Michael J. Mana

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.

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 drugs 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 drugs effects on ongoing patterns of neural activity, rather than to drug exposure per se.

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.

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 Korsakoffs 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 Korsakoffs 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 Korsakoffs disease by demonstrating that PTD rats display object-recognition deficits that are similar to those reported in Korsakoff amnesics.

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.

Contingent Tolerance to the Anticonvulsant Effects of Drugs on Kindled Convulsions

At the Kindling 3 Conference in 1985, we reported that the development of tolerance to ethanol’s anticonvulsant effect on kindled convulsions elicited in rats by amygdalar stimulation is greatly influenced by the temporal relation between the administration of ethanol and the convulsive stimulation (18). We reported that substantial tolerance developed to ethanol’s anticonvulsant effect if the kindled rats were stimulated following each of five bidaily (once every 48 hr) intraperitoneal ethanol injections, but not if they were stimulated before each injection (see also 16). The fact that both groups of rats received the same exposure to ethanol and the same number of stimulations suggested that the critical factor in the development of the tolerance was the administration of convulsive stimulation during the periods of ethanol exposure. On the basis of this and similar observations (e.g, 13;17;20), we have proposed a theory of tolerance that emphasizes the idea that functional drug tolerance is a reaction to the expression of a drug’s effect rather than to the mere presence of the drug in the system (19). Although a drug’s effects are often an inevitable consequence of drug exposure, in some instances they may not be fully expressed unless the nervous system (or other target tissue) is involved in a specific pattern of neural activity during periods of drug exposure (cf., 4).

Postshock learning and conditioned defensive burying

Natural sources of aversive stimuli are frequently well-defined material objects that are present both before and after the aversive event. In the present experiment, rats acquired information about such a source after the aversive event and used this information to guide their subsequent defensive reactions to it. The rats were shocked by one of two possible sources, either a black or a striped prod, mounted on opposite end walls of the test chamber. Immediately following the shock, the houselights were momentarily extinguished and the patterns on the two prods were automatically switched for subjects in the experimental condition or left unchanged for subjects in the control condition. The rats were left in the chamber for another 5 min with the patterns in their new positions before being removed for 2 min while the two prods were mounted on the side walls. During the ensuing test of conditioned defensive burying, the rats in the control condition directed the majority of their burying behavior at the prod exhibiting the pattern displayed by the shock source prior to and during the shock administration. In contrast, the rats in the experimental group buried the prod exhibiting the pattern displayed by the shock source during the postshock period more than they did the prod displaying the pattern present on the shock source prior to and during the shock administration.