Inhibition of the high affinity choline transporter enhances hyperalgesia in a rat model of chronic pancreatitis.

BACKGROUND: The mechanisms underlying chronic and persistent pain associated with chronic pancreatitis (CP) are not completely understood. The cholinergic system is one of the major neural pathways of the pancreas. Meanwhile, this system plays an important role in chronic pain. We hypothesized that the high affinity choline transporter CHT1, which is a main determinant of cholinergic signaling capacity, is involved in regulating pain associated with CP. METHODS: CP was induced by intraductal injection of 2% trinitrobenzene sulfonic acid (TNBS) in Sprague-Dawley rats. Pathological examination was used to evaluate the inflammation of pancreas and hyperalgesia was assessed by measuring the number of withdrawal events evoked by application of the von Frey filaments. CHT1 expression in pancreas-specific dorsal root ganglia (DRGs) was assessed through immunohistochemistry and western blotting. We also intraperitoneally injected the rats with hemicholinium-3 (HC-3, a specific inhibitor of CHT1). Then we observed its effects on the visceral hyperalgesia induced by CP, and on the acetylcholine (ACh) levels in the DRGs through using an acetylcholine/acetylcholinesterase assay kit. RESULTS: Signs of CP were observed 21 days after TNBS injection. Rats subjected to TNBS infusions had increased sensitivity to mechanical stimulation of the abdomen. CHT1-immunoreactive cells were increased in the DRGs from rats with CP compared to naive or sham rats. Western blots indicated that CHT1 expression was significantly up-regulated in TNBS-treated rats when compared to naive or sham-operated rats at all time points following surgery. In the TNBS group, CHT1 expression was higher on day 28 than on day 7 or day 14, but there was no statistical difference in CHT1 expression on day 28 vs. day 21. Treatment with HC-3 (60 µg/kg, 80 µg/kg, or 100 µg/kg) markedly enhanced the mechanical hyperalgesia and reduced ACh levels in a dose-dependent manner in rats with CP. CONCLUSION: We report for the first time that CHT1 may be involved in pain modulation in CP, as it plays an important role in pain inhibition. Increased CHT1 activity or the up-regulation of its expression may be used to treat pain in patients with CP.

Extracts of Cordyceps militaris lower blood glucose via the stimulation of cholinergic activation and insulin secretion in normal rats.

Previous studies have shown that Cordyceps militaris (CM) has a hypoglycemic effect, but the actual mechanism remains unclear. This study explored the hypoglycemic mechanism of aqueous extracts of CM in normal Wistar rats. First, the optimal dose of CM for lowering plasma glucose and insulin secretion was tested. Further, atropine and hemicholinium-3 (HC-3) were injected and a western blot was used to investigate insulin signaling. It was found that 10 mg/kg CM extracts had a stronger hypoglycemic effect than a higher dose (100 mg/kg); therefore, a dose of 10 mg/kg was used in subsequent experiments. In normal rats, CM extracts decreased plasma glucose by 21.0% and induced additional insulin secretion by 54.5% after 30 min. When atropine or HC-3 was injected, CM induced a hypoglycemic effect, but the enhancement of insulin secretion was blocked. By western blotting, significant increases in the insulin receptor substrate 1 (IRS-1) and glucose transporter 4 (GLUT-4) were observed after CM feeding. However, the elevation of these signaling proteins was abolished by atropine or HC-3. Taken together, these findings indicate that CM can lower plasma glucose via the stimulation of insulin secretion and cholinergic activation involved in the hypoglycemic mechanism of normal Wistar rats.

Post-retrieval effects of icv infusions of hemicholinium in mice are dependent on the age of the original memory.

CF-1 male mice were trained in an inhibitory avoidance task using a high footshock (1,2 mA, 50 Hz, 1 sec) in order to reduce the influence of extinction on retention performance. At 2, 7, 14, or 30 d after training, the first retention test was performed and hemicholinium (HC-3, 1.0 microg/mice), a specific inhibitor of high-affinity choline uptake in brain cholinergic neurons, was given intracerebroventricularly immediately after. Twenty four hours after treatment, mice were tested in an inhibitory avoidance task during five consecutive days, each 24 h apart. Retention performance was impaired by HC-3 when the first re-exposure took place at 2, 7, or 14 d, but the effect was no longer seen when re-exposure occurred 30 d after training. We did not find spontaneous recovery 21 d after training, when memory was retrieved 2 d after training and HC-3 was given immediately after. Although we cannot definitively discard a retrieval deficit, this lack of spontaneous recovery is in accordance with the storage-deficit interpretation. These results confirm and extend previous ones, suggesting that central cholinergic mechanisms are involved in the hypothetical reconsolidation memory processes of an inhibitory avoidance task in mice and also suggest that this participation depends on the "age" of the original memory trace. This implies that the vulnerability of a reactivated memory to a specific treatment, as the one used in this study, inversely correlates with the age of the original memory, and it is likely to determine memory reconsolidation processes.

Memory consolidation and reconsolidation of an inhibitory avoidance response in mice: effects of i.c.v. injections of hemicholinium-3.

The immediate post-training i.c.v. administration of hemicholinium-3 (HC-3) (1 microg), a specific inhibitor of the high-affinity choline uptake (HACU) in brain cholinergic neurons, impaired retention test performance of a one-trial step-through inhibitory avoidance response in adult male CF-1 mice. The effect was observed in mice that received a footshock (0.8 mA, 50 Hz, 1 s) on the learning trial, and not only 48 h after training, but also 7 days after it. After the completion of the retention test at each of the training-test interval that were studied, the HACU in the hippocampus of HC-3-treated mice was not significantly different from that of saline-injected (1 microl) control groups. Mice that were over-reinforced (1.2 mA, 50 Hz, 1 s) on the learning trial, exhibited a high retention performance 48 h after training. The immediate i.c.v. injection of HC-3 (1 microg) after the retention test, that is, after memory reactivation, significantly impaired retention performance over 4 consecutive days, whereas the saline-injected control group shown a slight, but significant performance decrease only at the last retention test. Retention performance was unchanged in HC-3-treated mice not undergoing memory reactivation session. These results, taken together, indicate that HC-3, not only impaired consolidation, but also reconsolidation of an inhibitory avoidance task in mice, suggesting a critical participation of central cholinergic mechanisms in both memory processes.

The role of ventrolateral striatal acetylcholine in the production of tacrine-induced jaw movements.

The anticholinesterase tacrine induces tremulous jaw movements in rats, and considerable evidence indicates that this response is dependent upon ventrolateral striatal mechanisms. Three experiments were conducted to study the relation between ventrolateral striatal acetylcholine and the production of tremulous jaw movements. In Experiment 1, intracranial microinjection of the acetylcholine synthesis inhibitor hemicholinium-3 into the ventrolateral neostriatum reduced tremulous jaw movements induced by 5.0 mg/kg tacrine. Microinjection of hemicholinium into a cortical site dorsal to striatum (Experiment 2) was without significant effect upon tacrine-induced tremulous jaw movements. In Experiment 3, rats were implanted with dialysis probes in the ventrolateral striatum to measure extracellular levels of acetylcholine during tacrine-induced jaw movements. Tacrine (2.5-5.0 mg/kg) increased both extracellular acetylcholine and tremulous jaw movements. The 5.0 mg/kg dose of tacrine produced a substantial increase in ventrolateral striatal acetylcholine levels (324% of baseline within 30 min). Across all tacrine-treated rats there was a significant linear correlation between tremulous jaw movements and acetylcholine levels (r = +0.56) during the first 30-min postinjection period. This correlation was largely due to the group that received 5.0 mg/kg tacrine; within this group, there was a very high correlation (r = +0.87) between tremulous jaw movements and acetylcholine levels in the first sample after injection. These data are consistent with the notion that tremulous jaw movements induced by tacrine are mediated by ventrolateral striatal acetylcholine. Moreover, these results suggest that dialysis methods could be used to monitor the relation between striatal acetylcholine and tremulous movements induced by a variety of different conditions.

Choline administration reverses hypotension in spinal cord transected rats: the involvement of vasopressin.

Intracerebroventricular (i.c.v.) choline (50-150 microg) increased blood pressure and decreased heart rate in spinal cord transected, hypotensive rats. Choline administered intraperitoneally (60 mg/kg), also, increased blood pressure, but to a lesser extent. The pressor response to i.c.v. choline was associated with an increase in plasma vasopressin. Mecamylamine pretreatment (50 microg; i.c.v.) blocked the pressor, bradycardic and vasopressin responses to choline (150 microg). Atropine pretreatment (10 microg; i.c.v.) abolished the bradycardia but failed to alter pressor and vasopressin responses. Hemicholinium-3 [HC-3 (20 microg; i.c.v.)] pretreatment attenuated both bradycardia and pressor responses to choline. The vasopressin V1 receptor antagonist, (beta-mercapto-beta,beta-cyclopenta-methylenepropionyl1, O-Me-Tyr2, Arg8)-vasopressin (10 microg/kg) administered intravenously 5 min after choline abolished the pressor response and attenuated the bradycardia-induced by choline. These data show that choline restores hypotension effectively by activating central nicotinic receptors via presynaptic mechanisms, in spinal shock. Choline-induced bradycardia is mediated by central nicotinic and muscarinic receptors. Increase in plasma vasopressin is involved in cardiovascular effects of choline.

Interaction of chlorphenvinphos with cholinergic receptors in the rabbit hypothalamus.

The purpose of this study was to find out whether chlorphenvinphos (CVP), an organophosphorous pesticide, interacts with the muscarinic cholinergic receptors in CNS. To attain this goal, the effects of intrahypothalamic injections of oxotremorine (Ox), a muscarinic agonist, and physostigmine (Phys), a carbamate anticholinesterase, were compared with those produced by intrahypothalamic injections of CVP in the rabbit. It was found that the infusion of Ox (20 micrograms) as well as Phys (200 micrograms) into the anterior hypothalamus leads to an increase in the 4-7 Hz theta rhythm in the hippocampus and to the appearance of behavioral symptoms suggestive of a threat response. In the case of Ox, the effects could be prevented by injections of 20 micrograms scopolamine, a muscarinic antagonist. Pretreatment of the hypothalamus with 100 micrograms hemicholinium (HC-3) did not prevent the effects of Phys injected 2 h later. (HC-3 prevents the resynthesis of acetylcholine by blocking choline reuptake. This leads to a gradual depletion of ACh stores and to an inhibition of the cholinergic transmission). It suggests that Phys activates directly postsynaptic muscarinic receptors. Intrahypothalamic injections of CVP in doses of up to 1360 micrograms produced no overt changes in behavior nor in the hippocampal EEG of the rabbit and did not prevent the effect of subsequent injections of Ox. This suggests that CVP is neither an agonist nor antagonist of the muscarinic receptors in the rabbit hypothalamus.

The role of alpha-adrenergic mechanisms within the area postrema in dopamine-induced emesis.

Intracerebroventricular injection of dopamine (0.5-4.0 mg) produced dose-dependent and short-lasting emesis (1-8 min) in cats, which was abolished after ablation of the area postrema. Relatively selective alpha 2-adrenoceptor antagonists (yohimbine and idazoxan) and a mixed alpha 1- and alpha 2-adrenoceptor antagonist (tolazoline), but not a non-selective alpha 1-adrenoceptor antagonist (prazosin), injected intracerebroventricularly inhibited the emesis induced by intracerebroventricular dopamine. However, dopamine receptor antagonists (chlorpromazine, droperidol, spiperone, domperidone, triflupromazine, sulpiride and metoclopramide), an antimuscarinic drug (atropine), a ganglionic blocking agent (mecamylamine), an opioid receptor antagonist (naloxone) and a 5-HT receptor antagonist (methysergide), all injected intracerebroventricularly, had no significant effect on emesis evoked by intracerebroventricular dopamine. The emetic response to intracerebroventricular dopamine was attenuated in cats pretreated with intracerebroventricular reserpine, 6-hydroxydopamine, alpha-methyl-p-tyrosine and hemicholinium-3. It is postulated that dopamine-induced emesis is mediated through the release of noradrenaline acting at alpha 2-adrenoceptors and that it depends on the integrity of monoaminergic and possibly cholinergic structures within the area postrema. It appears, therefore, that the emetic effect of intracerebroventricular dopamine is mediated by adrenergic rather than dopaminergic mechanisms in the area postrema, at least in the cat.

Effect of hemicholinium-3 on the hypothalamic concentration of a cytochemically detectable glucose-6-phosphate dehydrogenase-stimulating substance.

Hypothalamus and plasma of salt-loaded rats, spontaneously hypertensive rats (SHR), and hypertensive reduced renal mass rats (RRM), and the plasma of patients with essential hypertension and of Milan hypertensive rats contain an increased concentration of a cytochemically detectable glucose-6-phosphate dehydrogenase (G6PD)-stimulating substance that has properties similar to that of a possible choline derivative di-methyl methylene immonium ion. Intracerebroventricular (i.c.v.) administration of hemicholinium-3 (HC-3) selectively blocks high-affinity neuronal choline uptake, inhibits brain acetylcholine (ACh) synthesis, and decreases arterial pressure in SHR through an inhibiting effect on hypothalamic cholinergic function. The experiments were performed to study the effect of centrally administered HC-3 on the content of the cytochemically detectable cholinelike substance in hypothalamus and plasma of SHR. HC-3 or saline was infused into the lateral cerebral ventricle for 6 days with a minipump in 14 SHR. On day 7, the hypothalamic and plasma concentration of the cytochemically detectable substance was significantly reduced in rats that received HC-3. The hypothalamic concentration was 225 +/- 95.6 x 10(8) G6PD U per hypothalamus (range 38.2-775) in SHR that received saline and 1.037 +/- 0.45 x 10(8) G6PD U (range 0.112-3.61) (p < 0.05) in SHR that received HC-3. The respective plasma concentrations were 284.9 +/- 26 U/ml (range 192-374) and 72.7 +/- 14.7 U/ml (range 24-119) (p < 0.05). The findings are consistent with the physicochemical evidence, which suggests that the cytochemically detectable substance is a choline derivative.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of the hypertensive response to central injection of nicotine in conscious rats.

The purpose of this study was to examine the effects of nicotine administered directly into the CNS on mean arterial pressure (MAP) and heart rate to avoid the direct peripheral action of the drug. Also, because nicotine has been reported to enhance the release of endogenous brain acetylcholine, we sought to determine the role of this mechanism in mediating the cardiovascular response. Normotensive Wistar rats were previously implanted with indwelling intracerebroventricular (ICV) cannula guides and an arterial line (iliac artery) for central injection of drugs and measurement of MAP and heart rate, respectively. Rats received a series of increasing doses of nicotine (or saline vehicle) from 2-100 micrograms (in a 10 microliter volume) with each dose separated by at least 1 day. MAP increased immediately following all doses of nicotine; however, the maximal response was obtained following the 50 micrograms dose (higher doses actually produced lower responses). In general, the hypertensive response began immediately after injection, peaked within 2-3 min and returned to baseline within about 20 min. Heart rate changes were often not dramatic and highly variable. In order to examine the dependence of the pressor response to nicotine on brain acetylcholine, rats were pretreated with 20 micrograms (ICV) of hemicholinium-3 (HC-3) 1 h prior to nicotine to deplete endogenous acetylcholine. HC-3 pretreatment resulted in a significant reduction in the magnitude and duration of the pressor response to nicotine. Likewise, pretreatment with atropine inhibited the pressor response to subsequent injection of nicotine. Nicotine enhanced the release of [3H]acetylcholine from brain slices in vitro at concentrations likely achieved in the in vivo studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Attentional functions of the forebrain cholinergic systems: effects of intraventricular hemicholinium, physostigmine, basal forebrain lesions and intracortical grafts on a multiple-choice serial reaction time task.

Degeneration of the cholinergic magnocellular neurons in the basal forebrain and their cortical projections is a major feature of the neuropathology of Alzheimer's disease. In the present study, two experiments examined the disruptive effects on visual attentional performance of two different manipulations that reduce central cholinergic function. In Expt. I, pharmacological manipulation of the cholinergic system was investigated using icv administration of hemicholinium (HC-3), a high affinity choline uptake blocker, administered either alone or in conjunction with the anticholinesterase, physostigmine. The results revealed impairments in the ability of the rats to localize brief visual targets in a serial reaction time task, as shown in particular by a reduction in choice accuracy and lengthening of the latency to respond correctly to the visual stimulus. Cholinergic specificity was supported by the reversal of these behavioural impairments by pre-treatment with the anticholinesterase, physostigmine. In Expt. II, quisqualate-induced lesions of the basal forebrain produced behavioural deficits at 3 weeks post-lesion surgery similar to those observed following icv infusion of HC-3. In an attempt to restore the extrinsic cortical cholinergic innervation by reinnervation of the deafferented cortex, embryonic basal forebrain cholinergic cells were transplanted into the cortex of lesioned animals. After three months recovery, impairments in performance on the baseline schedule of the task were no longer apparent in lesioned animals. However, behavioural deficits, observed predominantly as a lengthening of correct response latency, could be reinstated in the lesioned animals by interpolation of distracting bursts of white noise during each trial, and this deficit was ameliorated by the cholinergic grafts. Furthermore, a non-specific effect of both cholinergic and non-cholinergic grafts in controlling the increase in perseverative time-out responses which occurred as a result of the basal forebrain lesion was consistently observed. These results suggest that cholinergic dysfunction can produce deficits in visual attention which can be ameliorated by cholinergic treatments such as physostigmine or cholinergic-rich cortical grafts. These data provide support for a role for the basal forebrain-neocortical cholinergic projection in attentional function.

Changes in urine volume and urinary electrolyte excretion after intracerebroventricular injection of arecoline and hemicholinium-3.

Activation of cholinergic neurons in specific brain regions evokes a hypernatriuretic response, which appears to be atropine-sensitive and, perhaps, independent from the renal innervation. However the role of cholinergic neurons in central control of renal function is not well understood. The purpose of this study was to further investigate whether brain acetylcholine stores are able to influence kaliuresis and natriuresis in conscious rats. Therefore, the renal response to cholinergic drugs was examined in Wistar rats which underwent to a 0.15 M NaCl solution (saline) load administered by gavage. Central injection of arecoline, a muscarinic agonist, produced a dose-dependent reduction in water diuresis and a highly significant increase in sodium excretion within two hours from the oral saline load. An intracerebroventricular (ICV) injection of methylatropine completely blocked both the antidiuretic and the natriuretic response induced by arecoline. Hemicholinium-3 (HC), centrally administered at a dose (34.8 nmol) known to be capable of inducing a maximal depletion of brain acetylcholine, elicited a time-dependent antidiuretic effect accompanied by a highly significant reduction in potassium and sodium urinary excretion. Therefore, we suggest that brain cholinergic neurons are involved in the regulation of the electrolyte balance.

Antidepressant-like action of nicardipine, verapamil and hemicholinium-3 injected into the anterior hypothalamus in the rat forced swim test.

Male Wistar rats, chronically implanted with cannulas into the anterior hypothalamus, were acutely injected with the calcium channel inhibitors, diltiazem, nicardipine and verapamil, or the choline uptake blocker hemicholinium-3 and tested in the forced swim test. Hemicholinium-3, nicardipine and verapamil markedly increased the duration of active swimming. This antidepressant-like effect did not appear to reflect merely a hyperactive state as the drug-treated rats did not differ from vehicle-injected controls in their open field motility scores. Diltiazem failed to influence rats' performance in either test. Since nicardipine and verapamil, but not diltiazem, share choline uptake property with hemicholinium-3, it seems that this action plays a role in the antidepressant-like effect of all three drugs in the forced swim test.

Intracerebroventricular injection of hemicholinium-3 prevents the ACTH-induced, but not the physostigmine-induced, reversal of hemorrhagic shock in rats.

In rats bled to hypovolemic shock, the intracerebroventricular injection of hemicholinium-3 (20 micrograms/rat) completely prevented the shock reversal induced by the intravenous injection of ACTH (1-24) (160 micrograms/kg), but had no influence on the shock reversal induced by the intravenous injection of physostigmine (70 micrograms/kg). These data indicate that brain cholinergic neurons are involved in the anti-shock effect of ACTH-peptides, but not in that of centrally acting cholinergic drugs.

Adaptation of the adrenocorticotropic response to chronic intermittent stress was altered by intracerebroventricular infusion of hemicholinium-3.

The role of diencephalic cholinergic neurotransmission in regulating hypothalamic-pituitary-adrenocortical (HPA) axis was investigated by means of administration of hemicholinium-3 (HC-3), a blocker of acetylcholine synthesis, in the third ventricle of hemispherectomized pigeons. Except for an early increase in ACTH and corticosterone levels following injection as a bolus of HC-3 that was ascribed to some stress-like situation, all data indicated that hypothalamic acetylcholine depletion resulted in inhibiting effects on the HPA axis. Twenty-four hours after injection of 6 micrograms of HC-3, the response to acute stress was markedly reduced in both magnitude and duration. Permanent instillation of HC-3 in the third ventricle at the rate of 0.25 microgram/h for 9 days led to lowered basal resting HPA activity and severely affected the development of adaptation to chronic intermittent stress. The anticipatory conditioned, endocrine response did not appear whereas attenuation of the poststress component was amplified. It is suggested that cholinergic mechanisms are involved in modulating the HPA function and particularly the conditioning process that takes place in the course of adaptation of the HPA response to chronic intermittent application of the same stressor.

Acetylcholine in the posterior hypothalamic nucleus is involved in the elevated blood pressure in the spontaneously hypertensive rat.

Intravenous injection of physostigmine, 40 and 80 ug/kg, in unanesthetized normotensive rats increased systolic blood pressure (SBP) by 21 +/- 3 and 42 +/- 7 mmHg. This pressor response was 80% inhibited by intracerebroventricular (icv) injection of hemicholinium-3 (HC-3), 20 ug. Simultaneous icv injection of HC-3 and choline (365 ug) prevented the inhibition of the pressor response by HC-3. In spontaneously hypertensive rats, injection of HC-3 either icv (20 ug) or bilaterally into the posterior hypothalamic nuclei (1 ug) decreased SBP by about 40 mmHg. The effect of intrahypothalamic HC-3 was completely blocked by simultaneous injection of choline (24.3 ug) into the same site. The hypotensive effect of icv HC-3 was completely blocked by icv choline (243 ug) and was inhibited up to 60% by injections of choline (24.3 ug) into the posterior hypothalamic nuclei.

Hypertension following intrathecal injection of cholinergic agonists in conscious rats: role of endogenous acetylcholine.

The spinal cord is capable of initiating a significant and long-lasting pressor response following intrathecal injection of cholinergic agonists in freely moving rats. The magnitude of the pressor response to the cholinesterase inhibitor, neostigmine, was greatest when the site of injection was restricted to the thoracic level. Intrathecal (i.t.) injection of neostigmine (1-10 micrograms) elicited a dose-related increase in mean arterial pressure of up to 45 mm Hg which remained elevated for almost 2 h. Significant inhibition of acetylcholinesterase was localized to the spinal cord, with the thoracic region exhibiting the greatest degree of inhibition. Also, depletion of spinal acetylcholine levels following i.t. injection of hemicholinium-3 (HC-3) resulted in a significant reduction in the magnitude of the neostigmine-induced pressor response. Carbachol, a direct-acting cholinergic receptor agonist also increased mean arterial pressure following i.t. injection. However, the pressor response to carbachol was not reduced following HC-3. For both agonists, cardiovascular changes were accompanied by significant behavioral changes characterized by tremor, scratching, tail biting and chewing. The appearances of these behaviors following neostigmine injection were reduced in frequency and intensity in HC-3-pretreated animals. These findings demonstrate the ability of spinal cholinergic neurons to mediate a significant hypertensive response. The presence of marked behavioral changes accompanying the cardiovascular response suggests the possibility that cholinergic neurons may be part of an ascending spinal system.

Suppression of hypertension during chronic reduction of brain acetylcholine in spontaneously hypertensive rats.

Experiments were conducted to determine the effects of chronic depletion of brain acetylcholine (ACh) on the development and maintenance of hypertension in spontaneously hypertensive rats (SHR). Synthesis of brain ACh was inhibited by chronic infusion of hemicholinium-3 (HC-3) into the cerebral ventricles, and systolic blood pressure was monitored by tail cuff occlusion. In 5-week-old SHR, infusion of HC-3 (0.25 micrograms/h) suppressed development of hypertension when compared to saline-infused control SHR during the 21 days of infusion (140 versus 190 mmHg on day 21). Hypothalamic and brain-stem ACh during this period was reduced by 50% and by 60-75%, respectively. In 18-week-old SHR with established hypertension, HC-3 (0.25 and 0.5 micrograms/h) reduced systolic blood pressure by 35-40 mmHg for 8 days, after which pressures returned to control hypertensive levels (191 mmHg) by day 14. The increase in blood pressure was accompanied by recovery of hypothalamic ACh levels to 75% of control. The specificity and physiological effectiveness of HC-3 was shown by its ability to inhibit the centrally mediated pressor response to physostigmine but not to oxotremorine. Infusion of HC-3 did not affect body growth, water consumption, body temperature or gross behavior. From this study, it can be concluded that brain cholinergic neurons are an important component in the development and the maintenance of hypertension in the SHR.

Increased central cholinergic activity in rat models of hypertension.

The role of brain acetylcholine (ACh) in the development and maintenance of experimental hypertension was evaluated. Single, acute injections of hemicholinium-3 (HC-3), 20 micrograms, were made into the lateral cerebral ventricle of unanesthetized rats during development of hypertension in the spontaneously hypertensive rat (SHR) or following induction of deoxycorticosterone-salt (DOCA), aortic coarctation (AoCo), and Grollman hypertension. HC-3 caused nonsignificant reductions in blood pressure (BP) when injected in SHR and Wistar-Kyoto (WKY) controls at 5 and 8 weeks of age. At 12 weeks in SHR, depressor responsiveness increased concomitantly with the development of hypertension and reached maximal stable effect during established hypertension (18-60 weeks). Intravenous (i.v.) infusion of sodium nitroprusside (NaNP) normalized BP of adult SHR, yet did not prevent the hypotensive response to HC-3. In DOCA hypertension, the HC-3 hypotensive effect (1 week postinduction) preceded establishment of significantly elevated BP (3 weeks), but then remained constant despite a continual rise in BP (2-6 weeks). In the AoCo model, onset of severe hypertension (day 2 postinduction) preceded the appearance of significant depressor responsiveness to HC-3 (day 10). Although BP remained uniformly elevated from 10 to 40 days, the magnitude of the depressor response was stable between 10 and 28 days, then increased further at day 40. At the only time studied, 4 weeks after induction of hypertension, HC-3 also lowered BP of Grollman rats. Marked bradycardia occurred in all models and controls. The results suggest that enhancement of central cholinergic neurotransmission is a common feature of experimental hypertension regardless of its etiology. Increased brain cholinergic function appears to be involved in the development (but not the initiation) and maintenance of elevated BP in the SHR, whereas it may play a role in the initiation of DOCA hypertension and in the maintenance of AoCo hypertension.