Chronic cold stress alters the basal and evoked electrophysiological activity of rat locus coeruleus neurons.

In vivo extracellular single-unit recording techniques revealed that chronic cold stress significantly alters both the basal and the evoked electrophysiological activity of noradrenergic neurons in the locus coeruleus of the anaesthetized rat. Following 17-21 days of chronic cold exposure (5 degrees C), the single-unit activity of histologically-identified locus coeruleus neurons in chloral hydrate-anaesthetized rats was recorded and analysed in terms of their basal firing rate and pattern of spike activity, as well as their response to footshock stimulation. There was no significant difference in the incidence of spontaneously active cells/electrode track between cold-stressed rats and control rats. However, the basal spike activity of locus coeruleus cells recorded from cold-stressed rats differed significantly from that of control rats along two dimensions: i) they displayed significantly higher basal firing rates (mean = 1.88 Hz vs 1.20 Hz, respectively); and ii) they frequently exhibited spontaneous burst-firing activity that was not observed in control rats (observed in 15/17 cold-stressed rats vs 1/26 control rats). The evoked spike activity of locus coeruleus cells in cold-stressed rats also differed significantly from that of control rats along two dimensions: i) they were more likely to respond to footshock stimulation (mean = 90.3% vs 74.4%, respectively); and ii) these responses were more likely to consist of multispike bursts of action potentials (mean = 8 bursts/50 stimulations vs 1 burst/50 stimulations, respectively). These results indicate that alterations in the electrophysiological activity of noradrenergic locus coeruleus neurons may contribute to the phenomenon of stress-induced sensitization of norepinephrine release that is thought to underlie some of the neuropathological changes that accompany long-term stress.

Impact of corticotropin-releasing hormone on extracellular norepinephrine in prefrontal cortex after chronic cold stress.

We have previously demonstrated that exposing rats to cold (5 degrees C) for 3-4 weeks potentiates the increase in extracellular norepinephrine (NE) in the medial prefrontal cortex produced by acute tail shock. In the present study, we used microdialysis to determine the duration of cold exposure required to produce this sensitization and explored the mechanism of the phenomenon. Tail shock elicited a twofold greater increase in extracellular NE in the medial prefrontal cortex of rats exposed to cold for 2 weeks than in naive control rats or in rats exposed to cold for 1 week and tested either immediately or after a 2-week delay. Local infusion of 10 microM D-amphetamine or 30 mM K+ increased extracellular NE in the medial prefrontal cortex (approximately 350 and 190%, respectively) comparably in control rats and rats exposed to cold for 3 weeks. In contrast, intraventricular administration of 3.0 microg of corticotropin-releasing hormone increased extracellular NE in the medial prefrontal cortex by 65% in rats exposed to cold for 2 weeks, but only 35% in control rats. These results indicate that an enhanced responsiveness of noradrenergic neurons to acute tail shock (1) requires approximately 2 weeks of cold exposure to develop and (2) may be mediated by a change at the level of the noradrenergic cell bodies rather than the nerve terminals.

Pyrithiamine-induced thiamine deficiency impairs object recognition in rats.

Pyrithiamine-induced thiamine deficiency (PTD) in rats is used to model the etiology, diencephalic neuropathology, and memory deficits of Korsakoff's amnesia. We assessed the performance of rats exposed to PTD on a test of object recognition--nonrecurring-items delayed nonmatching-to-sample (DNMS). PTD produced thalamic lesions similar to those of Korsakoff's amnesics and similar to those previously observed in PTD rats. PTD rats required more trials to master DNMS at a 4-s retention delay than did controls, and after they had done so, they performed more poorly than controls at delays of 15, 30, 60, and 120 s. DNMS deficits were also observed in PTD rats that received training prior to PTD treatment. These findings support the validity of the PTD rat model of Korsakoff's disease by demonstrating that PTD rats display object-recognition deficits that are similar to those reported in Korsakoff amnesics.

Tolerance to the anticonvulsant effects of carbamazepine, diazepam, and sodium valproate in kindled rats.

We assessed the development of tolerance to the anticonvulsant effects of carbamazepine (CBZ), diazepam (DZP), and sodium valproate (VPA) on convulsions elicited by amygdala stimulation in kindled rats in three similar experiments. In each experiment, amygdala-kindled rats were assigned to a drug group or to a corresponding vehicle control group. The rats in the three drug groups received a total of 10 bidaily (one every 48 h) IP injections of CBZ (70 mg/kg), DZP (2 mg/kg) or VPA (250 mg/kg) at a dose that initially blocked the forelimb clonus elicited by an amygdala stimulation (400 microA, 60 Hz, 1 s) administered 1 h after the injection. The rats in the three vehicle control groups were similarly treated except that they received injections of the saline vehicle. The drug tolerance test occurred 48 h after the final tolerance-development trial; the rats from each drug group and the corresponding vehicle control group received an injection of the appropriate drug followed 1 h later by the administration of a convulsive stimulation. The drug tolerance test revealed almost total tolerance in each of the three drug groups but no tolerance in any of the three vehicle control groups. Such large tolerance effects are inconsistent with the less dramatic effects reported in previous studies; possible reasons for this inconsistency were considered.

Contingent tolerance to the anticonvulsant effects of carbamazepine, diazepam, and sodium valproate in kindled rats.

The effect of convulsive stimulation during periods of drug exposure on the development of tolerance to the anticonvulsant effects of carbamazepine (CBZ), diazepam (DZP), or sodium valproate (VPA) was studied in three similar experiments. In each experiment, amygdala-kindled rats were assigned to one of three groups: one group received a drug injection (CBZ, 70 mg/kg, IP; DZP, 2 mg/kg, IP; VPA, 250 mg/kg, gavage) 1 h before each of a series of 10 bidaily (one every 48 h) convulsive stimulations, a second group received the same dose of the drug 1 h after each of the 10 stimulations, and a third group served as a vehicle control. The drug tolerance test occurred in each experiment 48 h after the 10th tolerance-development trial; every rat received the appropriate dose of CBZ, DZP, or VPA 1 h before being stimulated. In each experiment, only the rats from the drug-before-stimulation group displayed a significant amount of tolerance to the drug's anticonvulsant effect. Thus the development of tolerance to the anticonvulsant effects of CBZ, DZP, and VPA was not an inevitable consequence of drug exposure; the development of tolerance was contingent upon the occurrence of convulsive stimulation during the periods of drug exposure. These results support the idea that functional drug tolerance is an adaptation to a drug's effects on ongoing patterns of neural activity, rather than to drug exposure per se.

Effects of Ro 15-4513 on the motor impairment and hypnotic effects of ethanol and pentobarbital.

The selectivity of Ro 15-4513 in reversing the actions of sedative-hypnotic drugs was examined. The motor impairment induced by i.p. administration of 1.8 g/kg of ethanol was partially reversed by doses of Ro 15-4513 ranging from 2-8 mg/kg. Antagonism of the similar effect induced by 20 mg/kg of pentobarbital was observed only at the 8 mg/kg dose of Ro 15-4513. Treatment with 4 mg/kg of Ro 15-4513 shifted the dose-effect curve for motor impairment by ethanol to the right but did not affect the corresponding curve for pentobarbital. Ro 15-4513 also increased the onset latency and shortened the duration of sleep time induced by ethanol but not by pentobarbital. Blood ethanol and pentobarbital levels measured at 32 min after drug administration were not affected by Ro 15-4513. The selectivity of Ro 15-4513 and the mechanism(s) underlying its proconvulsant effect in reversing the actions of ethanol are discussed.

Attenuation of morphine analgesia by the S2 antagonists, pirenperone and ketanserin.

The involvement of serotonin type-2 (S2) receptors in morphine-induced analgesia was assessed by challenging the effect of 10 mg/kg of morphine sulphate (IP) with the S2 receptor blockers, pirenperone and ketanserin. Tail-flick latencies were assessed at 0, 30, 60, 90 and 120 min after injections by measuring the time that it took each rat to remove its tail from a 52 degrees C water bath. Pirenperone, at 0.08, 0.16, and 0.24 mg/kg (SC) attenuated morphine-induced antinociception. In contrast, only the high 10 mg/kg (SC) dose of ketanserin attenuated the effect of morphine. Because pirenperone easily enters the central nervous system whereas ketanserin does not, these results indicate the involvement of central S2 receptors in morphine-induced antinociception. The 10 mg/kg dose of ketanserin, however, did not attenuate the antinociception produced by 100 mg/kg of ketamine. Thus, the antianalgesic effect of S2 receptor blockers may be specific to opioid-mediated analgesia.

Evidence of Pavlovian conditioned fear following electrical stimulation of the periaqueductal grey in the rat.

Stimulation of the periaqueductal grey (PAG) has been used to support aversive conditioning in a variety of species with several experimental paradigms. However, it has not been clearly demonstrated whether the behavioral changes produced by PAG stimulation in these paradigms are mediated by associative or nonassociative mechanisms. The present studies demonstrate that electrical stimulation of the PAG in the rat may be used to support associative learning in a Pavlovian paradigm. In each experiment, a fully controlled conditional emotional response (CER) procedure was used to examine the unconditional aversive properties of PAG stimulation. In Experiment 1a, weak associative conditioning was observed when a light CS was paired with PAG stimulation over 6 conditioning trials. In Experiment 1b, robust associative conditioning was obtained with a light CS when 18 conditioning trials were used. In Experiment 2, robust associative conditioning was demonstrated with a tone CS when 6 conditioning trials were used. The results parallel those found when other aversive stimuli are used as a UCS (e.g., footshock or intraorbital air puff), and because the present experiments included the proper control procedures the results clearly indicate that the behavioral changes produced by PAG stimulation are mediated by associative Pavlovian learning mechanisms rather than nonassociative mechanisms such as sensitization or pseudoconditioning. The present technique may be useful for assessing the neuroanatomical and neurochemical substrates underlying the aversive effects of brain-stimulation, and for screening the effects of drugs on the conditional and unconditional responses produced by such stimulation.

Response contingency and the dissipation of ethanol tolerance.

Ethanol (1.5 g/kg) administered intraperitoneally to kindled rats blocks the seizures normally elicited by electrical stimulation of the amygdala. Tolerance to this anticonvulsant effect develops following a series of ethanol injections delivered at 48-hr intervals only when an amygdaloid stimulation is administered during each period of ethanol intoxication. In the present study, the response contingency was shown to also play a critical role in the dissipation of tolerance. There was no significant loss of tolerance over a 14-day retention interval in rats that received: 1) bidaily ethanol injections each followed 1 hr later by a convulsive stimulation; or 2) bidaily ethanol injections but no stimulation; or 3) neither ethanol nor stimulation. In contrast, tolerance dissipated completely in rats that received: 1) bidaily stimulations but no ethanol; or 2) convulsive stimulation 1 hr before each bidaily ethanol injection. Accordingly, the cessation of ethanol exposure was neither necessary nor sufficient for the dissipation of tolerance to the anticonvulsant effect of ethanol. The critical factor in the decline of tolerance was the elicitation of seizures in the absence of ethanol.

Contingent tolerance to the anticonvulsant effects of alcohol.

Ethanol (1.5 g/kg) administered intraperitoneally to kindled rats blocked the seizures normally elicited in these subjects by electrical stimulation of the amygdala. Tolerance to this anticonvulsant effect developed following a series of ethanol intubations delivered at 48-hr intervals only when an amygdaloid stimulation was administered during each period of ethanol intoxication. Subjects stimulated 1 hr following each intubation were tolerant to the intraperitoneal test dose after only five intubations (2 g/kg), whereas those stimulated 1 hr prior to each intubation displayed no tolerance during the course of 20 such trials. Even at high intubation doses (5 g/kg), significant levels of tolerance to the anticonvulsant effect of the intraperitoneal test dose were not observed in subjects unstimulated during each period of post-intubation intoxication. These findings emphasize the important role of response contingency in ethanol tolerance; tolerance develops readily for only those effects of alcohol that repeatedly manifest themselves during the periods of ethanol exposure.

Central angiotensin III-induced dipsogenicity in rats and gerbils.

Previous findings from our laboratory demonstrated [125I]angiotensin II (AII) binding to plasma membranes from rat but not gerbil circumventricular organs (CVOs), the presumed location of brain receptors for angiotensin-induced dipsogenicity. Since members of both species drink to intracranially applied AII, a degradation product of AII was suspected to be the active ligand in gerbils. High specific [125I]angiotensin III (AIII) binding capacity was presently determined in CVOs taken from both rats and gerbils. Nearly identical dose-response curves were obtained for members of each species following the intracerebroventricular injection of AIII; however, rats drank more water than gerbils following the administration of AII. These results were interpreted to suggest that the dipsogenically active ligand in gerbils is AIII or derived from AIII, and that this analogue also contributes to angiotensin-induced drinking in rats. Since the distribution of specific angiotensin binding capacity represented by gerbil closely approximates that seen in non-human primate brain, these findings are of particular relevance and encourage future efforts directed toward understanding the role of AII metabolites in the central control of dipsogenicity.