Tubero-infundibular dopamine neurons and the secretion of luteinizing hormone and prolactin: extrahypothalamic influences, interaction with cholinergic systems and the effect of urethane anesthesia

Combined in situ hybridization and immunocytochemistry in the assay of pharmacological effects on tyrosine hydroxylase mRNA concentration

An assay for tyrosine hydroxylase (TH) mRNA by in situ hybridization in combination with immunocytochemistry (ICC) for TH on the same section is described. The in situ hybridization protocol was optimized for [35S]cRNA (complementary RNA, i.e. anti-sense strand) probe concentration and time of hybridization. The specificity of hybridization was measured by several critera. The advantage of measuring grain density versus grains per cell is discussed for quantitation of in situ autoradiography. Finally, the reserpine-induced increase in adrenal TH mRNA was used to validate quantitative aspects of the in situ hybridization technique by comparison with blot hybridization. In contrast to the adrenal, reserpine did not increase TH mRNA in substantia nigra (s. nigra) neurons as measured by either technique.

Excitatory influence of the accessory olfactory bulb on tuberoinfundibular arcuate neurons of female mice and its modulation by oestrogen

The role of the accessory olfactory bulb in conveying pheromonal information to tuberoinfundibular arcuate neurons was examined electrophysiologically in chloral hydrate-anaesthetized, oestrogen (0.5 micrograms in silastic capsules)-treated and untreated ovariectomized Balb/c female mice. Electrical stimulation of the accessory olfactory bulb orthodromically excited part of tuberoinfundibular neurons which were antidromically stimulated from the median eminence and histologically verified as being located within the arcuate nucleus. No inhibitions followed accessory bulb stimulation. The excitatory response to accessory bulb stimulation was reversibly blocked by the local anaesthetic lignocaine infused into the amygdala. The percentage of tuberoinfundibular arcuate neurons responding to accessory bulb stimulation was significantly higher in oestrogen-treated than in untreated animals. There was no difference between the two groups for the antidromic activation threshold, spontaneous firing rate, absolute refractory period or frequency of successful antidromic propagation into the soma of tuberoinfundibular arcuate neurons. In oestrogen-treated preparations, tuberoinfundibular arcuate neurons responsive and unresponsive to accessory bulb stimulation could be distinguished by the frequency of successful antidromic propagation into the soma. These studies demonstrate that olfactory relay neurons in the accessory olfactory bulb act to enhance the activity of a subpopulation of tuberoinfundibular arcuate neurons via the amygdala and that this neural transmission is modulated by oestrogen.

Mode of action of chlorpromazine (CPZ) blockage on 13,14-dihydroprostaglandin F2-alpha formation in rat ovary

The mode of action of the inhibitory effect of CPZ on 13,14-dihydroprostaglandin F2-alpha (13,14H2-PGF2-alpha) formation in rat ovary was examined. The inhibition of 13,14H2-PGF2-alpha formation and of ovulation induced by a proestrus were completely recovered by an injection of hCG (25 IU/rat) or LH-RH (500 ng/rat) at 15:00 on the same day. 13,14H2-PGF2-alpha formation and ovulation were not inhibited by a single injection of prolactin (PRL:6 IU/rat) at 13:00 on the day of proestrus. Repeated injection of PRL inhibited cyclic ovulation and 13,14H2-PGF2-alpha formation. The estrus cycle of PRL treated animals showed a continuous state of diestrus. Although 13,14H2-PGF2-alpha formation and ovulation were inhibited by the repeated injection of CPZ, the repeated-simultaneous injection of CPZ and bromocriptine at 10:00 once a day for 3 days from the first day of diestrus partly restored both and entirely reversed the suppression of the cyclic-changes in the in the vaginal smear pattern. These results indicate that the inhibition of 13,14H2-PGF2-alpha formation induced by a single injection of CPZ probably occurs via the suppression of LH-RH release from the hypothalamus, whereas PRL secretion may participate in the inhibitory effects of repeated injections of CPZ.

Anatomical evidence for interactions between catecholamine- and adrenocorticotropin-containing neurons

Recent studies suggest that neurons containing adrenocorticotropin and catecholamines are localized to similar areas of the brain. In this immunocytochemical study, the distributions of neurons and terminals containing adrenocorticotropin and tyrosine hydroxylase, the first enzyme in the catecholamine biosynthetic pathway, were compared using the peroxidase-antiperoxidase technique. Neurons containing adrenocorticotropin and tyrosine hydroxylase formed overlapping hyperbolic lamina in the mediobasal hypothalamus. Although adrenocorticotropin and tyrosine hydroxylase containing neurons often formed small clusters, no double labeled cells were observed. Overlap also occurred between adrenocorticotropin and tyrosine hydroxylase terminal fields in several diencephalic nuclei including the periventricular hypothalamic gray and paraventricular thalamus. In contrast, other regions displayed striking compartmentalization of terminal fields; for example, in both the paraventricular hypothalamus and central nucleus of the amygdala, adrenocorticotropin was located in ventral and tyrosine hydroxylase in more dorsal aspects of the nuclei. Adjacent sections also showed a close correspondence between adrenocorticotropin terminals and tyrosine hydroxylase cell bodies in paraventricular, periventricular, dorsomedial and ventral hypothalamic nuclei. These data provide anatomical substrates for potential functional interactions between catecholamine and adrenocorticotropin systems in forebrain.

Distribution of catecholamine neurons in the hypothalamus and preoptic region of mouse

The distribution and morphology of cells containing tyrosine hydroxylase (TH) were mapped by using the immunoperoxidase technique in the hypothalamus and preoptic area in two strains of mouse, CBA/J and BALB/cJ. On the basis of rostral-caudal contiguities between cell aggregates, hypothalamic preoptic neurons were subdivided into three arbitrary groups: (1) dorsal, (2) intermediate, and (3) ventral. New or more prominent collections of TH cells were observed, and in some regions, cells were more complexly organized than originally described. In the dorsal group, a rostral collection of small ovoid cells, previously not described, were located in the anterior preoptic nucleus (APN) of Loo ('31) and extended rostrally and ventrally into the preoptic periventricular gray. The next constituent occupied the paraventricular nucleus (PVN), and was composed of two classes of cells: (1) a small ovoid cell within anterior and medial parvocellular PVN in contiguity rostrally with a similar cell in APN and (2) a larger, angular cell within and adjacent to the lateral PVN in contiguity caudally with cells in the zona incerta (ZI). Further caudally, a larger and more pleomorphic collection of TH neurons was localized in the medial ZI, particularly at midtuberal levels. These cells were not scattered, as previously reported, but were differentiated into two clear-cut densities, a larger medial island and a more elongated lateral island. Cells of ZI, both large and small, extended caudally into the dorsal hypothalamic and subparafascicular nuclei and periventricular gray. In contrast to previous descriptions, no cells were seen in the nucleus reuniens. In the intermediate group, the most rostral constituent occupied the preoptic periventricular gray, extended as far as the lamina terminalis, and merged dorsocaudally with cells in APN. While the variably shaped cells of the hypothalamic periventricular gray (PVG) were still present in the retrochiasmatic region, a striking absence of these cells was noted at midtuberal levels between the dorsomedial and arcuate hypothalamic nuclei. At this level, a new group of small-round TH cells, resembling those of the arcuate nucleus, was identified in the dorsomedial hypothalamic nucleus (DMN). At caudal tuberal levels, similar neurons were found in the posterior hypothalamic nucleus (PH). These neurons overflowed medially into the PVG and caudoventrally into the arcuate nucleus. In the ventral group, the most rostral constituent, composed of both small and ovoid cells in the retrochiasmatic area, appeared to represent the rostral commissural portion of the arcuate nucleus (Arc).(ABSTRACT TRUNCATED AT 400 WORDS)

Possible involvement of serotonergic neurons in the reduction of locomotor hyperactivity caused by amphetamine in neonatal rats depleted of brain dopamine

This experiment attempted to determine the mechanism by which amphetamine reduces locomotor hyperactivity in neonatal rats given brain dopamine (DA)-depleting 6-hydroxydopamine (6-OHDA) injections. Brain DA neurons were destroyed selectively in neonatal rats by intraventricular (i.v.t.) injections of 6-OHDA following desmethylimipramine (DMI) pretreatment. Control rats received DMI and i.v.t. injections of the 6-OHDA vehicle solution. Rats given the 6-OHDA treatment displayed 7-fold increases in locomotor activity compared to controls during days 16-55 of life. Throughout this period, amphetamine (1 mg/kg) reduced locomotor hyperactivity in 6-OHDA-treated rats but increased locomotor activity in control rats. The reduction of hyperactivity caused by amphetamine (0.5-4 mg/kg) was dose-related and was not accompanied by stereotyped behavior. Like amphetamine, methylphenidate (4 mg/kg) reduced locomotor hyperactivity in rats given 6-OHDA. The DA antagonist, spiroperidol (50-200 micrograms/kg) failed to attenuate the hyperactivity-reducing effect of amphetamine in 6-OHDA-treated rats at doses which abolished the stimulant effect of amphetamine in control rats. However, the serotonin antagonist methysergide (0.5-4 mg/kg) produced dose-dependent antagonism of the effect of amphetamine in 6-OHDA-treated rats. Pretreatment with propranolol ((5 mg/kg), phentolamine (5 mg/kg), atropine (0.5 mg/kg) or naloxone (10 mg/kg) failed to alter the reduction in locomotor hyperactivity caused by amphetamine. The serotonin releasing agent, fenfluramine (3 mg/kg), and the serotonin agonist, quipazine (0.5-4 mg/kg), both reduced locomotor hyperactivity in 6-OHDA-treated rats while not altering locomotion in control rats. These results confirm previous observations that amphetamine reduces locomotor hyperactivity caused by neonatal 6-OHDA administration and suggest that this effect is mediated by increased serotonergic neurotransmission.

The effect of benztropine mesylate on the prolactin response to haloperidol

The interaction between cholinergic and dopaminergic systems in regulating prolactin responses was investigated in normal men by administration of haloperidol (0.5 mg IM), with and without benztropine (1.0 and 2.0 mg IM). The acute prolactin response (at 30, 45, and 60 min) to haloperidol plus benztropine was significantly delayed compared to the response to haloperidol alone, in the same subjects. However, between 90 and 120 min after drug administration, prolactin responses achieved were similar in the three drug conditions. The results suggest a modest cholinergic-dopaminergic antagonism in the tuberoinfundibular system, apparent at low doses of neuroleptic and high doses of anticholinergic drugs.

Evidence for a cholinergic influence on catecholaminergic pathways terminating in the anterior and medial basal hypothalamus

Adult male Sprague-Dawley rats were given hourly injections of physostigmine for 1--4 h, and the effect of this treatment on dopamine (DA) and noradrenaline (NA) content or on DA and NA was estimated by measuring the decline in these amines produced following the inhibition of tyrosine hydroxylase with alpha-methyl-p-tyrosine (alpha MPT). In later experiments oxotremorine was administered instead of physostigmine at hourly intervals for 2 h. Physostigmine administration resulted in a highly significant increase in the depletion of NA produced by alpha MPT indicating that the turnover of NA was increased by this drug. This effect was observed in the medial basal hypothalamus and anterior hypothalamus but not in the telencephalon-thalamus. Oxotremorine also produced an increase in NA turnover, but this drug was effective in all three brain areas. Atropine pretreatment blocked the effect of both physostigmine and oxotremorine on NA turnover. However, in the case of physostigmine, atropine was only effective if it was given 30 min before each injection of physostigmine. Mecamylamine, a nicotine blocker, did not reverse the effect of physostigmine on NA turnover. These results suggest that there is a cholinergic input via muscarinic receptors which influences the activity of noradrenergic pathways terminating in the anterior or medial basal hypothalamus.

Changes in the distribution of non-neuronal elements in rat median eminence and in anterior pituitary hormone secretion after activation of tuberoinfundibular dopamine neurones by brain stimulation or nicotine

In order to investigate the posssibility of acute functional changes in non-neuronal elements (mainly tanycytes) of the median eminence, the proportion of portal capillary surface covered by such elements was measured by quantitative electron microscopy in ovariectomized, oestrogen-progesterone-pretreated rats. In some of these animals, the functional state of the tuberoinfundibular dopamine (DA) neurones was assessed by histochemical microfluorimetry. Serum concentrations of luteinizing hormone (LH), growth hormone (GH) and prolactin were determined by radioimmunoassay. Two different types of treatment, i.e. systemic administration of nicotine (1 mg/kg, s.c.) or electrical stimulation in the medial amygdaloid nucleus, markedly reduced the percentage of capillary surface covered by non-neuronal profiles within 20 and 15 min, respectively. At the same time, the tuberoinfundibular DA system responded by an increase in cellular fluorescence intensity, reflecting neuronal activation. Medial preoptic stimulation had basically the same effect but with more variability in the change in capillary coverage by tanycytes. The action of nicotine was prevented by pretreatment with the DA receptor blocking agent, pimozide (5 mg/kg), which indicates (1) that a dopaminergic mechanism was involved in the nicotine effect and (2) that the tanycyte response was elicited by DA released from nerve terminals acting at some receptor site. Nicotine also lowered serum levels of GH and prolactin. Pimozide antagonized only the effect on prolactin. While the reaction of DA neurones and capillary coverage by tanycytes were correlated with each other in individual rats, no statistically significant correlation was observed between tanycyte response and hormone levels, so that no conclusions can as yet be drawn as to the neuroendocrine significance of the tanycyte reaction. These results indicate that rapid changes in the proportion or portal capillary surface covered by non-neuronal profiles can be elicited by stimulation of extrahypothalamic brain areas or by activation of cholinergic mechanisms. The tanycyte response appears to be mediated at least in part by the tuberoinfundibular DA neurones.

The influence of the amygdala on the basal septum and preoptic area of the rat

Extracellular microelectrode recordings were made from single neurones in the medial preoptic area and basal septum of normal female rats anaesthetised with urethane and the effects of stimulation of the ipsilateral amygdala (AMY) was tested. 1. Quiescent cells were excited with latencies between 4 and greater than 40 msec with the majority being between 10 and 27 msec. Variable latency, changing latency and recruitment were observed and suggest that many pathways are oligo- or polysynaptic. 2. A minority of spontaneously active units (20.6%) were excited with latencies between 5 and 36 msec. The predominant initial response was inhibition (67.1%). The remaining 12.3% of neurones did not respond. 3. A predominantly positive complex field potential was associated with excitation, particularly of quiescent neurones. 4. Inhibitory responses were classified according to duration (short: 20-60 msec; long: 100-400 msec). All responses contained an inhibitory component. Responses were further described on the basis of presence and rhythmicity of post-inhibitory excitation. 5. The majority of action potentials evoked from quiescent neurones were of small amplitude (80-150 mu/V) and occurred in small groups. These characteristics and their occurrence at the onset of inhibition are consistent with the view that all or a proportion of them may be generated by inhibitory interneurones. 6. Only minor differences were observed between response characteristics evoked by stimulation of the corticomedial or basolateral AMY indicating that differential effects of these regions on adenohypophysial hormone secretion are determined at the septopreoptic level.

Catecholamine turnover alterations in discrete areas of the median eminence of the 4- and 5-day cyclic rat

Using quantitative microfluorimetry in combination with tyrosine hydroxylase inhibition (H44/68) the concentration and turnover of noradrenaline (NA) and dopamine (DA) was studied in the subependymal layer (SEL) and the medial (MPZ) and lateral palisade zone (LPZ) of the rat median eminence during the 4- and 5- day vaginal estrous cycle. Significant cyclic variations were only found in SEL and LPZ. The NA turnover in SEL was high on proestrous and low on all other days of the 4-day estrous cycle, whereas in the 5-day estrous cycle the NA turnover in SEL started to increase already on the second day of diestrous to reach a peak in the afternoon of proestrous. At that time also the NA concentrations in SEL were increased, although significantly only in the 5-day cyclic rats. The DA turnover in LPZ was low on proestrous and high on all other days in both 4-and 5-day cyclic rats. Apart from the median eminence cyclic variations in catecholamine metabolism were only found in the medial preoptic area, where NA turnover was high on proestrous and low on estrous-diestrous. The present findings give further support for the existence of a facilitatory noradrenergic and inhibitory dopaminergic mechanism in the control of gonadotrophin release. Furthermore, it is suggested that an acceleration of reticulo-hypothalamic NA turnover precedes the retardation of tubero-infundibular DA turnover found on proestrous and that the time lag between initial NA activation and subsequent DA inactivation is longer in the 5-than in the 4-day estrous cycle.

A quantitative correlation between single unit activity and fluorescence intensity of dopamine neurones in zona compacta of substantia nigra, as demonstrated under the influence of nicotine and physostigmine

In order to investigate the possible relationship between neuronal activity and cellular fluorescence intensity, extracellular recordings of single unit activity and determinations of fluorescence intensity of dopamine (DA) neurones by histochemical microfluorimetry were performed in the same (rostral) part of zona compacta of substantia nigra in male rats. In urethane anaesthesia, zona compacta neurones characteristically showed a slow and fairly regular type of firing. Nicotine (1 mg/kg s.c.) induced a transient decrease in unit activity for 1 min followed by a sustained increase in firing rate. During that stage, 4-5 neurones/rat were recorded at different anteroposterior levels, each during 200 sec. Microfluorimetric examination of the fluorescence intensity developed at the end of the 30-min observation period by the DA neurones of the same area revealed a marked rise in cellular fluorescence intensity. Similar results were obtained with a lower dose of nicotine and/or a shorter observation period. Additional microiontophoretic experiments supported the view that extracellular recordings of the correlative electrophysiological-microfluorimetric investigation belonged to DA neurones. Release of DA from terminals was indicated by an increase in HVA concentration of caudate-putamen in rats subjected to the same nicotine treatment. When tested on one cell during a prolonged period of time, physostigmine (0.25 mg/kg i.p.) caused an initial increase in firing rate of zona compacta neurones (5-10 min) followed by a decrease of unit activity (15-23 min). In agreement with previous observations in mice, fluorescence intensity of nigral DA neurones likewise showed a biphasic change with an initial rise and subsequent decrease (examined at 9.5 and 22-23.5 min, respectively). When mean unit activity and mean fluorescence intensity of individual rats out of various experimental groups were related to each other, a highly significant positive correlation between neuronal fluorescence intensity and firing rate was found. The results obtained with physostigmine demonstrate that mean intensity closely paralleled mean unit activity in time, so that this correlation was maintained. These findings indicate that cellular fluorescence intensity of DA neurone groups can be used as an index of the level of neuronal activity, except for cases where a drug treatment interferes directly with catecholamine synthesis or storage mechanisms.

Inhibition and facilitation of antidromically identified tubero-infundibular neurones following stimulation of the median eminence in the rat

1. Stimulation of the median eminence of female rats inhibited the spontaneous firing of antidromically identified tubero-infundibular units. Some units could be inhibited by stimuli subthreshold for the antidromic spike. A conditioning stimulus of subthreshold intensity for the antidromic spike also delayed or abolished the invasion of the soma and dendrites of the same unit by antidromic action potentials evoked by a suprathreshold stimulus given 2-10 msec later. 2. Although strychnine (0-2 mg/kg, i.v.) did not significantly alter the inhibition evoked by stimulation of the median eminence, it was abolished by picrotoxin (2-6 mg/kg, iv.). 3. In seven of the fifty-seven identified units examined stimulation of the median eminence facilitated the spontaneous firing. However, after an i.v. injection of picrotoxin the facilitory response was observed in thirty-seven of the forty-six units tested. Post-stimulus time histograms obtained from the same unit before and after an injection of picrotoxin demonstrated that the latency and duration of the facilitation did not always coincide with that of the inhibition. 4. After an injection of picrotoxin the field potential evoked by antidromic stimulation of the median eminence consisted of a small positive wave followed by a negative wave. Frequently the negative wave of the field potential was accompanied by a convulsive discharge. The latency of the negative wave appears to be identical to that of the facilitation seen in nearby single units. 5. The facilitation evoked by antidromic stimulation in the presence of picrotoxin was blocked by an i.v. injection of alpha-methyl-p-tyrosine (250 or 375 mg/kg). None of the nine units sampled from rats pre-treated with alpha-methyl-p-tyrosine injected twice I.P. (250 mg/kg for each) were facilitated by stimulation of the median eminence following I.V. picrotoxin, while eight of the eleven units sampled from control rats pre-treated with L-tyrosine could be facilitated by antidromic stimulation. 6. These results suggest that tuber-infundibular neurosecretory neurones are inhibited and facilitated by neural pathways which could involve the axon collaterals of the neruosecretory neurones which project to the external layer of the median eminence. It is also suggested that GABA-releasing neurones mediate the inhibition and catecholaminergic neurones are involved in the facilitory pathways. Presumably the facilitation is normally masked by the activity of the GABA-releasing neurones.

Acetylcholine induced responses of plasma LH and prolactin in normal and 6-hydroxydopamine-treated rats

The effects of intraventricular injection of acetylcholine at doses of 50 mug were studied in male rats. Under ether anesthesia, acetylcholine did not modify plasma prolactin and LH levels. Contrarily, in rats anesthetized with pentobarbital, acetylcholine at the same doses induced significant decreases of prolactin from 10 to 60 min postinjection. LH levels were not modified by treatment. Prolactin responses to acetylcholine were absent during the first 30 min postinjection in 6-hydroxydopamine-treated rats. Only a delayed response was detected. In these animals, icv administration of dopamine caused a clear decrease of plasma prolactin levels. A marked rise of LH at 30 and 60 min following acetylcholine injection was found in 6-hydroxydopamine-treated rats. Results suggest a catecholaminergic mediation of the prolactin and LH responses induced by acetylcholin in male rats.

Role of the blood-brain barrier in the anticholinergic differential effects on LH and prolactin release in proestrous rats

The effects of homatropine and atropine on plasma LH and prolactin (PRL) levels during the afternoon of pro-oestrus were investigated. Homatropine methylbromide was unable to block the spontaneous increase of LH but prevented the prolactin surge in pro-oestrous rats at doses of 450 and 700 mg/kg s.c. Atropine sulphate blocked both LH and PRL at doses of 450 and 700 mg/kg s.c. These two anticholinergics when injected into the 3rd ventricle (250 mug/rat) blocked the pro-oestrus increase of LH and inhibited PRL. Pharmacological experiments were performed in parallel and the injection of either homatropine or atropine (450 mg/kg s.c.) resulted in the abolishment of peripheral toxic signs induced by pilocarpine (50 mg/kg s.c.) whereas its central toxic signs were prevented by atropine but not by homatropine. Our findings suggest that the difference in action of systemic injections of atropine and homatropine on LH and PRL release is explained by a relatively low permeability of homatropine to the brain. Therefore, cholinergic LH-controlling mechanisms may be situated above the median eminence and those related to PRL placed outside the blood-brain barrier.

The catecholamine-containing tubero-infundibular system and the control of luteinizing hormone release in the rabbit

As determined by fluorescence histochemistry, the distribution of catecholamine-containing neurons in the hypothalamus of the female rabbit is similar to that seen in the rat. The fluorescence appearance of the hypothalamus was not appreciably different from normal at 0.25, 1, 4 or 24 h after mating, but in animals in which the synthesis of catecholamines was inhibited by the administration of the tyrosine hydroxylase inhibitor, H44/68, the number of fluorescent neurons seen in the nucleus periventricularis arcuatus following copulation was markedly reduced. However, the concentrations of luteinizing hormone (LH) in serum and in the pituitary glands of mated and unmated animals treated with H44/68 were not significantly different from those found in the corresponding control animals. This, together with the fact that ovulation followed mating in drug-treated rabbits suggests that a normal level of catecholamines in the tubero-infundibular system is not essential for the secretion of the amount of LH necessary for ovulation.

Response of nigral dopamine neurons to actue and prolonged morphine treatment: effect of exposure to cold, physostigmine and nicotine

The effects of morphine and acute exposure to cold on nigral dopamine (DA) neurons and possible interactions with cholinergic systems were studied by histochemical microfluorimetry in normal and partially morphine-tolerant mice. Morphine (40 mg/kg), cold (4 degrees C), nicotine (1 mg/kg) and physiostigmine (0.25 mg/kg) elicited a rapid rise and subsequent decrease in the fluorescence intensity of DA nerve cells with certain differences in time course. Fastest changes with a peak at 5 min, a marked subsequent drop below control levels and return towards control intensity after 40 min were seen after physostigmine. Antagonisms between various treatments were noted. DA responses correlated well with the time course of behavioural effects, especially after physostigmine. After 3 1/2 days of morphine treatment, the locomotor and analgesic effects of the drug were reduced. At this stage, the initial increase in fluorescence intensity after morphine and the biphasic pattern caused by physostigmine were delayed without any change in response magnitude. Responses to cold and nicotine remained unaltered both in magnitude and time course. Thus, partial tolerance affected the response of nigral DA neurons to some but not all funtional conditions and thereby markedly changed the interaction with cholinergic systems. The difference between physostigmine and nicotine suggests that neuronal circuits including mustcarinic mechanisms are possibly more susceptible to alteration.