Dopamine receptors in the ventral tegmental area affect motor, but not motivational or reflexive, components of copulation in male rats

Microinjection of apomorphine into the ventral tegmental area (VTA) of male rats was previously shown to delay the onset of copulation and slow its rate, presumably by stimulating impulse-regulating autoreceptors on cell bodies of the A10 mesocorticolimbic dopamine tract. Such stimulation would be expected to slow the firing rate of these neurons and, thereby, to impair locomotion and/or motivational processes. The present experiments tested whether the delayed onset and slowed rate of copulation were related to deficits in motor performance, sexual motivation, and/or genital reflexes. In X-maze tests the speed of running to all 4 goal boxes was slowed; however, the percentage of trials on which the male chose the female's goal box was not decreased. Examination of videotaped copulation tests revealed that the male showed fewer complete copulatory behaviors (mounts, intromissions, and ejaculations), but more misdirected or incomplete copulatory attempts after apomorphine in the VTA. There were also fewer scores of active, as opposed to inactive, behaviors, and the onset and rate of copulation were slowed. The total number of female directed behaviors was not different in apomorphine tests, compared to vehicle. Finally, tests of ex copula genital reflexes revealed no significant effects of apomorphine in the VTA on erections, penile movements, or seminal emissions. These data suggest a role of the VTA in the motor aspects and/or sensorimotor integration of copulation. Sexual motivation and ex copula genital reflexes appeared to be unaffected by apomorphine in the VTA.

Morphine and cocaine exert common chronic actions on tyrosine hydroxylase in dopaminergic brain reward regions

We studied levels of tyrosine hydroxylase immunoreactivity and phosphorylation state in the ventral tegmental area (VTA) and nucleus accumbens (NAc) in an effort to understand better the mechanisms by which these brain reward regions are influenced by opiates and cocaine. In the VTA, chronic, but not acute, administration of either morphine or cocaine increased levels of tyrosine hydroxylase immunoreactivity by 30-40%, with no change observed in the relative phosphorylation state of the enzyme. In the NAc, chronic, but not acute, morphine and cocaine treatments decreased the phosphorylation state of tyrosine hydroxylase, without a change in its total amount. In contrast, morphine and cocaine did not regulate tyrosine hydroxylase in the substantia nigra or caudate/putamen, brain regions generally not implicated in drug reward. Morphine and cocaine regulation of tyrosine hydroxylase could represent part of a common biochemical basis of morphine and cocaine addiction and craving.

Different regulations of dopaminergic (D1) receptors and neurotensinergic binding sites in the rat prefrontal cortex

The effects of the destruction of mixed dopamine/neurotensin (DA/NT) meso-cortical neurons were investigated by studying the development of denervation supersensitivity of DA (D1) and NT cortical post-synaptic binding sites using respectively [3H]SCH 23390 and [125I]NT as ligands. These neurons were destroyed bilaterally either by injection of 6-hydroxydopamine (6-OHDA) or by an electrolytic coagulation made in the ventral tegmental area (VTA). Five weeks later, both [3H]SCH 23390 and [125I]NT bindings were analysed by quantitative autoradiography on each lesioned animal and on corresponding controls. The chemical lesions of the VTA induced an increase in the density of the cortical NT binding sites but did not affect D1 binding sites. On the contrary, electrolytic lesions induced an increase in D1 binding sites and no change in NT binding sites. One possible explanation of these differences may be that, since chemical lesions of the VTA destroy noradrenergic (NA) ascending pathways while electrolytic lesions spare the cortical NA innervation, the observed modifications of D1 and NT cortical binding sites following their presynaptic denervation are dependent on the presence (increase in D1 receptors) or the absence (increase in NT binding sites) of the cortical NA innervation.

Behavioural and biochemical responses following activation of midbrain dopamine pathways by receptor selective neurokinin agonists

Preferential activation of mesolimbic and nigro-striatal dopamine (DA) pathways by receptor-selective and peptidase-resistant neurokinin (NK) agonists is reported. The DA cell body region of the mesolimbic pathway appears to be activated by NK agonists selective for NK-1 and NK-3 receptors whereas the DA cell bodies in the substantia nigra are under an excitatory NK-2 receptor-mediated influence. Stimulation of the mesolimbic DA pathway by NK-1 (Ava[L-Pro9,N-Me-Leu10]SP (7-11) [GR73632]) or NK-3 (Senktide) agonists increase locomotor activity. Additional studies showed that this elevated motor response observed after intra-VTA infusion of GR73632 was accompanied by a corresponding increase in DA turnover in the terminal fields of this pathway. Similarly, unilateral activation of the nigro-striatal DA pathway by NK-2 selective agonists (Ava (D-Pro9) SP (7-11) [GR51667] or [Lys3,Gly8,R-Lac-Leu9]NKA (3-10) [GR64349]) elicit contralateral rotational activity and an increase in DA turnover in the ipsilateral striatum. The rotational response was attenuated by prior administration of an NK-2 antagonist (cyclo (Gln, Trp, Phe, Gly, Leu, Met)] L-659877]) into the nigra. Peripheral injection of haloperidol, a DA antagonist, also blocked the NK-2 agonist induced rotations.

Ontogenetic changes in dopaminergic pre and postsynaptic elements in rat brain: effects of quinpirole and sulpiride

The effects of quinpirole and sulpiride on dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and accumulation of dihydroxyphenylalanine (DOPA) after inhibition of DOPA decarboxylase were determined for the striatum and tegmentum of 11- and 17-day-old rat pups. In both 11- and 17-day-olds, sulpiride enhanced striatal DOPAC and DOPA accumulation; whereas, quinpirole decreased DOPAC levels and increased DA levels. In the tegmentum, sulpiride enhanced DOPA accumulation in the 11-day-olds only. 3H-spiroperidol binding sites were also measured in the striatum, nucleus accumbens, tegmentum, medial frontal cortex, and the hippocampo-entorhinal area of 11- and 17-day-old pups. Age-dependent increases in binding site density were found in the striatum and nucleus accumbens.

Effects of tetrahydropapaveroline in the nucleus accumbens and the ventral tegmental area on ethanol preference in the rat

Numerous studies have attempted to identify sites of action in brain that are sensitive to the dopamine-aldehyde condensation product, tetrahydropapaveroline (THP). Recent studies performed by Myers and his co-workers have uncovered diverse sites of action in the brain. The present experiments were based on the hypothesis that the dopamine-aldehyde condensation products would be most reactive at those synapses that were dominated by the parent amine. To test this hypothesis, rats were implanted with bilateral cerebral cannulae in those brain regions that define the rostral-caudal aspects of two major dopaminergic systems: the mesolimbic system and the nigrostriatal system. Following surgery, rats were given single, bilateral injections of 1.0 microgram THP. The results suggest that a single injection of THP tends to augment the rat's preference for ethanol, when THP is placed in regions of the brain that correspond to the mesocorticolimbic system which in this study included certain aspects of the posterior hypothalamus. THP did not change the rat's preference for ethanol when the compound was injected into the nigrostriatal system.

The interstitial nucleus of Cajal of the cat. II. Effects of kainic acid lesion on vertical optokinetic nystagmus and after-nystagmus

The effects of bilateral lesions of the interstitial nucleus of Cajal (INC) by kaïnic acid on vertical optokinetic nystagmus (OKN) and after-nystagmus (OKAN) were studied in four cats: in three cats, in the acute stage from 1 to 60 days after the lesions; in the fourth cat, they were studied 3 years after the lesions were made. Histological control of lesions showed that the whole INC was bilaterally destroyed in two cats of the acute group and only the upper part of INC in the third cat. In the chronic cat, the density of cell bodies in both INC was lower than normal. In the acute group, the cats exhibited a spontaneous downward eye drift in light and in darkness. During an upward optokinetic stimulation, the effect of INC lesions was dramatic: upward slow phases and downward quick phases of OKN were abolished. Sixty days post lesions, small upward slow eye movements were again observed. During a downward optokinetic stimulation, the defect was much less; in particular, after a slight impairment of downward slow phases, during the first days post lesions, they recovered quickly. The secondary optokinetic after nystagmus (OKAN II) ensuing a downward OKN was cancelled and did not reappear 60 days post lesions. In the chronic stage, three years after the lesions, during an upward optokinetic stimulation, the cat showed upward slow phases with velocities close to normal. However, upward slow phases were curved: the velocity at the end of the slow phases was lower than at the beginning. After an upward OKN (the direction of slow phases gives the direction of the OKN and OKAN), the ensuing OKAN was present but abnormal.

Substance P injections into the ventral tegmentum affect unit activity in mesolimbic terminal regions

Microinjections of substance P (SP) into the ventral tegmental area (VTA) increase locomotor activity in rats, and this effect is thought to be produced by activation of the mesolimbic dopamine system. In the present study, firing rates of neurons in areas receiving projections from the mesolimbic dopamine system were recorded during injections of SP (3 microgram in 0.5 microliters saline) into the VTA of rats anesthetized with chloral hydrate. Significant changes in firing rates were observed in 84% of the units recorded in nucleus accumbens and olfactory tubercle. There were mostly decreases in nucleus accumbens (NAC, 21 of 25 units affected by SP) and mostly increases in olfactory tubercle (OT, 13 of 18 units affected by SP). In contrast, neither saline injections into VTA nor SP injections 2 mm dorsal to VTA had any effect on NAC or OT neurons. Haloperidol (0.5 mg/kg IV) blocked the effects of SP, suggesting that effects were mediated, at least in part, by the mesolimbic dopamine system. Results indicated that activation of dopaminergic neurons by SP injections into VTA can produce changes in the activity of neurons in NAC and OT, areas which receive mesolimbic dopaminergic projections.

Kynurenate blocks the acute effects of haloperidol on midbrain dopamine neurons recorded in vivo

The acute effect of systemic administration of the antipsychotic drug haloperidol on the activity of midbrain dopamine (DA) neurons was investigated with extracellular single cell recording in the chloral hydrate anaesthetized male rat. DA cells in the zona compacta-substantia nigra (SN) and ventral tegmental area (VTA) were excited by low doses of haloperidol. This excitation, which included increased firing rate and burst firing, was no longer present after treatment with the excitatory amino acid (EAA) antagonist kynurenate (1 mumol ICV). Kynurenate alone profoundly regularized the activity and abolished burst firing in VTA-DA neurons, while SN-DA neuronal activity was unaffected by this treatment. Thus, VTA-DA neurons, but not SN neurons, appear to be dependent on a tonic EAA input for their normal varied, burst-firing activity. The antagonism of haloperidol-induced effects by kynurenate suggests that the acute excitatory action of haloperidol on midbrain DA neurons is executed via EAA neurons, in the case of the VTA probably via a corticofugal EAA pathway from the medial prefrontal cortex.

Inhibition by talipexole, a thiazolo-azepine derivative, of dopaminergic neurons in the ventral tegmental area

A microiontophoretic study using rats anesthetized with chloral hydrate and immobilized with gallamine triethiodide was carried out to compare the effect of talipexole (B-HT 920 CL2:2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo [4,5-d]-azepine-dihydrochloride), a dopamine autoreceptor agonist, on dopaminergic neurons in the ventral tegmental area (VTA) to non-dopaminergic neurons in the VTA. VTA neurons were classified into two types according to the responses to antidromic stimulation of the nucleus accumbens (Acc): type I neurons with a long spike latency (8.69 +/- 0.24 msec) upon Acc stimulation and low spontaneous firing rate (6.80 +/- 1.34/sec), and type II neurons with a short latency (2.76 +/- 0.20 msec) and high spontaneous firing rate (26.77 +/- 7.05/sec), probably corresponding to dopaminergic and non-dopaminergic neurons, respectively. In type I neurons, microiontophoretic application of talipexole and dopamine inhibited antidromic spike generation elicited by Acc stimulation, and talipexole-induced inhibition was antagonized by domperidone (dopamine D-2 antagonist). In type II neurons, however, the antidromic spikes were not affected by either talipexole or dopamine. Furthermore, spontaneous firing was also inhibited by iontophoretically applied talipexole and dopamine in most type I neurons, but rarely affected by either drug. Inhibitory effects of talipexole were antagonized by domperidone. These results suggest that talipexole acts on dopamine D-2 receptors, thereby inhibiting the dopaminergic neurons in the VTA.

Paradoxical sleep and its chemical/structural substrates in the brain

As originally named for the ostensibly contradictory appearance of rapid eye movements and low voltage fast cortical activity during behavioral sleep, paradoxical sleep or rapid eye movement sleep, represents a distinct third state, in addition to waking and slow wave sleep, in mammals and birds. It is an internally generated state of intense tonic and phasic central activation that is contemporaneous with the inhibition of sensory input and motor output. In early studies, it was established that the state of paradoxical sleep was generated within the brainstem, and particularly within the pons. Pharmacological studies indicated an important role for acetylcholine as a neurotransmitter in the generation of this state. Local injections of cholinergic agonists into the pontine tegmentum triggered a state of paradoxical sleep marked by phasic ponto-geniculo-occipital spikes in association with cortical activation and neck muscle atonia. Following the immunohistochemical identification of choline acetyl transferase-containing neurons and their localization to the dorsolateral ponto-mesencephalic tegmentum, neurotoxic lesions of this major cholinergic cell group could be performed to assess its importance in paradoxical sleep. Destruction of the majority of the cholinergic cells, which are concentrated within the laterodorsal tegmental and pedunculopontine tegmental nuclei but extend also into the locus coeruleus and parabrachial nuclei in the cat, resulted in a loss or diminishment of the state of paradoxical sleep, ponto-geniculo-occipital spiking and neck muscle atonia. These deficits were correlated with the loss of choline acetyltransferase-immunoreactive neurons in the region, so as to corroborate results of pharmacological studies and single unit recording studies indicating an active role of these cholinergic cells in the generation of paradoxical sleep and its components. These cells provide a cholinergic innervation to the entire brainstem reticular formation that may be critical in the generation of the state which involves recruitment of massive populations of reticular neurons. Major ascending projections into the thalamus, including the lateral geniculate, may provide the means by which phasic (including ponto-geniculo-occipital spikes) and tonic activation is communicated in part to the cerebral cortex. Descending projections through the caudal dorsolateral pontine tegmentum and into the medial medullary reticular formation may be involved in the initiation of sensorimotor inhibition. Although it appears that the pontomesencephalic cholinergic neurons play an important, active role in the generation of paradoxical sleep, this role may be conditional upon the simultaneous inactivity of noradrenaline and serotonin neurons, evidence for which derives from both pharmacological and recording studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Alpha MSH-induced excessive grooming behavior involves a GABAergic mechanism

It has been shown that MSH administered in the ventral tegmental area (VTA) elicits excessive grooming behavior (EGB) by stimulating an acetylcholinergic pathway. The present work was performed in order to evaluate the possible participation of the GABAergic system in this behavior. VTA injection of GABA antagonist bicuculline stimulated the EGB (55.5 +/- 2.4). In contrast, this effect disappeared if the animals were pretreated with atropine (33.1 +/- 1.5). When bicuculline was injected before a 200 ng/microliters dose of MSH, the EGB increased (87.6 +/- 4.4) in comparison to MSH-treated rats (46.5 +/- 3.2). Our results suggest that GABA, ACh, and MSH interact in the VTA in the induction of EGB; an increase in MSH levels appears to stimulate cholinergic neurons. GABAergic fibers probably modulate the cholinergic discharge at the presynaptic level.

Mesolimbic dopamine mediates gastric mucosal protection by central neurotensin

Bilateral microinjection (1.0 microliter/side) of neurotensin (NT; 0.3, 1.5, and 3.0 micrograms/side) into the nucleus accumbens (NACB) and ventral tegmental area (VTA) but not in substantia nigra and striatum reduced gastric mucosal injury produced by 2 h of cold-water restraint (CWR). The minimal effective dose for NT-induced protection was 10-100 times lower when administered directly into NACB than into the lateral ventricle. These effects were blocked by pretreatment with the dopamine (DA) receptor antagonist, haloperidol (Hal; 0.5 microgram/side) given directly into NACB. Injection of 6-hydroxydopamine into VTA depleted endogenous DA and inhibited gastric mucosal protection against CWR-induced injury afforded by NT pretreatment. NT, given into either VTA and NACB, inhibited pentagastrin-stimulated gastric acid secretion. These results suggest that VTA and NACB, which represent the mesolimbic DA system, are important locations for interaction between NT and DA receptors to produce gastric mucosal protection against CWR-induced injury.

Cocaine effects in the ventral tegmental area: evidence for an indirect dopaminergic mechanism of action

Behavioral studies have implicated central dopaminergic systems, especially the ventral tegmental area of Tsai (VTA), in the mediation of the reinforcing effects of drugs of abuse such as cocaine. A brain slice preparation of the VTA was used to assess the direct effects of cocaine on the spontaneous activity of dopamine-type neurons. When superfused with 1-10 microM cocaine the firing rate of spontaneously active VTA neurons was decreased, with no corresponding change in spike height. While there was a considerable variability in the response to a given concentration of cocaine among the individual units, every cell inhibited by dopamine was also inhibited by cocaine. The local anesthetic lidocaine had variable effects on firing rate, but never potentiated the inhibitory effects of dopamine. Inhibitory responses to either dopamine or cocaine were blocked by the specific D2 dopamine receptor antagonist sulpiride. Small concentrations of cocaine (0.1-0.5 microM), which by themselves had little or no effect on spontaneous activity, potentiated the inhibitory effect of exogenously applied dopamine. Furthermore, the inhibitory action of apomorphine on spontaneous activity in the VTA was not potentiated by cocaine. These observations suggest that in low concentrations, cocaine can act as a dopamine reuptake inhibitor in the VTA, and that the resultant increase in extracellular dopamine acts upon dopamine autoreceptors to inhibit cellular activity.

Responses of presumed cholinergic mesopontine tegmental neurons to carbachol microinjections in freely moving cats

The effects of microinjections of a cholinergic agonist, carbachol (0.2 microgram/0.2 microliter), were examined on three different types of rostrally projecting tonic neurons that we have reported previously in the dorsal part of the pontomesencephalic tegmentum known to contain numerous cholinergic cell bodies: 1) tonic type I slow (Type I-S); 2) tonic type I rapid (Type I-R); and 3) tonic type II (Type II) (El Mansari et al. 1989). Microinjections of carbachol near unit recording sites in freely moving cats induced within a few minutes a complete suppression of the spontaneous activity and a marked reduction in orthodromic excitation of identified and non-identified type I-S neurons. These effect lasted for approximately 90-120 min and were reversed by local (0.4 microgram/0.2 microliter) or systemic (0.1-0.2 mg/kg, i.m.) administration of atropine sulfate. In contrast, the cholinergic agonist had no consistent effects on tonic type II nor on tonic type I-R neurons. In the light of these and other recent findings, we suggested the direct inhibition of central cholinergic neurons via muscarinic receptors, on the one hand, and the cholinergic nature of type I-S, but not type I-R nor type II neurons, on the other.

Responses of units in the mesolimbic system to olfactory and somatosensory stimuli: modulation of sensory input by ventral tegmental stimulation

It is well known that neurons of the nigrostriatal dopamine (DA) system respond to sensory stimuli, and our primary objective here was to ascertain if neurons in the terminal regions of the mesolimbic DA system respond to sensory input also. In addition, the effects of electrical stimulation of the ventral tegmentum, which contains the DA cells of origin of the mesolimbic system, on sensory-evoked responses in mesolimbic neurons was studied. In rats anesthetized with chloral hydrate, responses of single units to olfactory and somatosensory stimuli were recorded in 6 forebrain regions including nucleus accumbens and olfactory tubercle. Both increases and decreases in spontaneous firing rates were evoked in 225 of 336 units by one or more of the 8 types of sensory stimuli employed (5 olfactory, 3 somatosensory). Excitatory responses occurred twice as frequently as inhibitory responses, but a few units responded with excitatory responses to some stimuli and inhibitory responses to others. The proportions of units responsive to olfactory and/or somatosensory stimuli were different in different regions. After electrical stimulation of the ventral tegmentum, sensory-evoked responses were changed in 30 of the 49 units tested (61%). There were increases, decreases or combinations of a decrease followed by an increase in sensory-evoked responses, which persisted for 1-10 min after the application of a single electrical stimulus train. Haloperidol (0.3 mg/kg, i.p.) either blocked the effect of tegmental stimulation or decreased all responses. The present results demonstrated that units in the terminal regions of the mesolimbic DA system are responsive to sensory input and that these responses can be affected by prior electrical stimulation of the ventral tegmentum.

Effect of BHT 920 on monoaminergic neurons of the rat brain: an electrophysiological in vivo and in vitro study

BHT 920 was originally described as a dopamine autoreceptor agonist. In this study, the effect of this compound on the firing rate of noradrenergic locus coeruleus, serotonergic dorsal raphe and dopaminergic ventral tegmental area neurons was examined both in the anaesthetized rat and in rat brain slices. Extracellular recordings were performed in cells whose identity was determined by electrophysiological, pharmacological and histological criteria. In vivo, BHT 920 inhibited the firing of locus coeruleus neurons (ID50: 14.5 +/- 4.7 micrograms/kg, mean +/- SEM) and ventral tegmental area neurons (ID50: 7 +/- 3 micrograms/kg) at very low doses. As a comparison, the ID50 of clonidine on locus coeruleus cells was 5.5 +/- 0.6 microgram/kg and the ID50 of apomorphine on ventral tegmental area neurons was 13 +/- 3 micrograms/kg. BHT 920 also decreased the firing of dorsal raphe cells, but this effect was obtained at higher doses (ID50: 57 +/- 11 micrograms/kg). The in vitro study confirmed the results obtained in vivo. BHT 920 potently inhibited the firing of locus coeruleus cells (IC50: 71 +/- 28 nM) and was less potent than clonidine (IC50: 5.3 +/- 0.98 nM). The compound also inhibited the firing of ventral tegmental area neurons at very low concentrations (IC50: 21 +/- 3.3 nM), being more potent than apomorphine (IC50: 56 +/- 29 nM). BHT 920 only slightly decreased the firing rate of dorsal raphe neurons at 50 microM, showing that the drug has little direct effect on these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation by norepinephrine of neural responses to estradiol

Pharmacological studies have suggested that neurotransmitter activity impinging on steroid-concentrating cells can affect the steroid receptor system within those cells, modifying behavioral responses to the hormone. The present experiments revealed that the alpha 1-noradrenergic antagonist prazosin, administered to ovariectomized rats at the time of each of two pulses of estradiol, inhibited the appearance of sexual receptivity. Prazosin also substantially reduced the levels of estrogen receptors within hypothalamic cell nuclei following an injection of estradiol. Manipulation of noradrenergic inputs into the hypothalamus by lesioning brain stem norepinephrine cell groups with 6-hydroxydopamine (6OHDA) also reduced the level of nuclear estrogen receptors following an injection of estradiol. Although this effect of 6OHDA lesions was observed in two separate experiments, in other experiments 6OHDA had no effect on estrogen receptors. In some instances, there was a positive correlation between nuclear estrogen receptor levels in the hypothalamus and the levels of norepinephrine. The results are consistent with the hypothesis that brain stem inputs to the hypothalamus are able to modulate neural responses to steroids and specifically that noradrenergic inputs are able to modulate neural responses to estradiol. However, there are additional undiscovered variables that preclude statements of a simple relationship between norepinephrine levels and estrogen receptor levels.

Electrophysiological effects of ethanol on monoaminergic neurons: an in vivo and in vitro study

Monoaminergic neurons have been shown to play a role in both the intoxicating and chronic effects of ethanol. We present here the results of a study about the acute effects of ethanol on serotonergic raphe nucleus, noradrenergic locus coeruleus, and dopaminergic ventral tegmental area. These nuclei were investigated electrophysiologically by recording the spontaneous firing rate of single neurons using glass microelectrodes, both in vivo in chloral hydrate anesthetized rats and in vitro in brain slices. Ethanol was perfused intravenously at a rate ranging from 0.2 mg/kg/min to 0.2 g/kg/min in vivo, and at concentrations between 10(-8) M and 1 M in vitro. We observed that each monoaminergic nucleus had its own pattern of responses to acute ethanol perfusion, and that high and low concentrations have different actions, suggesting a biphasic effect. For example, in slices, ethanol concentrations higher than 10 mM induce an excitation in most raphe and ventral tegmental area neurons, and an inhibition of firing in locus coeruleus neurons. The results were comparable in the in vivo model, but much more heterogenous. We conclude that the effect of ethanol on the monoaminergic neurons is specific of the type of neuron, and that a biphasic effect is commonly found.

Chronic systemic administration of amphetamine increases food intake to morphine, but not to U50-488H, microinjected into the ventral tegmental area in rats

Chronic intermittent administration of amphetamine (AMPH) sensitizes rats to the stimulatory effects on feeding produced by systemic injections of either morphine (MORPH) or the kappa-opiate receptor agonist, U50,488H (U50). Both morphine and the putative kappa-receptor endogenous ligand, dynorphin, have been reported to stimulate feeding when administered into the ventral tegmental area (VTA). To evaluate whether the VTA is the site where AMPH produces sensitization to the feeding effects of opiates, rats were given daily IP injections of either saline or AMPH (3 mg/kg). The amount of powdered food ingested during the 5 h following the injections was measured. After 9 days of AMPH or saline administration, twice weekly tests were begun of the effects of either saline, MORPH (1-10 nmol) or U50 (10 pmol to 10 nmol) injected into the VTA; AMPH administration was continued on intervening days. MORPH produced a statistically significant greater increase in food intake in rats chronically treated with AMPH than in saline treated rats. No statistically significant effects were produced by U50. However, when U50 was administered systemically to the same animals, food intake increased, and the effect was significantly greater in the AMPH-pretreated group. Thus the sensitization to the feeding effects of MORPH and U50 produced by chronic AMPH administration appears to involve different systems; the mesolimbic dopamine system appears to mediate sensitization to the effects of the predominately mu-receptor agonist, MORPH, but not of the kappa-receptor agonist, U50.

Chronic cocaine treatment decreases levels of the G protein subunits Gi alpha and Go alpha in discrete regions of rat brain

A possible role for G proteins in contributing to the chronic actions of cocaine was investigated in three rat brain regions known to exhibit electrophysiological responses to chronic cocaine: the ventral tegmental area, nucleus accumbens, and locus coeruleus. It was found that chronic, but not acute, treatment of rats with cocaine produced a small (approximately 15%), but statistically significant, decrease in levels of pertussis toxin-mediated ADP-ribosylation of Gi alpha and Go alpha in each of these three brain regions. The decreased ADP-ribosylation levels of the G protein subunits were shown to be associated with 20-30% decreases in levels of their immunoreactivity. In contrast, chronic cocaine had no effect on levels of G protein ADP-ribosylation or immunoreactivity in other brain regions studied for comparison. Chronic cocaine also had no effect on levels of Gs alpha or G beta immunoreactivity in the ventral tegmental area and nucleus accumbens. Specific decreases in Gi alpha and Go alpha levels observed in response to chronic cocaine in the ventral tegmental area, nucleus accumbens, and locus coeruleus are consistent with the known electrophysiological actions of chronic cocaine on these neurons, raising the possibility that regulation of G proteins represents part of the biochemical changes that underlie chronic cocaine action in these brain regions.

Further studies of the effects of intranigral morphine on behavioral responses to noxious stimuli

Bilateral intranigral microinjection of morphine produces dose-related and naloxone reversible analgesic-like effects on the hot-plate and tail-flick tests. The main objectives of the present studies were to further characterize the analgesic-like effects of intranigral morphine, to determine whether these effects were related to a general impairment of sensory or motor function, and to assess their anatomical specificity. The principal findings are: (1) intranigral morphine (10 micrograms) suppresses pain-related behavior without altering responses to a variety of non-noxious auditory, visual, and somatic stimuli, and without producing motor impairment; (2) movement of injector needles approximately 1 mm rostral, dorsal, or medial to the active nigral site significantly reduces the analgesic-like effect of morphine on the tail-flick test; and (3) electrolytic lesions confined to the nigra significantly reduced the analgesic-like effect of morphine on the hot-plate test. It is concluded that the analgesic-like effects of intranigral morphine are mediated by the substantia nigra and that these effects are specifically related to pain.

Brainstem genesis of reserpine-induced ponto-geniculo-occipital waves: an electrophysiological and morphological investigation

Several experimental results indicate that the peribrachial (PB) cholinergic area of the pedunculopontine nucleus is the final relay for the transfer of brainstem-generated pontogeniculo-occipital (PGO) waves to the thalamus. However, the mechanisms underlying the PGO-related activity of PB neurons remain unknown. In order to study these mechanisms, single unit recordings in the PB area were performed in reserpinized cats. Because PGO waves are closely related to rapid eye movements, our microelectrode explorations were also aimed to some structures of the preoculomotor network, namely, the superior colliculus (SC) and parts of the central tegmental field (FTC). We have found several classes of PGO-on cells in the PB area, most of them descharging 80 ms or less before the peak of PGO waves. These cell-classes comprised high-frequency bursting cells, slow-frequency bursting cells, and neurons discharging single spikes or doublets. Intracellular recordings showed that PGO-on single spikes arise from conventional excitatory postsynaptic potentials. Among PGO-related cells in structures outside the PB limits, it was found that most SC cells discharge during or after the PGO, whereas FTC cells increase their discharge rate several hundreds of ms before PGO waves, thus indicating that PGO waves are elaborated long before the activation of PB neurons. Massive retrograde labeling was found in FTC following horseradish peroxidase injections into the PB area. We suggest that long-lead FTC neurons provide an excitatory input to PGO-on PB neurons.

Neurotensin and cholecystokinin microinjected into the ventral tegmental area modulate microdialysate concentrations of dopamine and metabolites in the posterior nucleus accumbens

Neurotensin and cholecystokinin, neuropeptides which coexist with dopamine in many ventral tegmental neurons, were microinjected into the ventral tegmental area during in vivo microdialysis in the posterior nucleus accumbens. Neurotensin significantly elevated concentrations of dopamine and its metabolites at doses of 10 pmol, 1 nmol, and 10 nmol, while cholecystokinin significantly elevated dopamine metabolite concentrations only at a dose of 10 nmol. These data suggest that neurotensin potently mediates the release of dopamine from the mesolimbic pathway via direct actions on the cell body.

Facilitation of sexual receptivity in hamsters by simultaneous progesterone implants into the VMH and ventral mesencephalon

Intracranial implantation experiments have shown that the ventromedial hypothalamus (VMH) is the most sensitive site for the facilitation of female sexual behavior by progesterone in estrogen-primed rats. However, similar implantation techniques have been much less successful in hamsters. Several lines of evidence indicate that both hypothalamic and midbrain structures are important for hamster lordosis. Therefore we compared the effect of progesterone (P) implants administered simultaneously to VMH and ventral midbrain on opposite sides of the brain to the effects of bilateral implants to each of these sites separately. Ovariectomized female hamsters were stereotaxically implanted with 24-gauge thin-wall guide tubes according to one of five patterns. Bilaterally symmetrical cannulae were aimed at VMH or ventral mesencephalon (vMES) or asymmetrical implants were aimed at one of the following pairs of sites, on opposite sides of the brain: VMH-vMES, VMH-preoptic area (VMH-POA), or anterior hypothalamus-anterior mesencephalon (AH-aMES). After recovery from surgery, females were primed with 10 micrograms estradiol benzoate and given pellets of P or cholesterol through a 30-gauge injector in the targeted sites. Latency, frequency, and duration of lordosis were recorded in 10-min tests with sexually active male hamsters. Sexual receptivity was significantly facilitated by simultaneous contralateral P implants into the VMH-vMES. P implants in any other combination of sites did not significantly facilitate lordosis compared to cholesterol control implants, nor did bilateral administration of this dose of P in either VMH or vMES have a reliable effect. The results support the hypothesis that P action is required in both VMH and vMES to reliably stimulate receptivity in hamsters.