acquired pendular nystagmus associated with the lesion of tegmentum mesencephali in a patient with probable multiple sclerosis]

A 42-year old woman was admitted to our hospital because of sudden onset of dizziness and oscillopsia. Neurologic examination revealed horizontal, binocular pendular nystagmus, which increase their amplitude on left lateral gaze. She also showed that mild right blephaloptosis, right facial spasms, increased tendon reflexes and positive pathological reflexes of four limbs and mild chorea-like movement of both feet. MRI showed an abnormal high intensity area on a T2weighted and proton density images located at the right tegmentum mesencephali. She was diagnosed as clinically probable multiple sclerosis according to the Poser's criteria. The nystagmus was suppressed by clonazepam and diazepam. To our knowledge, it is a first report of acquired pendular nystagmus associated with the lesion of tegmentum mesencephali. We speculate that the involvement of the tract of paramedian pontine reticular formation causes the nystagmus and the dysfunction of GABAnergic neurons might play an important role of the nystagmus.

The effects of halothane, isoflurane, and enflurane on thermoregulatory responses in the neuraxis of cats

BACKGROUND

Normal thermoregulatory function is believed to be modulated by thermosensitive neurons in the preoptic region of the anterior hypothalamus and other sites within the central nervous system including the spinal cord. Previous evidence has demonstrated modulation of segmental spinal cord thermoregulatory mechanisms from more rostral central nervous system sites. The ability of the volatile anesthetics to disrupt normal thermoregulatory function and produce shivering-like activity during emergence is well documented. The purpose of the current investigation was to examine the action purpose of the current investigation was to examine the action of the volatile anesthetics halothane, isoflurane, and enflurane on thermoregulatory responses produced at the preoptic region and spinal cord.

METHODS

Cats were chronically instrumented with bilateral cannulas allowing selective heating and cooling of the preoptic region. Electrodes were implanted in hindlimb and forelimb muscles for electromyographic (EMG) analysis. Animals underwent selective heating and cooling of the preoptic region in the awake state, during volatile agent anesthesia and during emergence. In a separate series of animals, pontine-transected cats with epidural thermodes and a thermocouple underwent alternate heating and cooling of the spinal cord. Heating and alternate heating and cooling of the spinal cord. Heating and cooling was performed in the nonanesthetized state, at graded concentrations of halothane, and during emergence. In all animals deep core peritoneal temperature, epidural spinal cord temperature, forelimb and hindlimb EMG activity were continuously recorded and digitally processed. EMG responses in both experiments were quantitated and analyzed for power spectral density.

RESULTS

In the chronically prepared animals, heating and cooling of the preoptic region in the conscious state resulted in appropriate thermoregulatory responses, including shivering-like activity and increased EMG power with preoptic region cooling. Halothane, isoflurane, and enflurane each abolished these thermoregulatory responses. During emergence from anesthesia, however, the typical spontaneous increases in EMG power observed at normothermia were significantly attenuated by heating of the preoptic region and augmented by cooling of the preoptic region. In the acutely prepared animals, cooling of the spinal cord produced graded increases in EMG activity. Increased concentrations of halothane dose-dependently diminished this response to cooling of the spinal cord. During emergence, cooling of the spinal cord resulted in a shivering response similar to those observed during control conditions.

CONCLUSIONS

The ability of preoptic region heating and cooling to modulate postanesthetic shivering implies that while thermoregulatory pathways remain intact, volatile anesthetics produce an imprecision in the control of thermoregulatory responses at the level of the anterior hypothalamus. Attenuation of shivering-like responses generated at spinal cord levels in pontine-transected cats implies a significant blunting action of thermoregulatory response mechanisms at the level of the spinal cord or lower brain stem.

Musical auditory hallucinations caused by a brainstem lesion

A man with left-sided deafness developed right-sided hearing loss after hypertensive hemorrhage at the right pontine tegmentum and began to experience ipsilateral musical hallucinations. Two weeks later, the right hearing returned, and the hallucinations disappeared. Auditory hallucinations due to brainstem lesions may be musical in nature and associated with hearing loss.

Sensitization and individual differences to IP amphetamine, cocaine, or caffeine following repeated intracranial amphetamine infusions

Rats that have a high locomotor response to novelty (HR) sensitize more readily to IP-administered amphetamine than rats with a low locomotor response (LR) to novelty. This experiment compared sensitization in HR and LR rats following amphetamine (3.0 micrograms/side for 5 days) infused bilaterally into either the nucleus accumbens (NACC), ventral tegmental area (VTA), or the medial frontal cortex (MFC). The subsequent locomotor response to IP-administered d-amphetamine sulfate (1 mg/kg), cocaine HCl (15 mg/kg), and caffeine benzoate (20 mg/kg) was also examined. No differences were observed between HR and LR rats following amphetamine infusion into either the MFC, NACC, or VTA. However, HR rats showed greater locomotor activity compared to LR rats following either IP amphetamine, cocaine, or caffeine for subjects cannulated in the NACC, MFC, or the VTA. Repeated infusions of amphetamine into the VTA increased the locomotor response to both IP amphetamine and cocaine, but not to IP caffeine, while repeated infusions of amphetamine into the NACC or MFC had no effect on locomotor response to any drug subsequently administered IP. The results support previous findings that changes induced by intra-VTA infusions, but not intra-NACC or MFC infusions, of amphetamine induce sensitization to IP-administered amphetamine and cocaine. Findings from the present experiment indicate the ability of the dopamine cell body region, but not the dopamine terminal fields, to produce locomotor sensitization to amphetamine and cocaine. The results from the present experiment also indicate the lack of localization to one of studied regions of individual differences.(ABSTRACT TRUNCATED AT 250 WORDS)

Analgesic actions of dynorphin A(1–13) antiserum in the rat brain stem

This study was performed to evaluate the effects of dynorphin A(1-3) antiserum when microinjected into an active hyperalgesic region within the rat brain stem. When administered within the dorsal posterior mesencephalic tegmentum (DPMT) of intact conscious rats, dynorphin A(1-13) antiserum produced rapid onset and persistent prolongation of a low intensity thermally evoked tail avoidance response (LITETAR). These analgesic actions of the dynorphin A(1-13) antiserum appeared to be dose dependent. These studies support previous hypotheses about the existence of tonically active brain stem opioid hyperalgesic process. Further, the results provide indirect evidence for a potential role of brain stem dynorphin(s) in facilitating pain.

Migration of astrocytes transplanted to the midbrain of neonatal rats

Previous studies have indicated that transplanted astrocytes are able to survive, express glial fibrillary acidic protein (GFAP), and migrate in the host brain, and that the pattern and speed of astrocyte migration is largely determined by the location of the graft. We examine here the pattern of astrocyte migration in the midbrain by transplanting CD-1 mouse corpus callosum (P2-3) into the midbrain of neonatal rats. The location of the grafts and the distribution of donor astrocytes were assessed by using a monoclonal antibody (anti-M2) specific for mouse astrocytes. A characteristic donor astrocyte distribution was seen. The highest density of cells was in the region of the substantia nigra (SN); lower numbers were found in the medial geniculate nucleus (MGN). Donor astrocytes were also found in the superior colliculus (SC) and central gray region, but only when the body of a graft was located nearby. [3H]thymidine studies showed that the concentrations of donor astrocytes in the SN were not the result of high levels of mitotic activity: all indications were that the proportion of dividing donor cells closely matched that of host glia. The pattern of astrocyte migration in the midbrain did not follow the course established by radial glia and was not influenced by axonal degeneration in the SC after removal of eyes. Moreover, donor cells failed to migrate along the course of axonal outgrowth from co-grafted retinae. Reciprocally, axonal elongation from retinal grafts did not follow the pathway of astrocyte migration, thus suggesting that astrocyte migration and neuronal outgrowth follow different cues.

[A case of brainstem vascular malformation with isolated trochlear nerve palsy as the initial symptom]

We report a 46-year-old, non-hypertensive man who suddenly developed isolated right trochlear nerve palsy. His diplopia was most prominent in the left lower gaze, and partially alleviated by head tilt to the left or by anteflexion of the neck. His CT scans showed a small high density area consistent with a hemorrhage in the lateral side of the right mesencephalic tectum. His MRI (T2-weighted images) showed a lesion consisting of mixed high- and iso-intensity areas with linear low intensity areas. The margin of the lesion was irregular and nodular. Cerebral angiography (prolonged injection) showed small feeding arteries (or capillaries) in the late arterial phase and dilated draining veins in the venous phase. No tumor stain, early draining veins, or capillary brushes were present. We thought he had an angioma (vascular malformation). AVM seemed unlikely. Review of the literature revealed that trochlear nerve palsy caused by a mesencephalic angioma is extremely rare. MRI and cerebral angiography (prolonged injection) seemed useful for the diagnosis of angiomas (Vascular malformations).

Differential response of striatal dopamine and muscarinic cholinergic receptor subtypes to the loss of dopamine. II. Effects of 6-hydroxydopamine or colchicine microinjections into the VTA or reserpine treatment

In the previous paper it was demonstrated that striatal dopamine (DA) D1 and D2 receptor subtypes and muscarinic M1 and M2 receptor subtypes show differing responses to lesions of the mesostriatal DA system. To examine this differential regulation further rats were given unilateral injections of 6-hydroxydopamine (6-OHDA) or colchicine into the ventral tegmental area (VTA), or treated chronically with reserpine or saline. Two weeks later the animals were tested for their behavioral response to a subthreshold dose of apomorphine and 24 h later their brains were removed and processed for quantitative autoradiography or for analysis of DA levels by high-performance liquid chromatography. The 6-OHDA-lesioned animals showed a supersensitive rotational response to apomorphine. The loss of DA, loss of DA uptake sites, regulation of DA D1 and D2 receptors and regulation of the muscarinic cholinergic system was similar to the previous paper. Injection of colchicine in the VTA resulted in incomplete loss of striatal DA (50%), [3H]mazindol binding (50%), and no behavioral supersensitivity to apomorphine. There was a small loss of presynaptically located D2 receptors (13%). Similar to the 6-OHDA lesions there was a loss of D1 (12%) and M1 receptors. Reserpine treatment produced an 86% decrease in DA levels, an enhanced stereotyped responsiveness to apomorphine, and an increase of both D2 (28%) and D1 receptors (26%). There was a loss of muscarinic M1 but not M2 receptors. Thus removal of DA terminals or blockade of transport of proteins in the mesostriatal axons can lead to a reduction in D1 receptor density in the striatum. In contrast, loss of DA without removal of DA terminals leads to a significant up-regulation of the D1 receptor. D2 receptors show increases following removal of DA or of DA terminals. Alteration in the muscarinic cholinergic system following damage to the mesostriatal DA system is a complex response not mimicked by either reserpine or colchicine treatment.

Morphine-dopamine interaction: ventral tegmental morphine increases nucleus accumbens dopamine release

Microdialysis and high-performance liquid chromatography with electrochemical detection were used to determine extracellular levels of dopamine following ventral tegmental morphine injections into chloral hydrate-anesthetized rats. Morphine (13.2 nanomoles in 0.5 microliter of Ringer's solution) caused 50-150% increases in nucleus accumbens dopamine and metabolites; latency for the effect was on the order of 15 min with peak effects occurring in 30-50 min. Contralateral dopamine levels were influenced only minimally. These data suggest opiate receptors in or near the ventral tegmental area as sites of the opioid action that is responsible for opioid activation of the mesolimbic dopamine system.

Effect of typical and atypical neuroleptics on the behavioural consequences of activation by muscimol of mesolimbic and nigro-striatal dopaminergic pathways in the rat

Direct injections of muscimol into the ventral tegmental area (VTA) or substantia nigra zona reticulata (SNR) have been used to selectively stimulate the mesolimbic and nigro-striatal dopamine pathways respectively. Such injections induced locomotor activity, rearing, sniffing and in some animals an intermittent grooming response. These responses were rapid in onset, dose-related and relatively short lasting (less than 40 min). Selective increases in dopamine turnover were seen in the nucleus accumbens and in the striatum following VTA and SNR injections of muscimol (100 ng) respectively. Haloperidol inhibited the behavioural consequences of VTA and SNR injections of muscimol with similar potency (ED50S 0.01-0.03 mg/kg IP), and fluphenazine did likewise (ED50S 0.05-0.16 mg/kg IP). However, thioridazine (ED50S VTA: 1.45-2.04 mg/kg IP, SNR 8.50-9.20 mg/kg IP) and in particular clozapine (ED50S VTA: 0.24-0.58 mg/kg IP, SNR: 6.10-9.70 mg/kg IP) were more potent at inhibiting the locomotor activity and sniffing responses due to VTA rather than SNR administered muscimol. Since dopamine D2 antagonists are believed to exert their anti-psychotic effects via an action on mesolimbic dopaminergic systems, and their ability to induce extrapyramidal side effects (EPS) is thought to be due to an action on nigro-striatal dopamine systems, these results suggest that the behavioural models described can be used to predict efficacy and side-effect liability of potential neuroleptic drugs.

Neurochemical consequences following injection of the substance P analogue, DiMe-C7, into the rat ventral tegmental area

The effect on forebrain catecholamine- and indoleamine-related neurochemical levels was investigated following stimulation of the rat ventral tegmental area with the substance P analogue, DiMe-C7. DiMe-C7 (6.0 micrograms) induced a marked hyperactivity in rats with maximal response between 15 and 30 min following the injection. Fifteen min following the DiMe-C7 injection levels of dopamine and/or its metabolites (3,4-dihydroxyphenylacetic acid, homovanillic acid) were significantly increased in the nucleus accumbens, amygdala, entorhinal cortex and striatum relative to vehicle-injected animals. Although the increase in dopamine metabolism in the nucleus accumbens is consistent with the behavioural hyperactivity, it is concluded that other forebrain nuclei may also be involved in the mediation of the hyperactivity response.

Neuronal sites mediating locomotor hyperactivity following central neurokinin agonist administration

The stable substance P analog, DiMeC7, increases spontaneous locomotor activity after infusion into the lateral ventricles or median raphe nucleus (MRN). The elevated locomotion observed after intracerebroventricular (ICV) infusion of DiMeC7 was attenuated, but not blocked, by bilateral 6-hydroxydopamine (6OHDA) lesions of the ventral tegmental area (VTA). In contrast, bilateral 6OHDA lesions of the nucleus accumbens (NAS) blocked the motor activity induced by ICV administration of DiMeC7. Similar lesions did not affect the increases in motor behavior observed after MRN infusions of DiMeC7. However, the hyperactivity following MRN microinjections of DiMeC7 was attenuated by intraperitoneal administration of the dopamine (DA) antagonist, haloperidol. The results suggest that ICV infusions of DiMeC7 increase locomotor activity by acting directly on neurons in the NAS and in part by influencing, directly or indirectly, the activity of DA cells in the VTA. The increased motor activity seen after MRN administration of DiMeC7 appears to depend on DA neurons but not on projections to the NAS.

MR demonstration of a unilateral olivary hypertrophy caused by pontine tegmental hematoma

Two patients with unilateral olivary hypertrophy (OH) following a pontine tegmental hematoma are presented. Both showed palatal myoclonus and one patient showed an extremity myorhythmia on the opposite side of OH. The magnetic resonance (MR) examination demonstrated an abnormal unilateral hyperintense lesion in the ventral part of medulla oblongata on T2-weighted images. The MR findings were highly suggestive of the pathological nature of the lesion, showing the value of MR.

[Left-sided extremity myorhythmia and rightward slow eye movement caused by a hemorrhage in the right pontine tegmentum]

A sixty-year-old man was admitted to our hospital on January, 1989. He had suffered a hemorrhage in the right side of pontine tegmentum on April, 1988. He had been in deep coma for about a week, and then he had showed diplopia, mild right deafness, right facial palsy, left hemiparesis, dysarthria, dysphagia, and urinary disturbance. He showed slight improvement of these symptoms and signs, but had developed thereafter extremity myorhythmia on the left side. On admission, rightward slow eye movement (absence of saccadic eye movement and preservation of pursuit eye movement of both eyes) was present. MRI revealed a hypointensity lesion with a hyperintensity spot on T2-weighted images showing an old hemorrhage in the right pontine tegmentum and a hyperintensity lesion on T2-weighted images showing an olivary pseudohypertrophy on the right. We concluded that the extremity myorhythmia in this patient was caused by the damage of the right central tegmental tract followed by right olivary pseudohypertrophy. The rightward slow eye movement was considered to be due to the damage of the right paramedian pontine reticular formation and/or its afferent fibers in the pontine tegmentum.

1-(3-Trifluoromethylphenyl) piperazine (TFMPP) in the ventral tegmental area reduces the effect of desipramine in the forced swimming test in rats: possible role of serotonin receptors

1-(3-Trifluoromethylphenyl)piperazine (TFMPP), a serotonin1 (5-HT1) receptor agonist, injected i.p. in doses of 0.1 and 0.6 mg/kg, did not modify the immobility time of rats in the forced swimming test but significantly antagonized the effect of a 7 days treatment with 10 mg/kg per day desipramine (DMI). A similar effect was found on infusing 1 and 5 micrograms/microliters TFMPP bilaterally into the ventral tegmental area (VTA). Infusion of 5 micrograms/microliters TFMPP into the nucleus accumbens or into the globus pallidus did not modify the effect of DMI. The effect of 5 micrograms TFMPP infused into the VTA was prevented by the i.p. administration of 5 mg/kg metergoline, a non-selective serotonin receptor antagonist. Infusion of 5 micrograms/microliters 8-hydroxy-2-(di-n-propylamino)tetralin, a specific 5-HT1A receptor agonist, into the VTA did not modify the effect of DMI. Besides acting as a 5-HT1B receptor agonist, TFMPP may also act on other 5-HT receptor types, but available evidence suggests that its former action is more important. It thus appears that 5-HT1 receptors in the VTA, presumably of the 5-HT1B type, act by preventing the anti-immobility effect of DMI. The role of VTA dopamine and non-dopamine cells in the effect of TFMPP is discussed.

Opiate influences on nucleus accumbens neuronal electrophysiology: dopamine and non-dopamine mechanisms

Single-unit recordings of nucleus accumbens septi (NAS) neurons in halothane-anesthetized rats revealed that microinfusions of morphine into the ventral tegmental area (VTA) primarily inhibited spontaneously active NAS units. These inhibitory effects were reversed by alpha-flupenthixol (s.c.), suggesting a role for dopamine (DA) in the observed opiate-induced effect. VTA opiate microinfusions also inhibited the evoked (driven) responses of silent cells (spontaneously inactive) in the NAS elicited by stimulation of hippocampal afferents to the NAS. In addition, this inhibition of driven response was reversed by naloxone (s.c.) but not by alpha-flupenthixol, implying a VTA-mediated non-DA mechanism. Morphine applied iontophoretically to cells within the NAS inhibited spontaneous activity but not fimbria-driven cellular activity, suggesting that the systemic effects of opiates on NAS activity can be mediated directly in the NAS as well as through VTA afferents. Moreover, since VTA-induced inhibition of fimbria-driven activity was reversed by systemic opiates, opiates also can exert effects through other, as yet unidentified NAS afferent systems.

Microinjection of neurotensin into the CNS induces hyperdipsia in the rat

Neurotensin (NT) is a neuropeptide and putative neurotransmitter that has been shown to exert a variety of effects on digestive and ingestive processes. In order to address the possibility that NT might play a role in the regulation of water intake as well, the peptide was infused into the lateral cerebral ventricle, amygdala, ventral tegmental area, lateral hypothalamus, and preoptic area of the anterior hypothalamus of rats deprived of water for 16 hours. Neurotensin produced a significant and dose-dependent increase in water intake when injected into the ventricular system but had no effect when it was applied to the other brain sites. It was concluded that this peptide may play a physiological role in the control of water ingestion and that central sites of action remain to be determined.

Infusions of cholecystokinin octapeptide into the ventral tegmental area potentiate amphetamine conditioned place preferences

Cholecystokinin (CCK) and dopamine (DA) coexist in both cell body and terminal areas of a mesolimbic pathway that projects from the ventral tegmental area (VTA) to the nucleus accumbens (N ACC). Autoradiography reveals extensive CCK binding sites in the N ACC, but not in the VTA. However, iontophoresis of CCK into the VTA results in activation or deactivation of DA neuronal firing rates, and bursting activity (depending on the dose of CCK administered). CCK could have neuromodulatory effects on mesolimbic DA neurons. In two studies, behavioral effects of infusions of CCK into the VTA were examined in the conditioned place preference (CPP) paradigm. The CPP paradigm is a behavioral test used to assess reinforcement induced by drug administration. Drugs with reinforcing properties can condition preferences for novel environments. CCK infusions into VTA (0.0, 0.04, 0.4, and 4.0 ng/cannula) potentiated amphetamine CPPs in a dose-dependent linear manner. CCK infusions by themselves did not have significant effects in the CPP paradigm. Results indicate a neuromodulatory role for CCK on the neuronal mechanisms that mediate the reinforcing effects of amphetamine. Results also implicate sites of action for CCK in the VTA.

Oral movements induced by interference with nigral GABA neurotransmission: relationship to tardive dyskinesias

Previous studies have shown that the emergence of spontaneous dyskinetic behaviors, such as vacuous chewing movements, following several months of neuroleptic treatment in the rat, is correlated with depletion of nigral GABA. To explore the specificity of this relationship, we acutely interfered with nigral GABA transmission pharmacologically, by microinfusing either the GABA receptor antagonist, bicuculline, or the GABA-depleting agent, isoniazid, bilaterally into substantia nigra. We found that both acute treatments induced vacuous chewing movements in rats. Moreover, the time to onset of action of each of these drugs corresponded to the onsets of their respective effects on GABA transmission. In addition, we found that the application of muscimol into the target field of the nigrotegmental projection, which has been shown to block gnawing elicited by nigral GABA receptor stimulation, completely abolished elicitation of vacuous chewing movements by intranigral isoniazid. In contrast, bilateral microinfusions of muscimol into the nigrocollicular target region, in the deep layers of superior colliculus, blocked elicitation of gnawing by intranigral muscimol, but completely spared elicitation of vacuous chewing movements by intranigral isoniazid. We conclude that qualitatively different dyskinetic syndromes can be produced by bidirectional perturbations of nigral GABA function and are differentially mediated by nigrotegmental and nigrotectal projections. These syndromes may represent animal models of distinct components of extrapyramidal side effects of chronic neuroleptic administration.

Neurotensin self-injection in the ventral tegmental area

Earlier work with the conditioned place-preference paradigm suggested that neurotensin (NT) acts as a behavioral reinforcer when microinjected into the ventral tegmental area (VTA) of the midbrain. We report here that animals will perform an operant task to obtain microinfusions of NT into the VTA. Rats reliably pressed a lever to obtain NT infusions while neglecting an identical but inactive lever. Substitution of saline for NT initiated response extinction; following the reintroduction of NT, reliable responding resumed. These results extend earlier work suggesting that NT in the VTA can be a positive reinforcer.

Experimental study on resting tremor and L-dopa-induced dyskinesia

Resting tremor and hypokinesia of unilateral limbs were produced in monkeys after making a lesion in the mesencephalic tegmentum. The administration of L-dopa or dopamine agonists relieved them and followingly induced dyskinesias. The same effects were produced by the direct injection of dopamine or its agonists into the dorsomedial part of caudate nucleus ipsilateral to the lesion, where spiroperidol binding to the D2 receptor was increased in the affinity. These results suggest that denervation supersensitivity at the postsynaptic D2 receptor is a basic condition for the development of dyskinesias, though they were slightly suppressed by the intracaudate injection of GABA, serotonin and met-enkephalin.

Behavioural effects of tachykinins and related peptides

Substance P (SP) and related tachykinins administered either intracerebroventricularly or directly into the ventral tegmental area of the mesencephalon of rat brain caused increased locomotor activity, grooming behaviour and wet dog shakes. Kassinin, eledoisin, neurokinin A and DiMe-C7, agonists with some selectivity for the SP-E-receptor elicited the greatest locomotor activity and wet dog shake responses, whereas SP and physalaemin which are more selective for the SP-P-receptor were most effective in eliciting the grooming response.

Dopamine agonist induced self-mutilative biting behavior in monkeys with unilateral ventromedial tegmental lesions of the brainstem: possible pharmacological model for Lesch-Nyhan syndrome

We have investigated the effects of various dopamine (DA) agonists on induction of abnormal involuntary movements (AIM) in a group of monkeys which had denervated nigro-striatal DA neurons for 10-14 years rendered by a unilateral surgical ventromedial tegmental (VMT) lesion of the brainstem. The surgical lesions were placed when the monkeys were 2-4 years old. The administration of mixed DA agonists, such as L-DOPA, apomorphine (Apo) and abeorphine 201-678, elicit a self-mutilative biting behavior (SMB) of the forelimb digits contralateral to the lesion, and spasticity of the contralateral hindlimb. These dysfunctions resemble, in some aspects, the neurological disturbances associated with Lesch-Nyhan syndrome. The SMB behavior was elicited by mixed DA agonists which predominantly stimulate D1, but not D2 DA receptors, and was prevented or abolished by the D1 DA antagonist SCH 23390 or by the D1 and D2 DA antagonist fluphenazine (Flu), but not by the D2 antagonist (+/-)sulpiride. These results suggest that DA agonist-induced SMB behavior is mediated by D1 and/or by both D1 and D2 DA receptor pathways. To study the relationships between HPRT, the defective enzyme in Lesch-Nyhan syndrome, and the DA neuronal systems, we have measured the effects of nigro-striatal DA degeneration and intrastriatal neuronal degeneration on HPRT activity. The unilateral 6-OHDA-induced nigro-striatal DA degeneration does not significantly alter the HPRT activity on the lesioned side of the striatum, while the quinolinic acid-induced intrastriatal neuronal degeneration significantly reduces the enzyme activity. These results suggest that HPRT is localized on intrastriatal neurons which are also known to contain DA receptors. It is postulated that HPRT deficiency in Lesch-Nyhan syndrome results in abnormal guanine nucleotide metabolism which may affect the regulation of DA receptors.

One-and-a-half syndrome in a patient with metastatic breast disease

This report reviews the clinical course of a patient with breast carcinoma presenting with the classic one-and-a-half syndrome. She had no saccades to the left and no leftward eye deviation on oculocephalic movement. Her clinical examination suggested involvement in the medial longitudinal fasciculus, paramedian pontine reticular formation, and abducens nucleus. There has only been one previous case reported of a metastasis producing this syndrome. A computed tomography scan confirmed the pontine location of the lesion.

Reversible parkinsonism from shunt failure

Parkinsonism developed as the result of shunt malfunction in a 16-year-old girl with aqueductal stenosis. Her symptoms responded to levodopa and shunt revision. Possible pathogenetic mechanisms are discussed.

Reinstatement of heroin and cocaine self-administration behavior in the rat by intracerebral application of morphine in the ventral tegmental area

In rats trained to self-administer heroin or cocaine intravenously, non-contingent priming injections of heroin or cocaine, respectively, reinstated responding after a period of extinction. In rats similarly trained to self-administer heroin or cocaine intravenously, morphine sulphate was applied centrally to sites in the ventral tegmental area (VTA), the periventricular grey (PVG) and the caudate nucleus following a period of extinction. Self-administration behavior was reinstated by morphine application to sites in the region of the VTA, but not to other sites, in both heroin and cocaine-trained animals. This priming effect of morphine was blocked or attenuated by prior administration of naltrexone, given intraperitoneally. Morphine in the VTA is known to activate mesolimbic dopamine neurons, suggesting that dopamine activity in this system may underlie the priming effects of both opiates and stimulants. Furthermore, the fact that the mesolimbic system is implicated in the positive motivational actions of both drug groups, suggests that morphine reinstates drug-taking behavior in these animals by activating appetitive motivational systems of the brain.

Neuroanatomical site specific modulation of spontaneous motor activity by neurotensin

Immunohistofluorescent neurotensin (NT) is found in the ventral tegmental area (VTA), and bilateral injection of NT into the VTA produces an increase in exploratory behaviors. The VTA also contains dopaminergic cell bodies with axonal projections to the nucleus accumbens. In this study it was shown that bilateral microinjection of NT (4.0 micrograms/side) into the nucleus accumbens blocked the behavioral hyperactivity produced by intra-VTA injection of NT (2.5 micrograms/side).