Involvement of PACAP receptor in primary afferent fibre-evoked responses of ventral roots in the neonatal rat spinal cord

The role of PACAP receptor in nociceptive transmission was investigated in vitro using maxadilan, a PACAP receptor selective agonist and max.d.4, a PACAP receptor selective antagonist. Potentials, from a ventral root (L3 - L5) of an isolated spinal cord preparation or a spinal cord - saphenous nerve - skin preparation from 0 - 3-day-old rats, were recorded extracellularly. In the isolated spinal cord preparation, single shock stimulation of a dorsal root at C-fibre strength induced a slow depolarizing response lasting about 30 s (slow ventral root potential; slow VRP) in the ipsilateral ventral root of the same segment. Bath-application of max. d.4 (0.01 - 3 microM) inhibited the slow VRP in a concentration-dependent manner. In the spinal cord - saphenous nerve - skin preparation, application of capsaicin (0.1 microM) to the skin evoked a depolarization of the ventral root. This response was also depressed by max.d.4 (1 microM). Application of maxadilan evoked a long-lasting depolarization in a concentration-dependent manner in the spinal cord preparation. In the presence of max.d.4 (0.3 microM), the concentration response curve of maxadilan was shifted to the right. Reverse transcription-polymerase chain reaction (RT - PCR) experiments demonstrated the existence of PACAP receptor and VPAC(2) receptor in the neonatal rat spinal cord and [(125)I]-PACAP27 binding was displaced almost completely by maxadilan and max.d.4, but not by vasoactive intestinal peptide (VIP). These data indicate that PACAP receptor is dominantly distributed in the neonatal rat spinal cord. The present study suggests that PACAP receptor may play an excitatory role in nociceptive transmission in the neonatal rat spinal cord.

Pituitary adenylate cyclase-activating polypeptide expression and modulation of neuronal excitability in guinea pig cardiac ganglia

Cardiac output is regulated by the coordinate interactions of stimulatory sympathetic and inhibitory parasympathetic signals. Intracardiac parasympathetic ganglia are integrative centers of cardiac regulation, and modulation of the parasympathetic drive on the heart is accomplished by altering intrinsic cardiac ganglion neuron excitability. The pituitary adenylate cyclase-activating polypeptide (PACAP)/vasoactive intestinal peptide (VIP) family of peptides modulates cardiac function, and in guinea pig heart, PACAP appears to act directly on intrinsic parasympathetic cardiac ganglia neurons through PACAP-selective receptors. A multidisciplinary project tested whether cardiac PACAP peptides act through PACAP-selective receptors as excitatory neuromodulators amplifying the parasympathetic inhibition from guinea pig cardiac ganglia. The in vivo sources of regulatory PACAP peptides were localized immunocytochemically to neuronal fibers and a subpopulation of intrinsic postganglionic cardiac neurons. RT-PCR confirmed that cardiac ganglia expressed proPACAP transcripts and have PACAP peptide biosynthetic capabilities. Messenger RNA encoding PACAP-selective PAC1 receptor isoforms were also present in cardiac ganglia. Alternative splicing of PAC1 receptor transcripts produced predominant expression of the very short variant with neither HIP nor HOP cassettes; lower levels of the PAC1HOP2 receptor mRNA were present. Almost all of the parasympathetic neurons expressed membrane-associated PAC1 receptor proteins, localized immunocytochemically, which correlated with the population of cells that responded physiologically to PACAP peptides. PACAP depolarized cardiac ganglia neurons and increased neuronal membrane excitability. The rank order of peptide potency on membrane excitability in response to depolarizing currents was PACAP27>PACAP38>VIP. The PACAP-induced increase in excitability was not a function of membrane depolarization nor was it caused by alterations in action potential configuration. These results support roles for PACAP peptides as integrative modulators amplifying, through PACAP-selective receptors, the parasympathetic cardiac ganglia inhibition of cardiac output.

Pituitary adenylate cyclase-activating peptide (PACAP) in the retinohypothalamic tract: a potential daytime regulator of the biological clock

The retinohypothalamic tract (RHT) relays photic information from the eyes to the suprachiasmatic nucleus (SCN). Activation of this pathway by light plays a role in adjusting circadian timing via a glutamatergic pathway at night. Here we report a new signaling pathway by which the RHT may regulate circadian timing in the daytime as well. We used dual immunocytochemistry for pituitary adenylate cyclase-activating peptide (PACAP) and the in vivo tracer cholera toxin subunit B and observed intense PACAP-immunoreactivity (PACAP-IR) in retinal afferents in the rat SCN as well as in the intergeniculate leaflet (IGL) of the thalamus. This PACAP-IR in the SCN as well as in the IGL was nearly lost after bilateral eye enucleation. PACAP afferents originated from small ganglion cells distributed throughout the retina. The phase of circadian rhythm measured as SCN neuronal activity in vitro was significantly advanced (3.5 +/- 0.4 hr) by application of 1 x 10(-6) M PACAP-38 during the subjective day [circadian time (CT)-6] but not at night (CT14 and CT19). The phase-shifting effect is channeled to the clock via a PACAP-R1 receptor, because mRNA from this receptor was demonstrated in the ventral SCN by in situ hybridization. Furthermore, vasoactive intestinal peptide was nearly 1000-fold less potent in stimulating a phase advance at CT6. The signaling mechanism was through a cAMP-dependent pathway, which could be blocked by a specific cAMP antagonist, Rp-cAMPS. Thus, in addition to its role in nocturnal regulation by glutamatergic neurotransmission, the RHT may adjust the biological clock by a PACAP/cAMP-dependent mechanism during the daytime.