Role of adenosine A2a receptors in the nucleus accumbens.

Adult male mice were stereotaxically implanted with permanent indwelling guide cannulae for bilateral injections into the nucleus accumbens (ACB). The effects on spontaneous locomotor activity of selective agonists for adenosine receptor subtypes were examined following bilateral injections into the ACB. Intra-ACB injections of CGS 21680, a potent and selective agonist at striatal adenosine A2a receptors, elicited pronounced, dose-related reductions in locomotor activity whereas similar bilateral dosages of CPA, a selective agonist at adenosine A1 receptors, did not significantly affect locomotor activity. The pronounced locomotor depression elicited by intra-ACB injections of CGS 21680 were completely blocked by I.P. pretreatment with DMPX, an adenosine receptor antagonist exhibiting selectivity for striatal A2 receptors, at a dosage which alone had no significant effect on locomotor activity. Adenosine A2a receptors in the nucleus accumbens may selectively modulate dopamine-mediated mesolimbic behavioral circuits involved in spontaneous locomotion.

Adenosine A2a receptors in the nucleus accumbens mediate locomotor depression.

The effects on locomotor activity (LA) of selective agonists for adenosine receptor subtypes were examined in mice following bilateral injections into the nucleus accumbens (ACB). The ACB is not only richly innervated by dopaminergic (DA) terminals but also exhibits the highest densities of adenosine A2a receptors in the brain. CGS 21680 (2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosi ne), a potent and highly selective adenosine A2a receptor agonist, elicited pronounced, dose-related reductions in LA (ID50 dosage = 0.0031 nmol/mouse). NECA (5'-N-ethylcarboxamidoadenosine), a mixed adenosine receptor agonist which exhibits high selectivity and potency at striatal A2a receptors, similarly elicited dose-related reductions in LA (ID50 dosage = 0.0023 nmol/mouse). In contrast, intra-ACB injections of CPA (N6-cyclopentyladenosine), a highly selective agonist for adenosine A1 receptors, did not exert any significant effects on LA, even at 2.0 nmol/mouse, a dosage at which both CGS 21680 and NECA depressed LA by almost 90% compared to vehicle controls. Further, the pronounced locomotor depression elicited by intra-ACB injections of both CGS 21680 and NECA, at approximately the ID65 dosage, was significantly antagonized by IP pretreatment with DMPX, (3,7-dimethyl-1-propargylxanthine), an adenosine receptor antagonist with selectivity for A2 receptors in the striatum, at a dosage (0.15 micromol/mouse) [corrected] which alone had no significant effect on LA. These observations provide support for the notion that adenosine may selectively modulate DA-mediated mesolimbic behavioral circuits via agonist actions at a population of A2a receptors densely concentrated in the ventral striatum.

An atlas of the rat subpostremal nucleus tractus solitarius.

The nucleus tractus solitarius (NTS) in the dorsal medulla is the principal visceral sensory relay nucleus in the brain. In the rat, numerous lines of evidence indicate that the caudal NTS at the level of the area postrema serves as a major integrating site for coordinating cardiorespiratory reflexes and viscerobehavioral responses. This region of the caudal NTS not only exhibits high densities of binding sites for an impressive array of transmitters and modulators but microinjections of many of these same neuroactive substances into the rat subpostremal NTS elicit pronounced cardiorespiratory and visceral response patterns. This report provides an abbreviated atlas of the rat subpostremal NTS consisting of a series of transverse, sagittal, and horizontal plates. Photomicrographs, together with their corresponding schematic drawings, are provided for the serial sections generated from each reference plane.

The adenosine analog, 5'-N-ethylcarboxamidoadenosine, exerts mixed agonist action on cardiorespiratory parameters in the intact but not decerebrate rat following microinjections into the nucleus tractus solitarius.

A limited occipital craniotomy was conducted on intact and decerebrate urethane-anesthetized, spontaneously breathing rats to expose the caudal medulla in the region of the obex. Microinjections of 5'-N-ethylcarboxamidoadenosine (NECA), a metabolically stable adenosine analog which exhibits mixed agonist properties for adenosine receptor subtypes, were made into the medial region of the caudal nucleus tractus solitarius (NTS) at the level of the caudal tip of the area postrema, an area of the NTS in which there is known to be a functional co-existence of cardiovascular and respiratory-related neuronal elements. Cardiorespiratory responses were subsequently recorded for a 30-min test period. In the intact rat, microinjections of NECA produced significant dose-related reductions in respiratory rate which were accompanied by dose-dependent increases in tidal volume and these pronounced effects on respiration persisted throughout the test period. On the other hand, microinjections of NECA into this region of the NTS of the intact rat elicited complex, bi-directional cardiovascular responses, producing hypotension (at lower doses) and pressor responses (at higher doses) in addition to bradycardia (at lower doses). In an effort to examine the functional interactions between the NTS and forebrain structures involved in cardiorespiratory control, microinjections of NECA in the identical dose range were made into the same NTS sites of a separate group of urethane-anesthetized, spontaneously breathing rats in which reciprocal connections between forebrain areas and the brainstem had been disrupted by acute supracollicular decerebration. A simulating electrode, placed in the paraventricular nucleus of the hypothalamus (PVH), was used to confirm complete transection during the experiment and to ascertain the integrity of reciprocal connections between the brainstem and rostral brain regions involved in cardiorespiratory control. Although decerebration at the supracollicular level negligibly affected basal cardiorespiratory parameters, microinjections of NECA into the NTS revealed dramatic differences in the cardiovascular response patterns between intact and decerebrate rats. Whereas cardiovascular responses elicited by microinjections into the NTS were significantly affected by supracollicular decerebration, respiratory responses were highly similar for both intact and decerebrate animals. Indeed, repeated measures MANOVA indicated that there were no significant differences in the time-related or dose-related responses in the depression of respiration between decerebrate and intact rats following NECA microinjections.(ABSTRACT TRUNCATED AT 400 WORDS)

Respiratory effects of cyclic AMP following injection into the nucleus tractus solitarius of rats.

Rats were anesthetized with urethane and a limited occipital craniotomy was performed to expose the caudal medulla in the region of the obex. Injections were made into sites in the brainstem of spontaneously-breathing rats through glass micropipettes. Tidal volume, respiratory frequency, minute volume, blood pressure and heart rate were recorded before and after the administration of 8-bromoadenosine 3',5'-cyclic monophosphate (Br-cAMP), an analog of cyclic AMP. Injections of Br-cAMP into the ventromedial portion of the caudal nucleus tractus solitarius (NTS) produced dose-related decreases in pulmonary ventilation due to effects on both respiratory frequency, as well as minute volume. In larger doses, Br-cAMP produced periodic apnea and irregular breathing. The respiratory depression was accompanied by transient hypotension and bradycardia. The data indicate that cyclic AMP may function as a second messenger in respiratory control regions in the brainstem.

Cardiovascular effects of microinjection of adenosine into the nucleus tractus solitarius.

Rats were anesthetized with urethane and limited occipital craniotomy was conducted to expose the caudal medulla in the region of the obex. Microinjections of adenosine were made into the nucleus tractus solitarius and heart rate and blood pressure responses recorded. Adenosine produced dose-related decreases in blood pressure and heart rate. The data indicate that adenosine may play a neuromodulatory role in central cardiovascular control areas.

Cardiovascular effects of microinjections of adenosine analogs into the fourth ventricle of rats.

Rats were implanted with chronic indwelling cannulae into the posterior region of the fourth ventricle. After recovery from surgery, acute experiments on blood pressure were conducted under urethane anesthesia. The blood pressure and heart rate responses following administration of two adenosine analogs, NECA and L-PIA were examined. Microinjections of both analogs produced dose-dependent reductions in blood pressure and heart rate. NECA was approximately 20-fold more potent than L-PIA in reducing blood pressure and depressing heart rate. The cardiovascular effects of both analogs were antagonized by parenteral injections of caffeine. These findings show that microinjections of analogs of adenosine into the fourth ventricle can influence areas of the central nervous system involved in cardiovascular control.